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Perfluoropolymer Surface for Shock-Free Homeotropic
Alignment of Smectic Liquid Crystals
By Soon Moon Jeong, Jin Ki Kim, Yoshio Shimbo, Fumito Araoka,
Surajit Dhara, Na Young Ha, Ken Ishikawa, and Hideo Takezoe*

In order to use liquid crystals (LCs) in practical applications suchas liquid crystal displays (LCDs) and photonic optical devices, a
uniform director orientation of LC molecules is essential. In
particular, vertical alignment (VA) of nematic LC molecules has
been studied and applied in VA-mode LCDs, owing to theirsuperior properties such as wide viewing angle and high contrast
ratio compared to the conventional twisted-nematic display.
Recently, an inorganic silicon oxide (SiOx) layer has been
intensely studied as a VA layer because of its robust properties
and good performance even under harsh conditions, such as
intense light and high temperature, that cause deterioration oforganic materials.[1–5] However, a vacuum process is not appropriate for large-area processing. Hence side-chain polyimide–coated surfaces are used for VA-LCDs. Here we demonstrate theadvantages of an amorphous perfluoropolymer as a very stable VA
layer for smectic LCs compared to conventional alignment layers.
Typically, low surface-energy substrates are preferred to obtainhomeotropic alignment of LC materials because the minimizedenergy of the system is obtained when the LC molecules are incontact with each other rather than with the surface. Usually the
homeotropic alignment of LCs is realized by the hydrophobic
surface if there is no preferential orientation or surface
roughness.[6,7] One way to achieve such alignment of LCs is toadsorb a surfactant monolayer on the surface to render ithydrophobic.[8,9] Geometrical factors can also be responsible; forexample, LC molecules align parallel to the flexible long chains ofa surfactant attached to substrate surfaces perpendicularly.[10]However, the surfactant alignment is not stable because the small
molecules of the surfactants gradually dissolve in the LCs. Henceside-chain polymers, in particular polyimides, are used inpractice, as mentioned above. It is well known that fluorine iseffective for lowering the surface free energy chemically because
fluorine has a small atomic radius and the largest electronegativity among the atoms, so that it forms a stable covalentbond with carbon.[11] Various fluorine polymers, in particularpolytetrafluoroethylene (PTFE), have been developed in many
fields, because of their ultimate resistance to hostile chemical
environments and high service temperatures owing to the highbonding energy of C–F (485 kJ mol1) and C–C (360 kJ mol1)bonds.[12] However, PTFE is insoluble, which makes vacuum
deposition indispensable.
The amorphous perfluoropolymer poly[perfluoro(4-vinyloxy-lbutene)] (PPFVB, also known under the commercial name Cytop)
has been used for antireflective coatings and optical fibers
because of its low refractive index of 1.34 and high transmittanceover a wide wavelength range (200 nm to 2 mm).[13,14] It also hashigh solubility in perfluorinated solvents and is highly resistant tochemical and thermal stimuli. These properties make deposition[*] Prof. H. Takezoe, Dr. S. M. Jeong,[+] J. K. Kim, Dr. Y. Shimbo,
Dr. F. Araoka, Prof. K. Ishikawa
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)
E-mail: takezoe.h.aa@m.titech.ac.jp
Dr. S. Dhara
School of Physics
University of Hyderabad
Hyderabad 500046 (India)
Prof. N. Y. Ha
Department of Physics and Division of Energy Systems Research
Ajou University443-749 Suwon (Republic of Korea)[+] Present address: Central Technical Research Laboratory, Nippon Oil
Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan

DOI: 10.1002/adma.200901777

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Figure 1. a) Schematic illustration of random (left) and vertical (right)
orientation of smectic LCs. b) Molecular structure of the alignment layer,
PPFVB, used in this work. Photograph (c) and transmittance spectra (d) of
bare and perfluoropolymer-coated ITO glass.

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Figure 3. a) Mechanical pressing test by tweezers. Spontaneous selfrecovery to vertically aligned SmA LCs is realized in a PPFVB-coated cell.
b) Magnified optical images before (left) and after (right) pressing for asilane-coated cell. Cracking is observed and there is no recovery.

Figure 2. a) Molecular structure and the phase sequence of the smectic LC(MBRA-8) used. b) Photomicrographs under crossed polarizers of MBRA-8
during cooling and heating processes in JALS204-, silane-, and
PPFVB-coated cells. Note that we manually increased the brightness of
the SmC* photos to visualize Schlieren textures. c) Photomicrograph of thephase transition from SmA to the isotropic state in a PPFVB-coated cellwithout polarizers. d) Photomicrographs of a vertically aligned nematic LC(CCN-47) in JALS204- and silane-coated cells at 40 8C.

of indium tin oxide (ITO) and nanoimprint lithography (NIL)
possible for photonic application in organic light-emitting diodes(OLEDs),[15] lasing,[16] and biological applications.[17] In thisCommunication we propose and demonstrate a new applicationof PPFVB as a VA layer for smectic LCs.
Smectic LCs have a one-dimensional (1D) positional order,
namely a layer structure, although they are liquid-like within eachlayer. Generally, the methods for homeotropic alignment ofnematic LCs can be applied for smectic phases. Therefore,
normally better alignment can be achieved in SmA LCs if materials
have the phase transition sequence IsoNSmA than forAdv. Mater. 2010, 22, 34–38

materials exhibiting the direct Iso–SmA transition.[18] However,alignment of smectic LC molecules is still very difficult and is easily
upset by mechanical shock, particularly in homogeneous (planar)alignment[19] because of the 1D periodic structure and limited
fluidity. So far, vertically aligned smectic LCs have not been applied
practically, although recently a display application using verticallyaligned smectic LCs such as bent-core SmAPR LCs,[20,21] andferroelectric SmC* LCs has been proposed.[22,23] However, it would
be useful to have a good alignment layer for vertically aligned
smectic LCs for future applications.
Figure 1a shows a schematic illustration of random orientation
and VA of SmA LCs. We employed a flat amorphous PPFVB layer
with low surface energy to achieve uniform VA. The molecular
structure of PPFVB is depicted in Figure 1b. PPFVB forms ahighly transparent film on an ITO-coated glass substrate owing to
its antireflection characteristic, as shown in Figures 1c and d.
To examine the superiority of the PPFVB layer over otheralignment layers for aligning smectic LCs, we prepared threekinds of substrates treated with a commercial homeotropicalignment layer (JALS204, Japan Synthetic Rubber), a silane
coupling agent (AY43-021, Dow Corning Toray), or PPFVB, andobserved the textures using a polarizing optical microscope (POM).
We chose a smectic LC material, 4-O-(2-methyl)-butylresorcylidene40 -octylaniline (MBRA-8), shown in Figure 2a. The material exhibits
the phase transition sequence Isotropic (56 8C) SmA (49 8C) SmC*
(36 8C) Crystal and was introduced into empty cells at atemperature above the isotropic phase transition (ca. 65 8C).

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www.advmat.deFigure 2b shows POM images of MBRA-8 sandwiched by
JALS204-, silane-, and PPFVB-coated substrates during coolingand heating. Broken fan-shaped texture was observed for
JALS204- and silane-coated cells. By contrast, we obtained very
good alignment in SmA if we used PPFVB-coated substrates.
Good alignment was retained even in SmC*, and then a typical
pattern of birefringent colors in random domains emerged in the
crystal phase. Quite surprisingly, the smectic layer seems torealign parallel to the PPFVB surface in SmC* and finally perfect
VA results in SmA. Such recovery is not possible using normal
homeotropic alignment agents, as shown by the textures in
JALS204- and silane-coated cells. To confirm that this dark image
represents vertically aligned SmA at 53 8C, and not the isotropic
phase, a texture of a PPFVB-coated cell was taken at 568C without
polarizers during the heating process. The phase transition from
SmA to the isotropic phase was clearly observed, as shown in
Figure 2c, indicating good VA in the SmA phase. It should be
noted that JALS204 and silane act as good alignment layers fornematic LCs, as exemplified by Figure 2d for the nematic LC
40 -trans-butyl-4-cyano-4-trans-heptyl-1,10 -bicyclohexane (CCN-47,
Merck) with negative dielectric anisotropy and the following
phase sequence: Isotropic (57.3 8C) N (28.2 8C) SmA (25.6 8C)
Crystal; both surfaces serve as good homeotropic alignment
layers.
Figure 3 shows how effective the PPFVB layer is as ahomeotropic alignment layer for the SmA phase of MBRA-8.
During POM observation, we used tweezers to exert mechanicalpressure on silane- and PPFVB-coated glass samples filled with
MBRA-8 to clarify the difference. While being pressed, thesilane-coated cell exhibited birefringence color because bendingof the upper glass substrate induced distortion of the smectic LC
layers. Bending-induced birefringence was also observed in the
PPFVB-coated cell. However, after the pressure had beenreleased, the layer deformation rapidly recovered to the original
vertically aligned state. As shown in the magnified picture in
Figure 3b, however, the smectic layer of MBRA-8 in thesilane-coated cell exhibited cracks after being pressed withtweezers and never returned to its original state. This is thenatural behavior arising because of the limited fluidity in SmA LC
cells with a conventional surface alignment layer. These results
can be explained by the different interfacial surface energies
between LCs and alignment layers. Owing to the low surface
energy of PPFVB, the interaction between the LC and thealignment layer decreases and intermolecular interaction among
LC molecules increases. Hence, vertical orientation of LC is much
more favorable on an alignment layer with low surface energy.[6,7]
Now we need to explain this phenomenon in more detailbecause the alignment of LC is induced by complex physicochemical interactions between the LC molecules and the aligned
films. To explain the orientational recovery to VA, several types ofinteractions must be considered, including p–p electron
coupling, geometric effect, dipole–dipole interaction, and vander Waals interaction. We assume that the dipole–dipole
interaction can be neglected because the polarity of the PPFVB

Figure 4. a) POM image of MLC6608 at 40 8C in a PPFVB-coated cell. b) Contact angles on JALS204-, silane-, and PPFVB-coated glass substrates. c)
Progress of the phase transition with time from crystal to SmC* phase. A–C) The transition starts from the grain boundary region. D–F) Realigned SmC*
regions are also formed (indicated by dotted arrows) and increase in volume over the whole sample area. Note that (D–F) were taken under an overexposed
condition compared with (A–C) to visualize Schlieren texture.

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surface is known to be very low. Low polar surface energy influorinated amorphous carbon supports our hypothesis.[24] We
also neglect the effects of p–p electron coupling and the geometriceffect because the spin-coated PPFVB layer was not rubbed and it
has a flat surface with high lubricity. The van der Waals
interaction is an attractive potential given by V ¼ –l/r6 between
molecules and by V ¼ –l/r3 between a surface and a molecule,[25,26] where r is the distance between two objects and l is aconstant depending on the properties of the objects. The value ofl is proportional to the product of the polarizabilities of thealignment layer and the LC medium.[3] As the polarizability isderived from its dielectric constants, (e(v) – 1)/(e(v) þ 2),[27]PPFVB has very low polarizability because of its low dielectric
constants (e ¼ 2.1). Hence, van der Waals interaction can also be
neglected.
Consequently, the smectic LC alignment on the PPFVB layer
cannot be explained by the competition of various interactions
between the alignment layer and LC molecules. Of course there
might be a preferred interaction, but this is insufficient to explain
homeotropic alignment of smectic LCs with limited fluidity
because the preferred force must be very strong. Recently, we
found that CCN-47 shows a discontinuous anchoring transition
from homogeneous to homeotropic with decreasing temperature;some nematic LCs are homeotropically aligned on PPFVB, andothers are not.[28] To examine the alignment of nematic LCs on
PPFVB, we also made POM observation of MLC6608 (Merck,
Nematic (90 8C) Isotopic, with negative dielectric anisotropy)
sandwiched between PPFVB-coated glass substrates. As shown in
Figure 4a, Schlieren texture was observed, indicating homogeneous alignment. However, PPFVB shows stronger hydrophobicity (contact angle 1148) than JALS204 (968) or silane
coupling agent (948), as shown in Figure 4b. These results
indicate that homeotropic alignment of nematic LCs cannot be
obtained simply by low surface energy of the alignment layer. On
the other hand, smectic LCs strongly favor homeotropicalignment, as mentioned above. Hence, the strong ability of
PPFVB to cause smectic LCs to align homeotropically must be
explained in light of the difference between smectic and nematicLCs; namely, different fluidity due to the existence or otherwise ofa layer structure. We simply employ a large driving force arisingfrom the different interfacial surface energies between the LCs
and alignment layers. The layered smectic plane is favorable tominimize contact area with PPFVB owing to its low surface
energy. It should be noted, however, that this can be different fornematic LCs, because the nematic phase usually has enoughfluidity that each molecule can interact with the alignment layerand LC molecules.
From this hypothesis the question arises as to how the crystal
phase is transformed to the SmC* phase with minimized freeenergy even though crystals have almost no fluidity. Figure 4cshows how the crystal transforms to SmC* with time. The
transition starts from the grain boundary region and the area
grows (A–C) because the grain boundaries are relatively unstable.
However, realigned regions (indicated by dotted arrows) also
appear and propagate to other grain regions to make SmC*
realigned over the whole sample area (D–F). (Note that D–F ofFigure 4c were taken under an overexposure condition comparedwith A–C of Figure 4c to visualize the textures.) This means that
LC molecules in layered planes reorient gradually with time to

Figure 5. a) Schematic illustration of a PPFVB-coated switching cell filledwith MBRA-8. Note that electric field was applied in the in-plane rubbing
direction. b,c) POM images of MBRA-8 at 40 8C in the PPFVB-coated cellwithout (b) and with (c) electric field.

decrease interfacial free energy with the layered structure
maintained. This results in perfect VA in the SmA phase over
the whole sample area.
Finally, we introduce a possible SmC* display device using
PPFVB as an alignment layer. Figure 5a shows a sample structure
filled with MBRA-8 sandwiched between PPFVB-coated glass
substrates. After antiparallel rubbing, two pieces of aluminum
foil were used as spacers to make a 13 mm thick cell as well as
electrodes with a 200 mm gap. The SmC* LC molecules aligned
vertically and formed a helical structure with the helix axis along
the surface normal, so that an optically isotropic view wasobserved when no voltage was applied, as shown in Figure 5b.
When a voltage was applied along the rubbing direction, polarswitching occurred to achieve uniform c-director orientation
perpendicular to the electric field. A uniform bright view was
obtained, since the crossed polarizers were 458 to the field
direction, as shown in Figure 5c. This device, which uses surface

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www.advmat.dealignment by PPFVB, has advantages over devices using othersurface alignment layers, namely, stable VA and self-recovery
from orientational damage caused by mechanical shock.
In summary, we have demonstrated VA of a SmA LC (MBRA-8)
with the direct Iso–SmA transition by employing an amorphous
perfluoropolymer (PPFVB) as an alignment layer. The selfrecovery to VA of SmA LCs was observed from deformed layer
orientations caused by crystallization or external pressure. The
high alignment ability for LC materials with Iso–SmA phasetransition and the spontaneous recovery effect are remarkable if
we consider the limited fluidity in the smectic phase. We believedthat these results have their origin in the strongly hydrophobic
surface of PPFVB, which has low polarity and low polarizability.

Experimental
The PPFVB used in our experiments is a mixture of PPFVB solution(CTX-809A, purchased from Asahi Glass Co.) and fluorinated solvent(CT-Solv.180). The solution CTX-809A contained 9 wt% PPFVB and was
diluted with the solvent to make a 3 wt% solution. The PPFVB alignment
layers were fabricated by spin-coating the solution onto the glass substrate
and were cured for 30 min at a temperature of 100 8C in an oven. The
thickness of PPFVB film was ca. 95 nm, as determined by a Dektak3ST(Veeco) surface profiler. The sample cell was made of PPFVB-coated glass
substrates and was supported by a spacer (ca. 5 mm). The LCs wereintroduced into an empty cell using capillary action in the isotropic phase.
The texture was observed under a polarizing microscope (Nikon,
Optiphot-Pol) equipped with a temperature-controlled Mettler-Toledo
FP90 hot stage. The cooling and heating speed was 2 8C min1. For
voltage-dependent POM images of the switching cell, a rectangular voltage(2 kV, f ¼ 3 kHz) was applied.
Received: May 27, 2009
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