RAPID COMMUNICATIONSPHYSICAL REVIEW E 79, 060701共R兲 共2009兲

Anchoring transitions of transversely polar liquid-crystal molecules on perfluoropolymer surfaces
Surajit Dhara,* Jin Ki Kim, Soon Moon Jeong,† Reiri Kogo, Fumito Araoka,
Ken Ishikawa, and Hideo Takezoe‡
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-42 O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
共Received 10 February 2009; revised manuscript received 13 May 2009; published 2 June 2009兲
We report a strong discontinuous orientational transition 共anchoring transition兲 of liquid-crystal moleculeswith a large transverse dipole moment. A perfluoropolymer was used as an alignment layer and the transition
was observed from planar to homeotropic with decreasing temperature in the nematic phase. Conversely agradual variation in tilt angle from homeotropic to conical was observed in a liquid crystal with a comparatively smaller transverse dipole moment on the same alignment layer. The experimental results clearly demonstrate the competition between a short-range dipolar force and long-range van der Waals force at theinterfacial region. Using discontinuous anchoring transition in the sample, we demonstrate a possible bistabledevice for memory and light-driven display.
DOI: 10.1103/PhysRevE.79.060701

PACS number共s兲: 64.70.⫺p, 68.35.Md, 68.47.Mn

Aligning liquid-crystal molecules to particular directions
at solid substrate surfaces is very important from technological as well as fundamental points of view 关1兴. Generally, twodifferent categories of alignment layers are used in liquidcrystal display devices 共LCDs兲, i.e., parallel 共homogeneousor planar兲 and vertical 共homeotropic兲 alignment of the director 共the average molecular direction兲. Apart from LCDs, depending on the strength of the anchoring energy, liquid crystals can also exhibit a rich variety of behaviors such as
memory effects 关2兴, surface melting 关3兴, orientational wetting
关4,5兴, etc. In appropriate surface conditions and interactionenergies with liquid-crystal molecules on the surfaces, anchoring transitions, i.e., spontaneous change in the preferred
director orientation, may occur. There are some theoreticaland experimental reports on various anchoring transitions关6–21兴. Among these few reports about the continuous anchoring transition on various alignment layers with longitudinal dipole moment as a function of temperature are known
关6,10,15,16兴. Recently Faget et al. 关7兴 demonstrated conicalplanar-anticonical transitions on grafted polystere surface
and also theoretically explained including the non-RapiniPapoular terms in the anchoring energy. A brief andqualitative report on discontinuous anchoring transitionin the mixture of liquid crystals with a longitudinal
dipole moment 关4-cyano-4⬘-n-octylbiphenyl 共8CB兲 and4-cyano-4⬘-n-octyloxycyanobiphenyl 共8OCB兲兴 as a functionof temperature was made by Birecki 关8兴. However no quantitative measurements were reported to reveal the role of permanent dipole moment and different surface forces.
Here we report two important experimental observations.
共1兲 A strong discontinuous anchoring transition of liquid-*Corresponding author. On leave of absence from School of Physics, University of Hyderabad, Hyderabad–500046, India.
sdsp@uohyd.ernet.in.
†Present address: Central Technical Research Laboratory, Nippon
Oil Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231–0815, Japan.
‡
Corresponding author: takezoe.h.aa@m.titech.ac.jp.
1539-3755/2009/79共6兲/060701共4兲

crystal director from planar to homeotropic with decreasing
temperature in a nematic liquid crystal with a large transverse dipole moment on an unrubbed as well as rubbedalignment layer with perfluoropolymers. 共2兲 The same perfluoropolymer surface also gives gradual variation in tilt
angle from homeotropic to conical in a liquid crystal with acomparatively smaller transverse dipole moment. Finally weshow that the discontinuous anchoring transition can be exploited to fabricate light-driven bistable memory devices.
We used spin-cast perfluoropolymer, poly关perfluoro共4vinyloxy-1-butene兲兴, known as CYTOP 共Asahi Glass Co.,
Ltd.兲 as an alignment layer, and two liquid crystals,
4⬘-butyl-4-heptyl-bicyclohexyl-4-carbonitrile 共CCN-47兲 andmethoxybenzilidene butylanline 共MBBA兲. CCN-47 exhibits
the following phase transitions: cry 28 ° C Sm-A 30.6 ° C N
54.4 ° C I, and has a large negative dielectric anisotropy
共⌬⑀ = −10.0 at 20 ° C兲. MBBA exhibits the following phasetransitions: cry 25 ° C N 38 ° C I, and has a small negative
dielectric anisotropy 共⌬⑀ = −0.6 at 30 ° C兲. The materials
were sandwiched by two glass substrates coated with perfluoropolymer 共upper right in Fig. 1兲.
Let us first show that texture changes with decreasing
temperature. CCN-47 showed a planar texture at 50 ° C asshown in Fig. 1共a兲. Interestingly the system mostly exhibits⫾1 / 2 strength wedge disclinations. The scarcity of ⫾1 disclinations is sometimes regarded as an evidence of biaxial
system 关22兴, but the present medium is definitely uniaxial, as
is clear by a complete dark view in the homeotropicallyaligned state 关Fig. 1共c兲兴. A similar texture was also observedin the nematic phase of a lyotropic liquid crystal 关23兴. The
physical implication is that the director is in the plane as any
tilting in such case is not allowed topologically, so that ⫾1
disclinations, if they exist, cannot escape from the line defects to point defects. The details of defect dynamics and thecorrelations will be reported elsewhere. On further coolingthe sample temperature below 45 ° C the dark region grows
abruptly and randomly 关Fig. 1共b兲兴 and the whole area becomes dark within a temperature range of ⬃1° 关Fig. 1共c兲兴. It
indicates that the director orientation spontaneously changes
from planar to vertical direction 共homeotropic alignment兲.
We measured the transmitted intensity of 545 nm light

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(a) 5
CF2

CF

CF

CF2

Transmittance (arb. units)

(a)
(a

n

CF2

O
CF2

(b)

(d)

coolingheating

4
3
2
1

N

0
42

(b)

Iso

46

50

54

58

46

50

54

58

Temperature ( C)

0.025

(e)

Birefringence (∆n)(c)

FIG. 1. 共Color online兲 Photomicrographs of the textures showing anchoring transitions in CNN-47 and MBBA under crossed polarizers 共white crossed arrows兲: 共a兲 CCN-47 at 50 ° C; 共b兲 CCN-47
during the first-order anchoring transition; 共c兲 CCN-47 at 40 ° C;
共d兲 MBBA at 35 ° C; 共e兲 MBBA at 25 ° C with flow direction 共whitearrow兲 making 45° with respect to the polarizer. The chemical
structure of perfluoropolymer CYTOP is also shown at the upper
right.

through a rubbed cell. Rubbing was made to have a uniformalignment in the planar state. The variation in the transmitted
intensity is shown as a function of temperature in Fig. 2共a兲.
The intensity in the nematic phase jumps at 45 ° C and
50 ° C on cooling and heating, respectively, with a large
hysteresis of 5 ° C. We also calculated the birefringencefrom the transmitted intensity using the formula, I共T兲d
= I0 sin2 ␲⌬n␭ , where ⌬n is the birefringence, d is the cell
thickness, and ␭ is the wavelength of the light used. We
show the variation in the birefringence as a function of temperature in both heating and cooling cycles in Fig. 2共b兲. It isnoted that the birefringence jumps from 0.022 to 0 on cooling and from 0 to 0.019 on heating. The birefringence of thesample was also measured as a function of temperature in an
independent cell treated with a homogeneous alignment
agent 共AL-1254, Nissan Chemicals兲. We confirmed that noanchoring transition was observed on this alignment layer.
The data agree well with the data measured on cells treatedwith the perfluoropolymer. It may be pointed out that birefringence measured on perfluoropolymer is slightly less thanthe values reported on other polyimides 关24,25兴 which aremeasured at 633 nm light. The lower value of birefringencemeasured on both perfluoropolymer and AL-1254 indicatesthat in the present cell 共CCN-47兲, a sufficient pretilt angle ofthe director is generated due to rubbing the plates.
In contrast to CCN-47, MBBA exhibits a continuous homeotropic to tilted transition. A typical homeotropic textureof compound MBBA at 35 ° C under a polarizing microscope is shown in Fig. 1共d兲. While cooling, the transmitted
intensity increases gradually without showing a birefringent

0.020

AL1254

0.015
0.010
0.005
0.000
42

Temperature ( C)

FIG. 2. 共Color online兲 共a兲 Transmitted light intensity 共in arbitrary units兲 of an orientated sample of CCN-47 between crossed
polarizers as a function of temperature. 共b兲 Calculated birefringenceobtained from 共a兲. Data for cooling and heating processes areshown by squares and circles, respectively. A large hysteresis intemperature is observed. Data corresponding to triangles are measured using the commercial homogeneous alignment agent AL1254, which shows a pretilt from the vertical direction and no anchoring transition.

color 关Fig. 1共e兲兴. The transmitted intensity gradually
increases/decreases with decreasing/increasing temperaturein the nematic phase range as shown in Fig. 3共a兲. Clearly nohysteresis was observed. The increase in the transmittanceindicates that the molecules are tilted with respect to thevertical direction. The transmittance varies by rotating thecell, showing the maximum when the flow direction in thesample introduction makes 45° with respect to the polarizer
direction 关Fig. 1共e兲兴, indicating that the molecules tilt along
the flow direction. In order to estimate the tilt angle we calculated the effective refractive index 共neff兲 as a function oftemperature by using the formula, I共T兲 = sin2 ␲共neff − no兲d / ␭.
The value of neff is used to calculate the tilt angle as afunction of temperature by using the equation ␪22= cos−1冑共n2e − neff
兲n2o / 共n2e − n2o兲neff, where no and ne are the ordinary and extraordinary refractive indices, respectively. The
variation in the tilt angle as a function of temperature isshown in Fig. 3共b兲. The isotropic to homeotropic nematictransition takes place at around 38 ° C. In the homeotropic
phase the director has a small pretilt of ⬃1.5° from the vertical direction 关Fig. 3共b兲兴. We found that the molecules
gradually tilt from 88.5° at the nematic-isotropic transition
point to 82.5° at room temperature.
Parsons 关26兴 theoretically predicted a continuous transition from homeotropic to tilted which could perhaps explain

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after irradiation

coolingheating

3

2

heat
pulse

1
0

24

28

32

N

Iso

36

40

Temperature ( C)(b)homogeneousTransmittance(a)Transmittance (arb. units)

ANCHORING TRANSITIONS OF TRANSVERSELY POLAR…

90

heat
pulse

Tilt Angle (deg)homeotropic
87

Temperature

84

81
24

28

32

36

40

Temperature ( C)

FIG. 3. 共Color online兲 共a兲 Transmitted light intensity of an orientated sample of MBBA between crossed polarizers as a functionof temperature. Data for cooling and heating processes are shownby squares and circles, respectively. No hysteresis is seen. 共b兲 Calculated tilt angle as a function of temperature obtained from 共a兲.

the transition in the case of MBBA in the present case. But
this theory cannot predict the discontinuous transition as observed in the case of CCN-47. Kimura 关9兴 studied the orientation of nonpolar rodlike molecules at hard flat walls using
the mean-field approximation. He predicted a discontinuous
planar-homeotropic transition and a continuous planar-tilthomeotropic transition with decreasing temperature by taking into account isotropic van der Waals interaction 共dispersion force兲 and steric interaction between liquid-crystal
molecules and surface. In addition to that polar contribution
must be included, since our two molecular systems withlarge and small transverse dipoles gave opposite results inthe present study. The steric interaction and the dispersion
force give planar and homeotropic orientations, respectively
关9,27,28兴. A short-range dipolar interaction tends to align the
dipole moments perpendicular to the surface, leading to different molecular orientations between molecules with atransverse dipole 共CCN-47兲 and a longitudinal one 共cyanobiphenyl兲. A long-range electrostatic interaction between thepolar groups of liquid-crystal molecules and surface also
gives a different orientation for these molecular systems, i.e.,surface tends to align the more electronically polarizable direction of liquid crystal parallel to the plane of the substrate.
Therefore, liquid-crystal molecules with transverse and longitudinal dipoles align perpendicular and parallel to the surface, respectively 关28兴. Thus these four contributions havedifferent orientational preference and show different temperature dependence. All of these factors could be responsible for anchoring transition in liquid crystals composed ofpolar molecules.

FIG. 4. 共Color online兲 Principle of a bistable switching device
using discontinuous anchoring transition and an actual image showing performance. On laser light irradiation to a cell at 49 ° C, ahomeotropic state 共closed circles兲 switches to a homogeneous state
共open circles兲 via thin black arrows. In the middle of a microscope
image, where light irradiation is made, schlieren texture isobserved.

In the present case both the liquid crystals exhibit negative dielectric anisotropy and CCN-47 has a strong dipole
moment of about 3.1 D 关25兴 along the transverse direction of
the long molecular axis. Both the strong dipolar interaction
and the steric interaction prefer a parallel 共homogeneous兲
alignment in CCN-47 just below the nematic-isotropic transition. As the temperature is lowered the electrostatic interaction becomes comparatively stronger due to the increased
polarizability anisotropy and the more polarizable direction
共short axes of the molecules兲 prefers to align parallel to thoseof the large polarizable direction of the alignment layer. As aresult the molecules are oriented vertically with respect tothe surface. In the case of MBBA, the dipolar interaction is
weaker compared to the van der Waals interaction due to thesmall values of dipole moment, and a vertical alignment ispreferred just below the nematic-isotropic transition. Smallcontinuous change to a tilted orientation may come from thetemperature dependence of these interaction forces. Thus therelative strength of the four forces may also determine thenature of the transition. We also observed small continuoustransition in penthylcyanobiphenyl 共5CB兲 from homeotropic
to planar with decreasing temperature. More detailed theoretical calculation accounting the physiochemical nature of
the interface is needed to understand the phenomenon completely. It may be pointed out that a stable homeotropicalignment was also observed in smectic liquid crystals without any anchoring transition. The details will be reported
elsewhere.
Finally we propose and demonstrate a possible device utilizing the bistability of homogeneous and homeotropic orientations in the CYTOP/CCN-47 system. The principle isshown in Fig. 4. We realized homeotropic orientation 共closed
circle兲 in the bistable temperature region at 49 ° C, which

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shows completely black under crossed polarizers. Then we
exposed the center of the cell to an Ar-ion laser light 共488nm兲 beam to increase the temperature above 50 ° C. Photoinduced transition from homeotropic to homogeneous orientations occurred, and the change was easily recognized underan optical microscope, as shown in Fig. 4. We confirmed that
this pattern remained unchanged at least for 1 day. We also
found that the irradiation intensity can be brought down to afew milliwatts to write a spot if we doped CCN-47 with asmall amount 共0.5 wt兲 of 4-共dicyanomethylene兲-2-methyl6共4-dimethylaminostryl兲-4H-pyran 共DCM兲. Further studies
for fabricating memory and light-driven display devices areunder way.
In conclusion, we showed two different anchoring behaviors on the same alignment layer depending on the strengthof dielectric anisotropy, i.e., discontinuous planar to homeotropic transition and gradually homeotropic to tilting state.
This is a report of strong discontinuous anchoring transitionon unrubbed and rubbed cells as well as opposite types oftransition in a single surface. These results can be qualitatively explained on the basis of four temperature-dependentcompeting forces at the liquid-crystal/solid substrate interface. The observation of discontinuous anchoring transition
in rubbed cell is unique in the sense that it provides an interesting system for application. The discontinuous firstorder anchoring transition with large hysteresis facilitates
bistable homogeneous and homeotropic orientations, which
can be controlled by heat or light pulses, providing us withmemory devices and light-driven display devices. Preliminary experiment has been made successfully. The details will
be reported elsewhere. Further experimental investigation on
the effect of electric field on the anchoring transitions and themeasurement of pretilt angle that arise due to rubbing are
under way.

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One of the authors, S.D., gratefully acknowledges the
support from the DST, Government of India for the Project
No. SR/FTP/PS-48/2006 under the SERC Fast Track
Scheme. We acknowledge Nissan Chemical Industries Ltd.
for supplying surface alignment agents, SE-1211 and AL1254, and Merck Japan Ltd. for supplying a liquid crystal
CCN-47.

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