Liquid Crystals

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Experimental studies on the phase diagram andphysical properties of mixture of calamitic anddiscotic nematic liquid crystals
D. Venkata Sai, Giorgio Mirri, Igor Musevic & Surajit Dhara
To cite this article: D. Venkata Sai, Giorgio Mirri, Igor Musevic & Surajit Dhara (2016)
Experimental studies on the phase diagram and physical properties of mixture ofcalamitic and discotic nematic liquid crystals, Liquid Crystals, 43:12, 1884-1888, DOI:
10.1080/02678292.2016.1199818
To link to this article: http://dx.doi.org/10.1080/02678292.2016.1199818

Published online: 27 Jun 2016.

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Date: 17 January 2017, At: 20:02

LIQUID CRYSTALS, 2016VOL. 43, NO. 12, 1884–1888
http://dx.doi.org/10.1080/02678292.2016.1199818

Experimental studies on the phase diagram and physical properties of mixtureof calamitic and discotic nematic liquid crystals
D. Venkata Saia, Giorgio Mirrib, Igor Musevicb and Surajit DharaaaSchool of Physics, University of Hyderabad, Hyderabad, India; bSoft Matter Laboratory, Department of Condensed Matter Physics, Jozef
Stefan Institute, Ljubljana, Slovenia

ABSTRACTARTICLE HISTORY

Physical studies on mixture of calamitic and discotic nematic liquid crystals are meagre although
they are potential for optimising physical properties. Here, we report experimental studies on thephase diagram and physical properties of mixtures of ambient temperature discotic and calamitic
nematic liquid crystals. A substantial decrease in several physical properties such as birefringence,dielectric anisotropy and elastic constants are observed with increasing wt% of discotic compound. On the other hand a large increase in the rotational viscosity is observed. Based on theexperimental results a simple model of mutual orientation of the rod-like and disc-like molecules
is proposed.

Received 18 May 2016
Accepted 7 June 2016

1. Introduction
Liquid crystals (LCs) used in display devices are mostly
the calamitic nematic, which are composed of rodshaped molecules. For a desired electro-optic response,
the physical properties are required to be optimised,
and that is usually achieved in the mixture of several
single component liquid crystals. Here we concentrateon the studies of mixture of a calamitic and a discoticliquid crystals, which are interesting from several
aspects. First, they have the opposite sign of the birefringence and dielectric anisotropy, which makes it
possible to tune the electro-optic response and
CONTACT Surajit Dhara

sdsp@uohyd.ernet.in© 2016 Informa UK Limited, trading as Taylor & Francis GroupKEYWORDS

Calamitic and discotic liquid
crystals; birefringence;dielectric; viscoelastic
properties

elasticity. Second, there may be a region of immiscibility of rod-like and disk-like nematics, resulting in aliquid dispersion of one phase in another. This should
result in a dispersion of microdroplets of one liquid
crystal phase in another liquid crystal phase, which
could be of great interest for applications in topologicalmicro-photonics [1] and for the fundamental studies of
topology in liquid crystals. In such a liquid crystal–liquid crystal dispersion, individual nematic droplets
might function as tunable optical microcavities [2,3]and microlasers [4], which could be elastically boundby topological defects [5,6] of the other nematic carrier
fluid (continuous phase). Third, immiscible liquid

LIQUID CRYSTALS

crystalline phases could provide a novel testbed forstudying topological defects in both phases, as well as
their interrelation across the interfaces. Understanding
and tailoring of mixtures of rod-like and disc-likenematic materials is therefore not only of high interest,
but could also lead to new research directions.
There are a very few reports on the phase diagramand physical properties of mixture of calamitic anddiscotic nematic liquid crystals. For example, Pratibha
et al. experimentally studied the phase diagram anddefect structure in the binary mixture of a calamitic
and a discotic nematic LCs [7,8]. Apreutesei et al.
experimentally showed a complete miscibility of disclike and rod-like compounds [9]. Andrzej et al. theoretically studied the elastic constants in the binary
mixture and reported a discontinuous change of elastic
anisotropy from disc-like to rod-like properties [10]. A
possibility of getting biaxial nematic and smectic
phases in such binary systems was discussed by G J
Vroege [11]. Recently, Parthasarathi et al. measured
several physical properties of binary mixtures of acalamitic and a columnar discotic liquid crystals [12].
They also observed an anchoring transition driven byshort-range ordering in the calamitic-discotic composites. In the above mentioned reports (experimental),
the compounds studied exhibit nematic phase above
the ambient temperature. To the best of our knowledge, there are no reports on the phase behaviour andphysical studies on the mixture of ambient temperature
discotic and calamitic nematic LCs. In this paper, we
report experimental studies on the phase diagram, andphysical properties such as birefringence (Δn),
dielectric anisotropy (Δ), curvature elastic constants(K11 & K33) and rotational viscosity (γ1) of the mixtureof a calamitic and a room temperature discotic nematic
liquid crystals.

2. Experimental
The experimental cells were made of two indium–tin–oxide (ITO)-coated glass plates with circularly patterned electrodes. These plates were spin coated withpolymide AL-1254 and cured at 180°C for 1 hour andrubbed antiparallel way for homogeneous alignment of
the sample. Cells were made by placing two plates
together such that the active electrode area overlaps.
The cell gap was controlled by glass-bead spacers of
average diameter of 8 μm. The thickness of the empty
cell was measured within ± 1% accuracy by an interferometric technique using a spectrometer (Ocean
Optics, HR-4000). The empty cell was heated and filledwith the sample in the isotropic phase. The phasetransition of the samples was observed using a1885

polarising optical microscope (OLYMPUS BX51) anda temperature controller (Instec, mK 1000). The temperature-dependent birefringence of a planar cell wasmeasured by using a phase modulation technique [13]with the help of a Helium–Neon laser, a photoelastic
modulator (PEM-100) and a lock-in amplifier. The
perpendicular component of the static dielectric constant (? ) was measured in a planar cell and the parallel
component of the dielectric constant (k ) was measured
by applying a sinusoidal voltage with a frequency of
10 kHz from 0.02 to 20 V in steps of 0.02 V using a
LCR meter (Agilent E4980A). The linear part of thedielectric constant is plotted against 1/V and extrapolated to 0 (1/V = 0) to obtain  k at various temperatures. The splay elastic constant (K11) was measured
directly from the Freedericksz threshold voltage (Vth)of the voltage dependent retardation data using therelation, K11 ¼ 0 Δ  ðVth =πÞ2 , where Δ ¼  k  ? , isthe dielectric anisotropy. The bend elastic constant(K33) was obtained from the fitting of voltage dependent optical retardation with theoretical variation[14–17]. The rotational viscosity (γ1) was measured ina planar cell by a phase-decay-time measurement technique [18,19].

3. Results and discussion
The sample E-18 (a room temperature calamitic nematic
mixture) exhibits the following phase transitions: Cr. 10°
C N 60°C I. The discotic nematic was synthesised in ourlaboratory (Figure 1(a)) and it has the following phasetransitions: N 83°C I. It also exhibits a glass transition
much below the room temperature [20,21]. We prepared
three different mixtures with increasing concentration of
discotic compound, namely; E-18+disc(2.5 wt%), E-18
+disc(5 wt%), E-18+disc(7.5 wt%). Physical appearanceof the mixtures is shown in Figure 1(b) and the phase
diagram is shown in Figure 1(c). From turbid and white,
it becomes brownish in colour with increasing wt% of
discotic compound. In pure E-18, there exists an isotropic–nematic coexistence (I + N) range of approximately
3°C and this range increases with increasing wt% of
discotic compound. In case of mixture with 7.5 wt%
discotic compound, while cooling, the I + N coexistence
range exists nearly up to 30°C. Below this temperature, it
exhibits a monodomain (no coexistence) nematic sample.
During heating, the coexistence reappears at 43°C. Thus,
larger temperature range of monodomain nematic is
obtained during heating. Hence we performed all themeasurements while heating and the physical measurements in the I + N coexistence region are excluded.
The temperature variation of birefringence (Δn) of
E-18 and various mixtures is shown in Figure 2(a). The

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D. V. SAI ET AL.

Figure 1. (colour online) (a) Chemical structure of the disc-like molecule. (b) Physical appearance of the samples with increasing wt% of the discotic compound in E-18. (c) Phase diagram of the mixtures along with a few representative photomicographs. The
transition temperatures are measured while heating the samples from the room temperature.

Figure 2. (colour online) Variation of (a) birefringence (Δn) and (b) dielectric anisotropy (Δ) as a function of shifted temperature.
Continuous lines are drawn as a guide to the eye. The shaded region indicates the nematic–isotropic (N + I) coexistence range. The
downward arrows indicate the decrease of the respective quantities with increasing wt% of the discotic compound.

birefringence of E-18 is large and positive whereas indiscotic nematic it is large but negative. For example, atroom temperature, in E-18, Δn ’ 0:21 and this is consistent with the previous measurement [22]. In purediscotic nematic, at room temperature, Δn is negativeand large ( ’ 0:2) [23]. In the mixtures, Δn decreases
with increasing wt% of discotic compound. For example,
at a fixed temperature (e.g. T  TNI = −26°C), Δn of the
mixture (E-18 + 7.5 wt%) decreases by 13% compared tothe pure E-18. This suggests that the mutual orientationof the rod and disc-like molecules is such that thepolarisability anisotropies are antithetically related.
The variation of dielectric anisotropy (Δ) of various
mixtures as a function of shifted temperature is shownin Figure 2(b). In pure E-18, Δ is positive and relatively
large whereas in discotic nematic it is small and negative[23]. For example, at room temperature in E-18, Δε ¼
13:5 and in pure discotic nematic, Δ ¼ 0:18 [23]. It isobserved that Δε decreases with increasing wt% of discotic compound. For example, at a fixed temperature(T  TNI = −30.5°C), Δ of the mixture, E-18 + 7.5 wt%
decreases by 12% compared to the pure E-18. ThoughΔ of pure discotic compound is very small and negative, it has a significant contribution in reducing thedielectric anisotropy of the mixtures.
The optical phase difference (Δϕ) of the samples wasmeasured as a function of applied voltage to measure
splay (K11) and bend elastic constants (K33)

LIQUID CRYSTALS

simultaneously. The temperature variation of K11 andK33 is shown in Figure 3. K33 is always greater than K11
and both the elastic constants decrease with increasing
wt% of discotic compound. At room temperature,(T  TNI = −30°C), K11 and K33 decreases by 25% and30%, respectively. According to the mean-field theory,
Kii / S2 / Δn2 . Since Δn decreases by about 13% (seeFigure 2(a)), the decrease of the elastic constants is
consistent with the prediction of the mean-field theory.
However, it may be noted that the relative decrease of
K33 is slightly larger (about 5%) than K11. This is due tothe fact, that in pure discotic nematic, the elastic anisotropy is negative i.e. K33 is less than K11 [23], and thisis opposite to the behaviour observed in pure E-18.
We also measured the rotational viscosity (γ1) of the
mixtures with temperature (see Figure 4). γ1 increases
with increasing wt% of the discotic compound. For
example, near the room temperature (T  TNI =−30°C), γ1 of the mixture, E-18 + 7.5 wt% is 73% larger
than that of the pure E-18. The rotational viscosity isgiven by; γ1 ¼ τ o K11 π2 =d2 , where d is the sample thickness and τ 0 is the relaxation time. In the mixtures, γ1
increases significantly in spite of the decrease of K11. So
the increase of γ1 is mainly due to the increase ofrelaxation time of the system. This is expected as therelaxation time of the pure discotic nematic is verylarge [23]. The birefringence, dielectric anisotropy
and the elastic properties clearly suggest that theplane of the disc molecules is parallel to the director
as shown schematically in Figure 5.

1887

Figure 4. (colour online) Variation of rotational viscosity (γ1) of
the mixtures as a function of temperature. The upward arrow
indicates the increase of γ1 with increasing wt% of the discotic
compound.

Figure 5. (colour online) Schematic representation of mutualorientation of rod-like and disc-like molecules in the nematic
phase.

4. Conclusion

Figure 3. (colour online) Variation of splay (K11) and bend (K33)
elastic constants of the mixtures as a function of shifted temperature. Continuous lines are drawn as a guide to the eye. The
downward arrows indicate the decrease of the respective
quantities with increasing wt% of the discotic compound.

In conclusion, we have determined the phase diagram,and measured several physical properties such as birefringence, dielectric anisotropy, curvature elastic constants and the rotational viscosity of mixture of E-18
and a discotic nematic liquid crystals. We find rather
broad temperature range of coexistence of the isotropic
and nematic phases. The temperature range ofnematic–isotropic coexistence increases with increasing
wt% of discotic nematic compound. At room temperature, with a small (7.5 wt%) addition of discotic compound, the birefringence and dielectric anisotropy
decreases by about 13%. The splay and bend elastic

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D. V. SAI ET AL.

constants decrease by 25% and 30%, respectively. On
the other hand rotational viscosity increases by about
73%. We have shown that the physical properties of thecalamitic nematic liquid crystals can be changed significantly by adding a small amount of discotic nematiccompound. This is therefore a very efficient method oftuning the electro-optic parameters of nematic mixtures. The experimental results suggest that the planesof the disc-like molecules are oriented parallel to thenematic director. Future studies should focus on thehigh concentration range of discotic component in thephase diagram (Figure 1), where a coexistence regionof two immiscible nematic phases was observed inpreliminary experiments above 10 wt% of discotic
component.

Acknowledgments
We gratefully acknowledge support from the DST (SR/NM/
NS-134/2010), CSIR (03(1207)/12/EMR-II), DST-PURSE
and UPE-II. Venkata Sai. D acknowledges UGC for fellowship. G. M. acknowledges the European Commission for a
Marie Curie Intra European Fellowship (Nemcode IEF-2012331350), G. M., I. M. and P. K. acknowledge a Marie Curie
ITN (Hierarchy ITN-2007-215851) both the projects were
funded within the 7th European Community Framework
Program.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the Department of Science andTechnology, Ministry of Science and Technology [DSTpurse].

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