Photoreaction of Photo-cross-linkable Methacrylate Polymer Films Comprising 2-Cinnamoyloxyethoxybiphenyl Side Group by Linearly Polarized Ultraviolet Light and Liquid Crystal Alignment on the Resultant Films

This paper presents photoreaction of methacrylate polymer films comprising a 2-cinnamoyloxyethoxybiphenyl mesogenic side group by a linearly polarized (LP) ultraviolet (UV) light and an alignment behavior of liquid crystal (LC) on the resultant films. The effect of methylene spacer length between the polymer backbone and the side group on the photochemical and the photoalignment behaviors was studied. When the film was irradiated near the clearing temperature of the polymer, a photoinduced thermal orientation process of the mesogenic groups was generated along the photo-cross-linked mesogenic groups that were parallel to the electric vector of the incident LP-UV light. The degree of the orientation was dependent on the spacer length and the thermotropic nature of the polymers. The uniform LC alignment was achieved on the LP-UV irradiated films, and the LC alignment direction changed from homeotropic to homogeneous with increasing the exposure doses. It needed longer irradiation time to achieve a homogeneous alignment when using a polymer with longer spacer. The alignment behavior could be explained by the (re)orientation of the polymer film and the interaction between the LC and the photo-cross-linked mesogenic group, methylene spacer or the polymer backbone.