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Effects of polymer doping on dielectric and electro-optical parameters of nematic liquid crystal.

INTRODUCTION

Nematic liquid crystals (NLCs) have been investigated for more than a century and their applications are omnipresent in our daily life. NLCs are very interesting mesophase materials for their relevance to basic physics and for the applications in the optoelectronic industry [1]. NLC having a low molecular weight has been developed as advanced materials for electro-optical applications [2]. The nematic phase shows dielectric anisotropy ([DELTA][epsilon] = [[epsilon].sub.[parallel]] - [[epsilon].sub.[perpendicular to]]) due to the anisotropic nature of LC molecules where [[epsilon].sub.[parallel]] and [[epsilon].sub.[perpendicular to]] are the components parallel and perpendicular to the long molecular axis, respectively. The anisotropy originates from the fact that molecular dipoles cannot take all possible orientations in LC phase as in isotropic phase [3]. Due to nematic order, some orientations are more probable and this gives a difference between [[epsilon].sub.[perpendicular to]] and [[epsilon].sub.[parallel]]. Nematic liquid crystals (NLCs) with a negative dielectric anisotropy ([DELTA][epsilon] < 0) play an important role in many electro-optical devices. Schiekel et al. has reported that a vertically aligned cell having negative [DELTA][epsilon] LC exhibits a high contrast ratio [4] which is particularly attractive for video applications.

In recent years, anisotropic composites consisting of LC and non-mesogenic materials have been used to develop a new kind of electro-optical materials for novel functions [5, 6). Various attempts have been made to improve the vital properties of the NLCs by doping them with non mesogenic materials like dyes, polymers and nanoparticles and so forth. This doping of nonmesogenic materials is based on the Guest-host interactions, that is, the interaction between the guest (dopant) molecule and host (NLC) molecules [7-9].

Polymers, because of their own strong molecular network can be used as dopants which can affect the alignment of the liquid crystal molecules up to a great extent. LCs and polymers represent an example of an interesting connection between two different research fields, both of which involve the enhancement of the performance of their respective products. Essentially, two types of polymer confinements in LCs are popular, that is, polymer doped LCs and polymer liquid crystal composites [10].

Dielectric and electro-optical properties of polymer doped NLC system are of immense practical importance. The incorporation of polymer not only increases the inherent mechanical strength of the material [11], but also changes the phase behavior along with its electro-optic properties. This slight change in molecular dynamics and ordering of liquid crystal produces a huge change in different physical properties like relative permittivity, dielectric strength and so forth. Dielectric and optical properties of such composites are completely different from those of pure polymer and pure liquid crystal.

In the present study, the investigation has been carried out on PiBMA polymer doped in EBHA NLC at various concentrations. The experimental measurements of dielectric permittivity, dielectric anisotropy, conductivity, rise time, and fall time of LC polymer composite have also been reported. All these parameters have been studied in the temperature range of 60-85[degrees]C. The experimental results indicate considerable improvement in various properties of the doped sample over the pure NLC. The vital parameters of the NLC like dielectric anisotropy and conductivity also show an improvement on adding a small amount of polymer in the NLC, whereas, the higher concentration of the polymer results in the reduction of these improvements.

EXPERIMENTAL DETAILS

Materials

The investigated NLC material used in the present study is p-ethoxybenzylidene p-heptylaniline (EBHA) (Frintron lab). Figure la shows the molecular structure of EBHA. The phase transition scheme of the NLC is

Crystal (52[degrees]C) [left right arrow] Nematic (85[degrees]C) [left right arrow] Isotropic.

The polymer PiBMA (Sigma-Aldrich) was first dissolved in ethanol and then it was doped in the pure NLC in 1,2, and 3 wt/wt%, termed as Mixture 1, Mixture 2, and Mixture 3, respectively. Figure lb shows the chemical formula of PiBMA.

Preparation for Sample Cells

The Dielectric study of the polymer doped NLC was conducted on the planar geometry. 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 [micro]m 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[degrees]C for 10 h before assembling the cell. The empty sample cells were calibrated using analytical reagent (AR) grade benzene ([C.sub.6][H.sub.6]) as standard reference for the dielectric study. To prepare the sample, an appropriate amount (in the weight ratio, i.e., 1.0, 2.0, and 3.0%) of polymer PiBMA were mixed into the pure EBHA and then homogenized with an ultrasonic mixer at 85[degrees]C for 1 h and uniform dispersion of polymer has been ensured by looking under a polarizing microscope (Radical RXLR-5). The pure and the doped samples were filled in the assembled cells at a temperature higher than the isotropic temperature of the NLC 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 a polarizing microscope. The composite system was cooled slowly from isotropic to room temperature under the applied electric field to get a well aligned cell. 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. It is a standard practice for obtaining a well aligned cell and has been reported by many other research groups [12-14].

Dielectric Measurements

The Dielectric measurements have been carried out by computer controlled Impedance/Gain Phase Analyzer (HP 4194 A) attached with a temperature controller in the frequency range 100 Hz to 10 MHz. The dielectric measurements have been carried out as a function of temperature by placing the sample on a computer controlled hot plate INSTEC (HCS-302). The temperature stability was better than [+ or -]0.1[degrees]C with the measurements. Measurements in the higher frequency range have been limited to 10 MHz because of the dominating effect of finite resistance of ITO coated on glass plates and lead inductance [15]. The real and imaginary part of dielectric permittivity is given by the following equation.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Here [sigma](dc) is ionic conductance and [[epsilon].sub.o] is free space permittivity, f is the frequency of relaxation, [delta][epsilon]' is the relaxation strength, [tau] is the relaxation time, [alpha] is the distribution parameter, [epsilon]'([infinity]) is the high frequency limit of the dielectric permittivity, [omega] is the angular frequency while n, m, and k are the fitting parameter. The term [epsilon](dc)/[f.sup.n] and [sigma](dc)/[[epsilon].sub.o][pi][f.sup.k] are added in the above equation for low frequency effects due to the electrode polarization, capacitance, and ionic conductance. The term [Af.sup.m] term is added in Eq. 2 for high frequency effect due to the ITO resistance and lead inductance [16, 17]. By the least square fitting of experimental data in the above equation the low and high frequency data have been corrected.

Electro-Optical Measurements

An arbitrary square wave (20Vpp and 1 Hz) has been applied to the cells using a function generator. He-Ne laser beam of wavelength 632 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[degrees] for maximum transmittance. Thus, 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 ([t.sub.on]) and fall time ([t.sub.off]) of pure and polymer doped NLC have been evaluated using the equation given below. Here, [t.sub.on] is the time required for the transmittance to rise from 10% to 90% and [t.sub.off] is the time required for the transmittance to fall from 90% to 10% [18, 19].

[[tau].sub.O] = [[tau].sub.ON] + [[tau].sub.OFF] x [[tau].sub.ON] = [[tau].sub.90] - [[tau].sub.10] & [[tau].sub.OFF] = [[tau].sub.10] - [[tau].sub.90] (3)

RESULTS AND DISCUSSION

To study the effect of doping polymer PiBMA into the pure EBHA the Polarizing Optical Microscope (POM) images have been recorded. The optical micrographs of pure and polymerNLC composite were taken using polarizing microscope under crossed polarizer condition in planar aligned cell as shown in Fig. 2. It can be seen from Fig. 2b that the doping of polymer into the pure NLC causes formation of polymer network.

The dielectric permittivity and dielectric loss of the planar aligned cell have been depicted with the variation of frequency in the Fig. 3 for the pure NLC and polymer doped NLC at a constant temperature of 72[degrees]C which is remote enough from both the crystallization and clearing points. It is clear from the figure that the dielectric permittivity is not changing across the frequency range of 1 KHz to 1 MHz, so all the dielectric parameters have been discussed at a particular frequency of 10 KHz. The variation of average dielectric permittivity with concentration of polymer at 72[degrees]C has been depicted in the Fig. 4. The figure clearly shows a decreasing trend of average dielectric permittivity.

The decrease in average dielectric permittivity is because of two reasons.

1. Guest-Host interaction between polymer and liquid crystal (LC) molecules (This interaction is dependent on the nature of both guest and host molecules and also on the concentration of guest molecule. The guest-host system may have a saturation concentration around which the various properties may have different nature. This saturation concentration is different for different properties).

2. Accumulation of ionic charges at LC-polymer interface.

In the present case, the guest-host interaction plays an important role and the formation of polymer network which spreads throughout the LC matrix affects various properties of host NLC sample. For the pure sample, the only interaction is LC-LC molecular interactions. When the polymer is doped into the NLC, another interaction, that is, the interaction between polymer and NLC molecules occurs and this interaction results in redistribution of intermolecular interaction energies. On account of this, for lower concentration of polymers, the NLC molecules feel some constraints which affect their molecular dynamics adversely. The presence of polymer network in the liquid crystal medium induces disorder and disturbs the LC molecular orientation around the polymer, as shown in Fig. 5. This decreases the overall order of the doped system resulting in the decrease in effective polarization of the doped system. This is the primary reason for the decrease in dielectric permittivity of the doped system.

Another reason for the decrement in the value of average dielectric permittivity is the accumulation of ionic charges at LC-polymer interface. At higher frequency of 10 KHz, when pure NLC is doped with PiBMA, the mobility of ionic charge carrier reduces so much that the charge carriers get easily trapped by the polymer network. It causes an accumulation of ionic charges at LC-polymer interface generating an internal electric field. This generated field opposes the applied electric field due to which the polarization decreases and hence the average dielectric permittivity decreases. The generated electric field would be localized but it would have some effect on the polarization though it will be a feeble one. Thus, the value of average dielectric permittivity decreases with increase in the concentration of the dopant [10, 14, 20].

The temperature dependence of electrical conductivity in the LC--polymer system shows a distinct behavior. The electrical conductivity is plotted with the variation of temperature for the pure NLC and polymer doped NLC at 10 KHz frequency in Fig. 6. It is clear from the figure that the conductivity for pure NLC and its composite system increases with increase in the temperature. As temperature increases, the charge carriers gain enough energy for the conduction. Hence, with increase in the temperature the conductivity increases. The figure shows that the value of conductivity for pure NLC is lesser in comparison to that of the doped system. The same as discussed in the dielectric permittivity section, the addition of polymer into the pure NLC offers some constraints in the form of polymer network. This interaction affects the molecular dynamics of NLC restricting the molecular movement which is reflected by the decrease in dielectric permittivity of the polymer doped system. This decrease in dielectric permittivity clearly indicates reduction of polarization. However, in addition to that the polymer network so formed provides a preferred path for the movement of charge carriers. These two factors play an important role in deciding the conductivity of the polymer-LC composite. For Mixture 1 and Mixture 2, the polymer network is not strong enough and provides a free path to the charge carriers resulting in increased conductivity. Although, for higher concentration of polymer, that is, for Mixture 3, this network is strong enough to reduce the polarization and this factor dominates. Therefore, the conductivity of Mixture 3 has slightly lower values in comparison to Mixture 1 and 2 but still it is higher than that of pure NLC. This type of behavior that after a particular concentration of non-mesogenic guest molecule showing opposite trend has also been reported by other research groups for different properties. This concentration may be termed as saturation concentration [14, 17, 21].

The dielectric anisotropy has been plotted with the variation of temperature in Fig. 7 for pure NLC and polymer doped NLC at a frequency of 10 KHz. The decreasing trend of negative dielectric anisotropy for both the pure NLC and the doped system on the temperature scale has been observed. The dielectric anisotropy is higher in the nematic phase as compared to the temperature near N-I transition indicating a decrease in orientational order near transition temperature. The disorder in the system increases with increasing temperature and hence the difference between [[epsilon].sub.[perpendicular to]] and [[epsilon].sub.[parallel]] decreases, that is, dielectric anisotropy decreases. The value of dielectric anisotropy for the doped system has increased in the negative order as shown in the Fig. 7. The influence of doping polymer on the dielectric anisotropy of nematic phase can be analyzed by applying Maier and Meier theory [22, 23], According to this theory, dielectric anisotropy is given as

[[epsilon].sub.[parallel]] = 1 + NhF/[[epsilon].sub.0] {[bar.[alpha]] + 2/3 [DELTA][alpha]S + F[[mu].sup.2]/3[K.sub.B]T [1 - (1 - 3[cos.sup.2] [beta])S]} (4)

[[epsilon].sub.[perpendicular to]] = 1 + NhF/[[epsilon].sub.0] {[bar.[alpha]] - 1/3 [DELTA][alpha]S + F[[mu].sup.2]/3[K.sub.B]T [1 + 1/2 (1 - 3[cos.sup.2] [beta])S]} (5)

[DELTA][epsilon] = [[epsilon].sub.[parallel]] - [[epsilon].sub.[perpendicular]] = NhF/[[epsilon].sub.0] [[DELTA][alpha] + [[mu].sup.2] F/2[K.sub.B]T (3[cos.sup.2] [beta] - 1)]S (6)

where [DELTA][alpha] is anisotropy of molecular polarizability, [mu] is the resultant dipole moment of the molecule, N is the molecular number density, [DELTA][alpha] is dielectric anisotropy, F is parameter depending on reaction field factor, and [beta] is the angle between the molecular axis and the direction of the off axis. The dielectric anisotropy depends upon the angle [beta] and the order parameter [24], The trend of dielectric anisotropy shown in Fig. 7 can be explained on the basis of interactions taking place between the polymer and NLC molecules. The NLC molecules are non-polar in nature and the polarization observed is an induced polarization obtained by applying an electric field. The presence of polymer network in the liquid crystal medium induces disorder and disturbs the LC molecular orientation around the polymer. This random orientation of LC molecules causes decrease in orientational order as discussed earlier. The decrease in orientational order of the doped system causes change in the effective polarization of the doped system. Therefore, the effective polarization of the doped system decreases which is clear from the Fig. 4. The decrease in the effective polarization of the doped system disrupts both the components of the dipole moment thereby changing the ratio between them. Thus, the effective angle [beta] of the doped system changes, due to formation of polymer network, as it depends upon the ratio of the two components of the dipole moment given by [beta] = [tan.sup.-1] ([[mu].sub.t]/[[mu].sub.1]), where [[mu].sub.t] and [[mu].sub.1] are the transverse and longitudinal components of the dipole moment of NLC molecule. It is clear from the figure that the dielectric anisotropy is increasing in negative order which indicates that the effective angle [beta] of the doped system increases and order parameter S decreases with increase in the concentration of the dopant. In Mixture 1, due to low concentration of the polymer, the effect of increase in effective angle [beta] dominates over the order parameter S. Therefore, negative dielectric anisotropy increases for Mixture 1. In Mixture 2, the increase in the concentration causes greater perturbations in NLC geometry. At this stage, the perturbation overcomes the effect of angle [beta], thereby decreasing the negative dielectric anisotropy for Mixture 2. A further increase in the concentration of polymer does not bring much change in its order, considering it as a type of saturation of polymer concentration in LC. The saturation concentration of polymer has already been discussed in the earlier part of the discussion. Thus, the effect of angle [beta] dominates over order parameter S due to which negative dielectric anisotropy increases.

The inset of Fig. 7 shows the variation of dielectric anisotropy as a function of concentration for two different temperatures 66 and 75[degrees]C. The nonmonotonic changes with polymer concentration at two different temperatures were observed for dielectric anisotropy. The probable reason behind this nonmonotonic trend of dielectric anisotropy with varying polymer concentration is the existence of impurities present in the sample. This impurity degrades the performance of NLC materials and resists the usual trend of the NLC parameters. It was determined that with the addition of different concentrations of polymer in the pure NLC system, the geometry changes itself in the composite system for all mixtures. This change in geometry depends upon the concentration of polymer, therefore, a nonmonotonic change in dielectric anisotropy at two different temperatures with polymer concentration has been observed.

Figure 8 depicts the variation of fall time with the concentration of dopant at 72[degrees]C. The figure shows that the fall time increases with an increase in the concentration of polymer in pure NLC. This indicates that the viscosity increases with an increase in the concentration of the polymer. The viscosity acts as a dominating factor for the fall time in the liquid crystals, thereby increasing the fall time.

The variation of rise time as a function of concentration at 72[degrees]C has been shown in the Fig. 9. The rise time for 1% of concentration (Mixture 1) has decreased in comparison to that of the pure NLC sample, whereas, for Mixture 2 and Mixture 3, it has slightly increased. This behavior can be explained by the rotational viscosity relation given as:

[tau] = [gamma]/[[epsilon].sub.0][DELTA][epsilon][E.sup.2] (7)

where E is the applied electric field, [gamma] is the rotational viscosity, and [DELTA][epsilon] is dielectric anisotropy. Equation 7 suggests that higher the anisotropy, smaller the electric field required to make the pure NLC and its composite system to respond, that is, the rise time will be decreased. It is well known that for smaller concentration, the viscosity does not play an effective role. As it is clear from the Fig. 7 that the value of dielectric anisotropy for Mixture 1 has increased but for Mixture 2, it has decreased. Therefore, rise time for Mixture 1 decreases and it increases for Mixture 2. For Mixture 3, the rise time increases though the value of dielectric anisotropy increases. This is due to the fact that on further increasing the concentration of polymer, the role of viscosity comes into play and becomes a dominating factor. The decrease in the value of rise time for the Mixture 1 indicates its utility in the field of display technology based on fast response time.

CONCLUSIONS

In pithiness, the effect of doping polymer PiBMA into the pure EBHA NLC has been investigated. The dopant concentration dependent properties of the pure NLC system have been analyzed and explained. It has been found that the electro-optical and dielectric parameters of the doped sample have changed in comparison to the pure NLC system. The study shows that the conductivity increases with an increase in the concentration of polymer. The dielectric anisotropic study shows that the NLC under investigation has negative dielectric anisotropy which has increased in negative order on increasing the concentration of polymer. The materials having a negative dielectric anisotropy and high conductivity have proven themselves as the highly interesting candidates for use in newly emerging superior display technologies. In addition to this, the more beneficial electro-optic character of doped system will prove its utility in display application due to fast rise time at a particular concentration and low charge consumable devices owing to the easy charge transportation phenomenon in the composite system.

ACKNOWLEDGMENTS

Author S.P. is sincerely thankful to UGC, New Delhi for providing financial assistance in the form of UGC-BSR Fellowship. T.V. is also thankful to UGC for financial assistance. Authors S.K.G. and D.P.S. are thankful to CSIR, New Delhi for SRF grant. All the authors are thankful to Indian Space Research Organization, India for the financial assistance for present work in the form of project.

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Shivani Pandey, Swadesh Kumar Gupta, Dharmendra Pratap Singh, Tripti Vimal, Pankaj Kumar Tripathi, Atul Srivastava, Rajiv Manohar

Liquid Crystal Research Lab, Physics Department, University of Lucknow, Lucknow, India-226007

Correspondence to: Rajiv Manohar; e-mail; rajiv.manohar@gmail.com

DOI 10.1002/pen.23907

Published online in Wiley Online Library (wileyonlinelibrary.com).
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Author:Pandey, Shivani; Gupta, Swadesh Kumar; Singh, Dharmendra Pratap; Vimal, Tripti; Tripathi, Pankaj Kum
Publication:Polymer Engineering and Science
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Date:Feb 1, 2015
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