Study on the molecular orientation of liquid crystals induced by the photoreactive polymers in various molecular environment

Abstract

Liquid crystal displays (LCD) are light-controlling devices which rely on the optical properties of liquid crystal materials to control the transmission of light from an external source. The successful operation of twisted nematic liquid crystal display requires uniform alignment of liquid crystal molecules on substrate, which is currently achieved by confining the liquid crystal between liquid crystal alignment layers formed on surface of ITO layer. Liquid crystal photoalignment, a nonrubbing liquid crystal alignment method, is a promising technology for manufacturing a variety of LCD. This technique avoids many drawbacks of the traditional rubbing technique for liquid crystal alignment, such as sample contamination and static charge generation. The liquid crystal photoalignment typically uses films of photo-cross-linkable polymers where photosensitive side groups are dimerized via illumination with linearly polarized UV light. Several types of photo-cross-linkable polymers have been used for liquid crystal photoalignment. Among them, polymers containing cinnamate and coumarin photo-cross-linkable groups were most widely studied. Photodimerization, appearing in such polymer films upon illumination with linearly polarized UV light, leads to the generation of surface anisotropy and unidirectional liquid crystal alignment. However, the thermal properties of this polymer are poor and the surface anisotropy is easily disrupted by thermal treatment. The thermal stability of liquid crystal orientation is very important factor for the commercial application of photoalignment method. In order to enhance the properties of liquid crystal orientation on photodimerizable polymers and to give new functionality on the alignment layer, other polymers were mixed with the photoreactive polymers and the modification of molecular structure of photoreactive polymers was studied. In this thesis, we studied the effect of molecular environment of photoreactive polymers on the liqui...