Influence of Alkylene Spacer Length on Thermal Enhancement of Photoinduced Optical Anisotropy in Photo-Cross-Linkable Liquid Crystalline Polymeric Films and Their Composites with Non-Liquid-Crystalline Monomers

Abstract

Thermally enhanced photoinduced reorientation in a photo-cross-linkable liquid crystalline polymethacrylate comprised of 4-(ω-cinnamoyloxyalkyloxy)biphenyl side groups is investigated by irradiating with linearly polarized ultraviolet (LPUV) light and subsequent annealing at elevated temperatures. An odd−even effect of the alkylene spacer length between the cinnamate end group and biphenyl group on the transition temperatures of the polymer, photoinduced optical anisotropy, and thermal amplification of the in-plane molecular reorientation of thin films are observed. Because of the higher straight-line characteristics of the mesogenic side groups with an odd number alkylene spacer, the axis-selectivity of the anisotropic photoreaction of the photoreactive groups is greater than that with an even number, which leads to a larger photoinduced optical anisotropy and effective molecular reorientation. Finally, the cooperative reorientation in composites of the polymer and nonliquid crystalline monomer is explored to amplify the reorientational order of the film. The resultant reoriented composite film exhibits an in-plane order parameter of 0.52 and birefringence at 633 nm of 0.15.