Ordering phenomena and photoorientation processes in photochromic thin films of LC chiral azobenzene-containing polymer systems

Abstract

The processes of mesogen ordering in nematic and cholesteric side-chain polymer systems were studied by polarized optical spectroscopy. For this purpose, nematic azobenzene-containing copolyacrylate was synthesized and its cholesteric mixture with chiral dopant was prepared. Thin films of copolymer and mixture were prepared by spin-coating using polyimide-coated and rubbed glass and quartz substrates. Film thickness was comparable or less than helix pitch value. It was found that azobenzene and phenylbenzoate side groups of nematic copolymer are oriented after annealing along a rubbing direction. Cholesteric mixture demonstrates a more complicated behaviour, the direction of orientation of the side-chain groups is rotated at certain angle depending on the film thickness. Dichroism in azobenzene groups’ absorbance is lower for cholesteric mixture and decreases with film thickness increasing. A possible mechanism of ordering of the side-chain groups in these thin films was proposed based on the helical order formation. Photooptical switching of the orientation and an influence of the nonpolarized and polarized light on the cholesteric thin films were studied demonstrating a possibility of using such thin films of photochromic cholesterics as promising photoswitchable materials.