Electrical and Optical Studies of Some Liquid Crystals

Transcription

1 Electrical and Optical Studies of Some Liquid Crystals THESIS SUBMITTED TO THE UNIVERSITY OF LUCKNOW FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS BY Pankaj Kumar Tripathi M.Sc. (Physics) Under the supervision of Dr. Rajiv Manohar Ph. D. (Physics) Associate Professor Department of Physics University of Lucknow Liquid Crystal Research Laboratory Department of Physics University of Lucknow Lucknow, U. P., India 2013

2 Dedicated to my grandfather Late Sri Badri Nath Tripathi for his eternal support

3 Acknowledgement Certificate List of Publication Abstract Glossary TABLE OF CONTENT i iv vi ix xiv Chapter 1- General Introduction Introduction 1.2. History of Liquid Crystal 1.3. Classification of Liquid Crystal Chemical Concept Lyotropic Liquid Crystal Thermotropic Liquid Crystal Molecular shape concept Calamitic Liquids Crystal Disc Shape Molecules Banana Shape Molecules Unusual Shape Molecules Molecular Arrangement Concept Nematic Liquid Crystals Chiral Nematic or Cholesteric Liquid Crystal Smectic Liquid Crystal Smectic A Liquid Crystal Smectic C Liquid Crystal Smectic B Liquid Crystal Smectic E Liquid Crystal Smectic I and F Liquid Crystal Smectic G, H, G and H Liquid Crystal Chiral Smectic or Ferroelectric Liquid Crystal Surface-Stabilized Ferroelectric Liquid Crystals (SSFLC) Antiferroelectric Liquid Crystal (AFLC) Ferrielectric Liquid Crystal (FLC) Twisted Grain Boundary Phase (TGB) The Chiral Line Liquid Bend Grain Boundary (BGB) Phase Reentrant Phase of Polar Liquid Crystals 1.4. Effect of Chemical Constitution on Mesomorphism 1.5. Physical Properties of Liquid Crystals 1.6. Applications of Liquid Crystals Display Applications of Liquid Crystals Thermal Mapping and Non-Destructive Testing Medicinal Uses Optical Imaging Liquid Crystal Solar Cell Reference

4 Chapter 2- Theoretical Background of Liquid Crystals Review of Liquid Crystal Basic Properties The Director The Order Parameter Tilt Angle Spontaneous Polarization 2.2. Basic Physical Properties The Anisotropy of Liquid Crystals Dielectric Anisotropy in Liquid Crystals Optical Anisotropy Of Liquid Crystals Uniaxial Versus Biaxial Media Flexoelectric Effect In Liquid Crystals 2.3. Visco-elestic Properties Viscosity and Rotational Viscosity Elasticity Theory Liquid Crystal Anchoring and Easy Axis Anchoring Energy Elastic Deformation of the LC Director 2.4. External Field effects Euler-Lagrange equations Alignment Threshold Voltage and Fredericksz Transition Response Time 2.5. Erickson-Leslie Equation 2.6. Interaction with Light: Propagation in Anisotropic Media 2.7. Phase Transition Landau-de Gennes Theory of Phase Transition Maier-Saupe Theory (Mean Field Approach) 2.8. Dielectric Spectroscopy Study The complex dielectric permittivity Debye type polarization mechanism Non-Debye polarization mechanism The Cole-Cole plot Parameters obtained from fitting of Cole-Cole equation Dielectric relaxation Non collective (molecular) relaxation process Collective relaxation process (A) Tilt angle fluctuations or soft mode (B) Phase angle fluctuations or Goldstone mode (C) Influence of dc bias electric field on the helix distortion mode 2.9. Guset host interaction Liquid crystal and nano composites Nanoparticles History

5 Properties Classification Characterization Nanoparticle morphology Liquid crystal and dye composites Liquid crystal and polymer composites References Chapter 3- Experimental Techniques Introduction 3.2. Sample holder fabrication techniques Electrode preparation Substrate cleaning Photolithography Photoresist application Soft baking Mask alignment and exposure Development and hard baking Etching Surface alignment Homogenous (planar) alignment Homeotropic alignment Hybrid alignment Tilted orientation Assembling, sealing and filling the cell 3.3. Instruments and experimental techniques used to characterize the liquid crystal materials Instruments used to characterize liquid crystal materials Experimental identification of liquid crystals 3.4. Spontaneous polarization 3.5. Optical transmittance 3.6. Response time measurement 3.7. Threshold voltage 3.8. Splay elastic constant and rotational viscosity 3.9. Temperature controller Dielectric spectroscopy study The stray capacitance problem Measurement How to cope with unwanted absorption Low frequency ionic contribution High frequency cell relaxation problem Data presentation References

6 Chapter 4 - Improved Dielectric and Electro- Optical Parameters of Zno Nano Particle (8% Cu 2+ ) Doped Nematic Liquid Crystal Introduction 4.2. Experimental details 4.3. Results and discussion 4.4. Conclusion References Chapter 5 - Abnormal Switching Behavior of Liquid Crystal Composite Introduction 5.2. Experimental details 5.3. Results and discussion 5.4. Conclusion References Chapter 6- Changes in Material Parameters for Dye Doped Ferroelectric Liquid Crystal Introduction 6.2. Experimental details 6.3. Results and discussion 6.4. Conclusion References Chapter 7-Goldstone and Soft Mode for Fluorescent Dye Doped Ferroelectric Liquid Crystal Introduction 7.2. Experimental details 7.3. Results and discussion 7.4. Conclusion References

7 Chapter 8-Reduction of Optical Response Time for Fluorescent Dye Doped Ferroelectric Liquid Crystal Introduction 8.2. Material and Methods 8.3. Results and discussion 8.4. Conclusion References Chapter 9-Final Conclusion and Discussion with Future Plans

8 i Acknowledgement The Research work presented in this thesis was carried out at the Department of Physics, University of Lucknow, Lucknow. I am sincerely thankful to ISRO for providing financial support during research period. I would like to thank many persons, who have helped and supported me during my studies leading to this thesis. First of all, I wish to express my highest gratitude to my supervisor Dr. Rajiv Manohar, without whom this work would have been impossible to accomplish. Throughout this research, he provided me with continuous scientific help, invaluable guidance, incessant encouragement, and unparallel affection which propelled me with the zeal and determination that were indispensable for this accomplishment. He constantly helped and encouraged me in difficult times. He also taught me several other things aside from physics. His compassion and kindness makes me realize how lucky I am to have him as my supervisor. I would like to specially thank Dr. K. R. Murli, Focal person, ISRO, SAC, Ahmadabad, for his help and for providing new ideas to experimental work during my research time. I would like to indebted my gratitude to Mrs. Manohar for her sustained hospitality and care during the long hour of discussion with my supervisor during complete Ph. D. course. My thankfulness also goes to Prof. Kirti Sinha, Head Department of Physics, University of Lucknow, Lucknow for making available the necessary facilities in this research pursuance. It is a great privilege to express my deep sense of gratitude to Prof. J.P. Shukla, Emeritus Scientist and Former Head, Physics Department, Lucknow University, Lucknow who kept me reminding that remain pragmatic in my endeavour.

9 ii I am also thankful to Dr. B. P. Pandey, Ex. Head, Department of Physics and Dr. P. K. Singh, M. L. K. (P.G.) College, Balrampur, who always helped and encouraged me to do much effort for their goal right from the beginning of my post graduation. I must express my thanks to my seniors Dr. Abhishek Kumar Srivastava, Dr. Abhishek Kumar Misra, Dr. Kamal Kumar Pandey and Dr. Satya Prakash Yadav for their suggestions and importunate encouragements. They also shared their thoughts, perspectives, insights and experiences, which have positively impacted the quality of my thesis. I would like to mention the huge amount of work done by the members of our research group who faultlessly and reliably have run loads of experiments for me. Therefore, many thanks are due to Dr. Satya Prakash Yadav. I will always remember with fondness the many nights that we worked late together, the early-morning dinners that we shared, and many words of encouragement when we couldn t understand the experimental results. I am also deeply indebted to Dr. Abhishek Kumar Misra. He was very kind to me and always concerned about my well-being. I am especially thankful to Dr. Abhishek Kumar Srivastava to help me from India and also from abroad. In addition to this, I could never overlook to acknowledge Dr. V. S. Chandel, Dr. S. P. Singh, Dr. Piyush Shukla, and Mr. Devesh Kumar Shukla (Research Scholar IIT Roorkee) for their persistent support. I wish to express my warm and sincere thanks to Dr. Prachi Tripathi, Mr. Sudhaker Dixit and my senior Dr. P.B. Chand and my colleague Mr. Swadesh Kumar Gupta and Mr. Dharmendra Pratap Singh for their support and their timely help and valuable discussions. I wish the best of luck to the two newest additions to the lab group Mrs. Tripti Vimal and Mrs. Shivani Pandey, Thanks to my friends of the research group for the great times that we have shared and the support that they have provided. The positive atmosphere in the lab, where everybody supports each other is an example for many other research labs. My final sincere appreciation goes to my family, my father Sri. Vijay Bahadur Tripathi, mother Smt. Gangotri Tripathi, and my brother Dilip Kumar Tripathi. Their love and emotional support have been the invisible motivation to

10 iii me. They have always prayed for my success, encouraged me, and been there whenever I need them. In the end of my acknowledgement, I would like to convey my message to the next generation that I have learned during the Ph.D. program, communicating by following lines of Swami Vivekananda :- All power is within you; you can do anything and everything. Swami Vivekananda Date - (Pankaj Kumar Tripathi)

11 iv CERTIFICATE This is to certify that all regulations necessary for the submission of Ph. D. thesis of Mr. Pankaj Kumar Tripathi have been fully observed. The contents of this thesis are original and have not been presented elsewhere for award of Ph. D. degree. (Dr. Rajiv Manohar) Associate Professor Supervisor

12 v CERTIFICATE This is to certify that all regulations necessary for the submission of Ph. D. thesis of Mr. Pankaj Kumar Tripathi have been fully observed. The contents of this thesis are original and have not been presented elsewhere for award of Ph. D. degree. Head Department of Physics (Dr. Rajiv Manohar) Associate Professor Supervisor

13 vi List of Publication Published Papers 1. Satya P. Yadav, Kamal K. Pandey, Abhishek K. Misra, Pankaj K. Tripathi and Rajiv Manohar, Journal of Physica Scripta, 83 (2011) (5pp). 2. Rajiv Manohar, Abhishek K. Srivastava, Pankaj K. Ttripathi and Dharmendra P. Singh, Journal of Materials Science, 46 (2011) Abhishek K. Misra, Satya P. Yadav, Pankaj K. Tripathi and Rajiv Manohar, Journal: Physics and Chemistry of Liquids, 50 (2012) Abhishek K. Misra, Pankaj K. Tripathi and Rajiv Manohar, Journal of Molecular Liquid, 175 (2012) Dharmendra P. Singh, Satya P. Yadav, Pankaj K. Tripathi, Prachi Tripathi and Rajiv Manohar, Journal: Soft Materials, 11 (2013) Pankaj K. Tripathi, Abhishek K. Misra, Shashwati Manohar, Swadesh K. Gupta and Rajiv Manohar, Journal of Molecular Structure, 1035 (2013) Abhishek K. Misra, Pankaj K. Tripathi and Rajiv Manohar, Journal: Phase Transitions, 86, 10 (2013) Swadesh K. Gupta, Dharmendra P. Singh, Pankaj K. Tripathi, Rajiv Manohar, Laxmi Sagar & Sandeep Kumar, Journal: Liquid Crystals, 40, 4 (2013) Pankaj k. Tripathi, Abhishek K. Misra, Kamal K. pandey, Satya P. Yadav & Rajiv Manohar, Journal: Phase Transitions, ID: , DOI: / Kamal K. Pandey, Abhishek K. Misra, Pankaj K. Tripathi, Satya P. Yadav and Rajiv Manohar Journal: Molecular Crystals and Liquid Crystals, 582 (2013) Abhishek K. Misra, Pankaj K. Tripathi and Rajiv Manohar, Journal: Non Crystalline solid, 376, 15(2013) Abhishek K. Misra, Pankaj K. Tripathi, Kamal K. Pandey & Rajiv Manohar. Journal: Molecular Crystals and Liquid Crystals ID: DOI: / Abhishek K. Misra, Pankaj K. Tripathi, and Rajiv Manohar, AIP Conf. Proc. 1536, 933 (2013); doi: /

14 vii 14. Pankaj K. Tripathi, Abhishek K. Misra, Shivani Pandey and Rajiv Manohar, AIP Conf. Proc. 1536, 885 (2013); doi: / Pankaj K. Tripathi, Abhishek K. Misra, K. K. Pandey and Rajiv Manohar, Chemical Rapid Communications (CRC) Vol.1 No.1. ISSN (2013) ISSN Conference Papers 1. Satya P. Yadav, Pankaj K. Tripathi, S. P. Singh, P. Kumar, R. Manohar and J. P. Shukla, Poster presentation in 16th NCLC held at University of Lucknow, Lucknow, Pankaj K. Tripathi and R. Manohar, Poster presentation in 17 th National Conference on Liquid crystals held at Surat, Participated in a NASI sponsored Workshop Writing Research paper held at Banaras Hindu University, Varanasi, June 10-11, Pankaj K. Tripathi, Sudhaker Dixit, Satya P. Yadav and Rajiv Manohar Poster presentation in a Soft Matter Chemistry Workshop held at Raman Research Institute, Bangalore, India, November 9-11, Abhishek K. Misra, Pankaj K. Tripathi and Rajiv Manohar, Poster presentation in a 19th National Conference on Liquid Crystals (NCLC-19) held at Thapar University, Patiala (Punjab), November 21-23, Pankaj K. Tripathi, Shivani Pandey, V. S. Chandel and Rajiv Manohar, Poster presentation in a 19th National Conference on Liquid Crystals (NCLC-19) held at Thapar University, Patiala (Punjab), November 21-23, Abhishek K. Misra, Pankaj K. Tripathi, Kamal K. Pandey and Rajiv Manohar, Poster presentation in a Conference On Condensed Matter And Biological Systems (CCMB13) held at Physics Department, Banaras Hindu University, Varanasi , India, January 11-14, Pankaj K. Tripathi and Rajiv Manohar, Poster presentation in a Conference On Condensed Matter And Biological Systems (CCMB13) held at Physics Department, Banaras Hindu University, Varanasi , India, January 11-14, Pankaj K. Tripathi, Abhishek K. Misra and Rajiv Manohar, Poster Presentation in the UCOST Sponsored National Conference held at

15 viii Department of Mathematics, Govt. P. G. College, Lansdowne (Jaiharikhal) Pauri Garhwal, Uttarakhand, INDIA, March 21-22, Pankaj K. Tripathi, Abhishek K. Misra and Rajiv Manohar, Poster Presentation in a 14 th International Conference on Ferroelectric Liquid Crystals (FLCC-2013) held at Otto-von-Guericke-University Magdeburg, Germany, September 01-06, 2013.

16 ix ABSTRACT Liquid crystals possess large dielectric and electro-optical properties owing to their large anisotropy coupled with the collective molecular reorientation. Doping dyes and nanoparticle into liquid crystals increases their optical responses significantly due to increased induced intermolecular torque, and other guest-host effects. The guest-host mixtures can be employed in display applications, optical storage devices. In this thesis, dielectric and electro- optical studies were carried out on nano-doped nematic liquid crystal cells and dye doped ferroelectric liquid crystal cells. The main objectives of the studies were to distinguish and characterize the several processes that can lead to the formation of dynamical response of different types in the samples, and to study the dielectric anisotropy, permittivity, splay elastic constant rotational viscosity and the orientational responses of these samples. Furthermore, we tried to explain and model the dynamical behaviors of the observed influence of non-mesogenic compound such as nanoparticle and dye on pure LC systems. Liquid crystals is a wide applications in the field of Physics, Chemistry and Biology and nanoparticle doped and dyes doped systems are used for variety of physical and biological processes due to their excellent fluorescence quantum yield and high sensitivity to the change in surrounding medium. In recent years materials with higher order optical nonlinearity are under investigation due to their applications in optical communications, image processing, switching, 3D data storage and optical limiting. The phenomenon of optical limiting is useful for automatic protection of human eyes and sensors against intense laser radiation (Tutt et al., 1993). Morphological, dielectric and electroptical data were the various tools used to study the behaviour of LCs on dye and nanoparticle addition. The dielectric and electro-optical properties have been studied for nanoparticle doped in nematic liquid crystals and ferroelectric liquid crystals as well as fluorescent dye have also used in doping. The present work shows how the presence of nanoparticle changes the dielectric properties of the LCs due to the strong interaction between LC and nanoparticles. Thus, we concentrate on the impact of ZnO nanoparticle and dye

17 x doped liquid crystal on the dielectric and electro-optical properties of LC composites systems. In the present thesis two nematic liquid crystals namely MBBA (pmethoxybenzylidene p-butylaniline) and p-butoxybenzylidene, pheptylaniline (BBHA) and two ferroelectric liquid crystals (FLC) namely Felix and Felix have been used for the investigation. The zinc oxide nanoparticles with 8% Cu 2+ doped, has been used as guest material. The influence of nanoparticles on the display parameters have been studies with the help of dielectric spectroscopy studies. The dielectric anisotropy, which is a crucial parameter for the use of these materials in displays, has been investigated with variation of frequency and temperature. The electro-optical investigation of the nematic liquid crystal i.e. MBBA and BBHA with nanoparticles has also been performed. The observed electro-optical parameters are threshold voltage (Vth), response time (), elastic constant and rotational viscosity (). The theoretical treatment of the energy functional of nanonematic suspension has been carried out. Chapter 1 is an introductory chapter and mainly deals with the physical properties of liquid crystals. Beginning with an overview of the liquid crystals and liquid crystal phase formation has been introduced in this chapter. The classification scheme has also been discussed in detail. The twist grain boundary phase, chiral line liquid phase, bend grain boundary phases of liquid crystal and ferroelectric liquid crystal has also been discussed. The chapter ends with the application of liquid crystals and their importance in the various fields. Chapter 2 ( Theoretical Background of Liquid Crystal ) describes brief discussion about basic terms related with liquid crystals such as director, order parameter, anisotropy etc, have been given in this chapter. The visco-elastic properties including viscosity, elasticity, response time etc. have also been discussed in the mid of the chapter. There are different methods for the measurement of dielectric behaviour. Some crucial and important methods have also been discussed to the point. How dielectric spectroscopy is helpful to the investigation of liquid crystals and what type of molecular motions in liquid crystals is associated with variation of frequency has also been discussed in brief. The phase transition theories of liquid crystals such as Landau-deGennes theory, Maier-Saupe theory etc

18 xi have also been discussed. In the last the description and classification of nanoparticles has been given and guest host interaction discussed in brief. Chapter 3 is an introduction to the experimental techniques used in the present thesis. Sample holder preparation is the most important part of the investigation. The complete fabrication technique and precautions related with each step has been discussed extensively. Different type of alignment techniques have been given in this chapter. Dielectric measurements suffer from many problems. These problems and how to cope with these problems has also been discussed in broad. Dielectric and electro-optical parameters have been determined in the Chapter 4, to understand the effect of 8% Cu 2+ doped ZnO nano particles in the pure nematic liquid crystal (NLC). The magnitude of dielectric anisotropy was found to increase in negative order and in turn it reduces the response time for nano-nematic composite system. However the threshold voltage has shown an increase for this composite system. The splay elastic coefficient, rotational viscosity and other parameter have also shown improvement. All the results i.e. related to the determination of dielectric and electro-optical parameters have been well explained by using existing theory of NLC. Chapter 5 deals with the changes in dipole dynamics with addition of nanoparticles in liquid crystal. The liquid crystal used here is BBHA and the zinc oxide nanoparticles are used. To study the effect of nanoparticles on the dipole dynamics and dielectric anisotropy of weakly polarized liquid crystal (BBHA), the electro-optic properties of LC have changed with varying concentration of ZnO nanoparticle. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 khz in the nematic and smectic phase were found to increase with increasing concentration of nanoparticle in liquid crystal. It has been established that the effect of nanoparticle on the dielectric anisotropy depends on the physical properties of LC; the nanoparticle disturb the orientation ordering of liquid crystals molecules. The nanoparticle also influences the switching behaviour, splay elastic constant, rotational viscosity and threshold voltage of pure LC. A small quantity of nanoparticle causes slight reduction of splay elastic constant and rotational viscosity of LC cells.

19 xii In Chapter 6, we have performed electro-optical measurement as a function of temperature for addition of the dichroic anthraquinone dye molecules in pure Ferroelectric Liquid Crystal (FLC) matrix results in many improvement in the various vital parameters of the pure FLC. However, addition of anthraquinone dye molecules in pure FLC matrix is not advantageous every time. There are certain constraints which are crucial for the application of these systems into many devices. In this chapter we have discussed the concentration and temperature dependence of vital properties of dye doped FLC. There is improvement in contrast ratio by dye doping due to enhancement in plane switching for dye doped FLC. In Chapter 7, dielectric and electro-optical study of FLC samples (Felix ) and its two different concentrations doped with fluorescent dye Benzo 2, 1, 3 Thiadiazole has been reported. All the dielectric and electro-optical properties changes effectively, after the addition of fluorescent dye molecules in the pure FLC system. The dielectric relaxation study indicates existence of Goldstone mode for 1% and 2% concentration of fluorescent dye doped mixtures. In addition to this it also shows high frequency mode i.e. soft mode for higher concentration of fluorescent dye i.e. for 2% concentration of fluorescent dye in pure FLC. The other important parameters i.e. spontaneous polarization, response time and rotational viscosity have also been evaluated by electro-optical study. The soft mode observed for high concentration fluorescent dye doped system has been explained and its nature and magnitude is found to be in agreement as reported for other FLCs. It has also been found that the all the parameters have been strongly changed by the variation of concentration of the fluorescent dye into pure FLC. In Chapter 8, electrical properties have been studied for dye doped ferroelectric liquid crystal systems. The additions of dichroic fluorescent dye (Benzo 2, 1, 3 Thiadiazole) in the pure Ferroelectric Liquid Crystal (FLC) improve various vital parameters of pure FLC. In this chapter we have studied and discussed temperature dependence of vital parameters of pure FLC Felix and its fluorescent dye doped mixture. It is found that the presence of fluorescent dye in the pure FLC changes the characteristics of pure geometry which causes the alteration in the physical parameters of the pure FLC system. One important observation is reduction of optical response time which may be useful for potential

20 xiii applications as slow optical response time has been a critical issue with the doped system. In the end, I conclude that our experimental findings backed by theoretical model and explanations presented in the thesis provides a broad information about the nano-nematic composite, nano particle doping FLC and fluorescent dye doping FLC their consequences on the dipole dynamics of liquid crystals including the applicability of the Guest host systems, and their limitations. Chapter 9 reports the final conclusions drawn from the experimental findings and some crucial suggestions.

21 xiv BASIC TERMS OF LIQUID CRYSTAL A A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Activation energy Alignment layer A layer and/or surface treatment applied to the boundary of a liquid crystal cell to induce a particular director orientation. For example, a layer of polyimide buffed in one direction induces alignment parallel to the buffing direction, or a surfactant may be polymerized on a boundary surface to induce perpendicular alignment. Amorphous Irregular; having no discernible order or shape. In the context of solids, the molecules are randomly arranged, as in glass, rather than periodically arranged, as in a crystalline material. Amphiphilic A molecule with a hydrophilic head and a hydrophobic tail. That is, a molecule that has one end which attracts water and one end which repels water. Anchoring Anisotropic Having properties which vary depending on the direction of measurement. In liquid crystals, this is due to the alignment and the shape of the molecules. Aromatic A compound containing a series of benzene (6 Carbon) rings; so named because many have a distinctive odour. Axial ratio The ratio of the length of the molecule to the diameter of the liquid crystal molecule. Azimuthal angle B Backflow Biaxial Possess two directions along which monochromatic light vibrating in any plane will travel with the same velocity. The optic axis lies just between these directions. Bilayer A double layer of amphiphilic molecules, arranged such that either the nonpolar ends are on the inside screened by the polar ends or the polar ends are on the inside screened by the nonpolar ends, depending on whether the solvent is polar or nonpolar. Birefringence Also called double refraction. The property of uniaxial anisotropic materials in which light propagates at different velocities, depending on its direction of polarization

22 xv Blue phases relative to the optic axis. A wave with polarization perpendicular to the optic axis will exhibit an "ordinary" index of refraction, n o (this is often referred to as the ordinary ray). In contrast, a wave with polarization parallel to the optic axis exhibits an "extraordinary" index, n e (the extraordinary ray). The ordinary index, n o, is isotropic with respect to direction of propagation while the extraordinary, n e, varies depending on the direction of propagation with a maximum value for light travelling perpendicular to the optic axis and, of course, polarized parallel to it. The difference n = n e - n o is also referred to as the birefringence or the optical anisotropy. Blue phases are special types of liquid crystal phases that appear in the temperature range between a chiral nematic phase and an isotropic liquid phase. Blue phases have a regular three-dimensional cubic structure of defects with lattice periods of several hundred nanometers, and thus they exhibit selective Bragg reflections in the wavelength range of light (visible part of electromagnetic radiation) corresponding to the cubic lattice. Bond orientational order Describes a line joining the centers of nearest-neighbor molecules without requiring a regular spacing along that line. Thus, a relatively long-range order with respect to the line of centres but only short range positional order along that line. C Chiral molecule A molecule that is not identical to its mirror image. This gives a chiral substance its characteristic twisted shape, due to the fact that its molecules do not line up when combined. Cholesteric liquid crystals Also known as Chiral Nematic. Similar to the nematic phase, however, in the cholesteric phase, molecules in the different layers orient at a slight angle relative to each other (rather than parallel as in the nematic). Each consecutive molecule is rotated slightly relative to the one before it. Therefore, instead of the constant director of the nematic, the cholesteric director rotates helically throughout the sample. Many cholesterol esters exhibit this phase, hence the name cholesteric. See also chiral. See also the classification of Liquid Crystal section. Cholesteric mesophase Nematic liquid crystals with chiral centers form in two dimensional nematic-like layers with directors in each layer twisted with respect to those above and below so that the directors form a continuous helix about the layer normal. Many cholesteric esters exhibit this phase, hence the name cholesteric. This mesophase exhibits circular dichroism and is optically active. Circular polarization A condition caused by two waves whose electric field components are 90 degrees out of phase, causing an effective rotation of the electric field about an axis in direction of propagation.

23 xvi Cis (configuration) A polymer configuration in which adjacent bonds are coplanar and on the same side of the carbon-carbon double bond. Columnar phase A liquid crystal phase characterized by disc-shaped molecules that tend to align themselves in vertical columns. Contrast ratio The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing. A high contrast ratio is a desired aspect of any display. Crystallinity The presence of three-dimensional order on the level of atomic dimensions. In polymers, the range of order may be as small as about 2 nm in one (or more) crystallographic direction(s) and is usually below 50 nm in at least one direction. Polymer crystals frequently do not display the perfection that is usual for lowmolecular mass substances. Polymer crystals that can be manipulated individually are often called polymer single crystals. D Defects A local break in the translational or orientational symmetry of the material. Deformation The condition where the director in a liquid crystalline material changes its orientation from one molecule to the next. Dendrimers Highly branched molecules that have several layers of branching. These molecules exhibit a characteristic spherical shape. Dichroism Refers to the selective difference in absorption between the two orthogonal components of the polarization state of light propagating in a given direction in an anisotropic medium. Generally applied to linearly polarized light. This is how a sheet polarizer works with one direction of polarization transmitted and the perpendicular directions absorbed. Circular dichroism describes the corresponding effect for circularly polarized light where one "handedness" of circularly polarized light is absorbed more strongly than the opposite "handedness." Solutions of chiral molecules will produce this effect as will cholesteric liquid crystals for wavelengths well removed from the pitch value. Dielectric anisotropy A condition arising when the dielectric constant parallel to the length of a mesogen, is different from that perpendicular to it. This occurs when the charge distribution along the molecule responds differently to the parallel component of the local electric field

24 xvii than the distribution perpendicular to the length does to the perpendicular component, yielding a difference in dielectric constants. See the LC properties section. Dielectric breakdown Dielectric breakdown is a sudden increase in current when the voltage exceeds a critical value V b. Dielectric loss The liquid crystal sample cell (parallel plate capacitor) treated as a RC network in which current is made up of two parts; capacitive current (out of phase part) and resistive current (in phase part). The resistive current is the property of dielectric and entirely due to the dielectric medium arises between the plates of sample cell. Therefore we characterize it by a component of the permittivity by defining relative permittivity as r = -j. The magnitude of the component is called dielectric loss. Dielectric strength For a given material, the insulating properties can be described in terms of the dielectric strength, expressed as the field which, when applied to the material, causes an uncontrollable current to flow through or across it. Dielectric susceptibility The dielectric susceptibility of the medium is given as the ratio of bound charge density to the free charge density. It is dimensionless constant. Dimer The liquid crystal compound is dimer which means that it is composed of molecules containing the two conventional mesogenic groups linked via a flexible spacer. This liquid crystal dimer shows quite different behaviour to the conventional liquid crystals. Dipole Two equal electric or magnetic charges of opposite sign, separated by a small distance. In the electric case, the dipole moment is given by the product of one charge and the distance of separation. Applies to charge and current distributions as well. In the electric case, a displacement of charge distribution produces a dipole moment, as in a molecule. Director The molecular direction of preferred orientation in liquid crystalline mesophases. Disclination Line defects arising from singularities in orientational order in a director field. Discotic liquid crystal The component disc-shaped molecules self-assemble in a way that resembles stacks of coins. The discs are stacked on top of each other to form columns, which in turn are packed on a two-dimensional, usually hexagonal, lattice. Dislocations Line defects arising from singularities in translational order in a crystalline lattice.

25 xviii Dispersion Domain A domain is a region where the spontaneous polarization is uniform. Adjacent domains can have different polarization, much like in ferromagnetism. E Elasticity Elastomers A class of polymers that have some degree of cross linking and are rubbery. Elastomers possess memory, that is, they return to their original shape after a stress is applied. Electro-optical materials Materials whose optical properties are changed under the application of an electric field. Emulsion A mixture of two mutually insoluble liquids such that one is dispersed in the other in droplets which often cause the solution to be cloudy or translucent. Enantiomers Molecules which exist in two nonsuperimposable mirror images, analogous to human hands. Chiral molecules are perfect physical and chemical models of each other with the exception of their rotation of polarized light and those interactions that involve other chiral systems, such as chiral molecular recognition. A racemic mixture contains equal amounts of two enantiomers and thus produces no rotation of the plane of polarization of light. Enantiotropic liquid crystal Exhibit the liquid crystal state both when the temperature rises from the solid state side or when it falls from the liquid state. Monotropic liquid crystals exhibit the liquid crystalline state only when the temperature changes in one direction. Extraordinary ray The optical ray which not follow the law of refraction in the double refraction phenomenon. See birefringence section. F Ferroelectric material One that produces domains of spontaneous polarization whose polar axis can be reversed in an electric field directed opposite to the total dipole moment of the lattice. Freedericksz transition The point at which a liquid crystal changes from an aligned to a deformed state under the influence of an external electric or magnetic field. Free radical A molecule with an unpaired electron, making it highly reactive.

26 xix G Grandjean texture A specific pattern of defects found in chiral nematic liquid crystals that is caused by the chiral nature of the crystal. Named for the French scientist F. Grandjean who worked with chiral nematic liquid crystals. H Helix The molecular conformation of a spiral nature, generated by regularly repeating rotations around the backbone bonds of a macromolecule. Homogeneous An uniform structure or composition throughout. Having or possessing the same properties. Homeotropic texture A mesogen configuration in which the molecules are aligned normal to the boundary surfaces, as at the faces of a liquid crystal cell, as illustrated. Consequently the director will be normal to the surface. This orientation is generally obtained by the application of an electric field normal to the surface but can be achieved through surface treatment. Homogeneous (planar) texture A mesogen configuration in which the molecules are aligned parallel to the boundary surfaces, as at the faces of a liquid crystal cell.

27 xx Hydrophilic "Water loving"; describes a molecule which is attracted to water. Hydrophobic "Water fearing"; describes a molecule which is repelled by water. I Index of refraction Ratio of the phase velocity of electromagnetic radiation in free space divided by the phase velocity in a given medium. It is greater than one except for rather special cases. Isomer A molecule which has an identical molecular formula to another molecule, but has a different structure. Isotropic Having properties that are the same regardless of the direction of measurement. In the isotropic state, all directions are indistinguishable from each other. See also anisotropic. ITO Indium Tin Oxide (indium oxide doped with tin). A transparent conductive material very commonly used for electrodes in displays or other applications which require conductivity along with light transmission. J K L Lipophilic Describes a molecule which is attracted to hydrocarbons. Liquid crystal A thermodynamic stable phase characterized by anisotropy of properties without the existence of a three-dimensional crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase, hence the term mesophase. Lyotropic Materials in which liquid crystalline properties appear induced by the presence of a solvent, with mesophases depending on solvent concentration, as well as temperature. M Macromolecule A very large molecule. Many polymers are composed of hundreds of thousands of atoms, and are thus characterized as macromolecules.

28 xxi Melting transition temperature The temperature at which the substance loses its translational and orientational order, changing from a solid phase to a liquid phase. Mesogen Rigid rodlike or disclike molecules which are components of liquid crystalline materials. Mesomorphic substance Another term for a liquid crystal material. Mesophase Equilibrium liquid crystalline phases formed with order less than three dimensional(like crystals) and mobility less than that of an isotropic liquid. Parallel orientation of the longitudinal molecular axes is common to all mesophases (longrange orientational order). See the Liquid Crystal introduction section Micelle A spherical formation caused by an amphiphilic substance in a solution. The lyophilic end of the molecule tends to orient itself toward the outside of the sphere while the lyophobic end tends to orient itself toward the inside of the sphere. Monolayer A layer of amphiphilic molecules on the surface of a solvent, arranged such that the ends attracted to the solvent are in contact with it and the other ends point into the air. Monomer The simple chemical unit which, when many are joined together, form a polymer. Monotropic A type of material which exhibits the liquid crystalline state only when the temperature changes in one direction. This is generally a result of the liquid crystal phase being below the melting temperature of the solid, where the liquid crystal phase is only observed if the liquid is supercooled below the melting point. N Nano particle In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. It is further classified according to size: In terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles, on the other hand, are sized between 1 and 100 nanometers. Nematic mesophase Liquid crystals are characterized by long-range orientational order and the random disposition of the centers of gravity in individual molecules. Nematics may be characterized as the simplest spontaneously anisotropic liquids. Nematic phases are composed of rod-shaped molecular aggregates that are arranged with parallel but not lateral order. See Liquid Crystal classification section. O

29 xxii Optical activity The plane of vibration of linearly polarized light rotates as it propagates through a medium. This rotation can occur in either a right or left handed direction. Since linearly polarized light can be regarded as the sum of right and left hand circularly polarized components, this optical activity corresponds to different indices of refraction for the two circular components(circular birefringence). Optic axis In a uniaxial material, a single direction of propagation along which double refraction does not occur. The index of refraction for both polarization directions is n o along this axis. This axis lies along the director for nematic liquid crystal. Order parameter S describes the orientational order of liquid crystalline material, allowing for the individual orientational deviation of the molecules from the director, which represents the average over the collection. Typically, S ranges from 0.3 to 0.9, depending on the temperature, with a value of unity for perfect order. See Introduction to Liquid Crystal phases section. Ordinary ray See birefringence section of chapter 2. Orientational order Measure of the tendency of the molecules to align along the director on a long-range basis. See order parameter. P Permittivity When a dielectric material is placed in an electric field already existing in a homogeneous medium, it has the effect of changing the distribution of the field to a degree depending upon its relative permittivity; i.e. the electric field intensity is a function of the medium in which it exists. In general the dielectric permittivity is defined as the ratio of the flux density to field. Piezoelectricity A mechanical strain produces dielectric polarization and vice versa, an applied electric field causes mechanical strain. Materials which lack a center of symmetry are piezoelectric. Pitch An important characteristic of the cholesteric mesophase is the pitch. The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix. (See LC Phase Section). Polarizability Relates the induced electric dipole moment, p, of an atom or molecule to the local electric field it experiences as α = p / E local, hence depending on the displacement by the field of the electronic charge from its equilibrium position in the atom or molecule.

30 xxiii Polarization The dipole moment per unit volume of the sample is called the polarization. Polymer liquid crystals Polymers that contain mesogen units and thus have liquid crystal properties. Positional order The extent to which the position of an average molecule or group of molecules shows translational symmetry. Prolate Elongated at the poles. Q R Relaxation A term used to mean all irreversible processes which bring a system back to equilibrium after it has been perturbed by some external force. For instance, if an electric field is applied to a fluid of polar materials a polarization will be induced, but this will disappear after the field is removed, because of the randomization of the molecular orientations produced by Brownian motions. Resonance A method of stabilizing a bond by delocalizing the electrons around the molecule. Rotational symmetry Consider the example of an unmarked billiard ball. Rotation of any amount about any axis through its center will take the ball into itself so it has complete rotational symmetry. However, the stitching on a baseball places several restrictions on the axes about which it can be rotated into itself. S Schlieren texture The texture that appears in the optical microscopy of nematic and related smectic C phases under crossed polarizers when the planarity of the phase is interrupted by defects. The schlieren, dark streaks or brushes, form in the liquid crystal, connecting the defect points. The dark streaks or brushes that are characteristic of this texture may also appear along disclinations in a liquid crystal. Self-assembly The aggregation of molecular moieties into more ordered structures that are thermodynamically stable and involve noncovalent bonds. Crystallization is an example of such self-assembly. Self-assembly is used to build nanostrucures such as inorganic clusters and lattices, nanotubes and channels, host-guest complexes, monolayers, hydrogen-bonded networks and systems of intertwined molecules.

31 xxiv Smectic mesophase The molecules organize themselves into layers. The smectic phases form a one dimensional periodic lattice in which the individual layers are two dimensional liquids. Now 12 different smectic phases have been identified. Spacer Flexible section of polymer chain between two mesogens or the mesogen and the backbone of a polymer. Steric hindrance A condition when the rotation of a given group is restricted due to the size of neighbouring groups. Super critical fluids A substance above its critical point on the temperature/pressure phase diagram. Above the critical point, the fluid is neither a gas nor a liquid but possesses properties of both. The viscosity of a supercritical fluid is at least one order of magnitude higher than the viscosity in the gaseous state, but is one or two orders of magnitude less than in the liquid state. Surfactants Surface active agents. Organic compounds consisting of two parts: a water-attracting (hydrophilic) portion and a water-resistant (hydrophobic) portion. Detergents may contain more than one kind of surfactant. The hydrophobic ends attach themselves to the soil particles or to the fabrics being washed while the hydrophilic ends are attracted to the water. The surfactant molecules surround the soil particles, break them up, force them away from the surface of the fabric, then suspend the soil particles in the wash water. Surfactants are classified by their ionic (electrical charge) properties in water. Symmetry The invariance of some properties of the object being investigated with respect to all the transformations considered. T Thermoplastics Linear plastics of finite molecular weight that can be fabricated into complex shapes by melting and injection molding. Thermotropic Liquid crystal molecules which exhibit temperature dependent liquid crystalline behavior. See also: lyotropic Topology A branch of mathematics concerned with those properties of geometric configurations which remain unaltered under very general kinds of elastic deformations (transformations such as stretching or twisting) where length, angles, and shapes are changed.

32 xxv Translational order A condition when molecules have some arrangement in space. Crystals have three degrees of translational order (each molecule is fixed in space with an x, y, and z coordinate) and liquids have no translational order. U Uniaxial materials Possess only one direction along which monochromatic light vibrating in any plane will travel with the same velocity. This direction is known as the optic axis. Unit cell The smallest, regularly repeating material portion contained in a parallelepiped from which a crystal is formed by parallel displacements in three dimensions. Unlike the case of low-molar mass substances, the unit cell of polymer crystals usually comprises only parts of the polymer molecules, and the regularity of the periodic repetition may be imperfect. V Vander Waal's forces Forces which act between molecules that are caused by small random fluctuations in the polarity of the liquid crystal molecules. Viscosity The internal resistance to flow existing between two liquid crystal layers when they are moved relative to each other. This internal resistance is a result of interaction between liquid crystal molecules in motion. Vulcanization A process by which a network of crosslinks is introduced into an elastomer to strengthen it. W X Y Z

33 Chapter 1 General Introduction

34 Chapter 1 General Introduction 1.1. Introduction 1.2. History of liquid crystals 1.3. Classification of liquid crystal Chemical concept Lyotropic liquid crystal Thermotropic liquid crystal Molecular shape concept Calamitic liquid crystals Disc shape molecules Banana shape molecules Unusual shape molecules Molecular arrangement concept Nematic liquid crystal Chiral nematic or cholesteric liquid crystal Smectic liquid crystal Smectic A liquid crystal Smectic C liquid crystal Smectic B liquid crystal Smectic E liquid crystal Smectic I and smectic F liquid crystal Smectic G, G, H and H liquid crystal Chiral smectic or ferroelectric liquid crystal Surface-stabilized ferroelectric liquid crystals (SSFLC) Antiferroelectric liquid crystals (AFLC) Ferrielectric liquid crystal (Ferri LC) Twisted grain boundary phases (TGB) The chiral line liquid Bend grain boundary (BGB) phases Reentrant phases of polar liquid crystals

35 1.4. Effect of chemical constitution on mesomorphism 1.5. Physical properties of liquid crystals 1.6. Applications of liquid crystals Display application of liquid crystals Thermal mapping and non-destructive testing Medicinal uses Optical imaging Liquid crystal solar cell References

36 INTRODUCTION Research on liquid crystal (LC) has involved in Chemistry, Physics, Biology, electrical engineering, electronic engineering and many other fields. The study of liquid crystals began in 1888 by Austrian Botanist F. Reinitzer when he prepared cholesteryl benzoate (the first liquid crystal); justified him to be called the grandfather of liquid crystal science [1-4]. Liquid crystal materials are unique in their properties and uses. In addition to display applications they are also promising materials for the photonic applications. WHAT ARE LIQUID CRYSTALS? The term Liquid Crystal seems to be a self-contradiction as it suggests that a substance is in two quite different state of matter at the same time. The two most common states of condensed matter are the isotropic liquid phase and the crystalline solid phase. In a crystal, the molecules or atoms have both orientational and three-dimensional positional order over a long range. In an isotropic liquid, however, the molecules have neither positional nor orientational order, they are distributed randomly. There is no degree of order, so three degrees of freedom are left. There is no preferred direction in a liquid, thus the name isotropic. The transition from one state to another normally occurs at a very precise temperature. When pure crystalline solid is heated beyond its melting temperature, it undergoes a single transition to isotropic liquid. e.g. ice-water is such a common phase transition. There are, however many organic compound that do not immediately transform to liquid phase when heated but exhibit more than a single transition from solid to liquid showing the existence of one or more intermediate phases, exhibiting the properties of both solids and liquids. For examples p-azoxy anisole when heated does not transform into the liquid state but adopts the structure (turbid condition) that is both birefringence and fluid the consistency varying with different compounds that of a paste to that of a freely flowing liquid. Transitions are definite and precisely reversible.

37 2 Materials undergoing such a phase transitions are called Liquid Crystal [5]. Condensed matters that exhibit intermediate thermodynamic phases between the crystalline solid and simple liquid state are now called LCs or mesophases as shown in figure 1.1. Figure 1.1. Liquid crystalline mesophases between the solid and isotropic liquid phase. LC molecules are normally constituted by aromatic ring attached with aliphatic tail as shown in figure 1.2. The aromatic ring provides the rigidity like solids whereas the aliphatic tail provides the fluidity to the LC molecules. Due to the presence of aromatic ring and aliphatic tail the LCs are capable to show the characteristics of both solids and liquids. Amphiphilic molecules, polar and non-polar parts form liquid crystal phases over certain concentration ranges when mixed with a solvent molecules consisting of a rigid core and flexible tail form liquid crystal phases over certain temperature ranges [6-12].

38 3 Figure 1.2. Liquid crystal molecules contain both the aromatic ring representing the crystalline properties and aliphatic tail representing the fluidity like liquid nature or a hydrophobic non polar tail (flexible) and hydrophilic polar head (rigid) HISTORY OF LIQUID CRYSTALS The discovery of liquid crystals is thought to have occurred nearly 150 years ago although its significance was not fully realized until over a hundred years later. Around the middle of the last century Virchow [13], Mettenheimer et al. [14] have found that the nerve fiber they were studying formed a fluid substance when left in water which exhibited a strange behavior when viewed using polarized light. They did not realize this was a different phase but they are attributed with the first observation of liquid crystals. Later, in 1877, further investigations of this phenomenon were carried out by the German physicist O. Lehmann [15] who observed and confirmed, using the first polarized optical microscope designed by him, the existence of crystals which can exist with softness that one could call them nearly liquid. He found that one substance would change from a clear liquid to a cloudy liquid before crystallizing but thought that this was simply an imperfect phase transition from liquid phase to crystalline phase. The first reported documentation of the LC state was through an accidental observation by an Austrian botanist, Friedrich Reinitzer [1] in 1888, working in the Institute of Plant Physiology at the University of Prague. He observed double melting" behavior of cholesteryl benzoate. The crystals of this material melted at C into a cloudy fluid, which upon further heating to C became clear. This discovery represented the first recorded documentation of the LC phase. He was the first to suggest that this cloudy fluid was a new phase of matter. He has consequently been given the credit for the discovery of the liquid crystalline phase.

39 4 Puzzled by his discovery, Reinitzer turned for help to the German physicist Otto Lehmann, who was an expert in crystal optics. Lehmann became convinced that the cloudy liquid had a unique kind of order. In contrast, the transparent liquid at higher temperature had the characteristic disordered state of all common liquids. Eventually he realized that the cloudy liquid was a new state of matter and coined the name "liquid crystal," illustrating that it was something between a liquid and a solid, sharing important properties of both. In a normal liquid the properties are isotropic, i.e. the same in all directions. In a liquid crystal they are not; they strongly depend on direction even if the substance itself is fluid. Till 1890 all the liquid crystalline substances that had been investigated were naturally occurring and it was then that the first synthetic liquid crystal, p-azoxyanisole, was produced by Gatterman and Ritschke[16]. Subsequently more liquid crystals were synthesized and it is now possible to produce liquid crystals with specific predetermined material properties. Maier and Saupe [17 formulated a microscopic theory of liquid crystals; Frank and later Leslie and Ericksen [18] developed continuum theories for static and dynamic systems and in 1968 scientists from RCA first demonstrated a liquid crystal display [19]. The interest in liquid crystals has grown ever since, partly due to the great variety of phenomena exhibited by liquid crystals and partly because of the enormous commercial interest and importance of liquid crystal displays. Today, thanks to Reinitzer, Lehmann and their followers. In the 1960s, a French theoretical physicist, Pierre-Gilles de Gennes, who had been working with magnetism and superconductivity, turned his interest to liquid crystals and soon found fascinating analogies between liquid crystals and superconductors as well as magnetic materials. His work was rewarded with the Nobel Prize in Physics The modern development of liquid crystal science has since been deeply influenced by the work of Pierre-Gilles de Gennes [3]. In 1922 the French scientist G. Friedel produced the first classification scheme of LCs [20], dividing them into three different types of mesogens (materials able to sustain mesophases), based upon the level of order the molecules possessed in the bulk material: 1. Nematic (from the Greek word nematos meaning "thread"), 2. Smectic (from the Greek word smectos meaning "soap"), and 3. Cholesteric (better defined as Chiral nematic) [21]. Following these first observations and discoveries, the scientific research turned attention towards a growing number of compounds, which displayed liquid

40 5 crystalline properties. In order to establish a relationship between the molecular structure and the exhibition of liquid crystalline properties, a series of systematic modifications of the structures of mesogens were undertaken, leading, in 1973 [22], to the discovery of the most technologically and commercially important class of LCs to date: the 4-alkyl-4'-cyanobiphenyl (CB) of which an example, 4-pentyl-4'- cyanobiphenyl (5CB) is illustrated in Figure 1.3. Figure 1.3. Molecular structure of 4-pentyl-4'-cyanobiphenyl (5CB). (The transition temperatures are expressed in 0 C). These are the materials, which still constitute the simple common displays found in calculators or mobile phones. Nowadays, LCs play a dominant role in a large part of the display technology CLASSIFICATION OF LIQUID CRYSTAL There are several concepts that have been introduced till date to classify the liquid crystals as shown in figure CHEMICAL CONCEPT Different types of molecules can form liquid crystalline phases. The common structural feature is that these molecules are form anisotropic: one molecular axis is much longer or wider than another one. The two major categories are: 1. Lyotropic LCs, whose mesophase formation is concentration and solvent dependent. 2. Thermotropic LCs, whose mesophase formation is temperature (T) dependent. Other is amphotropic Liquid Crystals, who has both thermotropic and lyotropic liquid crystals states e.g. salts of long chain aliphatic acids.

41 6 Figure 1.4. Classification scheme of liquid crystal LYOTROPIC LIQUID CRYSTAL Lyotropic liquid crystals were the first liquid crystals to be discovered (1850) [23]. Lyotropic liquid crystal transitions occur with the influence of solvents, not by a change in temperature. Lyotropic mesophases occur as a result of solvent-induced aggregation of the constituent mesogens into micellar structures. Lyotropic mesogens are typically amphiphilic, meaning that they are composed of both lyophilic (solvent attracting) and lyophobic (solvent-repelling) parts. This causes them to form into micellar structures in the presence of a solvent, since the lyophobic ends will stay together as the lyophilic ends extend outward toward the solution. As the concentration of the solution is increased and the solution is cooled, the micelles increase in size and eventually coalesce. This separates the newly formed liquid crystalline state from the solvent. For example the goo that sometimes collects in the bottom of your soap dish is a lyotropic liquid crystal phase of the soap/water mixture. There are several different type lyotropic liquid crystal phases. The concentration of material in solvent dictates the types of Lyotropic liquid crystals [9-10].

42 7 Figure 1.5. Temperature dependence of concentration of amphiphilic molecules in lyotropic liquid crystal. Lyotropic phases are classified into three different types viz. lamellar, cubic and hexagonal as shown in figure 1.5 [2]. Lyotropic liquid crystals are also extremely important because of their role in biological membranes. Membranes are composed of amphiphilic lipids - mostly phospholipids and cholesterol, with a small percentage of glycolipids THERMOTROPIC LIQUID CRYSTAL Thermotropic transition occurs in most liquid crystals, and they are defined by the fact that the transitions to the liquid crystalline state are induced thermally (figure 1.6). That is, one can arrive at the liquid crystalline state by raising the temperature of a solid and/or lowering the temperature of a liquid [24, 25]. At present thermotropics are mostly used for technical applications [26]. The essential requirement for a molecule to be a thermotropic LC is a structure consisting of a central rigid core (often aromatic) and a flexible peripheral moiety (generally aliphatic groups). This structural requirement leads to two general classes of LCs: calamitic LCs and discotic LCs, both of which have other molecular sub classes. Thermotropic liquid crystals can be classified into two types:

43 8 Figure 1.6. Liquid crystalline mesophases between the solid and isotropic liquid phase Enantiotropic liquid crystals These liquid crystals can be changed into the liquid crystalline state from either lowering the temperature of a liquid or rising of the temperature of a solid. Monotropic liquid crystals These liquid crystals can only be changed into the liquid crystal state from either an increase in the temperature of a solid or a decrease in the temperature of a liquid, but not both. Although the term thermotropic and lyotropic are widely used, Gray and Winsor [27] prefer the terms amphiphillic (for lyotropics) and non-amphiphillic (for thermotropics).

44 MOLECULAR SHAPE CONCEPT On the basis of molecular shape the liquid crystals are classified into three classes i.e. they are rod shape, disk shape and banana shape liquid crystals. A broad classification is shown in the figure 1.7. Instead of this broad classification the range of this concept is still growing. The most recent additions are lambda, hockey, U and Y shape molecules. Figure 1.7. (a) Rod Shape Liquid Crystals (b) Disc Shape Liquid Crystals (c) Banana Shape Liquid Crystals CALAMITIC LIQUID CRYSTAL Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape, responsible for the anisotropy of the molecular structure, as the result of the molecular length (l) being significantly greater than the molecular breadth (b), as depicted in the cartoon representation in Figure 1.8.

45 10 Figure 1.8. Cartoon representation of calamitic LCs, where length (l) >> breadth (b). Calamitic mesogens usually follow the general structural formula shown in Figure 1.9 Figure 1.9. General Structure of Calamitic LCs. R' and R" are often flexible terminal units such that at least one R group is an alkyl chain, A, B, C, and D are used to generally describe ring systems (phenyl, cyclohexyl, heteroaromatics, and heterocycles) and [L] represents the linking units, such as CH=N, COO or N=N that can increase the length and flexibility of the molecule, while preserving a compatible linear shape suitable for mesophase formation. Calamitic LCs can exhibit two common types of mesophases: nematic and smectic DISC SHAPE MOLECULES It was originally thought that the mesophase could only be generated by molecules of rod-like structure. The existence of mesophase generated by disk shaped molecules shown in figure 1.7(b) was theoretically predicted in 1970 and

46 11 mesomorphism in discotic materials was first reported by Chanderashekar in 1977 [4]. There are several different types of columnar mesophases exhibited by discotic materials; these arise because of the different symmetry classes of the two dimensional lattice of columns and the order or disorder of the molecular stacking within the columns. Different structures formed by disc shape molecules are shown in figure 1.7. Figure Different Structures formed by discotic liquid crystals phases. Figure 1.10 shows liquid crystal phases of disk-like molecules. Transformation between these phases with respect to the temperature is like the rodlike molecules. At high temperature molecules are in isotropic liquid phase. By decreasing the temperature the material transform into nematic phase which has orientational order but no positional order. When the temperature is decreased further, LC transforms into columnar phase where they have partial positional order. The molecules stack up in columns in which they have two-dimensional liquid behavior BANANA SHAPE MOLECULES Liquid crystal molecules with a bent molecular shape, so called banana-shaped liquid crystals, have attracted special attention. It has been known since 1923 that bent core molecules can show liquid crystalline phases [28]. However, at that time they were just called bad rods and no body anticipated the noise these substances would make later on due to the discovery of polar switching. Liquid crystals composed by banana shape molecules as shown in figure 1.7 (c) provide now a new and hot area of liquid crystal research, as such materials organize into fluid phases with polar order and supermolecular chirality [12,28] properties which are of current interest in different areas of science.

47 UNUSUAL SHAPE MOLECULES In addition to the conventional molecular shape of liquid crystals described above, many more molecular shapes have been added to the liquid crystals. As early as 1907 Vorlaender [29] proposed his rule that the liquid crystalline state is obtained for the most linear of ring and indeed a wide variety of rod-like mesogenic molecules have been synthesized. The substitution of the benzene ring in the 1, 2- or 1, 3-positions results in U-shaped or bent molecules that deviate significantly from the `ideal elongated lath-like structure. Consequently very few such systems have been reported to form liquid crystal phases. The first such report was by Vorlaender himself in conjunction with Apel [30]. Yoshizawa and Yamaguchi [31, 32] have reported layered structure in the nematic phase consisting of U shaped molecule. Yoshizawa et. al. [33] have designed novel Y-shaped (shown in figure 1.11 (A)) Liquid crystal oligomers in which three mesogenic units are connected via 3, 5-dihydroxybenzoic acid. The Y-shaped liquid crystal oligomers [33] were found to show a direct phase transition from isotropic liquid to anticlinic SmC phase. They observed that the Y- shaped compounds are quite different from that of the corresponding -shaped (Figure 1.11 (B)) mesogenic compounds. Since different possible orientations of the net molecular dipole moment are allowed at the air-water interface, one may speculate on the existence of monolayers and thin films of new structures. These compounds are also a good candidate for a better understanding of the correspondence between the phases of amphiphilic compounds in monolayers at the air-water interface and bulk mesophases. Figure 1.11 shows some of different shapes showing liquid crystalline behaviour like lambda, bowlic compound [34], hockey shape [35], star shape [36], U shape [37] and S shape [38]. O (CH 2 ) 6 O C 8 H 17 NC OCO O (CH 2 ) 6 O C 8 H 17 (A) N O(CH 2 ) n O C 8 H 17 N O N O O(CH 2 ) n O N C 8 H 17 NC (B)

48 13 (C) (D) (E)

49 14 O 00C N N OC 2 H 5 O OOC N N OC 2 H 5 (F) CN O (CH 2 ) n O O (CH 2 ) n O O (CH 2 ) n O O (CH 2 ) n O (G) Figure Molecular structure of the (A) Y shape (B) Lambda shape (C) 3BCN and 4BCD bowlic compounds (D) Hockey shape (E) Star shape (F) U shape (G) S shape MOLECULAR ARRANGEMENT CONCEPT In 1920 s French scientist George Friedel proposed a new classification scheme for the LCs. There are only two types of the LCs found basically i.e. Nematics and Smectics [20] NEMATIC LIQUID CRYSTAL The word nematic is derived from the greek word Nema meaning thread like.under the polarsing microscope, the nematic phase is seen as thread schlieren texture. This is the most liquid like structure in which, contrary to isotropic liquids, one or two molecular axes are oriented parallel to one another resulting in an orientational long range order and short positional order. The molecular long axis points on the average in one favored direction referred to as the director, figure Molecules can rotate by both the axes, the molecules have several possibility of intermolecular mobility. Because of the low viscosities, the nematic phases have high NC

50 15 mobility. They are anisotropic with respect to optical properties, viscosity, electrical and magnetic susceptibility, electrical and thermal conductivity. The classical example of LC displaying a nematic mesophase is the 5CB. In consequence, there is a macroscopic anisotropy in many material properties, such as dielectric constants and refractive indices. Figure Cartoon representation of Nematic phase. (a) (b) (c) Figure (a) Schlieren texture of a nematic film with surface point defects. (b) Thin nematic film on isotropic surface: 1-dimensional periodicity (c) Nematic thread-like texture. After these textures the nematic phase was named, as nematic Optically, a nematic phase can be uniaxial or biaxial [10]. The latter is formed by elongated lath like molecules. Conventional nematic liquid crystals formed by rod like molecules constitutes a uniaxial medium with non-polar symmetry. On optical examination of a nematic, one rarely sees the idealized equilibrium configuration. Some very prominent structural perturbations appear as threads. These threads are analogous to dislocations in solids and have been termed dislocations by Frank. Several typical textures of nematics are shown in figure The first one is a schlieren texture of a nematic film. This picture was taken under a polarization microscope with polarizer

51 16 and analyzer crossed. From every point defect emerge four dark brushes. For these directions the director is parallel either to the polarizer or to the analyzer. The colors are newton colors of thin films and depend on the thickness of the sample. Point defects can only exist in pairs. One can see two types of boojums with opposite sign of topological charge ; one type with yellow and red brushes, the other kind not that colorful. The difference in appearance is due to different core structures for these defects of different charge. The second texture is a thin film on isotropic surface. Here the periodic stripe structure is a spectacular consequence of the confined nature of the film. It is a result of the competition between elastic inner forces and surface anchoring forces. The surface anchoring forces want to align the liquid crystals parallel to the bottom surface and perpendicular to the top surface of the film. The elastic forces work against the resulting vertical distortions of the director field. When the film is sufficiently thin, the lowest energy state is surprisingly archived by horizontal director deformations in the plane of the film. The current picture shows a 1- dimensional periodic pattern. Many compounds are known to form nematic mesophase. A few typical examples are sketched in figure From a steric point of view, molecules are rigid rods with the breadth to width ratio from 3:1 to 20:1. From a rough steric point of view, p-azoxyanisole (PAA) is a rigid rod of length 20 Å and width 5 Å. The nematic state is found at high temperatures (between C and C at atmospheric pressure). In N-(p-methoxybenzylidene)-p-butylaniline (MBBA) nematic state is found at room temperatures (between 20 0 C to 47 0 C) Lacking chemical stability. The nematic state 4- pentyl-4 -cyanobiphenyl (5CB) is found at room temperatures (between 24 C and 35 C). Figure Typical compounds forming nematic mesophases: PAA, MBBA and 5CB.

52 17 Biaxial nematic A biaxial nematic is a spatially homogeneous liquid crystal with three distinct optical axes. This is different from simple nematic, which has a single preferred axis, around which the system is rotationally symmetric. The symmetry group of a biaxial nematic is D 2 h i.e. that of a rectangular right parallelepiped, having 3 orthogonal C 2 axes and three orthogonal mirror planes. In a frame co-aligned with optical axes the second rank order parameter tensor of a biaxial nematic has the form Where S is the standard nematic scalar order parameter T a measure of the biaxiality. The first report of a biaxial nematic appeared in 2004 [39, 40] based on a boomerang shaped oxadiazole bent-core mesogen. The biaxial nematic phase for this particular compound only occurs at temperatures around 200 C and is preceded by as yet unidentified smectic phases. It is also found that this material can segregate into chiral domains of opposite handedness [41] for this to happen the boomerang shaped molecules adopt a helical superstructure. In one azo bent-core mesogen as shown below in which a thermal transition is found from a uniaxial (Nu) to a biaxial nematic (Nb) mesophase [42]. This transition is observed on heating from the Nu phase with Polarizing optical microscopy as a change in Schlieren texture and increased light transmittance and from x-ray diffraction as the splitting of the nematic reflection. The transition is a second order with low energy content and therefore not observed in differential

53 18 scanning calorimetry. The positional order parameter for the uniaxial nematic phase is 0.75 to 1.5 times the mesogen length and for the biaxial nematic phase 2 to 3.3 times the mesogen length. Another strategy towards biaxial nematic is the use of mixtures of classical rod like mesogens and disk like discotic mesogens. The biaxial nematic phase is expected to be located below the minimum in the rod-disk phase diagram. In one study [43] a miscible system of rods and disks is actually found although the biaxial nematic phase remains elusive CHIRAL NEMATIC OR CHOLESTERIC LIQUID CRYSTAL The cholesteric phase is like the nematic phase in having long-range orientation order and no long-range order in positions of the centers of mass of molecules. It differs from the nematic phase in that the director varies in direction throughout the medium in a regular way. The configuration is precisely what one would obtain by twisting about the x axis a nematic initially aligned along the y axis. In any plane perpendicular to the twist axis the long axes of the molecules tend to align along a single preferred direction in this plane, but in a series of equidistant parallel planes, the preferred direction rotates through a fixed angle, as illustrated in figure The secondary structure of the cholesteric is characterized by the distance measured along the twist axis over which the director rotates through a full circle. This distance is called the pitch of the cholesteric. The periodicity length of the cholesteric is actually only half this distance since n and -n are indistinguishable. A nematic liquid crystal is just a cholesteric of infinite pitch, and is not really an independent case. In particular, there is no phase transition between nematic and cholesteric phases in a given material, and nematic liquid crystals doped with enantiomorphic materials become cholesterics of long (but finite) pitch. The

54 19 molecules forming this phase are always optically active, i. e. they have distinct right- and left-handed forms. (a) (b) (c) Figure The arrangement of molecules in liquid crystal phases. (a) The nematic phase. (b) The cholesteric phase (c) smectic A phase. Various colour changes can be observed by winding or unwinding the helix. This can be done by means of changing temperature, mechanical disturbance like pressure or shear. Liquid Crystals of this type are mostly optically active. The cholesteric liquid crystals are optically uniaxial with negative character, it can scatter the light to give bright colour and it shows strong rotatory power. Pitch An important characteristic of the cholesteric mesophase is the pitch. The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix as illustrated in the above figure A by product of the helical structure of the chiral nematic phase, is its ability to selectively reflect light of wavelengths equal to the pitch length. The effect is based on the temperature dependence of the gradual change in director orientation between successive layers (illustrated above), which modifies the pitch length resulting in an alteration of the wavelength of reflected light according to the temperature. The angle at which the director changes can be made larger and thus tighten the pitch, by increasing the temperature of the molecules, hence giving them

55 20 more thermal energy. Similarly, decreasing the temperature of the molecules increases the pitch length of the chiral nematic liquid crystal [7, 44, 45]. The pitch length is temperature dependent and hence so is the colour of the reflected light. This is the basis behind the commercially successful use of chiral nematic materials in thermo chromic thermometer devices and other devices that change colour with temperature e.g. articles of clothing, inks and paints. The wavelength of the reflected light can also be controlled by adjusting the chemical composition, since cholesterics can either consist of exclusively chiral molecules or nematic molecules with a chiral dopant dispersed throughout. In this case, the dopant concentration is used to adjust the chirality and thus the pitch [46]. Figure Schematic representations of the periodic helical structures of the (a) chiral nematic (cholesteric) phase and (b) chiral smectic liquid crystal phase. The pitch of the helix corresponds to the rotation of the director through SMECTIC LIQUID CRYSTAL The word smectic has been derived from the Greek word Smectos which means soap like [21, 47]. The smectic phase of a liquid crystal represents a higher state of ordering than nematics. In addition to the orientational ordering the molecules are arranged in layers [48]. A great variety of smectic phases can be observed depending on the molecular arrangements in the layers. The molecules may be upright or inclined to the layers and may or may not have long range positional ordering in

56 21 each layer, but long range orientational ordering is always present in all layers of the smectic liquid crystal. Some of the smectic liquid crystals have three dimensional long range positional order as in a crystal while some others have three dimensional long range bond orientational order without any long range positional order. The interlayer attractions are weaker than the lateral forces between molecules and hence the layers can easily slide over one another. Hence smectics have fluidity though these are much more viscous than nematics. The lamellar smectic state can be divided into four subgroups by considering the extent of the in-plane positional ordering of the constituent molecules, and the tilt orientational ordering of the long axes of the molecules relative to the layer planes. Two groups can be defined where the molecules have their long axes essentially normal to the layers. These two groups are distinguished from each other by the extent of the positional ordering of the constituent molecules. For example, smectic A and hexatic B are smectic liquid crystals in which the molecules have only short-range positional order, whereas crystal B and crystal E are smectic crystal modifcations where the molecules have long-range positional order in three dimensions. Two other classes can be distinguished where the molecules are tilted with respect to the layer planes. In smectic C, smectic I, and smectic F, the molecules have short-range orientational ordering, whereas in crystal G, crystal H, crystal J, and crystal K the molecules have long-range three-dimensional ordering [45, 49]. Thus, as already noted, smectics C, I, and F are essentially smectic liquid crystals, whereas G, H, J, and K are crystal phases. These latter phases, however, have somewhat different properties than normal crystals, for example, their constituent molecules are reorienting rapidly about their long axes (10 11 times s -1 ). Figure (a, b) Focal-conic fan texture of a smectic A liquid crystal (c) Focal-conic fan texture of a chiral smectic C liquid crystal. In general a various smectic phase, when placed between glass slides, does not assume the simple form. The layers, preserving their thickness, become distorted and

57 22 can slide over one another in order to adjust to the surface conditions. The optical properties (focal conic texture) of the smectic state arise from these distortions of the layers. Typical textures formed by smectics are shown in figure 1.17 [50] SMECTIC A LIQUID CRYSTAL In the smectic A phase the molecules are arranged in diffuse layers so that their long axis are on the average in perpendicular to the layer planes. The molecules are undergoing rapid reorientational motion about their long axis on a timescale of times per second, and also undergoing relaxations about their short axis but on a much longer timescale of 10 6 times per second. The molecules are arranged in such a way that there is no translational periodicity in the planes of the layers or between the layers. Therefore, the molecules have only short range hexagonal ordering extending over a few molecular centres at most. Although the phase has been described as having a layered structure, in the direction perpendicular to the layer planes the molecules are arranged in a one dimensional density wave, indicating that the layers are relatively diffuse [51]. As a consequence, the concept of a layered mesophase is somewhat misleading because the layers are so diffuse that in the macroscopic phase they are almost non-existent. Within these loosely constructed layers the molecules are arranged in such a way that they are often at slight angles to the layered planes. The long axes of the molecules can be tilted anywhere up to about 14 to 15 from the layer normal, and this makes the layer spacing, on average, slightly shorter than the molecular length. However, as this tilting occurs randomly across the bulk phase, the average direction of the long axes of the molecules is perpendicular to the layer planes. Consequently the director, n, is perpendicular to the layers, and the phase is therefore uniaxial. As the phase is uniaxial, it is also optically uniaxial, with the optic axis perpendicular to the layer planes [52].The sub-phases of smectic A are described as monolayer smectic A (SmA 1 ), bilayer smectic A (SmA 2 ), partially bilayer smectic A (SmA d ). Partial bilayer ordering is typically caused either by interdigitation or pairing of the molecules with partial overlap and found to happen in materials where the molecules have terminal polar groups. Another smectic modification called ribbon or antiphase has also been reported where an undulating bilayer is observed. Sub-structures of SmA phases are shown in figure Details regarding this polymorphism of smectic A have been reported by many authors [52].

58 23 Figure Bilayer and monolayer structures of the smectic A phase SMECTIC C LIQUID CRYSTAL In the smectic C phase the constituent molecules are arranged in diffuse layers where the long axes of the molecules are tilted at a temperature-dependent angle, q, with respect to the layer planes. The smectic C phase can be formed, via a first order phase transition, by cooling the isotropic liquid, the nematic, smectic A or the D phases, or via a second order phase transition from the smectic A phase. Typically, for first order phase transitions there is a jump in the value of the tilt angle at the transition, i.e., from a value of zero to a large finite value of usually more than 20. After the initial jump in the value of the tilt angle, which occurs over only a few degrees, the angle remains fairly temperature independent. For a second order transition to the smectic C phase, the tilt angle usually continues to rise with falling temperature over the entire temperature range of the mesophase. However at lower temperatures there is a tendency for the tilt angle to saturate. Mathematically the temperature dependence of the tilt angle takes the form (θ) T = (θ) 0 (T c - T) α

59 24 where (θ) T is the tilt angle at temperature T C, (θ) 0 is a constant, T c is the smectic A to smectic C transition temperature, T is the temperature and a is an exponent theoretically predicted to be equal to 0.5. This power law dependence of the tilt angle ensures that the value of the tilt angle will essentially saturate with falling temperature. The molecules within the layers are locally hexagonally close-packed with respect to the director of the phase; however, this ordering is only short range, extending over distances of approximately 15 Å. Locally the molecules may also have bond orientational ordering, but the extent of this structural feature has not yet been fully examined. Over large distances, therefore, the molecules are randomly packed, and in any one domain the molecules are tilted roughly in the same direction. Thus the tilt orientational ordering between successive layers is preserved over long distances [53, 54]. In SmC phase, when the constituent molecules are strongly axially polar, four sub phases are observed identical to those of the SmA phase, except that the molecules are tilted with respect to the layer planes. In this case an additional subgroup has been found where the tilt direction appears to flip as one moves from one layer to the other and is called alternating smectic C (SmC) [53-55] SMECTIC B LIQUID CRYSTAL Two distinct types of smectic B phase have been identified; one is called hexatic B (SmB hex ) [56-58] and the other one is crystal B (B) [7]. In SmB hex phase the molecular arrangement is close to that of the SmA phase, however, within a layer the molecules are arranged in close packed hexagonal symmetry. Although the positional ordering within the layer is short range, there is long range bond orientational order in this phase. If the line joining the centers of mass of a molecule and its nearest neighbor is called a bond then by long range bond orientational order it is meant that the orientation of the hexagonal packing array is of long range [58, 59]. In crystal B phase, additionally the molecules have long range positional order within the layers as well as along the layer normal. However, in crystal B [59] phase the inter-layer stacking sequence may be of mono- (AAAA type), bi- (ABAB type) and tri- (ABCABC type) layers. Even random ABCABC type packing is also reported. Structures of SmB hex and B phases have been shown in figure 1.19.

60 25 (a) (b) Figure Structure of the (a) Hexatic Smectic B phase (b) Crystal B phase showing ABC packing SMECTIC E LIQUID CRYSTAL The smectic E phase is now designated as crystal E (E) phase since it can also be considered as a soft crystal like the B phase [59, 60, 61]. The lath-like molecules rotate cooperatively about their long axes on a time scale of times per second but unlike in B phase the motion is not full free rotation rather of an oscillatory nature. SmE phase is also found to have bilayer structure as in B phase [60, 61] SMECTIC I AND SMECTIC F LIQUID CRYSTAL The structures of smectic I phase are similar to SmB hex but the molecules in this case are tilted within the layers [59], direction of tilt being towards an apex of the hexagonal packing net. Thus it has short range in-plane and quasi-long range out-ofplane positional order as well as long-range bond orientational order in three dimension. The only difference in the molecular arrangement of smectic F phase is that the tilt of the molecules is towards an edge of the hexagonal packing net. In addition slightly longer correlation has been observed in the in-plane positional ordering than that found in smectic I phase [59] SMECTIC G, G, H AND H LIQUID CRYSTAL All these smectic phases are now termed respectively as crystal G, crystal J, crystal H and crystal K phase since they have long range three dimensional order and they are like soft crystals. In these cases the molecules are tilted with respect to the

61 26 layer planes [59]. The crystal J and G modifications are like the tilted crystal B phase, direction of the tilt being that in SmI and SmF phases respectively. The best way to keep tracks of all these phases is to use a chart as shown in figure No single liquid crystal material is found to exhibit all the phases but many compounds are found to exhibit complex polymorphism, for example, the compound N-(4-npentyloxybenzylidene)-4 -n-hexylaniline possess the phases N, SmA, SmC, SmB, SmF and SmG phases. Current knowledge of phase sequencing with respect to temperature is found to be as follows: Iso, N O, N SC (or Ch), SmA, SmC, SmB hex, SmI, SmF, B, J, G, E, K, H, Crystal Decreasing temperature Increasing order Figure Basic molecular Structure of different liquid crystals CHIRAL SMECTIC OR FERROELECTRIC LIQUID CRYSTAL DOBAMBC (n-decyloxybenzylidene-n-amine-2-methyl butyl cinnmate) was the first synthesized ferroelectric liquid crystals (FLC) [62, 63, 64]. The possibility of a ferroelectric phase in liquid crystals was first envisaged by Saupe in 1969 [65]. Ferroelectricity, resulting from a spontaneous macroscopic electric polarization is a

62 27 property which was first reported by Meyer [66] to occur in a fluid liquid crystalline phase. Until recently, ferroelectricity in liquid crystals was based on a tilted arrangement of homochiral molecules in layers (e.g. smectic C phase). FLCs have attracted considerable interest of researchers because of their exceptional properties [67]. This phase was discovered by Robert Meyer [66] in 1975, who later demonstrated it in a synthesized chiral smectic C material DOBAMBC, [Figure 1.22(a) and 1.22(b)]. Figure (a) Schematic Representation of FLC (b) Schematic Representation of FLC in terms of pitch (c) Showing Tilt and Polar plane normal to C2 axis. The seven liquid crystal classes SmC, SmI, SmF, J, G, K and H are characterized by a tilt between the director and the normal to the smectic layers. If additionally, the molecules are chiral, the material become optically active and show ferroelectric properties. These phases are denoted by SmC*, SmI*, SmF*, J*, G*, K*

63 28 and H*. By virtue of their symmetry, FLCs are piezoelectric too, because polarisation in these materials can be induced by mechanical stress. Rod shaped mesogens, in smectic phase show a translational order as well as orientational order. In the smectic C phase, the periodic spacing of the mesogens along one axis, suppose Z axis, causes them to form layers in the X-Y plane. The director of the each layer is tilted at an angle from the normal [68]. This angle is temperature dependent if a smectic C to smectic A transition occurs with increasing temperature. When the molecule is chiral, successive smectic C layer shows a gradual change in the direction of the tilt, such that the director precesses about the Z axis from layer to layer, always lying on the surface of a hypothetical cone of angle 2 [figure 1.21 (a) and (c)]. The angle around the circle of precession is known as the azimuthal angle. This creates a helical structure in the chiral smectic C (Sm C*) mesophase with the pitch being the distance along the Z axis needed to reach the same molecular orientation [Figure 1.21 (c)]. In addition to producing this helical structure, chirality results in a spontaneous polarization. This polarization vector is perpendicular to the molecule and contained in the layer plane. Therefore, all possible directions for the polarization vector are tangent to the circle of intersection of the cone with the plane. A bulk SmC* sample, free to develop its helical structure, will not show ferroelectric behaviour since the spontaneous polarization will average to zero over one pitch (or polarization vectors go around an entire circle and cancel each other). This is often referred to as the helielectric phase [68]. The study of FLCs [67] has become important for their variety of applications such as in large area, high information content colour display devices and compounds having SmC* phase (both ferro and antiferro type) are mainly used for this purpose SURFACE-STABILIZED FERROELECTRIC LIQUID CRYSTALS (SSFLC) Although the molecular director in bulk ferroelectric liquid crystals (FLCs) adopts a helical structure, Noel Clark and S.T. Lagerwall found in 1980 that by confining the LC material between closely-spaced glass plates (spaced closer than the ferroelectric helix pitch), the natural helix could be suppressed. This principle is illustrated in the polarized micrograph above, where helix lines are largely absent in the thinner (upper right) part of the cell. Clark and Lagerwall found that the smectic

64 29 layers were oriented approximately perpendicular to the glass. Furthermore, they discovered that such cells could be switched rapidly between two optically distinct, stable states simply by alternating the sign of an applied electric field. The electrooptic properties of an SSFLC depend strongly on the layer geometry as well as on the nature of the orienting properties of the bounding glass plates [68, 69]. SSFLCs are being studied in many research laboratories throughout the world. They form the basis for the development of optical shutters, phase plates, and of high-resolution color displays ANTIFERROELECTRIC LIQUID CRYSTALS (AFLC) Antiferroelectric liquid crystals are similar to ferroelectric liquid crystals, although the molecules tilt in an opposite sense in alternating layers as show in figure. In consequence, the layer-by-layer polarization points in opposite directions as shown in figure Figure Schematic Representation of layer-by-layer polarization points in opposite directions in AFLC These materials are just beginning to find their way into devices, as they are fast, and devices can be made "bistable"[70-72]. The MHPOBC was the first AFLC found by the Japanese group [73-75]. Actually the MHPOBC was first reported as a new FLC in the Japan domestic LC meeting in 1985 by Inukai et. al. [76] but in the first international FLC conference in 1987 two groups pointed out the unusual behaviour of this compound. Hiji [77] reported a third stable state exhibiting dark

65 30 view between crossed polarizer while Furukawa [78] reported a very small relative permittivity and threshold behaviour in electro-optic response in the SmC* phase region suggesting the new phase. Figure Structure and phase sequence of MHPOBC material. Antiferroelectric switching was first reported for unichiral 4-[(1- methylheptyloxy) carbonyl] phenyl-4-octyloxy-4-biphenyl carboxylate [MHPOBC] with structure and phase sequence is shown in Figure This phase occurs in some materials at a temperature below the FLC phase. These materials, like FLCs are chiral and possess a spontaneous polarization. The difference is that in the AFLC phase, the director is tilted in opposite direction in alternate layers. The spontaneous polarization is depicted by the arrow. The director again is tilted thereby always lying on a theoretical cone. It may be noted that in the AFLC case, each subsequent layer the director is tilted in the opposite direction and the spontaneous polarization points in the opposite direction as shown in figure However, the director still precesses around the Z axis. For AFLCs the pitch is the distance for the director to precesses 180 degrees instead of 360 degrees as for FLCs [77-78]. This is because due to the opposite tilt in adjacent layers the director also has gone around half of the cone. As for the FLC, the AFLC helix must be unwound through a boundary constraint for the material to be used in displays. As shown in figure 1.24, the director always lies in the layer plane and the polarization vector perpendicular to it. In subsequent layers the directors is pointed in opposite directions and therefore so are the polarization vectors. Thus because of an equal number of polarization vectors pointing up and down, the spontaneous polarization averages out to zero even for the unwound (non-helical) state.

66 31 Figure Schematic Representation of AFLC in terms of polarization vector FERRIELECTRIC LIQUID CRYSTAL (FLC) Another phase closely related with SmC* phase have been discovered. This phase is somewhat like the frustrated smectic phase in which local dipoles are antiparrallel. This phase is known as the ferrielectric liquid crystal phase. Accordingly, the ferrielectric phase generates a spontaneous polarization which depends upon the degree of alternation of tilt directions. The ferrielectric liquid crystal phase is characterized by an uneven numbers of layers stacked with tilt to the left and tilt to the right shown in figure 1.25, therefore exhibiting a net but low spontaneous polarization, since there are an uneven number of molecules with polarization in two opposite direction. Recent work has shown that many shades of ferrielectric phase exist differing in proportion of layers with two different tilt directions [73, 79, 80]. (a) (b) (c) Figure Showing spontaneous polarization in FLC, AFLC and FerriLC.

67 TWISTED GRAIN BOUNDARY PHASES (TGB) Twisted grain boundary phases (TGB) have been known since 1988 [81] and have attracted great attention during the last 10 years. Chiral liquid crystals have tendency to form a cholesteric-like helical director field. On the other hand, the molecular interaction may favour a smectic layer structure. However it is impossible to realize a continuous structure which exhibits both a cholesteric director field and a smectic layer structure at the same time. The competition between these two structural features can result in frustrated structures containing a regular lattice of grain boundaries which in turn consist of a lattice of screw dislocations. This defected structure exhibits an interesting theoretical analogy to the flux line lattice which occurs in the type 2 superconductors. However the range of parameters determining the structure is larger in liquid crystals than in superconductors. Thus a large variety of new phases, such as the TGB A, TGB C, TGB 2q, melted grain boundary (MGB) phases [81, 82], antiferroelectric crystals of twist grain boundaries, and Smectic blue phases have been predicted and/or experimentally observed.

68 33 Figure (a) Structure of a phase which shows a local smectic order and a helical director field at the same time (b) Structure of the TGB A phase proposed by Renn and Lubensky. In addition to the structural features displayed in (a), the grain boundaries and the screw dislocations are shown. Characteristic lengths:d=smectic layer spacing, l b =thickness of the smectic slabs, l d =distance between neighbouring screw dislocations, and p=pitch of the director field. Twist grain boundary (TGB) phases usually appear in the temperature range between the cholesterics N* phases with short pitch and a smectic phase, typically a SmA or SmC*. One of their remarkable properties is the selective reflection of circularly polarized light. This feature shows that the director field has a helical structure similar to the cholesteric phase. On the other hand X-ray investigations of TGB phases indicate a layer structure as occurring in smectic phases. Chirality of the system is an essential precondition for the occurrence of TGB phases. Figure 1.26 shows the structure of the TGBA phase which consists of smectic slabs, separated by defect walls. Neighbouring slabs are tilted with respect to each other by an angle 0, thereby forming a helical structure. The helix axis h is perpendicular to the director. The table 1.1 shows some chemical compounds showing twist grain boundary (TGB) phases [82-85]:

69 34 Table 1.1. shows chemical compounds showing twist grain boundary (TGB) phases The other common examples of frustrated phase are the so called blue phases [26]. There are generally three types of blue phases BPI, BPII and BPIII. These are exhibited by materials that are highly chiral and occur at a temperature above a chiral nematic phase and exist for only a few 0 C before the material clears to the isotropic liquid. As shown before, the chiral nematic phase has a helical, twisted structure and the structure of the blue phase is similar except that a double twist exists. The axes of the two twists or helices shown are perpendicular, but in reality twist axes exist in all directions in the plane containing the two twist axes [6]. Blue phases are so called because when first discovered they appeared blue when viewed by eye as thin films. However the blue phases of many compounds exhibit other colors such as red and green when viewed by optical polarizing microscope THE CHIRAL LINE LIQUID The chiral line liquid N * L predicted by Kamien and Lubensky [85] is the analog of the flux line liquid occurring in type 2 superconductors with strong fluctuations. Instead of forming a regular array, the defects are rather disordered. * However the precise structure of the N L are still unknown [26].

70 BEND GRAIN BOUNDARY (BGB) PHASES Eleven years were necessary to recognize that the mixed state predicted by de Gennes [3] cannot only be induced by mechanical deformation of the director field, but can even occur spontaneously due to the presence of the chiral Molecules. The experimental evidence for the occurrence of the mixed state in the form of TGB phases has initiated a revival of the search for regular defect structures in nonchiral materials due to mechanical forces [83-85]. Figure Molecules with a bent molecular core are expected to show a high flexoelectric coefficient. Thus, an electric field-induced bend deformation may be suitable to generate a lattice of edge dislocation, thereby leading to a bend grain boundary phase REENTRANT PHASES OF POLAR LIQUID CRYSTALS In condensed matter physics phenomenon of reentrance of more ordered phases from less ordered phases by cooling or compressing can generally be argued to occur from competition of tendencies towards different orderings. Recently a molecular theory has been developed which exemplifies the potentialities of a microscopic approach in accounting for the observations of multiply re-entrant nematic and smectic phases in polar liquid crystals. The thermodynamic phases and phase transitions in these systems are very sensitive to details of the molecular structure. In some liquid crystals of molecules with strongly dipolar heads (-CN or

71 36 NO 2 ) nematic phases (orientational order) and smectic phases (smectic phase and partial positional order) reenter. Experimentally observed reentrances typically involve nematic (N), monolayer smectic A(A 1 ), and interdgitated partial bilayer smectic A(A d ) phases. The reentrance sequence include single reentrances (N- A d -N), double reentrances (N-A d -N-A d -N-A 1 ), and reentrance below A 1 (N-A 1 -N-A d (-A 1 )). On the theoretical side all of these reentrances have been obtained with the spin gas model of liquid crystals [86] EFFECT OF CHEMICAL CONSTITUTION ON MESOMORPHISM Most of the rod-like liquid crystalline compounds consist of two or more rings, which are directly bonded to one another or connected by linking groups. The chemical structure of many mesogens can be represented by the general formula shown in figure Figure General formula of Liquid Crystals Core group: usually aromatic or alicyclic; to make the structure linear and rigid. Linker: maintaining the linearity and polarizability anisotropic. Terminal Chain: usually aliphatic chain, linear but soft so that the melting point could be reduced. Without significant destroy the LC phase. Note that sometimes one terminal unit is replaced by a polar group to provide a more stable nematic phase. Side group: to control the lateral interaction and therefore enhance the chance for nematic. Note that large side groups will weaken the lateral interaction.

72 37 Effect of Core The major anisotropy of molecules, which is necessary for their mesogenity, results from the cores, which are also responsible for relatively high melting temperatures. The core consists of rings that are connected to one another either directly or by linking groups. Any ring that allows a stretched configuration of the molecules can be used. More complex ring systems cholesterol is also used. The oldest known liquid crystal have benzene rings as core. The increase in number of benzene rings generally results in the increase of melting temperatures. Also, the mesogenity of the compound increases with the number of linearly connected rings. Due to the large conjugated aromatic- systems, the intermolecular attractions of the molecules are very large giving rise to high melting temperatures.the cyclohexane ring is non-aromatic and flexible compared to benzene ring. The flexibility of the central ring has some negative influence on mesogenity. Bicyclooctane derivatives have much stronger nematogenity as compared to the cyclohexane core. Effect of Linking Groups Small chemical groups between the rings of liquid crystal molecule can increase the length of the molecule while preserving the linear shape. However, when the linking groups produce a bent molecular shape, the mesogenic potential of the molecule is diminished. Besides the geometry of the molecules, additional effects such as conjugative interaction of the linking groups with aromatic groups, effects due to polarity of the linking groups etc. also play an important role in liquid crystalline of a molecule. The effects of linking groups can be quite different in aromatic and nonaromatic compounds, as in the case of non- aromatic compounds, there are no conjugative effects, however, the effect of terminal substituents may sometimes overcome this effect. Effect of Terminal Substituents Terminally substituted compounds exhibit more stable mesophases compared to unsubstituted mesogenic compounds. The most common terminal substituents are the alkyl and alkoxy groups. The behavior within the homologous series shows that in general there is an alteration of T N-I temperatures. This can be explained by the alteration of the length to breadth ratio. Figure 1.29 shows a typical six-member ring, with an attached alkyl chain.

73 38 Figure Alteration effect in a terminal alkyl chain The attachment of an odd numbered carbon atom substituent increases the length to breadth ratio more than does the attachment of an even numbered carbon atom substituent. This principle behavior seen in alkyl chain can also be found in other flexible chains. X-rays and other methods have been used to show that compounds containing strongly polar groups like -CN and -NO 2 from double molecules that exist in equilibrium with single molecules [87-89]. Due to such dimerization, the breadth increases by the factor of 2 and length only by a factor of 1.1~1.4. Hence, the effective L/B ratio should be reduced. But, highly polar compounds have a much higher density than low polar compounds [90, 91]. This accounts for the increase in clearing temperature. The halogens and isothio-cyanato groups introduce relatively large positive dielectric anisotropy into the molecules however; there is no association [92]. Branched terminal substituent also affects mesomorphism. The effect of a branch depends substantially on its position in a chain. When the branch is nearer the centre of the molecules the clearing temperature is lowered. When -CH 2 group in the terminal chains are replaced by an oxygen atom, clearing temperature decreases. Oxygen atom seems to reduce the stiffness of the chain. The terminal group efficiency order which has been compiled for Smectic phase in rod-like aromatic system is: -Ph > -Br > -Cl > -F > -NMe2 > -Me > -H >-NO2 > -OMe > -CN and the nematic group efficiency order is, -Ph > -NHCOCH3 >-CN > -OCH3 >-NO2 > - Cl > - Br > - N (CH3)2 >-CH3 > - F. Intermolecular Hydrogen Bonding Intermolecular hydrogen bonding interactions have shown great potential in the preparation of new liquid crystalline systems especially thermotropic LCs [93, 94]. They have been used as links, connecting two independent molecular

74 39 components. These form anisotropic molecules, which complies with the main characteristic of liquid crystal molecules. Most of these systems are based on pyridine and acid derivatives [95]. The hydrogen bond in the liquid crystal field enables molecular components that do not themselves exhibit the property, form supramolecular species, which show the liquid crystal behaviour. Also these liquid crystal moieties have greatly enhanced mesomorphic range [96] PHYSICAL PROPERTIES OF LIQUID CRYSTALS Figure Physical properties of liquid crystals As a result of orientational order, most physical properties of liquid crystals are anisotropic [7, 97, 98] and must be described by second rank tensors shown in figure Examples are the heat diffusion, the magnetic susceptibility, the dielectric permittivity or optical birefringence [99]. Additionally, there are new physical qualities, which do not appear in simple liquids as e.g. elastic or frictional torques (rotational viscosity) acting on static or dynamic director deformations, respectively. The most remarkable features of liquid crystals with respect to applications are due to their anisotropic optical properties. Nematics, and SmA are uniaxial, SmC weakly biaxial. Cholesterics give rise to Bragg reflections if the helix pitch is in the magnitude of the light wavelength. As mentioned above these properties are carried by a fluid, soft material, and therefore are extremely sensitive against external

75 40 perturbations. Orientational order and hence birefringence can be manipulated easily e.g. with the help of rather weak magnetic, electric or optical fields, leading to huge magneto-optical, electro-optical and opto-optical effects [100,101]. The most successful application are liquid crystal displays well-known from wrist watches, pocket calculators or flat screens of laptop computer which take advantage of electrooptical effects. More recently, it turned out that orientational order can be also affected by optical fields leading to rather sensitive opto-optical effects and nonlinear optical properties, which are important e.g. for all-optical switching and other photonic devices in future optical information technologies [102, 103]. Birefringence in Liquid Crystals Liquid crystals are found to be birefringent, due to their anisotropic nature. That is, they demonstrate double refraction (having two indices of refraction). Light polarized parallel to the director has a different index of refraction (that is to say it travels at a different velocity) than light polarized perpendicular to the director. Thus, when light enters a birefringent material, such as a nematic liquid crystal sample, the process is modeled in terms of the light being broken up into the fast (called the ordinary ray) and slow (called the extraordinary ray) components shown in figure Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference. Figure Light traveling through a birefringent medium will take one of two paths depending on its polarization APPLICATIONS OF LIQUID CRYSTALS The dual nature and easy response of LCs provide great opportunities to use them in various electro-optical devices particularly in displays. By the end of 20th

76 41 century the market of LC displays has grown more than 20B$ and became a better option than the conventional CRTs. Now a days, applications of this nature s most delicate phase of matter not only limited to displays but spread over a broad area of science, engineering and medical and it is still growing and providing effective solution to many problems. Some applications of LCs are discussed as below DISPLAY APPLICATION OF LIQUID CRYSTALS The most common application of liquid crystal technology is liquid crystal displays (LCDs) [ ]. This field has grown into a multibillion dollar industry, and many significant scientific and engineering discoveries have been made. Liquid crystal display devices consisting of digital readouts are used in watches, calculators, and several other instruments like mobile and many household electric appliances [110]. Some liquid crystal substances could be useful in computer industry, for making new computer elements with high memory capacity. Liquid crystals displays (LCDS) [111] had a humble beginning with wrist watches in the seventies. Continued research and development in this multidisciplinary field have resulted in display with increased size and complexity. After three decades of growth in performance, LCDs now offer a formidable challenge to cathode ray tubes (CRT). Liquid crystal display (LCDs) have many adventages over other display types.they are flate and compact, possess extremely low power consumption (Microwats per square centimeter in the case of the twisted Nematic display), their colour and contrast does not fade with an

77 42 increase in the illumination intensity. They work both in transmitive and reflective modes in a wide operating temperature range and with a long life time. Because that, LCDs are the most economically produced display systems. LCDs have a brilliant future in high defination TV system, personal computer, measuring devices etc. The most widely used electro optics effects in display are the twist, super twist and guest host modes. There are many types of liquid crystal displays, each with unique properties. The most common LCD that is used for everyday items like watches and calculators is called the twisted nematic (TN) display. This device consists of a nematic liquid crystal sandwiched between two plates of glass. A special surface treatment is given to the glass so that the director at the top of the sample is perpendicular to the director at the bottom. This configuration sets up a 90 degree twist into the bulk of the liquid crystal, hence the name of the display. The underlying principle in a TN display (shown below) is the manipulation of polarised light. The left image shows that when light enters the TN cell, the polarisation state twists with the director of the liquid crystal material. For example, consider light polarised parallel to the director at the top of the sample. As it travels through the cell, its polarisation rotates with the molecules. When the light emerges, its polarisation has rotated 90 degrees from when it entered shown in figure Figure Principle of twisted nematic LCDs

78 43 The right image shows that the application of an electric current to these liquid crystals will "untwist" them to varying degrees, depending on the voltage. These liquid crystals are most popular for LCDs because they react predictably to electric current in such a way as to control light passage. Depending on the field strength, twisted nematic displays can switch between light and dark states, or somewhere in between (grey scale). How the molecules respond to a voltage is the important characteristic of this type of display The use of polymer liquid crystal (PLC s) in the display industries is an exciting area of research. A twisted nematic polymer liquid crystal cell can be used to make energy efficient displays. A layer is use to selectively melt portions of the display into the liquid crystal phase. The orientation of the cell is then chosen by applying a field across it, just as in an ordinary twisted nematic liquid crystal cell. When the polymer cools down and hardens into a glass, the mesogens will be locked in that configuration and the field can be turned off. Side chain polymer liquid crystal exhibit good properties for application in optically nonlinear devices including optical wave guides and electrooptic modulators in poled polymeric slab-waveguides THERMAL MAPPING AND NON-DESTRUCTIVE TESTING Chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the color reflected also is dependent upon temperature. Liquid crystals make it possible to accurately gauge temperature just by looking at the color of the thermometer. By mixing different compounds, a device for practically any temperature range can be built. More important and practical applications have been developed in such diverse areas as medicine and electronics. Special liquid crystal devices can be attached to the skin to show a "map" of temperatures. This is useful because often physical problems, such as tumors, have a different temperature than the surrounding tissue. Liquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the characteristic higher temperature. The sensitivity of cholesteric liquid crystals to react to pressure as well as temperature by colour change is used to make some very interesting publicity materials and toys. Cholesteric liquid

79 44 crystals can be used as an analytical tool to detect the presence of very small amounts of gases or vapours by colour changes to the extent of about 1 ppm. [112]. A film of cholesteryl liquid crystal [113] may be applied to large uneven area. This makes it an ideal tool for thermal mapping and non-destructive testing. The great deal of flexibility in the color play range allows for a great diversity in potential applications ranging from food processing to electronics and space applications e.g. thermo chromic paints have been used on primed circuit boards to examine overheating of components. The area in which liquid crystal thermograph is of use in non destructive testing continued to grow due to the development on new chiral smectic materials which offer improved performance over the cholesteryl esters used in early applications. Thermo chromic liquid crystals are extensively used in medical applications, forehead thermometers also known as fever strips are based on different thermo chromic liquid crystal materials. Thermal mapping of various areas of the body has been used as a diagnostic technique for a wide ranging group of medical conditions in which a temperature differential near the skin surface may be related to the disorder subcutaneous and intracutaneous malignant tumors are typically C warmer than the surrounding tissues. Therefore, thermograph is an interesting candidate for cancer screening MEDICINAL USES In gynecology, where there is a possibility that a cessarian section may be necessary, liquid crystals can be used to locate the plecenta, thus avoiding the need for X-ray. Hence it is useful in controlled drug delivery [114]. Recently their biomedical applications such as protein binding [115], phospholipids labeling [116] and inmicrobe detection [117] have been demonstrated. In psychology, cholesteric liquid crystals could be used in lie detectors OPTICAL IMAGING In this technology, a liquid crystal cell is placed between two layers of photoconductor. Light is applied to the photoconductor, which increases the material s conductivity. This causes an electric field to develop in the liquid crystal corresponding to the intensity of light. The electric pattern can be transmitted by an

80 45 electrode, which enables the image to record. This technology is still being developed. The other potential optical applications of liquid crystals are as follows (A) Light Shutters (B) Spatial Light Modulator (SLM) (C) Optical Telecommunication LIQUID CRYSTAL SOLAR CELL A new and promising application using liquid crystals is the liquid crystal semiconductor. Liquid crystals are organic molecules similar to polymers. In polymers containing conjugated systems (alternating single and double bond) the creation of a higher and lower pi-bond leads to the creation of a band gap similar to semiconductors. Figure Liquid Crystal Solar Cell. The use of such a liquid crystal in a device similar to the Grätzel cell can lead to new types of solar cells in Figure 1.33.

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87 Chapter 2 Theoretical Background of Liquid Crystal

88 Chapter 2 Theoretical Background of Liquid Crystal 2.1. Basic review of liquid crystal properties The director The order parameter Tilt angle Spontaneous polarization 2.2. Basic physical properties The anisotropy of liquid crystals Dielectric anisotropy in liquid crystals Optical anisotropy of liquid crystals Uniaxial versus biaxial media Flexoelectric effect in liquid crystals 2.3. Visco-elastic properties Viscosity and rotational viscosity Elasticity theory LC anchoring and easy axis Anchoring energy Elastic deformation of the LC director 2.4. External field effects Euler-Lagrange equations Alignment Threshold voltage and Fredericksz transition Response time 2.5. Erickson-Leslie equation 2.6. Interaction with light: propagation in anisotropic media 2.7. Phase transitions Landau-de Gennes theory of phase transitions Maier-Saupe theory (mean field approach) 2.8. Dielectric spectroscopy study The complex dielectric permittivity Debye type polarization mechanism Non-Debye type polarization mechanism The Cole -Cole plot

89 Parameters obtained from fitting of Cole- Cole equation Dielectric relaxation Non collective (molecular) relaxation process Collective relaxation process (A) Tilt angle fluctuations or soft mode (B) Goldstone mode (C) Influence of dc bias electric field on the helix distortion mode 2.9. Guest host interaction Liquid crystal and nano composites Nanoparticles History Properties Classification Characterization Nanoparticle morphology Liquid crystal and dye composites Liquid crystal and polymer composites References

90 REVIEW OF LIQUID CRYSTAL BASIC PROPERTIES THE DIRECTOR The continuous description of nematic liquid crystals (LCs) used in the literatures revolves around the director. As discussed in chapter one, liquid crystals are composed of long, bar like molecules. In the isotropic fluid phase, the orientations and positions of the molecules are random. In the nematic phase, the positions of the molecules are still random, but their long axis are oriented on the average along a particular direction specified by a unit vector n called the director. `` Figure 2.1. The Nematic Director making an angle with the long molecular axis of the molecule. This unit vector n, which lies parallel to the molecular axis is averaged over a small (but macroscopic) volume (figure 2.1). The n may vary, both in space (r), and in time (t), and one thrust of continuum theory is to determine differential equations which govern such variations. The term director profile is often used to refer to spatial variations in n. Since n is a unit vector, it can be described by two Euler angles, the tilt angle θ and the twist angle, φ. The twist angle is defined to be the angle between the projection of n onto the xy-plane, and the y axis (figure 2.2). Expressing n in terms of these angles, n = (sin θ cos φ, sin θ sin φ, cos θ) In most of the LCs both directions of the vector n, +n and n are equivalent. However, for molecules with permanent dipole moments this may not be the case, and the sign of n becomes important [1, 2].

91 53 Figure 2.2. Variation of director in space; where and denotes tilt and twist angles respectively. The LC director is the direction of the rod shaped LC molecule. If the particles have head-tail symmetry, the director itself as a vector is not useful in investigating the strength of order in the liquid crystal. Integration over all molecular axes, which is the first moment of an orientational distribution, would be zero because of this symmetry, although there is a preferential direction. One has to use the second moment of the orientational distribution, which is a second rank tensor, referred to as the second rank alignment tensor. It provides information about the degree of the nematic order as well as the principal axes of the liquid crystal, which e.g. refer to optical axes. The director corresponds to one of the principal axes of the alignment tensor [1-3]. Furthermore it is proportional to mechanical and electromagnetic properties of the liquid crystal like the polarizability etc (figure 2.3) THE ORDER PARAMETER The description of liquid crystals involves an analysis of order. A second rank symmetric traceless tensor order parameter is used to describe the orientational order of a nematic liquid crystal, although a scalar order parameter is usually sufficient to describe uniaxial nematic liquid crystals. The simplest way of defining the degree of orientational order is by using the order parameter S: 1 2 S 3cos 1 (2.1) 2 Where θ is the angle between the long axis of an individual molecule and the director, and the brackets imply a thermal average, is used to describe these fluctuations on a

92 54 macroscopic scale. It varies from 0 in the isotropic phase, through intermediate values in the nematic phase, to 1 in the ideal crystalline phase (figure 2.4). The tendency of the liquid crystal molecules to point along the director leads to a condition known as anisotropy. Figure 2.3. The order parameter: How does it affect display performance? (a) (b) Figure 2.4. (a)scalar order parameter S versus temperature: T c is clearing temperature. (b) The order parameter for particular cases of LC, where is the angle between the director and the long axis of each molecule. Typical values for the order parameter of a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature, as a result of kinetic molecular motion [1, 2, 3]. The most important classes of liquid crystals belong to the uniaxial case (the smectic C is a typical counter example). The order parameter for all such systems can be characterized by a magnitude S and a direction n, where the latter is the principal axis of the order parameter tensor.

93 TILT ANGLE For practical use, the director is usually specified by the twist and tilt angle as illustrated in figure 2.5. The azimuthal angle between the positive x-axis and the projection of the liquid crystal director on the xy-plane is defined as the twist angle or azimuthal angle of the director. Figure 2.5. Variation of tilt angle with the temperature for FLCs DOBAMBC [4] and definition of the twist angle and the tilt angle of the liquid crystal director n. The angle between the director and its projection on the xy-plane is called the tilt angle. The vectorial representation of the director is not unambiguous. Physically there is no difference between the director orientations n and n, since as many molecules in the material have the tail pointing upward or downward. Unless specified otherwise the XY-plane in this work will always coincide with the bottom glass substrate, with the z-axis pointing toward the top substrate. The temperature dependence of the tilt angle may be given by 1 2 C T) ( T (2.2) Where, T C represents the temperature at which FLC goes from SmC* to any less ordered liquid crystalline mesophases. Figure 2.5 represents the temperature dependence of tilt angle of FLCs SPONTANEOUS POLARIZATION The spontaneous polarization is the secondary order parameter for FLCs. FLCs have monopoly to have the spontaneous polarization due to an asymmetrical

94 56 structure of SmC* phase. The spontaneous polarization P s could be expressed via the FLC molecular parameters as [4]: Ps eff NA (2.3) M Here, M and N A are the density, molecular mass and Avogadro s number respectively also eff is the average molecular dipole moment averaged over a period of molecular rotation or over the molecular ensemble. The temperature dependence of the spontaneous polarization may be described as- PS (TC T) (2.4) Here P o is spontaneous polarization at absolute zero temperature, T C is the ferroelectric to paraelectric phase transition temperature and β is the fitting parameter. A typical graphical presentation of the spontaneous polarization for DOBAMBC has been shown in figure 2.6 [5]. Figure 2.6. Variation of spontaneous polarization with the temperature for ferroelectric liquid crystal DOBAMBC [3] BASIC PHYSICAL PROPERTIES Liquid crystal exhibits a certain degree of order in the molecular arrangement. As a result, there is anisotropy in the mechanical, electrical, magnetic, and optical properties. A number of unique characteristics make LC particularly suitable for displays. For electro-optic applications employing a nematic LC, birefringence, elastic constants, dielectric anisotropy, and rotational viscosity all play important role

95 57 affecting the device performance. The following sections will give an overview on the basic physical properties of LC materials THE ANISOTROPY OF LIQUID CRYSTALS The uniaxial symmetry around the director in the LC phase leads to an anisotropy in many physical properties. For example, the refractive index, the dielectric permittivity, the magnetic susceptibility, viscosity and conductivity have a different value parallel to the director n and perpendicular to it [3, 6] DIELECTRIC ANISOTROPY IN LIQUID CRYSTALS The useful property of nematic liquid crystals, Δε=ε -ε, causes the liquid crystal to interact with an external applied electric field. The dielectric anisotropy is a characteristic property of liquid crystal, due to linking group attached to aromatic ring of liquid crystals as show in figure 2.7. Dielectric properties of LCs are related to the response of LC molecules to the application of an electric field. Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium, it is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. In the LC materials consisting of non-polar molecules, there is only an induced polarization, which consists of two parts: the electronic polarization (which is also present at optical frequencies) and the ionic polarization [3-7]. In the LCs with polar molecules, there is in addition to the total induced polarization, the orientation polarization, due to the tendency of the permanent dipole moments to orient themselves parallel to the field. Considering the uniaxial LC phases in a macroscopic coordinate system x, y, z, with the z axis parallel to the director n, it is possible to distinguish two principal permittivities, parallel to the director ε = ε zz, and perpendicular to the director ε = 1/2(ε xx + ε yy ). Then the dielectric anisotropy Δε = ε - ε can take positive and negative values (figure 2.8). The graph of temperature dependence of dielectric permittivities for a typical LC (figure 2.9) shows that magnitude of Δε usually depends on temperature, Δε S(T).

96 58 m m Figure 2.7. The dielectric anisotropy liquid crystal and linking group attached to aromatic ring of liquid crystals Figure 2.8. Positive and negative dielectric anisotropy. For non-polar LCs the situation is relatively simple but in the case of polar molecules with permanent dipole moments, the orientation polarization must be added, which, by Maier and Meier, leads to an expression for the dielectric anisotropy [3] 2 F 2 3cos 1 (2.5) 2k BT where Δα is a polarizability anisotropy, μ is a molecular dipole moment, F is parameter depending on the reaction field factor, k B is a Boltzmann constant, is the angle between long molecular axis and net dipole moment of liquid crystal molecule

97 59 and T is temperature. Then Δε is determined by the angle β between the molecule axis and the direction of the off-axis molecular dipole moment μ. The dipole contribution to Δε is positive for β < 54.7 o and negative for β > 54.7 o, which may result in positive or negative Δε depending on the relative magnitude of two contributions. The larger the anisotropy the smaller electric field is needed to make the LC respond to it [6-9]. Figure 2.9. Temperature dependence of dielectric permittivity in liquid crystals with Δε > 0: Tc - clearing temperature; ε iso - permittivity of isotropic liquid OPTICAL ANISOTROPY OF LIQUID CRYSTALS Anisotropic materials produce different values for the index of refraction when the light is linearly polarised along the x-axis and when it is polarised along the y- axis. This phenomenon is referred to as birefringence.the anisotropy of LCs causes light polarized along the director n to propagate at a different velocity than light polarized perpendicular to it. Therefore, LCs are birefringent materials. A uniaxial LC has two principal refractive indices, ordinary refractive index n o and extraordinary refractive index n e. The first one, n o, is measured for the light wave where the electric vector vibrates perpendicular to the optical axis (ordinary wave) (figure 2.10a). The index n e is measured for the light wave where the electric vector vibrates along the optical axis (extraordinary wave) (Figure 2.10b). Then the birefringence is given by [10, 11] Δn = n e - n o. Usually, n e > n o and, therefore, Δn is positive and varies in the range from values close to zero to about 0.4. In the case of uniaxial LCs the optic axis coincides with the

98 60 director n. The indices n o and n e are measured for the light propagating along or normal to the optical axis. Figure Light propagation in LCs along and normal to optical axis: (a) ordinary beam; (b) extraordinary beam; n e - extraordinary refractive index; n o - ordinary refractive index. In the case when the direction of the light propagation is tilted with respect to the optical axis (figure 2.11), the refractive index for the ordinary wave is equal to n o, but the refractive index for the extraordinary wave is equal to some effective value given by neno n eff (2.6) n cos n sin e o where θ is an angle between the optical axis and the light propagation direction. Then birefringence is Δn = n eff - n o.

99 61 Figure Light propagating at some angle θ to optical axis. In the case of propagation along the optical axis, the refractive index is n o for both modes, ordinary and extraordinary. More formally, the refractive index is related to the response of matter to an electric field. Thus, in general, refractive index related to dielectric permittivity ε depending on the frequency of the applied field. When considering LCs we are interested in the optical frequency range, where n 2 a = ε a, a = x, y, z, Where n a is the refractive index along the direction. In general, birefringence Δn of LCs decreases as the wavelength of the incident light or the temperature increases, Δn S (T) (figure 2.12) [12, 13, 14]. Figure Temperature dependence of refractive index: Tc - clearing temperature; n iso - refractive index of isotropic liquid. The unpolarized light incident upon a LC is splitted to the ordinary and extraordinary waves, which travel with different velocities through the material.

100 62 Figure The LC molecule splitted the unpolarised light into ordinary and extraordinary beam with a phase difference Δ. They emerge from the LC with some phase difference that depends on the thickness of material d [14] (figure 2.13) Δφ = 2π Δn d/λ, (2.7) Where λ is a light wavelength. Thus, the variation in the thickness of the LC material produces different phase shifts and the outcoming light polarizations UNIAXIAL VERSUS BIAXIAL MEDIA The uniaxial media is the media with a single director and a single order parameter have been considered. However, liquid crystals may be biaxial - described by two mutually orthogonal directors, n and m, and two order parameters, S 1 and S 2. The molecules themselves might be biaxial - book-shaped, rather than rod-shaped, or a mixture of rod- and disc- shaped particles. Even if the molecules are uniaxial, they can form biaxial arrangements [3] FLEXOELECTRIC EFFECT IN LIQUID CRYSTALS The piezoelectric effects in nematics were proposed for the first time by Meyer in 1969 [15]. The flexoelctric theory was enunciated by analogy with the theory of piezoelectricity in solids. The basic idea of Meyer was that the macroscopic splay (div n) and the macroscopic bend (n rot n), where n is the unit vector showing the mean direction of the LC molecules along one preferred axis, called by

101 63 Frank [16] director, can lead to macroscopic polarization (fig 2.14). From this picture it is clear that the initial non-deformed state of the nematic is not polarized since the permanent electric dipoles of the molecules are locally compensated, which can be removed in two ways: (a) The confinement of the LC in a non-planar geometry with imposed Homogeneous boundary conditions leads to macroscopic polarization of the LC by the so-called direct piezoeffect, and (b) The exposition of the LC under the action of a DC voltage or AC voltage, when the frequency is sufficiently low, leads at appropriate conditions to the observation of piezodeformations caused by the so-called converse piezoeffect. Figure Schematic illustration of the Flexoelectric effect. This piezoelectric effect for both examples has been called by Meyer a dipolar piezoelectric effect. There are similarities and differences between the piezoelectricity in the crystals [17] and the piezoelectricity in the LCs. For instance, one similarity is that the splay or bend of either the LC or the piezoelectric crystal should lead to a macropolarization. A second similarity is that converse piezoeffect can exist in both: crystals and LCs. There is one important difference, which should be mentioned. The existence of macro-dislocations in the piezoelectric crystals should decrease the net polarization, since the piezoeffect in the crystals is connected with the crystal lattice, while around the disclinations in an insulating nematic, there should exist a very strong polarization [18, 19] (the situation is quite different, however, in a conductive LC due to the screening effect of the impurity ions. de Gennes suggested the term piezoelectricity to be replaced by flexoelectricity [20] and it is now

102 64 accepted in the terminology of the LCs (the term flexoelectricity can be found in the crystal terminology as well [6,10]) VISCO-ELASTIC PROPERTIES VISCOSITY AND ROTATIONAL VISCOSITY The viscosity of the liquid crystalline state is also an anisotropic property, depending on the direction of flow of an individual molecule with respect to the director at any one point within the medium. Three parameters are required to characterize the viscosity of the nematic state, due to the shape anisotropy of its constituent molecules. These are η 1, perpendicular to the direction of flow and parallel to the velocity gradient; η 2, parallel to the direction of flow and perpendicular to the velocity gradient; and η 3 perpendicular to the direction of flow and to the velocity gradient. These three coefficients are called Miesowicz viscosities (figure 2.15). Figure Viscosity: Shear flow viscosity Miesowicz coefficient. The bulk viscosity of an unaligned nematic liquid crystal is an average of these three viscosity coefficients [14, 21]. However, individual viscosity coefficients influence the optical response times in an electro-optic display device, due to the constrained anisotropic environment imposed by the boundary conditions and the unidirectionality of any applied electric field. Such an environment is represented by the rotational viscosity γ 1, [22-28] which, in the nematic phase, is associated with the movement of a molecule from a homogeneous planar conformation to a homeotropic conformation (figure 2.16).

103 65 Figure Viscosity: Rotational viscosity coefficient. For almost all LC-related applications, fast response time is critical issue. Therefore, low viscosity LC materials are essential for TFT-LCDs. Rotational viscosity γ 1 plays a critical role to LC dynamics. Both rise time and decay time are linearly proportional to γ 1. Thus, for most LC devices, low rotational viscosity γ 1 LC material is favorable. The temperature-dependent rotational viscosity could be written as [29]. E0 bsexp kt 1 (2.8) T 1 Tc S (2.9) Where E 0 is the activation energy, T is the Kelvin temperature and S is the order parameter shown in Eq. (2.9). From the molecular standpoint, the rotational viscosity depends on the molecular constituents, dimensions, molecular interactions, and moment of inertia. Thus, a linearly conjugated liquid crystal should exhibit a relatively low rotational viscosity. At the elevated temperature, γ 1 decreases dramatically. For every 10 C temperature increase, viscosity is decreased by ~2X. Low viscosity and high resistivity are essential for TFT-LCDs where the LC birefringence is about 0.1. The commonly used molecular structures are fluorinated cyclohexane-phenyl (CP) two-ring and CCP three-ring compounds. Using a low viscosity LC mixture or operating a LC mixture at an elevated temperature, a small visco-elastic coefficient can be obtained. As a consequence, response time is reduced.

104 ELASTICITY THEORY The phenomenological theory of the elasticity of nematic LCs has been developed over several decades and is now essentially quite complete. The basic difference between deformations in a LC and in a solid is that in LCs there is no translational displacement of the molecules on distortion of a sample. This is due to the slippage between liquid layers. A purely shear deformation of a LC conserves elastic energy. The elasticity of an isotropic liquid is related to changes density. In LCs, variations in density can also be characterized by a suitable modulus, but the elasticity which is related to the local variation in the orientation of the director is their principal characteristic [21, 30] LC ANCHORING AND EASY AXIS The bulk properties of LCs depend strongly on the molecular structure and intermolecular interactions. When the nematic phase comes in contact with a solid surface, the orientation of n in its proximity is primarily determined by its interaction with and the structure of the substrate. Anchoring may be defined as the phenomenon of orientation of a LC by a surface. The anchoring of LCs has a great importance, from the fundamental point of view, for understanding the anchoring mechanism and for improving the performance of the LC devices. A surface normally imposes a preferred direction on the director, called the anchoring direction, or, simply, easy direction (figure 2.17). The easy axis is then the direction of spontaneous orientation of n at the surface, in the absence of any external field/force [3, 31-33]. Figure Schematic digram of easy axis with respect to director(n).

105 ANCHORING ENERGY The energy of the interfacial region, between the LC and the substrate, depends on the orientation of the director relative to the easy direction. Figure Polar anchoring and Azimuthal anchoring of LC molecule. The origin of the surface alignment can be discussed in terms of anisotropic torques acting on the director and arising from either physico-chemical or geometrical factors. These torques deform the nematic director configuration which costs (elastic) energy. The term anchoring energy was introduced to describe the contribution due to deviation of the director at the surface from its preferred orientation determined by elastic torques in a constrained LC. The anchoring energy can be physically interpreted as the work that must be done to rotate the director away from the easy direction. Orientational anchoring can be classified as homeotropic, tilted, or planar depending on whether the preferred direction is perpendicular, tilted, or parallel to the surface. Furthermore, the last two cases can be monostable, bistable, multistable, or degenerate depending on the nature of the surface. The azimuthal anchoring coefficient Wφ is related to director deviations in the LC-substrate plane and zenithal (or polar) anchoring coefficient W θ is related to the director deviations in the direction perpendicular to the LC substrate boundary (figure 2.18) [3, 21,34]. The structure of liquid crystal directors in close proximity to an interface is different from that in the bulk, and this surface structure changes boundary conditions and influences the behaviour of the LC director in the bulk. Therefore, the anchoring energy is an important parameter for a LC cell. It affects not only the LC alignment but also the electro-optic properties such as threshold voltage and response time.

106 68 The anchoring energy depends on cell thickness. A thinner cell exhibits a higher anchoring energy. In contrast to the strong anchoring energy, weak anchoring can results in: 1) Lower threshold, 2) Rise time decreases, and 3) Fall time increases. We find that the LC response time depends on several factors, including the LC layer thickness, viscosity, temperature and surface treatment, as well as the driving waveform. The effects of these factors on the response time are listed in Table 2.1. To achieve a fast response time, low rotational viscosity (γ1) LC mixtures are preferred [35, 36]. Another straightforward approach is to use a thin cell gap filled with a high birefringence (Δn) and low viscosity LC mixture [37, 38]. High birefringence also enhances the display brightness and contrast ratio of polymerdispersed liquid crystal (PDLC) [39, 40], holographic PDLC [41], cholesteric LCD [42, 43] and LC gels [44-45]. Recently, many manufacturers have reported the display devices with reduced cell gaps of below 4 μm in order to achieve fast response time ELASTIC DEFORMATION OF THE LC DIRECTOR The director of a LC cell is like a spring. It has a preferred alignment which will minimize its elastic energy. There are three kinds of elastic deformations: bend, splay and twist (figure 2.19). They affect important LC properties such as threshold voltage and response time. In the continuum theory, the Frank s free energy density for a deformed LC is given by F K 2 11.n K22 n. n K33 n n (2.10) 2 2 2

107 69 n = director K 11 = splay distortion elastic constant K 22 = twist distortion elastic constant K 33 = bend distortion elastic constant Besides the volume density terms, there also exist surface contributions (anchoring surface elastic terms). The constants K i have the nematic degree of order incorporated and are proportional to S 2. One can note that the free energy density is a quadratic form for three basic deformation modes and obeys the n -n inversion symmetry. Many materials have similar elastic constants. Therefore, a common approximation for free energy volume density is to use only one average elastic constant that sets K 1 = K 2 = K 3 = K. Note that this elastic energy is similar to a stretched spring [46-49]: F kx (2.11) Usually, K 33 > K 11 > K 22 and they are all ~ N. For MBBA, K 33 /K 11 = 1.3 and K 33 /K 22 = 2.9. Figure Diagram to illustrate three kinds of director distortions. If there is a natural twist (Chirality) for the LC, then the twist term is biased by a permanent twist q o, and Frank s free energy becomes F K 2 11.n K22 n. n q0 K33 n n (2.12) 2 2 2

108 70 F q K K K n K n. n K n n n. n q (2.13) Noting that n = (sin cos, sin sin, cos ),it is easy to show that F can be written as Where K 1 () = K 11 sin 2 + K 33 cos 2 K 2 () = (K 22 sin 2 + K 33 cos 2 ) sin 2 K 3 () = 2q o K 22 sin 2 (2.14) F depends on, which are functions of z and so called a functional. The director distribution is determined by a minimization of F by varying the functions (z) and (z). This is the calculus of variations EXTERNAL FIELD EFFECTS Liquid crystals respond to even weak electric and magnetic fields with significant structural changes, showing a redistribution of the molecular director. The ordered structures of anisotropic molecules make the macroscopic physical properties also anisotropic. Because of this anisotropy, the dielectric permittivity and the magnetic permeability depend on the direction in which they are measured, so the electric and magnetic fields are Where D B i i E (2.15) ij ij j j ij ijh j H 4 (2.16) 1 4 Because of the cylindrical symmetry around n, the tensors ε ij and χ ij can be written in the form ij ij nin j (2.17) ij nin j (2.18) Introducing this in equations 1 and 2, we have D E n.en (2.19)

109 71 where 1 4 H 4n.H n B (2.20) and and the magnetic anisotropy respectively. The electromagnetic energy density is are the dielectric anisotropy 1 W E.D H.B (2.21) 8 To obtain the total free energy density in presence of external fields, we have to add this term to the deformation free energy density F TOT F W (2.22) d Because one can express 1 dw E.D H.dB (2.23) 4 F TOT is, therefore, a function of the independent variable D and B, of the temperature T and the volume V : the equilibrium is reached minimizing F TOT with T, V, D and B constant. In order to work with E and H constant, we define the new potential G as the function 1 G F E.D H.B F W (2.24) 4 Which is a function of E, H, T and V. Analogously to the situation without fields, we find that the molecular director is everywhere parallel to a molecular field, given by the sum of a distortion term, an electric term and a magnetic one: h hd he hm hd n.ee n.h H (2.25) 4 We can define the electric torque and the magnetic torque, e n he n.e n E (2.26) 4 n.h nh nh (2.27) m At equilibrium d e m 0 m Nematic liquid crystals might show both positive and negative value of the dielectric anisotropy:

110 72 Figure Orientation of an electric dipole by an electric field. In (a) the dipole is along the long axis of the molecule whereas in (b) it lies across the long axis. The presence of the electric field causes rotation of the molecule. if Δε > 0, the molecular director tends to dispose parallel to electric field, Figure 2.20(a) If Δε < 0, the molecular director tends to dispose perpendicular to electric field, Figure 2.20(b). The magnetic anisotropy Δχ is almost always positive, and the molecular director tends to align parallel to the magnetic field EULER-LAGRANGE EQUATIONS The alignment of the LC cell in the presence of an electric field is obtained by minimizing the total F. (2.28) ALIGNMENT The term alignment, or texture, refers to the orientation of liquid crystal molecules in the vicinity of a surface. Liquid crystals are usually confined between closely spaced plates with an alignment layer that forces the direction of the molecules near the surface: Planar alignment, the director aligns parallel to the cell surface, see Figure 2.21(a). Homeotropic alignment, the molecular director is perpendicular to the cell surface, see Figure 2.21(b).

111 73 Figure LC alignments inside a cell: (a) planar alignment; (b) homeotropic alignment THRESHOLD VOLTAGE AND FREDERICKSZ TRANSITION Let us consider what happens when a small amount of nematic liquid crystal is placed between two pieces of glass that have been treated to produce alignment of the director parallel to the surface (Figure 2.22 a). Near the two glass surfaces, the director is constrained to point in certain direction (parallel to the surface). Supposing that the NLC has positive dielectric anisotropy, Δε > 0, and an electric or magnetic field is applied perpendicular to the glass surfaces: the field tends to orient the director parallel to the field. The molecules near the surface are not free to reorient with the field like the one in the bulk of the cell. The electric or magnetic field thus causes the director to change its orientation in the middle of the cell, with diminishing change closer to the surfaces. This deformed structure is shown in Figure 2.22 b. This deformation does not occur gradually as the strength of the field is increased. In fact, if the electric field is strong enough to overcome the elastic torque, the small fluctuations of the director can be amplified by the electric field, resulting in a reorientation of n along E. For fields with strengths below a certain value, the LC remains undeformed. Then, at some threshold value of the field, the deformation begins. The geometrical transition from an undeformed structure to a deformed one, in function of the electric field, is called Freédericksz transition.

112 74 Figure Freedericksz transition: cell has planar alignment. When the field is below the threshold level the liquid crystal orientation is given by the alignment (a); above the threshold the field tends to align the director perpendicular to the surfaces (b). The presence of a threshold value E 0 in this reorientation process is known as Freédericksz effect: for E < E 0 no reorientation is observed; for E > E 0 reorientation begins, starting from the center of the sample, where the restoring torque exerted by the elastic forces is weaker (Figure 2.22). The dependence of the angular orientation of the molecular director on the electric filed is shown in Figure Figure Freedericksz transition. The total free energy density F = F d + F E and F d is elastic free energy density and F E is electric free energy density. 2 d 2 v 2 F n v ki E (2.29) 4 v1 d 4

113 75 As d is the thickness of the cell, As the system moves towards a stable state, the free energy F describing the deformed state must be negative for some values δn v. The deformation mode that requires the weakest field to be induced is the one with v= 1; this mode can be excited by any electric field respecting the inequality E z ki d The minimum of such values is the sought threshold field (2.30) E d 4 (2.31) 0i ki with i = 1 if the alignment passes from the planar to the homeotropic, i = 2 if the alignment remains planar but orthogonal to the initial one, i = 3 for the passage from homeotropic to planar. As expected, the first deformation mode corresponds to a higher distortion in the center of the sample. For higher electric fields, higher modes are excited and the distortion moves closer to the boundaries. It can be demonstrated that when the strong anchoring assumption is no more valid (weak surface anchoring), reorientation occurs even at the boundaries, and the threshold field may decrease. It is interesting that the field is proportional to d -1, so if we consider that the voltage to apply is V = Ed, one has that the critical voltage is independent to the cell thickness d: V c 4 3 ki (2.32) a RESPONSE TIME Response time is one of the most critical issues for nearly all liquid crystal (LC) devices involving dynamic switching. Based on the small angle approximation, Jakeman and Raynes derived the LC director reorientation times [50].The response time of LC molecules depends on elastic constants of LC system. Another LC property that is closely related to the response time is the viscosity and given by Where is the dynamic viscosity, is the density and is the kinematic viscosity. LC density is typically gm/cm 3, same as water.

114 76 What is Response Time? The transition time when LC materials are rotating on each of the required white/black or gray levels is called "rise time" and "fall time," respectively (figure 2.24). Figure The molecular orientation in the On and Off state of a LC display panel. THEORY: When the backflow and inertial effects are ignored, the dynamics of the LC director reorientation is described by the following Erickson-Leslie equation [51, 52]: K 11 0 cos 2 E K 2 33 sin 2 sin cos 1 K 33, t K 11 sin cos z 2 (2.33) Where γ1 is the rotational viscosity, K 11 and K 33 represent the splay and bend elastic constants, respectively, is the electric field energy density, ε 0 Δε is the LC dielectric anisotropy, and is the tilt angle of the LC directors. In general, Eq. (2.33) can only be solved numerically. However, when the tilt angle is small (sin ~) and K 33 ~ K 11 (so called small angle approximation) [50], the Erickson-Leslie equation is reduced to: 2 2 K33 0E 2 1 z t (2.34) Under such circumstances, both rise time and decay time have simple analytical solutions. DECAY TIME: When the electric field is switched off, i.e., E=0, Eq. (2.34) is further simplified as: 2 2 z K33 1 t (2.35)

115 77 The solution of Eq. (2.35) can be expressed as With z t z (2.36), t m sin exp d t 0 d K33 (2.37) Where m is the maximum tilt angle of the LC directors in the response of the applied voltage, d is the LC cell gap; z is the position of the oriented LC layer under discussion, and τ o is the LC director reorientation time (1 1/e). It should be pointed out that in the Erickson-Leslie equation the strong surface anchoring and zero pretilt angle at the surface boundaries are assumed. Under such conditions, the Freedericksz transition threshold exists [55]: K33 V th (2.38) 0 The time-dependent phase change associated with this angle change is described as follows: d 2 n n e o t no dz (2.39) n cos n sin 0 o e Where n e and n o are the refractive indices for the extraordinary and ordinary rays, respectively. One we can easily solve the optical decay time T decay (90% 10%) as follows [50-52] 1 o sin 0.9 sin 2 o T decay t 2 t1 ln (2.40) 2 1 o sin 0.1sin 2 Equation (2.40) correlates the optical decay time to the LC director reorientation time (τ o ). Similarly, the optical decay time from 100% to 10% can be derived easily and result is shown below: o o / 2 T decay t1 ln (2.41) 2 1 o sin 0.1sin 2

116 78 Based on equations (2.40) and (2.41), the optical decay time of a VA cell is linearly proportional to the director decay time. The initial phase retardation (δ o ) also plays an important role, but not too substantially. RISE TIME: Rise time is much more complicated to deal with than relaxation. The original small angle approximation used by Jakeman and Raynes for rise time is over simplified [50]. They have assumed that the LC director s tilt angle increases exponentially with time. This approximation is valid only in a very short time regime. Blinov has considered the second order term and the fact that the LC directors will eventually reach an equilibrium stage. Thus, equation (2.34) is rewritten as Where z 2 t 2 3 (2.42) 1 2 (2.43) oe K (2.44) oe The director s rise time r corresponding to the biased voltage r o E 2 1 d 2 2 K V V th o 2 1 (2.45) The rise time has been solve by assume the transmittance rises from I 1 to I 2 as the time increases from t 1 to t 2. We obtain the corresponding transmittance at 10% and 90% as follows [50-52] 2 o / 2 I 1 0.1I o sin (2.46) 2 2t1 1 1 exp 2 o r

117 79 2 o / 2 I 2 0.9Io sin (2.47) 2 2t exp 2 o r By solving t 1 and t 2 from Eqs. (2.46) and (2.47), we derive the optical rise time T rise (10% 90%) as: o / sin 0.1sin 1 2 o T rise t 2 t1 ln 2 / 2 (2.48) 2 V o o Vth sin 0.9 sin 2 Equation (2.48) correlates the optical rise time (T rise ) to the commonly used director rise time (τ r ) as described in Eq. (2.45). Basically, it is a linear relationship except for the additional logarithm term of the phase dependence ERICKSON-LESLIE EQUATION In the dynamic Freedericksz transition of a homogeneous nematic cell, the azimuthal angle is constant so that the LC director can be solely described by the tilt angle θ (z, t). The motion of the LC director is coupled with the flow ν in the x direction. The evolution of θ and ν under an applied voltage is governed by the following Erickson-Leslie equation [51, 52] which takes into account the balance of elastic and electric-field-induced torques: 2 I 2 t E o 1 2 t K cos K sin K K 11 sin cos 2 sin z 2 v cos z sin cos z 2 (2.49) where α i are the Leslie viscosity coefficients, I is the inertia of the LC, θ is the polar angle of the LC director (the angle between the LC director and the x-y plane, as depicted in Figure 2.25), are the K ii Frank elastic constants, Δε is the dielectric anisotropy, and v is the flow velocity.

118 80 Figure The coordinate system of LC director, where θ is the tilt angle of the LC director, which is the angle between the LC director and the x-y plane, and is the angle between projection of the LC director on the x-y plane and the x-axis. In general, the inertial effect is much smaller than the elastic and viscous torques and can be neglected. Neglecting the inertial term, the flow velocity is governed by: cos 2 sin z z sin cos 2 2 sin cos 2 4 0(2.50) v z Solving v/ z from Eq. (2.50), and substituting this into Eq. (2.49), we obtain the effective rotational viscosity * 1 as [53]: * sin 2 cos 2 2 sin 2 sin cos cos 2 4 (2.51) Where γ 1 is the rotational viscosity and α i are the Leslie coefficients. In the low voltage regime (V < V th ) of a homogeneous (splay-mode) cell, γ * 1 can be approximated by, γ * s where [53]. * s * (2.52) At a higher offset voltage (V >> V th ), the LC directors are reoriented perpendicular to the substrates except the boundary layers. Thus, the effective rotational viscosity (γ * B) for the bend-mode (θ = 90 ) is [53]:

119 * * 2 B (2.53) It is evident from Eqs. (2.52) and (2.53) that in a splay or bend mode, the director reorientation depends largely on α 2 and α 4 + α 5 ; the α 3 values are one to two orders of magnitude smaller and are neglected. Similar effect is also observed in the twisted nematic cell [54]. For laser beam steering, we need a pure phase modulator with phase change δ 2π. The pure phase modulation of a TN cell has been demonstrated in the low voltage regime; below the optical Freederisckz transition threshold [55]. However, to archive the required 2 phase change, the TN cell gap would be too large so that the response time would be too slow. Both homogeneous and homeotropic cells can be used for pure phase modulation. However, from a molecular design standpoint it is easier to obtain LC compounds with a large and positive Δε. Thus, here we focus on the LC directors deformation of a homogeneous cell with strong anchoring at surface boundaries INTERACTION WITH LIGHT: PROPAGATION IN ANISOTROPIC MEDIA The optical properties of liquid crystals are one of the most interesting and certainly the most beautiful features of the phase. How LCs effect light is also the basis for just about all the applications of liquid crystals. Two aspects are extremely relevant: due to anisotropy, light propagates in different way with respect to the molecular director orientation; Light itself can reorient the liquid crystal, because light is an electro-magnetic field, and this reorientation of the LC modify the propagation of light giving arise to non-linear optical effect. This situation will not be discussed in this thesis PHASE TRANSITIONS Phase transitions are ubiquitous in nature. Examples include a magnets, liquid crystals, superconductors, crystals, amorphous equilibrium solids, and liquid condensation. These transitions occur between equilibrium states as functions of temperature, pressure, electric field etc.; and define the nature of the matter.

120 LANDAU-de GENNES THEORY OF PHASE TRANSITIONS This theory was developed by Landau in the 1940's, originally to describe superconductivity [7]. The procedure is general, and is one of the most useful tools in condensed matter physics. Not only we can use Landau theory to describe and understand the nature of phase transitions among ordered (and disordered) states, but also we can use it as a starting point for understanding the behaviour of ordered states. The strengths of this theory are its simplicity and its ability to capture the most important elements of the nematic-isotropic transition. The Landau theory is a phenomenological theory initially developed to describe phase transitions of the second kind. It is assumed that near a second order phase transition point the free energy density F can be expanded in powers of the order parameter (S) characterising the phase with the lower symmetry. In the absence of any external field the expansion is as follows: F A B C 0 (2.54) P,T,S F S S S... Where F 0 is the free energy density when S = 0. The dependence of the order parameter near the phase transition point is then determined by minimising above equation with respect to S. The term linear in S is absent to ensure the stability of the higher symmetry phase. It can be nonzero when an external symmetry breaking field is introduced. A 0 ensures that S = 0 corresponds to a minimum in F for the higher temperature phase and A 0 corresponds to that of S 0 for the lower temperature phase. Landau assumed that A=a(T-T*) where T* is the transition temperature. B and C are normally assumed not to change with temperature. For a system in which the free energy density is independent of the sign of S i.e. F(S)=F(-S) the cubic and higher odd powers of S are not allowed as for example, in a ferromagnetic system. In this case for B=0, and for C0 a second order phase transition takes place between the states S=0 and S0 at T=T*. Minimising above equation with respect to S the temperature dependence of the order parameter (figure 2.26) is found to be a T T S (2.54) C

121 83 Figure Dependency of order parameter on temperature (Landau de-gennes Theory). Landau assumed that A=a(T-T*) where T* is the transition temperature. B and C are normally assumed not to change with temperature. For a system in which the free energy density is independent of the sign of S i.e. F(S)=F(-S) the cubic and higher odd powers of S are not allowed as for example, in a ferromagnetic system. In this case for B=0, and for C0 a second order phase transition takes place between the states S=0 and S0 at T=T*. Minimising above equation with respect to S the temperature dependence of the order parameter (figure 2.26) is found to be a T T S (2.54) C The above argument has been extended to describe weakly first order phase transitions. One way of obtaining a first order transition is to have a third order term. If the symmetry of the system prevents the presence of a third order term (i.e. B = 0) then a first order transition can be obtained by having C 0. In that case a stabilising sixth order term with coefficient E 0 is required. The detailed description of isotropic - nematic and nematic - smectic phase transition is given in the reference [57-60] MAIER-SAUPE THEORY (MEAN FIELD APPROACH) This is a mean field theory, in which the energy of a molecule does not depend on its particular environment. Energy is a function of the orientation distribution of the molecules, described by the order parameter S. It is assumed that this can be described simply by the P 2 term of Legendre polynomial expansion, but in reality

122 84 there will be higher order moments of the distribution which are ignored in this approach. Energy of j th molecule u j 1 2 CS 3cos j 1 (2.55) 2 Where C is a constant, assumed independent of T. Thus the interaction of a given rod depends on its particular orientation j, and how it interacts with the average orientation expressed by S. This is a purely geometric argument, and for instance, ignores, any particular dipolar interactions. So u 1 2 CS 3cos 1 2 j (2.56) u j CS 2 Or U = -1/2 N A CS 2 per mole (The factor of 1/2 arises to avoid double counting.) It shows that the energy decreases as the alignment increases i.e. as S1. Thus alignment decreases the contribution of U to the internal energy. The entropy and internal energy can be given as follows [61, 62] 2 cs s 1 1 cs3cos j G N AkT log exp dcos (2.57) j 2kT 0 2kT Plots of how G/N A kt varies with S show the trend of phase equilibrium shifts with temperature. 2 cs 1 cs3cos j s N Ak log exp dcos j (2.58) 0 2kT 2kT At certain temperature there are two minima with the same energy: one for S = 0 (i.e isotropic) and one for finite S. This corresponds to the liquid crystal to isotropic phase transition temperature, T LCI, Occurs for C/kT = 4.55 and S = Hence from the theory as you warm an LC phase up there is a critical value of S at which system transforms (in a universal way) to an isotropic fluid (figure 2.27). As with all mean field theories, this only works up to a point. Liable to be wrong in the vicinity of the phase transition, where critical exponents in particular will be incorrectly predicted; theory is ignoring fluctuations which will be very important here. Also the theory does not consider the specific shape of the molecules involved,

123 85 e.g. axial ratio while this is particularly important for polymers. For this reason other approaches suit polymers better, when both length and stiffness can be properly accounted for. One of the most successful models is the Flory Lattice Model, which builds on the more general Onsager theory. This is specifically a steric theory and takes no account of specific intermolecular attractions, so has a different range of limitations [57-60]. Figure Dependency of order parameter on temperature (Maier-Saupe Theory). The Onsager theory is essentially the rod-equivalent of a hard sphere fluid: the rod-rod interaction energy is zero except when they overlap in space where it is infinite. This leads to a reduction in translational entropy, as there is less space for the rods to explore. This excluded volume decreases as the rods align, and this factor provides the driving force for the formation of the nematic phase [22-24, 61, 62] DIELECTRIC SPECTROSCOPY STUDY The dielectric spectrum becomes interesting in the vicinity of the absorption frequencies of the polarization mechanism [14, 21] present in the material under investigation. It covers a broad timescale of motions, from about 10 4 s to s. When a sample is brought into a static external field, all charged particles experience forces tending to move them along the field in the appropriate direction. This results in dielectric polarization of the sample on several length scales. I have therefore measured the dielectric permittivity while scanning the frequency of the measuring field. This section deals with the dielectric measurement technique used in the present work and also incorporates other important points regarding dielectric spectroscopy.

124 THE COMPLEX DIELECTRIC PERMITTIVITY DEBYE TYPE POLARIZATION MECHANISM The dielectric permittivity is a microscopic quantity [56], and this is what makes it so useful, it relates the known electric field which is different from local microscopic field. Different mechanisms which contribute to the measured dielectric permittivity have been presented in table 2.2. Mechanism Type Method for investigation Electronic polarization Induced dipoles UV/VIS absorption spectroscopy Non-collective orientational polarization Permanent aligned dipoles Dielectric spectroscopy, IR absorption spectroscopy Collective orientational polarization Permanently aligned dipoles reoriented Dielectric spectroscopy Table 2.2. Character of Debye type polarization mechanisms possible in liquid crystals. The induced polarization P after turning on a static electric field E across the dielectric medium, after a long time, final saturation value P f is given by 0 Pf 0 E (2.59) Where (0) represents static susceptibility at zero frequency. It is reasonable to assume that, before reaching the equilibrium value P will change at a rate which is proportional to its deviation [63] therefore we write t 1 P P P P f f (2.60) On integration and simple calculation gives P f P t / P f e (2.61) which can also be written as

125 87 t / P P (1 e ) (2.62) f In this expression characteristic time constant is called the relaxation time [63]. After some time, when P has reached the value P f we would turn off electric field and we found that polarization decays to zero according to relation P / P e t (2.63) f If we now apply an AC field, E=E 0 e it relaxation time then can be written as to a dielectric material having P (2.64) it 0E0e In writing above equation it is assumed that induced polarization varies with the same frequency as the applied field, phase difference between E and P will be hidden in (), is complex in general. From equation (2.60) to (2.64) and simple mathematics gives ip i E 0 E E 0 0 (2.65) 0 Or 0 1 j (2.66) In terms of real and imaginary part 0 and (2.67) 1 The results was first published by Peter Debye in 1927 [64] and mechanism contributing to the polarization in dielectric material with equation (2.66) and (2.67), is therefore called Debye type mechanism [64]. In terms of permittivity, the Debye equations [64] thus take the form and (2.68) Graphs of equations (2.68) are shown in figure 2.28.

126 88 This is the official form of Debye equations which are mostly used in the field of liquid crystals. In this form static susceptibility is replaced by 0 and it is therefore written as and referred as relaxation strength. But in practice we do not deal with angular frequency and relaxation time, but with the frequency f an 1 relaxation frequency f R, related to relaxation time through the frequency domain equations are 1 2 f R. Using this f 2 and f 2 f 1 f R f (2.69) f R f 1 f R In our experiments we apply a weak AC measuring field over the sample to obtain its conductivity and capacitance, and from these the real and imaginary part of complex dielectric permittivity can be calculated. The behaviour of complex dielectric permittivity as a function of frequency is shown in figure Figure The real (continuous curve) and imaginary (dashed curve) part of Debye equation as a function of angular frequency.

127 89 Figure The behaviour of real (upper curve) and imaginary (lower curve) part of Debye equation as a function of frequency for the two cases of modes. The Debye analysis holds well only for some orientaional polarization and not all for electronic polarization NON-DEBYE TYPE POLARIZATION MECHANISM When applying the Debye results to actual experimental data, one soon discovers that it seldom works very well. For ferroelectric liquid crystal systems and in many other cases also in the low molar mass liquid crystals, a mode turns out to be characterized by distribution of relaxation times; therefore it is impossible to obtain good fit of equation (2.68) to the experimental data. In order to cope with this problem, the previous analysis has to be modified. Such types of modification was first introduced by K.S. Cole and R. H. Cole [65, 66], and their expression bears the name Cole -Cole equation in the frequency domain only * where 0 1 (2.70) f 1 j f R 1 For =0 the Cole-Cole equation reduces into Debye equation, considering that =1 corresponds to f/f R =1. The Cole-Cole function describes a symmetric

128 90 broadening of the dielectric function compared with equation of Debye. Separating into real and imaginary parts gives the following expression 1 f 1 Sin f R 2 f (2.71) 1 21 f f 1 2 Sin f R 2 f R 1 f Cos f R 2 f (2.72) 1 21 f f 1 2 Sin f R 2 f R For non zero values of called distribution parameter we get a distribution of relaxation times, which is in the dispersion plot ( vs frequency) gives less distinct steps, while in absorption plot ( vs frequency) results in broader and flatter curve as shown in figure Figure The effect of the distribution parameter in Cole-Cole equation is too broken and flattens the imaginary part of complex dielectric permittivity. Sometimes even the Cole-Cole equation will not be sufficient to model experimental data, and therefore this equation has been extended in a more or less

129 91 adhoc way. Davidson-Cole function [67] corresponds to an asymmetrical broadening of the dielectric function * Where 0 (2.73) f 1 j f R 1 In the Havrialik Negami equation [68] in which an additional parameter is added in order to account for asymmetries in the absorption curve. * Where 0 and 0 (2.74) f 1 j f R The shape parameters and describe the symmetric and asymmetric broadening of the relaxation peak, respectively. In the case of = 1, the Havriliak- Negami function [68] coincides with the Cole-Cole equation. When = 1, it transforms in the Davidson- Cole equation [67]. In general, the maximum value of the asymmetric Havriliak-Negami [68] and Davidson-Cole functions does not coincide with the relaxation time. However this equation is rarely used in the field of liquid crystal research, simply because the Cole-Cole equation normally does the job THE COLE -COLE PLOT If we eliminate the frequency in the set of equation (2.69) we have following expression [69] 2 2 (2.75) 2 2 The above equation represents a circle shown in figure 2.31 (a) centered around the point (,0) and a Debye type process should therefore produce a 2 semi circle due to no negative value of. If is plotted against, called Cole-Cole [64] plot after the researchers who introduced it in Debye cases, one again obtains the equation of circle, but with some important difference in relation to Debye case. 2

130 tan (2.76) cos 2 2 The radius of circle has now grown by a factor 1/cos/2 and centre has moved to tan ()/2 below the axis. If we can locate the centre of the circle, or measure the radius of it, we will get a value of the distribution parameter. Figure The Cole-Cole plot for the case of (a) one Debye type process (b) one distributed or Cole-Cole process PARAMETERS OBTAINED FROM FITTING OF COLE- COLE EQUATION One of the most informative ways of treating the experimental data is to fit Cole-Cole equation to the data [66]. At the end of fitting procedure one has hopefully obtained exact values of relaxation frequencies, relaxation strength and distribution parameter of the modes. It should be pointed out that obtaining a good fit can often be quite difficult and very time consuming. Fitting the equation to a data from one single phase is usually quite easy and can be automated to a high degree, but on passing phase transitions, one usually has to fit each set of data more or less manually. It is thus quite an effort to produce good fits to a large number of measurement series. In some cases, especially where distorted modes, induced by surface or field effect, show up, it may be impossible to get a realistic fit.

131 DIELECTRIC RELAXATION NON COLLECTIVE (MOLECULAR) RELAXATION PROCESS In case of liquid crystals, the non collective process is related to non-correlated motion of molecules [70] i.e. reorientations around the short and long molecular axes. The direction of the molecular dipole moment depends on chemical structure of the compound and it may have components both parallel and perpendicular to long axis of the molecule as shown in figure Figure The molecular dipole moment of liquid crystals may have an arbitrary direction in relation to the molecule and therefore non zero components in parallel and perpendicular to the molecular long axis. In a dielectric spectroscopy we apply a measuring field perpendicular to plane of cell i.e. along or perpendicular to director depending on if cell is homeotropic or planar, so only the dipole moment in this direction will interact with the field and give a contribution to measured dielectric permittivity. Since orientaional order is never perfect, both the longitudinal and transverse dipole moments will in principle have a projection both along and perpendicular to the director as shown in figure but one component usually dominates heavily [70]. A characteristic of non collective process in liquid crystal is that relaxation frequencies follow Arrhenius dependence on temperature [9] i.e. f Exp ( U/k T), where f R is relaxation frequency, k B is R B

132 94 Boltzmann constant and U is activation energy for the processes. The typical characteristics of non collective processes are summarized in table 2.3. Mechanism Alignment Relaxatio n frequency Susceptibilit y Distributio n parameter Temperatur e Vs Relaxation frequency Reorientatio n around short axis Planar GHz ~1 >0 Arrhenius Reorientatio n around short axis Homeotropi c khz-mhz ~1 0 Arrhenius Table 2.3 Characteristics of non collective liquid crystal modes COLLECTIVE RELAXATION PROCESS (A) TILT ANGLE FLUCTUATIONS OR SOFT MODE It is a general property of second order phase transitions that the magnitude of the order parameter fluctuation will diverge, the temperature dependence being characterized by a critical exponent on approaching the transition temperature from either side [71]. In case of the order parameter couples, direct or indirect, with an external field, the permittivity or susceptibility describing this interaction will also diverge. In achiral SmA or SmC system the order parameter is the tilt angle which can not be influenced by an external field. Hence we will not be able to observe the diverging behaviour of by means of dielectric measurement while in chiral version of this system there is a secondary order parameter, the polarization [4]. This interacts with electric field and we obtained diverging like behaviour at phase transition temperature. The tilt angle and polarization are related by following relation P=s [64], where s is called structure coefficient. Any fluctuation in tilt angle or polarization is linked to a fluctuation in polarization or tilt angle in well aligned sample. At the SmA/C transition as well as SmC*/A* transition, the elastic constant which constitutes the restoring force against tilt fluctuations, weakens or softens, and therefore these tilt fluctuations constitute a soft mode which grows in

133 95 strength as the transition is approached. In chiral system, where this mode can be observed via connected polarization fluctuation, the dielectric mode is referred to as the soft mode. As polarization is perpendicular to the director, the soft mode is observed in planar aligned samples. Figure Schematic representation of the director fluctuation in smectic A phase and its temperature dependence [38]. Figure 2.33 shows temperature dependence of the director fluctuation in smectic A phase. The soft mode is an amplitudnal mode since it is connected to fluctuation in the tilt angle. The relaxation times tends to infinity at the transition temperature, given by [4] T T C (2.77) Here is soft mode rotational viscosity. Experimentally, however, such a complete divergence is not observed. Many researchers show [72-75] that several terms must be added in above equation, the most important of which is a quadratic term in helicoidal wave vector q=2/p, where p is pitch directly below SmA* to SmC* transition. This explains why soft mode relaxation frequency in short pitch materials typically does not decrease below 1 khz.

134 96 (B) PHASE ANGLE FLUCTUATIONS OR GOLDSTONE MODE The phenomenon of the Goldstone mode was originated from the field of elementary particles in 1961 [76]. In condense matter physics Goldstone modes refer to a certain excitations above the lowest energy state of the system. Broken symmetry, Goldstone mode and Higgs mechanism are examples of concepts which have been taken over from condensed matter to field theory [77]. Broken symmetry means that lowest stable state does not contain the full symmetry of free energy. In liquid crystals we have a great number of cases of broken continuous symmetry. The most obvious one may be the isotropic to nematic phase transition, where continuous spherical symmetry of isotropic phase is broken and replaced by cylindrical symmetry of nematic phase. A compound with a SmA to SmC transition will have another kind of Goldstone mode, this time related to the appearance of the tilt. In a SmA phase the director is parallel to the layer normal, the tilt is zero and the azimuthal angle describing rotations around the director is degenerate both in energy function and physical ground state. At the tilt transition SmA* to Smc* in a corresponding chiral system a local non zero polarization P appears which is everywhere perpendicular, thus sterically bound to the director n. Therefore phase fluctuation also means polarization fluctuations. In the chiral SmC* phase, which in contrast to achiral one thus is of large interest for study by means of dielectric spectroscopy, the C director rotates in a helical fashion when moving along the layer normal i.e. phase angle changes with a constant amount from layer to layer. This spatial modulation has important applications in SmC* phase. The Goldstone mode i.e. slow and continuous change of phase angle, must in chiral SmC* phase correspond to rotation of the whole helix [78] as illustrated in figure Note that this is a rotation without distortion, otherwise the mode would not correspond to fluctuation between energetically equivalent ground state, which is a necessity for the Goldstone mode. As can easily be seen in figure, an arbitrary rotation of the helix is actually equivalent to translational alignment along the helix axis. Thus the Goldstone mode may also be regarded as a translational fluctuation of helical structure, mediated through the phase angle fluctuations i.e. molecules do not move along helix axis. If an external electric field is applied over the sample, distorting the helix but not unwinding it, the equivalence between rotation and translational character is absent, and only the translational Goldstone mode persists [78, 79, 80].

135 97 Figure The Goldstone mode in the helical SmC* system corresponds to a rotation an angle (denoted in figure) of the whole helix around its axis. The dielectric dispersion in SmC* phase due to the Goldstone mode appears in the frequency range of 1Hz to 5 khz and do not depend very strongly on temperature except near the SmC* to SmA* phase transition temperature. In 1990 Carlsson et. al. [79] discussed broadband dielectric spectroscopic of FLCs and suggested that the relaxation strength and relaxation frequency of the Goldstone mode in SmC* phase of liquid crystal can be written as- Δε G 1 2ε K S (2.78) 0 f 3 2 P q Sinθ K q 2 3 G 2 G (2.79) Here f G and G represent relaxation frequency and relaxation strength of Goldstone mode respectively. K 3 is twist elastic constant, q is the wave vector of the helix, G is the rotational viscosity and 0 is the permittivity of free space. Figure 2.35 represents the broad overview of relaxation processes of liquid crystals.

136 98 Figure Schematic presentation of overview of the dielectric spectrum of the Chiral Smectic C (SmC*) phase [40]. (C) INFLUENCE OF DC BIAS ELECTRIC FIELD ON THE HELIX DISTORTION MODE A state where helix is totally unwound may be of large interest and we therefore sometimes perform dielectric spectroscopy, where a constant DC bias is applied. If one for instance wants to study the soft mode behaviour within SmC* phase, this is a necessary as the soft mode otherwise is completely covered by the dominating phase angle mode. At intermediate DC field strength i.e. below the threshold for complete helix binding, we have a contribution due to the helix distortion mode but permittivity quickly decreases and relaxation frequency increases. This may be understood [79] by considering that a partially unwound structure consists of a number of layers with uniform director orientations, separated by walls in which the phase angle rotates shown in figure When field strength is raised, the walls turn thinner and thinner, resulting in a tighter and tighter twist with a corresponding increase in the effective elastic constant. Thus relaxation frequency increases, at the same time the region of the field locked phase angle grow, while regions where fluctuations are possible diminish resulting decrease in permittivity.

137 99 Figure On applying an electric field perpendicular to the SmC* helix i.e. along the smectic layers the helix will distort into periodic structure with discrete translational symmetry GUEST HOST INTERACTION Recently the study of composites based on anisotropic media has increasing attention, of them LCs have much focused. Guest Host displays were first reported by Heilimier and Zannoni in 1968 and later on in more detail by Heilimier, Castellano and Zannoni in 1969 [61]. Such type of displays has operated at high temperatures due to non-availability of room temperature liquid crystal at that time, yet they demonstrated the feasibility and potentiality of Guest Host dichroic displays. Dichroic LCDs have also many advantages over twisted nematic (TN) and Nematic LCDs such as fewer polarizers, wider viewing angle and more tolerance for alignment for cell spacing, easier construction and more brightness too. By the turn of the century liquid crystal displays will be the mode of the choice for most major applications, and will fuel a revolution based on powerful portable information processing, with Active Matrix Twisted Nematic and Ferroelectric Liquid Crystal (FLC) technologies leading the way. Ferroelectric liquid crystal displays have some positive advantages over these displays, because of their fast electro-optical response. But most of these displays are black and white in nature and suffer from low contrast ratio and small viewing angle.therefore, the physics of confined liquid crystals is an important subject from the technological point of view. Now days the range of Guest Host interaction has been extended very much. It covers the liquid crystals and dye composites, liquid crystals and polymer composites

138 100 and the latest addition is liquid crystals and nano-particle composites. The nature of confining on most materials (e.g. solids and liquids) has little influence on the internal properties, and for the most part, can be safely ignored even for small-scale samples. Liquid crystalline materials, on the other hand, are an exception to this rule. The confinement of liquid crystals can modify the internal properties up to a large extent. In the following sections we will discuss about the different Guest Host systems used to confine liquid crystals and their consequences on internal properties of liquid crystalline geometries [22-27] LIQUID CRYSTAL AND NANO COMPOSITES The addition of nano particles to organic substances makes it possible to fundamentally expand the range of mechanical, dielectric, magnetic, and optical characteristics of these material. As for back as 1970s, de-gennes et al proposed to add Ferromagnetic particles to LC to enhance its magnetic susceptibility. Later LC suspension of magnetic, dielectric, metal and ferroelectric nanoparticles has been studied. Small amounts of these particles substantially modify their Viscoelastic, dielectric,optical,electro-optical properties. Nematic LC turned out to be the ideal matrix for the preparation of highly dispersed orientative-ordered ensemble of nanoparticles with possible control of their orientation using external fields. Nanoparticle research is currently becomes an area of intense scientific research, due to its wide variety of potential applications in biomedical, optical, and electronic fields. LC and nano-particle composites are the latest addition in the Guest Host interaction phenomenon and fascinating researchers over the last decade. Liquid crystals when doped with inorganic nanoparticles may significantly affect the threshold voltage of Liquid crystal display [12, 82]. These enhancements in different electro-optical and dielectric properties of pure liquid crystals due to the presence of nano particles inspires scientist to explore the liquid crystal nano particle composites and deploy them for use in device form [82] NANOPARTICLES In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. It is further classified according to size: In terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles, on the other hand, are sized between 1 and 100

139 101 nanometers. Similarly to ultrafine particles, nanoparticles are sized between 1 and 100 nanometers, though the size limitation can be restricted to two dimensions. Nanoparticles may or may not exhibit size-related properties that differ significantly from those observed in fine particles or bulk materials. Nanoclusters have at least one dimension between 1 and 10 nanometers and a narrow size distribution. Nanopowders are agglomerates of ultrafine particles, nanoparticles or nanoclusters. Nanometer sized single crystals or single-domain ultrafine particles are often referred to as nanocrystals HISTORY Although nanoparticles are considered as an invention of modern science, they actually have a very long history. Specifically, nanoparticles were used by artisans as far back as the 9th century in Mesopotamia for generating a glittering effect on the surface of pots. Michael Faraday provided the first description, in scientific terms, of the optical properties of nanometer-scale metals in his classic 1857 paper "Experimental relations of gold (and other metals) to light." PROPERTIES Nanoparticles are of great scientific interest as they are a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and super paramagnetism in magnetic materials [22-24]. The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometre the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. The interesting and sometimes unexpected properties of nanoparticles are partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties. Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid (figure 2.37).

140 102 Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution. Figure Non synthetic method to make the nano LC suspension. Nanoparticles have a very high surface area to volume ratio. This provides a tremendous driving force for diffusion, especially at elevated temperatures. Sintering can take place at lower temperatures, over shorter time scales than for larger particles. This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate, complicates matters. The large surface area to volume ratio also reduces the incipient melting temperature of nanoparticles. Moreover nanoparticles have been found to impart some extra properties to various day to day products. Like the presence of titanium dioxide nanoparticles impart what we call as the self-cleaning effect, and the size being nanorange, the particles can't be seen. Nano Zinc Oxide particles have been found to have superior UV blocking properties compared to its bulk substitute. This is one of the reasons why it is often used in the sunscreen lotions. Clay nanoparticles when incorporated into polymer matrices increase reinforcement, leading to stronger plastics, verified by a higher glass transition temperature and other mechanical property tests [22-28] CLASSIFICATION At the small end of the size range, nanoparticles are often referred to as clusters. Nanospheres, nanorods, nanofibers, and nanocups are a few shapes that have been grown. Metal, dielectric, and semiconductor nanoparticles have been formed, as well as hybrid structures (e.g., core-shell nanoparticles). Nanoparticles made of semiconducting material may also be labeled quantum dots if they are small enough

141 103 (typically sub 10 nm) that quantization of electronic energy levels occurs. Such nanoscale particles are used in biomedical applications as drug carriers or imaging agents. Semi-solid and soft nanoparticles have been manufactured. A prototype nanoparticle of semi-solid nature is the liposome. Various types of liposome nanoparticles are currently used clinically as delivery systems for anticancer drugs and vaccines. The study of fine particles is Micromeritics. Ultrafine particles are also called colloids if they are suspended in mixture CHARACTERIZATION Nanoparticle characterization is necessary to establish understanding and control of nanoparticle synthesis and applications. Characterization is done by using a variety of different techniques, mainly drawn from materials science. Common techniques are electron microscopy [TEM, SEM], atomic force microscopy [AFM], dynamic light scattering [DLS], x-ray photoelectron spectroscopy [XPS], powder x- ray diffractometry [XRD], Fourier transform infrared spectroscopy [FTIR], Matrix- Assisted Laser-Desorption Time-of-flight mass spectrometry [MALDI-TOF], and Ultraviolet-visible spectroscopy [82] NANOPARTICLE MORPHOLOGY (a) (b) Figure (a)nano-stars of vanadium oxide (iv) and (b)nanorods of Zinc Oxide. Scientists have taken to naming their particles after the real world shapes that they might represent. Nanospheres, nanoreefs, nanoboxes and more have appeared (figure 2.38) in the literature. These morphologies sometimes arise spontaneously as an effect of a templating or directing agent present in the synthesis such as miscellar emulsions or anodized alumina pores, or from the innate crystallographic growth patterns of the materials themselves. Some of these morphologies may serve a

142 104 purpose, such as long carbon nanotubes being used to bridge an electrical junction, or just a scientific curiosity like the stars [26, 82] LIQUID CRYSTAL AND DYE COMPOSITES Dyes are those chemicals which show colour effect and absorbs certain wavelength strongly. Investigation of how the effect depends on the molecular structures of the Guest and Host showed that it is sensitive to the structural change of the substituent groups in the dye molecule, pointing to the importance of dipolar interactions between Guest and Host molecules. The dyes that cause the effect are known not to undergo photo induced conformational changes, as in the case with the azo dyes. A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. Both dyes and pigments appear to be colored because they absorb some wavelengths of light while reflecting others. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment. The dyes are obtained either by animal, vegetable or mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood but only a few have ever been used on a commercial scale. In a mixture of LC and dichroic dye the colletive orientation of the LC molecules under the action of an electric field is influenced by the presence of dye molecules. The presence of dichroic dye in an LC affects some of the properties of the host i.e. pre LC [9, 81] LIQUID CRYSTAL AND POLYMER COMPOSITES The dispersion of the LC in a polymer leads to a large surface to volume ratio, the electro-optical properties of the Polymer Dispersed LC (PDLC) films are essentially determined by interaction between the LC and the polymer. On the other hand, the PDLC composites are heterogeneous, and the interfacial effects are also of great significance. Due to the structural peculiarities of PDLC films one can expect an essential difference between the physical properties of these materials and those of pure LCs. Thermotropic liquid crystalline phases are also exhibited by some polymers. The basic monomer units are lower molecular weight mesogens with rod-

143 105 like or disc-like molecules, which are attached to the polymer backbone in the main chain [75], or as side groups. The nature of the mesophase depends rather sensitively on the backbone, the mesogenic unit and the spacers. With rod-shaped repeating units in the polymers, mesophases similar to the nematic, cholesteric, smectic types of calamitic liquid crystals are observed, whereas, with disc-shaped repeating units some new kinds of mesophase structures have been found like sanidic (or boardlike) nematic and columnar nematic phases. The introduction of polymer in a pure LC matrix not only increases the inherent mechanical strength of the material but also changes the phase behaviour along with its dielectric and electro-optical properties [83-84].

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148 Chapter 3 Experimental Techniques

149 Chapter 3 EXPERIMENTAL TECHNIQUES 3.1. Introduction 3.2. Sample holder fabrication techniques Electrode preparation Substrate cleaning Photolithography Photoresist application Soft baking Mask alignment and exposure Development and hard baking Etching Surface alignment Homogenous (planar) alignment Homeotropic alignment Hybrid alignment Tilted orientation Assembling, sealing and filling the cell 3.3. Instruments and experimental techniques used to characterize the liquid crystal materials Instruments used to characterize LC materials Experimental identification of liquid crystals 3.4. Spontaneous polarization 3.5. Optical transmittance 3.6. Response time measurement 3.7. Threshold voltage 3.8. Splay elastic constant and rotational viscosity 3.9. Temperature controller Dielectric spectroscopy study The stray capacitance problem Measurement How to cope with unwanted absorption Low frequency ionic contribution High frequency cell relaxation problem Data presentation References

150 INTRODUCTION The liquid crystal (LC) materials have proved themselves the most promising candidates in the field of highly growing electro-optical display devices as well as various non-display devices having tremendous impact on the scientific society and human beings. The liquid crystal has very interesting physical and optical properties. Therefore, a systematic study is very important to know the physical and optical chemical, electro-optical and dielectric properties of these materials. The research is continuing since long back in order to understand the unique and mysterious phase of matter, since their discovery in 1888 [1]. This chapter describes all the techniques, starting from the fabrication of the sample cell to the techniques which were used to determine the materials parameters, dielectric and electro-optic response of LC SAMPLE HOLDER FABRICATION TECHNIQUES The sample cells used for dielectric, electro-optical and optical measurement are fabricated in the laboratory ELECTRODE PREPARATION The highly conducting glass plates are seeing used for electrode to be used in all type of liquid crystal measurement. The coating on the glass substrates should have the following properties: 1. Optically transparent and Electrically conducting 2. High square resistance homogeneity 3. Uniform transmission homogeneity 4. Low roughness The transparent ITO coated glass substrates have been used for preparing cells in the present study. It has low resistance and got finely patterned displays and can be made very thin, and then the etched topography remains fairly smooth. Indium Tin Oxide (ITO) is a mixture of Indium (III) Oxide (In 2 O 3 ) and Tin (IV) Oxides (SnO 2 ), typically 90% In 2 O 3 and 10% SnO 2 by weight. The sheet resistance should have order of 10Ω/mm 2 and the visible transmission must be greater than 90% for best electric characteristics of the LC

151 111 sample. However there are few treatments which must be employed before using them as electrode, they are discussed as follows: SUBSTRATE CLEANING Glass cleaning is one of the most important steps in liquid crystal cell fabrication. Substrates are cleaned before processing in many subsequent steps as improper cleaning can result in electrical shorts, cell gap variations and poor alignment of the liquid crystal. It is therefore imperative to have an efficient cleaning process to remove all contamination from the glass surface. Figure 3.1. ITO coated glass plates. The ITO coated glass substrate has been washed with soap solution, to remove any traces for organic impurities. Then it is clean with acetone for traces of dust particles. The ITO coated glass plates used in present study has been shown in figure PHOTOLITHOGRAPHY After the glass plates have been cleaned, the photoresist is applied to the conducting surface of the ITO coated glass plate through spin coating technique. Photolithography is a process to transfer any particular pattern on the ITO coated glass plates. The photolithography comprises of many steps as follows and must be carried out in the dark room [2] PHOTORESIST APPLICATION After cleaning, photoresist is applied to the surface of substrate using spin coating technique. The substrate is held flat by a chuck, the solution is poured onto the substrate, and the substrate is then spun at high speed for seconds to obtain

152 112 a uniform film. The spin coating machine used in present study is shown in figure 3.2. The main drawback to spin coating is the amount of material required for a single substrate i.e ml can be for single substrate. As the substrate spins, the solution spreads to cover the entire substrate, and is pushed outward by the spinning. Ideally, the atmosphere above the substrate will saturate with solvent, preventing the film from drying before spinning is complete. When the spinning stops, the film relaxes back into a uniform thickness before being dried on a hot plate. A uniform coating of photoresist of desired thickness is obtained by varying the spinning rate. There are two types of photoresist: Positive and Negative. For positive photoresist, it is exposed with ultra violet light whenever the underlying material is to be removed. In these photoresists, exposure to ultraviolet light changes the chemical structure of the photoresist so that it becomes soluble in developer. The exposed photoresist is then washed away by the developer solution [2], leaving windows of the bare underlying material. The chemical structure and basic properties for both type of photo resists as shown in figure 3.3 and table. Figure.3.2. Spin coating mechanism. Figure.3.3. Chemical structure and properties of photoresists.

153 113 Negative photoresist behave in just opposite manner. Exposure to the ultra violet light causes the negative photoresist to become polymerized (hard) and more difficult to dissolve. Therefore, the negative photoresist remains on the surface whenever it is exposed and the developer solution removes only unexposed portions. Mask used for negative photoresist should therefore, contain the inverse (or photographic negative ) of the pattern to be transferred. The negative photoresist (chlorobezene, 2-ethoxyethanol cyclohexanane and polyvinyl cinnamate) has been used for making sample holder in the present study. The pattern used for the exposure of negative photoresist to UV light is shown in figure 3.4. The black portion shows the open part form where the negative photoresist is exposed to UV light SOFT BAKING After spin coating, the photoresist (positive or negative) is heated to a temperature of 90 0 C. This process of preheating the photoresist coated substrate before mask alignment is called soft baking. Soft baking prevents the light from reaching the sensitizer. Thus the time duration for soft baking is of utmost importance and in our case it has been optimized to 10 minutes MASK ALIGNMENT AND EXPOSURE After soft baking, the substrate is set against the photomask and exposed through the pattern on the mask with a high intensity ultra violet light. The ITO electrode pattern style and way of exposing the UV light is shown in the figure 3.4 and figure 3.5 respectively. The distance of the ultra violet light from the substrate and its duration of exposure have to be optimized. In our case it was 15 (approx.) and the exposure time was 45 seconds.

154 114 Figure 3.4. ITO electrode pattern. Figure 3.5. UV illumination for ITO etching DEVELOPMENT AND HARD BAKING The exposed substrate is now developed using developer of specific photoresist (negative here). It is then fixed using benzene. Benzene is used as a fixer to stop the reaction of the developer with the photoresist. The pattern obtained is now baked at C. This process of heating the developed pattern is called hard baking. This step is necessary in order to harden the photoresist and improve adhesion of the photoresist to the surface ETCHING Etching is a process which is used to obtain the conducting ITO pattern underneath the photoresist. Two types of etching process are well known: dry etching and wet etching. Dry etching dissolves the ITO using reactive ions or vapour phase etchant, whereas in wet etching, the ITO coating which is not covered with photoresist is dissolved when immersed in chemical solution. In our case wet etching method is employed. The choice for etchant depends on the material to be deposited on to the substrate. For indium tin oxide, the etchant can be HCl (15%) + Zn dust or HCl +HNO 3 [3]. Zinc dust is sprinkled uniformly into the dilute HCl (15%). After a few seconds, the masked glass substrates are immersed in HCl (15%) +Zn dust mixture. It is the action of nascent hydrogen produced during the reaction between the acid and zinc dust that reduces the oxides coating on the glass substrate. Zn + 2HCl ZnCl 2 + 2H + SnO 2 + 4H + Sn +4 +2H 2 O

155 115 In 2 O 3 +6H + 2In +3H 2 O Etching is stopped by dipping the glass substrate into the water. To insure that the extra ITO has been removed or not, the continuity of the substrate is checked using a multimeter. The substrates are now cleaned with soap solution and then with acetone to remove dust and organic impurities from the surface. Finally the desired conducting pattern is obtained on the glass substrate. Now the substrates are ready for the alignment and further procedures required for the fabrication of the sample cell SURFACE ALIGNMENT In the liquid crystal devices, one of the most important problems is the surface alignment of the liquid crystal molecules. There are different types of alignments for the characterization of liquid crystals (LCs), e.g. planar, homeotropic, hybrid aligned nematic (HAN), tilted etc (figure 3.6). However planar and homeotropic alignments are popular for the characterization of liquid crystals, pretilt is important for obtaining domain-free orientation under electric field. Various methods for obtaining uniform alignment have been proposed by many researchers [4-10]. The molecular alignment of liquid crystal is influenced by the forces acting between the LC molecules as well as between the LC molecules and the surface of substrate. Different alignments are used according to the requirement in display devices HOMOGENOUS (PLANAR) ALIGNMENT In the homogenous alignment (figure), the molecules are aligned parallel to the substrates. To obtain homogenous alignment, several methods such as rubbed polyimide (11,12], evaporation of silicon monoxide (SiO) and other similar dielectrics[13], Langmuir Blodgett films, magnetic field, photo alignment[14], etc have been utilized. Out of these methods, the rubbing of polyamide coating on the substrates has been extensively exploited due to its easiness, uniform alignment over large area, etc. In order to obtain homogenous alignment, unidirectional rubbing is necessary. Rubbing of substrates may create localized surface defects and static charge which may further degrade the quality of alignment. Therefore, order expensive methods such as evaporation of SiO, magnetic field, and photo alignment may be used to avoid any such problems. Oblique evaporation of SiO was first reported by Janning [13]. The tilt angle achieved by this method is controlled by the evaporation angle. In obliquely evaporated SiO film, the relationship between the

156 116 oblique angle of evaporation and the pretilt angle of LC orientation has been reported by Yamashita et al. [15] and Scheffer et al. [16]. Therefore, Goodman et al. proposed a columnar model based on the observation by transmission electron microscope (TEM) [17]. The resulting tilt angles either identically zero (for incident angle 60 0 C) or quite high (for incident angle 85 0 C). Neither case was especially useful for twisted nematic display: the high lilt case leads to reverse lilt. Johnson et al. obtained low lilt alignment by combining two evaporated layers of SiO at different angles of incidence [18]. In this thesis we have followed the rubbed polyimide technique in our experiments to achieve homogenous alignment and in this technique, Nylon 6/6 (polyimide) has been used as a polymer. The etched ITO plates have initially been dipped in an adhesion promoter such as silane solution for 10 minutes and then washed with propanol. After that the Nylon 6/6 (60% m-cresol and 40% methonal) has been spin coated on the substrates at a rotational speed of 50 rpm. The excess solvent present in our solution of Nylon 6/6 has been evaporated by keeping the coated substrates at C for one hour. After one hour both the glass plates have been rubbed unidirectionally with a good quality velvet cloth or cotton [2, 19]. Other different type of orientation are shown in the figure 3.6 Figure 3.6. Different types of nematic liquid crystal alignments [7] HOMEOTROPIC ALIGNMENT In homeotropic alignment, the molecules are aligned perpendicular to the substrates (figure above). For homeotropic alignment or vertical alignment in LC

157 117 materials, silanes or long alkyl side chain alcohols [10], oblique evaporation of SiO [20], ultraviolet (UV) light irradiation [21,22], magnetic field [23], ion beam exposure[24], different nanoparticles (NPs) [25-28], etc; have been extensively utilized. Almost all surfaces receiving homeotropic treatment by surface coupling agents have perpendicularly aligned alkyl chain or fluorocarbon chains. Kahn et al. proposed the use of n, n-dimethyl-n octadecyl-3-amino propyl trimethoxysilchloride (DMOAP) for homeotropic alignment [29]. Masumoto et al. proposed that tetrachloro- hydroxo--carboxyatodi chromium (III) complexes could be used to induce homeotropic alignment [30]. Hiroshima et al. proposed rotational oblique evaporation of SiO in order to obtain homeotropic alignment [31]. This method makes column structures perpendicular to the surface by oblique evaporation of SiO with rotation of substrate. LC molecules align in homeotropic configuration by the effect of geometrical structures. In the magnetic field-assisted alignment, the direction of applied external magnetic field to the LC sample (i.e. LC director) provides unique opportunity to achieve either homogeneous or homoetropic alignment [23]. In our case for homeotropic alignment, the substrates are cleaned with the acetone and then simply coated with the dilute solution of Lecithin. Solution of Lecithin has been prepared by dissolving Lechithin (Cetyl Trimethylammonium Bromide) in ethyl alcohol. The substrates have been dried at C for ten hours before assembling of the cell HYBRID ALIGNMENT In case of hybrid alignment the molecule are parallel to one substrate and perpendicular to other substrate, i.e., hybrid alignment is a mixture of homogeneous and homoetropic alignment configuration (Figure 3.6(d)). The hybrid alignment of LCs has been generally achieved by homogeneous alignment to one substrate and homoetropic to other one [32] TILTED ORIENTATION The method of pretilt angle generation includes a proper rubbing of polymer (polyimide) films. In this case, the chemical structure of the oriented nematic liquid crystals must be taken into consideration [33]. Avery important thing is the presence of alkyl branches in a rubbed polyimide film [33, 34]. The absence of alkyl branches results in a low pretilt angle (~2 0 ), polyimide films with a low density of alkyl

158 118 branches lead to a medium pretilt angle ~5 0 while a high pretilt angle (~20 0 )may be achieved for the high density of alkyl branches. The physical reason for this is evident. The presence of alkyl branches increases the tendency for a homeotropic orientation because they work on the polyimide surface in a manner similar to surfactants [35, 5]. Tilted orientation of the nematic liquid crystal molecules can also be achieved by using surfactants [4, 36]. The pretilt angle irreversibly increases with temperature due to the increase in the flexibility of the polymer side chains. The effect is more pronounced in case of larger dielectric constants of liquid crystals [34]. A high pretilt alignment ~20 0 of liquid crystal molecules can be achieved by an oblique evaporation technique of SiO 2 [35, 5] at present, the tilt angle produced by practical rubbing methods is about few degrees. Hence, a technique that induces higher tilt angles is required. The idea of nanodomains and the mixing of vertical and homogeneous alignment polyimide materials together to achieve high pretilt liquid crystal angles was developed [37]. Low and high liquid crystal pretilt angles can also be obtained by a photoalignment technique [6], in particular using a nonpolarized obliquely incident light (figure 3.7) Figure 3.7. Pretilt liquid crystal alignment induced by obliquely incident nonpolarized light [6]. The thermo polymerization stabilized the photo alignment produced by azo dye layers.

159 ASSEMBLING, SEALING AND FILLING THE CELL The patterned and pre-aligned substrates have been assembled to form a sample cell by placing Mylar spacers of known thickness just outside the conducting area. It is kept in such a way that there is an opening to inject the LC material inside. The other glass plate is placed over is taking care that the electrodes on them are over one another to overlap a perfect active display area. But before that, one important thing is to obtain uniform and accurate cell spacing. Uniform cell spacing is achieved by the use of spacers. Spacers are usually glass or polystyrene spheres, or glass fibers of a precisely controlled diameter; they are usually available in half micron increments. The spacers are distributed evenly throughout the cell. Cell thicknesses vary depending upon the type of display. The table 3.1 below shows gaps for several different types of displays. STN cells in particular are extremely sensitive to thickness differences. TYPE OF DISPLAY TYPICAL CELL GAP Twisted Nematic 5-10 microns Super Twisted Nematic 4-8 microns PDLC microns SSCT 4-5 microns PSCT microns Table 3.1. shows gaps for several different types of displays. Choice of spacer type is dependent upon desired properties as well as display materials. An advantage of plastic spacers is that their thermal expansion is similar to that of the liquid crystal material. This means that at higher temperatures when the liquid crystal expands and causes the cell to bow outward, the spacers will expand at the same rate, maintaining the cell gap. After application of spacers and then adhesive to one substrate, the cell is assembled by placing the other substrate on top. At this point, the two substrates can be aligned if necessary, before they are pressed together for the curing process. While curing, the substrates must be pressed together so that they rest on the spacers. Now liquid crystal cells are sealed using a thin line of adhesive around the perimeter of the cell. A small port is left on one side for liquid crystal injection. For capillary filling, two opposite sides are sealed.

160 120 Now before filling the liquid crystal material, the thickness of the cell is measured exactly by capacitance measurement. The capacitance measurement is very simple method in which the air capacitance (C 0 ) is measured through impedance phase analyzer (HP 4194A) and thickness of the cell can be calculated using the following relation: d = ε o A/C (3.1) Where d is the thickness of the cell, ε o is the absolute permittivity ( C 2 /Nm 2 ). A is the area of common conducting electrode (55 mm 2 ) and C 0 is the air capacitance of the cell. The last step in liquid cell fabrication is filling it with liquid crystal material. For this, the liquid crystal material is placed on the small opening between the spacers and by means of capillary action it is filled into the cell at its isotropic temperature. A well defined monodomain sample is obtained by cooling the cell slowly from isotropic phase to room temperature. Now LC sample cells are ready to characterize for different properties as shown in figure 3.8. Figure 3.8. The Detailed structure of the finally constructed liquid crystals sample cell. One difficulty encountered with the filling process is flow-induced alignment. With some alignment materials, the flow of the liquid crystal during filling is enough to induce an alignment different than desired. This can be eliminated by an annealing bake after cell sealing. This heats the liquid crystal to the isotropic state, and subsequent cooling yields alignment unaffected by flow. This tends to be a bigger problem with high pretilt polyimides. The filled sample holder used in present study is shown in figure 3.9.

161 121 Figure 3.9. Sample holder for the present study INSTRUMENTS AND EXPERIMENTAL TECHNIQUES USED TO CHARACTERIZE THE LIQUID CRYSTAL MATERIALS INSTRUMENTS USED TO CHARACTERIZE LC MATERIALS 1. Spin coater 2. Science Tech India Hot Air Oven 3. Physical Balance (Sartorius) 4. Polarizing Microscope (Radical, RXLr-5) 5. Instec HCS 302 Hot Plate 6. Impedance Gain/Phase Analyzer HP4194A 7. Impedance/Gain Phase Analyzer Solartron (SL 1260) 8. UV Visible Spectrometer ELICO SL Storage Oscilloscope HM Temperature Controller Julabo F Self Designed 3-In-1 Analyzer 12. Textronics Oscilloscope (Model Number TDS 2024C) 13. Textronics Function Generator (Model Number AFG3021B) 14. Photo Detector (Model PD02LI) 15. Hydraulic Press (MP -15) 16. Shimadzu Fourier Transform Infrared Spectrophotometer 17. Differential Scanning Calorimeter (DSC 200 F3 MAIA) 18. Photomultiplier Tube

162 122 The dielectric, electro-optical properties and materials constants of LCs can be determined with help of the above mentioned instruments. The sample cell is mounted onto the sample holder of the hot/cold stage and kept on the rotating table of the polarizing microscope. The sample can be viewed through the microscope. Figure 3.10 shows the photograph of all the instruments used to characterize the LC materials. Figure Photograph of instrument used in dielectric, electro-optical and physical parameters measurements for the thesis EXPERIMENTAL IDENTIFICATION OF LIQUID CRYSTALS In the course of research and commercial manufacturing, it is vitally important to be able to identify the types of liquid crystals phases that are exhibited by compound or a mixture of compounds [38, 39]. The most widely used techniques of liquid crystal phase identification are optical polarizing microscopy, which reveals that each different liquid crystal phase has a distinct optical texture. However this identification of liquid crystal phases

163 123 through optical polarizing microscopy is often difficult and requires a lot of experience. Differential scanning calorimetry (DSC) is nearly always employed as a complementary tool to optical microscopy and reveals the presence of mesophases and liquid crystals phases by detecting the enthalpy change that is associated with a phase transition. However this technique cannot identify the types of liquid crystals phase but the level of enthalpy change does give some information about the degree of molecular ordering within a mesophase. However the ultimate technique for the identification and classification of mesophases is X-ray analysis. X-ray analysis of a liquid crystal will map the positions of the molecules within the phase and hence determine the phase structure and classification to which the particular phase belongs. However to maximize the information aligned samples are required. Miscibility study is another method of identifying the mesophase. The material with unknown mesophase is mixed with a known material that possesses mesophases that have already been identified. If a particular mesophase of the unknown material is completely miscible with a known mesophase, then both the mesophases are identical. Other technique used to identify the structure of mesophase and liquid crystalline mesophase include neutron scattering and nuclear magnetic resonance. TEXTURAL OBSERVATIONS Everyone working in the field of liquid crystals will at some point need to use optical polarizing microscopy in the analysis of liquid crystals. The identification of mesophase through optical polarizing microscopy usually involves the magnified view of a thin sample of mesogenic material sandwiched between a glass microscope slide and glass cover slip [40]. Usually phase identification is carried out on glass slides that are not treated to obtain any particular alignment. Polarizers in the microscope are placed at 90 0 to each other, so no light passes through when isotropic liquid or no sample is analyzed [38]. But when birefringence material is present, optical textures appears that gives information relating to the arrangement of the molecules within the medium. Smectic phases that are generated by heating a sample from the crystal phase are often very difficult to identify unequivocally because their optical defect textures retain characteristics of crystal phase. Therefore material is heated into the isotropic

164 124 liquid phase and mesophases are identified from their defect textures generated on cooling. (A) (B) (C) Figure (A) Optical Polarizing Microscope. (B) And (C) Optical textures of some Liquid Crystals. DIFFERENTIAL SCANNING CALORIMETRY DSC reveals the presence of phase transition in a material by detecting the enthalpy change associated with each phase transition [5]. Eventhough the precise identity of the phase cannot be obtained, but the level of enthalpy change at the phase transition does provide some identification of the types of phase involved. Therefore optical microscopy and DSC are used in conjunction with each other. In general there are two types of phase transition- continuous (first order transition) and discontinuous (second order transition). If the entropy at phase transition shows a discontinuity, then a discontinuous first order transition has occurred. Most liquid crystal to liquid crystal transitions are discontinuous, but some such as SmC to SmA transition, are often continuous. (A) (B) Figure (A) Differential Scanning Calorimeter. (B) DSC Thermogram of well known Nematic Liquid Crystal (5CB).

165 SPONTANEOUS POLARIZATION Studies of the polarization reversal current in ferroelectric liquid crystals were begun by Martinot-Lagarde [41]. The motivation was to evaluate the spontaneous polarization from these experiments, but later attempts were made also to get information for the rotational viscosity. In order to measure the spontaneous polarization of ferroelectric liquid crystals polarization reversal current method is used. To switch the FLC molecule from P S to + P S one has to supply the charge of amount 2P S to every unit area, so for the complete sample it will be 2AP S, here A is the active area of the sample cell [42]. Thus this reversal of P S gives rise to a current i P = dq/dt. Therefore i P can be written as- d2aps ip (3.2) dt When P S makes an angle with the applied electric field E then the polarization charge on electrode is Q= P S Acos, therefore the i P can be written as- i P AP S d Cos d APS Sin dt dt (3.3) Using the dynamical equation for the FLC molecules in the presence of electric field we have i P AP S t τ e 0 so Sin d dt 2 (3.4) Thus by measuring the area under the polarization reversal peak one can determine the value of spontaneous polarization. There are number of methods suggested to measure the total charge transfer during the polarization reversal [43-46]. In the present work we have used the triangular wave method which has some unique advantages as use of this waveform separate the capacitive contribution from interfering with current due to polarization reversal. The resistive component is easy to extract as this contribution varies linearly with applied voltage [2]. The experimental setup has been shown in the figure A function generator (Scientific SM 5078) has been used to deliver triangular wave of frequency between 0.5 Hz to 10 MHz, with 20 volts peak to peak. A planar aligned cell can be regarded as a resistor (R) and capacitor C connected in parallel as shown

166 126 in figure. If a triangular wave of amplitude V is applied to the cell, then current I(t) induced in FLC is sum of three contributions (1) The first is due to the charge accumulation to the capacitor I C. (2) The second is due to polarization realignment I P. (3) The third is due to ion flow I I.. The total current I is given by dv dp V I IC IP II C (3.5) dt dt R Where P S is the amount of charge induced by the polarization realignment. Figure The experimental arrangement for the measurement of spontaneous polarization. With the suitable values of the resistance (two order smaller than that the FLC sample resistance) connected in series with the FLC cell, one can get the suitable current profile to subtract the ionic and capacitive current. Figure 3.14 shows current contribution of different component discussed earlier and the overall effect on the output current. When the electric field is applied on the planar aligned FLC sample, the whole FLC sample attains any one of the two polarization states (i.e. P S E or P S E). This results in accumulation of free charges of opposite polarity on both the electrodes and consequently a depolarising field generates which opposes the applied electric field. Hence the switching between two polarization states generates the polarization reversal current I P that appears as a hump shown in figure The estimation of area under this hump gives direct value of the P S [20]. To calculate the

167 127 area under the polarization reversal hump the wave form data has been transferred to the computer from oscilloscope and then processed by Microcal Origin scientific software, using equation below. I Pdt 2 AP S (3.6) ( ( ( ( ( Figure Illustration of current induced on application of a electric field of triangular wave to FLCs. (a) Applied triangular wave, (b) Contribution of charge accumulation in capacitor (c) Polarization reversal contribution (d) Contribution due to the resistance in the circuit (e) Overall current profile [4].

168 OPTICAL TRANSMITTANCE The optical transmittance measurement has been done by placing the cell between two-crossed polarizers of polarizing microscope model (Radical, RXLr-5) fitted with a hot stage. The complete experimental arrangement for the optical transmittance measurement is shown in figure Figure Optical transmittance measurement and texture study setup used in present work. The optical texture showing the texture clicked. The most fascinating thing about this method is the visualization and clicking of sample texture during measurement by camera (Model Prog Res CT3) fitted on one of the eyepieces of the microscope. The light intensity coming through one of the eyepieces has been measured by Photo Detector (Model PD02LI). The optical transmittance obtained from the photo detector (Instec-PD02LI) is directly fed to a digital storage oscilloscope (Tektronix TDS-2024C) in an electrical form. The output wave form intensity is then used to determine the transmittance. Now the sample holder has been placed in the path of incident light, under the crossed polarizeranalyzer position. The variation of light intensity coming out through the sample can easily be read on Photo Detector (Model PD02LI) in terms of arbitrary unit (at various temperatures), which gives the value of optical transmittance. The 0% and 100% optical transmittance have also been measured for empty and black ink filled cell to calculate the percentage optical transmittance [19]. Examination of a sample through this reveals information about the alignment of the optic axis. As the sample is rotated, annihilation of light is observed when the optic axis is parallel to the polarizer

169 129 or analyzer axis. At other orientations light is transmitted, the colour of which can give information about the birefringence and thickness of the sample. The transmitted light intensity (I) can be written as [47] 2 2Sin nd 2 I I Sin / 0 (3.7) Here I o is the incident intensity, is the angle of the optic axis relative to the anisotropy (Δn = n e - n o ), d is the sample thickness and is the wave length of the incident light [48]. The change in temperature of the thin layer of liquid crystal under crossed polarizer condition causes a significant change in optical texture due to the change in the intermolecular interaction field [49]. The phase difference between the extraordinary and the ordinary ray for a monochromatic light of wavelength l is found by integrating over the layer depth (d): (3.8) The intensity of the light passing through the cell depends on the angle jφ 0 between the polarization vector of the incident beam and the initial orientation of the director of the nematic liquid crystal (3.9) Where I 0 is the intensity of the plane-polarized light incident on the cell. Hence, the external magnetic or electric field changes the orientation of the director, θ=θ(e, z) and, consequently, the values of Δn(E, z) and ΔΦ(E). A change in the phase difference ΔΦ(E), in turn, results in an oscillatory dependence of the optical signal at the exit of the analyzer. The maximum amplitude of these oscillations corresponds to an angle φ 0 =45 0 and the maximum possible number of oscillations (e.g., the number of maxima during a complete reorientation of the director) is approximately (n -n ) d/λ RESPONSE TIME MEASUREMENT One of the most important parameters of a display device is its response time (). Response time of liquid crystal depends upon the cell thickness, field strength, surface anchoring etc. as well as material parameter like dielectric anisotropy(δε), rotational viscosity() and response time of ferroelectric liquid crystal is also depends

170 130 upon the polarisation (p), tilt angle (θ). It can be measured either by monitoring electrical response by applying square wave or triangular wave [49], arbitrary wave or by optical method [50]. The response time of liquid crystals can be measured by the experimental setup as shown in figure An arbitrary A.C signal voltage (20V pp &1Hz) was applied to the cells using function generator. He-Ne Laser beam with a wavelength 632 nm as the input signal is detected by new focus photo-detector (Instec-PD02LI) connected directly to a digital storage oscilloscope (Tektronix TDS- 2024C). From the detected shapes of the waveform in figure 3.17, we calculated the rise time ( on ) and fall time ( off ) for LC. Here, on is the time required for the transmittance to rise from 10% to 90% and off is the time required for the transmittance to fall from 90% to 10% [51, 52]. The sample cell is set at angle 45 0 crossed polarizer and analyzer for ensuring maximum optical transmittance. Thus the cell works as a phase retarder thereby altering the polarization of light. When voltage is applied, the LC molecules are aligned in applied field direction. Again when the voltage is switched off the LC molecules relax and return to their initial state. For the nematic, response time is defined as the time taken by the molecules to produce the change in transmission from 10 % to 90 % of its maximum value as shown in figure 3.17(b) [35, 6]. τ rise = t 90 -t 10 τ fall = t 10 -t 90 The total response time is given by τ total = τ rise + τ fall. Generally nematics shows slow switching in comparison to the FLCs. Figure Optical response time measurement setup (Block Diagram).

171 131 (a) (b) Figure (a) Applied waveform of a liquid crystals sample. (b) Detected waveform in oscilloscope THRESHOLD VOLTAGE To find the threshold voltage of liquid crystal display cells we have to use same setup as in response time measurement. In the setup the analyser and polarizer keep fix at crossed position. The input wave of voltage is applied to the LC cell. The output wave from LC cell is traces on the oscilloscope. Now we slowly increase the input voltage till the LC molecules just respond. This happening seen on the oscilloscope. The corresponding voltage treated as threshold voltage of the LC display and the phenomenon is Freedericksz transition SPLAY ELASTIC CONSTANT AND ROTATIONAL VISCOSITY The splay elastic constant (K 11 ) [35, 5] of the nematic liquid crystals is given by the following formula K 2 VTh 11 Δεε0 π (3.10) Where V th is the threshold voltage, Δε is the dielectric anisotropy of the LC sample, calculated from dielectric measurement discuss in next sections. The LC material possesses two types of viscosities (a) translational and (b) rotational viscosity (). Translational viscosity of LCs is just like isotropic liquids, due

172 132 to their translational motion or flow like properties while rotational viscosity () comes into the account electric. In LCD devices, we mainly concern about the rotational motion of LC molecules and hence rotational viscosity is important in this case. The rotational viscosity () of the nematics can be calculated with the help of following formula: τ0k γ d 11 2 π 2 (3.11) And the value of rotational viscosity () of FLC materials can be find out by using the formula: γ τ0 (3.12) p E s Here is rotational viscosity, τ 0 is the switching time of the sample, K 11 is splay elastic constant and d is the cell gap, P s is the spontaneous polarization, and E is the applied field, respectively TEMPERATURE CONTROLLER The accurate temperature measurement is very crucial point while studying the characteristics of any thermotropic liquid crystals. For the same purpose the INSTEC hot plate HCS 302 and Julabo temperature controller have been used in the present work. (A) Julabo F-25 Temperature Controller A microprocessor based temperature controller (model Julabo F-25 HD) is used shown in figure 3.18(a). It uses synthetic oils for maintaining the temperature of the sample holder. The oil circulates in a double walled jacket and a Pt-100 type sensor is placed in the jacket at the appropriate place to measure the temperature. With this temperature controller we can read a temperature variation of C. (B) Instec HCS 302 Temperature Controller The INSTEC HCS 302 heating/cooling stage is an instrument of very high precession for the temperature control. This package includes heating/cooling stage HCS 302, 1/16 DIN temperature controller STC 200 and computer software WinTemp. The hot stage HCS 302 uses single heater located underneath the sample chamber shown in figure 3.18 (b).

173 133 Figure (a) Julabo F-25 Temperature Controller (b) INSTEC HCS 302 heating/ Cooling stage (c) INSTEC STC 200 temperature controller. The HCS 302 is equipped with ports for the cooling accessories, such as liquid Nitrogen (LN2) pumps as well as gas purging of the sample chamber and defrost of the top and bottom windows (to prevent condensation during below ambient operation), and frame cooling (to prevent excessive external temperature during the high temperature operations) [53, 54]. These features of hot stage provide flexibility to a user to operate it in the wide temperature range from -190 C to 400 C. This hot stage also provides easy handling by the specially designed swing open cover. The STC 200 temperature controller is used to adjust the sample temperature for observation and their measurement. STC 200 is a very user friendly device which provides simple temperature operations using the four keys provided at the front panel. The simple isothermal setting can be achieved by up or down arrow keys on STC 200 CPU. The STC 200 temperature controller can be used in standalone mode as well as in remote mode with computer software Win-Temp. The STC 200 can be interfaced with the computer simply by RS232 port or IEEE card and can be operated with the help of Win-Temp software. The Win-Temp software provides easy single command or small program algorithms to operate STC 200 for different temperature operations. For example a small program algorithm is as follows- RAMP TO AT 0.50 HOLD AT FOR 5 BEEP HOLD AT FOR 10 BEEP

174 134 RAMP TO AT 0.50 HOLD AT FOR 5 BEEP HOLD AT FOR 10 The above program algorithm is a small part of the dielectric measurement program. The first command ramp the hot stage to 34 C with the heating rate of 0.5 C/min and after reaching at this temperature the second command holds the hot stage at the same temperature for 5 minutes so that sample can stabilize. After 5 minutes next command intimates us by the beeping alarm to start reading procedure of the HP 4194A which needs approximately 10 minutes to complete. The next command used to hold hot stage at the same temperature for 10 minutes in which reading process completes. Again an intimating beep alarm becomes active and this intimates that the reading process is completed. The preceding commands follow the same steps discussed above for the next temperature i.e. 30 C in the present case [51]. Similar type of small program algorithm has been developed for different set of experiments and performed successfully DIELECTRIC SPECTROSCOPY STUDY To study the dielectric properties of liquid crystals self fabricated cell with planar alignment have been used. But before discussing about the measurement techniques for dielectric spectroscopy, it is important discuss about the live capacitance THE STRAY CAPACITANCE PROBLEM The liquid crystal sample cell constitutes a parallel plate capacitor which we normally approximate as ideal in the sense that all measuring field lines go straight from one electrode to the other electrode. The value of the capacitance is within this approximation is given by C A d (3.13) Where A is the cell area, is dielectric permittivity of the material and d is cell thickness. The above relation does not quite reflect reality, because some field lines always bend out outside the cell active area and give a contribution to the capacitance which is often called stray capacitance [2, 51, 55], arises when it is subjected to an

175 135 electric field due to the lead attached. There is therefore a tiny part of the capacitor which is not affected by the introduction of dielectric material. If we write the empty capacitance as the sum of ideal empty capacitance and stray capacitance C ο = C L + C S (3.14) Here C L is the capacitance of the empty cell without the stray capacitance (C S ). For the measurement of stray capacitance of the constructed cell two standard liquids of known relative permittivity i.e. Benzene and CCl 4 are used [3]. The cell capacitance filled with any of these liquids can be written as C SL = SL C L + C S (3.15) By using the equation (3.14) and (3.15) we can calculate the live capacitance of the cell as- C L C ε SL SL C 0 1 (3.16) Here C SL represents the capacitance of the cell filled with the standard liquid of known relative permittivity ( SL). The value of stray capacitance can be calculated by using equation MEASUREMENT Dielectric spectroscopy is specially sensitive to intermolecular interaction and capable of monitoring cooperative process, while molecular spectroscopic provides a link between the investigation of the properties of individual constituents of complex material and the characterisation of its bulk properties. In present work dielectric studies have been carried out on a computer controlled Impedance / Gain Phase Analyzer model HP 4194A which is a highly accurate instrument that has the measurable frequency range from 100 Hz to 40 MHz.. The complete dielectric measurement setup is shown in figure 3.19.

176 136 Figure Experimental arrangement for the dielectric measurement. The cell has been loaded to hot stage (HCS 302) and terminal of the cell has been fed to HP A. Any particular frequency can be applied across the sample by simple operations as provided on the key pad on the bridge. The instrument employs microprocessor controlled built-in bridges and resonant circuits to cover a wide range of frequencies. The instrument can measure eleven different parameters, which are impedance (Z), admittance (Y), phase angle (θ), resistance (R), reactance (X), conductance (G), susceptance (B), inductance (L), capacitance (C), dissipation factor (D) and Q (= I/D) with 20 parameter combinations [13]. HP-4194A has an impedance measurement range from 0.1 m to 1.6 M with accuracy of 0.17%. The AC signal of the HP-4194A may vary between 10mV rms to 1V rms and DC signal may vary from 0 to + 40V. The default values of AC and DC signals are kept at 0.5 V rms and 0 V respectively. Various parameter values can be recorded in the tabular form and also can be seen in the form of the graph display on monitor. The Impedance / Gain Phase Analyzer Hewlett-Packard (4194A) works on the principle of auto balancing and consist of three sections (1) signal source section, (2) auto balancing bridge section and (3) vector ratio detector section. The signal source section consist a microprocessor based frequency synthesizer that generates the test signal of variable frequency with a high resolution of 1 mhz. These test signals are directly fed to the device under test (DUT). Internal reference signals are also generated in this section.

177 137 Figure Block diagram for the auto balancing section of the HP-4194A. Figure 3.20 shows the block diagram of the auto balancing section. The auto balancing section balances the range resistor current with the DUT current to keep the lower terminal at same zero potential. The detector D observes the potential at the lower terminal and manages the magnitude of the phase generated by oscillator OSC2 to bring potential back to zero. The vector ratio section measures two vector voltages, one across DUT (V DUT ) and other across the range resistor R r (V rr ) series circuit. Since the range resistor value is known, measuring two voltages will give the impedance vector Z x = R r (V DUT /V rr ). Each vector voltage is separated into its 0 and 90 components by a phase detector, and each component is measured using dual-slope A to D converter. Either V DUT or V rr signal is selected by a selector to that the V DUT and V rr signals follows identical path to eliminate tracking error between the two signals. To measure the dielectric properties of the liquid crystals filled cell terminals have been connected to Impedance / Gain Phase Analyzer by a text fixture (16047 D). To record the values of capacitance and conductance of the sample holder the Impedance / Gain Phase Analyzer has been interfaced with a P (IV) computer using a GPIB card. The values of the capacitance and conductance can be recorded by software. The beauty of this software is that we can program it to acquire the data according to our need and data can be processed directly by any scientific software like Origin or Mat Lab. The value of relative permittivity () and dielectric dispersion () can be directly calculated from recorded values of capacitance and conductance for with and without sample and by using following equations [56]-

178 138 C m C C L G m G 0 2fC 0 1 (3.17) L (3.18) Here C m and G m are the capacitance and conductance respectively for the cell filled with the liquid crystal material while C o and G o are capacitance and conductance for empty cell HOW TO COPE WITH UNWANTED ABSORPTION Instead of great accuracy of the measurement, the experimental data suffers from the two basic problems. Liquid crystal compounds are unfortunately never free from ionic impurities, and this has severe effects on dielectric spectroscopy measurement. There are two main types of ionic contributions i.e. ionic space charge polarization and charge build up at the cell electrodes [56, 57]. The first problem is in low frequency range which is due to the presence of free charge particles. The second is due to the sheet resistance of ITO coated sheet and lead inductance of the wires attached as terminals. These problems can be sorted out by the best theoretical fitting of some functions discussed below [35, 5] LOW FREQUENCY IONIC CONTRIBUTION Since the liquid crystals are improper dielectrics, they have some concentration of the charge carriers that can move freely in the presence of electric field. This conduction of charge carriers offers conductivity effect in the low frequency range and affects the dielectric results considerably. This conductivity effect can be resolved by assuming a resistance attached parallel to the cell as shown in figure 3.21 (a). Here R i represents the additional resistance and works in the inverse manner, subscript i refers to the ions. The measured data in low frequency range also shows an effect of ionic conductance, therefore for our fitting purpose it is therefore sufficient to model that the ionic contribution with simple inverse temperature dependence instead of separate Cole -Cole process [35, 5] n dcf & (3.19)

179 Ions dc 0 1 2f k (3.20) Here f is measurement frequency, 0 is vacuum permittivity,, n and k are fitting parameters. The parameter denotes the conductivity of the sample and m is needed because often absorption does not have a perfectly inverse linear dependence of frequency. The value should however normally be close 1. The conductivity is related 139 to the conductance, where A is area and l is length. According to Iwamoto [47], ionic contribution of space charge polarization decrease if applied voltage is increased. This is often seen when performing dielectric spectroscopy measurement with varying applied voltage, thus indicating that this kind of ionic contribution rather usual in chiral smectic liquid crystals. When performing measurements in these phases, one therefore has to find an optimum measuring voltage, where ionic contributions are suppressed as much as possible. The second type of ionic contribution due to surface charges at the electrodes is not affected by measurement voltage [58]. If such contribution is present, it will thus always give a constant contribution in low frequency end of the dielectric spectrum. R ITO R C C R R C C (a) (b) Figure Equivalent circuit of the sample cell in (a) low frequency range (b) in high frequency region. After adding low frequency terms effect due to the electrode polarization in Cole Cole equation, the real and imaginary part of the dielectric permittivity can be written as

180 140 n dcf 1 1 2f & dc Im k 0 2f 1 f 1 2 (3.21) (3.22) The first terms of both the equations are for the contribution due to the ionic conductivity, while bracket terms of the above equations represent real and imaginary (Im) part of the Cole-Cole equation HIGH FREQUENCY CELL RELAXATION PROBLEM The measurement cells used for dielectric spectroscopy, as well as for many standard liquid crystal characterization techniques, are normally coated with ITO electrodes which are transparent to visible light [50]. Due to the low resistivity of ITO unfortunately leads to a spurious absorption centred around a frequency f ITO in the dielectric spectrum. The effect is seen as a local increase in dielectric loss centred around a frequency f ITO and a corresponding decrease in dielectric loss above f ITO and it thus minimize a polarization processes in the dielectric spectrum. The frequency f ITO of this spurious mode is called cell relaxation decreased with decreasing cell thickness and with increasing electrode resistance. For this reason one must take care in using cells coated with low resistive ITO and for thin cell measurement it is actually worthwhile considering using really low resistive but unfortunately non transparent electrodes like gold or copper, in order not have the interesting part of the dielectric spectrum completely obscured. At the frequencies beyond 10 MHz, the ionic conduction contribution becomes small whereas the ITO sheet resistance becomes active and results in pronounced increase in the dielectric absorption at higher frequencies. The equivalent circuit of the measurement in high frequency region is as shown in figure 3.9 (b). In addition to the time constant of the dipolar reorientation, the cell also offers a cut off frequency given by the inverse of the R ITO C. Therefore the measured dielectric data shows problem only in absorption and after including the contribution of ITO the measured absorption can be written as Af m Im 1 1 2f (3.23)

181 141 Here A and m are the fitting parameters and depend upon the RC constant of the cell. Overall contribution due to the ionic conductivity and ITO resistance can be written as 0 n dcf dc 2f k Im f & Af m 1 2f (3.24) (3.25) R ITO R r C r C R i Figure Equivalent circuit for sample cell suitable for complete frequency range. Figure 3.22 shows the equivalent circuit valid for both high and low frequency regime and figure 3.23 shows a typical presentation of the fitting of above equations into the experimental data. By fitting equations (3.24) and (3.25) with the experimental data, it is possible to subtract background effects from the dielectric spectrum in the low and high frequency region [35, 5]. Therefore for the better results resistance of ITO should be as low as possible. This problem can be removed by the application of metal electrodes. However for liquid crystals it is necessary to use transparent electrodes which allow optical observations of measured structure at the same time. As mentioned in our case the resistance of the ITO sheet is 10/ which allows the measurement up to 10 MHz.

182 142 Figure An example of the best theoretical fitting of equation (3.10) into the experimental data. The open legends are presenting the experimental data, while the solid line represents the best theoretical fit [19] DATA PRESENTATION In the old literature the susceptibility for describing the contribution from any relaxation mode is given by 0. Many researchers introduced a shorthand version as dielectric strength which we should try to avoid because this is certainly a different thing and already well defined. The dielectric strength is well defined in the field of dielectrics as the maximum field strength of a material that it can withstand before dielectric breakdown occurs. Therefore to avoid this confusion one can use dielectric contribution or relaxation strength instead of dielectric strength. In the present thesis the relaxation strength has been used in place of dielectric strength [47].

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186 Chapter 4 Improved Dielectric and Electro- Optical Parameters of Zno Nano Particle (8% Cu 2+ ) Doped Nematic Liquid Crystal

187 Chapter 4 IMPROVED DIELECTRIC AND ELECTRO-OPTICAL PARAMETERS OF ZNO NANO PARTICLE (8% CU 2+ ) DOPED NEMATIC LIQUID CRYSTAL 4.1. Introduction 4.2. Experimental detail 4.3. Results and discussion 4.4. Conclusion References Journal of Molecular Structure 1035 (2013)

188 INTRODUCTION Liquid crystals exhibit molecular orientation in their ordered phase (the smectic and the nematic) that provide a path for charge carriers to move between electrodes. High photoconductivity in liquid crystals was discovered in 2001 [1, 2].Since then, interest of researchers has increased in the variation of possible use of the nematic liquid crystals. Nematic liquid crystals (NLCs) have shown profound impact on the versatile display as well as non display applications. The key point for all the possible applications is the alignment of liquid crystal molecules (i.e. the director) on the substrate [3, 4]. Homeotropic alignment of liquid crystals has applications in liquid crystal display technology, such as high information display devices, large area LCD TVs and digital display devices used in the medical field like digital medical imaging [5]. Liquid crystals (LCs) have received much attention in the recent years because of their ability to transfer their long range orientational order on the dispersed materials, such as carbon nano tubes, nano particles and various collides [6-12]. Many researchers are trying to improve the dielectric and electro-optical performance of LCDs by doping them with non-mesogenic material, such as polymer, dye, nanoparticle, carbon nano tubes (CNT) etc. The properties of the pure NLCs can be optimized by the addition of nano particles and CNT. The addition of nano particle may produce defects in the NLC phase and breaks its continuous rotational symmetry. The properties, which can be varied by doping of nano particles in the pure NLC, are dielectric permittivity, dielectric loss, dielectric anisotropy, threshold voltage, response time. Jeng et al. [13] has reported the effect of adding polyhedral oligomeric silsesquioxanes (POSS) nano particle in liquid crystals. They have reported enhancement in the switching properties of the POSS doped liquid crystals [13]. Martinez-Miranda et al. studied the properties of octylcyanobiphenyl (8CB) NLC doped with ZnO nano particle. They were able to enhance the electrical properties of the NLC sample [14]. Our group has also reported the effect of nano particles on some parameters of the ferroelectric liquid crystal [15]. The anisotropic ordering of NLCs has been utilized in organizing copper doped ZnO nano particle NLCs system. However most of the studies based on NLC and nano particle NLC composites have been concentrated on their electro-optical properties. But their electro-optical properties in turn depend upon dielectric properties and should be explained

189 147 accordingly. MBBA (N-(4-Methoxybenzylidene)-4-butylaniline) was one of the first synthetic liquid crystals to exhibit a nematic phase at room temperature and remains a popular subject of liquid crystal research to this day. During the early 1970's MBBA was used by UK liquid crystal researchers in early prototypes of liquid crystal displays (LCDs). But it was quick to hydrolyze and therefore any moisture entering these LCDs during fabrication was likely to degrade the MBBA into constituents that no longer exhibited an alignment response to an electric field [16]. The main drawback of this liquid crystal is very poor chemical stability. So, we have chosen the doping of nano particle in this liquid crystal. The dispersion of nano particle CNT and dyes improve the performance of electro-optical parameter of this NLC. Therefore, in the present chapter we have observed the change in the dielectric properties along with the change in electro-optical parameter. The present chapter is an effort to optimize dielectric and electro-optical parameters of pure NLC by the addition of nano particle. The dielectric and electro-optical parameters have been evaluated for pure and nano particle doped NLC. The temperature dependence of the above mentioned parameters has also been discussed with the help of different theories of NLC. It was observed that the doping of 0.5%, 1.0%, and 1.5% of nano particle (8% Cu 2+ in ZnO) has caused variation in almost all the properties of the NLC under consideration EXPERIMENTAL DETAIL Nematic material The NLC used in the present study is MBBA (p-methoxybenzylidene p-butyl aniline). Figure 4.1 represents the molecular structure of MBBA. Its approximate length and width are 20Å and 5Å [17]. The standard value of dielectric anisotropy and refractive index of pure MBBA liquid crystals are and ~ at 25 0 C temperature and dipole moment of MBBA is µ MBBA =7.34x10-34 C.m [18]. The phase transition of MBBA is Crystal 20 0 C Nematic 47 0 C Isotropic. The present study concentrates on the simplest phase of liquid crystal i.e. nematic phase.

190 148 Figure 4.1. Molecular structure of Nematic liquid crystal MBBA. Nano-nematic composite system is prepared by dispersion of 8% Cu 2+ doped Zinc oxide nano particles in pure MBBA. Three different concentrations of nanonematic composite system (0.5%, 1.0% and 1.5% wt/wt) have been prepared by dispersion of 8% Cu 2+ doped Zinc oxide nano particles in pure MBBA. Liquid crystal MBBA was mixed with Cu doped ZnO nano particles physically. The mixture was processed through repeated heating and cooling cycle for three times and mixed by ultrasonic mixer, to obtain a uniform dispersion. The mixture was viewed under polarizing microscope to assure homogeneous distribution of the nano particles. They were termed as mixture 1, mixture 2 and mixture 3 in the whole discussion of this chapter. Nano particle The Cu 2+ doped ZnO nano particles have been used in the present study, as they exhibit semiconducting piezoelectric and pyroelectric multiple properties. ZnO is an important (II-VI) semiconductor having a wide and direct band gap (as wide as 3.37ev) [19] and the dipole moment of ZnO nano particle µ ZnO =1.6x10-28 C.m [20]. It is an important material for the applications in different fields such as piezoelectric transducers, gas sensors photonic crystals, light emitting device, and transparent UV protection films. The synthesis of this copper doped ZnO nano particles have been given by P. K. Sharma et al. [21]. Figure 4.2 shows the X-ray diffraction (XRD) pattern for the ZnO:Cu 2+ nano particles used for doping in the NLC. The spectra shows broad peaks at the positions of 31.63, 34.50, 36.25º, 47.50, 56.60, 62.80, 66.36, and 68.91, as clear from the figure 3. The values are in good agreement with the standard JCPDS file for ZnO (JCPDS number , a = b = Å, c = Å) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P6 3mc [21]. The average particle size of ZnO:Cu 2+ nano particle was~13 nm estimated by the Debye-Scherer s

191 149 equation [21]. The uniform nano rod shape particle having diameter 12~15nm and length 40~80 nm were observed from Scanning Electron Microscopic (SEM) Images, shown in figure 4.3 [21]. Figure 4.2. X-ray diffraction spectra of ZnO:Cu 2+ (8% Cu 2+ doped ZnO)nano particles. Figure 4.3. SEM images of ZnO:Cu 2+ (8% Cu 2+ doped ZnO) nano particles. Preparation for sample cell The planar sample cells were used for dielectric and electro-optical studies. The sandwiched type (capacitor) cells was made using two optically plane glass substrates coated with conducting indium tin oxide (ITO) layers. The planar alignment was obtained by treating these plates with adhesion promoter and then coating it with polymer nylon (6/6). After drying the polymer layers two substrates were rubbed unidirectionally by velvet cloth. The substrates were then placed one over another to form a capacitor. The cell thickness was fixed by placing a Mylar spacer (6 µm in present case) in between the two substrates and then it is sealed by a

192 150 UV sealant. Sample cells were calibrated using analytical reagent (AR) grade CCl 4 and benzene as standard reference for dielectric measurement. The assembled cells were filled with the suspension and pure LC at temperature 10 0 C higher than the isotropic temperature of LC by capillary method. Above the threshold voltage (10V in present case), the molecular ordering was found to the homeotropic as seen by polarizing microscope. This condition has been used to measure the parallel component of dielectric permittivity ( ). Dielectric measurement The dielectric measurement of the pure NLC and copper doped ZnO nano particle in NLC were carried out by using an Impedance/Gain phase analyzer (HP4194A) in the frequency range 1KHz to 10MHz. The measurement in the high frequency range has been limited to 10MHz because of the dominating effect of the finite sheet resistance of ITO coating on the glass plate and lead inductance [15, 22, 23]. The temperature has been maintained by a computer controlled Hot plate (Model No.302, Instec Corporation USA). Experiment were performed by ramping the temperature at very slow heating rate of C min -1 and temperature stability was better than ±0.1 0 C [22]. The dielectric response of NLCs shows complex behavior as a function of both frequency and temperature. The dielectric relaxation phenomenon of the pure NLC and nano-nematic composite system has been analyzed by using Cole-Cole equation [22-24]. The low and high frequency data required a correction term as NLC are improper dielectric. On adding these correction terms and then separating the real and imaginary part of Cole-Cole equation one may get [15, 22, 23]. (1 ) ' '[1 (2f ) sin( / 2)] n '(dc)f '( ) 2(1 ) (1 1 (2f ) 2(2f ) ) sin( / 2) (4.1) & 0 (1 ) (dc) ' (2f ) cos( / 2) " Af k 2(1 ) (1 ) 2f 1 (2f ) 2(2f ) sin( / 2)) m (4.2) Here δε is the relaxation strength, is the relaxation time, α is the distribution parameter, ε() is the high frequency limit of the dielectric permittivity,(dc) is the ionic conductance, o is the free space permittivity and f is the frequency of

193 151 relaxation, while n, m and k are the fitting parameters [22,23]. By the least square fitting of experimental data in the above equation the low and high frequency data have been corrected. Electro-Optical measurement The switching behavior of a planar aligned cell as a function of applied voltage and temperature was studied by using electro-optical set up as shown in the figure 4.4. The response time of pure NLC and nano-nematic composite system have been measured by the optical switching method [25].In this method a square wave (frequency 0.5 Hz and amplitude 6 Vpp) has been applied to the planar aligned cell. The optical response of molecules obtained from the photo detector (Instec-PD02LI) is directly fed to a digital storage oscilloscope (Tektronix TDS-2024C) in an electrical form. The output wave form is then used to determine the response time. The total response time of pure NLC and nano-nematic composite system has been evaluated using equation [25]., & (4.3) O ON OFF ON OFF Here 90 and 10 are the time taken by the output waveform to reach 90% and 10% maximum of the output waveform for rise and fall of reference square wave signal. Figure 4.4. The experimental arrangement for the measurement of electro-optical parameters.

194 RESULTS AND DISCUSSION The perpendicular and parallel components of the dielectric permittivity ( & ) are depicted in the figure 4.5(a) and 4.5(b) respectively, for the pure NLC and its nano-nematic composite system at a constant temperature of 34 0 C. It is clear from the figure that the nature of the variation of perpendicular component of the dielectric permittivity ( ) as well as the parallel component of the dielectric permittivity ( ) are almost same for the pure NLC and its nano-nematic composite system. Figure 4.5. (a) Variation of perpendicular component of dielectric permittivity ( with frequency for pure and nano-nematic composite system. Figure 4.5.(b) Variation of parallel component of dielectric permittivity ( ) with frequency for pure and nano-nematic composite system. The magnitude of the perpendicular component of the dielectric permittivity ( ) for mixture 1, mixture 2 and mixture 3 is found to be higher in comparison to the

195 153 perpendicular component of the dielectric permittivity ( ) for the pure NLC. In nanonematic composite system, the dipole moment of the ZnO nano particle is higher as compared to dielectric moment of NLC molecule [18, 20]. Therefore, the value of the dielectric permittivity has increased for the nano-nematic composite system. In parallel alignment (perpendicular component of dielectric permittivity), the NLC molecules and nano particles are parallel to the surface of electrode. All the molecules (NLC and nano particles) are aligned to the direction of electric field when the electric field is applied on the NLC sample cells. So the dipole moment of nano particle affects the dipole moment of NLC molecule up to a higher extent. In the homeotropic alignment of NLC sample cells, the NLC molecule and nano particle are vertical to the direction of the surface electrode. Therefore, in this geometry the dipole moment of nano particle does not affect the dipole moment of NLC molecule much. Even the dipole moment of homeotropically aligned molecules is affected upto some extent for the mixtures with higher concentration of nano particles in NLC. But this effect is still lower in comparison to the planar alignment of the molecules. The dielectric permittivity of the mixture 2 shows a different trend above 1MHz frequency i.e. it is higher in comparison to the other mixtures and pure sample in this region. This can be explained with the help of three interactions taken place in the doped mixtures. They are as follows: 1- Nano-Nano particle interaction. 2- Nano-NLC molecule interaction. 3- NLC- Surface electrode. Out of these three interactions, in the higher frequency region the surface electrode interaction (i.e. third interaction) play an important role in the anchoring properties of NLC sample cells [26]. This interaction affects the dielectric parameter for all the three mixtures but for mixture 1(i.e. mixture with lowest concentration of dopant) the role of second interaction is higher as compared to third interaction and for mixture 3 first interaction is dominating as the sample has higher number of nanoparticles as compared to first two mixtures. Therefore the effect of this electrode interaction is prominent only for mixture 2 and show higher values of dielectric permittivity for the mixture in MHz region. One could expect that, with the increase in ZnO concentration, and accordingly the increase in molecular number density N, the dielectric permittivity

196 154 will increase linearly, if the other parameters remain unaffected by the doped nano particles. However the increment in the magnitude of the dielectric permittivity ( & ) for mixture 3 is more pronounced in comparison to the increment for the mixture1 and mixture 2. This is due to the fact that ZnO (8% Cu 2+ ) nano particles are semiconducting in nature. The higher concentration of nano particle increases the number of ions produced by the doping. Therefore, the magnitude of dielectric permittivity for mixture 3 i.e. the mixture with highest doping concentration has shown pronounced increment in comparison to the pure NLC and mixture with lower concentration. This is also supported by the fact that the net dipole moment of the system increases for the nano-nematic composite systems, which in turn increases the total polarizability () of the system, given by equation 2 d (4.4) K T 3 B Where is dipole moment, T is temperature, d is distortion polarizability and K B is Boltzmann constant. The dielectric anisotropy () has been plotted with variation of temperature in the figure 4.6, for the pure NLC and its nano-nematic composite system at a frequency of 10 khz. It was found that the magnitude of the dielectric anisotropy for the pure NLC and nano-nematic composite system decreases slightly with the rise in temperature. The decreases in dielectric anisotropy may be attributed to the strong dipolar interaction between the NLC molecule and nano particle and the change in the orientation of the NLC molecules. The dielectric anisotropy arises not only from dielectric effect but also has a contribution from the ions present in NLC. As discussed earlier the third interaction (i.e. surface electrode interaction) has a dominating role on mixture 2, its dielectric anisotropy slightly increases with the rise in temperature because of the change in number of ions present due to electrode interaction. The value of the dielectric anisotropy for nano-nematic composite system has increased in negative order as shown in the figure 5.6. The anisotropic properties of LCs are directly related to the response of LC molecules in the presence of applied electric field. NLC is a uniaxial phase in a macroscopic coordinate system x, y, z, with the z axis parallel to the director. Moreover, the electro-optical properties of NLC composite depend on the size, type, concentration and intrinsic characteristics of the

197 155 nano particles used for doping. It has been observed that the nano particles could significantly disrupt the order of the pure NLC. Thus the low doping concentration (0.5%) is usually more stable in comparison to 1% and1.5% nano-nematic composite system. When a rod shape nano particle is dispersed in the pure NLC, the nano particles try to fit in the sample geometry following intrinsic constraints. Therefore the nano particles disturb the orientational order of the NLC system. In order to analyze the influence of the doped nano particle on the dielectric anisotropy of the nematic phase, one can use the Maier and Meier theory [25, 27]. The dielectric anisotropy according to this theory is given as 2 NhF 2 F 1 S 1 1 3cos 0 3 3K BT 2 S (4.5) 2 NhF 1 F S 1 1 3cos S (4.6) 0 3 3K BT 2 2 NhF F 2 3cos 1 S (4.7) 0 2K BT Where is anisotropy of the molecular polarizability,µ is the resultant dipole moment of the molecules, N is the molecular number density, is dielectric anisotropy, F is parameter depending on the reaction field factor and β is the angle between the molecular axis and the direction of the off-axis. Usually, the compounds with permanent dipole at one end such as CH=N group have a tendency of increasing dielectric anisotropy due to doping because of the increase in the value of β but in some cases such molecules tend to pair up with antiparallel dipole moments, effectively decreasing the dielectric anisotropy. The dielectric anisotropy depends upon the angle and order parameter. The contribution of dipole moment in parallel and perpendicular direction for the NLC making angle with long molecular axis is given by 1 t tan, where µ t and µ l are the transverse and longitudinal l component of dipole moment for NLC molecule. In the present case ZnO nano particle disrupts both the component of the dipole moment and changes the ratio of dipole moments. Figures 4.5(a), 4.5(b) and 4.6 show increase in both the permittivity values and therefore an increase in negative dielectric anisotropy has been observed. It indicates that the value of angle has increased which satisfies the equation 4.5, 4.6

198 156 and 4.7. Thus the doping of ZnO nano particle supports the perpendicular component of dipole moment more than the parallel component. Thus, the value of has increased more than to the value of. The higher the anisotropy, smaller the electric field required to make the pure NLC and nano-nematic composite system to respond. This suggests that the response time should reduce for the doped system. The same trend was found in experimental measurement of response time. Figure 4.6. Temperature dependence of dielectric anisotropy for the pure and nano-nematic composite system at a frequency of 10 Hz. Figure 4.7. Variation of response time for pure nematic and nano-nematic composite systems at 6 volt and 0.5Hz square wave.

199 157 Figure 4.7 shows the variation of response time with temperature for the pure NLC and nano-nematic composite system. The response time decreases with the increases in the temperature for the pure NLC and nano-nematic composite systems as reported for other NLCs by different groups [23, 28]. Above the threshold value (response time), a linear dependence of the response time on the temperature is observed. This is due to the fact that viscosity is maximum at low temperature and acts as a dominating factor for the response time in the liquid crystals. The viscosity decreases with increase in the temperature and thus response time decreases. At higher temperature when viscosity becomes small, the dipole moments of the NLC molecules acts as the governing factor and a linear dependence of response time on the temperature are observed. Thus the results are in agreement with response time and rotational viscosity relation given by (4.8) 2 0E Where E is the applied electric field, γ is the rotational viscosity and is dielectric anisotropy. The inset of figure 4.7 shows the variation of response time with the concentration of the nano particles in the pure NLC. It has been observed that the response time decreases for nano-nematic composite system. The linear relation between response time and concentration has been observed for the sample at 30 0 C temperature as shown in inset of figure 4.7. It is clear that fast response is observed for the higher concentration of nano particles in the pure NLCs. The addition of nano particle provides supportive interaction to the dipole moment of NLC molecules. Therefore, the higher concentration of nano particles causes to decrease the response time of composite system. The mechanics of ZnO nano particle in liquid crystals is based on the orientiational distribution of the dipole moment of nano particle and this distribution is characterized by an orientiational order parameter. The nano particle interacts with the orientiational order of the liquid crystals enhancing the coupling strength to increase the response for liquid crystals and stabilizes the nematic phase. Therefore, the response time of mixture 2 and mixture 3 is generally constant for all temperature. The same trend of the response time has been noticed with the variation of applied voltage as shown in figure 4.8. The value of response time improves with the

200 158 addition of nano particles and gives small values at low applied voltage as compared to the pure NLC. It indicates that the use of nano particles in NLC improves the response of NLC system at low field which is very suitable for low power operating devices. Figure 4.8.Variation of response time with applied field for pure nematic and nano-nematic composite systems at 30 0 C temp and 0.5Hz square wave. Figure 4.9. Variation of threshold voltage with concentration of nano particle. The threshold voltage is another important factor for the NLCs. Figure 4.9 shows the variation of threshold voltage with concentration of nano particles in the pure NLC. The threshold voltage was determined by measuring intensity of

201 159 transmitted light by a given cell as a function of applied voltage. By applying step change ac voltage and then detecting the change in the transmitted light intensity, we have determined the threshold voltage (the voltage, under which the transmitted intensity changed by the 10%). The increase in the threshold voltage strongly depends upon the thickness of the cell and size of nano particle. The threshold voltage has increased with addition of nano particle in the pure NLC because of the increased charge density of the nano-nematic composite system which creates an energy barrier. The doping of ZnO (8% Cu 2+ ) nano particle in the LC layer generates free electron, ZnO and Cu ions, these generated free electrons enter into the LC layer and therefore charge density increases near the interface. This results in higher electric field and thus a larger threshold voltage is required. The magnitude of threshold voltage also depends upon the value of dielectric anisotropy (). The nature of threshold voltage can be seen from equation given below 1/2 V th Δε (4.9) Figure 4.10 and 4.11 shows the variation of splay elastic coefficient and rotational viscosity with concentration of nano particles in the pure NLC. The splay rotational coefficient (K 11 ) and rotational viscosity were determined for the pure NLC and nano-nematic composite systems by using following formula [25, 27, 29]. K 2 Vth 11 (4.10) 0 K d (4.11) Here symbols have their usual meaning. It has been observed that the splay elastic coefficient of nano-nematic composite systems is higher as compared to the pure NLC. The earlier results [29] and above equations clearly shows that the K 11 depends upon the threshold voltage while rotational viscosity (γ) depends upon the K 11 and response time of the system The change in the values of K 11 and γ are due to the interactions taking place between nano particles and LC molecules. The value of K 11 and γ increases with the addition of the nano particle in pure NLC. The presence of nano particles increases charge density in LC layer and causes hindrance for the molecular movements due to imposing additional constraints on the LC molecules. Therefore rotational viscosity increases for nano-nemtaic composite system. The

202 160 rotational viscosity has increased for nano-nematic composite system as compared to the pure NLC due to the increasing order in negative sign of dielectric anisotropy. Figure Variation of splay elastic coefficient with variation of concentration of nano particle. Figure Variation of rotational viscosity with concentration of nano particle CONCLUSION The present chapter demonstrates the enhancement in the dielectric and electro-optical parameters by doping of nano particles in the pure NLC, while preserving the other properties of the pure NLC. The dielectric anisotropy of the ZnO (8% Cu 2+ ) nano particle doped NLC has been increased in negative order. The increment in negative dielectric anisotropy is reflected by the increase in the value of threshold voltage. It has been observed by the determination of splay elastic constant and rotational viscosity that the rotational viscosity is higher for the nano- nematic composite system, which is dependent on the charge density of LC layer. The elastic

203 161 constant has been found to increase due to the perturbation produced in the LC order by the addition of nano particles in the presence of electric field. Thus, response time reduces with the addition of nano particles in NLC and shows an improved switching behavior.

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206 Chapter 5 Abnormal Switching Behavior of Liquid Crystal Composite

207 Chapter 5 ABNORMAL SWITCHING BEHAVIOR OF LIQUID CRYSTAL COMPOSITE 5.1. Introduction 5.2. Experimental details 5.3. Results and discussion 5.4. Conclusion References Phase Transitions

208 INTRODUCTION Liquid crystals (LCs) are a mesostate between solids and liquids, it share the anisotropic properties of optical (uniaxial and biaxial) crystals and the fluid properties of isotropic liquids [1, 2]. These materials are extremely sensitive to the small external factors (electric and magnetic fields, surface effects, temperature, etc.) and possess order and mobility at microscopic and macroscopic levels [3]. The thermotropic LCs are technologically most important among all the LC mesophases and nematic mesophase is one of them which have broad applications in many engineering devices. Another important application of nematic LC is their utilization in holographically formed polymer-dispersed LCs. These switchable diffraction grating have broad engineering applications: video displays, switchable focus lenses, photonic time-delay generators for optically assisted phased-array radar [3-6]. The remarkable advances in LC technology have led to the appearance of LC based spatial light modulators (SLMs) and display applications [7]. The correct performance of all the above mentioned devices require LC materials that are stable over a long period of time having high thermal stability and thermal range, high dielectric and optical anisotropy, and low switching voltage and switching times. These various requirements are achieved by using chemical synthesis and carefully designed mixtures of different LC materials such as dye, polymer and nanoparticle [8, 9]. It is important to note that in the optimizing one property usually result in changes in other properties, mostly in an undesirable direction. Therefore, designing right material for commercial applications is a challenging task. During the past four decades the doping of nanomaterials in liquid crystals has been extensively studied [9] and this area of research is now completely mature and can be used for device fabrication. The interpretation of measured properties of LCs and the proper understanding of LCs device depends on the knowledge of the geometry of LC system and anchoring energy. The doping of BaTiO3 Nanoparticles, Silica nanoparticles and cadmium telluride quantum dots has been used in recent time for application in high quality performances of the devices [10-13]. Most of the groups have presented results of doping in the ferroelectric liquid crystals (FLCs) especially for display applications [14-15]. They observed enhancement of the E-O properties is attributed to the variation of the order parameter and dielectric studies of BaTiO3 nanoparticles dispersed in a FLC matrix [14]. Gold nanoparticles change the

209 165 elastic parameter and rotational viscosity of the FLCs. Gold nanoparticles doping creates a strong intrinsic field inside the sample generating high tilt and a reproducible observation of memory effect [16]. Each type of these nanoparticles has its own effect on alternation of the LC materials properties. It has been established that the impurity ions in LC systems strongly impact the device performances reducing the liquid crystal display quality particularly at higher temperature. Therefore in the present chapter we have observed the change in the nature of dielectric anisotropy with changing the electro-optical parameters. We have used ZnO nanoparticles, which provided best alignment of liquid crystals molecules in our cells. The switching behavior of LC molecules have affected due to presences of nanoparticle in pure LC. The dielectric and electro-optical parameters have been evaluated for pure and nanoparticle doped LC. The temperature dependence of the different dielectric and electro-optical parameters such as dielectric anisotropy, threshold voltage, splay elastic constant and rotational viscosity have also been discussed with the help of different theories of LC EXPERIMENTAL DETAILS Materials The investigated LC used in the present study is p-butoxybenzylidene, p- heptylaniline (BBHA). Phase sequence of BBHA is Smectic C - Nematic C Isotropic C. The chemical structure of the BBHA is shown in the figure 5.1. The use of nematic LC for photonic applications is still of great interest, and so it can be electrically modulated within this wide working temperature range. Figure 5.1. Molecular structure of liquid crystal BBHA. The Cu 2+ doped ZnO nanoparticles have been used in the present study; ZnO (wide band gap) is one of the most important materials for blue and ultra-violet

210 166 optical device applications. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O 2- and Zn 2+ ions, heaped alternatively along the c-axis. The tetrahedral coordination in ZnO leads to a non-central symmetric structure, which is one of the most important structural characteristics of wurtzite nano-structured materials. ZnO shows strong electromechanical coupling due to its unique structure, which results in strong piezoelectric and pyroelectric properties. The other important structural characteristic of ZnO is its polar surfaces. ZnO nanoparticle has a normal dipole moment and spontaneous polarization along the C-axis [17]. The synthesis of this copper doped ZnO nanoparticle has been given by our synthesis group P. K. Sharma et al [17]. Figure 5.2 shows the X-ray diffraction (XRD) pattern for nanoparticle used for doping in the LC. The spectra shows broad peaks at the positions of 31.63, 34.50, 36.25º, 47.50, 56.60, 62.80, 66.36, and 68.91, as also clear from the figure 2. The values are in good agreement with the standard JCPDS file for ZnO (JCPDS number , a = b = Å, c = Å) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P6 3mc [17]. The average particle size of ZnO:Cu 2+ nanoparticle was~13 nm estimated by the Debye-Scherer s equation [17,18]. The uniform nano rod shape particle having diameter 12~15nm and length 40~80 nm were observed from Scanning Electron Microscopic (SEM) Images, shown in figure 5.3 [17]. Figure 5.2. X-ray diffraction spectra of ZnO:Cu 2+ ( 8% Cu 2+ doped ZnO).

211 167 Figure 5.3. SEM images of ZnO:Cu 2+ (8% Cu 2+ doped ZnO) nanoparticles. Preparation for sample cell The planar as well as homeotropic sample cell have been used in the present study. The sandwiched type (capacitor) cells was made by using two optically plane glass substrates coated with conducting indium tin oxide (ITO) layers. Planar alignment has been achieved by treating conducting layers with adhesion promoter and coated with polymer nylon (6/6). After drying the polymer layers both the substrates were rubbed unidirectionally by velvet cloth [18-22]. The substrates were then placed one over another to form a capacitor. The cell thickness was fixed by placing a Mylar spacer (2.5µm in our case) in between the two substrates and then it is sealed by a UV sealant. Similarly for homeotropic alignment, the glass substrates were coated with dilute solution of lecithin (Cetyl trimethyl ammonium Bromide). The substrates have been dried at C for 10 hour before assembling the cell. Sample cells were calibrated using analytical reagent (AR) grade carbon tetra chloride (CCl 4 ) and benzene (C 6 H 6 ) as standard reference for dielectric study. We prepared two mixtures of different concentration of nanoparticle, i e, 0.5% and 1.0% wt/wt in liquid crystal. The nanoparticle dispersed in pure liquid crystals physically. Each mixture was heated at room temperature from smectic to isotropic phase and cooled back to room temperature. Then the heating- cooling cycle was repeated. The mixture was viewed under polarizing microscope to assure homogeneous distribution of the nanoparticle. The assembled cells were filled with the suspension and the pure NLC at temperature higher than the isotropic temperature of NLC by capillary method [20-22]. Nanoparticle composite system is prepared by dispersion of 8% copper doped Zinc oxide nanoparticles (8W) in pure BBHA liquid

212 168 crystal. These samples are denoted as mixture 1 and mixture 2 in the discussion of the present chapter. Dielectric Measurement The dielectric measurements have been carried out by using a computer controlled impedance/gain phase analyzer (HP4194A) in the frequency range 100Hz to 10MHz. The measurements in the high frequency range have been limited to 10MHz because of the dominating effect of the finite sheet resistance of ITO coating on the glass plates and the lead inductance [21-23]. The temperature has been maintained by using a computer controlled hot plate (Instec Corporation USA). Experiments were performed by ramping the temperature at a very slow heating rate of C min -1 and temperature stability was better than ±0.1 0 C [23]. Electro-optical measurement for the Rise and fall time An arbitrary A.C signal voltage (20V pp &1 Hz) was applied to the cells using function generator. He-Ne Laser beam with a wavelength 632 nm as the input signal is detected by new focus photo-detector (Instec-PD02LI) connected directly to a digital storage oscilloscope (Tektronix TDS-2024C). From the detected shapes of the waveform in figure 5.4, we calculated the rise time ( on ) and fall time ( off ) for pure LC and nanoparticle doped LC. Here, on is the time required for the transmittance to rise from 10% to 90% and off is the time required for the transmittance to fall from 90% to 10% [7, 23]. Figure 5.4. (a) Applied arbitrary wave and (b) detected shapes of the waveform in oscilloscope.

213 169 The cell is set at angle 45 0 crossed polarizer and analyzer for ensuring maximum optical transmittance. Thus the cell works as a phase retarder thereby altering the polarization of light. When voltage is applied, the LC molecules are aligned in applied field direction. Again when the voltage is switched off the LC molecules relax and return to their initial state RESULTS AND DISCUSSION Dielectric anisotropy has been calculated by using dielectric data of planar as well as hometropic alignments. The value of dielectric permittivity for both the alignments has been required correction due to sheet inductances and lead inductances of the cell [20-22]. By least square fitting we have evaluated corrected dielectric permittivity for both alignment and then dielectric anisotropy was calculated by [7] using = -. The values and represent the dielectric permittivity for the applied electric field E parallel and perpendicular to the macroscopic molecular orientation n, respectively. The extracted dielectric anisotropy for pure liquid crystals and nanoparticle composite system is plotted as a function of temperature in figure 5.5. The value of dielectric anisotropy is enhanced for nanoparticle composite system. The dielectric anisotropy is generally constant for one phase (smectic) and suddenly decreases for other phase (nematic) for pure liquid crystals [24, 25]. Figure 5.5. Dielectric anisotropy of pure liquid crystal and ZnO nanoparticle doped composite system at different temperatures.

214 170 Figure 5.5 shows the little change in magnitude of dielectric anisotropy after doping of nanoparticle in the pure LC. It can be seen that the value of dielectric anisotropy increases with the increase in the temperature for pure liquid crystals in negative order, while its value is in positive order for nanoparticle composite system. We consider that the pure liquid crystal molecule exhibits a continuous symmetry breaking phase transition on varying a control parameter. The dielectric anisotropy was found to increase with increasing concentration of nanoparticle in pure liquid crystal. The dielectric anisotropy (Δε) increases significantly for pure liquid crystal in negative order, but ultimately changes its sign for nanoparticle composite system. The dielectric anisotropy (Δε) of liquid crystal are mainly depends upon change of effective dipole moment of liquid crystal molecule [26]. The change in molecular polarizability of nanoparticle composite system will also change the dielectric anisotropy according to this equation 2 NhF F 2 3cos 1 S (5.1) 0 2K BT Here N is the number density, h and F are the internal field factors, ε 0 is the permittivity of vacuum, is the dielectric anisotropy & Δα is the anisotropy of the polarizability, S is the orientational order parameter, K B T is the thermal energy, and β is the angle between the molecular net permanent dipole moment and the long molecular axis of the molecule. The resultant dipole moment of the liquid crystal molecule attained a new orientation with respect to the long molecular axis. Therefore it can be concluded that the 8W nanoparticle significantly influenced the geometrical orientation of the used BBHA molecule in suspension. The contribution of the electronic polarizability to dielectric permittivity is greater in the direction along the molecular long axis than perpendicular to it. Consequently is positive, as there are additional contributions from dipolar relaxation because the ZnO ananoparticle with diameter 12 nm possess dipole moment >100 D which is much larger than that of a LC molecule. This large value of dipole moment on ZnO nanoparticle interacts strongly with dipolar species present in the LC mixtures. This dipolar interaction enhances the anchoring and hence the

215 171 ordering of LC molecules which surround the ZnO nanoparticle. For composite system the dipole moment of ZnO nanoparticle, which contributes slightly more in the parallel case than in perpendicular component. For nanoparticle composite system a reversal in the sign of the dielectric anisotropy has been seen both in nematic and smectic A (SmA) phase [24]. The change in sign is caused by a decrease in the value of with increasing the concentration of nanoparticle in liquid crystal on entering the smectic and nematic phase, whereas increases anomalously. The decrease of in nanoparticle system results from the smaller distance between the nanoparticle and LC molecules, leading to an increased antiparallel correlation between the components of the dipole moments along the molecular long axis. Consequently, the effective dipole moment in this direction is reduced, causing a decrease in. The absolute value of the static dielectric permittivity for liquid crystal with a weak dipole moment is much smaller than for ZnO nanoparticle with a strong dipole moment. The electro optical properties have been studied on planar alignment for pure liquid crystals and nanoparticle composite system. The alignment layer greatly affects the electro-optical (EO) properties of the liquid crystals [27]. The more confined and uniform alignment quality leads to a better liquid crystal device performance. It is known that at applied external field, EO response of LC is related to the surface anchoring energy which depends on the boundary surface energy [7, 27]. The alignment of the LC molecule director is determined by the competition among the surface interactions, bulk interactions, visco-elastic properties and the externally applied electric field. As the surface anchoring strength decreases the LC response time also decreases; the EO switching becomes faster [7]. The threshold voltage V th of the LC is given by the following equation [7, 28]. K 11 V th (5.2) 0 Here K 11 is the splay elastic constant. The response time from OFF state to ON state is given by on and from ON state to OFF state is given by off given respectively by following equations [7, 28]. on 2.d 2 V V (5.3) 0 2 th

216 172.d 2 off 2 (5.4) K11 Here d is cell thickness and is the rotational viscosity. All these parameter are proportional to the order parameter of LC. From this equation the rise time depends on both applied voltage and threshold voltage. However, the threshold voltage depends on the LC dielectric anisotropy, which is dependent on the applied frequency. Thus the rise time depends on the applied frequency as well. Therefore, for a cell of fixed thickness and specific LC, the rise time (or the switch on time) depends mainly on driving frequency and applied field strengths. The figure 5.6 shows variation of the rise time with temperature in nematic phase for the pure LC and nanoparticle composite systems. Figure 5.6. Variation of the rise time with temperature for pure LC and nanoparticle composite systems. The figure indicates that from 60 0 C to 71 0 C the rise time is constant for pure LC, however the rise time for mixture 1 and mixture 2 are decreases in this temperature range. The rise time decreases from 72 0 C to 83 0 C temperature for all systems. It is also clear that the rise time basically depends upon the viscosity and order parameter. The viscosity and order parameter always decreases with increase in temperature. The rise time is greater for nanoparticle composite system as compared to the pure LC. The rise time of mixture 2 is less than mixture 1 but its value is higher

217 173 as compared to the pure LC. The reason is that the nanoparticle produces the maximum viscosity and interaction of surface boundary. So the rise time increases for concentration of nanoparticle in LC. The other reason is that the interaction of nanoparticle and LC molecule is weak, but interaction of LC molecule and surface electrode is strong. So the anchoring strength is strong for mixture 1. While the interaction of nanoparticle and LC molecule is strong as compared to interaction of LC molecule and surface electrode for mixture 2. Therefore, anchoring strength of mixture 2 is weak as compared to mixture 1. Since the response of LC molecule is depends upon the anchoring strength of the sample cell. The response of LC molecule depends upon the surface anchoring of the alignment layer. But nanoparticle also increases the response time. Therefore, the doping of such nanoparticle in this liquid crystal is not perfect requirement in the application of display technology. If any how we can develop a process to decrease the rise time of LC sample, then this process should be useful for display devices. The figure 5.7 shows the temperature variation of fall time in nematic phase for the pure LC and nanoparticle composite systems. Figure 5.7. Variation of fall time with temperature for pure LC and nanoparticle composite systems. Generally the fall time does not depend upon the applied field but it depends upon the elastic constant and rotational viscosity. If rise time decreases then fall time increases with variation of temperature [7]. The fall time of mixture 1 and mixture 2 are larger as compared to the pure LC, because the nanoparticle opposes to the LC molecules to reach their initial position. Therefore the LC molecule could not easily

218 174 attain original position. The rise time, fall time, rotational viscosity, splay elastic coefficient, saturated voltage, threshold voltage and dielectric anisotropy have evaluated experimentally as given in table 6.1 at 72 0 C temperature. Threshold voltage increases for mixture 1 and it decreases for mixture 2. Theoretically the threshold voltage depends upon the magnitude of dielectric anisotropy as shown in equation 5.1. Sample 72 0 C temp Dielectr ic Anisotr opy () Thickn ess (d) Thresho ld voltage (V th ) in volt Saturati on Voltage (V sat ) in volt Rise Time ( on ) in milliseco nd Fall Time ( off ) in milliseco nd Splay Elastic Coefficie nt (K 11 ) in order of magnitu de Pure µm Mixture 1 Mixture µm µm Rotation al Viscosity () in poise Table 5.1. Experimental value of the rise time, fall time, rotational viscosity, splay elastic coefficient, saturated voltage, threshold voltage and dielectric anisotropy for pure LC and nanoparticle composite systems at 72 0 C temperature. The rise time and fall time also have measured as a function of the applied voltage. Figure 5.8(a) and 5.8(b) shows the applied field strength effects for the pure LC cell and nanoparticle doped cells at a frequency 1Hz. The field strength increased from 5 volt to 10 volt. The rise time strongly depends on the applied voltage according to equation (5.3). When the applied voltage is just above the threshold voltage for mixture 1, mixture 2 and pure LC, the rise time is comparatively higher. However, the rise time is reduced effectively just by increasing the applied voltage. The decrement in the rise times found in this study are ms, ms and 10.6 ms for mixture 1, mixture 2 and pure LC respectively as shown in dotted line in figure. At below to 7 volt, the rise time is very large for mixture 2 in comparison to the pure LC and mixture 1.

219 175 Figure 5.8. (a) Variation of rise time with applied voltage for pure LC and nanoparticle composite systems. (b) Variation of fall time with applied voltage for pure LC and nanoparticle composite systems. The threshold voltage is minimum for mixture 2 as compared to mixture 1 and pure LC. In this system the maximum rise time achieved due to presence of nanoparticle, while some LC molecules respond at this voltage. But higher concentration of nanoparticle disrupts the alignment of LC molecules. Other reason is that the surface anchoring of the alignment layer produced due to interaction of nanoparticle between the substrates. The threshold voltage behavior exists, when the pretilt angle is zero. However, in most LC device a non-zero pretilt angle is required in order to avoid domain formation during molecular reorientation. In nanoparticle composite system, some LC molecules also tilted due to presence of nanoparticle. In addition to this the nanoparticle are also generates multi domain in the bulk LC cells. From 7 volt to 10 volt the rise time have small dependence for mixture 1, mixture 2 and pure LC. Above the 7 volt, the nanoparticle aligns in the direction of electric field and do not easily reach to own position. Therefore, rise time is reduced for nanoparticle doped systems. Figure 5.8(b) shows the fall time with variation of applied electric field (voltage) for pure LC and nanoparticle doped system. The fall time increases with applied field for mixture 1 and suddenly increased from 6 volt to 8 volt and it little change for pure LC. This suggested that the rise time are minimum at 8 volt and 9 volt, it means the LC molecules show very fast response at that condition and molecules does not immediately attain to initial position. The fall time decreases with rise voltage 5 volt to 7 volt for mixture 2 and after that the fall time saturates with increasing voltage from 7 volt to 10 volt. At this applied field the nanoparticle

220 176 opposes the response of LC molecule. They do not come to initial position. Above the 8 volt the fall time of mixture 2 saturates with applied field. In this condition the nanoparticle fully aligned to the direction of applied electric field and only L C molecules are responses to it. The response of LC molecule is related to rotational viscosity. The rotational viscosity have also discussed in last paragraph. The connectivity of the liquid-crystal cavity network has some effects on the switching times (rise & fall time). In conventional nanoparticle composite system, fall times typically exceed rise times, since relaxation is driven only by elastic energy, with no electric field. The fall time is proportional characteristic size of nanoparticle and liquid crystal molecule. This model predicts that isolated, spherical liquid-crystal droplets should have anomalously long fall times [29-31]. Figure 5.8(a) and 5.8(b) provides a comparison of rise and fall times for various applied electric fields for both the pure liquid crystal and nanoparticle composite system. For the nanoparticle composite system, the shorter rise vs fall times agree with expectations. In a continuum of this study, we have also determined the rotational viscosity and splay elastic constant for pure LC and nanoparticle composite system. The splay elastic constant is obtained from the Frederick s threshold voltage with the help of the equation (1) [32]. Figure 5.9 and figure 5.10 show the temperature behavior of rotational viscosity and splay elastic constant for the pure LC and nanoparticle composite system [33, 34]. One can conclude from the figure that they would have both changed in the similar way with respect to the nanoparticle concentration and this similar change is proportional to order parameter. Splay elastic constant remains constant for all samples. The value of splay elastic constant of nanoparticle composite system have found to less as compared to the pure LC. The splay elastic constant basically depends upon threshold voltage and dielectric anisotropy. The magnitude of dielectric anisotropy of pure LC is high as compared to the nanoparticle composite system. In addition to this the value of splay elastic constant of mixture 2 is less in comparison to the mixture 1.

221 177 Figure 5.9. Temperature behaviour of rotational viscosity for the pure LC and nanoparticle composite system. Figure Temperature behavior of splay elastic constant for the pure LC and nanoparticle composite system. The rotational viscosity of the pure LC and nanoparticle composite system has been determined with the help of the equation (5.3). Rotational viscosity of aligned liquid crystals represents an internal friction among LC directors during the rotation process. The magnitude of rotational viscosity depends on the detailed molecular constituents, structures, intermolecular associations and temperature. As temperature increases rotational viscosity decreases rapidly. Rotational viscosity is an important parameter for many electro-optical applications employing liquid crystals, because the response time of the LC is linearly proportional to the rotational viscosity.

222 178 According to the molecular theory developed by Osipov and Terentjiv [7, 35] the rotational viscosity of mixture 1 is larger as compared to the pure LC. It is also clear that the molecular association between nanoparticle and LC molecule is strong as compared to LC-LC molecule. But for mixture 2 the intermolecular association of nanoparticle is very strong as compared to nanoparticle-lc molecule. In higher concentration of nanoparticle in LC, the nanoparticle is more effective as compared to LC molecule, so the interaction of nanoparticle-nanoparticle is very strong for mixture 2. Therefore LC with weak intermolecular association reduces the rotational viscosity significantly. The another reason is that the relaxation time is proportional to the viscosity, size of nanoparticle and splay elastic constant in liquid crystals cell. The splay elastic constant decreases with the addition of nanoparticle in pure LC. The reason is that the number of nanoparticle increases in LC system, thus the splay elastic constant is decreased for nanoparticle composite system. According to the mean field theory [7, 28], the splay elastic constant is depends upon the number and size of the molecules. C S K11 (5.5) V C n 1 3 3A L 2 2 m (5.6) 1 Where C 11 is called the reduced splay elastic constant, V n is the mole volume, L is length of molecule, m is the number of molecule, A is constant and S is order parameter CONCLUSION Nanoparticle induced electro-optical parameters and dielectric anisotropy has been demonstrated in the liquid crystals. The dielectric anisotropy has been found to be of positive order for nanoparticle composite system and negative order for pure liquid crystals. In addition to this the rise time and fall time has also been measured for both pure and nanoparticle composite system with variation of voltage and temperature. Rise time has increased for nanoparticle liquid crystal as compared to the pure liquid crystal. The electro-optical performance of these doped systems is not good for application point of view. The threshold voltage and saturation voltage have

223 179 decreased for nanoparticle composite system. The dielectric anisotropy increases with an increase in concentration of the nanoparticle doped in liquid crystal indicating that nanoparticle changes the order parameter of rod like liquid crystal molecule. The splay elastic constant remains constant to mixture 2 with variation of temperature, whereas it increases significantly with increasing temperature of mixture 1 and pure LC suggesting the coupling of the liquid crystal due to distortion of nanoparticle.

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226 Chapter 6 Changes in Material Parameters for Dye Doped Ferroelectric Liquid Crystal

227 Chapter 6 Changes in Material Parameters for Dye Doped Ferroelectric Liquid Crystal 6.1. Introduction 6.2. Experimental details 6.3. Results and discussion 6.4. Conclusion References Phase Transitions, 86, 10 (2012)

228 INTRODUCTION Liquid crystal systems have been studied in the last few years because of their wide spectrum of applications to the modern devices [1-4]. Ferroelectric liquid crystals (FLCs) are exciting class of materials having various applications in display and other electro optic devices because of their low operating voltage and fast response time [5]. There are many applications in which material scientists have been exploring role of novel Guest host materials such as turn off switching, multi purposes Liquid Crystal Displays (LCDs) with switchable viewing angle and reflective color displays [6,7]. To optimize these properties of FLC materials, it usually requires knowledge of material parameters for FLC material such as dielectric permittivity, relaxation time, spontaneous polarization and response time. Response characteristics of any display are mainly determined by relaxation process of the FLC molecules [8]. Rotational viscosity is also one of the important factors for designing of fast response FLC devices [9-12]. To improve the various FLC material parameters which are crucial for their use in display devices and other useful applications, efforts have been made by different research groups [13, 15]. These parameters are contrast ratio, vision angle, spontaneous polarization etc. Introduction of different non-mesogenic molecules like dyes and nanoparticles can improvise practically important material parameters of FLC systems. One of the approaches to modify the material parameters of the FLCs is doping of anthraquinone dye into FLC material [16-19]. Various researchers have reported changes in the different parameters of FLCs due to the addition of dye molecules [12, 14]. The study of dye doped systems is also important to understand the molecular dynamics of such systems and how the presence of dye molecules affects the molecular properties of the pure FLC. The present chapter reports the effect of anthraquinone dye on some dielectric and electro-optical parameters of pure and dye doped FLC [14, 16-19]. The different dielectric and electro-optical parameters viz. dielectric permittivity, relaxation time, spontaneous polarization and response time have been measured for both pure and dye doped FLC and behaviour of these parameters with variation in temperature have also been discussed. All the properties are found to show strong dye concentration dependence [17-20]. The results indicate that concentration of dye in liquid crystals also play important roles in improvising different material parameters [16]. Present

229 183 chapter also provides the broad information about the modifications taking place in the intermolecular interaction field and molecular geometry of pure FLC matrix due to the addition of dye molecules and how it changes with the change in the concentration of dye. The rotational viscosity has been determined by measuring spontaneous polarization and response time [11, 12]. In addition to that we have also compared relaxation time and response time for both pure and dye doped FLC material EXPERIMENTAL DETAILS The sample used for this study is Felix purchased from Clariant Chemicals Co. Ltd with the phase sequence of Cr- SmC* -SmA - N*- Iso at C, 74 0 C, 82 0 C and 95 0 C respectively. The value of Spontaneous polarization at 25 0 C is 5.9 nc/cm 2 and value of rotational viscosity at 25 0 C is 82 mpoise. The anthraquinone dye used for doping is D5 with structure as shown in figure 6.1. Three mixtures of anthraquinone dye with 1%, 2% and 3% wt./wt concentration have been prepared by dispersion of anthraquinone dye [17, 19] into the pure FLC. We named these mixtures as mixture 1, mixture 2 and mixture 3 respectively for the whole discussion. Figure 6.1. Molecular structure of Anthraquinone dye D5 used for present study. For dielectric and electro-optical study, we used sandwiched type sample holder of ITO coated glass plates having cell thickness of 7.5 m. The cells were prepared by using ITO (Indium Tin Oxide) coated glass plates (sheet resistance is 10 Ohm/mm 2 and the visible light transmission is more than 90 %) having active area 25 mm 2, as electrodes. Both the electrodes were treated with adhesion promoter and polymer Nylon 6/6 and then rubbed unidirectionaly for planar alignment. Cell

230 184 thickness was maintained by means of a Mylar spacer. The cells were calibrated using AR grade CCl4 and benzene. The material was introduced into the cell by capillary action at a temperature slightly higher than its isotropic temperature. A well-aligned cell was obtained by applying an electric field in the slow cooling cycle from the isotropic phase to room temperature. The macroscopic orientation for all the mixtures was checked by placing sample cell between two-crossed polarizers of polarizing microscope CENSICO The details regarding preparation of the sample cell has been given in chapter 3. The dielectric data have been obtained by measuring capacitance and dissipation factor with the help of an impedance/gain phase analyzer HP-4194A. The complete experimental measurement technique has been given in chapter 3. Spontaneous polarization has been measured by standard polarization reversal current method, while response time has been determined by using optical switching method [19, 22]. Rotational viscosity has been determined by spontaneous polarization data and response time using following relation [19,20] t ep (6.1) P E S Here t, ep are optical response time and electro-optical rotational viscosity of the sample respectively and E is applied electric field. The contrast ratio measurement has been performed by applying a square wave to the sample cell placed between the crossed polarizers. The experimental setup is shown in figure 6.2 [20]. The corresponding maximum and minimum intensity have been recorded by light dependent resistance (LDR) and storage oscilloscope HM 407. Tilt angle has been measured by well known pulse electro-optical techniques proposed by Baikalov et.al [22].

231 185 Figure 6.2. The schematic diagram of the apparatus for measurement of contrast ratio RESULTS AND DISCUSSION Figure 6.3 shows variation of effective dielectric permittivity with temperature. Here term effective is used for the dielectric permittivity for which we have removed contribution of sheet resistance, ITO coatings and lead inductance of the cell. For this purpose we require correction in the Cole-Cole equation [20-23]. The Cole-Cole equation is given δε' ε* ε'( ) (6.2) 1α 1 (i2πfτ ) Where ε is the dielectric strength and ε() is high frequency limit of the dielectric permittivity, f is frequency and is the relaxation time. After adding correction terms due to ionic conductivity and electrode polarization problem in low frequency region and ITO sheet resistance and lead inductance problem in high frequency region data, the real and imaginary parts of the above equation become as follows (1 ) ' '[1 (2f ) sin( n '(dc)f '( ) 2(1 ) (1 1 (2f ) 2(2f ) ) / 2)] sin( / 2) (6.3) and 0 (1 ) (dc) ' (2f ) cos( / 2) " Af k 2(1 ) (1 ) 2f 1 (2f ) 2(2f ) sin( / 2)) m (6.4)

232 186 Here (dc) is the dc ionic conductance, o is free space permittivity and f is the frequency while n, m and k are the fitting parameters. The terms ı (dc)f n and (dc)/o2f k are added in (6.3) & (6.4) for additional contribution due to the electrode polarization and ionic conductance in the low frequency effect [16,19, 20]. The Af m term is added in (6.4) in the high frequency effect due to the contribution of ITO sheet resistance and lead inductance of sample cell. By the least square fitting of experimental data we remove these contributions and we got effective dielectric permittivity. Figure 6.3. Variation of effective dielectric permittivity with temperature for all the mixtures. It is clear from the figure that the value of effective dielectric permittivity is reduced very much after the doping of anthraquinone dye, while the nature of variation of dielectric permittivity with temperature is same for dye doped system as compared to the pure FLC. This is due to the reason that when dye is doped into the pure FLC, dye molecules are free to move into the pure FLC system. The movements of dye molecules in pure FLC are in such a way that the dye molecules tend to arrange themselves with their long molecular axis aligning along the FLC director [16, 19]. As the concentration of dye is increased the value of effective dielectric permittivity is reduced to a lower value in comparison to the dielectric permittivity of the pure FLC suggesting dipole moment of dye and FLC molecules are counteracting. Figure 6.4 shows the relaxation time at different temperatures in SmC* phase for the pure and dye doped FLCs. The relaxation phenomenon of FLC with a planar alignment is normally due to the collective dielectric processes such as the Goldstone

233 187 mode and soft mode [11, 12, 16, 20]. In the present study only Goldstone mode has been observed, while soft mode could not detected because of low spontaneous polarization of FLC. The relation between relaxation time and relaxation frequency is given as 1 (6.5) (2Relaxation Frequency) Figure 6.4. Temperature dependence of relaxation time for all the mixtures. The relaxation band of the Goldstone mode shifts towards the lower frequency side for the dye doped system in comparison to the pure FLC. The mixture with lower concentration of dye i.e. mixture 1 shows the relaxation band shift towards low frequency range as compared to pure FLC. Mixture 2 and mixture 3 (sample with higher concentration of dye) show shift towards the higher frequency region as compared to mixture 1 but when these are compared with pure FLC the relaxation band is shifted to the lower frequency region. The nature of variation of relaxation frequency with temperature is almost similar for pure and dye doped system but the value of relaxation frequency for all the dye doped mixtures shifts in lower side as compared to the pure FLC. The higher value of relaxation time in dye doped mixtures means that molecular motion of FLC molecules is hindered by the presence of dye

234 188 molecules [14, 16]. Addition of dye in pure LC system offers some new constraints due to new interactions taking place. There are 1. Interaction between dye and FLC molecules. 2. Interaction between the dye dye molecules. It appears that for mixture 1 interaction of dye molecule with FLC molecules dominate over the interaction of dye-dye molecules but as concentration of dye is increased i.e. for mixture 2 and mixture 3 interactions between dye-dye molecules also become prominent. This indicates that different behavior of relaxation time for the mixture 1 as well as mixture 2 and mixture 3. This typical behaviour of relaxation time can be explained with the help of the following equation [20] K q (6.6) d 2 3 Here represents relaxation time; K 3 is twist elastic constant, q is the wave vector of the helical pitch and d is the dielectric rotational viscosity of mixture. It is clear from the equation (6.6) that the relaxation time is directly proportional to the rotational viscosity of the system. Addition of dye into pure FLC alters the rotational viscosity of the system and result in the change in relaxation time. The nature of rotational viscosity with temperature and concentration of dye is discussed in the later part of the chapter. Figure 6.5 shows temperature variation of spontaneous polarization for the pure and dye doped FLC mixtures. From figure it is clear that magnitude of spontaneous polarization is continuously decreasing and it vanishes at SmC* -SmA phase transition point or near this phase transition point for dye doped as well as for pure FLC system [19,20]. The magnitude of spontaneous polarization is continuously decreasing with the increase in the concentration of dye. It is also support by dielectric permittivity behavior.

235 189 Figure 6.5. Temperature dependence of spontaneous polarization for all the mixtures. The effect of dye on FLC response time has been obtained by determining the values of response time are presented in figure 6.6 with variation of temperature. The magnitude of response time is increasing with increment in the concentration of dye. The higher values of response time in the dye doped system suggest there is no improvement in response time after dye doping as the dye restricts the molecular switching. The behaviour of relaxation time and response time are just opposite for mixture 2 and mixture 3 while it is same for pure FLC and mixture 1. The response time is time taken to reach intensity from 10% to 90% of its maximum value in which optical response of molecule is involved while effect of electric field giving rise to relaxation time. Figure 6.6. Variation of response time with temperature for all the mixtures.

236 190 The electro-optical rotational viscosity is plotted with variation of the temperature in the figure 6.7, while inset shows concentration dependence of electrooptical rotational viscosity. The temperature dependence of electro-optical rotational viscosity is as similar as reported by our group [12, 18] but the magnitude of electrooptical rotational viscosity increase with the increase in the concentration of the dye. This type of behaviour of rotational viscosity is due to the combined effect of spontaneous polarization and response time also indicated by the equation (6.1). As we know rotational viscosity is related to the rotation of molecules and higher value of response time suggest that the present dye offers more hindrance of FLC molecule for the dye doped FLC system. Therefore, magnitude of electro-optical rotational viscosity increases after dye doping. This is also clear from the equation (6.1) that rotational viscosity is directly proportional to response time and spontaneous polarization. Thus any change in these parameters cause changes in the magnitude of rotational viscosity. Therefore, magnitude of rotational viscosity depends upon combined effect of these material parameters [18]. The net effect of these parameters i.e. response time and spontaneous polarization is increment in rotational viscosity because of the small decrement in the magnitude of spontaneous polarization and increment in the magnitude of response time. Figure 6.7. Variation of electro-optical rotational viscosity with temperature for all the mixtures while inset shows variation of rotational viscosity with concentration of dye at 35 0 C.

237 191 The behavior of electro-optical rotational viscosity has also been confirmed by the nature of rotational viscosity as determined by dielectric parameters. The behavior of both the rotational viscosities as measured by electro-optical and dielectric method is same but their magnitude is slightly different [12, 18]. The magnitude of dielectric rotational viscosity is given as 2 PS d (6.7) Here, P S is spontaneous polarization, γ d is dielectric rotational viscosity whereas represents tilt angle. The magnitude of dielectric rotational viscosity is directly proportional to (P S /θ) term. Therefore value of this term has been evaluated for pure and dye doped system at constant temperature and presented in table 6.1. The magnitude of (P S /θ) term increase with increase in concentration of dye, hence magnitude of dielectric rotational viscosity increases. The increment of (P S /θ ) term in the dye doped system implies that the amplitude of tilt is decreasing at a much faster rate than spontaneous polarization which is due to the appearance of tilt distribution within smectic layer for the dye doped systems as compared to the pure FLC. Sample P S /θ ( x 10-9 ) (Coulomb per degree/cm 2 ) Tilt Angle Contrast ratio Pure Mixture Mixture Mixture Table 6.1. Variation of different material parameters with concentration of dye at constant temperature of 35 0 C. The tilt angle could also vary due to the presence of dye molecules in the pure FLC system and depends upon the concentration of dye. Table 6.1 present the value of tilt angle () for all the samples at constant temperature 35 0 C. The decrease in the values of tilt angle () for the dye doped mixtures confirms that after dye doping in the pure FLC system there is suppression of randomization of tilt distribution.

238 192 The contrast ratio is another crucial material parameter of FLC and was calculated for the pure and the dye doped FLC by usual experimental techniques and its value is shown in the table 6.1. The contrast of any device is defined as the difference between the luminance in on and off state, while the contrast ratio is defined as the ratio of greater luminance to lesser luminance. Contrast ratio shows an increase for mixture 1 as compared to pure FLC, while for mixture 2 and mixture 3 it slowly reduces to lower values as compared to mixture 1. The higher value of contrast ratio for mixture 1 and 2 as compared to pure FLC may be due to enhancement of in plane switching for dye doped system CONCLUSION In the present chapter we have explored anthraquinone dye D5 doped FLC system. The relaxation time shifts toward lower side for the mixture 1 (mixture with small concentration of dye) as compared to the pure FLC. For mixture 2 and mixture 3 (mixtures with high concentration of dye) relaxation time shifts towards higher side as compared to the mixture 1 suggesting that with increasing in the concentration of dye, the motion of molecules slows down. The magnitude of spontaneous polarization continuously decreases with increase in the concentration of dye. It indicates that molecular alignment is not good which is also clear from the behavior of effective dielectric permittivity. The material parameters such as rotational viscosity tilt angle and (P S /θ) term show strong dependence on concentration of the dye used. There is improvement in contrast ratio by dye doping due to enhancement in plane switching for dye doped FLC system. The values of tilt angle () decreases for the dye doped mixtures confirm that there is suppression of randomization of tilt distribution in dye doped mixture as compared to pure FLC system.

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241 Chapter 7 Goldstone and Soft Mode for Fluorescent Dye Doped Ferroelectric Liquid Crystal

242 Chapter 7 GOLDSTONE AND SOFT MODE FOR FLUORESCENT DYE DOPED FERROELECTRIC LIQUID CRYSTAL 7.1. Introduction 7.2. Experimental details 7.3. Results and discussion 7.4. Conclusion References Journal of Non-Crystalline Solids 376 (2013) 7 11.

243 INTRODUCTION Nowadays the application range of liquid crystals has been greatly expanded from small size displays to their use in monitors and television or display motion pictures [1-3]. Major advances have been made in infrastructure for high quality image services in media technology such as broadcasting, communications and computers [2-4]. All these applications required wide viewing angle, high contrast ratio and slow response time. The optical performance of any device is determined by their dielectric and electro-optical parameters such as dielectric permittivity, relaxation time, rotational viscosity and response time. The high accuracy in these parameters is necessary to design a high quality display. It has been noticed that Ferroelectric Liquid Crystals (FLCs) are most promising material in the entire class of liquid crystals for displays as well as non display applications [4]. The response to the external field gives detailed information about molecular dynamics of FLC materials. The response time of LCDs is limited due to slow behavior of liquid crystal molecules [5]. Although several methods have been proposed to accelerate the response time but all of them have disadvantages such as requirement of bias voltage causing decrement in light transmittance and complicated fabrication process [6]. The key point in these applications is based on our understanding of their molecular motion in response to external applied electric field. However it is equally important to study the influence of temperature on the orientational order and response time of liquid crystalline phases. In recent years fluorescent liquid crystals displays are considered to hold promise for next generation flat panel displays due to their low voltage operation and high contrast ratio [7-9]. The dielectric spectroscopy has a special importance due to their resulting readily stable information in the electro-optical devices [10-13]. Measurement of dielectric response enables one to determine the relaxation time which is an important dynamical quantity of molecules [11]. The present chapter is an effort to enhance electro-optical parameters and width of SmC* phase in FLC due to addition of Fluorescent dye into pure FLC as it reduces randomized arrangement of molecules at SmC* to SmA phase transition point. In the present chapter we have observed changes in the various properties caused by doping of fluorescent dye into the pure FLC [8, 14]. Therefore, dielectric and electro-optical study has been done for both pure and Fluorescent dye doped FLC

244 196 system. The different parameters have been calculated by using dielectric and electrooptical data. The temperature dependence of different parameters such as dielectric permittivity, relaxation time, relaxation strength, spontaneous polarization, response time and rotational viscosity have been discussed for pure and Fluorescent dye doped FLC system EXPERIMENTAL DETAILS The FLC sample used for the present study is Felix purchased from Clariant Chemicals Co. Ltd with the phase sequence as Cr- SmC* -SmA - N*- Iso C, 70 0 C, 76 0 C and C respectively. The basic parameters of the FLC have already been given in our research group [13, 14]. The fluorescent dye used for doping is Benzo 2, 1, 3 Thiadiazole purchased from Sigma Aldrich with the chemical structure as shown in figure 7.1. We have prepared mixture of FLC sample with two concentration (i.e. 1% wt./wt and 2% wt./wt) of the fluorescent dye and denoted these samples as Mixture 1 and Mixture 2 in the whole manuscript [15]. The literature indicates that the present dye is highly fluorescent in the liquid as well as solid state [8]. Figure 7.1. Molecular structure of the fluorescent dye Benzo 2, 1, 3 Thiadiazole used for investigation. Sandwiched type sample cell of glass plates has been used for the dielectric and electro-optical study having cell thickness of 10 m. The cells were prepared by using ITO (Indium Tin Oxide) coated glass plates (sheet resistance is 10 Ohm/mm 2 and the visible light transmission is more than 90 %) having active area 25 mm 2, as

245 197 electrodes. Both the electrodes were treated with adhesion promoter and polymer Nylon 6/6 and rubbed unidirectionaly for planar alignment [13-16]. procedure for preparation of the sample cell has been discussed in our chapter 3. The full The dielectric data have been determined by measuring the value of capacitance and the dissipation factor measured with the help of impedance/gain phase analyzer HP-4194A in the frequency range 100 Hz to 10 MHz. In this measurement there is accuracy of % for the measurement of parameters. The temperature control is very essential for the FLC because of the different phases exist at different temperatures. The temperature has been maintained with the help of Instec Hot Plate HCS 302 with temperature stability of K. The spontaneous polarization (Ps) has been measured by the polarization current reversal peak method by using function generator (Tektronix AFG3021B) and storage type oscilloscope (Tektronix TDS2024C). For the spontaneous polarization measurement, a triangular wave of amplitude of 20Vpp and frequency 10Hz has been used. The spontaneous polarization is determined by the relation Ps was calculated by using the following relation [13, 15] 1 Ps i ( t) dt (7.1) 2A Where, i ( t) dt is the area under the current bump and A is the active area of the sample. For the measurement of response time (τ), a square wave signal of 10 Hz and 20V PP was used. The response time has been determined by using optical switching method [13, 17]. In this method, a square wave of frequency 1 Hz and amplitude 20V peak to peak has been applied to the sample cell. The optical response of molecules as observed by the detector is fed to a storage oscilloscope (HM 407) in electrical form. The output wave form is now used to determine the response time [20]. The response t90 t 10 time of FLCs has been evaluated using equation, 1.8 Here t 90 and t 10 are the time taken by the output wave form to reach 90% and 10% of the maximum of the output wave form. Rotational viscosity has been calculated by using spontaneous polarization data and response time as follows [13, 14, 18-20] γ t, Here t, are optical response time and rotational viscosity of the sample P E S

246 198 respectively, P S is spontaneous polarization of the material and E is applied electric field RESULTS AND DISCUSSION Fig. 7.2 shows temperature variation of the effective dielectric permittivity for the pure FLC and fluorescent dye doped FLC system. Here term effective is used for the dielectric permittivity for which we have removed contribution of sheet resistance, ITO coatings and lead inductance of the cell [18, 21]. Therefore, the data require correction in the Cole-Cole equation [20-22]. The Cole-Cole equation is given as [19] δε' (7.2) 1 (i2πfτ ) ε* ε'( ) 1α Where ε is the dielectric strength and ε() is the high frequency limit of the dielectric permittivity, f is frequency, is the distribution parameter and is the relaxation time. Figure 7.2. Temperature variation of effective dielectric permittivity for the pure FLC, Mixture 1 and Mixture 2. After adding correction terms due to the ionic conductivity and electrode polarization problem in the low frequency region while ITO sheet resistance and lead inductance problem in the high frequency region data, the real and imaginary parts of the above equation become as follows [20-22]

247 199 (1 ) ' '[1 (2f ) sin( / 2)] n '(dc)f '( ) (7.3) 2(1 ) (1 1 (2f ) 2(2f ) ) sin( / 2) and 0 (1 ) (dc) ' (2f ) cos( / 2) " Af k 2(1 ) (1 ) 2f 1 (2f ) 2(2f ) sin( / 2)) m (7.4) Here (dc) is the dc ionic conductance, o is free space permittivity and f is the frequency while n, m and k are the fitting parameters. The terms ı (dc)f n and (dc)/o2f k are added in the equation (7.3) and (7.4) for additional contribution due to the electrode polarization and ionic conductance in the low frequency effect [14,15,21]. The Af m term is added in equation (7.4) in the high frequency effect due to the contribution of ITO sheet resistance and lead inductance of sample cell. By the least square fitting of experimental data these contributions have been removed. The value of effective dielectric permittivity increases with increase in temperature showing maximum value in SmC* phase and then decreasing continuously. The nature of effective dielectric permittivity with variation in temperature is similar as reported for other FLCs. After addition of fluorescent dye into pure FLC there is drastic change in the dielectric relaxation process. The higher value of effective dielectric permittivity is due to the presence of Goldstone mode for Mixture 1 and mixture 2 in SmC* phase appears. The Goldstone mode gives finite contribution in SmC* phase showing a drop but still being finite as transition temperature is approached. The value of effective dielectric permittivity is higher for mixture 2 as compared to mixture 1 showing presence of more than one relaxation process i.e. Goldstone mode and other modes. It is well known that high dielectric permittivity value in SmC* phase is due to the Goldstone mode contribution [18]. The soft mode is observed for mixture 2 arises from the fluctuations in the magnitude of tilt is well studied both theoretically and experimentally [21]. From Cole-Cole theory [22] and theoretical fitting of the experimental data we have evaluated different dielectric parameters such as relaxation frequency and relaxation strength. There is one relaxation mode i.e. Goldstone mode for pure FLC and Mixture 1 while for Mixture 2 there is two type of relaxation i.e. Goldstone mode and soft mode. Fig. 7.3(a) shows variation of Goldstone mode relaxation frequency with temperature for pure FLC and fluorescent dye doped FLC i.e. Mixture 1 and Mixture 2. From figure we observed that nature of Goldstone mode relaxation

248 200 frequency is same for all the samples. In deep SmC* phase the Goldstone mode relaxation frequency is increased slowly approximately temperature independent but near the phase transition temperature it starts increasing rapidly. The relaxation frequency is shifted towards higher side for fluorescent dye doped samples as compared to the pure FLC due to the doping of fluorescent dye into pure FLC. When fluorescent dye is doped into pure FLC then there is disturbance created by fluorescent dye molecules into pure system. This disturbance is more when we increased the concentration of fluorescent dye i.e. for Mixture 2. In other words we can say that presence of additional interaction between fluorescent dye and pure FLC molecules there is a competition between FLC-FLC and dye-dye interaction causes formation of domains due to which our dye doped system becomes more flexible therefore relaxation frequency increase to reorient the FLC molecules. Figure 7.3. (a) Temperature variation of Goldstone mode relaxation frequency for the pure FLC, Mixture 1 and Mixture 2. The observed temperature shift of KHz relaxation frequency in the vicinity of SmA to SmC* or SmA phase side are related to the fluctuation in the coupled over layers manifested as tilt order [19-21]. However tilt order fluctuations start in the right of SmA phase before they converge at SmA to SmC* phase transition temperature. As such tilt is considered as the relevant long range order that grows in the SmA phase itself before it gets stabilized as a changed parameter in the SmC* phase by analysis of high frequency. Hence this mode can be studied by high frequency KHz

249 201 relaxations. The observed temperature dependence of the soft mode relaxation frequency for Mixture 2 is shown in Fig. 7.3 (b). Figure 7.3. (b) Variation of Soft mode relaxation frequency with temperature for Mixture 2. The nature, range and magnitude of this observed relaxation are found to an agreement with the reported values for other FLCs compounds. The soft mode relaxation frequency for FLC material is given by [19, 21] * 2 Kq T TC fs (7.5) 2 Here, K is the elastic constant, q is the wave vector of helix, γ is the rotational viscosity and β is the proportionality constant. In the SmC* phase, relaxation frequency of soft mode increases with increasing temperature which may be due to dominating effect of the first term Kq 2 with temperature approaching T * c. In SmA phase the variation in the value of relaxation frequency is very small which gradually disappears at higher temperature. Fig. 7.4(a) shows temperature variation of Goldstone mode relaxation strength for pure FLC and fluorescent dye doped FLC i.e. Mixture 1 and Mixture 2 while soft mode relaxation strength has also been plotted for mixture 2 is shown in Fig. 7.4(b).

250 202 Figure 7.4. (a) Temperature variation of Goldstone mode relaxation strength for the pure FLC, Mixture 1 and Mixture 2. Figure 7.4. (b) Variation of Soft mode relaxation strength with temperature for Mixture 2. The magnitude of Goldstone mode relaxation strength for mixture 1 and mixture 2 sharply increases with temperature in comparison to the pure FLC. The magnitude of Goldstone mode relaxation strength for mixture 2 is much higher than mixture 1 and pure FLC. This is due to the reason that for higher concentration of fluorescent dye interaction between FLC molecule and fluorescent dye molecule dominates over FLC and FLC molecule interaction. The magnitude of soft mode relaxation strength increases with increase in temperature for Mixture 2 as compared to Goldstone mode relaxation strength for pure FLC, Mixture 1 and Mixture 2. Variation of spontaneous polarization with temperature for the pure FLC and fluorescent dye doped FLC is shown in the Fig The behavior of spontaneous

251 203 polarization with variation in the temperature is same as reported for other FLCs [18-21, 23] i.e. the magnitude of spontaneous polarization is continuously decreasing and it vanishes at SmC* -SmA phase transition point or near this phase transition point. It is clear from the figure that the magnitude of the spontaneous polarization is increases after fluorescent dye doping. The relation between spontaneous polarization and pitch value of the FLC material is given by [24, 25] P S fq z (7.6) Here P S is spontaneous polarization, θ is tilt angle, q (=2/L, L is pitch of material) is wave vector of helix, f and z are flexoelectric and piezoelectric coupling coefficients between P S and θ. Figure 7.5. Temperature variation of spontaneous polarization for the pure FLC, Mixture 1 and Mixture 2. The behavior of helicoidal pitch of the material may also depends upon other parameters such as f, and z. The appearance of soft mode in the vicinity of SmC* phase to SmA phase transition temperature can be correlated with gradual increment in fluorescent dye doped system. This gradual increment in spontaneous polarization value for fluorescent dye doped system suggests that molecules are more aligned because of reduction in phason modes. The increment in magnitude of the spontaneous polarization suggests that there is improvement in the molecular alignment of FLC sample after fluorescent dye doping. It is also clear that the magnitude of the spontaneous polarization is increases after fluorescent dye doping and therefore, observed higher value of spontaneous polarization for Mixture 1 and Mixture 2 is due to the contribution of transverse dipole moment in comparison to the pure FLC.

252 204 Fig. 7.6 shows variation of the response time with temperature for the pure FLC, Mixture 1 and Mixture 2. The nature of response time is same for all the samples [18, 23, 24]. The overall slow response time are considered a major drawback which needs to improve in order to display of high quality moving pictures. In the present chapter we have improved response time after fluorescent dye doping as compared to the pure FLC. The magnitude of response time is found to be smaller as comparison to the other doped system. It may be argued that addition of fluorescent dye into pure FLC leads to the faster response which is a critical issue with the dye doping into pure FLCs. Figure 7.6. Temperature dependence of optical response time for the pure FLC, Mixture 1 and Mixture 2. The nature of the response time is explained by well known relation between response time, spontaneous polarization and rotational viscosity [14, 23-25]. According to this relation response time is directly proportional to rotational viscosity while inversely proportional to spontaneous polarization. In present chapter spontaneous polarization is increasing for fluorescent dye doped FLC due to which response time should be slowly decreased but the magnitude of response time is rapidly decreasing for fluorescent dye doped system as compared to the pure FLC. Therefore, rotational viscosity is also important parameter which also governs the magnitude of response time. In the present case rotational viscosity is decreasing for fluorescent dye doping. Therefore the net effect of rotational viscosity and spontaneous polarization, response time is decreasing due to decrement in rotational viscosity and increment in spontaneous polarization.

253 205 To verify this rotational viscosity is plotted with variation of the temperature in the Fig. 7.7 for pure FLC, Mixture 1 and Mixture 2.The magnitude of rotational viscosity is decreasing due to the suppression of randomization in the fluorescent dye doped materials. Figure 7.7. Temperature dependence of rotational viscosity measured from electro-optical method for pure FLC, Mixture 1 and Mixture 2. The intermolecular interaction is dominated by probable dipole-dipole interaction between fluorescent dye and FLC molecule hence randomized structure is obtained whereas such randomization is not allowed to the conventional FLCs. Therefore, enhancement in the dielectric and electro-optical properties has been observed. The fluorescent dye molecule act as mediator between FLC molecule which clamp them through the probable dipole-dipole interaction and it increases optical tilt angle. Therefore, rotational viscosity is decreased for fluorescent dye doped system i.e. Mixture and Mixture CONCLUSION The comparative study of the pure FLC and fluorescent dye doped system are described in present chapter. We have found one relaxation mode i.e. Goldstone mode for pure FLC and Mixture 1 but for Mixture 2 soft mode of relaxation has also been found in addition to Goldstone relaxation mode. The relaxation frequency is shifted towards higher side for Mixture 1 and Mixture 2. The behavior of the rotational viscosity for fluorescent dye suggests that rotational viscosity decreases with

254 206 temperature implying that energy barrier for rotation increases significantly due to the formation of randomized structure. The fluorescent dye molecule act as mediator between FLC molecule which clamp them through the probable dipole-dipole interaction increases optical tilt angle therefore rotational viscosity for fluorescent dye doped system decreases. We have also improved response time for Mixture 1 and Mixture 2 which is considered as a drawback for fluorescent dye doped system to use in display of high quality moving pictures.

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256 208 [20] L. M. Blinov, V. G. Chigrinov, Electro-Optical Effects in Liquid Crystal Materials, Springer-Verlag, New York, Vol. 3 (1994). [21] F. M. Gouda, Dielectric Relaxation Spectroscopy of Chiral Smectic Liquid Crystals, A Ph.D Thesis, Chalmers University of Technology, Goteberg, Sweden, (1992). [22] K.S. Cole, R.H. Cole, J. Chem. Phys. 9 (1941) 341. [23] S. Essid, M. Manai, A. Gharbi, J. P. Marcerou, J. C. Rouillon, Liq. Cryst. 32 (2005) 307. [24] E. P. Pozhidaev, D. Ganazke, V.Y. Zyryanov, S. L. Smorgon, W. Haase, Liq. Cryst. 29 (2002) [25] D. M. Potukuchi, A. K. George, C. Carboni, S. H. Al-Harthi, J. Naciri, Ferroelectrics. 300 (2004) 79.

257 Chapter 8 Reduction of Optical Response Time for Fluorescent Dye Doped Ferroelectric Liquid Crystal

258 Chapter 8 REDUCTION OF OPTICAL RESPONSE TIME FOR FLUORESCENT DYE DOPED FERROELECTRIC LIQUID CRYSTAL 8.1. Introduction 8.2. Material and methods 8.3. Results and discussion 8.4. Conclusion References Journal of Molecular Liquids 175 (2012)

259 INTRODUCTION Ferroelectric liquid crystals (FLCs) are the most promising materials for displays as well non-display applications in the modern technology [1-4]. The FLC materials are useful in these applications due to their lower driving voltage, grey scale generation capability, easily achievable alignment and fast optical response [2]. The liquid crystal displays (LCDs) are more popular due to their low weight, thin shape and low power consumption. The displays based on Guest host mode can work in the active and the passive modes but the choice of guest material is vital for practical utilization of such Guest host LCDs [3, 5-7]. Passive full colours LCDs consume appreciable electric power for back light systems. The drawbacks of these FLCs based display devices are their low brightness and energy deficiency due to the use of polarizers and absorbing color filters. The fluorescent Guest hosts LCDs are useful in the mobile communication tool which requires less energy [4-7]. Doping of fluorescent dye in pure FLC may be useful for enhancement of dielectric and electrooptical parameters. Recently contrast ratio and colour shift of FLC displays have been improved greatly by dye doping. The change in colour intensity is obtained by controlling the direction of FLC molecules. However the slow optical response still remains a critical issue in LCDs [8, 9]. The turn on time could be reduced by overdriving while there is no practical solution for reducing turn off time due to the limitations of slow relaxation of FLCs [9]. Therefore, it is important to investigate effect of the fluorescent dye on the dielectrical and electro-optical properties of the pure FLC. In the present chapter we report dielectric and electro-optical parameters of pure FLC and fluorescent dye doped FLC. The used dye is highly fluorescent in solution as well as in solid state [5]. The presence of fluorescent dye causes deformation of FLC matrix which affects orientation of domains drastically, changing the properties of dye doped system. The dielectric study has been performed in the frequency range 100 Hz to 10 MHz with temperature variation. The various parameters such as dielectric permittivity, dielectric loss, relaxation time, relaxation strength and distribution parameter have been evaluated by using dielectric spectroscopy for pure FLC and fluorescent dye doped FLC. The temperature dependence behavior of above mentioned parameters have been discussed in the light of different theories and models related with the pure FLC. Moreover the optical response of fluorescent dye doped system in presence of

260 210 an external applied filed may result in new physical behavior as well as formation of self assembled structure MATERIAL AND METHODS The FLC used for the present study is Felix purchased from Clariant Chemicals Co. Ltd with the phase sequence as Cr- SmC* -SmA - N*- Iso at C, 74 0 C, 82 0 C and 95 0 C respectively. The basic parameters of the FLC have already been given in our earlier papers [10-14]. The fluorescent dye used for doping is Benzo 2,1,3 Thiadiazole purchased from Sigma Aldrich with the chemical structure as shown in fig We have prepared a mixture of FLC sample with 1% wt./wt concentration of the fluorescent dye [11,12]. Figure 8.1. Molecular structure of the fluorescent dye Benzo 2, 1, 3 Thiadiazole used for investigation. The sandwiched sample cell of conducting glass plates having cell thickness of 10 m has been used for the dielectric and electro-optical study. The cells were prepared by using ITO (Indium Tin Oxide) coated glass plates (sheet resistance is 10 Ohm/mm 2 and the visible light transmission is more than 90 %) having active area 25 mm 2, as electrodes. Both the electrodes were treated with adhesion promoter and polymer Nylon 6/6 and then rubbed unidirectionaly for planar alignment. The thickness of the cell was maintained at 5 mm by means of a Mylar spacer. The cells were calibrated using AR grade CCl4 and benzene. The material was introduced into the cell by capillary action at a temperature slightly higher than its isotropic temperature. A well-aligned cell was obtained by applying an electric field in the slow cooling cycle from the isotropic phase to room temperature. The detailed procedure for preparation of the sample cell has been discussed in chapter 3.

261 211 The dielectric data have been determined by measuring the value of capacitance and the dissipation factor measured with the help of impedance/gain phase analyzer HP-4194A in the frequency range 100 Hz to 10 MHz. The temperature of the samples has been maintained with the help of Instec Hot Plate HCS 302. Spontaneous polarization has been measured by standard polarization reversal current method, while optical response time has been determined by using optical switching method [13,15, 16]. Rotational viscosity has been calculated by spontaneous polarization data and optical response time using following relation [13] t γ E P E (8.1) S Here t, E are optical response time and electro-optical rotational viscosity of the sample respectively, P S is spontaneous polarization of the material and E is applied electric field. The contrast ratio measurement has been performed by applying a square wave to the sample cell placed between the crossed polarizers. The corresponding maximum and minimum intensity have been recorded by light dependent resistance (LDR) and storage oscilloscope HM RESULTS AND DISCUSSION The effective dielectric permittivity with variation in temperature for the pure FLC and fluorescent dye doped FLC is shown in the fig Here term effective is used for the dielectric permittivity for which we have removed contribution of sheet resistance, ITO coatings and lead inductance of the cell. For this purpose we require correction in the Cole-Cole equation [10-14]. The Cole-Cole equation is given as δε' ε* ε'( ) (8.2) 1α 1 (i2πfτ ) Where ε is the dielectric strength and ε() is the high frequency limit of the dielectric permittivity, f is frequency, is the distribution parameter and is the relaxation time.

262 212 Figure 8.2. Temperature dependence of effective dielectric permittivity for the pure FLC and fluorescent dye doped FLC. After adding correction terms due to the ionic conductivity and electrode polarization problem in the low frequency region while ITO sheet resistance and lead inductance problem in the high frequency region data, the real and imaginary parts of the above equation become as follows (1 ) ' '[1 (2f ) sin( / 2)] n '(dc)f '( ) (8.3) 2(1 ) (1 1 (2f ) 2(2f ) ) sin( / 2) And 0 (1 ) (dc) ' (2f ) cos( / 2) " Af k 2(1 ) (1 ) 2f 1 (2f ) 2(2f ) sin( / 2)) m (8.4) Here (dc) is the dc ionic conductance, o is free space permittivity and f is the frequency while n, m and k are the fitting parameters. The terms ı (dc)f n and (dc)/o2f k are added in the equation (8.3) and (8.4) for additional contribution due to the electrode polarization and ionic conductance in the low frequency effect [10-13]. The Af m term is added in equation (8.4) in the high frequency effect due to the contribution of ITO sheet resistance and lead inductance of sample cell. By the least square fitting of experimental data we have removed these contributions and we got effective dielectric permittivity.

263 213 It is claimed from fig. 8.2 that the nature of variation of effective dielectric permittivity with temperature for the pure FLC and fluorescent dye doped FLC is similar as reported in the literature for other FLCs [10, 17, 18]. The magnitude of the effective dielectric permittivity for the fluorescent dye doped FLC is reduced to the lower value as compared to the pure FLC. In other words after doping of fluorescent dye in pure FLC there is drastic change in the dielectric relaxation process. The observed relaxation process involves two types of fluctuations in the FLCs, first is the tilt fluctuations (i.e. tilt tuned by coupling of the longitudinal dipole moment) second is azimuthal angle fluctuation i.e. fluctuations tuned by coupling of the transverse dipole moment. The former relaxation is observed in the higher frequency range i.e. in the khz -MHz region while later relaxation is observed in the lower frequency region. The dielectric relaxation is observed in the lower frequency region for both the samples due to the contribution of Goldstone mode. The lower value of effective dielectric permittivity is due to the suppression of Goldstone mode for fluorescent dye doped FLC. Lower value of effective dielectric permittivity also shows that dipole moment of dye molecules and dipole moment of pure FLC are in the opposite direction. When fluorescent dye is used for the doping into pure FLC then there are three interaction takes place (1) Interaction between the FLC and FLC molecules (2) Interaction between the fluorescent dye and FLC molecules (3) Interaction between the fluorescent dye and fluorescent dye molecules Due to the presence of two additional interactions dipole moment of dye and FLC molecules are randomly distributed in such a manner to reduce the value of effective dielectric permittivity for fluorescent dye doped FLC. We have evaluated different dielectric parameters such as relaxation time, relaxation strength and distribution parameter from Cole-Cole theory [19] and theoretical fitting of the experimental data. Fig. 8.3 shows variation of the relaxation time for the pure FLC and fluorescent dye doped FLC. We observed from figure that nature of the relaxation time is same for both the samples but for fluorescent dye doped FLC relaxation time is shifted towards higher side as compared to the pure FLC. When fluorescent dye is doped into pure FLC then molecules tend to fit according to the position of pure FLC molecules. This indicates that after dye doping system becomes rigid therefore relaxation time increases to reorient the FLC molecules.

264 214 Figure 8.3. Temperature variation of relaxation time for the pure FLC and fluorescent dye doped FLC. The nature of the relaxation time is also explained by well known equation given below [10, 11, 14] (8.5) K D 2 3q Here K 3 is twist elastic constant, q is the wave vector of the helical pitch and D is the rotational viscosity of mixture measured from dielectric method. Relaxation time is directly proportional to the rotational viscosity of mixture while inversely proportional to the twist elastic constant and wave vector of the helical pitch as seen by above equation. Change in the relaxation time indicates that the rotational viscosity of the system should be altered after addition of fluorescent dye into the pure FLC. Therefore, we have evaluated rotational viscosity for pure FLC and fluorescent dye doped FLC. The nature of rotational viscosity with temperature is discussed in the later part of the present chapter. In addition to this we have also evaluated spontaneous polarization and relaxation strength for fluorescent dye doped FLC and then compared to the pure FLC. Fig. 8.4 shows temperature variation of the Goldstone mode relaxation strength for the pure FLC and fluorescent dye doped FLC. The value of relaxation strength sharply increases for fluorescent dye doped FLC as compared to the pure FLC.

265 215 Figure 8.4. Temperature variation of the Goldstone mode relaxation strength for the pure FLC and fluorescent dye doped FLC. We see that phase transition temperature shifted towards higher side after fluorescent dye doping in compared to pure FLC from figure. Such behavior of the Goldstone mode relaxation near phase transition temperature in the SmC* phase is due to the realignment of randomization of FLC molecules which contribute to the Goldstone mode of the FLC sample. This degradation randomization of FLC molecules is due to the doping of fluorescent dye. When interactions between FLC and fluorescent dye molecules come into picture it dominates over molecular interaction or random fluctuation and hence aids in realignment of randomization of FLC molecules. Due to this reason relaxation strength increases for the fluorescent dye doped FLC in comparison to the pure FLC. Variation of spontaneous polarization with temperature for the pure FLC and fluorescent dye doped FLC is shown in the fig The behavior of spontaneous polarization with variation in the temperature is same as reported for other FLCs [20-23] i.e. the magnitude of spontaneous polarization is continuously decreasing and it vanishes at SmC* -SmA phase transition point or near this phase transition point. It is clear from the figure that the magnitude of the spontaneous polarization is increases after fluorescent dye doping. The increment in magnitude of the spontaneous polarization suggests that we have improved the molecular alignment of FLC sample after fluorescent dye doping.

266 216 Figure 8.5. Variation of spontaneous polarization with the temperature for the pure FLC and fluorescent dye doped FLC. The materials like liquid crystals, materials with long molecular chains show broader dispersion of relaxation time. Therefore, in 1941 Cole-Cole [19] has suggested expression for distribution parameter, also defined as degree of distribution of relaxation time. Temperature dependence of the distribution parameter for the pure FLC and fluorescent dye doped FLC is shown in the fig It is also observed that Goldstone mode of relaxation in the SmC* phase at any temperature is influenced by an applied electric field. Figure 8.6. Temperature variation of distribution parameter for the pure FLC and fluorescent dye doped FLC.

267 217 The low value of distribution parameter at lower temperature in SmC* phase indicates that the Goldstone mode predominates over other relaxation modes. The other relaxation modes comes into existence near the vicinity of the SmC* to SmA phase transition point. The higher value of distribution parameter near the vicinity of SmC* to SmA phase transition point for the Goldstone mode of relaxation reveals that relative fixture of the transverse dipole moment in the SmC* phase. Fig. 8.7 shows variation of the optical response time with temperature for the pure FLC and fluorescent dye doped FLC. The nature of optical response time is same for both pure FLC and fluorescent dye doped FLC however magnitude of optical response time for fluorescent dye doped FLC is lower as compared to the pure FLC. This indicates that we have achieved slow optical response time by fluorescent dye doping, which otherwise has been a critical issue with the dye doping into pure FLCs. The nature of the optical response time [11, 14] is explained by equation (8.1). Figure 8.7. Temperature dependence of optical response time for the pure FLC and fluorescent dye doped FLC. We see that optical response time depends upon rotational viscosity and spontaneous polarization of the sample from equation (8.1). Spontaneous polarization is increasing for fluorescent dye doped FLC due to which optical response time

268 218 should be decreased. But rotational viscosity is also important parameters which governs the magnitude of optical response time. In the present case rotational viscosity is decreasing after fluorescent dye doping. Therefore the combine effect of rotational viscosity and spontaneous polarization, optical response time is decreasing due to decrement in rotational viscosity and increment in spontaneous polarization. This has been verified by plotting electro-optical rotational viscosity with variation of the temperature shown in the fig. 8.8 for pure FLC and fluorescent dye doped FLC. Figure 8.8. Temperature dependence of rotational viscosity measured from electro-optical method for pure FLC and fluorescent dye doped FLC. The temperature dependence of electro-optical rotational viscosity is as similar as reported in the earlier papers for other FLCs [12, 13] but the magnitude of electrooptical rotational viscosity for fluorescent dye doped FLC decrease as compared to the pure FLC. Hence the dielectric rotational viscosity is also decreasing for fluorescent dye doped FLC in comparison to the pure FLC. The behavior of both the rotational viscosities as measured by electro-optical and dielectric method is same but their magnitude is slightly different [10, 13]. The magnitude of dielectric rotational viscosity is related with (P S /θ) term as [10, 14] 2 PS D (8.6)

269 219 Here represents tilt angle. The magnitude of dielectric rotational viscosity is directly proportional to (P S /θ) term. Therefore value of this term has been evaluated for pure and fluorescent dye doped system at constant temperature and shown in table 1. As seen from equation (8.6) value of (P S /θ) term depend upon magnitude of rotational viscosity therefore the magnitude of (P S /θ) term decreases after fluorescent dye doping, is also clear from table 8.1. Sample P S /θ ( x 10-9 ) (Coulomb per Tilt Angle Contrast Ratio degree/cm 2 ) Pure FLC % Fluorescent Dye Doped FLC Table 8.1. Variation of different material parameters for the pure FLC and fluorescent dye doped FLC at constant temperature of 35 0 C. The decrement of (P S /θ) term in the fluorescent dye doped system implies that the amplitude of tilt (Tilt Angle) is decreasing at much faster rate than spontaneous polarization which is due to the appearance of tilt distribution within smectic layer for the fluorescent dye doped systems as compared to the pure FLC. The contrast ratio is another crucial material parameter of FLC and was calculated for the pure FLC and the fluorescent dye doped FLC by usual experimental techniques and its value is shown in the table 1. The contrast of any device is defined as the difference between the luminance in on and off state, while the contrast ratio is defined as the ratio of greater luminance to lesser luminance. The higher value of contrast ratio for fluorescent dye doped FLC as compared to pure FLC may be due to in plane switching for doped system CONCLUSIONS The comparative study of the pure FLC and fluorescent dye doped FLC are described in the present chapter. The entire study may be summarized in the following points:

270 The value of the effective dielectric permittivity for fluorescent dye doped FLC have been reduced due to the presence of two additional interactions (dye -dye and dye -FLC). Therefore, dipole moment of dye and dipole moment of FLC molecules are randomly distributed to reduce the value of the effective dielectric permittivity. 2. It has been observed that relaxation time after adding fluorescent dye shift towards higher side due to the rigidness of the system. It causes more obstruction of movement of fluorescent dye in unwinding the helix. 3. The Goldstone mode relaxation strength and spontaneous polarization are changed after doping of fluorescent dye. This is due to the fact that helicoidal motion in the fluorescent dye doped FLC has been suppressed and as a result Goldstone mode is remarkably reduced. 4. The behaviour of the rotational viscosity for fluorescent dye doped mixture suggests that rotational viscosity decreases with temperature implying that energy barrier for rotation increases significantly. 5. In addition to this we have achieved slow optical response time for fluorescent dye doped FLC which has been a critical issue for the dye doping into pure FLCs. The contrast ratio also shows improvement for fluorescent dye doped system.

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273 Chapter 9 Final Conclusion and Discussion with Future Plans