Reprinted fromRAPID COMMUNICATION

Perfluoropolymer as Planar Alignment Layer for Liquid Crystal Mixtures
Tatipamula Arun Kumar, Hideo Takezoe, and Surajit Dhara

Jpn. J. Appl. Phys. 50 (2011) 040203# 2011 The Japan Society of Applied Physics

Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.

RAPID COMMUNICATION

Japanese Journal of Applied Physics 50 (2011) 040203
DOI: 10.1143/JJAP.50.040203

Perfluoropolymer as Planar Alignment Layer for Liquid Crystal Mixtures
Tatipamula Arun Kumar, Hideo Takezoe1 , and Surajit Dhara
School of Physics, University of Hyderabad, Hyderabad 500046, India
1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro, Tokyo 152-8552, Japan
Received October 29, 2010; accepted January 20, 2011; published online April 5, 2011
We report a perfluoropolymer (CYTOP) provides planar alignment to liquid crystal mixtures (e.g., ZLI-2293) that are used in display. We found thatthe anchoring energy of ZLI-2293 on CYTOP is almost eighteen times less and the pretilt angle is slightly larger than on AL-1254. It is expectedthat the low surface anchoring and high transmission in CYTOP cell will make CYTOP a suitable aligning agent for LC displays as well as manyother LC devices. # 2011 The Japan Society of Applied Physics

T

he molecular alignment of liquid crystals (LCs) on
the treated surfaces of solid substrates is of great
importance for the basic understanding of the interfacial phenomena as well as technological applications.1)
The bulk LC and the interfacial properties strongly inﬂuence
the electro-optic properties of liquid crystal displays(LCDs).2,3) It is well known that weak anchoring of theLC director at the interface with alignment layer can inprinciple reduce the operating voltages and can improve thesteepness of the electro-optic response of LCDs.4) For theperformance of multistable optical switching, weak anchoring is also desirable. Fast response times less than several
hundreds of microseconds are desired for high performance
LCDs. The main objective of this work is to show that a
perﬂuoropolymer (CYTOP) which is commonly used as anantireﬂection coating in organic light emitting devices
provides good planar alignment to a liquid crystal mixture(ZLI-2293). We measured some surface and bulk physicalproperties like anchoring energy, pretilt angle and rotational
viscosity at room temperature and compared with results
using another alignment layer AL-1254.
The nematic mixture ZLI-2293 was obtained from Merck.
We used amorphous perﬂuoropolymer poly[perﬂuoro(4-vinyloxy-1-butene)] (PPFVB, commercially known as
CYTOP). It has been used widely for antireﬂective coatingsand optical ﬁbers because of its low refractive index (1.34)
and high transmittance over a wide wavelength range(200 nm to 2 m).5,6) High solubility in ﬂuorinated solvent
makes it suitable for easy coating on substrate for various
applications in photonics such as nanoimprint lithography(NIL), organic light-emitting diodes (OLEDs7,8)), lasing,and in biological applications.9) Very recently we reported
several interesting physical phenomena related to anchoringtransition10,11) in nematic liquid crystals and also demonstrated bistability in dye-doped samples.12,13) In case ofsmectics it was shown to provide a shock-free homeotropicalignment.14)
The glass plates with indium–tin-oxide (ITO) werespin-coated with polyimide AL-1254 or perﬂuoropolymer(CYTOP), and were cured at 180 and 100  C for 1 h or
30 min, respectively. After curing, the plates were rubbed byusing a homemade rubbing machine in which commerciallyavailable soft velvet cloth with long ﬁbres was used. Theplates are assembled such that the rubbing directions wereset antiparallel. The rubbing strength were kept ﬁxed for all
the plates by adjusting the distance between the plate and theE-mail address: sdsp@uohyd.ernet.inroller. Milar spacers and glass beads were used to make thick(100 m) and thin (10 m) cells. The thickness of the cellis measured using ﬁber optic specrometer (Ocean Optic).
The cell of thickness 10 m was used for measuring
anchoring energy and rotational viscosity. Cells of thicknesses in the ranges 100 to 150 m were used for measuring
pretilt angle. The sample was injected into the empty cellthrough the capillary action in the nematic phase at room
temperature.
We adopted the crystal rotation method (CRM)15) tomeasure the pretilt angle. The transmitted intensity wasmeasured with photodiode as a function of the incidence
angle from 45 to +45 about the axis perpendicular to thelight beam and the rubbing direction. The transmitted
intensity was ﬁtted to the theoretical equation.15)
In order to measure anchoring energy we adopted a highﬁeld technique proposed by Yokoyama and Sprang.16) A
computer controlled setup was used to measure the optical
retardation (R) and sample capacitance (C) simultaneously.
A He–Ne laser of wavelength 632.8 nm was used as sourcefor optical measurements and the cell is placed between two
crossed Glan Thampson polarizers. A photoelastic modulator (PEM-100) was used to measure the retardation veryaccurately. The temperature of the cell was maintained by aInstec (mK 1000) temperature controller. A LCR meter(Agilent E4980A) was used to measure the cell capacitanceas a function of voltage. The transmitted light intensity I wasmeasured using a photodiode. The anchoring strength (W) isestimated from the following equation:16,19)RIð; ; Þ 2K11
;¼R0CVWdð1Þ

where R0 is the retardation at V ¼ 0 and R=R0 is thenormalized phase retardation. The parameters , , and  arerelated to dielectric constants, elastic constants and refractive indices respectively. Hence Ið; ; Þ depends on thematerial parameters. Here,  is a constant dependent on thegeometrical factor of the cell (the area and the thickness).
K11 and d are the splay elastic constant and thickness of thesample respectively. W can be determined from the interceptof the plot R=R0 as a function of 1=CV , provided that K11and d are known.
To measure rotational viscosity ( 1 ) we adopted relaxation method.17) We measured the transmitted light intensity
through a planar aligned LC cell, to obtain the optical phaseas a function of time. The phase change ðtÞ as a function oftime t was measured from the time dependent intensity, IðtÞ,using the following equation: IðtÞ ¼ I0 sin2 ½ðtot  ðtÞÞ=2,040203-1# 2011 The Japan Society of Applied Physics

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Jpn. J. Appl. Phys. 50 (2011) 040203

T. A. Kumar et al.

Fig. 1. (Color online) (a) ITO plates coated with AL-1254 (left plate withgreen dot at the right corner) and CYTOP (right plate with red dot at theright corner). Photomicrographs of CYTOP coated cell at room temperature:(b) rubbing direction ¼ 0 and (c) rubbing direction ¼ 45 .

where I0 is the maximum intensity change. The decay
time is given by the following equation:17,18) ðtÞ ¼ð0Þ expð2t= 0 Þ, where ð0Þ is the total phase change of
the LC cell under a bias voltage VB . The slope of the linear
plot ln½ð0Þ=ðtÞ is 2= 0 , which yields the relaxation time(0 ). The rotational viscosity ( 1 ) of the sample is given by 1 ¼  0 K11 2 =d 2 . All the experiments were performed atroom temperature (27  C).
Figure 1(a) shows the visible diﬀerence between the AL1254 and CYTOP coated on ITO substrate. CYTOP coatedplate appears more transparent than AL-1254 due to the hightransmission of CYTOP. The texture of nematic phase ofZLI-2293 in rubbed cell under polarizing optical microscopewith the rubbing direction ¼ 0 and 45 are also shown inFigs. 1(b) and 1(c), respectively. A complete dark state( ¼ 0 ) and a uniform bright state ( ¼ 45 ) indicate good
uniform alignment of director in the entire nematic phase.
In order to determine pretilt angle, transmittance intensity
through a thick cell was measured as a function of incident
angle. The result on a CYTOP coated cell is shown in Fig. 2.
A good ﬁt to the theoretical equation gives pretilt angle
 2 . In case of AL-1254 it is  1 , which is negligibly
smaller than CYTOP coated cell. To measure the anchoringenergy, the retardation (R) and the sample capacitance (C)were measured simultaneously. Typical variation of normalized retardation (R=R0 ) and capacitance C=C0 as a functionof applied voltage is shown in Fig. 3. We obtain thethreshold voltage Vth ¼ 1:16 V and estimated splay elastic
constant K11 ’ 8:9  1012 N. In Fig. 4 we show the variation of R=R0 with 1=CV in the voltage range of 8–17 V.
Good linear ﬁts were obtained in the above voltage range.
From the intercept to the ordinate axis we obtain theanchoring strength; W ’ 1:0  104 J/m2 in a CYTOP celland W ’ 18:3  104 J/m2 in a AL-1254 cell. Note that W
in a CYTOP cell is more than eighteen times smaller than W
in a AL-1254 cell. Similar values of anchoring energy(W  17  104 J/m2 ) of ZLI-2293 on other polyimide atroom temperature was reported by You et al.19) Low surface anchoring properties of 4-cyano-40 -pentyl-1,10 -biphenyl(5CB) on Langmuir–Blodgett ﬁlms of similar polymer(perﬂuoropolyether) was reported by Russel-Tanner et al.20)

Fig. 2. (Color online) Transmitted intensity versus incident angle for
ZLI-2293 in a CYTOP coated cell at room temperature. The circles
represent the experimental data points and the solid line denotes the best ﬁt
to the theoretical equation.15)

Fig. 3. (Color online) Variation of normalized retardation (R=R0 ) and theratio of the sample capacitance to the empty cell capacitance (C=C0 ) as afunction of voltage in a CYTOP coated cell at room temperature.

Fig. 4. (Color online) The variation of R=R0 versus 1=CV in AL-1254
and CYTOP coated cells at room temperature. The data are well ﬁtted toeq. (1).

Furthermore, CYTOP also provides perfect homeotropicalignment of smectic liquid crystals exhibiting its low
surface anchoring properties.14)

040203-2# 2011 The Japan Society of Applied Physics

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Jpn. J. Appl. Phys. 50 (2011) 040203

T. A. Kumar et al.
Table I. Summary of the physical properties of ZLI-2293 on bothalignment layer, CYTOP and AL-1254.

W(104 J/m2 )(deg)

1(Pa s)

CYTOP

1.0

20.08

AL-1254

18.3

10.10

performances using many other nematic mixtures should bestudied for further details.

Fig. 5. (Color online) ln½ð0Þ=ðtÞ versus time t for ZLI-2293 at roomtemperature. The open circles represent the experimental data for bothalignment layers and the solid lines represent the best ﬁt to the theoreticalequation ln½ð0Þ=ðtÞ ¼ 2t=0 . Cell thickness d ¼ 9:5  0:5 m (AL1254) and 10:6  0:5 m (CYTOP), respectively.

In Fig. 5 we show the variation of ln½ð0Þ=ðtÞ as afunction of time t. A good ﬁt of the experimental data withthe theoretical equation is obtained. The slope of this linear
plot is 2= 0 , which yields the relaxation times 107 and99 ms in CYTOP and AL-1254 coated cells, respectively.
The calculated rotational viscosities are 0:08  0:01 and0:10  0:01 Pa s respectively, and agree within an experimental error due to the cell thickness measurement, theaccuracy of which is within 5%. Rotational viscosityof this compound was also measured by using a transientcurrent method by Imai et al.21) and was slightly larger(0.14 Pa s) than our results. Finally we present the summaryof the obtained quantities, anchoring strength (W), pretilt
angle ( ), and rotational viscosity ( 1 ) in cells coated withCYTOP and AL-1254 in Table I. It is noted that theanchoring energy in CYTOP coated cell is signiﬁcantly low
compared to AL-1254 coated cell.
In conclusion, our measurements show that CYTOP is agood candidate for planar alignment of ZLI-2293. It provides much lower surface anchoring energy than AL-1254.
The low anchoring energy is attributed to the low surface
free energy and is supported by the fact that CYTOP
provides larger contact angle than other alignment layersfor liquid crystals.14) The pretilt angles are not signiﬁcantly
diﬀerent and hence suitable for the application. It is expectedthat the advantage of high transmission through CYTOP cell
will be very useful for better performance. However, display

Acknowledgements One of the authors Surajit Dhara gratefully acknowledges the support from the Department of Science and Technology, Governmentof India for the project number SR/FTP/PS-48/2006 under the Science andEngineering Research Council Fast Track Scheme and Center for Advanced
Studies, School of Physics. We acknowledge Ashai Glass for supplying CYTOP.
Tatipamula Arun Kumar acknowledges Center for Advanced Studies, School ofPhysics for the fellowship.

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