Optical birefringent film by photoreactive high- and low-molecular liquid crystal composites

Abstract

New optical birefringent film for liquid crystal display (LCD) is described using composite materials consisting of photo-cross-linkable polymer liquid crystal (PLC) and photoreactive bifunctional monomers. The alignment of mesogenic groups in both PLC and monomers was achieved by irradiation with linearly polarized ultraviolet (LPUV) light and subsequent annealing. Axis-selective photo-cross-linking reaction of the mesogenic group in the PLC occurred by exposure with LPUV light, although the induced birefringence was very small. The annealing process generates a thermal reorientation of monomers in a direction parallel to the photo-cross-linked mesogenic groups, resulting in a reversion of the orientation direction and thermal enhancement of the alignment of the film. Since the orientation direction is parallel to the polarization direction of LPUV light, three-dimensional orientation can be feasible by oblique irradiation with LPUV light, and the angle of the molecular orientation is controlled by changing the exposure angle. By the use of this optical birefringent film, the improvement of the viewing characteristics of twisted-nematic (TN) LCD is demonstrated.