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 liquid crystal orientation. We prepared the various types of photoalignment materials and investigated the liquid crystal orientation on these polymers. The liquid crystal orientation was dominated by the reaction of photosensitive group in polymers and the reaction of this group was related with the molecular environment surrounding the photosensitive group.
Various types of polyimides are blended with photoreactive polymers due to the higher thermal stability of polyimide. Polyimide plays a role of matrix for photoreactive groups, such as cinnamate and coumarin, and the thermal stability of liquid crystal orientation of the blends was affected by the matrix. The relaxation of photodimers of photoreactive groups induced by thermal energy was responsible for the low thermal stability of liquid crystal orientation and the thermal relaxation was suppressed by rigid polyimide matrix. Besides the enhancement of thermal stability of liquid crystal orientation, the surface modification of blend alignment layer was possible by using the fluorinated polyimides. The change of surface energy of alignment layer was related with the polar anchoring of liquid crystals and fluorine has effect on the generation of pretilt angle of liquid crystals. However, the polyimides are also known for the photodecomposition of imide group and the possibility of application for photoalignment layer was reported. It was found that the photodecomposition of polyimide matrix of blend alignment layer had an effect on the liquid crystal orientation and the competition between photoreactive polymer and polyimide was occurred. The photoreactivity of polyimide was dominant factor for determining liquid crystal orientation of photoreactive polymer/polyimide blends and the control of photoreaction of polyimide was possible by modulating the wavelength of irradiation light.
However, polymer blend system has fundamental problems such as a miscibility of component polymers and a side effect of matrix polymer on liquid crystal orientation. In order to overcome the problems of blend system, we prepared novel photoreactive polymer with various molecular structure. Polymer backbone is important for the determining the characteristic properties of polymers.
We modified the polymer backbone of photoreactive polymers and investigated the effect of flexibility of polymers on the photoreaction of photosensitive groups and liquid crystal orientation. First, we introduced the rigid polyimide on photoreactive polymers in order to enhance the thermal stability of liquid crystal orientation. However, the rigid polymer backbone has a negative effect on the dimerization behavior of photoreactive groups and the reactivity of them was significantly decreased. The reduced photoreactivity of the polymer could not induce the anisotropic distribution of photodimers and the liquid crystal orientation was significantly restricted. In order to investigate the effect of flexibility of polymers on the reactivity of photosensitive groups, we employed the flexible e thylene c hains a s a p olymer b ackbone and p repared m ore flexible photoreactive polymers. In spite of the low glass transition temperature of flexible polymer, the thermal stability of liquid crystal orientation was excellent. This extraordinary phenomenon was caused by the dimerization reaction of photoreactive groups by thermal treatment. Due to the flexibility of photoreactive polymer, the photoreactive groups could possess the enough mobility for the dimerization reaction of themselves. After the anisotropic photoreaction of photoreactive groups, remaining photoreactive groups was additionally dimerized by thermal heating and the anisotropic distribution of photodimers was fixed. The flexibility of photoreactive polymer was also related with the liquid crystal orientation direction and the pretilt angle of liquid crystal was affected by the rigidity of photoreactive polymer.