Langmuir 1991, 7, 1314-1315
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Multifarious Liquid Crystalline Textures Formed on a Photochromic Azobenzene Polymer Film? Yuji Kawanishi,' Takashi Tamaki, Takahiro Seki, Masako Sakuragi, Yasuzo Suzuki, and Kunihiro Ichimura' Research Institute for Polymers and Textiles, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan
Koso Aoki Toda Kogyo Corporation, 4-1 -2 Funairi-Minami, Naka-ku, Hiroshima 730, Japan Received February 27, 1991. In Final Form: April 23,1991 The photochromic reaction of an azobenzene (Az) pendent polymer film regulated the alignment and the texture of the nematic liquid crystalline(LC) phase. The nematic phase exhibitedthree characteristic textures, marbled, homeotropic, and schlieren textures, and one of which was alternatively obtained on the photochromic Az polymer surface by controlling the exposure. The texture change was accompanied by the birefringence change of the nematic phase and the absorbance change of the doped dichroic dye. Disappearance of homogeneously aligned domains of the nematic phase corresponded to deformation of an oriented structure of the Az polymer. The multifarious LC textures are discussed in regard to the morphological changes of the oriented Az surface. The alignment of the liquid crystalline (LC) phase is strongly affected by the surface to which LC is attached.* A clean mica surface induces perpendicular (homeotropic) alignment, while a rubbed glass induces parallel alignment in general. We have recently demonstrated the regulation of the nematic LC alignment by the photo~ ~ LC chromic reaction of surface azobenzenes ( A z ) . The alignment was changed from the perpendicular state to the parallel state by the trans cis photoisomerization of Az, and vice versa. Here, we report new characteristics of the LC alignment change. The LC phase is regulated photochemically to display three distinctive textures: marbled, homeotropic, and schlieren textures. A thin film of poly(ll-(4-(4-hexylphenylazo)phenyl0xy)undecyl methacrylate), P(C&OC110)MA, was formed on a quartz plate by spin-coating and baked at 130 "C for 1 h. A nematic mixture of phenylcyclohexanecarboxylate derivatives DON103 (Rodic Co.), which has a negative dielectric anisotropy, was placed between a pair of the plates with 8 km spacers. Structures of and abbreviations for chemicals are shown in Figure 1. The texture of the LC phase was studied by polarizing microscopy. The LC phase initially displayed the parallel, marbled texture (A in Figure 2) consisting of many domains, each with its own homogeneous axis and tilting angle. The cell was exposed alternately to UV (365 nm) and visible (440 nm) light of a 500-W high-pressure Hg lamp. The LC texture was changed from the marbled into the homeotropic a t first (Bin Figure 2), then into the schlieren (C in Figure 2) along with UV exposure. The trans isomerchange was reversible: subsequent cis ization of the Az surface by visible light exposure gave the reverse process. The homeotropic B state appeared between two distinctive parallel textures. The B state was stable unless the cell was overexposed.
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* To whom correspondence should be addressed.
+ Reversible Alignment Change of Liquid Crystals Induced by Photochromic Molecular Films: Part 7. For Part 6, cf. ref 4. Present address: Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan. (1)Cognard, J. Alignment of Nematic Liquid Crystals and Their
Mixtures (Molecular Cryrtals and Liquid Crystals, Supplement Series I ) ; Gordon and Breach: New York, 1982. (2) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir
1988, 4, 1214. (3) Ichimura, K.; Suzuki, Y.;Seki,T.; Kawanishi, Y.; Aoki, romol. Chem., Rapid Commun. 1989, 10, 5.
K. Mak-
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LCD118 Figure 1. Chemical structures of and abbreviations for Az polymer, nematic DON103, and dichroic LCD116 The alignment change can be detected as the birefringence change of the cell (Figure 3). When the LC phase contains dichroic dye LCD118 (Nippon Kayaku) as a guest molecule, the absorbance of the dye is changed with the LC alignment change (Figure 4). The absorbance of the cell was changed in the order of C > A > B, which should be equivalent to the order of the tilting angle. The raise in the tilting angle could be related to energy increase of the Az surface along the trans cis isomerization. However, the tilting angle in the B state is smaller than in the A state despite higher cis-Az concentration in the B state. A mechanism of the LC photoregulation was considered with regard to the polymer morphology. The film was opaque initially, and it became transparent after UV exposure of short duration. Judging by the appearance of the film, the polymer seems to have a highly ordered structure like crystals when the trans-Az is dominant. The film loses the structure after cis-Az is produced to some
0743-7463/91/2407-1314$02.50/0 0 1991 American Chemical Society
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Letters
Langmuir, Vol. 7, No. 7, 1991 1315
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Figure 3. Polarizingtransmittance (retardation) chhngesof the photoresponsiveLC cell at 633 nm upon alternate exposure to UV and visible light.
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Figure 2. Polarizing micrographs of the photoresponsive LC textures: (A)marbled texture observed before exposureor after sufficientvisible light exposure; (B)transient homeotropictexture obtained during UV exposure; (C) schlieren texture obtained after sufficientV V exposure. The bar represents 100 pm.
Figure 4. Absorption spectra of the guest-host LC cell upon alternate exposureto W and visible light at the marbled texture (A), at the homeotropictexture (B), and at the schlieren texture (C). The cell contains 1.0 w t 96 of dichroic dye LCD118. extent by UV exposure. The ordered structure of the polymer probably originates from orientation of trans-Az units. Such morphological variation in the polymer surface should reflect on the multifarious LC textures. It has been proven that Az polymer surfaces often acquire LC photoregulating ability after the LC phase is heated to isotropic melt to form an effective interface in which the long axes of Az units are parallel to those of LC molecules.4 When the trans-Az units form the oriented structure, the effective parallel interaction between Az and the LC molecules becomes difficult, and the alignment is rather affected by the macroscopic structure and/or other interactive sites of the oriented polymer surface. Thus, the parallel marbled texture is induced by the oriented trans-Az surface. The appearance of the transient homeotropic B state during UV exposure is related with liberation of trans& units from the oriented structure. Prolonged UV exposureincreasesthe cis-Az concentration in the polymer, and thus, the random schlieren texture of the LC phase appears on the resulting polar and structureless polymer surface. Understanding the LC aligning mechanism is urgently required, not only for the present LC photoregulationeffect but also for all systems involving the LC molecules. Morphological analysis of the surface and the LC phase should be carried out. Further experimentsare in process. (4) Kawanishi, Y.; Seki,T.;Tamaki, T.; Ichimura, K.; Red4 M.;Aoki, K. Polym. Ado. Technol.,in press.
