THERMAL AND OPTICAL STABILITIES OF PHOTOISOMERIZABLE POLYIMIDE LAYERSFOR NEMATIC LIQUID-CRYSTAL ALIGNMENTS

We have investigated the thermal and optical stabilities of the photoa lignment layers for nematic liquid crystals (LCs). For the photoalignm ent layers, three kinds of photoisomerizable polymer systems were stud ied; a polyamic acid doped with azobenzene molecules, a side-chain-sub stituted polyamic acid with azobenzene units, and a main-chain-substit uted 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 spect ra 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 po lyimide system exhibited good unidirectional LC alignment and excellen t thermal (300 degrees C for 1 h) and optical stabilities. Moreover, w e suggest a new procedure, in which the photoillumination process is c arried out before the thermal imidization process, to achieve a stable photoalignment layer.