POLYMER STABILIZED AND DISPERSED BLUE PHASES

Transcription

1 POLYMER STABILIZED AND DISPERSED BLUE PHASES A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY By Emine Kemiklioglu December, 2014

2 Dissertation written by Emine Kemiklioglu B.S., Celal Bayar University, 2004 M.S., Celal Bayar University, 2007 Ph.D., Kent State University, 2014 Approved by, Chair, Doctoral Dissertation Committee (Liang-Chy Chien, PhD ), Members, Doctoral Dissertation (Hiroshi Yokoyama, PhD) Commitee (Robin Selinger, PhD) (John West, PhD) Accepted by, Director, Department of Chemical Physics (Liang-Chy Chien, PhD) Interdisciplinary Program, Dean, College of Arts and Sciences (James L. Blank, Ph.D.)

3 Contents List of figures...v List of tables...x Acknowledgement... xii Abstract... xiii Chapter 1 Introduction and Background Chirality in Molecules Cholesteric Liquid Crystals Blue Phases Polymer Stabilized Blue Phase Liquid Crystals Polymer Dispersed Liquid Crystals Polymer Dispersed Blue Phase Liquid Crystals Dissertation overview...18 Chapter 2 Characterization Methods Characterization Methods Polarizing Optical Microscopy Bragg Reflection Spectra Electro Optical Measurement Scanning Electron Microscopy Contact Angle Measurement...34 Chapter 3 Polymer Stabilized Blue Phase Liquid Crystals Introduction Polymer stabilization of blue phase with UV curable nanoparticles Cell preparation...42 i

4 3.2.2 Observation of blue phase temperature range Theoretical analysis of temperature expansion temperature dependence of Bragg reflection of the polymer stabilized blue phase Kerr effect of polymer stabilized blue phase Response time of polymer stabilized blue phase Polymer Morphology Polymer system effect on blue phase stabilization Polymer system effect on thermal stability of polymer stabilized blue phase Polymer system effect on the Bragg peaks of polymer stabilized blue phase Polymer system effect on the electro-optical proporties of polymer stabilized blue phase Polarity spectrum of monomers Polymer system dependence of Electro-optical behavior of polymer stabilized blue phase Summary...83 Chapter 4 Carbon Nanotube Doped Polymer Stabilized Blue Phase Liquid Crystals Introduction Materials and Cell Preparation Temperature range observation of CNT-doped polymer stabilized blue phase Reflection spectra of CNT-doped polymer stabilized blue phase Electro-optical behavior of CNT-doped polymer stabilized blue phase Voltage holding ratio of CNT-doped polymer stabilized blue phase Summary Chapter 5 Stabilization Effect of Bent Core Molecules on Blue Phases Introduction ii

5 5.2 Bent-Core Concentration Effect on Blue Phase Stabilization Materials Texture Analysis Refraction Spectra of Blue Phases with Dpoed Bent-Core Molecules at Different Concentration Bent-Core Structure Effect on Blue Phase Stabilization Different Bent Core Effect on the Refraction Spectra of Blue Phases Electro-Optical Behaviors of Different BCLC Doped Blue Phases Conclusion Chapter 6 Polymer Dispersed Blue Phase Liquid Crystals Polymer Encapsulated Blue Phase Liquid Crystals Materials and Cells Preparation Temperature Range of Polymer Encapsulated Blue Phase Liquid Crystals Reflection Spectra of Polymer Encapsulated Blue Phase Liquid Crystals Electro-optical proporties of Polymer Encapsulated Blue Phase Liquid Crystals Morphological Examination of Polymer Encapsulated Blue Phase Liquid Crystals Contact angle measurement of Polymer Encapsulated Blue Phase Liquid Crystals Conclusion Polymerization-Induced Phase Seperation Blue Phase Liquid Crystals Materials and Cell Preparation Temperature Range Determination of Polymerization of Polymerization- Induced Phase seperated BPLC Reflection Spectra of Polymerization-Induced Phase Seperated Blue Phase Liquid Crystals iii

6 6.2.4 Electro-optical Resuts of Polymerization-Induced Phase Seperated Blue Phase Liquid Crystals Conclusion Chapter 7 Polymer Stabilization of Polymer Encapsulated Blue Phase Liquid Crystals Introduction Materials and cell preparation Material Characterization Electro-Optical Performances of Polymer Stabilized Blue Phase Liquid Crystals Morphological Studies of Polymer Encapsulated and Polymer Stabilized Blue Phase Liquid Crystals Conclusion Chapter 8 Summary and Future Work Summary Future work References iv

7 List of Figures Figure 1-1: Simulaton of a chiral molecule...2 Figure 1-2: Simulation of formation of the helical structure of a chiral nematic (cholesteric) phase....3 Figure 1-3: An illustration of cholesteric liquid crystal with a pitch P....3 Figure 1-4: Formation of helical chiral nematic phase using chiral dopant....4 Figure 1-5: The simulation of f a double twist cylinder structure of blue phase....6 Figure 1-6: Blue phase structures in (a) BPI (b) BP II....7 Figure 1-7: Simulated polymer network structure within a cubic lattice of a polymer stabilized blue phase Figure 1-8: Working principle of Polymer Dispersed Liquid Crystal a) off state-light scattering mode b) on-state-light transmitted mode...15 Figure 2-1: Schematic illustration of Snell s law Figure 2-2: Propagation of light through a sample between crossed polarizers Figure 2-3: An example of a) BPII b) BPI c) texture colorful platelet texture of BP liquid crystal mixtures formed using 55 wt% R Figure 2-4: Schematic illustration of bragg scattering Figure 2-5: a) simple cubic b) body centered c) face centered cubic Figure 2-6: The (100) planes of a) simple cubic b) body centered c) face centered cubic. 27 Figure 2-7: a) simple cubic b) body-centered cubic c) face centered cubic...28 Figure 2-8: Experimental setup for measuring the electro-optical response Figure 2-9: Schematic illustration of scanning electron microscope...33 Figure 2-10: The SEM micrographs of polymer morphology of (a) PSBP cell of sample with 10% polymer on the surface of a substrate with electrode (the arrow indicates electrode direction) (arrow shows the preferential orientation) (b) PDBP films show the top view of droplets at the surface of a substrate with an electrode of 22-µm IPS cell Figure 2-11: Pictorial illustration of contact angle of the droplets v

8 Figure 2-12: Pictorial illustration of the goniometer setup Figure 3-1: a) A chemical structure of Polyhedral oligomeric silsesquioxanes (PSS- PMH), and b) RM257 (bis[acryloyloxy-(4-propoxyl(1,4-pheny benzoate))] Figure 3-2: A chemical structure IR Figure 3-3: A pictorial illustration of in-plane switching (IPS) cell Figure 3-4: POM images of (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 3*,(e) sample 4 after polymerization under crossed polarizers Figure 3-5: a) Reflection spectra for pure BPLC without the monomer Figure 3-5: (b) The plot of wavelength versus temperature for sample 1, 2, 3 and 4 after the polymerization Figure 3-6: Normalized transmittance-voltage (TV) curves of samples 1 (@45 o C), 2 (@45 o C), 3 (@45 o C) and 4 (@35 o C and sample of the mixture of BL006 and o C without monomer) Figure 3-7: a) POM images of PSBP cell of sample 1 under applied voltage of (a) 0 V, (b)20 V, (c) 30 V, (d) 40 V, (e) 50 V, (f) 60 V, (g) 70 V, (h) 80 V, (i) 90 V, (j) 100 V at 45 o C, and b) POM images of PSBP cell of sample 4 under applied voltage of (a) 0 V, (b)20 V, (c) 30 V, (d) 40 V, (e) 50 V, (f) 60 V, (g) 70 V, (h) 80 V, (i) 90 V, (j) 100 V at 35 o C. The black scalar bar is 50 µm...57 Figure3-8: Kerr constant of PSBP samples versus polymer concentration Figure 3-9: The plot of transmittance versus response time of sample Figure 3-10: The SEM micrographs of polymer morphology of PSBP (a) cell of sample 1, (b) cell of sample 2, (c) cell of sample 3, (d) cell of sample the surface of a substrate with electrode (the arrow indicates electrode direction) (arrow shows the preferential orientation), (d-1) the surface of a substrate without electrode. The yellow and black scale bars represent 20 and 2 µm with respectively. (e) Simulated polymer network structure within a cubic lattice of a polymer stabilized blue phase (small cubes in the right picture represents PSS- PMH) Figure 3-11: The chemical structure of JC1041XX, 5CB, and ZLI Figure 3-12: POM images of pure BP mixture with nematic liquid crystals of JC1041XX (44.5% wt) and 5CB (34% wt), and the chiral dopant of ZLI 4572 (10% wt). Scalar bar is 100µm Figure 3-13: POM images of polymer stabilized BP samples of a) with TMPTA b) with BTMATD c) with BMATD d) with HDDA, e) with HFBA, f) with HBA, monomers at various temperatures vi

9 Figure 3-14: Reflection spectra for pure BPLC without the monomer Figure 3-15: Reflection spectra for polymer stabilized BP samples of a) with TMPTA b) with BTMATD c) with BMATD d) with HDDA, e) with HFBA, f) with HBA monomers at various temperatures Figure 3-16: The simulation of the dipole moment of a linear molecule Figure 3-17a: Trifunctional monomer: TMPTA and 3.17b: Difunctional monomers of a) BMATTD, b) BMATD, c) BBA, and d) HDDA.. 77 Figure 3-17c: Monofunctional monomers of a) HFBA and b) HBA Figure 3-18: Dipole moments of monomer dopants Figure 3-19: The transmittance-voltage curve of PSBP samples Figure 3-20: Kerr constant of PSBP samples with different monomer Figure 3-21: Response time of PSBP samples Figure 4-1: a) Phase diagram showing transition temperatures vs. concentration of CNTs in the PSBPLC mixture, b) POM images of sample PSBP-2a at (1) 50 C and (2) 8 C after polymerization, and c) POM images of sample PSBP-2b at (1) 52 C and (2) 29 C after polymerization. The yellow scalar bar corresponds to 20 μm...91 Figure 4-2: a) Reflection spectra for SWCNT doped BPLC without polymer, b) reflection spectra for MWCNT doped BPLC without polymer, and c) plot of wavelength versus temperature after polymerization of CNT doped PSBPLC Figure 4-3: a) Kerr constants and on-stage voltage for the BPI as a function of concentration of CNTs and b) Voltage-transmittance curves of PSBP-1c, PSBP-2a, and PSBP-2b cells Figure 4-4: Response times of PSBP-1c, PSBP-2a, and PSBP-2b cells Figure 4-5: Voltage holding ratios of CNT undoped PSBP, PSBP-2a, and PSBP-2b cells versus time Figure 5-1: Chemical structure of bent core molecule Figure 5-2: POM textures of the mixtures by doping with BCLC with the concentration of a) 0% wt. b) 1% wt. c) 3% wt. d) 5% wt. e) 7% wt Figure 5-3: Phase diagram obtained by cooling for BP mixtures as a function of bent core molecule concentration vii

10 Figure 5-4: Bent-core concentration effect on the temperature range of blue phase for a) 0wt% BC Figure 5-4: Bent-core concentration effect on the temperature range of blue phase for b) 1wt% BC, c) 3wt% BC, d) 5wt% BC, e) 7wt% BC Figure 5-5: POM textures of a) sample 1, b) sample 2, and c) sample Figure 5-6: Simulations of the chemical structures of a) the bent-core molecule in sample 1, b) the bent-core molecule in sample 2, c) the bent-core molecule in sample Figure 5-7: Bragg reflection spectra of a) sample 1, b) sample 2, c) sample Figure 5-8: Normalized voltage-transmittance curves and POM images of the cells as a function of applied voltage of a) pure BP sample without BC molecules, b) Sample 1, c) Sample 2, d) Sample Figure 6-1: Schematic representation of a PEBP film with blue phase droplets dispersed in a polymer matrix and laminated inside of an IPS cell Figure 6-2: Photomicrographs of (a) PEBP1 (b) PEBP2, and (c) PEBP3 films prepared from a mixture of NeoRez/BPLC at 27 C. The white-colored scalar bar has a length of 50 µm Figure 6-3: Plot of average size of droplets versus temperature Figure 6-4: Reflection spectra for PEBP sample Figure 6-5: (a) Threshold and turn on voltages of PDBP1 as a function of IPS cell thickness and (b) Response time of PDBP1 as a function of IPS cell thickness Figure 6-6: The normalized transmittance versus switching time of PDBP1 sample in an IPS cell with 15 µm cell gap Figure 6-7: The plots of normalized transmittance versus applied voltage of (a) PEBP1 (b) PEBP2 and (c) PEBP3 films in an IPS cell with 15 µm cell gap at 27 C Figure 6-8: SEM images of (a) PEBP1 (b) PEBP2 and (c) PEBP3 films show the top view of droplets at the surface of a substrate, and (d) cross-section of the PEBP3 film viewed at an oblique angle (30 degrees from normal) with an electrode of 22-µm IPS cell Figure 6-9: Contact angle measurements of DI water, liquid crystal (E-31), polymer (Neorez), blue phase LC and encapsulated blue phase on the glass substrates with different surface treatments for planar or homeotropic alignment Figure 6-10: Chemical structures of the monomer of hydroxybutyl acrylate (HBA) viii

11 Figure 6-11: Composition of blue phase and monomers of HBA and PN Figure 6-12: POM images of revealing the textures of PDBP sample at a) isotropic state (30 o C), b) blue phase state (room temperature), c) cholesteric state (21 o C). Scalar bar is 20 µm Figure 6-13: a) The plot of reflectance of the PDBP samples versus temperature, b) The plot of the wavelength the PDBP samples versus temperature Figure 6-14: Normalized transmittance-voltage (TV) curves of PDBP liquid crystal samples a) without photoinitiator of IR651, b) with photoinitiator of IR651, c) Picture of the cell the sample of PDBP* under the applied field Figure 6-15: Contrast ratio dependence of PDBP liquid crystal samples both with photoinitiator of IR651 and without photoinitiator of IR Figure 6-16: Response time of PDBP liquid crystal samples a) without photoinitiator of IR651, b) with photoinitiator of IR Figure 7-1: Chemical structures of a) Polyvinyl alcohol and b) Triton-X Figure 7-2: POM images and phase sequence of sample A a) before polymerization and b) after polymerization (White scalar bar represents 100 µm) Figure 7-3: POM images and phase sequence of sample B a) before polymerization and b) after polymerization (White scalar bar represents 100 µm) Figure 7-4: POM images and phase sequence of sample C a) before polymerization and b) after polymerization (White scalar bar represents 100 µm) Figure 7-5: POM images and phase sequence of sample A* a) before polymerization and b) after polymerization (White scalar bar represents 100 µm) Figure 7-6: Reflection spectra of the sample A* as a function of temperature Figure 7-7: Transmittance curves as a fnction of applied voltage of a) Sample A, b) Sample B, c) Sample C, and d) Sample A* Figure 7-8: Response time of Sample A* Figure 7-9: SEM image of encapsulated PSBP droplets in the IPS cell of the a) sample A on the substrate without alignment layer, b) Sample A* in the IPS cell with surface alignment layers which wraps around the interconnected holes that vary in size and distribution, c) the polymer morphology on the different position at the same substrate surface of the sample A* ix

12 List of Tables Table 3.1. The compositions of monomer mixtures and transition temperatures of BPLCs and PSBPs Table 3.2. The chemical structure of monomers Table 3.3. Component fractions of PSBPLC mixtures and transition temperatures...67 Table 4.1. Compositions of CNT doped blue phase liquid crystal mixtures Table 5.1. Compositions of BCLC doped blue phase liquid crystal mixtures Table 5.2. Compositions of mixtures with different BC molecules and polarity of BC molecules Table 6.1. The compositions of three polymer encapsulated BPLCs Table 6.2. The measured threshold (Vth) and turn on voltages (Von), rise and decay times, and calculated Kerr constants of the encapsulated LC droplets for three different PEBP samples at 27 C Table 6.3. Measured contact angles of the materials on the different substrates Table 6.4. Measured contact angles on the planar alignment substrates Table 6.5. Measured contact angles on the homeotropic alignment substrates Table 6.6. The compositions of the polymerization induced phase separated BPLC droplets Table 7.1. Compositions of materials of encapsulated polymer stabilized BPLC x

13 To my family xi

14 Acknowledgement Foremost, I would like to express my deepest gratitude to my dissertation advisor, Prof. Liang-Chy Chien, for his excellent guidance, patience, encouragement, and providing me with a great ambience for doing research. Besides my advisor, I would like to express my special thanks to my dissertation committee members Prof. John West, Prof. Robin Selinger, Prof. Hiroshi Yokoyama and Prof. Mark Manley. I thank all my classmates and friends from LCI. Special thanks to Young Ki Kim for his help and support during class times. I would like to thank all LCI people and staff, especially Liou Qui for helping me during my SEM study, and Douglas Bryant for the time and effort for cleanroom process. I am also grateful to the Ministry of Education of the Republic of Turkey for their financial support throughout my PhD study. I would like to thank to the Educational Attache in NY for helping make the official procedures easier for me. I would like to express my greatest appreciation to my mother, brother and father for their unconditional love, encouragement and moral support throughout my study. xii

15 Abstract Blue phase liquid crystal (BPLC) materials have potential for advanced applications of display material and technology based on their optical behaviors, such as field-induced birefringence and sub-millisecond response time, which is at least one order of magnitude faster than the present nematic liquid crystal based displays. Since blue phases appear in the narrow temperature range between the chiral nematic and the isotropic phases, there is a temperature range limitation for the application of blue phase liquid crystal. In this dissertation, we have developed blue phase liquid crystal materials with a wide temperature range and low driving voltage. The first goal was to develop widetemperature range blue phase liquid crystal materials using several stabilization methods notably polymer stabilization, doping of carbon-nanotubes and bent-core molecules. The temperature range could be expanded more than 54 o C via the polymer stabilization. The second goal was to explore the polymer dispersed blue phase liquid crystal combining the advantages of the polymer dispersion method and blue phase materials. Polymer encapsulated blue phase films showed a large Kerr constant, low switching voltage and fast response time. Moreover, the temperature range of encapsulated blue phase films were successfully expanded from 9 o C to 54 o C. xiii

16 Chapter 1 Introduction and Background 1.1 Chirality in Molecules Chirality was discovered by Lord Kelvin in 1894 [1] and it was described as a property of a molecule that cannot be superimposed over its own mirror image. The main feature that gives rise to chirality at a molecular scale is the presence of an asymmetric ally handed carbon atom. A chiral molecule typically has a carbon atom in the center of the molecule surrounded by four different substituents (Figure 1-1). Two mirror images of the chiral molecule are commonly called enantiomers and pairs of enantiomers are determined as right (for Latin rectus, right, R) and left (for Latin sinister, left, S) handed. Each chiral center is labeled as R or S related to the priority of the substituents of the molecule based on their atomic numbers. To determine the handedness of the molecule, first the chiral center is determined according to the lowest priority of the four substituents. If the priority of the other three substituents decreases in clockwise direction, it is called R (right handed), if it decreases in counterclockwise direction, it is called S (left handed). Moreover, an enantiomer can be named by its ability to rotate the plane of plane-polarized light (+/ ). The enantiomer is labeled (+), if it rotates light in a clockwise direction. If it rotates the light counterclockwise direction, it is labeled as (-). Liquid crystals may have multiple chiral centers with handedness and configuration. 1

17 Figure 1-1: Simulation of a chiral molecule which cannot be superimposed with its mirror image. 1.2 Cholesteric Liquid Crystals Chiral nematic liquid crystals are a type of liquid crystal that has a helical structure due to the molecular chirality of its components (Figure 1-2). Esters of cholesterol were examples of the first observed cholesteric liquid crystal materials. Cholesteric liquid crystals organize within layer without any positional ordering in the layer whereas the director axis rotates with the layers as shown in Figure 1-3. The rotation of the director axis is periodic and its full rotation of 360 o is called the pitch, p. 2

18 Pitch plays an important role in the reflection of the wavelength of the incident light, as a result of the periodic structure of cholesteric liquid crystals. Figure 1-2: Simulation of formation of the helical structure of a chiral nematic (cholesteric) phase. Figure 1-3: An illustration of cholesteric liquid crystal with a pitch P. 3

19 Cholesteric liquid crystals have the ability to reflect a handedness of circularly polarized light when the pitch has in the same wavelength of visible light [2]. The light will be circularly reflected if it is the same handedness as that of the cholesteric liquid crystal, whereas it will be circularly transmitted with opposite handedness as that of the cholesteric liquid crystal [3]. Furthermore, the use of chiral dopants in an achiral nematic forms new chiral materials with specific helical pitches (Figure 1-4). = + P Addition of Chiral dopant Chiral Nematic (Cholesteric)LC Nonchiral Nematic LC Figure 1-4: Formation of helical chiral nematic phase using chiral dopant. Chirality in liquid crystals can be described with regards to inverse pitch, and the material with a shorter helical pitch has a higher chirality. The chiral dopant's ability to induce helicity is the helical twisting power (HTP) (the normalized reciprocal of the pitch p -1 ) of a molecule can be defined [4]. 4

20 HTP = 1 c p (1.1) where p is helical pitch in microns and c is concentration of chiral dopant in the cholesteric liquid crystal mixture. In terms of the long-range distortions induced in the nematic by a molecule, qo is the pitch in the ground state is given by qo= 2π/P (1.2) and the pitch introduces in a scalar quantity of the free energy of cholesteric phases [3] : (1.3) where n is the director, K11 is splay elastic constant, K22 twist elastic constant and K33 is bend elastic constant. If the pitch is around 100 nm, in other words if the chirality of a material is high enough, then another phase becomes energetically favorable, which is called Blue Phase with three dimensional double twist structure [5]. 1.3 Blue Phases Blue phases were first observed in 1888 by Reinitzer who noted a brief hazy blue color that appeared in the narrow temperature range between the chiral nematic (cholesteric) and the isotropic phases [5]. Blue phases are locally isotropic fluids in which the molecules are organized themselves in complex three-dimensional (3D) structures 5

21 characterized by crystallographic space group symmetry. Hence, the blue phases form as double-twisted cylinders separated by defect lines (Figure 1-5). Effectively, it is the network of the defect line that characterizes the blue phases. Three network states are known - denoted as BP I, II and III with increasing temperature. The BPI and BPII form soft, frequently coagulating platelete-domains of micrometer to sub-millimeter size. The Bravais lattice is body-centered for BPI and simple cubic for BPII [7] (Figure 1-6). Indeed, light is selectively reflected with scattering vectors forming a reciprocal lattice of a cubic periodic system. The lattice constant is a few 100 nm depending on the radius of double twisted helix and photonic band, mostly in the blue wavelength range, is of the same order of magnitude as the cholesteric pitch. The BPIII has a cloudy and an amorphous appearance and is called blue fog. Figure 1-5: The simulation of a double twist cylinder structure of blue phase. 6

22 Figure 1-6: Blue phase structures in (a) BPI (b) BP II. Since 1980, BPs have been comprehensively studied based on the characteristics of the selective reflections which can be explained by Bragg scattering [7-8]. Previously, the blue phases have been a challenge to the experimentalists since they exhibit in a narrow temperature range [9-10]. They require extreme temperature stabilization. Thermodynamically stable blue phases of low-chirality chiral nematic behavior have been predicted using Landau theory [11] and demonstrated that the planar helix structure generally arises when the cholesteric phase becomes unstable at the temperature near the transition point. As explained by Meiboom et al. [12] on the basis of the Oseen-Frank elasticity equation, as an alternative, the temperature range of the blue phase liquid crystal (BPLC) can be estimated according to Meiboom s defect model. For blue phases, the presence of the defect lines is important for existence of the lattice structure and the energy cost of the defects is low enough to stabilize the entire phase for narrow range 7

23 temperatures. In the blue phase, the free energy per unit length for the disclination line can be described as: Fdiscl = Fcore +Fint Fsurf + Fel, (1.4) where Fdiscl is the total free energy per unit length of the disclination, Fel is the elastic energy related to the defect, Fsurf is the free energy at the disclination surface and Fint is the energy related to melting of area to the isotropic core. For blue phase double-twist cylinder lattices, the free energy calculations of Meiboom et al. include Fcore as the only temperature-dependent term: Fcore = α(tiso-t)πr o2, (1.5) where Tiso is the isotropic transition temperature, R0 is the defect core radius size and the difference in free energies of the isotropic and ordered phases at temperature T is represented by α(tiso-t). The surface energy at the interface between core and cholesteric is characterized by a surface tension, σ (Eq.1.6); Finterface = 2σπRo (1.6) Fsurf can be turned into a surface integral and ignored since surface terms do not scale competitively with the bulk terms, therefore they are negligible. Therefore the interior 8

24 surface of the disclination must be taken into consideration. In that case the solution includes the energy per unit length of the disclination line (Eq.1.7): Fsurf = -π(k22+k24) = -πk (1.7) Fel is the elastic energy where K is the elastic constant and R max is the radius of the double twist cylinder and Ro is the defect core radius. In order to extend the blue phase temperature range, one must minimize the energy cost of the disclination lines. Isotropic particles, such as nanoparticles or monomers, are expected to move towards isotropic areas of a liquid crystal for the purpose of serving to minimize the core energy. Adding nanoparticles into an isotropic sample and cooling to the blue phase causes accumulation of the isotropic particles in the defect lines. Furthermore, the nanoparticles will disrupt any tendency towards orientational order inside the core according as temperatures decreased into the blue phase. However, the surface energy at the interface between core and cholesteric was simplified and assumed zero during the energy minimization for the system. Recently, Coles group used a mixture of nematic bimesogenic liquid crystals [13] to stabilize the defect structures against the temperature variation and obtain the blue phases over a range of about 50 o C. This stabilization resulted from the flexoelectric 9

25 coupling between polar order and curvature of the director and was attributed for the change in blue phase properties [14,15]. Unfortunately, the report does not address the question of whether BPI appears at 16.5 o C on heating from the smectic phase. This would most clearly indicate thermodynamic stability of BPI. Kikuchi et al. [16-18] developed a technique for broadening the BP temperature range with a polymerized polymer network, referred to as the polymer-stabilized blue phase (PSBP). The polymers in the PSBP were found to form a remarkably unique aggregation structure, selectively concentrated in the disclination cores, which has been identified by synchrotron small angle X-ray scattering measurements [19]. This result clearly corroborates the mechanism of the stabilizing effect of BPI arising from the immobilization of the disclination in the blue phase by polymers. Alternatively, Yoshizawa et al. [20] could expand the temperature range of the BPs more than 10 o C using chiral T-shaped compounds. Yelamaggad et.al. [21] stabilized the BP more than 20 o C using chemically linked bent core molecules. Wang et al [22] not only introduced BPs in a wide temperature range using ZnS nanoparticles, but also showed the stability of the cubic structures against the electrical field. Recent studies on BPs with an extended temperature range make them more attractive for applications since the BPs show some specific electro-optical (E-O) properties. More interesting E-O properties are the fast response time [23], wide viewing angle and also any surface treatments are not necessary for the BPs. 10

26 This dissertation discusses different types of stabilization methods of BPs, such as polymer stabilized BPs, addition of carbon nanotubes and bent-core molecules, in order expand the thermal stability minimizing the free energy of the disclination lines. The stabilization of liquid crystal blue phases can be obtained using polymerizable siliconbased nanoparticles due to the potential to modulate interfacial properties. 1.4 Polymer Stabilized Blue Phase Liquid Crystals Polymer-stabilized blue phase (PSBP) liquid crystals have been studied commonly for two decades [24-32] because of their great potential for use in display devices or as an optical modulator. Based on the concept of the polymer stabilized liquid crystal the orientation of liquid crystal directors can be stabilized by a crosslinked network dispersed in a liquid crystal [24]. The liquid crystal director can be kept at certain states of polymer network due to the strong anchoring interaction between the polymer and liquid crystal. The polymer network was obtained by in situ-polymerization in the BP, and results in the stabilization of the disclinations in the lattice (Figure 1-7). The polymer network plays a fundamental role in enhancement of temperature range, giving rise to the thermodynamic stabilization of BP [24-27]. In the polymer stabilization of BP liquid crystals, either thermally polymerizable [28] or photopolymerizable [24-32] monomers can be used and the photoinitiator decomposes into free radicals after it is excited electronically via the absorption of UV radiation during the photopolymerization process. A three- 11

27 dimensional polymer network is formed by the reaction of benzoyl radicals with the double bonds of the diacrylate monomer through a chain reaction [28]. Furthermore, molecular mobility of the network polymers obtained in the PSBP affects the stability of PSBP [29]. Additionally, the electro-optic properties can be improved due to the variation of the flexibility of the molecule, the length of the rigid core and the polymerizable functional group of reactive monomer [30]. Figure 1-7: Simulated polymer network structure within a cubic lattice of a polymer stabilized blue phase (blue, orange and red colors shows the disclination lines in BP). Polymer-stabilized blue phase (PSBP) liquid crystal [ ] has advantage; it does not require any surface alignment layer, and it has submilisecond response time, and wide viewing angle, as well as cell gap insensitivity in an in-plane switching (IPS) cell. Due to 12

28 these advantages, PSBP becomes an attractive material as a next generation display technology [36-38]. On the other hand, its wide-spread application has some limitations due to a high operating voltage, and a low-contrast ratio due to residual birefringence and hysteresis [34,35]. In order to overcome the high operating voltage, there are two common approaches. One of these approaches is using a small electrode gap to produce a strong electric field [37,38]. The second approach is electric-field-induced birefringence known as the Kerr effect [39]. Moreover, PSBP with large Kerr constant was reported by Kikuchi et al. [34] and Wu et al. [38,40-42]. In the absence of the electric field BPs do not have birefringence and induced birefringence occurs with the applying electrical field. The electro-optical behavior of BP can be explained by the Kerr effect. Induced birefringence is linearly proportional to E 2, where E is the electric field and this linear relationship is valid only in the low field region. 1.5 Polymer Dispersed Liquid Crystals Polymer dispersed liquid crystals (PDLCs) have become the topic of considerable interest during the last decades, because of their potential applications such a smart windows, flexible displays, projection displays, and holographic gratings [43-47]. The PDLC films have been widely studied as a candidate for the large area display because of the simplification of the preparation process and because their light transmittance is higher than conventional LCs in the absence of polarizer by the reason of their light scattering nature [48-53]. PDLC films are a mixed phase of micron-sized liquid crystal 13

29 droplets, which are randomly dispersed inside a polymer matrix [54]. In general, the polymer weight concentration is between 30% and 60% [54]. There are four general methods for the fabrication of PDLCs. The first method used in the preparation of PDLC films is the encapsulation (emulsification) of the liquid crystal inside an aqueous solution of film-forming polymer. The second method includes the solvent-induced phase separation (SIPS), solvent is used to dissolve the liquid crystal and thermoplastic polymer and creates a single phase. Then solvent was evaporated at a certain rate to induce phase separation. The third method is called thermally induced phase separation (TIPS). In this method, the liquid crystal and thermoplastic polymer are heated to obtain a melt and mixed to form a single phase. When the mixture is cooled at a controlled rate, the liquid crystal phase separates into droplets. The fourth method is known as polymerizationinduced phase separation (PIPS) and includes the liquid crystal that is dissolved into the monomer. This method uses ultraviolet radiation to initiate the free radical polymerization of monomers [54]. One of the main advantages of this method is the possibility to form a composite directly between two glass substrates coated with Indiumtin-oxide (ITO) without any requirement of laminating procedure. The above methods produce a wide size distribution of liquid crystal domain size [3]. PDLCs are operated based on the micron-sized LC droplet dispersion inside the polymer matrix and the scattering performance of the PDLC film is determined by the LC droplet size. The operation principle of the PDLC films - electrically switchable between light scattering and transparent states or vice versa depends on the refractive indices matching between guest and host materials. [55-56] At zero applied voltage, the PDLC films normally appear 14

30 milky and scatter incident ambient light because the LC molecules orient randomly inside of droplets. Upon applying a voltage across the PDLC film, the LC directors align in the direction parallel to the applied field. Due to matching in indices of refraction between polymer and liquid crystal molecules under the electric field, PDLC film becomes transparent at normal viewing direction (Figure 1-8). Additionally, H-PDLC, which is the another type of PDLC, includes liquid crystal droplets smaller than that of PDLC [57] and they are staged in varying planes in accordance with the polymer. There are two modes of H-PDLCs, which are called transmissive and reflective. In the transmissive mode, diffraction occurs by an applying voltage and light is reflected in the absence of electric field. In the reflective mode, light is reflected in the absence of electric field, with the applying voltage it transmits through the display. Figure 1-8: Working principle of Polymer Dispersed Liquid Crystal a) on state-light transmitted mode b) off-state-light scattering mode. 15

31 1.6 Polymer Dispersed Blue Phase Liquid Crystals Polymer dispersed liquid crystals (PDLCs) are composite materials comprised of liquid crystal droplets of micron-sized embedded in a polymer matrix. These composites have been used to fabricate flexible and transparent displays [58,59] as well as switchable windows [60]. Importantly, the electro-optical properties of PDLCs can be altered by changing the concentration of liquid crystal and polymer [54,61,63]. PDLC films can be switched between two states by applying voltage. The PDLC films show scattering at zero voltage due to the mismatching refractive indices of polymer and liquid crystals and with an applied voltage they switch to the transparent state. One of the advantages of PDLCs is that they don t require polarizers [62]. Blue phase liquid crystal (BPLC) materials have aroused great interest based on their optical properties and potential for advanced applications for display material and technology. These advantages include field-induced birefringence of BPLC due to its sub-millisecond response time, which is at least one order of magnitude faster than the present nematic liquid crystal based displays. However, BPLCs have wide and symmetric viewing angle because the voltage off stage is optically isotropic and the voltage on state forms multi domain structures of BPLC. Therefore, blue phase liquid crystal can be a potential candidate for polymer encapsulated liquid crystal films due to their fast switching properties. In this dissertation, the polymer dispersed blue phase liquid (PDBP) crystal droplets were reported first time. We reported a new form of PDLC E-O films composed 16

32 of BPLC and polymer prepared either using the solvent evaporation method or polymerization-induced phase separation methods. Polymer dispersed blue phase liquid crystal droplets showed two switching modes as reverse-mode PDBP and normal-mode PDBP. In the reverse-mode PDBP, there is a refractive index matching between BPLC and polymer phases since blue phases are randomly oriented and optically isotropic in the absence of the electric field. Therefore, they have a very small difference in the refractive index between neighboring droplets and PDBP droplets will be transparent in the absence of the electric field. When an electric field applied across the PDBP film, the molecules of BP will align and light scattering occurs as a result of a mismatching in refractive index between BPLC and polymer. The normal-mode PDBP obtained with the increasing BPLC concentration. Details will be discussed in Chapter 6. Besides, there is a temperature range limitation for the application of polymer dispersed blue phase liquid crystal. In order to solve this issue, we also stabilized the encapsulated blue phase droplets in a wide temperature after forming the blue phase droplets by emulsification method that has been not reported before. The electro-optical (E-O) performances of these devices were investigated by taking of the advantages of great E-O properties of blue phases. 17

33 1.7 Dissertation overview The dissertation will cover electro-optical applications of polymer-stabilized and polymer dispersed blue phase liquid crystals. The introduction part describes studies of Bragg reflected colors as well as the transmittance in PSBP and PDBP liquid crystals as a function of applied field. Our goal is to understand, develop and characterize how organic-inorganic nanocomposites can be used to stabilize blue phase and systematically modify the surface energy at the interface between core and cholesteric which was previously simplified. Chapters 2 through 7 describe experimental details including the stabilization of blue phase liquid crystals. Chapter 2 presents characterization methods. Chapter 3 covers the blue phase stabilized by polymer network based on the variation in concentration of monomer. This chapter presents studies of a new organicinorganic composite that may ultimately allow us to achieve the above benchmarks of broadening of blue phase temperature range and enable the development of a high performance Kerr device. Chapter 4 describes doping of carbon nanotubes (CNTs) doped into the mixture of monomer and blue phase liquid crystals to enhance the temperature range and improve the electro-optical properties in an in-plane switching (IPS) cell of polymer stabilized blue phase (PSBP) liquid crystals. We study the effect of doping CNTs on temperature 18

34 range and the threshold voltage as well as the decay time due to enhancement of the elastic constant of CNT-doped PSBP liquid crystals. Chapter 5 covers bent-core doped blue phase liquid crystal based on the variation in concentration and chemical structures of bent-core molecules. Chapter 6 presents an exploration of a new form of PDLC electro-optical films comprised of blue phase liquid crystal and polymer prepared by either the solvent evaporation and polymerization-induced methods. The compositions, film preparations, physical and morphological behaviors, and electro-optical properties of PDBP films are described. The electro-optical (E-O) performances of PDBP films are characterized by analyzing transmittance as a function of switching voltages and response times using an in-plane field switching cells. Chapter 7 describes stabilization of polymer dispersed blue phase liquid crystals Chapter 8 discusses a discussion the possibilities on the future of polymer stabilized and polymer dispersed blue phase liquid crystals. 19

35 Chapter 2 Characterization Methods 2.1 Characterization Methods Identification of the blue phases is a challenging due to its narrow temperature range. Polarizing optical microscopy (POM) was used for characterization of blue phase liquid crystals such as texture identification and phase range determination. Other characterization methods used in this study include spectroscopy and electro-optic characterization. Scanning electron microscopy is used in order to examine the morphology of polymer network in the polymer stabilized blue phase liquid crystal samples and image analysis of size of the polymer droplets in polymer dispersed blue phase liquid crystal samples. Additionally, contact angle measurement was carried out to determine surface tension and anchoring behavior of the polymer encapsulated blue phase (PEBP) liquid crystal droplets Polarizing Optical Microscopy Polarizing Optical Microscopy (POM) is a crucial characterizing technique for the texture identification of new liquid crystal material. A polarizing microscope is equipped with a polarizer placed in the light path and an analyzer positioned in optical pathway 20

36 between the objective rear aperture and camera port. A polarizer is a filter that only allows the light in the specific direction along its polarizing direction to pass through. In order to obtain an image using the POM, there are some important parameters of electromagnetic radiation such as intensity, frequency (wavelength), polarization and angular momentum. Intensity can be described as power per unit area with the unit of watts per meter squared (W/m 2 ). It is possible to control over this property with variable power bulbs. The primarily used wavelengths of these filters are 589 nm, 486 nm, and 656 nm, which can be called D line, F line, and C line, respectively. When visible light is incident on a material, the light can be transmitted through the material, refracted from the surface, or both. The law of refraction states that the angle of refracted light is equal to the angle of incident light. Figure 2-1: Schematic illustration of Snell s law. 21

37 Additionally, the speed of light is certainly not a constant in all mediums. The ratio of the speed of light in vacuum to that in a medium is named the refractive index. Mathematically, refraction complies with Snell s law (Figure 2-1): where nr is the refractive index of the refracting material, ni is the refractive index of the incident material, and θi is the angle of the light incident with the perpendicular direction to the interface and θi is the refractive angle. This basic law describes the principle relationship for the manipulation of light by glass lenses and in microscopes. In POM, light cannot transmit in crossed polarizers. When an anisotropic material is inserted between a polarizer and an analyzer, linearly polarized light pass through a sample with thickness d and birefringence n (Figure 2-2). Randomly (unpolarized) light Polarizer Transmitting axis Analyzer d θ Transmitting axis I=I o Cos 2 θ Figure 2-2: Propagation of light through a sample between crossed polarizers. 22

38 Light will travel along the sample in two paths at different speeds, one along the extraordinary and one along the ordinary axes of the sample. The phase retardation of light which travels through a materials is expressed by where Io is the intensity of transmitted light, φ is the angle between the local director and the polarization direction of the incoming light, d is the sample thickness, θ is the angle between the direction of light propagation and the axis of a filter, λ is the wavelength of light, and ne and no are the extraordinary and ordinary refractive indices of LC material. Although it is quite simple to imagine the repeating patterns in a cubic system such as sodium chloride, observation of the blue phases is slightly more difficult. Blue phases don t have any order because they are optically isotropic and randomly oriented. We clearly cannot arrange the repeating structure based on individual molecules at spaces along the axes. Blue phase liquid crystals can be determined with POM due to Bragg reflection of different domains of the cubic lattice. The blue phase domains show different reflection wavelength and colors under the POM such as blue, green or mosaic structure, depending on the orientation and dimension of the crystal lattice as well as the sizes of the domains, as seen in Figure

39 The POM used to image the BPLC samples was a Nikon Transmission/Reflection Polarized Optical Microscope equipped with an Amscope MU100 camera with 1832 x 1374 resolution and textures analyzed using Toup View 64 Software. a) b) c) Figure 2-3: An example of a) BPII b) BPI c) texture colorful platelet texture of BP liquid crystal mixtures formed using 55 wt% R Bragg Reflection Spectra The first optical spectrum I(λ) experiment of blue phase liquid crystal based on cholesteryl ester was carried out by measuring the wavelength (λb) of a Bragg peak in the spectrum as a function of temperature by Stegemeyer et al. [64,65]. Their results are comparable with that of Gray et al. [66], which refer to cholesteryl esters having at least two blue phases of cubic symmetry. The cubic symmetry structures of the two blue phases were called BPI and BPII. These results were later confirmed by Meiboom and Salmon [67,68], who measured the spectrum of light transmitted across polycrystalline samples comprised of crystallites smaller than the diameter of the light beam and oriented 24

40 randomly. Marcus investigated these polycrystalline samples with a reflecting microscope [69-70]. Although there were many polygonal crystallites in the sample, he recognized that only a few of these crystallites were shining and showing monochromatic Bragg reflection at the blue wavelength (λb). In the other words, blue phase liquid crystals exhibited brilliant colored reflection of circularly polarized light, which can be explained by Bragg scattering. Bragg scattering defines the angles for coherent and incoherent scattering from a crystal lattice and was first discovered by W. H. Bragg and W. L. Bragg in 1913 [71-74]. They describe Bragg peaks as the intensive peaks of reflected radiation of these crystals at particular wavelengths and incident angles [75-77]. W. L. Bragg modeled the crystal as a set of distinct parallel planes separated by a constant parameter d by proposing that the incident X-ray radiation would generate a peak when their reflections from the different planes interfered constructively. According to Bragg s law, the reflection wavelength is simply related to incident angle as given by, where n is an integer, λ is the wavelength of incident wave, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes (Figure 2-4). 25

41 d θ θ Figure 2-4: Schematic illustration of Bragg scattering. Due to the cubic lattice, the reflection wavelength of a blue phase liquid crystal is different from cholesteric liquid crystals. This is governed by Bragg's law, which defines the condition for constructive interference from consecutive crystallographic planes of the crystalline lattice: where a is the lattice spacing of the cubic crystal, and h, k, l are the Miller indices of the Bragg plane. The different crystal planes are described by their Miller indices. Miller indices are a symbolic vector representation for the orientation of an atomic plane in a crystal lattice and constructed by determining the intersection of the plane with the x, y, z axes and then taking the reciprocals, by convention written as (hkl). Figure 2-5 shows the intercepts of the plane of red atoms are (1,0,0), (0,1,0) and (0,0,1) on going from left to right. The Miller indices (100), (010) and (001) were determined by taking the 26

42 reciprocals of each intercept. However, the expression (100) may be used to specify either an individual plane or the whole set of planes. Figure 2-5: a) simple cubic b) body-centered c) face-centered cubic. The structural arrangement of three cubic structures within a plane and between different planes may be different. Figure 2-6 shows the (100) planes of simple, body-centered and face-centered cubic lattices. Figure 2-6: The (100) planes of a) simple cubic, b) body-centered, c) face-centered cubic. 27

43 The lattice parameter of the cubic crystal is simply the length of the cube edge and denoted as a. The surface structure is easily calculated from the lattice parameter. As an example, the (100) surfaces of the three cubic structures are shown in Figure 2-7a. Those of simple and body-centered lattices are identified with the square lattices of lattice parameter a. Whereas, the face-centered lattice is identified with a/ 2. Figure 2-7b shows the diagram of the surface lattices obtained from a cut along a (111) plane of each of the cubic lattices. The resulting lattice parameters of simple and body-centered lattices are 2 a, and it is given by a/ 2 for face-centered lattices structure. Figure 2-7: a) simple cubic b) body-centered cubic c) face-centered cubic. 28

44 As a consequence, the reflection wavelength for light normally incident on a crystal plane is given as in Eq. (2.7): Electro-Optical Measurement The field-induced birefringence, Kerr effect, and fast optical response are some of the important electro-optical properties of blue phases. In order to determine the Kerr effect, an in-plane switching (IPS) cell is placed between a pair of polarizers crossed at 90 o and switched from dark state to bright state. The maximum brightness is obtained when the IPS cell stripes are at 45 o between the crossed polarizers. The external electrical field was produced using a power supply. The experimental setup used in our experiment is shown in Fig A He-Ne laser light beam (λ=632.8 nm) was passed through the polarizer with optical axis at 0 o, IPS cell with electrode aligned at the 45 o and the analyzer with optical axis at 90 o The transmitted light was read by the photodiode detector and recorded with a computer. 29

45 Figure 2-8: Experimental setup for measuring the electro-optical response Scanning Electron Microscopy The scanning electron microscope (SEM) is a type of electron microscope that is used to produce high-resolution images of objects and to display spatial variations in the samples. The SEM generates images of a sample by using a focused beam of high energy electrons. As a result of the interaction of these electrons in the sample, a variety of signals at the solid surface can be detected. These signals contain information about the external morphology (texture), chemical composition and crystalline structure of the sample. A resolution of better than 1 nanometer can be accomplished using a conventional SEM. SEM generates different types of signals, which are called secondary electrons (SE), back-scattered electrons (BSE), characteristic X-rays, and light 30

46 (cathodoluminescence) (CL). Secondary electrons are commonly used for imaging the morphology and topography on samples, while backscattered electrons are most useful for showing contrasts in composition of multiphase samples. X-rays are generated as the result of inelastic collisions between the incident electrons and electrons in discrete orbitals of atoms in the sample. X-rays are produced at a fixed wavelength because of returning of the excited electrons to lower energy states (this is because of the energy level differences of electrons in the different shell for a given sample). Essential components of all SEMs are schematically illustrated in Figure 2-9. An electron gun equipped with a tungsten filament cathode emits the electron beam thermionically. Tungsten is mostly used in thermionic electron gun due to its highest melting point and lowest vapor pressure of all metals. The electron beam with an energy ranging from 0.2keV to 40keV is focused to spot about 0.4 nm to 5 nm in diameter via condenser lens. Electrons lose energy as a result of the interaction between the primary electron beam and sample via random scattering and absorption within the interaction volume of the sample. The size of the interaction volume is affected by the electron's landing energy, and the atomic number and density of the sample. SEMs includes detector (usually a secondary electron detector) and the detector determines the beam absorbed by the sample and produces an image using electronic amplifiers to amplify the signals. SEM has an advantage of fast data acquisition for many applications. However, SEM has some limitations of not only requirement of low vacuum but also suitable 31

47 samples size to the microscope chamber. Moreover, this method is not applicable for the examination of wet samples. For our polymer stabilized and polymer encapsulated blue phase mixtures, SEM is especially helpful in examining the morphologies such as shape, size, and density of polymer network and polymer droplet. Figure 2-10 shows the pictures of polymer network in PSBP and polymer droplet in PEBP samples. The morphological studies of the samples were performed by extracting blue phase liquid crystal from the cell with a mixture of dichloromethane and hexane at ratios of 20% and 80%, respectively.after the cell was opened and dried under reduced pressure, the polymer was sputtered with a thin layer of gold and examined with a Hitachi S-2600N scanning electronic microscope (SEM). 32

48 Figure 2-9: Schematic illustration of scanning electron microscope. 33

49 Figure 2-10: The SEM micrographs of polymer morphology of (a) PSBP cell of sample with 10% polymer on the surface of a substrate with electrode (the arrow indicates electrode direction) (arrow shows the preferential orientation) (b) PDBP films show the top view of droplets at the surface of a substrate with an electrode of 22-µm IPS cell Contact Angle Measurement The contact angle describes the angle at which the phase boundaries of different substances meet. The substrates encountered are gas-solid and liquid. The contact angle is measured at the line of the intersection of these three phases simultaneously (Figure 2-11). The relationship between contact angle and other surface properties was reported by Mankowich 79,80]. Contact angle measurements have been used for many years in order to evaluate the surface energy, roughness, and wettability of surfaces. Contact angles less than 90 o means that wetting of the surface is favorable, therefore the fluid will spread over on the surface. While contact angles larger than 90 o indicate that wetting of the surface is unfavorable and the fluid will reduce its contact with the surface. In other 34

50 words, hydrophilic surfaces are characterized by angles smaller than 90 o, whereas contact angles larger than 90 o correspond to hydrophobic surfaces. Superhydrophobic surfaces occur if the water contact angle is greater than 150 o. In that case, there is no contact angle between the liquid drop and the surface; this phenomena is called the lotus effect [81]. Figure 2-11: Pictorial illustration of contact angle of the droplets. There is a close relationship between contact angle and surface tension. The fluid will spread over the surface due to the decreasing of liquid surface tension when molecules in liquid and molecules on the surface have similar forces. Therefore, contact angles are affected by the interactions between atoms and surface at the liquid-solid-vapor interfaces. The liquid droplets are formed with different shapes due to these intermolecular forces, which is called the surface tension [82-84]. The contact angle θ of a liquid droplet on a solid surface is determined by svv cos, (2.8) sl lv 35

51 where sv, sl and lv represent the solid-vapor, solid-liquid and liquid-vapor interfacial tensions, respectively. Furthermore, the wetting process is not only a static state and a droplet shows many metastable states. Therefore the observed angles are usually not equal to θ. The contact angle produced is described a dynamic contact angle when the three-phase contact line is in real motion. Principally, the contact angles construct by expanding and contracting the liquid are defined as the advancing contact angle θa and the receding contact angle θr, respectively. The contact angle will increase by adding a small amount of liquid drop where the contact line will be pinned. Likewise, by removing a small amount of liquid drop, the contact angle will decrease. The difference between the advancing (maximal) angle and the receding (minimal) angle is named the hysteresis (H): H = θa θr. Contact angle measurement of PEBP samples was performed using a Goniometer (Figure 2-12). 36

52 Figure 2-12: Illustration of the goniometer setup. 37

53 Chapter 3 Polymer Stabilized Blue Phase Liquid Crystals 3.1 Introduction Blue phases (BPs) represent the most interesting self-organized structures of liquid crystals, which generally appear over a narrow temperature range between the chiral nematic (cholesteric) and isotropic phases [85]. BPLCs have the potential for varied applications because of their electro-optical properties, such as fast response time, wide and symmetric viewing angle, and lack of requirement of any surface alignment layer. However, the narrow temperature range of BPs is one of the main limitations for their practical applications [24, 86,87]. Recently, two separately reported methods to broaden the BP temperature range have drawn attention to blue phase materials, which have thereafter become a subject of extensive research in exploiting applications in new optics, photonics, and information displays because of the outstanding electro-optical properties of BPLCs. The first method uses a small amount of polymer for polymerization that is phase-separated to the defects of the blue phase. It has been reported that polymer stabilization aids the extension of the BP temperature range to more than 60 K including room temperature with an ultrafast response time [24]. The second reported approach is to use the nematic bimesogenic liquid crystal mixtures to stabilize the defect structures of the blue phase, and it has been possible to extend the blue phase temperature range to 38

54 over 50 C [13]. The first method proposed that the reason of the increment of the BP temperature range is the polymer network, which is concentrated in the isotropic defect core and localizes the disclination core of BP. Therefore, molecular reorientation of liquid crystal directors is hindered via crosslinked network of the polymer, which is formed by in-situ polymerization [24]. In this chapter we explore the stabilization of cholesteric blue phases using polymerizable silicon-based nanoparticles to modify the interfacial properties of disclination cores and broaden the blue phase temperature range. Concentration of polymer plays an important role in the thermodynamic stability of modulated liquid crystal blue phases. The low surface energy feature of the inorganic polymer leads to significant reduction of the switching voltage of the corresponding device. It dramatically decreases the switching voltage from about 140V to 40V [14,15,25,88]. The significant reduction in the switching voltage and widening of blue phase temperature range are useful for new electro-optical applications. 3.2 Polymer stabilization of blue phase with UV curable nanoparticles The temperature range of blue phase was successfully expanded to more than 100 o C in the polymer/blue phase liquid crystal composite system by Kikuchi et al [24]. In order to understand, develop, and characterize how organic-inorganic nanocomposites can be used to stabilize blue phase and systematically modify the surface energy at the interface 39

55 between defect core and cholesteric, which was previously simplified. To address this issue, we used a mixture of monomers consisting of a monomeric nanoparticle with an acrylate polymerizable group, polyhedral oligomeric silsesquioxane (PSS-(1- Propylmethacrylate)-Heptaisobutyl (PSS-PMH, Aldrich Chemical), and a commercially available mesogenic diacrylate RM257 (Merck). The chemical structures of the monomers are shown in Fig Polyhedral oligomeric silsesquioxanes (PSS-PMH) are structurally well-defined compounds with the empirical formula RSiO1.5, where R is a hydrogen atom or an organic functional group, such as, alkyl, alkylene, or their derivative groups [89-91]. PSS-PMH may be described as a silica nanoparticle comprised of a silica cage and organic functional groups connected to the corners of the cage (Fig.3-1). PSS- PMH molecules can be named monofunctional or multifunctional depending on the numbers of reactive organic groups [89-90]. The diameters of PSS-PMH molecules are in the range 1-3nm [89]. Additionally, POSS molecules can be dissolved in the polymer matrix at the molecular level unlike of traditional fillers. The incorporation of PSS-PMH derivatives into the polymeric systems may provide significant improvements in polymer properties involving expansions in use temperature range, mechanical properties and viscosity during process [92-94]. Unlike many other nanoparticles, PSS-PMH nanoparticles, which are expected to have low surface tension, are easily dispersed in a wide range of polymer matrices. The inorganic PSS-PMH segments covalently bound to the polymer network provide unique opportunities to stabilize the blue phases, due to the size and Van der Waals interactions of the polyhedral oligomeric silsesquioxane functional groups with cholesteric liquid crystal molecules. We expect the polymerized nanoparticles will 40

56 accumulate in the defect cores due to the size of the nanoparticle, which will lower the interfacial energy. Furthermore, the nanoparticles will disrupt any tendency towards orientational order inside the core according as temperatures decreased into the blue phase. Figure 3-1: a) A chemical structure of Polyhedral oligomeric silsesquioxanes (PSS- PMH), and b) RM257 (bis[acryloyloxy-(4-propoxyl(1,4-pheny benzoate))]. Small amounts of photoinitiators are added to the reactive LC formulations to increase the sensitivity to UV light. A typical free-radical initiator used for polymerization of the LC diacyrlates is α,α-dimethoxydeoxybenzoin (Irgacure 651, 41

57 CIBA) is shown in Fig Such sensitized LC formulations are photopolymerized by exposure to UV light in the region between nm. Figure 3-2: A chemical structure IR Cell preparation A representative BPLC material comprises of 55wt% of a commercially available nematic liquid crystal (BL006 and MLC6080 (Merck)) and 45wt% of chiral dopant (20% CB15, 20% R811 and 5% R1011). BL006 and MLC6080 belong to the group of the low viscosity liquid crystals. Although the nematic liquid crystals are similar, whose differences include the extra florine atoms attached to the benzene ring of mesogen in nematic mixture MLC6080. A diacrylate mesogenic monomer (bis[acryloyloxy-(4- propoxyl-(1,4-phenybenzoate))]) RM257 (Merck), PSS-PMH monomer and a small amount of photoinitiator (0.01 wt. % of Irgacure 651 by the weight of reactive monomer and BPLC mixture) were added to the BPLC mixture. Transition temperature and reflection spectra measurements were determined using planar glass cells with a cell gap of 15 m separated by glass ball spacers. All samples were heated to the isotropic phase 42

58 and cooled to room temperature at a rate of -0.2 C/min. The optical textures were observed with a polarizing optical microscope (POM) and the samples were placed on a hot stage with a programmable temperature controller and positioned between a pair of polarizers crossed at 90 degrees. Reflection spectra measurements were determined with an Ocean Optics spectrometer at various temperatures. In-plane-switching (IPS) cells were prepared with inter-digitated indium tin oxide (ITO) electrode, which was lithographically patterned to give 10 m electrode width and 10 m electrode space on one glass substrate. The IPS cells were assembled by placing a second substrate without ITO electrode on top of the first substrate, and the two substrates were separated with 5 m glass ball spacers to give a cell gap of 5 m (the cell configuration is referred as 10/10/5) as shown in Fig The mixtures of reactive monomer and BPLC were filled into the cells, and the cells were exposed to a UV lamp (365 nm, 0.4 mw/cm 2 ) from the electrode side for 40 minutes at the BPII temperature of the BP mixtures before polymerization. An in-house assembled E-O apparatus consisting of a helium-neon laser with light emission at 633 nm, a pair of polarizers crossed at 90 o with respect to the polarization axes, a diode detector, a computer-controlled function generator, and an amplifier were used for data acquisition. The acquisitions of light transmittance as a function of applied voltage curves and response times were carried out at BP phase temperatures. 43

59 Figure 3-3: Illustration of in-plane switching (IPS) cell Observation of blue phase temperature range In order to investigate the polymer network effects on blue phase behavior, samples with different monomer concentrations were formulated and the compositions of these samples are listed in Table 3.1. Samples 1, 2, and 3 contain the nematic BL006, while sample 4 consists of a mixture of nematic liquid crystals BL006 and MLC6080 at the ratio of 1 to 1 mixed at the same ratio of nematic to chiral dopant. The phase transitions of pure blue phase mixtures of samples 1, 2, and 3 show the isotropic-to-bp transition at 41.8 o C and BP to cholesteric phase at 32 o C. By using the mixture of MLC6080 and BL006 (sample 4), blue phase was observed in the wider temperature range. The phase transitions of this mixture show the isotropic-to-bp transition at 43.8 o C 44

60 and BP to cholesteric phase at 30.7 o C. Several research groups have studied the thermal stability of BPI theoretically and experimentally [86,24,95-99]. The elastic constant of host nematic liquid crystal was varied gradually by adding bent core molecules to the conventional nematic LC and demonstrated that the thermal stability of BPI depends on the elastic constant of nematic LC. In our system, the elastic constant ratio of bend to splay (K33/K11) of nematic BL006 is The average ratio of bend to splay (K33/K11) of sample 4 was (( )/2) = The temperature ranges of BPI of the samples 1-3 is between o C, whereas that of the sample 4 was 24 o C. These results suggest that blue phase temperature range is associated with values of the K33/K11 ratio [95]. When the value of K33/K11 ratio is decreased, BPI becomes more stable. The purpose of recognizing the stabilization effect of blue phase, PSS-PMH was replaced with RM 257 at the concentration of 10% (sample 3* in Table 3.1). The use of RM257 didn t have an important effect on the blue phase stabilization. PSS-PMH nanoparticles are easily dispersed in a wide region of blue phase mixture due to a branched structure of the side alkyl group in the polymer. 45

61 Table 3.1. The compositions of monomer mixtures and transition temperatures of BPLCs and PSBPs. Sample Monomer conc. (wt%) (RM/PSS- PMH) BP temp. range before polymerization ( o C) ( T) ( o C) BP temp. range after polymerization ( o C) ( T) ( o C) 1 6 (1:2) (1:1) (1:4) * 10(10:0) (1:4) Figure 3-4 shows the textures of POM images of PSBP samples at various temperatures after polymerization. Blue phase textures of all PSBP samples were observed initially with small colored domains and the domain size increased with the decrease in temperature as the result of distortion of the cubic lattice. According to the POM observations, the BP temperature ranges of samples 1-4 were expanded after polymerization. The stabilization had a remarkable influence on the temperature ranges of the BP samples. The temperature range was expanded to more than 30 o C for sample 4 by the stabilization. 46

62 Figure 3-4: POM images of (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 3*,(e) sample 4 after polymerization under crossed polarizers. In order to confirm the temperature range of BP, we used a theoretical estimation method reported in Ref. [12] for all samples. The temperature range was found to be 10.3 o C for 47

63 the first three samples according to the theoretical calculation, and this theoretical value was quite well fitted with the experimental value of 9.8 o C. The theoretical value of the temperature range of sample 4 was calculated as 13.1 o C, which matches the experimental value of 13.1 o C. After polymer stabilization, the BP range expands to 17.3 o C for sample 4. Detailed theoretical analyses of temperature expansion were as follows Theoretical analysis of temperature expansion For blue phases, the presence of the defect lines is important for existence of the lattice structure and the energy cost of the defects is low enough to stabilize the entire phase for narrow range temperatures. In the blue phase, the free energy per unit length for the disclination line is described by Fdiscl = Fel +Fsurf +Fcore +Fint, (3.1) where Fdiscl is the total free energy per unit length of the disclination, Fel is the elastic energy related to the defect. For BP double-twist cylinder lattices Meiboom, et al. s free energy calculations include Fcore as the only temperature-dependent term: Fcore = α(tiso-t)πr o 2 (3.2) where Tiso is the isotropic transition temperature, R0 is the core radius size, and the difference in free energies between the isotropic and ordered phases is represented by α(tiso-t) at temperature T. 48

64 The interfacial free energy at the interface between core and cholesteric is characterized by a surface tension, σ (Eq. 3.3); Finterface = 2σπRo (3.3) The K24 saddle-splay surface elastic constant can be turned into a surface integral and ignored since surface terms do not scale competitively with the bulk terms, therefore they are negligible. For that reason, the interior surface of the disclination must be taken into consideration. In that case, the solution includes the energy per unit length of the disclination line (Eq. 3.4): Fsurf = -π(k22+k24) = -πk (3.4) In order to extend the BP temperature range, one must minimize the energy cost of the disclination lines. Isotropic particles, such as nanoparticles or monomers, are expected to move towards isotropic areas of a liquid crystal on the purpose of minimizing the core energy. The corporations of nanoparticles or monomers into an isotropic sample and cooling to the BP cause accumulation of the isotropic particles in the defect lines. Meiboom et al. minimize the free energy of the disclination of blue phase with respect to the defect core radius and found an equation to express the relationship between temperature range and defect core as a function of chirality. In that case, from a provided estimate the minimum free energy can be obtained when the formula 3.5 [12] was used. 49

65 T = T 1 T = K 8aR 2 (3.5 Then the temperature range can be estimated from a provided formula in Ref.[100] as follows, T = 500xKxHTPxc2 a, (3.6) where K is the typical elastic constant of bend in the BP mixture (K=K33), HTP is the helical twisting power of the chiral dopant, p is the pitch of liquid crystal, c is the weight concentration of the chiral dopant, and a is coefficient that can be estimated from the latent heat of cholesteric-isotropic phase transition. The value of a can be adopted as the value of 8x10 4 erg/k*cm 3[3] Temperature dependence of Bragg reflection of the polymer stabilized blue phase Figure 3-5 shows the reflected wavelengths as a function of temperature for pure BP and PSBP liquid crystal samples. The reflected wavelength of the pure BPLC sample was blue-shifted or red-shifted when the temperature was ascending or descending, respectively, as seen Fig. 3-5a. This causes a distortion or tilting of the lattice, which 50

66 results in a shift of the Bragg reflection wavelength. For the PSBP sample, the reflected wavelength was pinned at the same wavelength when the temperature was ascending or descending accordingly as seen in Fig.3-5b. Evidently, the PSBP cell shows good thermal stability against temperature fluctuation as a result of stabilization of disclination lines by polymer network, which are expected to be more concentrated in the disclination lines. Figure 3-5: a) Reflection spectra for pure BPLC without the monomer. 51

67 Figure 3-5: (b) The plot of wavelength versus temperature for sample 1, 2, 3 and 4 after the polymerization Kerr effect of polymer stabilized blue phase Polymer stabilized blue phase is emerging based on the faster electro-optical response and simplified the fabrication process for PSBP samples [101,23,26]. The physical mechanism of electric field birefringence of PSBP is called the Kerr effect [105]. Figure 3-6 shows the E-O properties of the studied PSBP samples. By applying a sufficiently high 52

68 in-plane voltage through the cells, we were able to switch the cells from an initial dark (field-off) state to a bright (field-on) state. For the pure BPLC sample, the threshold voltage was 14.16V, whereas the turn-on voltage was 36V. The threshold voltages for PSBP samples 1, 2 and 3 are 29V, 22V, and 11V, respectively. Furthermore, the threshold voltage of PSBP sample 4 is 4.5V. The results indicate that the threshold voltage, V10 (voltage required for achieving 10% transmission of the cell), decreases with the concentration of PSS-PMH since the anchoring energy of polymer network decreases. The turn-on voltages at 45 o C are 88V, 78V, and 89V for the PSBP samples 1, 2, and 3, respectively. However, the turn-on voltage for the PSBP sample 4 is 30V at 35 o C and for a pure BPLC with a mixture of two different nematic LCs is 91.4V at 40 o C. It is clear from the results that sample 4 shows the lowest turn on voltage due to the anchoring energy of polymer network formed by PSS-PMH. 53

69 Figure 3-6: Normalized transmittance-voltage (TV) curves of samples 1 (@45 o C), 2 (@45 o C), 3 (@45 o C) and 4 (@35 o C and sample of the mixture of BL006 and MCL6080@40 o C without monomer). Sample 4 has a lower switching voltage not only than samples 1, 2, and 3 but also the reported PSBPs that were switched at around 140V [5,6,11,12]. The low switching voltage of reported PSBP samples arise from the polymerized nanoparticles, which disrupt any tendency towards orientational order inside the core of the blue phase and enable a weak anchoring at the surface of the inorganic nanoparticles. These results of low switching 54

70 voltage distinguish this new system from those of previously studied materials [106]. The advantages of reported PSBP material are the temperature independence of Bragg reflection and low switching voltage. Figure 3-7 shows the POM images of samples 1 and 4 in response to an applied electric field. Green and the deep blue phase colors are switched to a bright state for sample 1 and sample 4, respectively, at a low applied voltage. For sample 1, there is an induced phase transition from BPI to cholesteric at an applied E-field of 16 V m -1 in the direction perpendicular to the surface as shown in Fig. 3-7a. For sample 4, the selective reflection of BPI is switched from an initial deep blue color to reflect a blue-green color at the voltage 6 V m -1 and 1 khz. The POM images show a few dark lines at bright state due to lithographically damaged electrodes. At applied voltages between 6 V m -1 and 16 V m - 1, the light transmission depends on the field strength. Continued increase in applied electric field results in a small spectral shift from blue to green, which is known as the extended Kerr effect in a PSBP composite material [41]. The voltage-transmission curves of sample 4 in Fig. 3.5 validate this phenomenon. When the field is applied in the normal direction to the electrode stripes, the cell starts to switch from an initial dark state at 40 V and achieves a bright state at 60 V. According to the Kerr effect there is a linear relationship between the square of the external electric field and induced birefringence. In the small electric field region, with the increasing electric field, the LC composite becomes birefringent due to the reorientations of molecules within double twisted cylinders. The induced birefringence of PSBP under an applying electric field is 55

71 governed by equation 3.7, where nind is the induced birefringence of PSBP, λ is the wavelength of the probe, K is the Kerr constant, and E (=V/l, where V is the applied voltage and l is the distance between electrodes) is the amplitude of the electric field. nind = λke 2 (3.7) Equation 3.8 is valid only in the low electric field region. If the external electric field keeps increasing, the induced birefringence will gradually saturate. In that case, a saturation field has been introduced and modified as follows [41] nind = n (E/Es) 2, (3.8) where n represents the maximum induced birefringence of the PSBP liquid crystal. 56

72 Figure 3-7: a) POM images of PSBP cell of sample 1 under applied voltage of (a) 0 V, (b)20 V, (c) 30 V, (d) 40 V, (e) 50 V, (f) 60 V, (g) 70 V, (h) 80 V, (i) 90 V, (j) 100 V at 45 o C, and b) POM images of PSBP cell of sample 4 under applied voltage of (a) 0 V, (b)20 V, (c) 30 V, (d) 40 V, (e) 50 V, (f) 60 V, (g) 70 V, (h) 80 V, (i) 90 V, (j) 100 V at 35 o C. The black scalar bar corresponds to 50 µm. 57

73 Figure 3-8 shows the Kerr constant of PSBP samples. The Kerr constants of samples 1-3 are in the range of ~ V 2 m, while sample 4 generates considerably large Kerr constants of V 2 m, (at 633 nm), which is about 10 times higher than that of the reported PSBPs [107]. With the decreasing applied voltage, the Kerr constant is increasing. Polymer concentration in the PSBP samples leads to lower switching voltage due to disruption of any tendency towards orientational order of BP, which causes a weak anchoring at the surface of the inorganic nanoparticles. Figure 3-8: Kerr constant of PSBP samples versus polymer concentration. 58

74 3.2.5 Response time of polymer stabilized blue phase Polymer stabilized blue phases have considerably faster response times on the order of s when compared to those of the conventional liquid crystals device with the response time of 10-2 s [97]. The response times of the pure BPLC and PSBP samples were studied by switching between 10 and 90 percent light transmittance of the cells for the rise time and 90 to 10 percent light transmittance of the cells for the fall times, which were obtained from Fig The response time was determined by switching sample 4 between its corresponding voltages V10 and V90 at 35 o C. The measured rising time ( rise) for sample 4 is 6.2ms and fall time ( fall) is 4.5ms. The measured rising time ( rise) for sample 4 without monomer is 1.1ms and fall time ( fall) is 0.7ms. There is a linear relationship between the monomer overall concentration and response time. Higher monomer concentration gave a faster response time for samples

75 1.0 rise fall Normalized Transmittance Time (sec) Figure 3-9: The plot of transmittance versus response time of sample Polymer Morphology Polymer network morphology was visualized using a SEM. The SEM image of the substrate with patterned electrodes viewed at 90 o angle revealed a porous structure for sample 1, whereas sample 2 and sample 3 exhibited a sponge and sheet-like structures (Fig. 3-10), respectively. Sample 4 displayed cubic-like patterns formed with submicronsized polymer sticks with preferential orientation aligned along underlying inter-digitated electrodes as shown in Fig. 3.10(d). This may be due to the grating effect of the UV light 60

76 impinged on the reactive monomers. Therefore, the formation of polymer becomes anisotropic. These polymer sticks are between 300 and 600 nm in length and approximately 60 nm in diameter. By contrast, the SEM image of polymer at the opposite substrate surface without electrodes showed connected hollow channels with denser polymer intertwined in the bulk (Fig (d-1)). Because we placed a mirror in the back of the cell, the reflected light may have caused interference and destroyed anisotropy of light pattern. So the morphology is irregular. The size of empty tunnels ranged from a few hundred nanometers to 0.88 µm [108]. Figure 3-10(e) shows a simulated polymer network structure within a cubic lattice of a polymer stabilized blue phase. The right picture of Fig. 3-10(e) shows that PSS-PMH molecules are located at joints of the polymer network because PSS-PMH monomer has a low surface energy compared to that of the RM257 and phase-separates into the region of defects. 61

77 (d-1) (a) (c) (d) (d-1) (e) Figure 3-10: The SEM micrographs of polymer morphology of PSBP (a) cell of sample 1, (b) cell of sample 2, (c) cell of sample 3, (d) cell of sample 4 at the surface of a substrate with electrode (the arrow indicates electrode direction) (arrow shows the preferential orientation), (d-1) the surface of a substrate without electrode. The yellow and black scale bars represent 20 and 2 µm respectively. (e) Simulated polymer network structure within a cubic lattice of a polymer stabilized blue phase (small cubes in the right picture represents PSS- PMH). 62

78 3.3 Polymer system effect on blue phase stabilization The dependence of a blue phase temperature range on a nematic mixture and chiral dopant as well as different monomers was investigated. Various monomers at different polarities were added to evaluate the relationship between blue phase stability and molecular structure of components. The effect of stability of a blue phase has been extended by formation of the polymer network. Fundamental electro-optic properties such as Kerr constant, switching time, and operating voltage of a polymer-stabilized blue phase were investigated. The liquid crystals in this study included a nematic mixture of JC1041XX (Chisso Co, Ltd.) and 4-cyano-4 -pentylbipenyl (5CB) at the weight ratio of 50%:38.5%. ZLI 4572 (HTP~30µm -1 ) with concentration of 11.5% added in the LC mixture as a chiral dopant. In order to broaden the temperature range of the blue phase liquid crystal, four different monomer dopants and difunctional monomer RM 257 were added as well as a small amount of photoinitiator (IR651,0.5 wt%). The chemical structures of liquid crystals and the chiral dopant are indicated in Fig For the cell preparation, the BP sample was mixed for 1.5 hours at room temperature after adding achiral dopant in the nematic liquid crystals. After adding monomers in the BP samples, all the samples were mixed for an additional 1.5 hours at room temperature. These polymerizable mixtures were heated up to an isotropic state and then filled into an in-plane switching (IPS) cells to carry out the E-O measurements. The IPS cells were assembled with a second glass substrate without 63

79 ITO electrode placed on top of the first substrate and separated with 5 m glass ball spacers to give a cell gap of 5 m (the cell configuration is referred as 5/5/5). Figure 3-11: The chemical structure of JC1041XX, 5CB, and ZLI

80 In order to address the question of how monomer functionality, specifically the number of reactive double bonds as well as polarity, affects the network formation and the electro-optical properties of polymer stabilized blue phase, seven different monomers were chosen. The monomers used to form precursors are TMPTA (trimethylolpropane triacrylate), HFBA (1H,1H heptaflorobutyl acrylate), BMATTD (1 3-bis(3- methacryloxypropyl)tetrakis(trimethylsiloxy)disiloxane), BMATD (1,3-bis(3- methacryloxypropyl) tetramethyldisiloxane), HBA (4-Hydroxybutyl Acrylate), BBA (benzoic acid acrylate), and a soft polymer HDDA (hexanediol diacrylate). The chemical structures of the monomers are indicated in Table

81 Table 3.2. The chemical structures of monomers. Type Name Chemical structure Tri-functional monomer TMPTA Di-functional monomer BMATTD BMATD BBA H 2 C O O O (CH 2 ) 6 O C OH HDDA Mono-functional monomer HFBA HBA For the purpose of comparing the various polymer systems in identical conditions, the mixtures were prepared with these precursors whose overall monomers concentration 66

82 were controlled 11wt% and chiral pitches were controlled at the same value as listed in Table 3.3. The observed phase transition temperatures of the pure BP, and the PSBP samples are also shown in Table 3.3. Table 3.3. Component fractions of PSBPLC mixtures and transition temperatures Sample BPLC (%) RM 257 (%) Dopant monom er (%) Dopant monomer name BP Temp. Range ( T) before poly. ( o C) Heating Cooling BP Temp. Range ( T) after poly. ( o C) Heating Cooling TMPTA < > BMATTD < > BMATD < > HDDA < > BBA HFBA < > HBA Polymer system effect on thermal stability of polymer stabilized blue phase The mesomorphic behavior of PSBP samples was studied using a hot stage with a programmable temperature controller and positioned between a pair of polarizers crossed at 90 degrees. The optical textures were confirmed with a POM before and after the polymerization. The phase transition temperature of pure BPLC is N* 54.7 BP 55.5 ISO and ISO 53.1 o C BP 51.7 o C N* during heating and cooling process, respectively (Fig. 3-67

83 12). Domain sizes were quite big with the range of 30 µm and 150 µm. The deep blue color was observed for the pure BP sample under crossed polarizers at the BPII phase. With the decreasing temperature, the color changed to green bluish color due to tilt of the BP lattice structure. Figure 3-12: POM images of pure BP mixture with nematic liquid crystals of JC1041XX (44.5% wt) and 5CB (34% wt), and the chiral dopant of ZLI 4572 (10% wt). Scalar bar is 100µm. In order to investigate the polymer system effect on the thermal stability, PSBP samples were irradiated with a UV light of 8 mw/cm 2 (measured at 365nm) at the blue phase. After 20 minutes exposure, the samples were placed into the hot stage and heated up to the isotropic state at a rate of 1 o C/min. and then cooled to the cholesteric state at a rate of 0.2 o C/min. Figure 3-13 shows the platelet textures of the samples after polymerization. 68

84 a) 23 o C 0 o C 45 o C 53 o C 0 o b) Iso BPII BPI 53.2 o C 45 o C >0 o C 53 o C 48 o C 23 o C 0 o C c) Iso BPII BPI 53.2 o C 48 o C >0 o C d) Iso BPII BPI 53 o C 45 o C >0 o C 54 o C 45 o C 23 o C 0 o C e) Iso BPII BPI 54.7 o C 45 o C >0 o C 53 o C 46 o C 23 o C 0 o C Iso BPII BPI 53.7 o C 46 o C >0 o C 69

85 f) 56 o C 54 o C 40 o C 39 o C Iso BPII BPI Ch 57 o C 50 o C 39 o C Figure 3-13: POM images of polymer stabilized BP samples of a) with TMPTA b) with BMATTD c) with BMATD d) with HDDA, e) with HFBA, f) with HBA, monomers at various temperatures. A bluish green color was observed for the samples, which include BTMATTD, HFBA monomers. The samples that contained TMPTA and BMATD monomers exhibited a green color, whereas samples included the monomers of HDDA and HBA showed a deep blue color under crossed polarizers. PSBP samples showed different colors due to the tilt of lattice structure as well as different domain sizes of the samples. Moreover, polymers played an important role in stabilizing the orientation of liquid crystal directors and thus, wideninig the temperature range of BP. The BP temperature ranges of all samples were expanded to more than 53 o C except the samples with HBA monomer. This may be because of the higher miscibility of HBA due to a polar end group for forming hydrogen bonding with liquid crystal molecules than the other monomers in the liquid crystal JC1041-XX. Therefore, HBA could be homogenously dispersed in a whole region of the BP, instead of stabilizing the defect lines. However, there is no stabilization effect when 70

86 BBA monomer was used. During the polymerization process, the conditions were carefully controlled for all the cells to keep from phase transitions since the temperatures of BP-N* and BP isotropic phase transitions can occur easily. While checking the cell with BBA monomer with a polarizing optical microscope equipped with a hot stage during the polymerization process, a BP isotropic phase transition was observed in the early stage of polymerization. The polymerization-induced time may be shorter for the sample with BBA monomer at this concentration of the components; therefore, the polymer cannot be stabilized in a periodic structure. In addition, the local concentration of the monomer on the defect lines is not high enough to form a polymer network for the stabilization of BP with this BP-monomer combination Polymer system effect on the Bragg peaks of polymer stabilized blue phase Reflection spectra of the polymer stabilized blue phase liquid crystal samples were determined at normal angle from cells with an Ocean Optics spectrometer at various temperatures. Figure 3-14 shows the reflected wavelengths as a function of temperature for pure BP liquid crystal and polymer stabilized BP liquid crystal samples. The Bragg reflection wavelength shifted to a longer wavelength due to the distortion or tilting of the lattice in response to a decrease of temperature for pure BPLC sample. 71

87 Figure 3-14: Reflection spectra for pure BPLC without the monomer. By contrast, polymer stabilized blue phase liquid crystal samples have good thermal stability in reflected colors in responding to the temperature changes. Bragg reflection wavelengths of PSBP samples are temperature independent, as seen in Fig From Fig. 3-15a to 3-15e, PSBP samples have different type of polymers at the same concentration. The sample with TMPTA showed a decrease in the reflectance with respect to the other samples. This may be attributed to the oversaturation of the polymer in the defect regions of the cubic lattice of BP. However, samples with HFBA, and HBA have wider spectra peaks than the other PSBP samples due to the non-uniform distribution of polymers in these samples which leads to a large deformation of BP lattice depending on the polymer network. 72

88 Figure 3-15: Reflection spectra for polymer stabilized BP samples of a) with TMPTA, b) with BMATTD, c) with BMATD, d) with HDDA, e) with HFBA, f) with HBA monomers at various temperatures. 73

89 3.3.3 Polymer system effect on the electro-optical properties of polymer stabilized blue phase The influence of polymer effect due to its polarity, functionality as well as flexibility on the electro-optical properties of polymer stabilized blue phase liquid crystal was also investigated Polarity Spectrum of Monomers Polar molecules have a permanent electric dipole moment due to the charge separation in the molecules as a result of differences in electronegativity. For the nonpolar molecules, induced electric dipoles were generated by means of an applied electric field, which induced the separation of positive and negative charges of molecules [109]. Dipolar interaction between atoms in the molecules symbolizes a large amount of internal energy. The atoms bond to each other in the polar molecule and give rise to high surface energy for the molecule. When the sizes of molecules are similar, the larger the dipole moment of a molecule is, the stronger intermolecular forces for the molecules are. The surface energy depends on the polarity of the molecules, which is affected by the type and number of the functional groups. For example, polar groups, such as hydroxy or carboxy groups, increase the surface tension, while nonpolar groups, such as long alkyl chains, siloxanes, ease it. The electric dipole moment (µ, measured in Debye (D)) can be described a product of magnitude of charge (q) and the distance between the charges (r) as seen in Figure

90 r Figure 3-16: The simulation of the dipole moment of a linear molecule. When the molecule possesses more than two charges, the molecular dipole moment is the sum of the individual dipole moments and the resultant is a vector quantity [110]. The dipole moment for the vector r points from the negative to positive charges is given by µ=q.r (3.9) If the molecule is a polyatomic molecule, dipole moment is equal sum of individual contributions. Dipole moments for two dipole moments µ1 and µ2 that make an angle θ governs by Eq μ μ μ μ μ cosθ (3.10) However, there is a linear relationship between dipole moment of the molecule and electrostatic potential, according to the Equation The dipole contribution to the electrostatic potential is given by 75

91 E = 4 q 3 πr3 4 3 πr3 4πε o r 2 = qr 4πε o R 3 = μ 4πε o R 3 (3.11) where q is the charge distribution s dipole moment, which is an intrinsic (vector) property of the source and does not depend on r. The chosen precursors include the monomers of TMPTA (trimethylolpropane triacrylate), HFBA (1H,1H heptaflorobutyl acrylate), BMATTD (1 3-bis(3- methacryloxypropyl)tetrakis(trimethylsiloxy)disiloxane), BMATD (1,3-bis(3- methacryloxypropyl) tetramethyldisiloxane), HBA (4-Hydroxybutyl Acrylate), BBA (benzoic acid acrylate), and a soft polymer HDDA (hexanediol diacrylate) as well as reactive monomer of RM 257. In order to determine the polarities of monomers were used, the dipole moments of monomers were calculated using the software Code Marvin. Simulations of monomers are shown in Fig Figure 3-17a: Trifunctional monomer: TMPTA. 76

92 Figure 3-17b: Difunctional monomers of a) BMATTD, b) BMATD, c) BBA, and d) HDDA. Figure 3-17c: Monofunctional monomers of a) HFBA and b) HBA. 77

93 According to the calculation, the dipole moments of the monomers are as shown in Fig Figure 3-18: Dipole moments of monomer dopants. The monomer HDDA is a non-polar molecule due to its symmetrical structure whereas the dipole moment of the monomer BBA has the highest value of 9.78 Debye (D). The dipole moment of monomer of TMPTA is higher than that of the molecules with disiloxanes due to the electronegativity of oxygen atoms in the monomer of TMPTA. When the two different disiloxane monomers are compared, the dipole moment of monomer BTMATTD is higher than that of BTMAD because BMATTD has a lower surface energy due to the large number of methyl groups. The dipole moment of HFBA is 78

94 smaller than of HBA due to the high degree of overlap between outer orbitals of fluoride. The small size of flourine atoms gives rise to low surface tension and dipole moment [111] Polymer system dependence of Electro-optical behavior of polymer stabilized blue phase Voltage-transmittance (V-T) curves of polymer stabilized blue phase liquid crystal cells were measured in the IPS cells (the cell configuration is referred as 5/5/5) by applying a square wave voltage of 1 KHz. Figure 3-19 shows the V-T curves of PSBP samples at room temperature. For the pure BP mixture, the threshold voltage was 45V and turn on voltage was 110V. Compared the PSBP samples, sample 1 has the highest turn on voltage, whereas sample 2 has the lowest turn on voltage. It is hard to switch sample 1 as a result of the high anchoring energy of polymer network formed by TMPTA. The crosslink between TMPTA and RM257 monomer mixture is much stronger than the others due to high monomer functionality, resulting in an increased elastic torque of the BPLC composites. Strong polymer networks in the disclination cores may restrain molecular reorientations, causing high turn on voltage. On the other hand, polymer forms an aggregation in BP due to stronger dipole-dipole interactions and causes high turn on voltage. The benefit of low surface energy property of the polymers leads to a reduction in switching voltage for samples of BMATTD and BMATD compared to the switching 79

95 voltage of the sample with TMPTA. Disiloxanes have lower dipole moments, which cause a lower surface energy than TMPTA. However, BMATTD may have lower surface energy because of the large number of methyl groups and small interactions between the siloxane hydrophobes. The sample with HFBA has a lower turn on voltage when the samples are compared. The sample with HDDA has the lowest turn on voltage. HDDA is a non-polar molecule, and due to weak dipole-dipole interactions, polymer HDDA forms a weak polymer network and reduces turn on voltage. According to our results, polymer structure has a considerable effect on anchoring strength and turn on voltage. a) 1.0 Normalized Transmittance TMPTA HFBA BMATTD BMATD Normalized Transmittance b) Sample 5 (HDDA) Voltage (V) Voltage (V) Figure 3-19: The transmittance-voltage curve of PSBP samples. When an electric field is applied to a BPLC, the symmetry is broken and leads to an increase in birefringence. The relationship between the transmittance (birefringence) and the electric field in a BP system is given by [29] n(e) = λke 2, where K is the Kerr constant, λ is the probe wavelength, and E (=V/l, where V is the applied voltage and l is the distance between electrodes) is the applied electric field. The estimated Kerr 80

96 constants for the PSBP samples are shown in Fig The Kerr constants don t show a large variation except for the sample with HDDA. The large Kerr constant of the sample with HDDA is due to the decreasing of turn on voltage. Kerr Constant (x10-9 m/v 2 ) Sample 1 Sample 2 Sample 3 Sample 4 Sample Samples Figure 3-20: Kerr constant of PSBP samples with different monomer. 81

97 To measure the response time, all the cells were switched between their corresponding voltages V10 and V90 obtained from Fig The measured response times of samples are shown in Fig The measured response time for the sample with TMPTA is slightly higher than those of other samples. The slow response time is due to the higher anchoring energy of the polymer network. Lower surface energy of the polymer network causes a weak anchoring and fast response time. The fastest response time is obtained for the sample with HDDA due to the weak anchoring of the polymer network. Figure 3-21: Response time of PSBP samples. 82

98 3.4 Summary We have demonstrated stabilization of cholesteric blue phases using polymerizable silicon-based nanoparticles to modify the interfacial properties of disclination cores and broaden the blue phase temperature range. The polymerized polymer broadens the blue phase temperature range and stabilize the Bragg reflection wavelength against temperature change. Our experimental results show that concentration of PSS-PMH polymer plays an important role in the thermodynamic stability of modulated liquid crystal blue phases. The low surface energy feature of the inorganic PSS-PMH polymer leads to significant reduction of the switching voltage of the corresponding device. It dramatically decreases the switching voltage from about 140V to 40V. The significant reduction in the switching voltage and widening of blue phase temperature are useful for new electro-optical applications. We have also demonstrated broadening of blue phase temperature range by using a different nematic liquid crystal and polymer system with different chemical structures to modify the interfacial properties of disclination cores. Our experimental results show that polymer plays an important role in the thermodynamic stability of modulated liquid crystal blue phases. Polymer structure and polarity have a considerable effect on anchoring strength and turn on voltage. The interface energy between BPLC and polymer has an effect on the electro-optical properties of polymer stabilized BP. Polymer with low surface energy leads to significant reduction of the switching voltage of the 83

99 corresponding device. Lower surface energy of polymer network causes a weak anchoring and lower turn on voltage as well as fast response time. 84

100 Chapter 4 Carbon Nanotubes Doped Polymer Stabilized Blue Phase Liquid Crystals 4.1 Introduction Carbon nanotubes are anisotropic nanoparticles, which exhibit metallic or semiconducting behavior based on the diameter and helicity of the carbon rings [112]. Carbon nanotubes include two morphologies, namely, single-wall carbon nanotubes (SWCNT) and multiwall carbon nanotubes (MWCNT). SWCNTs are anisotropic nanoparticles characterized by a typical length of the order of submicron to microns. The diameter being in the range from 0.5 a nm to 2 nm leads to a high aspect ratio of the tubes, and they show exceptional tensile strength depending on their high aspect ratio and rigidity [113]. In addition, MWCNTs have electronic properties similar to those of SWCNTs because of weak coupling between cylinders. Since the discovery of the carbon nanotubes (CNT) by Iijima in 1991, [114] the dispersion of CNTs inside LCs has become an important research topic because of the extraordinary electrical properties and strong interactions of the CNTs with the mesogenic units of LCs [115]. Recently, different groups have reported studies on the alignment and characterization of CNTs in nematic liquid crystals [ ] as well as on the dielectric [119,120] and electro-optical properties [121,122] of CNTs. Different textures of CNTs were observed not only in the nematic LCs but also 85

101 when the nematic LC droplets were embedded in a polymer matrix medium [ ]. By applying a high electric field, various LC textures based on the field-induced movement of CNTs inside the nematic LC medium were observed [ ]. Using a very small concentration of CNT dopant, it has been demonstrated that the rising time of CNT doped nematic liquid crystals decreases the threshold voltage in the twisted nematic (TN) and in-plane switching LC cells [121,122,127,128]. In addition, the application of CNTs in the optical controlled birefringence cells resulted in a fast response time because of the increase in anchoring energy of the alignment layer by CNT doping [95]. To investigate the effect of CNT doping on the temperature range of blue phase and electro-optical performance of polymer stabilized blue phase, carbon nanotubes were added into a mixture of monomer and BPLCs, and the mixtures were polymerized in IPS cells for determination of electro-optical properties such as Kerr constant, switching voltages, and response times of the polymer stabilized blue phase (PSBP) liquid crystals. 4.2 Materials and Cell Preparation The BPLC material was prepared using a mixture of 55wt% of commercially available nematic liquid crystals BL006 and MLC6080 (both from Merck) and 45wt% of chiral dopants (20% CB15, 20% R811, and 5% R1011). The polymerizable mixture contains monomers of RM257 (bis[acryloyloxy-(4-propoxyl(1,4 phenyl benzoate))]) (Merck) and PSS-PMH(PSS-(1-Propylmethacrylate)-Heptaisobutyl substituted) (Aldrich) at the weight percentage of 2% and 8%, respectively, as well as a small amount of 86

102 photoinitiator (0.01wt.% of Irgacure 651, (CIBA)), and commercially available singlewall CNTs (Carbolex) and multiwall CNTs (SES Research) were used for doping the BPLCs. Six different mixtures were prepared by adding a small amount of SWCNT or MWCNT from 0.01% to 0.001% by the weight of the BPLC. The compositions of CNT doped PSBP liquid crystal mixtures, and the temperature ranges of the mixtures before polymerization are listed in Table 4.1. Liquid crystal and chiral dopant components were mixed by stirring in an ultrasonic bath for 1.5 hrs at room temperature. After adding the reactive monomers and the photoinitiator, the resulting mixtures were sonicated for 1.5 hrs. The mixture of reactive monomers and CNT doped BPLCs were filled in the cells by a capillary action at the isotropic states of the mixtures and cooled to room temperature slowly after filling. The blue phase temperature range observations before and after polymerization were carried out in a cell consisting of two plain glass substrates without indium-tin oxide (ITO) and with a 10 µm cell gap maintained by spherical glass spacers. 87

103 Table 4.1. Compositions of CNT doped blue phase liquid crystal mixtures. PSBP Sample SWCNT (%) MWCNT (%) Monomer (%) (RM257/ PSS- PMH) BPLC (%) (BL006/MLC6080) (2/8) 55 (1/1) 1a (2/8) 55 (1/1) 1b (2/8) 55 (1/1) 1c (2/8) 55 (1/1) 1d (2/8) 55 (1/1) 2a (2/8) 55 (2/1) 2b (2/8) 55 (2/1) Temp. range before polymerization ( C) Temperature range observation of CNT-doped polymer stabilized blue phase The BP temperature ranges were determined using a polarizing optical microscope (POM) with the cells placed on a temperature-controlled stage and positioned between a pair of polarizers crossed at 90. The samples were heated to the isotropic state and cooled at the rate of 0.2 C/min to a cholesteric state. The polymer-stabilized blue phase liquid crystal samples were prepared by illuminating the cells with UV light (

104 mw/cm 2, 365 nm) for 40 min, and the temperature at which large domains of blue phase are observed and the Bragg wavelength reflected a blue color was chosen as the curing temperature. The blue phase temperature range of the SWCNT doped blue phase mixtures PSBP1a, PSBP1b, PSBP1c, PSBP1d, and PSBP2 increased with an increase in the SWCNT concentration. By changing the ratio of the liquid crystals between MLC6080 and BL006 (PSBP1a-d and 2a) as seen in Table 4.1, the blue phase temperature range widened because of a decrease in the bend to splay elastic constant ratio (K33/K11). Blue phase temperature range widening because of a decrease in the bend to splay elastic constant ratio has been reported earlier [129]. Furthermore, the blue phase temperature range of SWCNT doped PSBPs (22 C, PSBP2a) is larger than that in MWCNT doped PSBPs (9.8 C, PSBP2b). Figure 4-1a shows the phase diagram after the polymerization in which the phasetransition temperatures for CNT doped PSBP liquid crystal samples are plotted for different concentrations of carbon nanotubes. The phase transition of pure blue phase mixture of sample 1 shows the isotropic to BP transition at 41.8 o C and BP to cholesteric phase at 32 o C, whereas sample 2 shows the isotropic to BP transition at 43.8 o C and BP to cholesteric phase at 30.7 o C. The results of PSBP samples before doping of CNT are discussed in Chapter 3. When the BP samples doped with CNT ranging from 0.05% to 0.001%, the mixtures show a significant increase in the BP temperature range after polymerization. The blue phase temperature range for the sample PSBP-2a is 42 C after polymerization, which is wider than that of other PSBP samples. Nanotubes nucleate and 89

105 position themselves at the defect regions of blue phase liquid crystals and help stabilize the blue phase against temperature fluctuation. The blue phase stabilizes because of the removal of a part of the volume occupied by the disclination lines and their subsequent replacement with the polymer. Figures 4-1b(1) and 4-1b(2) show the POM images of the SWCNT doped PSBPLC cell when cooling from the isotropic state to 50 C and 8 C, respectively. Also, Figures 4-1c(1) and 4.1c(2) show the POM images of the MWCNT doped PSBPLC cell when cooling from the isotropic state to 52 C and 29 C, respectively. We observe that the blue phase domains of the MWCNT doped BPLC are larger when compared to that of SWCNT doped BPLCs. In accordance with the POM images, the temperature range of the PSBP liquid crystal mixtures decreased as the concentration of SWCNT increased from 0.001% to 0.01%. This phenomenon can be explained with Onsager s theory for suspensions of rigid rods in a liquid crystal phase [130]. According to Onsager s theory, nanotubes tend to align because of their large aspect ratio leading to the reduced volume. This reduced volume causes a phase transition from the isotropic to the cholesteric state as a function of nanotube concentration. Therefore, the temperature range decreases with an increase in the CNT concentration. The blue phase temperature range decreased to 24.4 C when the SWCNT was replaced with 0.001% concentration of MWCNT (sample PSBP-2b in table 4.1). Contrary to the SWCNT, the MWCNT did not provide an extension of the blue phase. These results may be associated with the lower aspect ratio and rigidity of MWCNTs, which hindered them from packing into the defect cores contrary to SWCNTs [129]. 90

106 Figure 4-1: a) Phase diagram showing transition temperatures vs. concentration of CNTs in the PSBPLC mixture, b) POM images of sample PSBP-2a at (1) 50 C and (2) 8 C after polymerization, and c) POM images of sample PSBP-2b at (1) 52 C and (2) 29 C after polymerization. The yellow scalar bar corresponds to 20 μm. 91

107 4.4 Reflection spectra of CNT-doped polymer stabilized blue phase Reflection spectra of the CNT doped liquid crystal samples were determined at right angles from the cells with an Ocean Optics spectrometer at different temperatures. To compare the effect of CNTs on the Bragg reflection property of the BPLCs and polymer stabilized liquid crystal samples, another cell was filled with CNT doped BP liquid crystal, which has the same ratio of nematic to chiral dopants but without the polymer. Figure 4-2a shows the Bragg reflected wavelengths as a function of temperature for a SWCNT doped BP liquid crystal sample. The reflected color changes from deep blue (491 nm) to green (535 nm) as the temperature decreases from 47 C to 43 C. This causes a distortion or tilt of the lattice, which results in shift of the Bragg reflection wavelength. In the case of the MWCNT doped BP liquid crystal sample, the reflected color exhibits a small shift from 443 nm (blue) to 438 nm (green), when the temperature decreases from 51 C to 46 C, as shown in Fig. 4-2b. In contrast, the CNT doped polymer stabilized blue phase liquid crystal samples have good thermal stability for reflected colors in response to temperature changes and the Bragg reflection wavelengths are thereby temperature independent, as can be seen in Fig. 4-2c. 92

108 50 a) Reflectance (%) o C 44 o C 45 o C 46 o C 47 o C Reflectance (%) b) o C 47 o C 49 o C Wavelength(nm) Wavelength(nm) Wavelength(nm) c) Temperature( o C) PSBP-1a PSBP-1b PSBP-1c PSBP-1d PSBP-2a PSBP-2b Figure 4-2: a) Reflection spectra for SWCNT doped BPLC without polymer, b) reflection spectra for MWCNT doped BPLC without polymer, and c) plot of wavelength versus temperature after polymerization of CNT doped PSBPLC. 93

109 4.5 Electro-optical behavior of CNT-doped polymer stabilized blue phase Electro-optical (E-O) measurements of field induced birefringence were conducted using in-plane-switching (IPS) cells with a patterned ITO having 10 m electrode width and 10 m electrode space on one substrate; the IPS cells were assembled with a second glass substrate without the ITO electrode and separated by 5 m ball spacers. The E-O apparatus included a helium neon laser with light emission at 633 nm, a pair of polarizers crossed at 90 with respect to the polarization axes, a diode detector, and an amplifier, which were used for data acquisition. To carry out the E-O measurements, the IPS cells were placed at 45 angle between the polarizers crossed at 90. The light transmittance as a function of the applied voltage and response times at the BP temperatures was studied. Macroscopically, BPLCs are an isotropic medium in the absence of an external electric field. With an external electric field, BPs become anisotropic along the direction of the applied electric field and the induced birefringence viz. Kerr effect is a product of wavelength, Kerr constant, and electric field [39]. In the IPS cell, the Kerr constant (K) is given by n(e) = λke 2, (4.1) where λ is the probe wavelength and E (= V/l, where V is the applied voltage and l is the distance between the electrodes for the IPS cell) is the applied electric field. Figure 4-3a shows a plot of Kerr constant and Von for PSBP1 as a function of concentration of carbon nanotubes. The Kerr constant decreases with an increase in the applied voltage Von when the CNT concentration increases from 0.005% to 0.01%. A high concentration of CNT 94

110 results in an increase in the number of LC molecules anchoring around the CNTs because of the strong binding energy between the CNTs and LC molecules [131]. As a result of the strong anchoring of the LC molecules, a larger voltage is necessary to induce their reorientation. Moreover, the strong interactions between CNTs and LC molecules increase the elastic constant of the CNT doped PSBP liquid crystal samples with an increase in CNT concentration [131]. The turn-on voltage is proportional to the elastic constant in the IPS cell and can be expressed by [122] V on = πl d K 33 ε o ε, (4.2) where l is the distance between the electrodes, d is the cell gap, K is the elastic constant, εo is the dielectric permittivity in vacuum, and ε is the dielectric permittivity of the liquid crystal. From Eq.4.2, the change of Von is directly related to K33 of the CNT doped PSBP, when all cells have the same d and ε values [132]. In addition, the magnitude of the Kerr constant for the CNT doped PSBP liquid crystal mixtures in the BPI state is 10 2 times greater than in conventional LCs [133]. Figure 4-3b shows the light transmittance-voltage curves of the CNT doped PSBP liquid crystal samples as a function of the applied 1 KHz AC rectangular voltage. The IPS cells were switched between dark (field-off) and bright (field-on) states by applying an in-plane voltage through the cells at the BPI temperatures for all the samples. The threshold voltage, 95

111 which is required for achieving 10% transmission of the cell, for the sample PSBP-1c (34 V) is higher than that for the PSBP-2a (23.76 V) and PSBP-2b (26.80 V) samples. The turn-on voltage for PSBP-1c (70 V) is also higher than that for the PSBP-2a (46.58 V) and PSBP-2b (46.14 V) samples at 35 C. The threshold and turn-on voltages for the PSBP-2a and PSBP- 2b samples are lower than those for the PSBP-1c sample because the PSBP-1c sample has a slightly higher bend elastic constant than the PSBP-2a and PSBP-2b samples. Also, the elastic constant ratio of bend to splay (K33/K11) is decreased for the samples PSBP-2a and PSBP-2b using a different ratio of liquid crystal as given in Table 4.1. Therefore, the PSBP- 2a and PSBP-2b samples have lower threshold and turn-on voltages than PSBP-1c because of their smaller bend to splay elastic constant ratios. Furthermore, with an increase in the CNT concentration the strong interactions between CNTs and LC molecules increase the elastic constant of the CNT doped PSBP liquid crystal samples. 96

112 Figure 4-3: a) Kerr constants and on-stage voltage for the BPI as a function o concentration of CNTs and b) Voltage-transmittance curves of PSBP-1c, PSBP-2a, an PSBP-2b cells. 97

113 To identify the effect of rotational viscosity on the response time of the CNT doped PSBP samples, the PSBP-1c, PSBP-2a, and PSBP-2a samples were switched between their corresponding threshold and turn-on voltages (V10 and V90) obtained from Fig. 4-3a. The response times of the PSBP samples were measured at 35 C as seen in Fig The measured rise time ( rise) and decay time ( decay) for PSBP-1c are 1.83 ms and 1.0 ms, respectively. The rise and decay times for PSBP-2a are 1.43 ms and 1.02 ms, respectively, and the rise and decay times for the sample PSBP-2b are 4.45 ms and 4.33 ms, respectively. The response time for the cell of PSBP-1c is slower than that for the cells of PSBP-2a and PSBP-2b. The response times of the CNT doped PSBP cells improved because the rotational viscosity was modified by CNT doping of the blue phase liquid crystal mixtures. τ on = γ 1 d 2 ε o ε V 2 K eff π 2, (4.3) τ off = γ 1 d 2 K eff π 2, (4.4) As evident from Eqns. (4.3) and (4.4), the rise time will increase and decay time will decrease with an increase in the elastic constant of the CNT doped BPLC samples [122]. Increasing the CNT concentration in the PSBP samples leads to an increase in the effective elastic constant. Therefore, the rise times of the CNT doped PSBP cells will increase. The MWCNT doped PSBP sample has higher rise and decay times than the 98

114 SWCNT doped PSBP samples. The MWCNT doped PSBP sample gives rise to a smaller increment in the elastic constant and has a lower rotational viscosity because of the aspect ratio of the multiwall carbon nanotubes used in this study. Figure 4-4: Response times of PSBP-1c, PSBP-2a, and PSBP-2b cells. 99

115 4.6 Voltage holding ratio of CNT-doped polymer stabilized blue phase The voltage holding ratio (VHR) is an important measure of the LCD performance. For determining the voltage holding ratio, the VHR value of the empty IPS cell was measured to be 97%. After the samples of CNT doped BP liquid crystals and reactive monomers were filled into the cell, the measured VHR value was 98%. Subsequently, the cells were illuminated with a UV lamp and the variation of the data with time was recorded. Observations were made over two weeks in order to examine the degradation of the cell indicated by a decrease in VHR over time. The voltage-holding ratio is defined as the ratio of the initially selected voltage (Vo) to the relaxed voltage in the nonselected period (V). For the nonselected periods, by applying the electric field to the CNT doped PSBP samples along the direction perpendicular to the substrates, the decay of voltage over the layer of liquid crystal sample can be determined [134] V = exp t V o CR t = exp ερ (4.5) where C is the capacitance, R is the resistance, t is the characteristic lifetime, ε is the dielectric constant, and ρ is the resistivity of the liquid crystals. Therefore, the larger the resistivity of the liquid crystals, the larger is the holding ratio because the product of capacitance and resistance of the liquid crystals becomes larger as can be seen from Eqn. (4.5). The voltage decay can be attributed to a change in the internal electric field caused by ion polarization and the capacitance increases because of reorientation of the liquid crystals. Figure 4-5 shows the time-evolved VHR curves for the pure BPLC (undoped), 100

116 SWCNT, and MWCNT doped PSBP cells. Although the VHR values of the PSBP-2a and PSBP-2b samples initially have small fluctuations, the ion polarization becomes persistent within 5 days. As seen in Fig. 4-5, by adding SWCNT and MWCNT into the PSBP mixture, the VHRs increased from 0.93 to 0.99 and from 0.93 to 0.95, respectively, yielding a substantial increase of 2.2% and 6.5%, respectively. Notably, CNT doping reduces ion concentration in the PSBP samples cells. Figure 4-5: Voltage holding ratios of CNT undoped PSBP, PSBP-2a, and PSBP-2b cells versus time. 101

117 4.7 Summary We have demonstrated wide blue phase temperature range and low switching voltage using both single-wall and multiwall carbon nanotube (CNT) doped polymer stabilized blue phase (PSBP) liquid crystal. A small amount of single-wall CNT doping significantly widens the blue phase temperature range over 42 C after the polymerization and stabilizing the reflection wavelength against temperature changes. A lower threshold voltage is obtained for a higher CNT doping concentration. A lower Kerr constant was obtained for a higher CNT concentration because of the strong interactions between chemically-modified SWCNTs and LC molecules, which increase the bend elastic constant of the CNT doped PSBP samples with an increase in the CNT concentration. The response times of the PSBP cells have been significantly improved with CNT doping because of a reduction in the rotational viscosities. Furthermore, the CNT doping has significantly improved the voltage holding ratios of PSBP samples. 102

118 Chapter 5 Stabilization Effect of Bent-Core Molecules on Blue Phases 5.1 Introduction Bent core (BCLC) molecules are banana shaped molecules that are usually composed of five benzene rings linked in different chemical groups and ending with a carbon tail. After the first discovery of BCLC molecule by D. Vorlander in 1903 [135,136], several groups have made modifications by changing the attached groups as well as adjusting the length and symmetry of the carbon chain. It has been proved that the different structural modes of BCLC molecules may have different mesophases, which are known as B1-B7 [ ]. It is important to note that the phases can be tilted or non-tilted structures without the requirement of chirality at the molecular level. However, molecular chirality leads to a macroscopic polarization due to the compact packing of the molecules [141] and causes ferroelectric or anti-ferroelectric polar structure in the tilted smectic phase. In the presence of spontaneous polarization, a tilted smectic phase of an achiral bent-core molecule shows chirality. Furthermore, the direction of the spontaneous polarization can be reversed by external stimuli [137, ]. Recently some experimental results were reported on the interesting properties of the nematic phase of BCLCs, such as flexoelectricity [144,146], and significant Kerr effect [147]. Additionally, recent studies show that the splay constant (K11) of bent-core nematicsis can be reduced by a factor of 103

119 two when the small amount of BCLC molecules are dissolved in the nematic LC. Moreover, some researchers have achieved a wide temperature range for blue phase by doping BCLC due to the low bend/splay elastic ratio of BCLC molecules [148,95]. All these features of bent-core molecules make them attractive for researchers to stabilize the cubic BPs. In this work, our aim was to see the effect of BCLC doping on the stabilization of blue phases. All bent-core molecules were synthesized by our laboratory [149]. As a first step, we checked the concentration effect bent-core molecule on the stabilization of BP. For that purpose we used the bent-core molecule derivative of 4,6-dichloro-1,3-phenylene bis-[4-(4-n-octyloxyphenyliminomethyl)benzoate],with H compound (whose chemical structure is shown in Fig.1). As a second step, we investigated the structure effect on the stabilization of BP by varying the substituent (X= Cl and F, chemical structures are as shown in Table 5.2) on the same structure. 5.2 Bent-Core Concentration Effect on Blue Phase Stabilization Materials We have studied stabilization of blue phase by doping a BCLC in the presence of chiral dopants. A bent-core materials presented here are five ring bent-core molecules containing substituent in the third position. The two kinds of nematic liquid crystals were used in this work were BL006 and MLC6080. The chiral dopants used in this mixtures 104

120 were CB15 (20%), R811 (20%), and R1011 (5%) and the total chiral dopant concentration was kept at 45% by the weight of the chiral dopant in this mixture while the concentrations of NLCs and BCLC were varied. The chemical structure of BCLC is shown in Fig Five different mixtures were prepared by doping BCLC with various concentrations and their compositions are listed in Table 5.1. All the mixtures were prepared by mixing in the ultrasonicator for 1.5 hrs. at room temperature. The mixtures of BCLC doped blue phases were filled in the cell at 56 o C and cooled it to room temperature The blue phase temperature range identified in a cell that consisted of two plain glass substrates without indium-tin oxide (ITO) and with a 15 µm cell gap without any surface treatment such as rubbing. Figure 5-1: Chemical structure of bent-core molecule. 105

121 Table 5.1. Compositions of BCLC doped blue phase liquid crystal mixtures. Sample BL006 (wt%.) MlC6080 (wt%.) BCLC (wt%.) Chiral dopant mixture (wt%.) a b c d Texture Analysis The phases of mixtures were identified using a polarizing optical microscope (POM) with crossed polarizers. The temperature of samples was controlled using a hot stage. The samples were heated to the isotropic state with a heating rate of 1 o C/min and then cooled to a smectic state with a cooling rate of 0.2 o C/min to carry out the texture observation. Figure 5-2 illustrates both POM images of blue phase platelets and phase sequences of the BP mixtures doped by BCLC ranging from 0% and 7% by weight. The results were also obtained during cooling. According to the POM images, the BP mixture before BCLC-doping shows bluish green color independent of temperature variation. The 106

122 BP mixtures doped by BCLC molecules exhibit deep blue color under crossed polarizer. With doping of BCLC molecules, the color of BP mixtures changes from bluish green to blue because the chirality of the system is increased with BCLC doping. An explanation of this increase in chirality is that achiral bent-core molecules become structurally chiral resulting from the interactions with the chiral host, which induces tilt and polar order of bent-core molecules. 107

123 Cooling Rate 0.2 o C/min a) b) 50 o C 42 o C Iso BPII BPI 51.3 o C 47 o C 40.7 o C 40.7 o C c) 47 o C 43 o C 40.3 o C Iso BPII BPI 50.9 o C 46 o C 40.3 o C d) 59 o C 52 o C 50 o C Iso BPII BPI 59.4 o C 54 o C 50 o C e) 54 o C 50 o C 46 o C Iso BPII BPI 55 o C 49 o C 46 o C 50.4 o C 42 o C 37 o C Iso BPII BPI 50.4 o C 43 o C 37 o C Figure 5-2: POM textures of the mixtures by doping with BCLC with concentrations of a) 0% wt., b) 1% wt., c) 3% wt., d) 5% wt., e) 7% wt. 108

124 On the basis of POM images we have obtained a phase diagram. Figure 5-3 shows the phase diagram for bent-core doped BP mixtures as a function of bent-core molecule concentration. As bent-core molecule concentration increases, the temperature range of bent-core doped BP increases. This observation indicates that the temperature range of BP increased from 11.3 o C to 13.4 o C with doping of BCLC, whereas the temperature range of BPII increased from 3.3 o C to 6.4 o C with the increasing concentration of BCLC molecule from 0% to 7% by the weight. Although a wide temperature range of BPI and BPIII [148,151,152] doped by bent-core molecules have been reported by many researches, stabilization of BPII by doping bent-core molecules has been reported by Park et.al [150]. Figure 5-3: Phase diagram obtained by cooling for BP mixtures as a function of bent core molecule concentration. 109

125 5.2.3 Refraction Spectra of Blue Phases with Doped Bent-Core Molecules at Different Concentrations Reflection spectra of pure BP sample without BC doping and BP samples after being doped by BC at different concentration were determined as a function of temperature. Reflection spectra were measured in a cell with a cell gap of 15µm. The cell was composed of two plain glass substrates without any electrode and alignment layer. In order to obtain the data, an Optics spectrometer was used. Figure. 5-4 shows the Bragg reflected wavelengths as a function of temperature for pure and BCLC-doped BP liquid crystal samples. By mixing bent-core molecule with BPLC, the initial helical pitch of BPLC was slightly decreased and the Bragg reflection wavelength shifted to a shorter wavelength. Figure 5-4: Bent-core concentration effect on the temperature range of blue phase for a) 0wt% BC. 110

126 Figure 5-4: Bent-core concentration effect on the temperature range of blue phase for b) 1wt% BC, c) 3wt% BC, d) 5wt% BC, e) 7wt% BC. 111

127 5.3 Bent Core Structure Effect on the Blue Phase Stabilization After we optimized the BCLC concentration at 7% by weight, we compared the effect of different BLCL molecules on the stabilization of BP. These two bent-core molecules represent the first samples having a change of polar properties. The chemical structures, compositions, and dipole moments of the samples of BCLC molecules are given in Table 5.2. We studied a mixture made by doping a BCLC at 7 wt% in the LC mixture of BL006/ MLC6080 with concentrations of 17 wt%, and 32 wt%, respectively. The chiral dopant was comprised of CB15, R811and R1011 at concentrations of 20 wt%, 20 wt% and 5 wt%, respectively. Table 5.2. Compositions of mixtures with different BC molecules and polarities of BC molecules. Sample BL006 (wt%) MLC6080 (wt%) BC (wt% 7) Chiral dopant mixture(wt%) Dipole moment(d) of BC molecules Cl Cl Sample O O O O H N N H O O Cl Cl O O Sample O O Cl N N Cl O O Sample Cl Cl O O O O F N N F O O 112

128 In order to examine the phase transition, textures of the phases of mixtures were observed at various temperatures using a polarizing optical microscope (POM) with crossed polarizers with a cooling rate of 0.2 o C/min. Figure 5-5 shows a sequence of POM images of BP samples with different bent-core molecules. All the mixtures revealed deep blue color with decreasing temperature when viewed under a crossed polarizer as seen in Fig The evaluated temperature range of blue phase was 13.4 o C, 10.1 o C, and 8.8 o C and for the BP sample 1, sample 2, and sample 3, respectively. BCLC doped BPLCs could stabilize the defect cores depending on the molecule shape of BCLC and elastic constants of resultant mixtures [151]. 113

129 Figure 5-5: POM textures of a) sample 1, b) sample 2, and c) sample 3. The difference in the temperature range of BP samples is because of their different chemical structures (Figure 5-6). There is a correlation between temperature range and bend angle as well as L/W ratio of BC molecules, as mentioned by Takezoe and Takanishi [152]. As a result of the strong dipole-dipole interactions of Flourine atoms, there is a tendency of an increase at the bending angle between the two legs of the molecule 114

130 (Fig. 5-6c). The temperature range of BP increases with the increasing the ratio of L/W, where L and W are length and width of the BC molecule, respectively. A larger bend angle gives rise to a wider BP temperature range. a) L/W=3.2 b) L/W=2.7 c) L/W=1. Figure 5-6: Simulations of the chemical structures of a) the bent-core molecule in sample 1, b) the bent-core molecule in sample 2, c) the bent-core molecule in sample

131 5.3.1 Different Bent Core Effect on the Refraction Spectra of Blue Phases Figure 5-7 shows the Bragg reflected wavelength of the BCLC-doped BP samples during the cooling (0.2 o C/min) as a function of temperature. As seen in Fig. 5-7, the reflection peaks gradually shifted the shorter wavelengths with the increasing in temperature. The reflected wavelength is blue-shifted from long wavelength to shorter wavelength for the BPI structure, whereas the reflection peak is red-shifted from shorter wavelength to longer wavelength for the BPII. 116

132 a) o C 39 o C 48 o C 44 o C Reflectance (%) Wavelength (nm) Reflectance (%) b) o C 35 o C 36 o C 37 o C 38 o C 39 o C 40 o C Wavelength (nm) c) o C 36 o C 38 o C 40 o C 26 Reflectance (%) Wavelength (nm) Figure 5-7: Bragg reflection spectra of a) sample 1, b) sample 2, c) sample

133 5.3.2 Electro-Optical Behaviors of Different BCLC Doped Blue Phases The electro-optical performance of the BP samples containing 7 wt% bent-core molecules were investigated in an IPS cell with a patterned ITO having 10 m electrode width and 10 m electrode space on one substrate. The IPS cells were assembled with a second glass substrate without the ITO electrode and the cell gap between the two substrates was maintained at 5 m by using ball spacers. Figure 8 shows the electrooptical behaviors of the BCLC-doped BP samples under a square bias field (1 khz). The striped electrodes of the IPS cells were placed at a 45 angle between the polarizers crossed at 90, so with an applied field, the initial dark state of the cell can be switched to a bright state. The threshold and turn on voltages for the cell of pure BP are 18V and 78V, respectively. On the other hand, the threshold voltages of the sample cells of 1, 2, and 3 are 10V, 39V, and 14 V and the turn-on voltages are 74V, 55V, and 30V, respectively. The threshold and turn-on voltages are lower for the bent-core doped BP samples because of their smaller bend-to-splay elastic constant ratios. Additionally, the turn-on voltages for sample 2 and sample 3 are lower than for sample 1 because of the dipole interactions of the molecules. Moreover, bent-core molecules reduce the interfacial energy. Because of the large molecular volume of bent-core molecules, BCLC extends the distance between the molecules and decreases the Van der Walls energy of the system [152]. Additionally, Fig. 5-8 shows POM images of the BPII phase for the pure BP and bent-core BP mixtures under in-plane electric field geometry. Without the electric field, a dark state was obtained for BPII, as shown in Fig. 5.8a. A bright state can be 118

134 observed under the electric field for pure and all bent-core doped BP samples. A fieldinduced phase transition was observed for the bent-core doped BPLC samples in response to a high electric field. The transmittance gradually increased with the applied voltage and the BP underwent a deformation induced phase transition due to the change in the lattice constant or a local re-orientation of the molecules. 119

135 Figure 5-8: Normalized voltage-transmittance curves and POM images of the cells as a function of applied voltage of a) pure BP sample without BC molecules, b) Sample 1, c) Sample 2, d) Sample

136 5.4 Conclusion In conclusion, we have investigated the achiral bent-core molecules doped blue phase liquid crystals. Surprisingly, in our system, the widest temperature range increase was to from 3.3 o C to 6.4 o C for the stable BPII at the optimal concentration of bent-core molecules of 7 wt%. When the bent-core nematic molecules with different substituent group were used, the observed BP temperature range increased more than 13 o C. There is a coupling between chirality and molecular shape resulting in a stabilization of blue phase structure. We also observed good E-O performance of the BP by driving in-plane electric field geometry. The E-O behavior observed for the field-induced Kerr effect corroborates the deformation of the cubic structure either via change at the lattice constant or a re-orientation of the molecules. 121

137 Chapter 6 Polymer Dispersed Blue Phase Liquid Crystals Polymer dispersed liquid crystals (PDLCs), i.e., dispersions of micron-sized LC droplets inside a polymer matrix, have formed a class of important electro-optical (E-O) materials since their discovery by Fergason [54,61]. The PDLC films were fabricated either by solvent evaporation, thermal induction, or polymerization-induced phase separation. [153] The first method used in the preparation of PDLC films is the encapsulation (emulsification) of the liquid crystal inside an aqueous solution of filmforming polymer [61]. After water evaporated at a certain rate to induce phase separation, the film is laminated between two conductive electrode coated substrates. The second method includes the solvent-induced phase separation (SIPS), in which solvent is used to dissolve the liquid crystal and thermoplastic polymer and create a single phase. Then solvent is evaporated at a certain rate to induce phase separation. The third method is called thermally induced phase separation (TIPS). In this method, the liquid crystal and thermoplastic polymer are heated to obtain a melting and then mixed to form a single phase. When the mixture is cooled at a controlled rate, liquid crystal phase separates into droplets. The third method is known as polymerization-induced phase separation (PIPS) containing the liquid crystal, monomer, and a small amount of catalyst. After exposing 122

138 the prepolymer mixture to an external stimulius, for example, light or heat, the monomer gels into a polymer matrix and liquid crystal phase separates into droplets. The operation principle of the PDLC films is to be electrically switchable between light scattering and transparent states due to index matching between guest and host materials. [43,58,154] With no applied voltage, the PDLC films normally appear milky and scatter incident ambient light because the LC molecules orient randomly inside of the droplets. Upon applying a voltage across the PDLC film, the LC directors align in the direction parallel to the applied field. Due to index matching between polymer and LC molecules in the presence of the electric field, the PDLC film becomes transparent when viewed along the normal direction. PDLC films have significant advantages for electrooptical device applications, including having no need for polarizers or alignment films, and their high light transmittance. [154] A number of reports have appeared recently suggested useful applications of PDLCs ranging from switchable light modulators [39,155], smart windows [154], information displays [156], and holographically formed optical elements and devices [ ]. The PDLC behavior in electro-optic devices such as displays and smart windows can be improved by the presence of BPLCs. The dispersed or encapsulated blue phase liquid crystal leads to change of its original optical and electro-optical properties with an external field. Polymer encapsulated blue phase liquid crystal films are a strong candidate for the next generation of displays and spatial light modulators due to the outstanding electro-optical properties of blue phases. 123

139 This chapter reports an exploration of a new form of PDLC electro-optical (E-O) films composed of BPLC and polymer prepared by the solvent evaporation method as well as by polymerization-induced phase separation methods. The compositions, film preparations, physical and morphological behaviors, and E-O properties of polymer encapsulated blue phase (PEBP) and polymerization induced phase separated blue phase (PDBP) films are described. The E-O performances of these films were characterized by analyzing transmittance as a function of switching voltage and response time, using inplane field switching cells. 6.1 Polymer Encapsulated Blue Phase Liquid Crystals Encapsulation is one of the major methods used in the fabrication of PDLC films. Emulsion-based PDLC films are formed of small liquid crystal droplets into the aqueous solution of the polymer. The continuous aqueous phase can be water soluble polymer [164,165] or a colloidal suspension of a water insoluble polymer [61,166]. The required energy input for the droplets formation and droplets break-up generally arises from the chemical potential of components or from mechanical devices. The emulsion system is obtained by high shear; for example, by ultrasonication or high-pressure homogenizers. Factors such as the rate of solidification and polymer solubility also play a role in the yield. Microspheres formed by rapid solidification of the polymer may give a higher yield due to encapsulation of some of the soluble fractions in the matrix [107,133].In order to 124

140 make the PDLC film, the emulsion is coated on a conductive substrate and laminated with the second conductive substrate after allowing the water to evaporate. The evaporation of water leaves thin polymer films containing liquid crystal droplets dispersed in a matrix. Based on the evaporation process, the droplets become spherical or oblate in the polymer film [39,43,58-60, ]. This shape deformation affects the alignment of LC inside the film cavities, which has a significant effect on the physical properties of PDLC films. The orientation of the LC director inside the droplet structure depends on anchoring energies, the elastic properties of the liquid crystal, as well as on shapes and the sizes of droplets [61]. However, the size distribution of the liquid crystal droplets in the emulsion can be modified by the preparation process and materials used to produce the emulsion, for example, the stirring time and speed, viscosities of polymer and liquid crystal. With increase in time of mixing in an ultrasonic cleaner, the droplet size of emulsion decreases [167]. The other factor that has an influence on the size, stability, and polydispersity of the droplets is the type and concentration of surfactant. The droplet size and polydispersity index decreased with increase in surfactant concentration [169]. The size and size distribution of encapsulated LC droplets can have a significant effect on the electro-optical properties of the films. Large area applicability of the emulsion system enhances the range of its application in displays and light modulating devices [165]. 125

141 6.1.1 Materials and Cells Preparation A representative BPLC mixture was formulated with a cyanobiphenyl-based nematic mixture E31 (Merck, n = 0.2, ne = 1.792, no = 1.533, ε ~ E7 = 13.8) and a chiral dopant R811 (Merck) with a moderate helical twisting power (10 µm 1 ), and with a weight ratio of 62% and 38% for E31 and R811, respectively. Commercially available water-based polyurethane latex NeoRez 967 (Royl DSM) with a refractive index (np) of 1.51 (at λ = 589 nm) was used as the polymer matrix. The polyurethane latex NeoRez 967 contains 60 wt% water as solvent. The PEBP1, PEBP2, and PEBP3 samples consist of NeoRez 967 latex at concentration of 68%, 50%, 34% and a blue phase liquid crystal at 32%, 50%, 66% weight ratio, respectively. The compositions are listed in Table 6.1. Table 6.1. The compositions of three polymer encapsulated BPLCs. Sample BPLC Conc. (wt%) Latex Conc. (wt%) PEBP PEBP PEBP

142 PEBP samples were formed by stirring the BPLC and NeoRez 967 latex in an ultrasonic bath at room temperature for 2 hrs. The mixture was again stirred with a vortex shaker for 3 min at room temperature to remove air bubbles. Subsequently, a small drop of the mixture was deposited on a piece of ITO-coated glass with a pipette. A big drop of latex was deposited on the substrate and spread to cover the whole surface with another glass substrate. The solvent was evaporated under reduced pressure in a vacuum desiccator at room temperature for 45 min. The top-down E-O cells were assembled by placing the second ITO-coated substrate on top of the first substrate. The cell gap was maintained by dispersing 22 µm ball spacers between the substrates. To study the field-induced birefringence effect of blue phase, PEBP films were deposited in between substrates used for in-plane-switching (IPS) cells, as shown in Fig The IPS cells were made of one substrate without the transparent electrode and the other substrate with lithographically prepared electrode patterns with 10-µm electrode line width, 10 µm spacing between electrodes, and a cell gap was maintained by ball spacers. Figure 6-1: Schematic representation of a PEBP film with blue phase droplets dispersed in a polymer matrix and laminated inside of an IPS cell. 127

143 6.1.2 Temperature Range of Polymer Encapsulated Blue Phase Liquid Crystals To determine the temperature range and observe the textures of the BPLC droplets of the PEBP films, a polarizing optical microscope (POM) equipped with a computer-controlled hot stage, and a video camera with a magnification factor of 200 was used. All samples were heated to the isotropic phase and then cooled to room temperature at a rate of C/min. The PEBP samples showed isotropic-to-blue-phase transition at 32 C and blue-phase-to-cholesteric-phase transition at 22 C. Figure 6-2 shows the photomicrographs of phase sequence of PEBP1 (Fig. 6-2a), PEBP2 (Fig. 6-2b), and PEBP3 (Fig. 6.2c) films in E-O cells under crossed polarizers. The observed textures of PEBP1 and PEBP2 samples show a uniform, reflecting, bluish-green color of the BPI at room temperature. As seen in Fig. 6-2, the droplets in the mixture have both cholesteric and BP textures. The small droplets tend to have cholesteric texture whereas bigger droplets have BP texture. The reason may be because of the anchoring energy of different size of the droplets. Due to strong anchoring energy of the small droplets, the phase transition of small droplets becomes more difficult. 128

144 Figure 6-2: Photomicrographs of (a) PEBP1 (b) PEBP2, and (c) PEBP3 films prepared from a mixture of NeoRez/BPLC at 27 C. The white-colored scalar bar has a length of 50 µm. The POM images also show two distinct droplet-sized groups of 20 µm and 55 µm for PEBP1 and two distinct droplet-sized groups of 40 µm and 65 µm for the PEBP2 as seen in Fig For the PEBP3 sample, the droplets exhibit an average size of 16 µm (Fig. 6-3). The droplets formed in clusters resulting from coalescence and inter-connected or partially merged small droplets arose with boundary lines clearly being exhibited across the surface of the droplets. 129

145 Average Droplet size ( m) PEBP1 PEBP2 PEBP Temperature ( o C) Figure 6-3: Plot of average size of droplets versus temperature Reflection Spectra of Polymer Encapsulated Blue Phase Liquid Crystals Reflection spectra were measured with an Ocean Optics spectrometer at various temperatures. The reflected wavelengths as a function of temperature for three PEBP samples are shown in Fig The Bragg reflection wavelengths of samples were blueshifted as the temperature was increased from the BPI to the BPII phase indicating a deformation or tilting in the cubic lattice of BP. 130

146 Figure 6-4: Reflection spectra for PEBP sample Electro-optical properties of Polymer Encapsulated Blue Phase Liquid Crystals To study the electro-optics of blue phase, PEBP films were deposited on in-planeswitching (IPS) cells with 10-µm electrode line width, with 10-µm spacing between electrodes, and the cell gap was maintained by ball spacers. Two types of PEBP samples were prepared; one is the normal mode and the other is the reverse mode. The PEBP1 131

147 sample was a reverse-mode PDLC that switched from bright (field on) to dark (field off) states. In order to optimize the cell gap for the electro-optical measurements, three different cell gaps with 10 µm, 15 µm, and 22 µm were used. Figure 6-5 (6-5a and 6-5b) shows the plots of threshold, turn-on voltage, and response time of the PEBP1 sample as a function of the thickness of IPS cell. The threshold voltage (since PEBP1 is a reverse mode device, threshold voltage is described as V90, required for achieving 10% reduction in transmission) of PEBP1 sample decreased from 90.5 V to 34 V as the cell gap decreased from 10 µm to 22 µm. It is known that the threshold voltage of an IPS cell is governed by equation (6.1) [168] V th= πl d K ε o ε, (6.1) where l is the distance between electrodes, d is the cell gap, K is the bend elastic constant, εo is the dielectric permittivity in vacuum, and ε is the dielectric anisotropy of LC. 132

148 Figure 6-5: (a) Threshold and turn on voltages of PDBP1 as a function of IPS cell thickness and (b) Response time of PDBP1 as a function of IPS cell thickness. In case of the turn-on voltages, since PEBP1 is a reverse mode device, the turn on voltage is described as V10 required for achieving 90% reduction in transmission for PEBP1 films. The turn-on voltage increases from 7 V to 22 V with an increase in cell gap from 10 µm to 22 µm for PEBP1 films. PEBP films with small droplets (10 µm sample) require higher fields to overcome strong surface anchoring at the LC polymer interface when compared to those of PEBP films with larger droplets (22 µm sample). Response time depends on the strength of the applied electric field that orients the LC molecules and the viscoelastic property that resists reorientation to the initial state at zero voltage [170].The response times of PEBP samples were determined by switching the PEBP samples at 27 C between corresponding values of V10 to V90 and V90 to V10. The rise time ( rise) of PEBP1 decreased from 0.91 ms to 0.15 ms, and the decay time ( decay) 133

149 increased from 0.10 ms to 0.14 ms with an increase in cell gap from 10 µm to 22 µm (see Fig. 6-5(b)). Figure 6-6 shows measured rise of 0.14 ms and decay of 0.11 ms for the PEBP1 sample in an IPS cell with a 15 µm cell gap. As seen in Fig. 6-6, rise is longer than decay because of the higher energy required for the deformed on-state. The response time of the PEBP2 and PEBP3 samples were measured in IPS cells with a 15 µm cell gap by switching between their corresponding V10 and V90 voltages at 27 C. The rise of the PEBP2 and PEBP3 samples are ms and ms whereas the decay of the PEBP2 and PEBP3 samples are ms and ms. The results are summarized in Table 6.2. In general, the decay time is faster than the rise time because of higher elastic energy of the deformed state. Figure 6-6: The normalized transmittance versus switching time of PEBP1 sample in an IPS cell with 15 µm cell gap. 134

150 The relation between the rise and decay times and droplet size of the cholesteric liquid crystal is described by the equations 6.3 and 6.4. Since small droplet films require higher fields to overcome higher surface anchoring energy at the LC polymer interface, it is reasonable to assume there is an inversely proportional relation between decay time and droplet size of BP droplets. If we consider switching of PEBP samples between V10 (State 1) and V90 (State 2) where the free energy of LC is F1 (State 1) and F2 (State 2), then the transition time of the LC molecules between States 1 and 2 is given by [107] where γ is the viscosity and V is the volume. The free energy of the LC is F1 = Felas1 + Felec1(E) in State 1 and F2 = Felas2 + Felec2(E) in State 2. The electrical energy, given by F = εεo(e 2 Ec 2 ) 4/3πa 2 b, [107] overcomes the elastic energy to switch the LC molecules from State 1 to State 2. In that case, the rise time is given by equation 6.3 τ γ rise = εεo E 2 E2 c a 2 b/a 2 b = γ εεo E 2 E2 c (6.3) where a and b are the semi-major and semi-minor axes, respectively, and δ= 1/(a 2 b 2 ) is the aspect ratio of the droplet. When the LC molecules switch from State 2 to State 1, 135

151 elastic energy, which is equal to F= F2 F1= Felas, compensates for the electrical energy and decay time is given by [56] A persistent effect was observed when an electric field was applied to the PEBP samples [61]. This phenomenon can be explained as follows. If one of the disclination points of the two interconnected droplets becomes confined in a local energy minimum, then the droplets may get confined in high field. After the removal of the electric field, the BPLC molecules inside the droplets cannot easily relax back to their original state until the disclination escapes from the confinement. Relaxation of the droplets to the zero field requires some activation energy, so the transmittances of the PDLC films take several minutes to reach a full-scattering state [61]. Furthermore, a slight mismatch between the ordinary refractive index of nematic in BPLC and the refractive index of NeoRez 967 gives rise to light scattering in the on-state of the PEBP films. To compare the E-O results of a 15 µm PEBP1 sample, the light transmittance as a function of applied voltage curves and response times of the PEBP2 and PEBP3 samples were also studied with IPS cells having a 15 µm gap. The field-induced birefringence of the PEBP films was studied with cells used for inplane-switching mode of operation. The transmittances of the PEBP films in IPS cells were measured as a function of applied voltages and shown in Fig The PEBP1 and PEBP2 samples are reverse-mode PDLCs that switch from bright (field on) to dark (field 136

152 off) states, whereas the PEBP3 sample is a normal mode PDLC that switches from dark state to a bright state. The reverse mode PEBP1 sample has a threshold voltage of 11 V and a turn-on voltage of 46 V. On the other hand, the threshold voltage for the normal mode PEBP2 sample is 9.66 V and its turn-on voltage is 44.8 V. PEBP1 sample with the average droplet size of 55 µm requires higher Vth and Von than PEBP 2 sample with the average droplet size of 65 µm. Larger LC droplet size gives rise to lower Vth and Von as a result of weak anchoring strength. The normal mode PEBP3 sample has a threshold voltage of 14.0 V and turn-on voltage of 30.3 V, as shown in Fig. 6.7(a). Although PEBP3 has a smaller droplet size, it should be perceived that the reverse mode sample has a lower threshold voltage, whereas the turn on voltage is higher than the other sample. The differences in the optical and E-O behavior of the PEBP samples enable different modes of operation. Furthermore, the results also lead to the conclusion that the shape and size of the droplet are affected by the composition and speed of phase separation (rate of solvent evaporation). Differences in switching voltages of PEBP samples are also a consequence of strength of surface anchoring of BPLC molecules at the droplet walls. On the other hand, neither the external field may induce the alignment direction of the liquid crystal without changing the director configurations, nor the external field forces the director configuration to change from one form to another. 137

153 Figure 6-7: The plots of normalized transmittance versus applied voltage of (a) PEBP1 (b) PEBP2 and (c) PEBP3 films in an IPS cell with 15 µm cell gap at 27 C. An applied electric field E breaks the symmetry and gives rise to birefringence, which describes the Kerr effect. The relationship between the transmittance (birefringence) and the electric field in a system of encapsulated LC droplets is given by [39] n(e) = λke 2, (6.5) where K is the Kerr constant, λ is the probe wavelength, and E (=V/l, where V is the applied voltage and l is the distance between electrodes) is the applied electric field. The Kerr constant is calculated as a function of voltage by fitting the data with Eq The Kerr constant is inversely proportional to the applied electric field. Low electric field plays an important role to increase Kerr effect. As shown in Table 6.2, PEBP samples generate considerably large Kerr constants in the range of ~ V 2 m (at 633 nm), which are about 10 times higher than that of the reported PSBPs [171]. This 138

154 difference in the Kerr constant may be because of domain size differences of blue phase structures in the PSBP and PEBP samples. The lattice constant of a blue phase structure is a few 100 nm depending on the radius of double twisted helix of the blue phase. For polymer encapsulated blue phase, samples with bigger blue phase domains or droplets require lower electrical field and larger Kerr constant. However, the Kerr constant is linearly proportional to the dielectric permittivity of the BP system according to (6.6) [39] : In our system, the polymer is neorez polyurethane latex. Latex has a much bigger dielectric permittivity (~24) than that of LC. Neorez increases the dielectric properties of the emulsion via an increase in the dielectric constant. For that reason, the Kerr constant increases as a function of increasing dielectric permittivity in our system when compared to PSBP system. 139

155 Table 6.2. The measured threshold (Vth) and turn on voltages (Von), rise and decay times, and calculated Kerr constants of the encapsulated LC droplets for three different PEBP samples at 27 C. Sample Vth Von τrise τdecay K (V) (V) (ms) (ms) (m/v 2 ) PEBP x10 8 PEBP x10 8 PEBP x Morphological Examination of Polymer Encapsulated Blue Phase Liquid Crystals The morphology of the PEBP samples was examined by using a Hitachi S-2600N scanning electron microscope (SEM) with an acceleration voltage of 22 kv. SEM samples of PEBP films were prepared by extracting BPLC with a mixture of dichloromethane and hexane at a ratio of 20% and 80%, respectively, for 24 h at room temperature in sealed vials. The cells were removed from the vials, the solvent was allowed to evaporate, and the cells were opened carefully. The films were deposited with a thin layer of gold under vacuum to enhance the contrast and resolution of the images. 140

156 Figure 6-8 shows the SEM images of PEBP films in an IPS cell with a film thickness of 22 µm. The SEM images of the three PEBP samples reveal a dense polymer network, and the polymer density decreases with a decrease in polymer concentration. With high BPLC concentration, the morphological behavior of the polymer shows a change from discrete droplets to coalesced droplets [171]. The droplets on the substrate with patterned electrodes viewed at normal angle exhibit two discrete size groups; one group of droplets has an average size around 10 ± 3 µm for three samples, whereas the other group of droplets has size around 50 ± 5 µm for PEBP1, 40 ± 3 µm for PEBP2, and 20 ± 3 µm for PEBP3 samples, as shown in Fig As seen in Fig. 6-8(c), the droplets formed in the film via the encapsulation method established with an oblate spheroid shape due to the shrinkage of the polymer in the film drying process. During the process, depending on water evaporation from the system, the composite films tend to shrink [61]. The SEM image of cross-section view of the PEBP film exhibits small droplets that are pinned to the substrates surface, whereas droplets with size comparable to or larger than the film thickness are deformed in the direction parallel to the plane of film as shown in Fig. 6-8(d). 141

157 (a) (b) (c) (d) 100 µm Figure 6-8: SEM images of (a) PEBP1 (b) PEBP2 and (c) PEBP3 films show the top view of droplets at the surface of a substrate, and (d) cross-section of the PEBP3 film viewed at an oblique angle (30 degrees from normal) with an electrode of 22-µm IPS cell. 142

158 6.1.6 Contact angle measurement of Polymer Encapsulated Blue Phase Liquid Crystals Another study focused on the contact angle measurement to understand the relation between the anchoring strength and free energy. The shape of the cavity including the liquid crystal, the elastic constants of the bulk, and the external field all play an important role on the anchoring of the liquid crystal at the polymer surface. The anchoring energy on the interface of the liquid crystal and polymer binder is given by Rapini-Papolar free energy density equation 6.7 [172] where g represents the isotropic liquid crystal-interface interaction, θo refers to the preferred angle of the director field at the droplet surface, (θ-θo)represents the actual director field from preferred angle, φ-φo is the difference between the azimuthal orientation of the director field and the preferred field, and WQ and Wφ are the polar and azimuthal energy coefficients, respectively. For the weak anchoring, as a result of the competition between the elastic energy of the bulk and the surface free energy of the droplet, the tilt angle of the liquid crystal director can change spatially within the droplet to minimize curvature in the bulk of the droplet. In the case of the strong anchoring interactions between polymers and liquid crystal, the contact angle is either 0 o or 90 o at the droplet surface. However, the chemical nature of 143

159 the polymer surface plays an important role on the strong anchoring or weak anchoring of the system. This section gives our results for the contact angle to understand the surface tension properties of the polymer encapsulated liquid crystal droplets on the substrate with different alignment. A goniometer (Model 250 Standard G/T) was used to measure the contact angle of DI water, LC, polymer and PEBP droplets on glass substrates coated with different polyimides; for example, PI2555 for planar alignment and SE1211 for homeotropic alignment as well as plain and ITO glass substrates (Fig. 6-9). The results are summarized in Table 6.3. There was PI coating on the ITO and non-ito glass substrates. Figure 6-9: Contact angle measurements of DI water, liquid crystal (E-31), polymer (Neorez), blue phase LC and encapsulated blue phase on the glass substrates with different surface treatments for planar or homeotropic alignment. 144

160 Table 6.3. Measured contact angles of the materials on the different substrates. Substrate of DI water of E-31 of BP of Neorez of PEBP1 Non-ITO Glass 26.5± ± ± ± ±0.07 ITO Glass 46.5± ± ± ± ±0.05 PI ± ± ± ± ±0.02 SE ± ± ± ± ±0.09 Water has higher contact angles than LC due to high surface energy of water. Molecules in liquid and molecules on the surface have very different forces on the same substrate. Thus, liquid surface tension is decreasing and surface energy is increasing for the same liquid. In order to examine the effect of surface alignment effect on the surface tension, contact angles were measured on the substrates with the planar polyimide PI2555 and homoetropic polyimide SE1211 which are rubbed different times listed in Tables and 6.4 and

161 Table 6.4. Measured contact angles on the planar alignment substrates Rubbing times of DI water of E-31 of BP of Neorez of PEBP 1 52±0.02 8± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.04 Table 6.5. Measured contact angles on the homeotropic alignment substrates Rubbing times of DI water of E-31 of BP of Neorez of PEBP ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.03 As seen in Tables 6.4 and 6.5, when the surface energy of a substrate is relatively low, the intermolecular forces among LC molecules are stronger than the forces across the interface. The LC with perpendicular alignment to the surface maximizes their intermolecular interactions. And while the surface tension of a solid is more than the surface tension of LC molecules, the force across the interface dominates. Therefore, surface free energy is minimized if LC molecules are aligned parallel to the substrates. 146

162 6.1.7 Conclusion To conclude, the reported PEBP films demonstrate a new electro-optic application of the BPLCs with two switching modes: normal and reverse modes. First, we have demonstrated that PEBP films laminated between two indium-tin-oxide-coated conductive substrates enable switching between light scattering and transparent states in response to an applied electric field across the film. Second, PEBP films also allow inplane switching to induce birefringence between crossed polarizers at low switching voltage and fast response time. Using an encapsulated BPLC device and an IPS switching mode, a PEBP film shows a very large Kerr constant, which is about 10 times higher than that of conventional PSBPs. The advantages of the PEBP films are easy sample preparation, no alignment layers, and no polarizers, therefore, the PEBP films have a potential for a wide range of applications such as displays and spatial light modulators. 6.2 Polymerization-Induced Phase Separation Blue Phase Liquid Crystals The liquid crystal was mixed with low molecular-weight monomers to form a polymerized PDLC film. When the polymerization occurs under convenient conditions, the growing polymer chains phase separate from the LC to form a polymer wall surrounding LC domains. This method uses the ultraviolet radiation to initiate the free radical polymerization of monomers. During the polymerization process, one end of the double bond is chemically activated and it can covalently link to another double bond to form a new one that is reacting with another monomer. Activated species are produced as 147

163 a result of the photo activation of the initiator. The initiation process completes after the reaction of produced species with the monomer. The propagation step leads a rapid chain growth in consequence of the reaction of the activated species with monomer. The termination step completes the reaction and form a polymer gel. The reaction rate is controlled by a number of factors such as the number of active initiators and monomer concentration. The phase separation process is affected by temperature, light intensity, and the solubility characteristics of polymer and LC materials. The variations in the mixture composition or phase separation process lead to difference in droplet morphology, [173] which has a significant effect on the electro-optical performance of the resulting film [174,57]. The electro-optical performance of the PDLCs depends upon the size and morphology of the liquid crystal droplets as well as the type and functionality of monomers and crosslink density [57,175]. The droplet size was determined by the composition ratios between the LC and polymer [120,48], UV light intensity, curing time, and temperature [61,176]. The size of liquid crystal droplets with the increasing content of LC inside the prepolymer mixture was reported by Aleksander and Klosowicz [171]. The decrease of size with the increasing exposure time was investigated by Henry et al [ ]. The polymerization-induced phase separated blue phase (PDBP) liquid crystal samples were prepared with a monomer/crosslinking agent/blue phase composite. The effects of the relative weight ratio between monomer, crosslinking agent and BP, as well 148

164 as the effect of the adding of photoinitiator on the electro-optical properties of PDBP liquid crystal samples were systematically investigated Materials and Cell Preparation The polymer dispersed blue phase liquid crystal films were produced by the polymerization-induced phase separation process. The PDBP liquid crystal mixtures were prepared using mixtures of a UV-curable material (crosslinking agent, PN393) and a monomer hydroxybutyl acrylate (HBA) (Fig. 6-10) and a blue phase (BP) liquid crystal mixture. PN393 consists of acrylate monomers and it forms a cross-linked network via curing with ultraviolet (UV) light. Figure 6-10: Chemical structures of the monomer of hydroxybutyl acrylate (HBA). The blue phase liquid crystal mixture used in this study was formulated with a cyanobiphenyl-based nematic mixture E31 (Merck, n = 0.2, ne =1.792, no = 1.533; ε ~ E7= 13.8) and a chiral dopant R811 (Merck) at the weight ratio of 62% and 38%, respectively. PDBP liquid crystal samples were formed by stirring the 149

165 monomer/crosslinking agent/initiator/bplc mixture with a vortex shaker for 10 minutes at room temperature. PDBP films were produced by placing mixtures of the monomers and blue phase liquid crystal mixtures in a thin gap between ITO coated glass plates. The cell gap was maintained by dispersing 10 µm ball spacers between the substrates. The PDBP films were exposed to UV light with a wavelength of 365 nm and irradiation intensity of 0.6 mw/cm 2. The monomer materials were stored in dark medium and under reduced room lights to protect any unwanted photo-polymerization or phase separation; and under these conditions the cells were cured for 30 minutes. The concentration and the ratio between the diacrylate crosslinking agent, monomer and BPLC are shown in Fig Figure 6-11: Composition of blue phase and monomers of HBA and PN

166 To understand better the photoinitiator effects, the samples were prepared either with photoinitiator or without photoinitiator. The compositions of these samples are listed in Table 6.6. Table 6.6. The compositions of the polymerization induced phase separated BPLC droplets. Sample name HBA conc. (wt%) PN393 conc. (wt%) BPLC conc. (wt%) EK-PDBP EK-PDBP1 * EK-PDBP EK-PDBP2 * EK-PDBP EK-PDBP3 * EK-PDBP EK-PDBP4 * * the sample without photoinitiator 151

167 6.2.2 Temperature Range Determination of Polymerization-Induced Phase separated BPLC The transition temperature of PDBP films was determined in the top-down electro-optical (E-O) cells with a cell gap of 22 µm. All the samples were placed on a hot stage with a programmable temperature controller and were heated to the isotropic phase at a rate of 1 C/min and then cooled to room temperature at a rate of -0.2 C/min for the determination of transition temperatures of the PDBP liquid crystal films. The optical textures were observed using a polarizing optical microscope (POM). Blue phase textures of all PDBP liquid crystal samples were observed with small colored domains. According to the POM observations, the phase transitions of PDBP liquid crystal mixtures show the isotropic to BP transition at 30 o C and BP to cholesteric phase at 22 o C. Figure 6-12 shows the POM images of the PDBP sample at room temperature. a) b) c) 30 o C 23 o C 21 o C Figure 6-12: POM images of revealing the textures of PDBP sample at a) isotropic state (30 o C), b) blue phase state (room temperature), c) cholesteric state (21 o C). Scalar bar is 20 µm. 152

168 The electro-optical characteristics of PDBP liquid crystal films were determined mainly by the property of the liquid crystals dispersed in the polymer matrix. The E-O properties and the size of the liquid crystals in the PDBP liquid crystal samples depend on the relative contents of the E31 liquid crystal, PN393 crosslinking agent, HBA monomer, and the existence of the photoinitiator IR651 in the prepolymer mixture Reflection Spectra of Polymerization-Induced Phase Separated Blue Phase Liquid Crystals Reflection spectra measurements of PDBP films were carried out in the top-down electro-optical (E-O) cells (22 µm) by using an Ocean Optics spectrometer at various temperatures. Figure 6-13 shows the reflectance of the PDBP liquid crystal samples either with IR651 or without IR651 with changes in the temperature and wavelength of the incident light in the absence of electrical field. As seen in Fig. 6-13a, the reflectance of the PN393 sample (without HBA content) has a higher reflectance than the other samples at all values of the incident wavelength. However, the addition of 12.5 wt% HBA causes a significant decrease in the reflectance. Additionally, with the increasing weight ratio of HBA to PN393, the reflectance continues to decrease. These results show that the concentration ratio between single functional monomer (HBA) and multifunctional diacrylate crosslinking agent has an important effect on the light scattering properties of the blue phase liquid crystals. Figure 6-13.b shows that the 153

169 wavelength value of the PDBP liquid crystal samples are stable versus the changing temperature. PDBP cells have good stability versus temperature changes.. Figure 6-13: a) The plot of reflectance of the PDBP samples versus temperature, b) The plot of the wavelength the PDBP samples versus temperature. 154

170 6.2.4 Electro-optical Results of Polymerization-Induced Phase Separated Blue Phase Liquid Crystals The electro-optical experiments were measured at room temperature by a setup that has an in-house assembled E-O apparatus consisting of a helium-neon laser with light emission at 633 nm, and an iris to collect the transmitted light. Then the forwardtransmitted light was focused by a lens onto a diode detector. A computer controlled function generator and an amplifier were used for data acquisition. A gated, 1 khz, square voltage was applied to the PDBP film. The E-O properties of the PDBP liquid crystal samples with cells used in top-down mode of operation were studied and the results are shown in Fig The PDBP liquid crystal samples switch from dark (fieldoff) to bright (field-on) states. The results indicate that the threshold voltage, V10 (voltage required for achieving 10% transmission of the cell), and turn-on voltage, V90 (voltage required for achieving 90% transmission of the cell), are dependent on the rate of polymerization-induced phase separation - sensitive to not only prepolymer composition but also existence of photoinitiator. Figure 6-14a shows the PDBP liquid crystal samples without photoinitiator of IR651 and Fig. 6-14b shows the samples with photoinitiator of IR651. As shown in Figures 6-14a and 6-14b, values of V10 and V90 generally decreased with increasing the HBA monomer concentration in the prepolymer mixtures. This effect shows that the operating voltage of the PDBP film is significantly influenced by HBA monomer content. The addition of HBA monomer produces in a significant improvement on the phase separation and increases the size of the liquid crystal domain. Liquid crystal 155

171 droplet size plays an important role in the determination of electro-optical characterization of polymer dispersed liquid crystal system. Threshold voltage is inversely proportional to the droplet size according to [56], where d, R, K, ω, and ε represent film thickness, droplet radius, elastic constant (K33) of BP liquid crystal, aspect ratio, and the dielectric anisotropy of the liquid crystal, respectively. On the other hand, the values of V10 and V90 are increase with addition of the photoinitiator of IR651 as seen in Fig.6-14b. Polymerization rate becomes faster due to addition of the photoinitiator and the faster polymerization rate results in smaller droplet size. The smaller droplet size yields the higher V10 and V90 values. However the additive can change the dielectric anisotropy of the samples because HBA monomer causes a large dielectric anisotropy. Transmittance-voltage (T-V) curves different from each other due to differences in the dielectric anisotropy value, described by [61] 156

172 where Eeff and Ea are the effective electric field across the droplets and the applied field, respectively [38]. The εp and εlc are the dielectric constants of the polymer and liquid crystal. Figure 6-14: Normalized transmittance-voltage (TV) curves of PDBP liquid crystal samples a) without photoinitiator of IR651, b) with photoinitiator of IR651, c) Picture of the cell the sample of PDBP* under the applied field. 157

173 Figure 6-15 shows the contrast ratios of either the PDBP liquid crystal samples without photoinitiator of IR651 or the samples with photoinitiator of IR651. The contrast ratio (CR) for the PDBP liquid crystal is characterized by the difference between transparent and opaque states as given by [61] : Figure 6-15: Contrast ratio dependence of PDBP liquid crystal samples both with photoinitiator of IR651 and without photoinitiator of IR

174 The PDBP samples without the photoinitiator of IR651 show a higher contrast ratio. Moreover, the contrast ratio of the PDBP liquid crystal samples increases with the increase of HBA monomer concentration in the prepolymer mixtures. The contrast ratio reaches a maximum value at 12.5 wt% HBA monomer concentration. The addition of HBA monomer improves the contrast ratio of the PDBP liquid crystal samples with and without IR651 due to the strong polar terminal group of HBA. As a consequence of the chemical affinity, hydrogen bonding with the cyanobiphenyl of E31, the PDBP samples without the photoinitiator of IR651 have low miscibility with LC and fully phaseseparated state of the PDBP liquid crystal cells can be obtained. Figure 6-16a shows the response times of the PDBP liquid crystal samples without photoinitiator of IR651 whereas Fig. 6-16b shows the samples with photoinitiator of IR651. For the polymer dispersed blue phase liquid crystal, decay times are longer than rise times as in the conventional PDLC [61]. Rise and decay times increase with the decreasing HBA monomer as a function of the viscosity of the samples. The samples have a lower viscosity giving rise to a faster rise time and slower decay time with the increasing viscosity [61]. Furthermore, the response time increases with the adding of the photoinitiator of IR 651in the PDBP liquid crystal samples, as seen in Fig The smaller droplets due to higher polymerization rate lead the larger rising time, which can be explained by Eqs and 6.12 [48]. However, the prefactor should be considered, which is related to the field-accumulating effect due to a dielectric or conductivity mismatch between the blue phase liquid crystal droplets and the surrounding polymer 159

175 matrix, as well as the viscosity and droplet shapes of the blue phase liquid crystal [ ]. Moreover, rise and decay times of the PDBP liquid crystal samples depend on the field strength molecular orientation and on the viscoelastic parameters that influence the forces that cause such orientation. τ off = γ 1 a 2 K(l 2 1, (6.12) where 1 is a rotational viscosity coefficient, a is the characteristic droplet radius, εo is the vacuum permittivity, K is the elastic constant (K33) of BP liquid crystal, ρp and ρlc are the resistivity of polymer and liquid crystal, respectively, and l is the ratio of the largest to the smallest radii, assuming the drops to be ellipsoidal. V is the applied voltage and ε is the dielectric anisotropy of the liquid crystal. According to Eqs. (6.11) and Eq. (6.12), the response time is dependent on the shape and size of the liquid crystal and interfacial interaction between BPLC droplet interface and surrounding polymer matrix, as well as on the viscosity and the measurement wavelength [2,41]. Also, the response time of 160

176 polymer dispersed blue phase liquid crystal samples is faster than that of the conventional nematic liquid crystal samples [61]. Figure 6-16: Response time of PDBP liquid crystal samples a) without photoinitiator of IR651, b) with photoinitiator of IR

177 6.3 Conclusion We have demonstrated that the polymer encapsulated blue phase and polymer dispersed blue phase. The thermal stability and electro-optical properties were investigated for the polymerization-induced phase separated blue phase (PDBP) liquid crystal samples, which consist of monomer, crosslinking agent, and blue phase composite. The electro-optical properties of PDBP liquid crystal samples as a function of the relative weight ratio between monomer (HBA), crosslinking agent (PN393) and BP, as well as the addition of photoinitiator in this system were systematically investigated with electro-optical cells having both top-down and in-plane-switching operation. In the presence of HBA, the contrast ratio is improved for the PDBP samples with and without photoinitiator due to the strong terminal polar group of HBA. It was shown that the response time increases with decreasing HBA concentration as a function of decreasing viscosity. These polymer dispersed BP materials are promising to expand blue phase technology into new display, smart glass, light shutter, by combining the advantages of both polymer dispersion method and unique electro-optic properties of blue phase liquid crystal such as fast response time and low switching voltage. 162

178 Chapter 7 Polymer Stabilization of Polymer Encapsulated Blue Phase Liquid Crystals 7.1 Introduction Recently, blue phase liquid crystal (BPLC) materials have generated great interest due to their optical properties [23,26,175] and potential for advanced applications in displays and photonic devices. One of the greatest advantages of BPLCs is field-induced birefringence due to their sub-millisecond response time, which is at least one order of magnitude faster than the present nematic LC-based displays. BPLCs do not require any surface alignment layer; thus, the device fabrication process is greatly simplified. Another significant advantage of BPLCs is their wide and symmetric viewing angle due to the fact that their voltage off state is optically isotropic and the voltage on state forms multidomain structures [35,36]. BPLC can be a substantial candidate for polymer encapsulated LC films due to their fast switching properties. Blue phases, i.e., self-organized three-dimensional structures formed by doubletwisted cylinders of cholesteric liquid crystals (LCs), appear in the narrow temperature range between the chiral nematic (cholesteric) and isotropic phases [6]. The narrow temperature range is one of the most significant limitations restricting their potential applications [3,39,176,12,177]. One of the first methods used to create blue phases stable 163

179 enough to use for practical applications [20,24,86,178] is the stabilization of the defects by polymer chains. It is suitable for use in applications of liquid crystal/polymer composites in displays, light shutters, and switchable windows [61,30,43,179]. However, liquid crystal/polymer composites can be divided into two distinct groups as polymer dispersed liquid crystal (PDLC) and polymer stabilized liquid crystal (PSLC). Both PDLC and PSLC methods are usually operated between a transparent state and an opaque state. In the PDLC systems, droplets of liquid crystal are dispersed in a polymer film, which can be switched from scattering state to transparent state or vice versa with an applied electric field. This electro-optical performance results from mismatching of the refractive indices in the field-off state and matching of the refractive indices the liquid crystal and polymer in the field-on state. The motivation for this chapter originated from the stabilization of encapsulated blue phase droplets based on their wide temperature range and outstanding E-O properties. We used encapsulation process starting from an emulsion of the mixture of monomer and BP liquid crystal in a polymer solution. After we obtained the encapsulated BP/monomer droplets, we stabilized the droplets. Our goal was to create a wide temperature range for the device of the encapsulated PSBP droplets and investigate the E- O performance of these devices by taking of the advantages of great E-O properties of blue phases. 164

180 7.2 Materials and cell preparation We prepared a mixture of nematic liquid crystals JC1041-XX (Chisso) and 5CB (Merck), which has a positive dielectric anisotropy, and a chiral dopant ZLI4572 (HTP~30µm -1 ) at the weight ratio of 50%:38.5%:11.5%. The content of the mixture was sonicated for 1.5 hours at room temperature. After adding a blend of reactive liquid crystalline di-functional monomers RM257 (7.1 wt%) and HDDA (5.4%) (Merck) and a small amount of the photoinitiator (IR651, 0.5 wt%) the samples were mixed for an additional 1.5 hours at room temperature. The thermal stabilities and E-O performances for that BPLC mixtures were discussed in Chapter 3. In order to form the encapsulated PSBPLC droplets using emulsification method, we used a water-soluble Polyvinyl Alcohol (PVA) solution of 48.5 wt% including three different concentrations of PVA (Table 7.1) and 3wt% surfactant (triton-x100) (the chemical structures are shown in Fig.7-1) and 48.5 wt% blue phase liquid crystal/monomer mixture. The samples A, B, and A* include low molecular weight PVA (50,000 g/mol), whereas sample C has high molecular weight PVA (115,000 g/mol). Compositions of four different emulsions comprising BPLC, PVA and surfactant are summarized in Table 7.1. For comparison, sample A* contains 3% of gelatin to control the droplet size of the emulsion. Then a mixture of BPLC/monomers and surfactant was added to a PVA aqueous solution at 20 o C while stirring at 1500 rpm for 20 min. to form an emulsion. Then a emulsion was heated to 35 o C and the mixture was left under atmospheric pressure for 10 hrs. to evaporate the solvent. Finally, an aqueous dispersion 165

181 of BPLC/monomer inside PVA microcapsules was obtained. Then a small drop of the emulsion was coated on a glass substrate using another glass substrate. The top-down E- O cells were assembled by placing the second ITO-coated substrate on top of the first substrate and, in order to maintain a uniform thickness, we used paper clips to attach two substrates. The cell gap was controlled by 15 μm spacers. The cell was placed in the vacuum oven for 2 hrs. For the purpose of carrying out the stabilization process, encapsulated BPLC droplets were irradiated with a UV light of 10mW/cm 2 (measured at 365 nm) at the blue phase. After 30 minutes exposure, the samples were placed under a polarizing microscope with crossed polarizers to observe the droplets. Figure 7-1: Chemical structures of a) Polyvinyl alcohol and b) Triton-X

182 Table 7.1. Compositions of materials of encapsulated BPLC. Sample BPLC (wt%) PVA solution (% wt) Water PVA (% wt) (% wt) Surfactant (Triton- X100) (% wt) Sample A Sample B Sample C Sample A* Material Characterization Encapsulated blue phase droplets were identified before and after the polymerization by placing the sample under a microscope with crossed polarizers. Samples that transmitted light from the polarizing optical microscope were captured by a digital camera and microphotographs are shown in Fig All samples were heated to the isotropic phase and then cooled to room temperature at a rate of -0.2 C/min. For sample A, the blue phase temperature was found between 40 o C and 32 o C before the polymerization and the size of droplets sizes were in the range of 30 µm and 100 µm. After the polymerization, the isotropic to blue phase transition temperature increased to 55 o C and the phase was also blue phase state at the room temperature with the droplets size between 20 µm and 100 µm. Moreover, after the polymerization process, it was 167

183 recognized that some of the small droplets were in a cholesteric phase whereas the bigger droplets were in blue phase. The strong anisotropic interactions between polymer and LCs caused a delay in their phase transition to the BP state. With increasing temperature, pairs of droplets merged together to form a single larger droplets. This effect occurred the polymer matrix was gelling and loss of a wall allowed the droplets to coalesce. The PVA concentration not only affects the viscosity of the aqueous phase but also helps stabilize the LC droplets against agglomeration [61]. Furthermore, the ratio of LC to polymer plays an important role for the thickness of the walls that separates the LC domains. Drzaic et al., showed that films with higher polymer concentrations have higher durability against the deformations [180]. As a next step, by following the outcome of this report, sample B was prepared with a higher PVA concentration. The POM images of the sample B validate the findings, as shown in Fig The BP temperature range for sample B was between 38 o C and 31.5 o C before polymerization with a droplet size range from 10 µm to 100 µm. After the polymerization, the BP to isotropic transition temperature increased to 52 o C and the phase was also blue phase state at room temperature with the droplets size between 5 µm and 100 µm. With increasing temperature, the PSBP droplets in PVA became highly interconnected. 168

184 a) 40 o C 38 o C 36 o C 32 o C Iso 40 o C BPII 37 o C BPI 32 o C Ch b) 55 o C 50 o C 40 o C 36 o C 23 o C Iso 55 o C BPII 37 o C BPI 0 o C> Figure 7-2: POM images and phase sequence of sample A a) before polymerization and b) after polymerization (white scalar bar represents 100 µm). 169

185 a) 38 o C 36 o C 34 o C 31.5 o C Iso BPII 38 o C 34 o C BPI 31.5 o C Ch b) 52 o C 45 o C 42 o C 30 o C 23 o C Iso BPII BPI 52 o C 43 o C 0 o C> Figure 7-3: POM images and phase sequence of sample B a) before polymerization and b) after polymerization (white scalar bar represents 100 µm). The experiments were extended to find the effect of varying the molecular weight of PVA in the solvent, as mentioned before. In sample C, we used the PVA with high molecular weight (150,000 g/mol) at the concentration of 3 wt%. Since it has a high molecular weight, there is a solubility problem above the concentration of 3 wt% of PVA. Figure 7-4 shows the photomicrographs of sample C. The BP temperature range for sample C was determined to be between 38 o C and 31.5 o C before polymerization with the droplet size range 5 µm to 40 µm. After polymerization, the BP to isotropic transition temperature increased to 53 o C and the phase was also blue phase state at room temperature with the droplets size between 10 µm and 40 µm. The droplet sizes became 170

186 smaller by using PVA with high molecular weight due to the increasing interfacial viscosity. a) 38 o C 36 o C 32 o C 31.5 o C Iso BPII 38 o C 34 o C BPI 31.5 o C Ch b) 52 o C 48 o C 39 o C 52 o C 23 o C Iso BPII BPI 52 o C 39 o C 0 o C> Figure 7-4: POM images and phase sequence of sample C a) before polymerization and b) after polymerization (white scalar bar represents 100 µm). We also examined the addition of gelatin into the PVA solution as a protective agent to prevent coagulation of the polymer encapsulated PSBP droplets. Sample A* was prepared with a small amount of gelatin (3 wt%). Figure 7-5 shows the POM images of sample A* before and after polymerization. As seen in Fig.7-5a, the BP was observed between 35 o C and 42 o C before polymerization and droplet size was in the range of 3 µm 171

187 to 30 µm. After polymerization, the BP to isotropic transition temperature increased to 54 o C and the phase was also blue phase state at room temperature with the droplets size between 5 µm and 40 µm. However, our results showed that the droplet size was not sensitive to the PVA concentration for the encapsulated PSBP samples. The droplet density increased with adding of gelatin into the PVA solution. a) 38 o C 37 o C 34 o C 31.5 o C Iso BPII 38 o C 34 o C BPI 31.5 o C Ch b) 54 o C 52 o C 45 o C 40 o C 23 o C Iso BPII BPI 52 oo C 39 o C 0 o C> Figure 7-5: POM images and phase sequence of sample A* a) before polymerization and b) after polymerization (white scalar bar represents 100 µm). 172

188 Reflectance (%) o C 23 o C 30 o C 40 o C 50 o C Wavelength (nm) Figure 7-6: The reflection spectra of sample A* as a function of temperature. The Bragg reflection peak was temperature independent after the polymer stabilization. As seen in Fig.7-6, the reflected wavelength appeared at 447 nm for BPII and 444 nm for BPI as a result of stabilization of disclination lines by polymer network in the encapsulated droplets. Since Sample A* had more uniform and dense droplets, its Bragg reflection intensity was stronger than that of the other samples. 173

189 7.4 Electro-Optical Performances of Polymer Encapsulated and Polymer Stabilized Blue Phase Liquid Crystals In order to study field-induced birefringence effect of polymer encapsulated and polymer stabilized BPLCs, all PEPSBP samples were sandwiched between substrates used for in-plane switching (IPS) cells. The IPS cells consisted of one substrate, which had lithographically prepared electrode patterns with a 10 µm electrode line width, and 10 µm spacing between electrodes, and a second substrate without the electrode. The cell gap between these two substrates was maintained at 15 µm by ball spacers. Figure 7-7 shows the measured voltage dependence of transmittance (V-T) curves of the polymer encapsulated and polymer stabilized BP cells. All measurements were performed at room temperature (25 o C) controlled by a hot stage. Threshold voltage and turn-on voltages are defined as the voltages that are required for achieving 10% and 90% transmission in the cell, respectively. The incident angle of the He-Ne laser made an angle of 90 o with respect to the substrate surface of the IPS cell. Upon application of an electric field, the cell of the encapsulated PSBP sample exhibited a birefringence. For sample A, the threshold voltage was 103.5V, whereas the turn-on voltage was 188V. The threshold voltages for PEPSBP samples B and C were 74.5V and 76V, respectively. Furthermore, the turn-on voltages of PEPSBP samples B and C are 190V and 192V, respectively. For sample A*, the threshold voltage was 45V, and the turn-on voltage is 92V. However, both samples B and C showed similar E-O features, as seen in Fig. 7-7a and b. Since these cells included focal conic texture as well as BP texture droplets with different sizes, the cells were in the BP and focal conic texture at low voltages and transmittance was 174

190 low. When the voltage was increased above the threshold; the focal conic texture was not stable and changed with time rapidly, which resulted in fluctuation in transmittance [181]. The other reason for the fluctuation in transmittance is because of the large droplet size dispersion. Although most of the droplets were quiet large, some of the droplets were very small. They had different anchoring strengths due to their sizes and this may be the other reason for the fluctuation on the curve. Moreover, the results show that turn-on voltage decreases with the adding of gelatin due to the increasing conductivity of the PVA solution according to where do represents the film thickness, σ1 and σ2 are the conductivities of polymer solution and LC, respectively. l(a/b) is the aspect ratio of the droplet, where a and b are the lengths of the semi-major and semi-minor axes, respectively [48]. 175

191 Figure 7-7: Transmittance curves as a function of applied voltage of a) Sample A, b) Sample B, c) Sample C, and d) Sample A*. 176

192 The response times of the E-O performance of this cell of the sample A* was found to be 0.03ms and 0.009ms for the rise and decay time, respectively. Figure 7-8 shows the response time of the cell, which was switched between the corresponding voltages 45V (V10) and 92V (V90). Figure 7-8: Response time of Sample A*. 177

193 7.5 Morphological Studies of Polymer Encapsulated and Polymer Stabilized Blue Phase Liquid Crystals Scanning electron microscopy (SEM) was performed for the PEPSBP samples to examine the polymer network. Before performing the scan, BPLC l was extracted from the cell with the mixture of dichloromethane and hexane at ratios of 20% and 80%, respectively, at room temperature for 24 h in sealed vials. The cell was removed from the vial and opened carefully after the solvent was allowed to evaporate. Then the cell was sputtered with a thin layer of gold and placed in the SEM for examination of the polymer morphology. Figure 7-9 shows the SEM photographs of sample A and sample A*. Morphological analyses show that the encapsulated PSBP droplet distribution was inhomogeneous even on the same substrate of the cells, as seen in Fig. 7-9(a). The polymerization of monomers leads to trap of the orientation of LC molecules at the polymer/lc boundary with a different structure (Fig.7-9b). It should be noticed that the polymer and LC boundaries in the droplets were not perfectly spherical, because of shrinkage of the polymer film during evaporation of the solvent. However, it was possible to observe small polymer droplets inside some cavities in sample A* due to the polymerization of monomers trapped in BPLC (Fig. 7.9c). 178

194 Figure 7-9: SEM image of encapsulated PSBP droplets in the IPS cell of a) sample A on the substrate without alignment layer, b) sample A* in the IPS cell with surface alignment layers, which wraps around the interconnected holes that vary in size and distribution, c) the polymer morphology on the different position at the same substrate surface of sample A*. 7.6 Conclusion We have investigated for the first time polymer encapsulated and polymer stabilized blue phase liquid crystals and devices. The encapsulated droplets were formed in a polymer solution by using emulsification method and stabilized as blue phase droplets. The temperature range of blue phase was remarkably increased more than 53 o C by polymer stabilizing effect. Moreover, the effect of the PVA concentration on the droplet shape was investigated. The results showed that droplets tend to aggregate and droplet size was decreased by using PVA with high molecular weight as a result of the increasing interfacial viscosity. In order to prevent coagulation of the polymer encapsulated PSBP droplets, gelatin was added into the PVA solution. With the addition 179