Abhishek Kumar Misra a, Pankaj Kumar Tripathi a & Rajiv Manohar a. To link to this article:

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1 This article was downloaded by: [University of Lucknow ] On: 05 October 2012, At: 02:10 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: Changes in material parameters for dye-doped ferroelectric liquid crystal Abhishek Kumar Misra a, Pankaj Kumar Tripathi a & Rajiv Manohar a a Liquid Crystal Research Laboratory, Physics Department, University of Lucknow, Lucknow , Uttar Pradesh, India Version of record first published: 05 Oct To cite this article: Abhishek Kumar Misra, Pankaj Kumar Tripathi & Rajiv Manohar (2012): Changes in material parameters for dye-doped ferroelectric liquid crystal, Phase Transitions: A Multinational Journal, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Phase Transitions 2012, 1 10, ifirst Changes in material parameters for dye-doped ferroelectric liquid crystal Abhishek Kumar Misra, Pankaj Kumar Tripathi and Rajiv Manohar* Liquid Crystal Research Laboratory, Physics Department, University of Lucknow, Lucknow , Uttar Pradesh, India (Received 14 July 2012; final version received 4 September 2012) Addition of the dichroic anthraquinone dye molecules in pure ferroelectric liquid crystal (FLC) matrix results in many improvement in the various vital parameters of the pure FLC. However, addition of anthraquinone dye molecules in pure FLC matrix is not advantageous every time. There are certain constraints which are crucial for the application of these systems into many devices. In this article, we have discussed the concentration and temperature dependence of vital properties of dye-doped FLC. In this study there is improvement in contrast ratio by dye doping due to enhancement in plane switching for dye-doped FLC. Keywords: ferroelectric liquid crystal; dichroic anthraquinone dye; tilt angle; response time 1. Introduction Liquid crystal systems have been studied in the past few years because of their wide spectrum of applications to the modern devices [1 4]. Ferroelectric liquid crystals (FLCs) are exciting class of materials having various applications in display and other electrooptic devices because of their low-operating voltage and fast response time [5]. There are many applications in which material scientists have been exploring role of novel guest host materials such as turn-off switching, multi-purposes liquid crystal displays with switchable viewing angle and reflective color displays [6,7]. To optimize these properties of FLC materials, it usually requires knowledge of material parameters for FLC material such as dielectric permittivity, relaxation time, spontaneous polarization, and response time. Response characteristics of any display are mainly determined by relaxation process of the FLC molecules [8]. Rotational viscosity is also one of the important factors for designing of fast-response FLC devices [9 12]. To improve the various FLC material parameters which are crucial for their use in display devices and other useful applications, efforts have been made by different research groups [13,14]. These parameters are contrast ratio, vision angle, spontaneous polarization, etc. Introduction of different non-mesogenic molecules such as dyes and nanoparticles can improvise practically important material parameters of FLC systems. One of the approaches to modify the material parameters of the FLCs is doping of anthraquinone dye into FLC material [15 18]. Various researchers have reported changes in the different parameters of FLCs due to the addition of dye molecules [12,19]. The study *Corresponding author. rajiv.manohar@gmail.com ISSN print/issn online ß 2012 Taylor & Francis

3 2 A. Kumar Misra et al. of dye-doped systems is also important to understand the molecular dynamics of such systems and how the presence of dye molecules affects the molecular properties of the pure FLC. This study reports the effect of anthraquinone dye on some dielectric and electrooptical parameters of pure and dye-doped FLC [15 19]. The different dielectric and electro-optical parameters namely dielectric permittivity, relaxation time, spontaneous polarization, and response time have been measured for both pure and dye-doped FLC and behavior of these parameters with variation in temperature have also been discussed. All the properties are found to show strong dye concentration dependence [16 18]. The results indicate that concentration of dye in liquid crystals also play important roles in improvising different material parameters [15]. This study also provides the broad information about the modifications taking place in the intermolecular interaction field and molecular geometry of pure FLC matrix due to the addition of dye molecules and how it changes with the change in the concentration of dye. The rotational viscosity has been determined by measuring spontaneous polarization and response time [11,12]. In addition to that we have also compared relaxation time and response time for both pure and dye-doped FLC material. 2. Experimental details The sample used for this study is Felix purchased from Clariant Chemicals Co. Ltd with the phase sequence of Cr SmC* SmA N* Iso at 20 C, 74 C, 82 C, and 95 C, respectively. The value of spontaneous polarization at 25 Cis 5.9 nc cm 2 and value of rotational viscosity at 25 C is 82 mpoise. The basic parameters of this sample have already been given in our earlier published papers [17,19,20]. The anthraquinone dye used for doping is D5 with structure, as shown in Figure 1. Three mixtures of anthraquinone dye with 1%, 2%, and 3% wt/wt concentration have been prepared by dispersion of anthraquinone dye [16,18] into the pure FLC. We named these mixtures as mixture 1, mixture 2, and mixture 3, respectively, for the whole discussion. For dielectric and electrooptical study, we used sandwiched type sample holder of indium tin oxide (ITO) coated glass plates having cell thickness of 7.5 mm. The cells were prepared using ITO coated glass plates (sheet resistance is 10 Ohm mm 2 and the visible light transmission is more than 90%) having active area 25 mm 2, as electrodes. Both the electrodes were treated with adhesion promoter and polymer nylon 6/6 and then rubbed unidirectionally for planar alignment. Cell thickness was maintained by means of a Mylar spacer. The cells were Figure 1. Molecular structure of anthraquinone dye D5 used for this study.

4 Phase Transitions 3 calibrated using AR grade CCl4 and benzene. The material was introduced into the cell by capillary action at a temperature slightly higher than its isotropic temperature. A wellaligned cell was obtained by applying an electric field in the slow cooling cycle from the isotropic phase to room temperature. The macroscopic orientation for all the mixtures was checked by placing sample cell between two-crossed polarizers of polarizing microscope CENSICO The details regarding preparation of the sample cell has been reported earlier by our group [15 18,20,21]. The dielectric data have been obtained by measuring capacitance and dissipation factor with the help of an impedance/gain phase analyzer HP A. The complete experimental measurement technique has been reported in our earlier papers [20,21]. Spontaneous polarization has been measured by standard polarization reversal current method, while response time has been determined using optical switching method [18,22]. Rotational viscosity has been determined by spontaneous polarization data and response time using the following relation [18,20] t ¼ ep ð1þ P S E where t, ep are the optical response time and electro-optical rotational viscosity of the sample, respectively, and E the applied electric field. The contrast ratio measurement has been performed by applying a square wave to the sample cell placed between the crossed polarizers. The experimental setup is shown in Figure 2 [20]. The corresponding maximum and minimum intensity have been recorded by light dependent resistance and storage oscilloscope HM 407. Tilt angle has been measured by well-known pulse electro-optical techniques proposed by Baikalov et al. [22]. 3. Results and discussion Figure 3 shows variation of effective dielectric permittivity with temperature. Here term effective is used for the dielectric permittivity for which we have removed contribution of Figure 2. The schematic diagram of the apparatus for measurement of contrast ratio.

5 4 A. Kumar Misra et al. Figure 3. Variation of effective dielectric permittivity with temperature for all the mixtures. sheet resistance, ITO coatings and lead inductance of the cell. For this purpose, we require correction in the Cole Cole equation [20 23]. The Cole Cole equation is given " ¼ " 0 " 0 ð1þ þ 1 þði2fþ 1 ð2þ where " 0 is the dielectric strength and " 0 (1) the high-frequency limit of the dielectric permittivity, f the frequency, and the relaxation time. After adding correction terms due to ionic conductivity and electrode polarization problem in low-frequency region and ITO sheet resistance and lead inductance problem in high-frequency region data, the real and imaginary parts of the above equation become as follows and " 0 ¼ " 0 ðdcþ f n þ " 0 " 0 ½1 þð2fþ ð1 Þ sinð=2þš ð1þ þ 1 þð2fþ 2ð1 Þ þ 2ð2fÞ ð1 Þ sinð=2þ " 00 ¼ ðdcþ " 0 2f k þ " 0 ð2fþ ð1 Þ cosð=2þ 1 þð2fþ 2ð1 Þ þ 2ð2fÞ ð1 Þ sinð=2þþ þ Af m ð4þ where (DC) is the DC ionic conductance, " o the free space permittivity, and f the frequency while n, m, and k are the fitting parameters. The terms " 0 (DC)f n and (DC)/ " o 2f k are added in Equations (3) and (4) for additional contribution due to the electrode polarization and ionic conductance in the low-frequency effect [15,18,20]. The Af m term is added in Equation (4) in the high-frequency effect due to the contribution of ITO sheet resistance and lead inductance of sample cell. By the least-squares fitting of experimental data we remove these contributions and we got effective dielectric permittivity. It is clear from the figure that the value of effective dielectric permittivity is reduced very much after the doping of anthraquinone dye, while the nature of variation of dielectric permittivity with temperature is same for dye-doped system as compared to the pure FLC. This is due ð3þ

6 Phase Transitions 5 Figure 4. Temperature dependence of relaxation time for all the mixtures. to the reason that when dye is doped into the pure FLC, dye molecules are free to move into the pure FLC system. The movement of dye molecules in pure FLC are in such a way that the dye molecules tend to arrange themselves with their long molecular axis aligning along the FLC director [15,18]. As the concentration of dye is increased the value of effective dielectric permittivity is reduced to a lower value in comparison to the dielectric permittivity of the pure FLC suggesting dipole moment of dye and FLC molecules are counteracting. Figure 4 shows the relaxation time at different temperatures in SmC* phase for the pure and dye-doped FLCs. The relaxation phenomenon of FLC with a planar alignment is normally due to the collective dielectric processes such as the Goldstone mode and soft mode [11,12,15,20]. In the present study only Goldstone mode has been observed, while soft mode could not detected because of low spontaneous polarization of FLC. The relation between relaxation time and relaxation frequency is given as 1 ¼ ð5þ ð2 relaxation frequencyþ The relaxation band of the goldstone mode shifts toward the lower frequency side for the dye-doped system in comparison to the pure FLC. The mixture with lower concentration of dye, i.e., mixture 1 shows the relaxation band shift toward low-frequency range as compared to pure FLC. Mixtures 2 and 3 (sample with higher concentration of dye) show shift toward the higher frequency region as compared to mixture 1 but when these are compared with pure FLC the relaxation band is shifted to the lower frequency region. The nature of variation of relaxation frequency with temperature is almost similar for pure and dye-doped system but the value of relaxation frequency for all the dye-doped mixtures shifts in lower side as compared to the pure FLC. The higher value of relaxation time in dye-doped mixtures means that molecular motion of FLC molecules is hindered by the presence of dye molecules [15,19]. Addition of dye in pure LC system offers some new constraints due to new interactions taking place. There are: (1) Interaction between dye and FLC molecules. (2) Interaction between the dye dye molecules.

7 6 A. Kumar Misra et al. Figure 5. Temperature dependence of spontaneous polarization for all the mixtures. It appears that for mixture 1 interaction of dye molecule with FLC molecules dominate over the interaction of dye dye molecules but as concentration of dye is increased, i.e. for mixtures 2 and 3 interactions between dye dye molecules also become prominent. This indicates that different behavior of relaxation time for the mixture 1 as well as mixtures 2 and 3. This typical behavior of relaxation time can be explained with the help of the following equation [20] ¼ d K 3 q 2 ð6þ where represents the relaxation time; K 3 the twist elastic constant, q the wave vector of the helical pitch, and d the dielectric rotational viscosity of mixture. It is clear from Equation (6) that the relaxation time is directly proportional to the rotational viscosity of the system. Addition of dye into pure FLC alters the rotational viscosity of the system and result in the change in relaxation time. The nature of rotational viscosity with temperature and concentration of dye is discussed in the later part of this article. Figure 5 shows temperature variation of spontaneous polarization for the pure and dye-doped FLC mixtures. From figure it is clear that magnitude of spontaneous polarization is continuously decreasing and it vanishes at SmC* SmA phase transition point or near this phase transition point for dye-doped as well as for pure FLC system [18,20]. The magnitude of spontaneous polarization is continuously decreasing with the increase in the concentration of dye. It is also support by dielectric permittivity behavior. The effect of dye on FLC response time has been obtained by determining the values of response time are shown in Figure 6 with variation of temperature. The magnitude of response time is increasing with increment in the concentration of dye. The higher values of response time in the dye-doped system suggest there is no improvement in response time after dye-doping as the dye restricts the molecular switching. The behavior of relaxation time and response time are just opposite for mixtures 2 and 3 while it is same for pure FLC and mixture 1. The response time is time taken to reach intensity from 10% to 90% of its

8 Phase Transitions 7 Figure 6. Variation of response time with temperature for all the mixtures. Figure 7. Variation of electro-optical rotational viscosity with temperature for all the mixtures while inset shows variation of rotational viscosity with concentration of dye at 35 C. maximum value in which optical response of molecule is involved while effect of electric field giving rise to relaxation time. The electro-optical rotational viscosity is plotted with variation of the temperature in Figure 7, while inset shows concentration dependence of electro-optical rotational viscosity. The temperature dependence of electro-optical rotational viscosity is as similar as reported in the earlier papers [12,17] but the magnitude of electro-optical rotational viscosity increase with the increase in the concentration of the dye. This type of behavior of rotational viscosity is due to the combined effect of spontaneous polarization and response time also indicated by Equation (1). As we know rotational viscosity is related to the rotation of molecules and higher value of response time suggest that the present dye offers

9 8 A. Kumar Misra et al. more hindrance of FLC molecule for the dye-doped FLC system. Therefore, magnitude of electro-optical rotational viscosity increases after dye-doping. This is also clear from Equation (1) that rotational viscosity is directly proportional to response time and spontaneous polarization. Thus, any change in these parameters cause changes in the magnitude of rotational viscosity. Therefore, magnitude of rotational viscosity depends upon combined effect of these material parameters [17]. The net effect of these parameters, i.e., response time and spontaneous polarization is increment in rotational viscosity because of the small decrement in the magnitude of spontaneous polarization and increment in the magnitude of response time. The behavior of electro-optical rotational viscosity has also been confirmed by the nature of rotational viscosity as determined by dielectric parameters. The behavior of both the rotational viscosities as measured by electro-optical and dielectric method is same but their magnitude is slightly different [12,17]. The magnitude of dielectric rotational viscosity is given as d / P 2 S ð7þ where P S is the spontaneous polarization and d the dielectric rotational viscosity, whereas represents tilt angle. The magnitude of dielectric rotational viscosity is directly proportional to (P S /) term. Therefore, value of this term has been evaluated for pure and dye-doped system at constant temperature and presented in Table 1. The magnitude of (P S /) term increase with increase in concentration of dye, hence, magnitude of dielectric rotational viscosity increases. The increment of (P S /) term in the dye-doped system implies that the amplitude of tilt is decreasing at a much faster rate than spontaneous polarization which is due to the appearance of tilt distribution within smectic layer for the dye-doped systems as compared to the pure FLC. The tilt angle could also vary due to the presence of dye molecules in the pure FLC system and depends upon the concentration of dye. Table 1 presents the value of tilt angle () for all the samples at constant temperature 35 C. The decrease in the values of tilt angle () for the dye-doped mixtures confirms that after dye-doping in the pure FLC system there is suppression of randomization of tilt distribution. The contrast ratio is another crucial material parameter of FLC and was calculated for the pure and the dye-doped FLC by usual experimental techniques and its value is presented in Table 1. The contrast of any device is defined as the difference between the luminance in on and off state, while the contrast ratio is defined as the ratio of greater luminance to lesser luminance. Contrast ratio shows an increase for mixture 1 as compared to pure FLC, while for mixtures 2 and 3 it slowly reduces to lower values as Table 1. Variation of different material parameters with concentration of dye at constant temperature of 35 C. Sample P S / (10 9 ) (Coulomb deg. 1 cm 2 ) Tilt angle ( ) Contrast ratio Pure Mixture Mixture Mixture

10 Phase Transitions 9 compared to mixture 1. The higher value of contrast ratio for mixtures 1 and 2 as compared to pure FLC may be due to enhancement of in plane switching for dye-doped system. 4. Conclusion In this study, we have explored anthraquinone dye D5 doped FLC system. The relaxation time shifts toward lower side for the mixture 1 (mixture with small concentration of dye) as compared to the pure FLC. For mixtures 2 and 3 (mixtures with high concentration of dye) relaxation time shifts toward higher side as compared to the mixture 1 suggesting that with increasing in the concentration of dye, the motion of molecules slows down. The magnitude of spontaneous polarization continuously decreases with increase in the concentration of dye. It indicates that molecular alignment is not good which is also clear from the behavior of effective dielectric permittivity. The material parameters such as rotational viscosity tilt angle and (P S /) term show strong dependence on concentration of the dye used. There is improvement in contrast ratio by dye doping due to enhancement in plane switching for dye-doped FLC system. The values of tilt angle () decreases for the dye-doped mixtures confirm that there is suppression of randomization of tilt distribution in dye-doped mixture as compared to pure FLC system. Acknowledgments Abhishek Kumar Misra is thankful to UGC, New Delhi for the grant of Dr D.S. Kothari Post Doctoral Fellowship no. F.4.2/2006 (BSR) /2008(BSR). References [1] J. Prakash, A. Kumar, T. Joshi, D.S. Mehta, A.M. Biradar, and W. Haase, Spontaneous polarization in smectic a phase of carbon nanotubes doped deformed helix ferroelectric liquid crystal, Mol. Cryst. Liq. Cryst. 541 (2011), pp [2] A. Kumar, P. Silotia, and A.M. Biradar, Effect of polymeric nanoparticles on dielectric and electro-optical properties of ferroelectric liquid crystals, J. Appl. Phys. 108 (2010), pp [3] G. Singh, G. Vijaya Prakash, A. Choudhary, and A.M. Biradar, Electro-optic studies in conventional and pure/ethanol mixed de Vries ferroelectric liquid crystals, Liq. Cryst. 39 (2012), pp [4] A. Chaudhary, P. Malik, R. Mehra, and K.K. Raina, Electro-optic and dielectric studies of silica nanoparticle doped ferroelectric liquid crystal in SmC* phase, Phase Transitions 85 (2012), pp [5] P. Malik, A. Chaudhary, R. Mehra, and K.K. Raina, Electro-optic, thermo-optic and dielectric responses of multiwalled carbon nanotube doped ferroelectric liquid crystal thin films, J. Mol. Liq. 165 (2012), pp [6] C.P. Chen, C.G. Jhun, T.H. Yoon, and J.C. Kim, Optimal design of omni-directional viewing angle switching panel, Opt. Express 15 (2007), pp [7] S.A. Jewell, T.S. Taphouse, and J.R. Sambles, Rapid switching in a dual-frequency hybrid aligned nematic liquid crystal cell, Appl. Phys. Lett. 87 (2005), pp [8] B. Bahadur, Liquid Crystals: Application and Uses, Vol. 2, World Scientific, Singapore, 1994.

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