Dielectric and electro-optical properties of polymer-stabilized liquid crystal system

Transcription

1 Appl. Phys. A (2016) 122:217 DOI /s Dielectric and electro-optical properties of polymer-stabilized liquid crystal system Mukti Pande 1 Pankaj Kumar Tripathi 1 Abhishek Kumar Misra 2 Shashwati Manohar 2 Rajiv Manohar 2 Shri Singh 1 Received: 12 July 2015 / Accepted: 7 October 2015 Springer-Verlag Berlin Heidelberg 2016 Abstract In this work, we report the results of dielectric and electro-optical properties as a function of temperature for both pure liquid crystal matrix and polymer-stabilized liquid crystal (PSLC). The threshold and saturation voltages have been determined from transmission voltage curves. We have studied the polymer domains formation in PSLC with variation of concentration of polymer in liquid crystal matrix. It is observed that the dielectric anisotropy of PSLC is significantly influenced by the polar order present in the polymer domains environment. A delicate interplay between the orientational order of liquid crystal and polymeric domains determines the molecular orientations of PSLC with respect to the director of the LC system. 1 Introduction In recent years, polymer-stabilized liquid crystal (PSLC) system, i.e., incorporation of polymers in liquid crystalline materials, has attracted considerable attention due to their wide applications in the area of optoelectronics and photonics [1 5]. These applications, like switchable windows, display and other devices, spatial light modulators, holographically formed light modulators, temperature sensors, flexible display, are based on their wide optical birefringence arising due to the anisotropic nature of molecules & Pankaj Kumar Tripathi pankajtripathi19@gmail.com 1 2 Department of Physics, Banaras Hindu University, Varanasi , India Department of Physics, University of Lucknow, Lucknow , India combined with the high sensitivity of the molecular orientations to externally applied fields. De Gennes [6] first contemplated that liquid crystal polymers can form composite material and can exhibit useful electro-optical properties. The recent advances in LC materials applications are proceeding in two direction, synthesis of new LC materials and tailoring the existing LC materials by dispersing/doping dyes [7, 8], nanoparticles [9, 10], polymers [11, 12], etc. The introduction of polymer embedded in a pure LC matrix changes the phase behavior, for example dielectric and electro-optical properties. The investigation on PSLC systems is extremely important from the points of view of both the fundamental understanding and applications. The purpose of this work is to study the influence of polymer species on the dielectric and electro-optical properties of the pure LC matrix. We determine the various electro-optical and dielectric parameters, viz. dielectric anisotropy, rotational viscosity and splay elastic coefficient along with response time and threshold voltage for both pure LC matrix and PSLC systems. We have found that the response time decreases, whereas threshold voltage increases in the PSLC as compared to the pure LC matrix. We also investigated the effect of increasing concentration of polymer on pure LC matrix. 2 Experimental detail 2.1 Materials We used pure nematic liquid crystals 4-pentyl-4 0 -cyanobiphenyl (5CB) with positive dielectric anisotropy and disperse with poly, n-butyl methacrylate (PBMA) polymer. The pure 5CB exhibits the sequence: crystalline -18 C

2 217 Page 2 of 9 M. Pande et al. nematic -35 C isotropic. The molecule is about 20 Å long. The polymer material PBMA has been used as a guest dopant in host liquid crystal, which is high molecular weight (300,000 g/mole). The polymer PBMA (Sigma- Aldrich, USA) was first dissolved in ethanol, and then, it was dispersed in the pure nematic LC in 1 and 3 wt./wt. %, respectively. 2.2 Preparation for sample cells The dielectric study of the pure LC matrix and PSLC system was carried out on the planar and homeotropic geometries. The sandwiched-type (capacitor) cells were made using two optically flat glass substrates coated with indium tin oxide (ITO). The planar alignment was obtained by treating these glass plates with the adhesion promoter and is then coated with polymer nylon (6/6). After drying the polymer layer, substrates were rubbed in a unidirectional antiparallel way. The substrates were then placed one over another to form a capacitor. The cell thickness was fixed by placing a Mylar spacer (2.5 lm in our case) in between and then sealed with UV sealant. Similarly, for homeotropic alignment, the glass substrates were coated with a dilute solution of lecithin (cetyltrimethyl ammonium bromide). The substrates were dried at 220 C for 6 h before assembling the cell. The empty sample cells were calibrated using analytical reagent grade benzene (C 6 H 6 )as standard reference for the dielectric study. To prepare the PSLC sample, an appropriate amount (in the weight ratio, i.e., 1.0 and 3.0 %) of polymer PBMA was mixed into the pure 5CB and then homogenized with an ultrasonic mixer at 36 C for 1 h. Each sample was heated from nematic to isotropic phase and cooled back to room temperature. The pure and the dispersed samples were filled in the assembled cells at a temperature higher than the isotropic temperature of the LC sample by capillary method. After filling the sample in a cell, it was cooled slowly under the applied AC electric field and the alignment of the sample was checked under the crossed position of polarizers in a polarizing optical microscope. After cooling under low electric field, the molecules tend to become more regular and oriented. The slow cooling of the sample ensures a better alignment of the molecules. A low electric field has been applied to the cell to seize the LC molecules from switching which occurs on applying a higher electric field. The usual method used by us for obtaining a well aligned cell is given elsewhere [13, 14]. The mixture was viewed under polarizing optical microscope (POM) (Radical RXLR-5) to ensure homogeneous distribution of the polymer. Optical textures taken in nematic phase for pure 5CB sample and PSLC sample are given in Fig Dielectric measurements The dielectric measurements have been taken by computercontrolled impedance/gain-phase analyzer (HP-4194 A) attached with a temperature controller in the frequency range 100 Hz 10 MHz as a function of temperature by placing the sample on a computer-controlled hot plate INSTEC (HCS-302). The temperature stability was better than ±0.1 C. The dielectric properties of the pure LC and PSLC system were determined from the measurement of capacitance, conductance and dissipation factor (tan d), phase angle (h) and impendence (z) for planar and homeotropic alignments of the cells. Some worker have adopted the simple calculation by the relative dielectric permittivity using the following relation [15, 16] e 0 ¼ C p ¼ C p c ð1þ C 0 A e 0 where C p is parallel capacitance, d is the thickness of the cells, and A is the area of electrode. However, we have calculated the dielectric parameters of pure LC and PSLC systems by the following relations. A test cell has been calibrated using nonpolar material, i.e., Fig. 1 Optical texture obtained from a polarizing optical microscope using 910 objective. a Pure 5CB nematic LC b 1 % PBMA, polymerstabilized liquid crystal (PSLC)

3 Dielectric and electro-optical properties of polymer-stabilized liquid crystal system Page 3 of benzene. The geometrical capacitance (C G ) of the test cell has been determined by the relationship [17], C G ¼ C b C a e 0 b 1 ð2þ where C a and C b are the capacitances of empty and benzene-filled dielectric cells, respectively. The dielectric permittivity of materials has been determined by the relation, e 0 ¼ C m C a þ 1 ð3þ C G and dielectric loss e 00 ¼ G m G a ; tan d ¼ e00 2pfC G e 0 ð4þ where C m and C a are the capacitance of the dielectric cell with and without a materials, respectively. G m and G a are the conductance of the dielectric cell with and without materials; respectively. The parallel and perpendicular components of dielectric permittivity (e 0 \ and e 0 k) were measured by planar and homeotropic aligned cells for pure and polymer-stabilized liquid crystals (PSLC) system. Measurements in the higher-frequency range have been limited to 10 MHz because of the dominating effect of finite sheet resistance of ITO coated on glass plates and lead inductance of the cell [18].The real and imaginary parts of dielectric permittivity is given by the following equations, e 0 ¼ e 0 ðdcþf n þ e 0 ð1þ de 0 ½1 þð2pf sþ ð1 aþ sin ðap=2þ þ ð5þ 1 þð2pf sþ 2ð1 aþ þ 2ð2pf sþ ð1 aþ sin ðap=2þ and e 00 ¼ rðdcþ e 0 2pf k þ de 0 ð2pf sþ ð1 aþ cosðap=2þ 1 þð2pf sþ 2ð1 aþ þ 2ð2pf sþ ð1 aþ sinðap=2þþ þ Af m ð6þ Here r(dc) is ionic conductance, e o free space permittivity, f the frequency, de 0 the relaxation strength, s the relaxation time, a the distribution parameter, and e 0 (?) the highfrequency limit of the dielectric permittivity while n, m and k are the fitting parameters. The term e(dc)/f -n and r(dc)/ e o 2pf k are added in the above equations for low-frequency effects due to the electrode polarization, capacitance and ionic conductance. The term Af m in Eq. (2) takes care of high-frequency effect due to the ITO resistance and lead inductance. By the least square fitting of experimental data in the above equations, the low- and high-frequency data have been corrected. 2.4 Electro-optical measurements A suitable square wave (20 V pp and 5 Hz) has been applied to the cells using a function generator. He Ne laser beam of wavelength nm as the input signal is detected by a photodetector (Instec-PD02LI) connected directly to a digital storage oscilloscope (Tektronix TDS-2024C). The cell is placed between the polarizer and analyzer which are in a crossed position. The cell is then set at an angle of 45 for maximum transmittance. The LC sample cells were held at different temperatures using control hot plate. The cell works as a phase retarder, thereby altering the polarization of light. The output waveform is then used to determine the rise time and fall time. The rise time (s on ) and fall time (s off ) of pure and PSLC system have been evaluated using Eq. (3) [19]. s o ¼ s on þ s off ; s on ¼ s 90 s 10 and s off ¼ s 10 s 90 ð7þ Here, s on is the time required for the transmittance to rise from 10 to 90 %, and s off is the time required for the transmittance to fall from 90 to 10 %. 3 Result and discussion In this work, we have observed the domain formation of polymer networks by microscopic observation in the polarized light. Figure 2 shows the alignment of LC molecules and polymer network without and with applied electric field. It can be seen that when electric field is applied perpendicular to electrodes, domains are formed. These domains are separated by the polymer network and in various domains orientations, of LC molecules are different. The maximum number of domains has been obtained for the high concentration of PBMA polymer in pure LC matrix. Dielectric studies are concerned with the response of liquid crystals to the application of an electric field. In particular, the dielectric properties are induced by the change in the LC molecule orientations. In fact, it depends upon how an electric field is applied to a LC matrix and PSLC. Dielectric permittivity depends on how the molecules are oriented relative to the electric field distribution. The perpendicular and parallel components of dielectric permittivity (e \ and e k ) for pure LC matrix and PSLC are shown in Fig. 3a, b at constant temperature 28 C. In Fig. 3a, it is observed that perpendicular component of dielectric permittivity (e \ ) changes unusually with the addition of PBMA polymer. For low concentration of polymer in pure LC matrix (1 % PBMA), the dielectric permittivity (e \ ) is lower as compared to pure LC, whereas for 3 % PBMA, it is higher as compared to pure LC matrix

4 217 Page 4 of 9 M. Pande et al. Fig. 2 Liquid crystal in a planar-aligned sample cell without and with applied electric field for PSLC system Fig. 3 Frequency dependences of a perpendicular component of dielectric permittivity and b parallel component of dielectric permittivity for pure nematic LC and PSLC systems and 1 % PBMA polymer. Dielectric permittivity (e \ ) increases on approaching the higher concentration of polymer; such feature is attributed to the polymer polymer interaction and increase in the chain length of LC molecules. Figure 3a, b shows that e \ decreases very slightly in low-frequency region, remains constant at values of frequency lying between 300 Hz to Hz and decreases rapidly at frequencies more than Hz. The decrease in dielectric permittivity in high-frequency region suggests that there occurs some relaxation for pure LC matrix and PSLC. In low-frequency region, the ionic polarization contributes for pure LC matrix and PSLC system. But Fig. 3a shows that the dielectric relaxation occurs at high frequency for PSLC system. In homeotropic geometry, the dielectric relaxation is almost constant at Hz frequency. Therefore, it can be inferred that the polymer is more affected in planar geometry as compared to homeotropic geometry. For homeotropic geometry, the permittivity has been found to decrease for PSLC system as compared to pure LC matrix. The presence of polymer network in the LC matrix induces disorder and disturbs the LC molecules orientations around the polymer. In LC materials with PBMA polymer, in addition to the induced polarization, an orientation polarization also occurs, due to the contribution of dipole moments to orient themselves parallel to the field. Figure 4 shows the dielectric anisotropy change as a function of temperature at a frequency 1 khz. Only the magnitude of the dielectric anisotropy changes with increasing polymer concentration, but its orientation remains unaffected. The interaction between a liquid crystal and an electric field depends on the magnitude of the dielectric permittivity measured parallel e k and perpendicular e \ to the director and to the difference between them. The nature and magnitude of the dielectric anisotropy (De = e k -e \ ) depend on the anisotropy of the induced polarizability and the direction of permanent dipole moments. With less polymer present in the PSLC system, the domain size is larger, and at a higher polymer concentration, the domains do not combine or coalesce.

5 Dielectric and electro-optical properties of polymer-stabilized liquid crystal system Page 5 of Fig. 4 Temperature dependences of dielectric anisotropy for pure LC and PSLC systems Therefore, the amount of bulk polymer present in PSLC system leads to a smaller contribution to dipole moment in parallel component of dielectric permittivity. That means the decrease in e k in case of 3 % PBMA polymer is supported by the formation of polymeric domains leading to a suppression of dielectric response along short molecular axis, whereas there is an effective enhancement of e \ due to the polymer polymer network and LC polymer network interactions. A decrease in the concentration of polymer in pure LC matrix may lead to a significant increase in polymer network and LC molecules interaction. As a result, LC molecules align along the cell substrates. By examining the difference between the interactions of the LC molecule and the PBMA polymer, we can better understand anchoring difference between bipolar and radial polymer domain configurations. The dielectric anisotropy increases for 1 % dispersion of polymer. This indicates that the angle b (=tan -1 l \ /l k; l \ and l k are, respectively, the perpendicular and parallel components of dipole moment) changes such that e k increases and e \ decreases. The dielectric anisotropy depends upon the angle b and order parameter. The order parameter changes due to the reorientation process of the LC matrix and the polymeric domains formation in the PSLC system. However, for 3 % concentration of polymer, the dielectric anisotropy is even below the value of dielectric anisotropy of pure LC matrix. This indicates that high dispersion decreases angle b, sode is decreased. It is clear that polymer network exhibits a significant influence on the liquid crystal molecules. It is shown that the liquid crystal polymer network interaction increases at low polymer concentrations in LC matrix. In other words, the polymer network domains formed due to electric field show strong influence on the orientation of liquid crystal molecules [20]. The dielectric anisotropy (De) of the LC matrix is governed by the induced polarization, which is proportional to the order parameter and the orientation polarization, which varies as S/T, where S is order parameter and T temperature. For pure LC matrix, the De first increases and then decreases with increasing temperature. The dipole moment of pure 5CB material is approximately large due to linking group CN directed along long molecular axis. Such system leads to a large positive dielectric anisotropy as compared to low concentration of polymer in pure LC matrix. This suggests that the induced polarization of polymer contributes more to the dipole moment of perpendicular component of dielectric permittivity. But at higher concentration of polymer, the polymer polymer interaction is more prominent as compared to low concentration. The induced polarization of PSLC is responsible for the methacrylate group of polymer materials. The dielectric anisotropy of 3 % PBMA is decreased as compared to pure LC matrix due to the dominant contribution of perpendicular component of dipole moment of polymer domains. The maximum numbers of polymer domains are formed with 3 % PBMA, but in this case, the size of domain is very small as compared to low (1 %) concentration case. The dipole moment also depends upon the size of dominant. At above the critical temperature, the pure LC transits to isotropic state. We found that the dielectric anisotropy changes rapidly for 3 % PBMA compared to 1 % PBMA system at the phase transition temperature 35 C. The polymer network LC molecule interaction is more prominent as compared to LC LC molecules interaction. So at higher concentration of polymer, the former interaction dominates the result. Thus, the PSLC system has very promising optical and electro-optical applications. The electro-optical response of liquid crystal molecules under the influence of electric field is the major characteristics utilized in industrial applications. To estimate the performance of the liquid crystal dispersions for display device application, we measured response time of the sample. The study of response time is another key aspect for the evaluation of liquid crystal display performance to an applied electric field. Rise time (s R ) and fall time (s F ) depend on the relative strength of the applied field and the elastic reorientation forces. The response time of LC depends on the different factors such as shape, size of domains and dielectric permittivity, conductivity of polymer and LCs interfacial area and elastic energy [21], 1 ¼ 1 s R c De V2 þ k 11 L 2 ð 1Þ a 2 ð8þ cd 2 s F ¼ k 11 ðl 2 1Þ ð9þ

6 217 Page 6 of 9 M. Pande et al. where c is the rotational viscosity of liquid crystals, a and L are, respectively, the major axis and the aspect ratio (major dimension/minor dimension) of liquid crystal polymer domain, and k 11 is the splay elastic constant. The fall time of the PSLC is proportional to the square of the distance between polymer networks and LC layer. Figure 5a shows the variation of response time with temperature for pure LC matrix and PSLC system at a frequency 5 Hz. The response time decreases with increasing the temperature for pure LC matrix and PSLC system, but for PSLC system, the response time increases near the phase transition. At higher temperature, the interaction of polymer polymer network is more dominant as compared to LC LC interaction for PSLC system. It is suggested that rotational viscosity increases due to interaction of polymer polymer network interaction. Such higher values of response time above 32 C temperature are explained by considering the formation of polymer domains due to anchoring energy for PSLC system. Response time of PSLC system have been found 1/6 time less than the response time of pure LC matrix. At 32 C temperature, the response of LC molecule is not affected in PSLC because the LC phase changes to isotropic phase. Therefore, the response time is increased near the phase transition temperature for PSLC system. However, the PSLC system shows the orientation of LC molecules in applied field which does not relax back over time on switching the field off. Even though there are no long-range nematic interactions in the isotropic phase, the interaction between LC polymer networks still exists. For PSLC, the methacrylate functional group of PBMA polymer is interacting with the mesogenic core of alkyl chain. Therefore, the electro-optical properties of mesogenic compound changes with coupling interaction of polymer and LC molecules. The electric field causes a reorientation of the nematic director in each domain such that the optical axis is aligned parallel to the field vector (for a LC with positive dielectric anisotropy). As well known, the sign of the dielectric anisotropy (De) depends upon the parallel alignment (De [ 0) of the electric field and the director is favored or a mutually perpendicular orientation (De \ 0) of the field. Figure 5 b shows the variation of response time with applied electric field (20 V pp and 5 Hz) for pure LC matrix and PSLC system at a temperature 30 C. The response time decreases with increasing the applied field. As a particular voltage, all molecules are fully rotated in the field direction for pure LC matrix. This is clear that polymer network generates liquid crystal polymer domains. All domains are easily affected by the minimum field, so response time is decreased with the addition of polymer material. The response time is also related to the rotational viscosity of liquid crystals. The rotational viscosity is defined as torque momentum of LC director with field. This can be more clearly seen for 1 % PBMA and 3 % PBMA, and the size and numbers of polymer domains are different in both systems. A better polymer domains uniformity provides larger electric dipole moment in PSLC system. There are a number of forces that interplay with each other to decide what kinds of configuration will be adopted by the LC director field within a LC droplet. The most important factors are the alignment properties of the LC at the polymer network, the shape of the polymeric domains containing the liquid crystal, the elastic constants of the bulk LC and the presence of external electric field. The most important factor in determining the polymeric domain configuration is usually the preferred alignment of the LC at the boundary of the polymer network. As with all liquid crystal devices, switching on Fig. 5 a Response time with variation of a temperature and b applied voltage for pure LC and PSLC systems

7 Dielectric and electro-optical properties of polymer-stabilized liquid crystal system Page 7 of time is faster than switching off time. This is due to the fact that during switching off, there is no electric driving field to help LC directors to rotate to their original alignment, and instead, they rely upon the restoring force of the anchored LC molecules at the polymer network boundary. The LC directors have different angles with respect to the applied electric field. Therefore, they will rotate by different angles under an electric field. Those which are rotated by a larger angle will have slower response time. Moreover, the anchoring force provided by the polymer network is not uniform over the entire LC matrix. For example, the LC director close to the polymer network experiences a stronger anchoring force, while those in the center experience a weaker force. A stronger anchoring force helps to restore the LCs with faster response time. As a result, the responses of LC directors are different and there are multiple decay processes going on. Here, we take an average effect of the pure LC matrix and PSLC. As the electric field increases but below the critical field, the LC rotation angle increases, but remains unaffected within a certain limit. From our results, the response time becomes slower as the electric field exceeds the critical field. Therefore, it is very important to keep the maximum electric field below this critical field in order to achieve fast response time and good performance of liquid crystal devices. The threshold voltage is important factor for the LCs. Figure 6a, b shows the variation of threshold and saturation voltages with the concentration of polymer in the pure LC matrix. These voltages were determined from measuring the intensity of transmitted light by given sample cells as a function of applied voltage. By applying step change AC voltage and then detecting the change in transmitted light intensity, we have determined the threshold voltage (the voltage under which the transmitted intensity is changed by the 10 %) and saturation voltage (the voltage under which the transmitted intensity remains constant 90 %). The increase in the threshold voltage strongly depends upon the thickness of the cell and size and number of the LC domains. The threshold voltage has increased with the addition of polymer in pure LC due to increase in the size and number of LC domains of the PSLC. The magnitude of threshold voltage also depends upon the value of dielectric anisotropy (De), [18]. V th / p 1 ffiffiffiffiffi ð10þ De These experimental results suggest that adding polymeric material to nematic liquid crystals not only causes substantial changes to the dielectric anisotropy of the liquid crystal, but also influences the elastic constants. Figures 7 and 8 show the variation of splay elastic coefficient and rotational viscosity with the variation of temperature for pure LC matrix and PSLC system. The rotational viscosity of the pure LC and polymer dispersions in pure LC were estimated by measuring the fall time of the LC cells with planar alignment. In this study, however, we just aimed to compare the dispersions to the pure LC matrix, so we have described their performance with a single, effective response time constant and have introduced rotational viscosity. The splay elastic coefficient and rotational viscosity were determined for pure LC and PSLC system with the relations [18], K 11 ¼ v th 2Dee0 ð11þ p and c ¼ s 0K 11 p 2 d 2 ð12þ Fig. 6 a Threshold voltage and b saturation voltage with concentration of polymer in pure LC matrix

8 217 Page 8 of 9 M. Pande et al. Fig. 7 Splay elastic coefficient with variation of temperature for pure LC and PSLC mesogenic materials, respectively. From the behavior of rotational viscosity as shown in Fig. 8, it can be observed that its value has decreased with the dispersion of polymer in pure LC matrix. The value of rotational viscosity is generally constant in nematic phase, and then, sharp decrease is observed on further increase in the temperature for pure LC matrix. In PSLC system, the rotational viscosity is sharply increased near the phase transition temperature and decreases with the rise in the temperature. This result is explained on the basis that the LC polymeric domains are diminishing near the phase transition temperature. Further, increasing the concentration of polymer in pure LC, the role of viscosity comes into play and becomes a dominating factor. The value of rotational viscosity is higher near the phase transition temperature due to response time increase for PSLC system. The elastic constant is the force necessary to deform the molecular orientations. Increasing the elastic constant of the liquid crystal materials can be achieved by strengthening the interaction between the LC molecules [22]. In this work, it is obvious that the interaction of polymer network and LC molecules is strong because the elastic constant increases for PSLC system. 4 Conclusions Fig. 8 Rotational viscosity with variation of temperature for pure LC and PSLC The balance of elastic forces within the domain is the most important factor in determining the director configuration. These elastic forces determine whether the structure within the droplet is simple or complex. It has been observed that the splay elastic coefficient of PSLC is higher as compared to the pure LC matrix. In principle, lower viscosity should give rise to faster fall time and rise time. However, the response time are also affected by other parameters such as domain shape, thickness of the cell and anchoring of boundary layer. Due to addition of polymer in LC matrix, the viscosity of liquid crystal may not be the same as it was originally, with the effect that the viscosity and fall time may be correlated. Viscosity and rotational viscosity are related to linear momentum and torque momentum of In summary, polymer PBMA was dispersed in the pure 5CB liquid crystal. The dielectric spectroscopy was performed to investigate the dielectric anisotropy with the variation of temperature. The threshold and saturation voltages were calculated from the variation of different concentration of polymer in pure LC matrix. Polymer chain may play an important role in the enhancement of the electro-optical response of liquid crystal molecules. Higher values of splay elastic coefficient and lower value of rotational viscosity reveal the weak anchoring occurring between the LC molecules and polymer network. The influence of liquid crystal polymeric domains due to dispersion of the polymer in liquid crystal has been studied for both planar and hometropic alignments. Our study also demonstrates that in addition to good dispersal of the polymer in the LC, it is necessary to establish the polymeric network of the PBMA to observe any enhancement in the orientational order of the host liquid crystals. The polymeric materials greatly improve the response of liquid crystal molecules and offer an opportunity to design novel optical and electro-optical devices. Acknowledgments P.K. Tripathi is thankful to UGC, New Delhi, for providing grant of Dr. D.S. Kothari Post-Doctoral Fellowship No. F.4-2/2006 (BSR)/PH/13-14/0080. We are grateful to DST, New Delhi, for the financial support. We are extremely grateful to both the referees for very useful comments/suggestions.

9 Dielectric and electro-optical properties of polymer-stabilized liquid crystal system Page 9 of References 1. J. Sun, S.T. Wu, Recent advances in polymer network liquid crystal spatial light modulators. J. Polym. Sci. B Polym. Phys. 52, (2014) 2. I. Dierking, Recent developments in polymer stabilised liquid crystals. Polym. Chem. 1, (2010) 3. J. Zou, J. Fang, Adhesive polymer-dispersed liquid crystal films. J. Mater. Chem. 21, (2011) 4. J. Yan, L. Rao, M. Jiao, Y. Li, H.C. Cheng, S.T. Wu, Polymerstabilized optically isotropic liquid crystals for next-generation display and photonics applications. J. Mater. Chem. 21, (2011) 5. J. Yan, S.T. Wu, Polymer effect on the electro-optic properties of blue-phase liquid crystals. SID Int. Symp. Dig. Tech. Pap. 42, (2011) 6. P.G. de Gennes, Scaling Concept in Planar Science (Cornel University press, Ithaca, 1979), chapter 5, p D.O. Krimer, G. Demeter, L. Kramer, Pattern-forming instability induced by light in pure and dye-doped nematic liquid crystals. Phys. Rev. E 66, (2002) 8. R. Kumar, K.K. Raina, Electrically modulated fluorescence in optically active polymer stabilised cholesteric liquid crystal shutter. Liq. Cryst. 41, (2014) 9. M. Inam, G. Singh, A.M. Biradar, D.S. Mehta, Effect of gold nano-particles on switch-on voltage and relaxation frequency of nematic liquid crystal cell. AIP Adv. 1, (2011) 10. O. Buluy, S. Nepijko, V. Reshetnyak, E. Ouskova, V. Zadorozhnii, A. Leonhardt, M. Ritschel, G. Schönhense, Yu. Reznikov, Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals. Soft Matter 7, (2011) 11. S. Singh, J.K. Srivastava, R.K. Singh, in Polymer Dispersed Liquid Crystals, ed. by V.K. Thakur, M.R. Kessler. Liquid Crystalline Polymers: Fundamentals and Applications (Springer, 2015), Chapt P.S. Dzaic, Liquid crystal dispersion series on liquid crystals (World Scientific, Singapore, 1995) 13. S. Pandey, S.K. Gupta, D.P. Singh, T. Vimal, P.K. Tripathi, A. Srivastava, R. Manohar, Effects of polymer doping on dielectric and electro-optical parameters of nematic liquid crystal. Polym. Eng. Sci. 55, 414 (2015) 14. S. Kaur, A.K. Thakur, S.S. Bawa, A.M. Biradar, Phys. B 344, (2004) 15. S.S. Parab, M.K. Malik, P.G. Bhatia, R.R. Deshmukh, J. Mol. Liq. 199, (2014) 16. R.R. Deshmukh, A.K. Jain, Liq. Cryst. 41, (2014) 17. M.B. Pandey, R. Dhar, K. Agrawal, R.P. Khare, R. Dabrowski, Phase Transit. 76, (2003) 18. P.K. Tripathi, A.K. Misra, S. Manohar, S.K. Gupta, R. Manohar, Improved dielectric and electro-optical parameters of ZnO nanoparticle (8 % Cu 2? ) doped nematic liquid crystal. J. Mol. Struct. 1035, (2013) 19. L.M. Blinov, V.G. Chigrinov, Electro-Optical Effects in Liquid Crystal Materials, vol. 3 (Springer, NewYork, 1994) 20. P. Archer, I. Dierking, M.A. Osipov, Landau model for polymerstabilized ferroelectric liquid crystals: experiment and theory. Phys. Rev. E 78, (2008) 21. S.T. Wu, Design of a liquid crystal based tunable electro-optic filter. Appl. Opt. 28, 48 (1989) 22. N. Koide, The Liquid Crystal Display Story (Springer, Tokyo, 2014). doi: /