Reversion of alignment direction in the thermally enhanced photoorientation of photo-cross-linkable polymer liquid crystal films

Reversion of the in-plane reorientation direction of mesogenic groups has been observed for the first time in novel polymethacrylate liquid crystal (PLC) films substituted with a 4-methoxy-cinnamoyloxybiphenyl side group. The reversion was generated by irradiation with linearly polarized ultraviolet (LPUV) light and a subsequent annealing, Irradiation with LPUV light induces negative optical anisotropy of the films as a result of an axis-selective photoreaction of the side groups. The direction of the thermally enhanced reorientation is dependent on the degree of photoreaction and the distribution of photoproducts, while the induced orientational order in both directions, S, was larger than 0.5. The distribution of photoproducts in PLC films has been analyzed to elucidate their contribution to the thermally enhanced reorientation behavior. Initially upon photoreaction, thermal enhancement of the photoinduced negative optical anisotropy was observed. However, when the degree of photodimerization was 15% or greater, the direction of the thermally enhanced reorientation was found to be parallel to the polarization direction (E) of LPUV light. It is concluded that a small amount of photoproduct plays a role in the thermal amplification of the photoinduced negative optical anisotropy in a manner identical to that of PLC with azobenzene side groups. In contrast, photodimerized mesogenic groups generated reversion of the orientational direction and enhancement of positive optical anisotropy of the film through annealing.