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

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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.