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Alignment of unconventional nematic liquid crystals
a

b

b

b

T. Arun Kumar , K.V. Le , Jin Ki Kim , H. Takezoe & Surajit DharaaaSchool of Physics, University of Hyderabad, Hyderabad, Indiab

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Japan

Available online: 12 Jul 2011

To cite this article: T. Arun Kumar, K.V. Le, Jin Ki Kim, H. Takezoe & Surajit Dhara (2011): Alignment of unconventional
nematic liquid crystals, Liquid Crystals, 38:7, 917-924
To link to this article: http://dx.doi.org/10.1080/02678292.2011.585522

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Liquid Crystals,
Vol. 38, No. 7, July 2011, 917–924

Alignment of unconventional nematic liquid crystals
T. Arun Kumara , K.V. Leb , Jin Ki Kimb , H. Takezoeb and Surajit Dharaa *aSchool of Physics, University of Hyderabad, Hyderabad, India; b Department of Organic and Polymeric Materials, Tokyo Instituteof Technology, Japan

Downloaded by [University of Hyderabad] at 20:34 12 July 2011

(Received 23 February 2011; final version received 1 May 2011)
Planar (homogeneous) and homeotropic alignment of some unconventional nematic liquid crystals with both
positive ( > 0) and negative ( < 0) dielectric anisotropies are studied on planar (AL-1254) and homeotropic(JALS-204, SE-1211) alignment layers. Irrespective of the sign of dielectric anisotropy and molecular structure, uniform planar alignment of the director is observed in Al-1254 coated cells whereas no stable homeotropic alignmentof bent-core compounds with  < 0 is observed in cells coated with JALS-204 or SE-1211. Alignment propertiesof these two types ( > 0 and  < 0) of unconventional nematic liquid crystals are explained considering thesteric, short-range dipolar and long-range van der Waals interactions at the interface. A bent-core compound (< 0) which has no higher-order liquid crystalline phase except for the nematic exhibits a narrow temperature rangeof homeotropic state (on JALS-204) while cooling from the isotropic phase followed by an anchoring transition
to a planar state as the temperature is further lowered. The measured polar anchoring energy of this compound
on JALS-204 is significantly lower than many known calamitic liquid crystals and is responsible to the anchoringtransition observed.
Keywords: bent-core liquid crystals; planar alignment; homeotropic alignment; dielectric anisotropy

1. Introduction
Uniform alignment of the liquid crystal director(denoted by n̂, the average orientation of the longaxes of the molecules) is technologically very important. It is also essential for any quantitative measurements of various physical properties. Commonly
two categories of alignment layer are used, which
promotes either planar or homeotropic alignment of
the director. These alignment layers are very stablein the sense that the director orientation does notchange with time and temperature. Most of the liquid crystals with rod-like molecules (calamitic) canbe aligned easily in the nematic phase both in planarand homeotropic cells, and the majority of the liquid crystal display devices (LCDs) make use of thesealignment layers.
The unconventional nematic liquid crystals so
far have not been used in LCDs. Nevertheless,recently bent-core nematics has drawn much attention because of their many uncommon physical properties such as an emerging biaxial nematic phase[1], unconventional electroconvection [2, 3], flexoelectric [4] and Kerr effects [5], sign inversion ofelastic anisotropy [6], larger shear and rotational viscosities [7], low pretilt angle [8], when comparedto calamitic liquid crystals. In many of these measurements the director is aligned in the planar cell
∗ Corresponding author. Email: sdsp@uohyd.ernet.in

ISSN 0267-8292 print/ISSN 1366-5855 online
c 2011 Taylor & Francis

DOI: 10.1080/02678292.2011.585522
http://www.informaworld.com

uniformly. There are some reports that bent-corenematic liquid crystals do not align homeotropically with the conventional alignment layers [9, 10].
However, the effect of dielectric anisotropy, and inparticular the role of the dipole moment and stericeffect, on the alignment behaviour has not been
investigated. In this paper we report that most of theunconventional nematic liquid crystals, irrespectiveof their sign of dielectric anisotropy and molecular
structure, can be aligned in planar cells and bentcore nematic liquid crystals with negative dielectricanisotropy in general cannot be aligned homeotropically. Further, we show that a bent-core nematicwhich has no higher-order liquid crystalline phase atlower temperature exhibits a continuous anchoringtransition from homeotropic to planar on JALS204 as the temperature is reduced. We show thatthe polar anchoring energy for bent-core liquid crystals with negative dielectric anisotropy is low andcould be responsible for the inability to provide stablehomeotropic alignment.
2.

Experimental details

We used AL-1254 as planar and JALS-204 and SE1211 as homeotropic alignment layers. Indium tinoxide (ITO) coated glass plates were spin coatedwith AL-1254 and cured at 180◦ C for one hourDownloaded by [University of Hyderabad] at 20:34 12 July 2011

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T. Arun Kumar et al.

for planar alignment. Similarly JALS-204 and SE1211 spin coated ITO glass plates were cured at200◦ C and 180◦ C respectively for one hour for the
homeotropic alignment. The AL-1254 coated plates
were rubbed in an antiparallel direction to obtain
planar alignment of the liquid crystal director. An
ultraviolet (UV) curable adhesive mixed with ∼5 µm
spacer was used to make cells. The sample wasfilled in the isotropic phase and cooled slowly acrossthe nematic–isotropic transition. Texture observation
was made by using a polarising optical microscope(Olympus BX-51) connected with a Instec hotstage(Instec Inc.) and digital camera.
The polar anchoring energy of the compoundwith  = (|| − ⊥ ) < 0 was measured in the
homeotropic cell by using a technique proposed byNie et al. [11]. When the applied voltage exceeds athreshold, the liquid crystal director in the bulk isreoriented and the reorientation at the surface occurs
at much higher voltage due to the finite surface
anchoring. Nie et al. [11] showed that the anchoring energy in a homeotropic cell can be measured
from the voltage-dependent retardation data whenother material parameters are known. The sample capacitance and the retardation were measured
simultaneously as a function of voltage. The sample was placed between two crossed Glan–Thompson
polarisers and a He-Ne laser (632.8 nm) was used asa light source. A photoelastic modulator (PEM-100,
Hinds Instruments) was placed between the polariser
and the sample making the retardation axis parallel to the rubbing direction. The first and the second
harmonics of the optical intensity were measured byusing a photodiode and lock-in amplifier (SRS-830).
The retardation was estimated from the measured
intensities. A LCR meter (Agilent E4980A) at 4111
Hz was used to measure the capacitance. All the measurements were controlled with the help of a computer. The anchoring strength (W ) was estimatedfrom the following equation [11]:
1 ξ
2K33
R
=−
I(b, γ , ν, π/2) + (1 +
),
R0
CV n
Wd

(1)

where

ξ = (0 ⊥ A/d)π K33 /
andI(b, γ , ν, θ ) =

2
π

 θ
0

1 − ν + (1 − ν)1/2
1 − ν cos2 φ + (1 − ν cos2 φ)1/2(1 + γ cos2 φ)2 (1 + b sin2 φ)1/2
dφ.
sin φ

R0 is the maximum retardation and R/R0 is the normalised phase retardation. C, V , d and A are thesample capacitance, applied voltage, cell thickness
and electrode area. In Equation (1), ν = (n2e − n2o )/n2e ,where ne and no are the extraordinary and ordinary refractive indices, γ = (K11 − K33 )/K33 and b =()/0 ⊥ . K 33 is the bend elastic constant and ismeasured from the Freedericksz threshold voltage(Vth ). (ξ/n)I(b, γ , ν, π/2) depends on the material
parameters and is independent of voltage. The polaranchoring energy, W , is determined from the intercept of the plot R/R0 as a function of 1/CVrms .

3. Results and discission
Several compounds with unconventional molecular structures and in various temperature rangesof the nematic phase were chosen for the experiment. The textures obtained in the planar andhomeotropic cells for liquid crystals with positive
dielectric anisotropy ( = || − ⊥ > 0) are shownin Figures 1 and 2. The dielectric anisotropy ofthe compound in Figure 1(a) is  ∼ 1 at TNI –T = 20◦ and exhibits a large temperature rangeof the nematic phase. Various physical measurements on this compound have already been reported[6, 12–15]. Uniform planar and homeotropic alignments of the nematic director are obtained in planar(Figure 1(b)) and homeotropic (Figure 1(c)) cells,
respectively. The homeotropic state is very stable in
the entire nematic phase except tilting of the director
at much lower temperature in some parts of the sample is observed as reported earlier on various otherhomeotropic alignment layers [12, 15]. The molecular structure of the compound in Figure 1(d) ishockey-stick shaped and exhibits a positive dielectric anisotropy ( ∼ 1.6 at TNI – T = 4◦ ). The
detailed physical measurement on this compound
will be reported elsewhere [16]. Uniform planar anddark homeotropic states are observed in the planar (Figure 1(e)) and homeotropic (Figure 1(f)) cells,
respectively. The compound in Figure 2(a) is composed of branched molecules and exhibits positive
dielectric anisotropy ( = 0.9 at TNI – T = 4◦ )[17]. The molecule appears to be T-shaped although
the optimised structure of a single molecule showsthat the different parts of the molecule are not in thesame plane [17]. It also shows good planar (Figure2(b)) and homeotropic (Figure 2(c)) alignment on therespective alignment layers.
The textures of compounds with  < 0 are
shown in Figures 3 and 4. The dielectric anisotropyof the compound in Figure 3(a) is  ∼ −1 at TNI –T = 3◦ [8]. Uniform planar alignment of the director is observed in planar cells (Figure 3(b)) in this919

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Figure 1. (a) Molecular structure and phase transition temperatures of the compound ( > 0). Textures observed in the (b)planar and (c) homeotropic cells at a temperature of 170◦ C. Cell thickness: 6.9 µm and 8 µm for the planar and homeotropic
cells, respectively. (d) Molecular structure and phase transition temperatures of the compound ( > 0) with hockey-stick
shaped molecules. Textures observed in (e) planar and (f) homeotropic cells at a temperature of 120◦ C. Arrows in (b) and (e)denote the rubbing directions. Cell thickness: 7.1 µm and 4.2 µm for the planar and homeotropic cells, respectively (colour
version online).

compound. In homeotropic cells (Figure 3(c) and3(d)) the textures appear mostly dark and exhibit
no schlieren texture suggesting that the sample ispartially homeotropic. The molecule in Figure 3(e)has a larger number of phenyl rings and exhibitsa high-temperature nematic phase. No Freedericksz

transition in the nematic phase was observed in aplanar cell suggesting that the dielectric anisotropy isnegative. The director is uniformly aligned in a planar cell (Figure 3(f)) whereas no homeotropic alignment is observed in homeotropic cells (Figure 3(g)and 3(h)). In Figure 4 we show the textures obtainedDownloaded by [University of Hyderabad] at 20:34 12 July 2011

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T. Arun Kumar et al.

Figure 2. (a) Molecular structure and phase transition temperatures of the compound ( > 0) with branched (T-shaped)
molecules. Textures observed in the (b) planar and (c) homeotropic cells at a temperature of 117◦ C. Cell thickness: 5.2 µm and6 µm for the planar and homeotropic cells, respectively. The arrow in (b) denotes the rubbing direction (colour version online).

in a relatively low temperature and well characterised
bent-core nematic liquid crystal which has no higher
order liquid crystalline phase at lower temperature[2, 3, 18–20]. It shows uniform planar alignment in aplanar cell (Figure 4(b)) as seen in other samples. No
homeotropic alignment is observed in cells coatedwith SE-1211 (Figure 4(c)). In JALS-204 coated cells,
initially they show good homeotropic alignment up
to 70◦ C (Figures 4d(i)) when the sample is cooled
from the isotropic phase and exhibit a continuous
anchoring transition (Figures 4(e)(ii)–4(e)(iv)) as the
sample is further cooled down, finally becoming
almost planar at 51◦ C. A similar continuous anchoring transition of a calamitic nematic mixture (E7) on
perfluoropolymer was reported by Patel et al. [21].
Thus from the texture observation of the compounds
studied it appears that unconventional nematic liquid crystals show planar alignment irrespective of
the sign of the dielectric anisotropy. The compoundwith positive dielectric anisotropy can be aligned
uniformly in homeotropic cells whereas bent-corenematic materials with negative dielectric anisotropy
in general cannot be aligned in homeotropic cells.
There are several interfacial forces that are
responsible for the orientation of the liquid crystal
molecules on the substrate surfaces, such as stericinteractions and short-range dipolar and long-range
van der Waals interactions. Steric interactions preferplanar alignment of the director [22, 23]. Longrange van der Waals interactions prefer the large
polarisability direction (here the bow axis denotedby P in Figure 5) parallel to the substrate surface[24, 25]. Thus both the steric and van der Waals
interactions in bent-core liquid crystals always prefer planar alignment of the director. The short-range
dipolar interactions tend to make dipoles normal
to the interface to maximise their interaction withthe charges at the surface and have an opposite

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921

Figure 3. (a) Molecular structure and phase transition temperatures of the compound ( < 0). Textures observed in the(b) planar and (c),(d) homeotropic cells at a temperature of 117◦ C. Cell thickness: 5.4 µm for planar and 5.7, 5.3 µm forhomeotropic cells, respectively. (e) Molecular structure and phase transition temperature of the compound with n = 7 ( < 0).
M denotes an unidentified smectic phase. Textures observed in the (f) planar and (g), (h) homeotropic cells at a temperature of207◦ C. Cell thickness: 5.3 µm for the planar and 4.5, 5 µm for the homeotropic cells, respectively. Arrows in (b) and (f) denote
the rubbing directions.

alignment effect depending on the strength and orientation of the resultant dipole moment with respect
to the long molecular axis. They tend to align thelong molecular axis (P) perpendicular to the surface
where the direction of the resultant dipole moment(denoted by D in Figure 5(a)) is parallel to P. On
the other hand, they tend to orient the bow axis(P) parallel to the substrate where the direction of
dipole moment (D) is perpendicular to P. In the caseof compounds with  > 0, D is parallel to P and
has opposite alignment effect. However, if the dipolar interactions are relatively strong compared to thatof the long-range van der Waals interactions they
can lead to homeotropic alignment of the bent-coremolecules as schematically shown in Figure 5(c). In
the case of bent-core compounds with  < 0, D isparallel to the arrow axis, i.e. almost perpendicular to
P (Figure 5(b)) and both D and P have similar alignment effects, i.e. planar alignment of the molecules(Figure 5(d)). As a result, dielectrically-negative-type
bent-core nematic compounds in general do notexhibit homeotropic alignment. The anchoring transition in Figure 4 can be considered as a result ofcompetition between these two major interactions asDownloaded by [University of Hyderabad] at 20:34 12 July 2011

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T. Arun Kumar et al.

Figure 4. (a) Molecular structure and phase transition temperatures of the compound ( < 0). Textures observed in the (b)planar (AL-1254) and (c) homeotropic (SE-1211) cells at 72◦ C. Textures observed in the homeotropic cell (JALS-204) at various
temperatures when the rubbing direction is parallel ((d)(i)–(d)(iv)) and rotated by 45◦ ((e)(i)–(e)(iv)) with respect to the polariser.
Arrows in the figure denote the rubbing directions. Cell thickness: 7.1 µm for the planar and 4.2, 5.2 µm for the homeotropic
cells, respectively (colour version online).

discussed. Since most of the bent-core liquid crystals
possess several phenyl rings, the polarisability is large
and is strongly temperature dependent. Because ofincreased polarisability at lower temperatures the vander Waals interactions can dominate over the dipolarinteractions and an anchoring transition can occur.
The compound in Figure 4(a) shows an unstablehomeotropic state (from 75.3◦ C to 70◦ C ) in the sensethat below 70◦ C it exhibits a continuous anchoring transition to the planar state. We measured
the anchoring energy exploiting the homeotropicalignment of this compound. Retardation (R) andcapacitance (C) were measured simultaneously tomeasure the anchoring energy (W ) at a particulartemperature. The variation of retardation and capacitance as a function of applied voltage is shownin Figure 6(a) and Figure 6(b), respectively. Below
the Freedricksz threshold voltage (Vth = 0.47V )the sample retardation is zero (homeotropic state)
and the director tilts as the voltage is increased
and finally becomes almost planar at higher voltages. The bend elastic constant was calculated byusing the relation K 33 = (Vth /π )2  o where is the dielectric anisotropy. At 72◦ C,  || = 5.5 and923

Figure 5. Schematic representation of bent-core molecules. The direction of the resultant dipole moment and the large polarisability directions are denoted by D and P, respectively. (a) Both D and P are parallel in materials with  > 0. (b) D and P are
mutually perpendicular in materials with  < 0. (c) Homeotropic alignment of the director with  > 0. (d) Planar alignmentof the director with  < 0.

(a)(b) 220
Capacitance (pF)

800
Retardation (nm)600
(72 °C)

400
200

200
(72 °C)
180

160
0
0

4

8

12

16

0

20

4

Vrms

8

12
Vrms

16

20

(c)
1.2

R/R°

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Liquid Crystals

Intercept = 1.007

0.8

0.4

0.0

0.00040.00080.00120.0016

1/CVrms (× 1012 coulomb–1)

Figure 6. (a) Retardation as a function of applied voltage measured in the homeotropic cell (at 72◦ C). (b) Sample capacitanceas a function of applied voltage at 72◦ C. (c) Normalised retardation R/Ro as a function of 1/CVrms at 72◦ C. The continuousline is the theoretical fit by Equation (1). Cell thickness: 6.1 µm.

Downloaded by [University of Hyderabad] at 20:34 12 July 2011

924

T. Arun Kumar et al.

 ⊥ = 6.9, which were measured from the extrapolation of C with 1/Vrms . The estimated K 33 fromthe present Freedericksz threshold measurement is
0.3 × 10−12 N, which is comparable in magnitudewith the reported values [26]. A similar value ofK 33 near the nematic–isotropic transition has also
been reported for other bent-core nematic liquid
crystals [6]. The variation of the normalised retardation (R/Ro ) as a function of 1/CVrms is shown in
Figure 6(c). The experimental data were fitted to thetheoretical Equation (1) by using a least-squares fitting method to estimate the intercept. The anchoringenergy, W , was estimated to be 1.4 × 10−5 J/m2 ,which is significantly low compared to many known
calamitic liquid crystals. The anchoring transition
observed in this compound could be attributed to the
low anchoring energy.
4. Conclusion
We have studied alignment properties of somebent-core, hockey-stick and T-shaped nematic liquid crystals with both positive and negative dielectric anisotropy. We found that these unconventional
nematics with both positive and negative dielectric anisotropy can be aligned easily in planar cells
whereas bent-core materials with negative dielectricanisotropy in general cannot be aligned homeotropically. The experimental results are explained by considering the steric, short-range dipolar and van der
Waals interactions. The polar anchoring energy ofa material with negative dielectric anisotropy is verysmall and perhaps leads to the anchoring transition
as observed. Further, it will be interesting to study
and measure the surface-related properties of bentcore nematic compounds with different shapes on
various other alignment layers.
Acknowledgements
Surajit Dhara gratefully acknowledges support fromthe Center for Advanced Studies, School of Physics.
Tatipamula Arun Kumar acknowledges the Center for
Advanced Studies, School of Physics for a fellowship. We
thank Professors B.K. Sadashiva and C.V. Yelamaggad
and Dr A.K. Prajapati for providing compounds with
bent-core and T-shaped molecules.

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