Japanese Journal of Applied Physics
Vol. 46, No. 9A, 2007, pp. 5920–5923#2007 The Japan Society of Applied Physics

Experimental Investigation on Pretilt Angle in Binary Liquid Crystal
Composed of Highly Polar Molecules
Surajit D HARA, Khoa Van LE1 , Yoichi TAKANISHI1 , and Hideo TAKEZOE1
School of Physics, University of Hyderabad, Hyderabad, India
1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan(Received April 10, 2007; accepted June 4, 2007; published online September 7, 2007)

We report the measurement of dielectric constants, birefringence, and pretilt angle in binary mixtures of nematic liquid
crystals (CP7B and CCN-47). CP7B has a strong longitudinal dipole moment and exhibits a large positive dielectricanisotropy (" ¼ "k  "? > 0) and CCN-47 has a strong transverse dipole moment and exhibits a large negative dielectricanisotropy. In the mixtures, "k increases with increasing CP7B concentration, whereas "? remains almost constant. We showthat the pretilt angle increases with " and birefringence (n ¼ ne  no ) much rapidly than reported earlier. It is suggested
that this eﬀect may arise due the presence of strong permanent dipole moments in the molecules.
[DOI: 10.1143/JJAP.46.5920]
KEYWORDS: nematic liquid crystal, highly polar molecule, dielectric anisotropy, birefringence, pretilt angle

1.

Introduction

The uniform alignment of liquid crystal (LC) molecules
on a substrate treated with an alignment layer is important
both from the technological and fundamental points ofview.1,2) There are several techniques for obtaining auniform orientation of nematic LC director n^, which is theunit vector along the average orientation of the long axes ofthe molecules. Among all techniques, the most familiar andwidely used technique is the rubbing of the substrate treatedwith a suitable polyimide. The rubbing provides not only auniform alignment but also a certain pretilt angle p , i.e., theacute angle between the rubbing direction and n^ at theboundary surface. The performance of LC display devices isstrongly aﬀected by p . For example, a twisted nematic (TN)display made of low p gives rise to undesirable lightscattering because of the introduction of a reverse twist
structure and the associated boundaries between two twistedstructures. On the other hand, displays made of high p show
disturbing interference colors and have a low multiplexing
capacity. Therefore, the dependence of p on various factors
should be known to realize optimized display performance.
The important factors that determine p are the chemicalstructures and physical properties of the LCs as well as thealignment layers. There have been several reports on theeﬀect of the chemical structure of the alignment layer andrubbing strength on p .3–7) Some studies have also been
carried out to understand the eﬀect of molecular structure,physical properties such as dielectric anisotropy (" ¼"k  "? ), birefringence (n ¼ ne  no ), and the anchoringenergies of the LCs on p .3–5) The subscripts in the dielectric
constant refer to the ﬁeld direction with respect to n^, andthose in n represent extraordinary and ordinary refractive
indices, respectively.
In most of these studies, several mixtures of nematic LCs
made of rodlike molecules have been used. Recently, wehave measured p in a discotic nematic LC and found that pin such a negative material is low.7) More recently, we havealso shown that a LC made of banana-shaped molecules withnegative dielectric anisotropy in the nematic phase exhibits avery low p (less than 1 )8) on a rubbed polyimide surface
designed to give a high p . These ﬁndings motivated us
to carry out systematic investigations on the relationshipamong pretilt angle, molecular dipole, dielectric constants,and birefringence in calamitic LCs. For this purpose wechose several binary mixtures composed of two highly polar
molecules. The ﬁrst compound has a strong longitudinal
dipole moment and exhibits a large positive dielectricanisotropy and the second one has a strong transverse dipole
moment and exhibits a large negative dielectric anisotropy.
In this paper, we report the measurement of the parallel ("k )and perpendicular ("? ) components of dielectric constant,birefringence, and pretilt angle as a function of the mixing
ratio of the binary mixtures and show the eﬀect of dielectricanisotropy, and birefringence on p .
2.

Experiments

To realize the eﬀect of a strong dipole moment on p , wehave chosen two highly polar compounds. The chemicalstructures of the compounds are shown in Fig. 1. The
compound p-cyanophenyl p-n-heptylbenzoate (CP7B) exhibits the following sequence of phase transitions: Cry 41.2
N 53.7 I. The nematic phase can be super cooled down tothe ambient temperature. The compound 40 -butyl-4-heptylbicyclohexyl-4-carbonitrile (CCN-47) exhibits the following
sequence of phase transitions: Cry 25.6 SmA 28.2 N 57.3 I.
Both compounds were obtained from Merck and usedwithout further puriﬁcation. Glass plates with indium tinoxide (ITO) were spin-coated with polyimide (Nissan SE7492) and cured at 180  C for 30 min for the homogeneousalignment of the director. After curing, the plates wererubbed and assembled such that the rubbing directions wereset antiparallel. The rubbing strength was kept ﬁxed for all
the cells. The sample was introduced into the cells in the5920

C7H15

CN

COO
CP7BH

C7H15CN

C4H9CCN-47Fig. 1. Chemical structures of compounds used.

Jpn. J. Appl. Phys., Vol. 46, No. 9A (2007)
Table I.
used.

S. DHARA et al.

Nematic to isotropic phase transition temperatures in mixtures

CP7B(wt %)CCN-47(wt %)N–I transition
temperature( C)

p"k"?

" ¼ "k  "?

nMixture-1
Mixture-2

8.5
6.9

29
18.1

910

20
8.1

0.148
0.117

100

053.7

Mixture-3

5.1

14.1

9.1

50.073

Mixture-2

75

25

43.6

Mixture-4

5.1

9.4

8.8

0.6

0.064

Mixture-3

5050

32.5

Mixture-5

2.1

4.0

9.7

5:70.032

Mixture-4
Mixture-525075100

37.957.3

Results and Discussion

The physical properties of the pure compounds and binary
mixtures are summarized in Table II. The variations in"k and "? are shown in Fig. 2 as a function of the wt %of CP7B. Note that "k increases practically linearly withincreasing wt % of CP7B in the mixture, whereas "? remains
almost constant. The increase in "k with increasing wt % of
CP7B is expected as the concentration of molecules withlongitudinal dipole moment increases in the mixtures. In themixtures, the wt % of CCN-47 decreases with increasingconcentration of CP7B (see Table I); hence "? is expected toReal Part of Dielectric Constant (ε )Mixture-1

4030

ε ||20ε⊥1000204060

80100

Fraction of CP7B (wt%)
Fig. 2. Variations in parallel ("k ) and perpendicular ("? ) components ofdielectric constant as functions of wt % of CP7B. Solid lines are drawn as
guides to the eyes.

∆ε160.24
0.18

80.12

∆n0Birefringence (∆n)

0.30

24
Dielectric Anisotropy (∆ ε )isotropic phase and cooled slowly to the nematic phase. For
homeotropic alignment, glass plates were spin-coated withpolyimide (JSR JALS 204) and cured under the same
condition. The temperature of the sample was controlled
within an accuracy of 0.1  C. Texture observation was
carried out using a polarizing microscope. All the measurements were made while cooling the sample from theisotropic phase.
We employed a conventional crystal rotation method(CRM) for measuring the pretilt angle, which is generally
used for uniaxial nematic LCs made of rodlike molecules.9)
A He–Ne laser (632.8 nm) was used to illuminate a 100-mmthick cell and the transmitted intensity was measured as afunction of the incidence angle from 45 to þ45 about
the axis perpendicular to the light beam and the rubbingdirection. The transmitted intensity was ﬁtted to a theoreticalequation [eq. (2) in ref. 9] and p was obtained as a ﬁtparameter. The birefringence of the sample was measured ata wavelength of 550 nm using a polarizing spectrophotometer (ORC TFM-120AFT-PC) at various mixtures in thenematic phase. To determine the birefringence, the transmittance spectra were ﬁtted to a well-known dispersion
equation for nonabsorbing birefringent media where thedispersion of the refractive indices was taken care. The
dielectric measurements were conducted using an impedance analyzer (Yokogawa-Hewlett-Packard 4194A) for
homogeneous and homeotropic cells. A typical cell thickness of 5 mm was used for measuring the birefringence andthe dielectric constants. The nematic to isotropic (N–I)
transition temperatures of the binary mixtures are shownin Table I. We could not measure the nematic to crystal
transition temperatures because for most of the mixtures
they were close or below the ambient temperature. Since wefound that p s in all the mixtures were independent oftemperature (see later), all the experiments were carried out
at a ﬁxed temperature of 30  C and the dielectric constants
were measured at a ﬁxed frequency of 3981 Hz.
3.

Table II. Pretilt angle, dielectric constant, dielectric anisotropy, andbirefringence in the mixtures used.

0.06
-8020

4060

80100

Fraction of CP7B (wt%)
Fig. 3. Variations in dielectric anisotropy (" ¼ "k  "? ) and birefringence (n ¼ ne  no ) as functions of wt % of CP7B. Note the change ofsign in " nearly at 25 wt % of CP7B. Solid lines represent linear ﬁts tothe data points.

decrease. However, with increasing concentration of CP7B,
a transverse component of the dipole moment also contributes to the increase in "? as CP7B has also a COO group at
the transverse position and as a result "? remains almost
constant in the mixtures (see Fig. 2).
The variation in dielectric anisotropy as well as birefringence as a function of the wt % of CP7B is shown in Fig. 3.
We use a least-squares method to ﬁt the experimental dataand notice that both quantities vary linearly. We also noticethat the dielectric anisotropy changes sign nearly at 25 wt %of CP7B. In Fig. 4, the variation in p is shown as a functionof "k in the mixtures. Note that initially p increases with"k and tends to saturate further. A similar observation was5921

Jpn. J. Appl. Phys., Vol. 46, No. 9A (2007)

S. DHARA et al.

10
Pretilt Angle (θp in deg.)

Pretilt Angle (θp in deg.)

1296300102030

8642040

Parallel Component of Dielectric Constant ( ε || )

0.04

0.08

0.120.16

Birefringence (∆n)

Fig. 4. Variations in pretilt angle as function of parallel component ofdielectric constant ("k ). A solid line is drawn as a guide to the eyes.

Fig. 6. Variations in pretilt angle as function of birefringence. Opencircles correspond to data of our binary mixtures and data corresponding
to the open squares are obtained from ref. 4. Solid lines represent linear
ﬁts to the data points.

Pretilt Angle (θp in deg.)

129630-1001020
Dielectric Anisotropy (∆ε )30Fig. 5. Variations in pretilt angle as function of dielectric anisotropy (").
Open circles represent the data points corresponding to our binary
mixtures. Data corresponding to the open squares are collected fromref. 11. Solid lines are drawn as guides to the eye.

reported by Sawada et al.10) in several multicomponent
mixtures of nematic LCs. They suggested that this eﬀect wasdue to the dipole–dipole interaction between the nematic LC
and the alignment ﬁlm. The generation of the pretilt angle innematic LCs with negative dielectric anisotropy and cyano
mixtures on a rubbed polyimide surface was studied bySeo et al.5) They reported that p is independent of theperpendicular component of the dielectric constant insystems with positive dielectric anisotropy. The variationin p as a function of " is shown in Fig. 5. We observe that
p increases with increasing dielectric anisotropy and tends
to saturate further at higher values of the dielectricanisotropy of the mixtures. A similar observation wasreported by Becker et al.11) They have measured p using amagneto-optical null method on several mixtures, namely,ZLI-1085, ZLI-1957/5, ZLI-1132, RQTN-605, and ROTN403. It has also been shown by Nishikawa et al.4) that pincreases linearly with ". However, in their experiments,
the exact composition of LCs was unknown.
Summarizing all these data including the present ones, wecan suggest that p is aﬀected by the interaction between thestrong permanent and induced electric dipole moments in theLC molecules and the dielectric anisotropy of the polymer,
as mentioned by Becker et al.11) We compare our pretiltangle data (open circles) with their reported values (opensquares) against " in Fig. 5. In our binary systems, therelative slope of p =" is much larger. We can interpret that
this eﬀect is enhanced due to the presence of strong dipole
moments in the molecules used. In Fig. 6, we show p as
a function of n (open circles). We use a least-squares
method to ﬁt the experimental data and notice that p
increases linearly with n. A similar observation (opensquares) was also reported by Nishikawa et al.4) in multicomponent mixtures obtained from Merck. We ﬁnd that theslope, i.e., p =n is much larger in our binary mixtures.
Nakayama et al.12) reported that p of a nematic LC
increases with increasing n, suggesting the importance ofthe – interaction or the dispersion force between thenematic LC and the alignment ﬁlm. In our systems, both
two LC molecules possess strong dipole moments, so thatthe polarizability anisotropy and hence the birefringenceincreases more rapidly. Thus we believe that the dispersionforce is also enhanced causing relatively larger slope ofp =n in our mixtures.
Finally, we want to comment on the temperature dependence of p . As mentioned at the end of the introduction, noessential temperature dependence of p was observed. Forinstance, p s were 5.08 (30  C), 5.14 (25  C) and 4.93
(22  C) for Mixture-3 with an N–I transition temperature of
32.5  C. The anisotropic physical properties, e.g., birefringence and dielectric anisotropy among others show a strong
temperature dependence particularly near the N–I transitiontemperature. This is because of the change of the orderparameter of the director orientation. For example, byincreasing the temperature of the sample in the nematicphase, the distribution of the long axis of the molecules
becomes broader, resulting in decreasing anisotropy. However, the average alignment direction of the molecules, i.e.,the director, remains almost unaﬀected even if the orderparameter changes, provided that the temperature dependence of the orientational change at both surfaces isneglected. The temperature dependence of pretilt angles
was reported by Toda et al.13) They reported a tendency
wherein p decreases with increasing temperature; however,they failed to explain the temperature dependence of pusing that of the order parameter and tentatively ascribed the5922

Jpn. J. Appl. Phys., Vol. 46, No. 9A (2007)

S. DHARA et al.

temperature dependence of p to the surface orientationalchange. In any event, the temperature dependence of thepretilt angle is not so large when its value is smaller than10 , although considerable temperature dependence emerges
for large pretilt angles. Recently, we have also reported thatthe pretilt angle for a banana nematic LC is very small anddoes not change with temperature in the entire nematicrange.8) In this way, the use of data at 30  C in this study
does not pose any serious problem in grasping the important
relationship between p and dielectric anisotropy andbirefringence.
4.

Conclusions

In conclusion, the eﬀects of dielectric anisotropy andbirefringence on pretilt angle in binary mixtures of two
highly polar compounds, namely, CP7B and CCN-47 havebeen investigated. It is found that the pretilt angle strongly
depends on the dielectric anisotropy as well as thebirefringence. The variation in pretilt angle with birefringence is almost linear, whereas it shows a saturating
tendency with dielectric anisotropy at a higher concentrationof CP7B. The much larger pretilt angle in our mixtures isattributed to the strong dipole–dipole interaction, the –interaction or the dispersion force between the nematic LC
and the alignment ﬁlm.
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