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.

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Polarization-selective photosensitization and molecular reorientation in photo-crosslinkable polymer liquid crystal (PMCB6M) films, based on polarization-selective triplet energy transfer using LP-365 nm and LP-405 nm light with different photosensitizers, are described. A dramatic improvement in the photoreactivity of the film was observed with a small amount of photosensitizer; and a high degree of molecular reorientation and slantwise reorientation were generated when annealing the resulting films at elevated temperatures. The efficiency of reorientation depended on the photoinduced dichroism of the film.

Large birefringence and polarization holographic gratings formed in photocross-linkable polymer liquid crystals comprising bistolane mesogenic side groups

Thermally enhanced photoinduced in‐plane molecular reorientation in new photo‐cross‐linkable polymer liquid crystalline (PPLC) films comprising 4‐[ω‐(4‐methoxycinnamoyloxy)alkyloxy]biphenyl side groups is explored using linearly polarized ultraviolet (LPUV) light exposure and subsequent annealing. The influence of the alkylene spacer length between the photo‐cross‐linkable group and the biphenyl mesogenic moiety is investigated. The straight‐line characteristics of the photoreactive mesogenic side group and the 4‐methoxycinnamoyl group play important roles in the high photoreactivity and the large thermally enhanced molecular reorientation, where the in‐plane order and the birefringence of the reoriented film are 0.61 and 0.18 are obtained. Finally, cooperative molecular reorientation of dichroic dyes doped in PPLC films is also achieved to fabricate a linear polarizer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4712–4718, 2008