The Surface Morphology Effects on Liquid Crystal Alignment After Photo-Agitation and Crosslinking of Polyimide

The photo-reactive activators are highly reactive radical generators upon the ultraviolet (UV) light illumination. The photo-reactive initiators produced nitrogen radical and alkyl radical after releasing carbon dioxide. The radicals could react with polyimide (PI) main chains. These reactions enforced the alignment layers and exhibited high azimuthal anchoring energy. The thickness of photo-irradiated PI alignment layers were reduced dramatically by photo-induced crosslinking, which induced surface wrinkling and roughness. The carbon dioxide gases released from the thin films produced many micro-pores, which provides tight anchoring of liquid crystal (LC) molecules. The azimuthal anchoring energy obtain by photo-alignment was better than that obtain by rubbing method with the same PI. The maximum value was 6.92×10⁻⁵ J/m². Small aliphatic hydrocarbons, such as methane and propene, were released during photo-decomposition reaction from the PI surface. The polarity of film surface became more hydrophilic. The photo-alignment of LC was perpendicular to the polarization axis of UV light. On the basis of high anchoring energy, the rough surface, hydrophilic surface, and rapid photo-reactions, the photo-alignment mechanism is proposed.