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Sign inversion of dielectric anisotropy in nematic liquid crystal by dye doping
Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Sign inversion of dielectric anisotropy in nematic liquid crystal by dye doping Rajiv Manohar n, Kamal K. Pandey, Abhishek K. Srivastava, Abhishek K. Misra, Satya P. Yadav Physics Department, University of Lucknow, Lucknow 226007, India
a r t i c l e in fo
abstract
Article history: Received 25 November 2009 Received in revised form 12 April 2010 Accepted 19 May 2010
The present paper reports sign inversion in dielectric anisotropy of a nematic liquid crystal, i.e. 5CB, as an effect of doping dye (solvent green 3) in small amount. It is the result of strong variation of the parallel component of dielectric permittivity with temperature for a dye doped sample. This behavior is attributed to the interaction taking place between the nematic liquid crystal molecule and the dye molecule. This behavior of dielectric anisotropy has been explained on the basis of interaction between the dye (guest) and the liquid crystal molecules (host). & 2010 Elsevier Ltd. All rights reserved.
Keywords: D. Dielectric properties A. Non-crystalline materials
1. Introduction Liquid crystals have been studied for several decades not only because of their technological importance but also because of their extraordinary physical properties such as dielectric and optical anisotropy, flow properties, response to external fields and ability to transmit static torque [1–3]. Quantitative knowledge of orientational ordering in liquid crystal is necessary for the development of improved materials for application. Thus, attempts are being continuously made to study the material properties of both pure compounds as well as their mixtures for a better insight into the basic understanding of the liquid crystalline behavior. Most applications of liquid crystals depend upon the possibilities of changing orientation of the liquid crystal molecules by the applied electric fields, which in turn depends on the visco-elastic and electrical properties of such materials [4]. The most important electrical property of the liquid crystals that affects its switching behavior is the dielectric anisotropy (De). Anisotropy of all the physical properties of liquid crystals mainly arises due to their long-range molecular ordering. The study of the anisotropy of the electrical and optical properties of nematic liquid crystals is not only the basis of their application in many electro-optical devices [5–7] but also very important from the molecular physics point of view. The study of dielectric anisotropy is important and it may provide significant information about the arrangement of molecules within the system. The dielectric anisotropy of liquid crystals is manifested as a difference in the value of electric permittivity measured along the parallel direction to the long molecular axis e99 and perpendicular n
Corresponding author. Tel.: + 91 9451 000 687. E-mail address: [email protected] (R. Manohar).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.05.011
to the long molecular axis e?. The value and the sign of dielectric anisotropy (De ¼ e99 e? ) depend on the anisotropy in anisotropic distribution of the molecular dipoles in liquid crystal phases or, more precisely, on the distribution of dipole moments of a polar group with respect to the long axis of the molecule, i.e. director [8]. The sign of dielectric anisotropy may be positive or negative, varying from sample to sample. The value of De can also be significantly altered by the presence of guest molecule. This guest molecule may be a dye, a polymer or a nanoparticle. This phenomenon is called the guest–host interaction [9]. In the present paper we report the sign inversion of De of a standard nematic liquid crystal 5CB on the addition of small traces of dye molecules. For this purpose temperature dependency of e99, e? and De has been investigated. Variation of dielectric anisotropy with concentration of dye in the nematic phases has been explained. Sign inversion of dielectric anisotropy with frequency and temperature has been investigated and reported by various groups [10,11] but inversion due to the presence of guest entity, i.e. dye, is not reported in the literature.
2. Experimental details 2.1. Material The investigated system consists of a room temperature nematic liquid crystal material 5CB having high positive dielectric anisotropy and chemical stability. Such features make 5CB (commercial, Merck) very interesting for fundamental investigations on nematic liquid crystals [12–18]. The dye used in our investigation is solvent green 3 (SG3) purchased from Across Chemicals. To study the guest–host system, we used 5CB doped at a dye concentration of about 1% wt/wt ratio. The doped mixture
1312
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
3. Result and discussion
C 5 H 11
CN
4′-n-Pentyl-4-cyanobiphenyl (5CB) Cr-----(22.5°C)------ nematic ---(35.3°C)-------isotropic
O
HN
Addition of guest molecule in a pure LC matrix produces changes in the intermolecular interaction field and may cause phase separation or complete phase loss in pure LC. Therefore optical transmittance and response time measurements were performed under crossed polarized condition [19]. To analyze the phase behavior, the optical transmittance for planar aligned cell has been plotted against the applied electric field at 331C in Fig. 2. From Fig. 2, it is clear that the Freedricksz transition appears in both pure and doped mixtures. That means nematic mesophase exists in doped LC, but nematic–isotropic phase transition temperature is reduced almost by 11C for the guest–host system. The dielectric response of liquid crystals shows complex behavior as functions of both frequency and temperature. Dielectric data have been analyzed by the Cole–Cole equation as follows [7,19–21]:
e ¼ e0 ð1Þ þ O
HN
Solvent green 3 (Mol. Wt. 418) Fig. 1. Molecular structure of liquid crystal and dye used for study: (a) 5CB nematic with transition scheme and (b) solvent green 3 dye.
was prepared by dispersion of dye in the nematic liquid crystal. The transition temperatures of the pure compound, their structural formula and the chemical names are shown in Fig. 1.
de0
ð1Þ
1 þ ðiotÞ1a
where de0 is the dielectric strength, e0 (N) the high frequency limit of dielectric permittivity, f the frequency, t the relaxation time and a the distribution parameter. Errors of experimental data in low and high frequency range due to one types of sample holder are quite common; therefore experimental results require low and high frequency correction. On separating the real and imaginary part of Eq. (1) one may get, after adding high and low frequency correction parameters [7,19–21]
e0 ¼ e0 ðdcÞf n þ e0 ð1Þ þ
d e0 ½1 þ ð2p f tÞð1aÞ sinðap=2Þ 1þ ð2p f tÞ2ð1aÞ þ 2ð2p f tÞð1aÞ sinðap=2Þ ð2Þ
Sandwiched type cells were prepared for the measurement of dielectric permittivity using transparent and highly conducting ITO (Diamonds Coating UK) coated glass substrate. ITO coated glass plates have a sheet resistance of 10 O/&. Both electrodes of the cell were treated with adhesion promoter and polymer (nylon 6/6; Sigma Aldrich) and rubbed unidirectionally to get planar aligned cell. For homeotropically aligned cell lecithin (Sigma Aldrich) was used as the coating material. Thickness of the cell was maintained at 10 mm with the help of Mylar spcer (Liquid Crystal Technologies, USA). The cell was calibrated using AR grade carbontetra-chloride (CCl4) and benzene. The material was introduced into the cell by capillary action at a temperature 10 1C above the isotropic temperature of the sample. 2.3. Dielectric permittivity study The dielectric response of the sample was studied in the frequency range 100 Hz–10 MHz using a computer controlled impedance/gain phase analyzer (HP-4194A). The temperature of the sample holder was achieved using an Instec hot plate (HCS-302) with an accuracy of 70.11C. The sample was left for 10 min before starting the measurement at a particular temperature. Dielectric permittivity is very sensitive to impurities as reported by Murakami and Naito [12]. They report that dielectric permittivity of 5CB exhibits a slow but appreciable drift (within a month) probably due to the injection of impurities from the contacting media (glue, polyimide layers, etc.). Therefore a complete set of measurement was made using a fresh sample and maintaining the total measurement time within a 3-day period, which is much smaller than the degradation time observed by Murakami and Naito [12].
and
e00 ¼
sðdcÞ de0 ð2pf tÞð1aÞ cosðap=2Þ þAf m þ e0 2pf k 1 þ ð2pf tÞ2ð1aÞ þ 2ð2pf tÞð1aÞ sinðap=2Þ
where s(dc) is the ionic conductance, e0 the free space permittivity, f the frequency of relaxation and k the fitting parameter. The terms e0 (dc)/fn and s(dc)/2peofk are added in the above equation for low frequency effect due to electrode polarization, capacitance and ionic conductance. The term Afm is added in the equation for high frequency effect due to ITO resistance and lead inductance of the cell. By the least square
% OPTICAL TRANSMITTANCE (Normalized)
2.2. Cell preparation
5CB PURE 5CB+1% SG3
100
80
60
40
20
0 0
5 10 15 APPLIED VOLTAGE (VOLTS)
20
Fig. 2. Optical transmittance dependence on applied electric field.
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
12
10 1
0
-1 24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 5. Variation of dielectric anisotropy of pure and dye doped samples as a function of temperature.
5CB+1% SG3
18
5CB+1% SG3 5CB PURE
Δε
fitting of experimental data in the above equation the low and high frequency data have been corrected. In order to see the effect of the dye on the dielectric properties of the liquid crystal, e99, e? and De have been measured for the pure and 1% dye doped sample. The results are shown in Fig. 3, Fig. 4 and Fig. 5. All the graphs have been plotted at the frequency of 10 kHz. Fig. 6 shows the change of molecular arrangement with the addition of dye. Fig. 7 shows the variation of dielectric anisotropy with frequency. The temperature range has been taken as 25–351C. Fig. 3 shows variation of e99 with temperature for both pure and dye doped samples. Dielectric permittivity of the guest–host system decreases almost 3 times than that for pure 5CB for a particular temperature. As the temperature of the sample increases, pure 5CB shows a fall in the value of e99 while the dye doped sample shows a slight increase. Fig. 4 shows variation of e? with temperature for both pure and dye doped samples. A slight
1313
5CB PURE
ε//
16
8
6
24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 3. Parallel component of dielectric permittivity (e99) of pure and dye doped sample as a function of temperature.
6.1
5CB+1% SG3 5CB PURE
6.0
ε⊥
5.9
5.8
5.7
5.6 24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 4. Perpendicular component of dielectric permittivity (e?) of pure and dye doped samples as a function of temperature.
increase has been observed for the 1% dye doped sample in comparison with the dielectric permittivity value of pure 5CB. Fig. 5 shows variation of De for both pure and dye doped samples with temperature. Decrease in dielectric anisotropy of the dyed sample is observed in comparison with pure 5CB. An interesting result has been observed for dielectric anisotropy for the 1% dye doped sample. Dielectric permittivity e99 decreases for the 1% dye doped sample drastically, while the perpendicular component e? does not show much effect of dye concentration on itself. This leads to change of e99/e? 41 to e99/e? o1 for the dye doped sample. Sign inversion for dielectric anisotropy has been observed in the case of 1% dye doped sample. These negative anisotropy materials are of prime importance in the flat-panel display industry, forming the basis of ‘‘patterned vertical alignment’’ technology [22,23]. The above results can be explained on the basis of interactions taking place between the dye and 5CB molecules. When a rod shaped dye molecule is used to dope in pure nematic LC, the dye molecules try to fit in the sample geometry and follow intrinsic geometry constraints. This type of behavior for the rod shaped dye molecule in LC matrix has already been reported by many researchers in recent years [24,25]. The dye used in the present investigation is not rod shaped. Therefore, presence of this dye in pure matrix disturbs the pure LC molecular orientation around the dye molecules, as shown in Fig. 6. This random orientation of LC molecules around the dye molecule results in decrease in orientation order. Usually the contribution of dipole moment in parallel and perpendicular directions for the LC molecule making an angle b with long molecular axis is given by ml ¼ m cos b and mt ¼ m sin b. In the present case the randomness in the pure LC matrix, around the dye molecules, causes change in net dipole moment, i.e. decrease in e99 and increase in e? . In addition to this at higher temperature the interaction between dye and LC molecules reduces; consequently, the randomness in LC molecular orientation will also reduce at higher temperatures. Therefore, at higher temperatures small increment in e99 has been observed for the dye doped system. The effect of dye on e? at higher temperatures is lesser too but it is insignificant. The permanent and the induced dipole moments may be combined in an analytical expression for the dielectric anisotropy as follows [7,26,27]:
De ¼ eJ e? ¼
N0 hF
e0
Da
F m2 ð13cos2 bÞ S 2KT
ð3Þ
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R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
LC MOLECULE
DYE MOLECULE
Fig. 6. Change of orientation of nematic liquid crystal molecule with the addition of dye in a pure liquid crystalline system.
5CB+1% SG3 5CB PURE
Δε
10
0
1000
10000
100000
1000000
1E7
Frequency (Log) Fig. 7. Variation of dielectric anisotropy of pure and dye doped samples as a function of frequency.
Here F and h are constants of proportionality called the reaction field factor and cavity factor, respectively. Density r of the materials enters the expression in the particle number N. It is obvious that both S and T enter the expression, leading to a direct proportionality of De with the order parameter S. It is obvious from the above equation that De strongly depends on b, S and density of the material. The presence of guest molecules in pure LC matrix results in disturbance in the pure LC matrix. This disorder also affects the order parameter. The order parameter for pure LC is 0.67 but after adding dye it becomes 0.64. The change in the value of order parameter is not very prominent. Therefore, this small change in S cannot be responsible for the sign reversal in De. We are surprised from the above experimental findings that dielectric anisotropy and threshold voltage decrease simultaneously. Usually both of them cannot decrease together [7,8]. In our view, the presence of the dye molecules in the liquid crystal medium induces disorder around the dye molecules. This disorder becomes more prominent if the molecular structure of the dye is other than the rod shape as in the present case. The dye molecule disturbs the LC geometry in such a manner that it induces a small tilt in the medium. This induced tilt in the LC system is more prominent near the dye molecules and spreads throughout the
doped LC system. Due to this tilt, the energy required for the molecular movement decreases. This decrease in required energy for molecular movements and thick cell may be one of the reasons for the decrease in threshold voltage for the dye doped LC. The induced tilt also affect the response time of the system. The optical response of the dye doped LC reduces as compared to that of LC. This reduction in optical response time confirms that induced tilt is dispersed throughout the dye doped LC suspension. Sign inversion is also observed with variation of frequency. This result has been plotted in Fig. 7. At the critical frequency both pure 5CB and the dye doped sample show sign inversion of dielectric anisotropy. In this case, as a result of low frequency dispersion in e99 at a certain frequency fc a change in sign of dielectric anisotropy of the nematic liquid crystal occurs. The dual frequency phenomenon observed in special nematic liquid crystals arises from molecular dielectric relaxation, having a relaxation frequency of a few decades of kilohertz, rotation around their short axes. The sign of dielectric anisotropy as a function of frequency changes near the relaxation frequency [28]. Therefore, by changing the frequency of the applied field, the direction of the director can be controlled by coupling torque of the applied field with dielectric anisotropy in both rising and falling signals.
4. Conclusions In this paper we have reported the dielectric anisotropic behavior of the dye doped 5CB (doped with solvent green 3). Dielectric properties of the nematic liquid crystal 5CB doped with dye (1% solvent green 3) have been measured in the temperature range 25–351C at 10kHz frequency. We have observed that dielectric anisotropy changes its sign when 1% of dye, solvent green 3, is introduced into the liquid crystals matrix. Director alignment of the nematic liquid crystal is found to be sensitive to the presence of the guest molecule. Here the interaction taking place between the liquid crystal molecule and the dye molecule alters director alignment of the nematic liquid crystal sample. We can say that molecular alignment of the liquid crystal molecule in dyed samples is controlled by the guest molecule to some extent while in the pure sample it is controlled only by confining walls of the electrodes. This feature opens up a new field of application such as developing a liquid crystal–solid particles interface that makes the role of the liquid crystal surface layers important in the determination of material properties.
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
Acknowledgements The authors are sincerely thankful to UGC, New Delhi, India, for financial assistance in the form of project for the present work. References [1] K. Kocevar, I. Muscvic, Liq. Cryst. Today 12 (3) (2003) 3–8. [2] F. Simoni, L. Lucchetti, D.E. Lucchetta, O. Francescangeli, Opt. Exp. 9 (2001) 85–90. [3] L. Lucchetti, M.D. Fabrizio, O. Francescangeli, F. Simoni, J. Nonlin. Opt. Phys. Mater. 11 (2002) 13–23. [4] G. Vetrogen, W.H. Jeu, The Physics of Liquid Crystals Fundamentals, Springer, Berlin, 1988. [5] P.G. de Gennes, J. Prost, The Physics of Liquid Crystals, Oxford University Press, New York, 1997. [6] S.T. Wu, D.K. Yang, Fundamentals of Liquid Crystal Devices, Wiley, New York, 2006. [7] L.M. Blinov, V.G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials, Springer-Verlag, New York, 1994. [8] B. Bahadur, Liquid Crystals Applications and Uses, Vol. 1, World Scientific, Singapore, 1990. [9] G.H. Heilmeier, L.A. Zanoni, Appl. Phys. Lett. 13 (1968) 91–92. [10] M. Okutun, S.E. San, O. Koysal, Dyes Pigm. 65 (2005) 169–174. [11] M. Gu, Y. Yin, S.V. Shiyanovskii, O.D. Lavrentovich, Phys. Rev. E 76 (2007) 061702-1–067702-12.
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[12] S. Murakami, H. Naito, Jpn. J. Appl. Phys. 36 (1997) 2222–2225. [13] K.T. Schell, R.S. Porter, Mol. Cryst. Liq. Cryst. 188 (1990) 97–103. [14] S. Urban, A. Wurflinger, in: I. Prigogine, S.A. Rice (Eds.), Advances in Chemical Physics, vol. 98, John Wiley and Sons, 1997, pp. 143–216. [15] A. Ferrarini, Mol. Cryst. Liq. Cryst. 395 (2003) 233–252. [16] B.A. Belyaev, N.A. Drokin, C.F. Shabanov, V.A. Baranova, Phys. Solid State 46 (2004) 574–578. [17] B.A. Belyaev, N.A. Drokin, V.F. Shabanov, V.N. Shepov, Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. A 366 (2001) 305–312. [18] S.A. Rozanski, G.P. Sinha, J. Thoen, Liq. Cryst. 33 (2006) 833–840. [19] R. Manohar, S.P. Yadav, A.K. Srivastava, A.K. Misra, K.K. Pandey, P.K. Sharma, A.C. Pandey, Jpn. J. Appl. Phys. 48 (2009) 101501-1–101501-6. [20] A.K. Srivastava, A.K. Misra, P.B. Chand, R. Manohar, J.P. Shukla, Phys. Lett. A 371 (2007) 490–498. [21] A.K. Misra, A.K. Srivastava, R. Manohar, J.P. Shukla, Phys. Scr. 78 (2008) 065602-1–065602-7. [22] S. Gauza, M. Jiao, S.T. Wu, P. Kula, R. Dabrowski, X. Liang, Liq. Cryst. 35 (12) (2008) 1401–1408. [23] M. Gu, I.I. Smalykuh, O.D. Lavrentovich, Appl. Phys. Lett. 88 (2006) 061110-1– 061110-3. [24] A.V. Ivanshchenko, Dichoric Dyes for Liquid Crystal Displays, CRC Press, Florida, 1994. [25] I.C. Khoo, Liquid Crystals, Wiley, New York, 2007. [26] Z. Ran, H. Jun, P. Zeng-Hui, X. Li, Chin. Phys. B 18 (7) (2009) 2885–2892. [27] M. Hang, S. Rui-Zhi, L. Zhen-Xin, L. Yu-Fang, Chin. Phys. B 17 (1) (2008) 0255–0258. [28] S.W. Kang, L.C. Chien, Macromol. Res. 15 (5) (2007) 396–402.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Sign inversion of dielectric anisotropy in nematic liquid crystal by dye doping Rajiv Manohar n, Kamal K. Pandey, Abhishek K. Srivastava, Abhishek K. Misra, Satya P. Yadav Physics Department, University of Lucknow, Lucknow 226007, India
a r t i c l e in fo
abstract
Article history: Received 25 November 2009 Received in revised form 12 April 2010 Accepted 19 May 2010
The present paper reports sign inversion in dielectric anisotropy of a nematic liquid crystal, i.e. 5CB, as an effect of doping dye (solvent green 3) in small amount. It is the result of strong variation of the parallel component of dielectric permittivity with temperature for a dye doped sample. This behavior is attributed to the interaction taking place between the nematic liquid crystal molecule and the dye molecule. This behavior of dielectric anisotropy has been explained on the basis of interaction between the dye (guest) and the liquid crystal molecules (host). & 2010 Elsevier Ltd. All rights reserved.
Keywords: D. Dielectric properties A. Non-crystalline materials
1. Introduction Liquid crystals have been studied for several decades not only because of their technological importance but also because of their extraordinary physical properties such as dielectric and optical anisotropy, flow properties, response to external fields and ability to transmit static torque [1–3]. Quantitative knowledge of orientational ordering in liquid crystal is necessary for the development of improved materials for application. Thus, attempts are being continuously made to study the material properties of both pure compounds as well as their mixtures for a better insight into the basic understanding of the liquid crystalline behavior. Most applications of liquid crystals depend upon the possibilities of changing orientation of the liquid crystal molecules by the applied electric fields, which in turn depends on the visco-elastic and electrical properties of such materials [4]. The most important electrical property of the liquid crystals that affects its switching behavior is the dielectric anisotropy (De). Anisotropy of all the physical properties of liquid crystals mainly arises due to their long-range molecular ordering. The study of the anisotropy of the electrical and optical properties of nematic liquid crystals is not only the basis of their application in many electro-optical devices [5–7] but also very important from the molecular physics point of view. The study of dielectric anisotropy is important and it may provide significant information about the arrangement of molecules within the system. The dielectric anisotropy of liquid crystals is manifested as a difference in the value of electric permittivity measured along the parallel direction to the long molecular axis e99 and perpendicular n
Corresponding author. Tel.: + 91 9451 000 687. E-mail address: [email protected] (R. Manohar).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.05.011
to the long molecular axis e?. The value and the sign of dielectric anisotropy (De ¼ e99 e? ) depend on the anisotropy in anisotropic distribution of the molecular dipoles in liquid crystal phases or, more precisely, on the distribution of dipole moments of a polar group with respect to the long axis of the molecule, i.e. director [8]. The sign of dielectric anisotropy may be positive or negative, varying from sample to sample. The value of De can also be significantly altered by the presence of guest molecule. This guest molecule may be a dye, a polymer or a nanoparticle. This phenomenon is called the guest–host interaction [9]. In the present paper we report the sign inversion of De of a standard nematic liquid crystal 5CB on the addition of small traces of dye molecules. For this purpose temperature dependency of e99, e? and De has been investigated. Variation of dielectric anisotropy with concentration of dye in the nematic phases has been explained. Sign inversion of dielectric anisotropy with frequency and temperature has been investigated and reported by various groups [10,11] but inversion due to the presence of guest entity, i.e. dye, is not reported in the literature.
2. Experimental details 2.1. Material The investigated system consists of a room temperature nematic liquid crystal material 5CB having high positive dielectric anisotropy and chemical stability. Such features make 5CB (commercial, Merck) very interesting for fundamental investigations on nematic liquid crystals [12–18]. The dye used in our investigation is solvent green 3 (SG3) purchased from Across Chemicals. To study the guest–host system, we used 5CB doped at a dye concentration of about 1% wt/wt ratio. The doped mixture
1312
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
3. Result and discussion
C 5 H 11
CN
4′-n-Pentyl-4-cyanobiphenyl (5CB) Cr-----(22.5°C)------ nematic ---(35.3°C)-------isotropic
O
HN
Addition of guest molecule in a pure LC matrix produces changes in the intermolecular interaction field and may cause phase separation or complete phase loss in pure LC. Therefore optical transmittance and response time measurements were performed under crossed polarized condition [19]. To analyze the phase behavior, the optical transmittance for planar aligned cell has been plotted against the applied electric field at 331C in Fig. 2. From Fig. 2, it is clear that the Freedricksz transition appears in both pure and doped mixtures. That means nematic mesophase exists in doped LC, but nematic–isotropic phase transition temperature is reduced almost by 11C for the guest–host system. The dielectric response of liquid crystals shows complex behavior as functions of both frequency and temperature. Dielectric data have been analyzed by the Cole–Cole equation as follows [7,19–21]:
e ¼ e0 ð1Þ þ O
HN
Solvent green 3 (Mol. Wt. 418) Fig. 1. Molecular structure of liquid crystal and dye used for study: (a) 5CB nematic with transition scheme and (b) solvent green 3 dye.
was prepared by dispersion of dye in the nematic liquid crystal. The transition temperatures of the pure compound, their structural formula and the chemical names are shown in Fig. 1.
de0
ð1Þ
1 þ ðiotÞ1a
where de0 is the dielectric strength, e0 (N) the high frequency limit of dielectric permittivity, f the frequency, t the relaxation time and a the distribution parameter. Errors of experimental data in low and high frequency range due to one types of sample holder are quite common; therefore experimental results require low and high frequency correction. On separating the real and imaginary part of Eq. (1) one may get, after adding high and low frequency correction parameters [7,19–21]
e0 ¼ e0 ðdcÞf n þ e0 ð1Þ þ
d e0 ½1 þ ð2p f tÞð1aÞ sinðap=2Þ 1þ ð2p f tÞ2ð1aÞ þ 2ð2p f tÞð1aÞ sinðap=2Þ ð2Þ
Sandwiched type cells were prepared for the measurement of dielectric permittivity using transparent and highly conducting ITO (Diamonds Coating UK) coated glass substrate. ITO coated glass plates have a sheet resistance of 10 O/&. Both electrodes of the cell were treated with adhesion promoter and polymer (nylon 6/6; Sigma Aldrich) and rubbed unidirectionally to get planar aligned cell. For homeotropically aligned cell lecithin (Sigma Aldrich) was used as the coating material. Thickness of the cell was maintained at 10 mm with the help of Mylar spcer (Liquid Crystal Technologies, USA). The cell was calibrated using AR grade carbontetra-chloride (CCl4) and benzene. The material was introduced into the cell by capillary action at a temperature 10 1C above the isotropic temperature of the sample. 2.3. Dielectric permittivity study The dielectric response of the sample was studied in the frequency range 100 Hz–10 MHz using a computer controlled impedance/gain phase analyzer (HP-4194A). The temperature of the sample holder was achieved using an Instec hot plate (HCS-302) with an accuracy of 70.11C. The sample was left for 10 min before starting the measurement at a particular temperature. Dielectric permittivity is very sensitive to impurities as reported by Murakami and Naito [12]. They report that dielectric permittivity of 5CB exhibits a slow but appreciable drift (within a month) probably due to the injection of impurities from the contacting media (glue, polyimide layers, etc.). Therefore a complete set of measurement was made using a fresh sample and maintaining the total measurement time within a 3-day period, which is much smaller than the degradation time observed by Murakami and Naito [12].
and
e00 ¼
sðdcÞ de0 ð2pf tÞð1aÞ cosðap=2Þ þAf m þ e0 2pf k 1 þ ð2pf tÞ2ð1aÞ þ 2ð2pf tÞð1aÞ sinðap=2Þ
where s(dc) is the ionic conductance, e0 the free space permittivity, f the frequency of relaxation and k the fitting parameter. The terms e0 (dc)/fn and s(dc)/2peofk are added in the above equation for low frequency effect due to electrode polarization, capacitance and ionic conductance. The term Afm is added in the equation for high frequency effect due to ITO resistance and lead inductance of the cell. By the least square
% OPTICAL TRANSMITTANCE (Normalized)
2.2. Cell preparation
5CB PURE 5CB+1% SG3
100
80
60
40
20
0 0
5 10 15 APPLIED VOLTAGE (VOLTS)
20
Fig. 2. Optical transmittance dependence on applied electric field.
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
12
10 1
0
-1 24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 5. Variation of dielectric anisotropy of pure and dye doped samples as a function of temperature.
5CB+1% SG3
18
5CB+1% SG3 5CB PURE
Δε
fitting of experimental data in the above equation the low and high frequency data have been corrected. In order to see the effect of the dye on the dielectric properties of the liquid crystal, e99, e? and De have been measured for the pure and 1% dye doped sample. The results are shown in Fig. 3, Fig. 4 and Fig. 5. All the graphs have been plotted at the frequency of 10 kHz. Fig. 6 shows the change of molecular arrangement with the addition of dye. Fig. 7 shows the variation of dielectric anisotropy with frequency. The temperature range has been taken as 25–351C. Fig. 3 shows variation of e99 with temperature for both pure and dye doped samples. Dielectric permittivity of the guest–host system decreases almost 3 times than that for pure 5CB for a particular temperature. As the temperature of the sample increases, pure 5CB shows a fall in the value of e99 while the dye doped sample shows a slight increase. Fig. 4 shows variation of e? with temperature for both pure and dye doped samples. A slight
1313
5CB PURE
ε//
16
8
6
24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 3. Parallel component of dielectric permittivity (e99) of pure and dye doped sample as a function of temperature.
6.1
5CB+1% SG3 5CB PURE
6.0
ε⊥
5.9
5.8
5.7
5.6 24
25
26
27
28 29 30 31 Temperature (°C)
32
33
34
35
Fig. 4. Perpendicular component of dielectric permittivity (e?) of pure and dye doped samples as a function of temperature.
increase has been observed for the 1% dye doped sample in comparison with the dielectric permittivity value of pure 5CB. Fig. 5 shows variation of De for both pure and dye doped samples with temperature. Decrease in dielectric anisotropy of the dyed sample is observed in comparison with pure 5CB. An interesting result has been observed for dielectric anisotropy for the 1% dye doped sample. Dielectric permittivity e99 decreases for the 1% dye doped sample drastically, while the perpendicular component e? does not show much effect of dye concentration on itself. This leads to change of e99/e? 41 to e99/e? o1 for the dye doped sample. Sign inversion for dielectric anisotropy has been observed in the case of 1% dye doped sample. These negative anisotropy materials are of prime importance in the flat-panel display industry, forming the basis of ‘‘patterned vertical alignment’’ technology [22,23]. The above results can be explained on the basis of interactions taking place between the dye and 5CB molecules. When a rod shaped dye molecule is used to dope in pure nematic LC, the dye molecules try to fit in the sample geometry and follow intrinsic geometry constraints. This type of behavior for the rod shaped dye molecule in LC matrix has already been reported by many researchers in recent years [24,25]. The dye used in the present investigation is not rod shaped. Therefore, presence of this dye in pure matrix disturbs the pure LC molecular orientation around the dye molecules, as shown in Fig. 6. This random orientation of LC molecules around the dye molecule results in decrease in orientation order. Usually the contribution of dipole moment in parallel and perpendicular directions for the LC molecule making an angle b with long molecular axis is given by ml ¼ m cos b and mt ¼ m sin b. In the present case the randomness in the pure LC matrix, around the dye molecules, causes change in net dipole moment, i.e. decrease in e99 and increase in e? . In addition to this at higher temperature the interaction between dye and LC molecules reduces; consequently, the randomness in LC molecular orientation will also reduce at higher temperatures. Therefore, at higher temperatures small increment in e99 has been observed for the dye doped system. The effect of dye on e? at higher temperatures is lesser too but it is insignificant. The permanent and the induced dipole moments may be combined in an analytical expression for the dielectric anisotropy as follows [7,26,27]:
De ¼ eJ e? ¼
N0 hF
e0
Da
F m2 ð13cos2 bÞ S 2KT
ð3Þ
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R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
LC MOLECULE
DYE MOLECULE
Fig. 6. Change of orientation of nematic liquid crystal molecule with the addition of dye in a pure liquid crystalline system.
5CB+1% SG3 5CB PURE
Δε
10
0
1000
10000
100000
1000000
1E7
Frequency (Log) Fig. 7. Variation of dielectric anisotropy of pure and dye doped samples as a function of frequency.
Here F and h are constants of proportionality called the reaction field factor and cavity factor, respectively. Density r of the materials enters the expression in the particle number N. It is obvious that both S and T enter the expression, leading to a direct proportionality of De with the order parameter S. It is obvious from the above equation that De strongly depends on b, S and density of the material. The presence of guest molecules in pure LC matrix results in disturbance in the pure LC matrix. This disorder also affects the order parameter. The order parameter for pure LC is 0.67 but after adding dye it becomes 0.64. The change in the value of order parameter is not very prominent. Therefore, this small change in S cannot be responsible for the sign reversal in De. We are surprised from the above experimental findings that dielectric anisotropy and threshold voltage decrease simultaneously. Usually both of them cannot decrease together [7,8]. In our view, the presence of the dye molecules in the liquid crystal medium induces disorder around the dye molecules. This disorder becomes more prominent if the molecular structure of the dye is other than the rod shape as in the present case. The dye molecule disturbs the LC geometry in such a manner that it induces a small tilt in the medium. This induced tilt in the LC system is more prominent near the dye molecules and spreads throughout the
doped LC system. Due to this tilt, the energy required for the molecular movement decreases. This decrease in required energy for molecular movements and thick cell may be one of the reasons for the decrease in threshold voltage for the dye doped LC. The induced tilt also affect the response time of the system. The optical response of the dye doped LC reduces as compared to that of LC. This reduction in optical response time confirms that induced tilt is dispersed throughout the dye doped LC suspension. Sign inversion is also observed with variation of frequency. This result has been plotted in Fig. 7. At the critical frequency both pure 5CB and the dye doped sample show sign inversion of dielectric anisotropy. In this case, as a result of low frequency dispersion in e99 at a certain frequency fc a change in sign of dielectric anisotropy of the nematic liquid crystal occurs. The dual frequency phenomenon observed in special nematic liquid crystals arises from molecular dielectric relaxation, having a relaxation frequency of a few decades of kilohertz, rotation around their short axes. The sign of dielectric anisotropy as a function of frequency changes near the relaxation frequency [28]. Therefore, by changing the frequency of the applied field, the direction of the director can be controlled by coupling torque of the applied field with dielectric anisotropy in both rising and falling signals.
4. Conclusions In this paper we have reported the dielectric anisotropic behavior of the dye doped 5CB (doped with solvent green 3). Dielectric properties of the nematic liquid crystal 5CB doped with dye (1% solvent green 3) have been measured in the temperature range 25–351C at 10kHz frequency. We have observed that dielectric anisotropy changes its sign when 1% of dye, solvent green 3, is introduced into the liquid crystals matrix. Director alignment of the nematic liquid crystal is found to be sensitive to the presence of the guest molecule. Here the interaction taking place between the liquid crystal molecule and the dye molecule alters director alignment of the nematic liquid crystal sample. We can say that molecular alignment of the liquid crystal molecule in dyed samples is controlled by the guest molecule to some extent while in the pure sample it is controlled only by confining walls of the electrodes. This feature opens up a new field of application such as developing a liquid crystal–solid particles interface that makes the role of the liquid crystal surface layers important in the determination of material properties.
R. Manohar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1311–1315
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