Journal of Molecular Liquids 312 (2020) 113410

Contents lists available at ScienceDirect

Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq

Studies on binary mixtures of nematic liquid crystals made of strongly
polar molecules with identical cores and antagonistic orientation of
permanent dipoles
Dasari Venkata Sai a,b, Tae Hoon Yoon a,⁎, Surajit Dhara b,⁎ab

Department of Electronics Engineering, Pusan National University, Busan 609-735, Republic of Korea
School of Physics, University of Hyderabad, Hyderabad 500046, Indiaa r t i c l ei n f oArticle history:
Received 9 April 2020
Received in revised form 15 May 2020
Accepted 20 May 2020
Available online 01 June 2020a b s t r a c t
We report experimental studies on optical (birefringence, Δn), dielectric (Δε) and bend-splay elastic anisotropies(ΔK = K33 − K11) of a few mixtures of two nematic liquid crystals, namely CCH-7 and CCN-47, made of highlypolar molecules with identical cores and antagonistic orientation of permanent dipoles. In particular, the polargroup (-CN) attached to the bicyclohexane core of CCH-7 is oriented along the longitudinal direction whereas,in CCN-47, it is oriented along the transverse direction. We show that apart from the signiﬁcant contribution
to the optical and dielectric anisotropies, the antagonistic orientation of strongly polar groups plays a crucialrole in determining the bend-splay elastic anisotropy. The elastic properties are explained based on a model proposed by Priest, considering the effect of intermolecular association and the resulting length-to-width ratio of themolecules.
© 2018 Elsevier B.V. All rights reserved.

1. Introduction
The electrooptical response of liquid crystals (LCs) mostly depends
on the birefringence, dielectric, elastic properties, and the rotational viscosity [1–4]. Depending upon the applications, a set of desired physicalproperties is required. Since all the desired properties are unavailable inany single compound, usually several liquid crystals are mixed in appropriate proportions to tune the physical properties for commercial devices. Further, studies on liquid crystal mixtures with a variety ofshapes and structures have always proven to be rewarding from boththe technological and fundamental perspectives. Several new phaseshave been discovered, which are absent in the parent compounds[5–7]. In the context of binary mixtures made of symmetric and highly
asymmetric molecules, such as mixtures of rod-like and bent-shaped
molecules have resulted in several new physical properties [8–13].
However, physical studies on the binary mixtures of nematic liquid
crystals with identical core structures and highly antagonistic dipole
orientation are meagre. High dipole moments of the molecules are
known to make an important contribution to the intermolecular interactions and give rise to distinct physical properties of liquid crystals[14–18].
In this paper, we report experimental studies on the binary mixturesof two low molecular weight nematic liquid crystals, namely CCH-7 and⁎ Corresponding authors.
E-mail addresses: thyoon@pusan.ac.kr (T.H. Yoon), sdsp@uohyd.ernet.in (S. Dhara).

https://doi.org/10.1016/j.molliq.2020.113410
0167-7322/© 2018 Elsevier B.V. All rights reserved.

CCN-47, having identical (bicyclohexane) cores and antagonistic dipole
orientation with respect to their long axes (see Fig. 1). In particular, thedirection of permanent dipole moment is oriented parallel and perpendicular to the long axis in case of CCH-7 and CCN-47, respectively[19,20]. We have prepared a few mixtures with varying composition
and measured physical properties such as birefringence, dielectric andelastic anisotropies as a function of temperature. We show that the orientation of the polar group (-CN) not only contributes to the optical anddielectric anisotropies, but it also signiﬁcantly affects the bend-splay
elastic anisotropy.
2. Experimental
The experimental cells are made of two parallel indium-tin-oxidecoated glass plates with circularly patterned electrodes. To obtain theplanar alignment of the director (the mean molecular orientation), theplates are coated with polyimide AL-1254 and cured at 180 °C for 1 h.
The polyimide coated plates are rubbed in an antiparallel way for homogeneous or planar alignment of the director. For homeotropic alignment, the plates are coated with polyimide JALS-204 and cured at200 °C for 1 h. The typical cell thickness used in the experiments isabout 8 μm. The phase transition temperatures of the samples are obtained using a polarising optical microscope (Nikon, LV100 POL) and atemperature controller (Instec, mK1000). The temperature-dependent
birefringence of the mixtures is measured by a polarisation modulation
technique, using a home-built electro-optic setup, involving a He-Ne

2D.V. Sai et al. / Journal of Molecular Liquids 312 (2020) 113410
Table 1
Phase transition temperatures of binary mixtures of CCN-47 and CCH-7. TNI: nematic toisotropic; TNS: N-SmA transition temperatures. ‘–’ indicates no N-SmA transition is observed in the experimental temperature range.

Sample-1
Sample-2
Sample-3
Sample-4Sample-5wt% CCN-47wt% CCH-7TNI (°C)

TNS (°C)

1007555
2300254577100

59.6
46.5
47.6
71.5
85.0

30.2
–
39.2
–
–of negative dielectric anisotropy materials are measured with an accuracy of 5% and 10%, respectively.
3. Results and discussion
Fig. 1. Chemical structures and the phase transition temperatures of the liquid crystals.
Red arrows indicate the direction of permanent dipole moment. (For interpretation of
the references to colour in this ﬁgure legend, the reader is referred to the web version ofthis article.)laser (λ=632.8nm), photo-elastic modulator (PEM) and a lock-in ampliﬁer [21–23]. A schematic diagram of the experimental setup isshown in Fig. 2. Planar cells are used for all the measurements on samples with positive dielectric anisotropy. We measured voltagedependent dielectric constant of the samples form 0.02 to 18 V insteps of 0.03 V, using an LCR meter (Agilent E4980A) at a frequency of4111 Hz. For samples with positive dielectric anisotropy the dielectric
constant below the Freedericksz's threshold voltage [24] is equal to ε⊥.
To measure ε‖, the linear part of dielectric constant at higher voltages
plotted against 1/V and extrapolated to 0 (V → ∞ ⇒ 1/V → 0). This experiment is repeated at different temperatures to get temperature dependent dielectric anisotropy, Δε. A computer controlled LabVIEW
program is used to control the experiments.
The same procedure is followed for the samples with negative dielectric anisotropy, but in homeotropic cells [19]. The splay and bendelastic constants of all the samples are measured following the procedure described in ref. [19, 25, 26]. The birefringence and dielectric data
are measured with an accuracy of 2%. The splay and bend elastic constants of positive dielectric anisotropy materials are measured with anaccuracy of 5% and 8%, respectively. The bend and splay elastic constants

The chemical structure of the pristine compounds is shown in Fig. 1and the phase transition temperatures of the compounds are presented
in Table 1. Apart from nematic, CCN-47 also exhibits a nematic tosmectic-A (N-SmA) phase transition at 30.2 °C (TNS). The molecules ofboth CCN-47 and CCH-7 have identical bicyclohexane cores. The polargroup (-CN) in CCN-47 is directed along the transverse direction and exhibits negative dielectric anisotropy, whereas in CCH-7, the -CN group isdirected along the longitudinal direction and exhibits positive dielectricanisotropy [19,27,28].
We prepared three mixtures with different wt% of these compounds
as shown in Table 1. Sample-1 and Sample-5 are pure CCN-47 and CCH7 compounds. All the mixtures show a nematic phase. Sample-3 shows
relatively a shorter temperature range of nematic (8.4 °C) and a nematic
to smectic-A phase transition. The birefringence (Δn) of the samples as afunction of shifted temperature is shown in Fig. 3(a). At a shifted temperature T − TNI = − 12∘C, the birefringence of pristine CCN-47 isΔn ≃ 0.027 and for pristine CCH-7, it is Δn ≃ 0.047 and agrees wellwith the previous reports [19,27,28]. At a ﬁxed shifted temperature,
Δn increases with increasing wt% of CCH-7 in the mixtures as expected.
Since the molecules have identical core structure, the difference in thebirefringence (i.e., ΔnCCH−7 − ΔnCCN−47 ≃ 0.02) arises mostly due tothe antagonistic orientation of the -CN groups. The variation of dielectricanisotropy (Δε) of the samples is shown as a function of shifted temperature in Fig. 3(b). The dielectric anisotropy of pristine CCH-7 is a large
positive (Δε ≃ 3.8), whereas for CCN-47, Δε is a large negative(Δε ≃ − 4.2) at T − TNI = − 12∘C. The core structures of the two molecules are identical, hence the large difference in Δε of the pristineFig. 2. Schematic diagram of the experimental setup. The orientations of the polariser, analyser, photoelastic modulator (PEM) axes and the director are shown by red arrows. (Forinterpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)D.V. Sai et al. / Journal of Molecular Liquids 312 (2020) 113410

3Fig. 3. Variation of (a) birefringence (Δn) and (b) dielectric anisotropy (Δε) of themixtures as a function of shifted temperature.

samples is mostly due to the difference in the orientation of the -CNgroups. This indicates that the antagonistic orientation of the -CNgroups in mixture tends to cancel out their contribution on the dielectricanisotropy. In fact, Δε of Sample-3, which is composed of nearly 55 wt%
of CCN-47 is very close to zero (Δε ≃ 0.03).
The variations of splay (K11) and bend (K33) elastic constants of thesamples as a function of shifted temperature are shown in Fig. 4(a,b).
Both K11 and K33 increase with decreasing temperature as they are proportional to the square of the orientational order parameter S [1]. In
pristine CCN-47 (Sample-1) both K11 and K33 tend to diverge near theroom temperature due to the short-range presmectic order effect[19,29]. Fig. 4(c) shows that in pristine CCN-47, the bend-splay elasticanisotropy (ΔK = K33 − K11) is negative whereas, in pristine CCH-7,
ΔK is positive. It changes sign between 25 wt% to 50 wt% of CCH-7.
Fig. 5(a) shows the ratio of the two elastic constants (K33/K11) of thesamples as a function of shifted temperature. It is observed that for
Sample-1 and Sample-2, K33/K11b1, whereas for Sample-3, Sample-4
and Sample-5, K33/K11N1. Fig. 5(b) shows that at a ﬁxed shifted temperature (e.g., T − TNI = − 8∘C) the ratio, K33/K11 increases with increasing
wt% of CCH-7.
The variation of K33/K11 can be qualitatively explained based on amolecular theory proposed by Priest [30], considering the effective
length-to-width ratio of the molecules. The ratio K33/K11 is related tothe molecular properties and given by [30]:

K 33 1 þ Δ þ 4Δ0 P 4 =P 2
¼K 111 þ Δ−3Δ0 P 4 =P 2

ð1ÞFig. 4. Variation of (a) splay (K11) and (b) bend (K33) elastic constants with shifted
temperature. (c) Variation of K33 and K11 with wt% of CCH-7 at a relative shifted
temperature T − TNI = − 8∘C.

where Δ = (2R2 − 2)/(7R2 + 20), Δ′=9(3R2 − 8)/16(7R2 + 20) andR = (L − D)/D. L and D are the length and width of the spherocylindricalmolecules. The calculated ratio of K33/K11 for various values of P 4 =P 2 isshown in Fig. 5(c). It is observed that the ratio K33/K11 increases or decreases depending on the sign of P 4 =P 2 . For positive values of P 4 =P 2 ,the ratio K33/K11 increase with L/D.
In the X-ray measurements of nematic phase of CCH-7, an antiparallel local ordering of the molecules was reported due to the antiparallelcorrelation of the longitudinal polar group (-CN) [20]. Bradshaw et al.,showed the antiparallel local ordering of the dipoles increases the4D.V. Sai et al. / Journal of Molecular Liquids 312 (2020) 113410

effect of increasing the molecular length. The best comparison to the experimental data with calculation is obtained with a length-to-width
ratio of L/D ≃ 7 at a value of the ratio P 4 =P 2 ¼ 0:5. In the X-ray diffraction
studies of CCH-7, the effective L/D was reported to be ≃6.5 [20]. This isvery close to the value at which our experimental result resembles
with the calculations (see Fig. 5(c)). It would be very useful to have experimental measurements of P 4 =P 2 of these samples. It may be mentioned that both the rotational and translational entropy of themolecules with longitudinal dipole moment is expected to get reduced
due to the antiparallel correlation. This effect should be absent in molecules with transverse dipole moment due to the lack of antiparallel correlation. As a result of which, the orientational order parameter andeventually the dielectric and elastic properties could differ in the respective systems. A detailed computer simulation may be useful for getting
quantitative information.
4. Conclusion
In conclusion, we have measured Δn, Δε and ΔK of pristine as well assome mixtures of CCH-7 and CCN-47 liquid crystals, possessing identicalcore structures and antagonistic dipole orientation. Both Δn and Δε ofthe mixtures decrease systematically with increasing wt% of CCN-47.
In particular, at 55 wt%, Δn is reduced by 33% of the pristine CCH-7 sample and Δε becomes almost zero. Since the core structures are identical,
the large change in the optical and dielectric anisotropies of the mixtures are mostly due to the antagonistic orientation of the dipolargroup (-CN). The bend-splay elastic anisotropy ΔK changes sign fromnegative to positive beyond ~37wt% of CCH-7. The analysis suggeststhat the antiparallel correlation of longitudinal dipoles and the resulting
molecular association, which is absent in pristine CCN-47 becomes signiﬁcant beyond 37 wt% of CCH-7 in the mixture. Thus, the orientation of
the polar group (-CN) with respect to the molecular axis inﬂuences theelastic properties signiﬁcantly, in addition to the optical and dielectricproperties.
CRediT authorship contribution statement
Dasari Venkata Sai: Data curation, Formal analysis, Investigation.
Tae Hoon Yoon: Project administration,Funding acquisition. Surajit
Dhara: Conceptualization, Supervision, Writing - review & editing.

Declaration of competing interest
We have no conﬂict of interests in connection with this publication.
Acknowledgments

Fig. 5. (a) Ratio K33/K11 (a) as a function of shifted temperature (b) as a function of wt% ofCCH-7 at a relative shifted temperature T − TNI = − 8∘C. (c) Variation of calculated K33/K11

This work was supported by National Research Foundation of Korea(NRF) grant funded by the Korean government (MSIT) (No.
2020R1A2C1006464). SD acknowledges support from DST-FIST-II,
School of Physics, University of Hyderabad.
References

effective length of CCH-7 molecules [31]. As a result of which the ratio
K33/K11 is greater than 1 (see Fig. 5(a)). In our previous studies, fromthe energy minimised DFT calculations, the shape of the CCN-47 molecule was found to be bent-shape [19]. Due to such a shape, the effective
length-to-width ratio could be relatively smaller. In addition, they do
not exhibit strong antiparallel correlation due to the transverse orientation of the polar group (-CN) [32]. This could result in, K33/K11b1 forpure CCN-47. As we increase the wt% of CCH-7, beyond about 25 wt%,
the antiparallel correlation of permanent longitudinal dipoles (-CN) ofCCH-7 develops, as a result of which the length-to-width ratio is increased. Thus, increasing wt% of CCH-7 in the mixture is an equivalent[1] P.G. de Gennes, The Physics of Liquid Crystals, 2nd ed. Clarendon, Oxford, 1993.
[2] S. Chandrasekhar, Liquid Crystals, Cambridge University Press, Cambridge, 1977.
[3] W.H. de Jeu, Physical Properties of Liquid Crystal Materials, Gordon and Breach, NewYork, 1980.
[4] L.M. Blinov, V.G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials, Springer,
New York, NY, 1994.
[5] P.A. Pramod, R. Pratibha, N.V. Madhusudana, Curr. Sci. 73 (1997) 9.
[6] S. Dhara, R. Pratibha, N.V. Madhusudana, Ferroelectrics 277 (2002) 13.
[7] R. Pratibha, N.V. Madhusudana, B.K. Sadashiva, Science 288 (2000) 2184.
[8] J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J.T. Gleeson, A. Jakli, Phys.
Rev. Lett. 97 (2006), 157802. .
[9] B. Kundu, R. Pratibha, N.V. Madhusudana, Phys. Rev. Lett. 99 (2007), 247802. .
[10] P. Sathyanarayana, B.K. Sadashiva, S. Dhara, Soft Matter 7 (2011) 8556.

D.V. Sai et al. / Journal of Molecular Liquids 312 (2020) 113410[11] P. Sathyanarayana, V.S.R. Jampani, M. Skarabot, I. Musevic, K.V. Le, H. Takezoe, S.
Dhara, Phys. Rev. E 85 (2012), 011702. .
[12] M.R. Dodge, C. Rosenblatt, R.G. Petschek, M.E. Neubert, M.E. Walsh, Phys. Rev. E 62(2000) 5056.
[13] M.R. Dodge, R.G. Petschek, C. Rosenblatt, M.E. Neubert, M.E. Walsh, Phys. Rev. E 68(2003), 031703. .
[14] N.V. Madhusudana, Braz. J. Phys. 24 (1998) 4.
[15] R.W. Sobha, D. Vijayaraghavan, N.V. Madhusudana, Europhys. Lett. 44 (1998) 296.
[16] J. Prost, Toner, Phys. Rev. A 36 (1987) 5008.
[17] P.E. Cladis, Phys. Rev. Lett. 35 (1975) 48.
[18] J. Ananthaiah, Rasmita Sahoo, M.V. Rasna, Surajit Dhara, Phys. Rev. E 89 (2014),022510. .
[19] D. Venkata Sai, K.P. Zuhail, R. Sarkar, S. Dhara, Liq. Cryst. 42 (2015) 328.
[20] G.J. Brownsey, A.J. Leadbetter, J. Physique Lett. 42 (1981) L–135.
[21] Oakberg, SPIE Proc. 3121 (1997) 19.

5[22] P. Sathyanaraya, M. Mathews, Q. Li, V.S.S. Sastry, H. Takezoe, Surajit Dhara, Phys.
Rev. E 81 (2010) 010702(R).
[23] P. Sathyanarayana, M.C. Varia, A.K. Prajapati, B. Kundu, V.S.S. Sastry, Surajit Dhara,Phys. Rev. E 82 (2010) 050701(R).
[24] V. Freedericksz, V. Zolina, Trans. Faraday Soc. 29 (1933) 919.
[25] H. Gruler, T.J. Scheffer, G. Meier, Z. Naturforsch. 27a (1972) 966.
[26] S.W. Morris, P. Palffy, D.A. Balzarini, Mol. Cryst. Liq. Cryst. 139 (1986) 263.
[27] I.H. Ibrahim, W. Haase, Mol. Cryst. Liq. Cryst. 66 (1981) 189.
[28] R. Pohl, R. Eidenschink, J. Krause, Phys. Lett. 65A (2) (1978) 169 022510.
[29] D.V. Sai, T.A. Kumar, W. Haase, A. Roy, S. Dhara, J. Chem. Phys. 141 (2014), 044706. .
[30] R.G. Priest, Phys. Rev. A 7 (1973) 720.
[31] M.J. Bradshaw, E.P. Raynes, Mol. Cryst. Liq. Cryst. 72 (1981) 35.
[32] M. Praveen Kumar, D. Venkata Sai, Surajit Dhara, Phys. Rev. E 98 (2018), 062701. .