Thermal and Optical Stabilities of Photoisomerizable Polyimide Layers for Nematic Liquid Crystal Alignments

Abstract

We have investigated the thermal and optical stabilities of the photoalignment layers for nematic liquid crystals (LCs). For the photoalignment layers, three kinds of photoisomerizable polymer systems were studied; a polyamic acid doped with azobenzene molecules, a side-chain-substituted polyamic acid with azobenzene units, and a main-chain-substituted polyamic acid with azobenzene units. Photoinduced anisotropy was produced by illumination with linearly polarized near-UV light from a mercury lamp, and evaluated by measuring polarized UV absorption spectra and optical birefringence. The nematic LC-alignment properties were investigated using the photoalignment layers processed under various thermal and optical conditions. It was observed that the main-chain polyimide system exhibited good unidirectional LC alignment and excellent thermal (300°C for 1 h) and optical stabilities. Moreover, we suggest a new procedure, in which the photoillumination process is carried out before the thermal imidization process, to achieve a stable photoalignment layer.