Photoinduced alignment of azobenzene side-chain polymers with short spacer: Biaxial orientation and fast response

Side-chain azobenzene-containing liquid crystalline polymer (ALCP) of Poly(6-[4-(4-cyanophenylazo)phenoxy] x-methylene methacrylate)(Px, x is 2 or 6) was synthesized and used to study its photoinduced alignment behavior irradiated by a linearly polarized laser at room temperature. The relationships between transmittance and irradiation time as well as transmittance and various incident angles were studied in detail. The oversaturation phenomenon of P-2 film was found to be higher than 1 mW/cm(2) under irradiation energy density. A model of the biaxial orientation was introduced to explain the phenomenon: it is the rigid short spacer that makes it possible for mesogen to coexist in a way of out-of-plane and in-plane orientation (biaxial orientation), in which the type of photoinduced motions is a single mesogen motion in the film of P-2, whereas in the film of P-6 with longer spacer, the motion is a microdomain motion. The whole domain motion restricted the out-of-plane orientation, which shows a slow orientation rate and all mesogens aligned within the film plane. A novel dynamic fitting was also presented to describe the orientation of P2, which was discussed in terms of the fitting parameters. (c) 2006 Wiley Periodicals, Inc.