ABSTRACT
MOONHOR REE,4 SEUNG WOO LEE, SANG IL KIM, WOOYOUNG CHOI and BYEONGDU LEE Department of Chemistry, Center for Integrated Molecular Systems, BK21 Program, and Polymer Research Institute, Division of Molecular and Life Sciences, Pohang University o f Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, The Republic o f Korea
Abstract-New soluble, photosensitive polyimides containing cinnamoyl and coumarin moieties as side groups were synthesized. Cinnamoyl group can undergo both [2+2] photodimerization and trans-cis photoisomerization but coumarin group undergoes only [2+2] photodimerization. Photoinduced alignment behaviors of the polyimide chains were investigated by irradiating with linearly polarized ultraviolet light (LPUVL). Both polymers in film forms gave a maximum dichroic ratio and optical retardation at 0.25-0.50 J/cm2 exposure dose, indicating that the photoreaction induced polymer chains to orient preferentially. Such oriented polymers in a film form were found to induce nematic liquid-crystals (LCs) to align along a direction of 97-99° with respect to the electric vector of LPUVL, regardless of the kind of photosensitive side group. Such LC alignment behavior was observed even on films treated by multiple exposures with changing electric vector of LPUVL. In these cases, the director of LC alignment was governed mainly by the first exposure rather than the subsequent exposures with changing electric vector. The results suggest that the homogeneous, uni axial alignment of LCs on the polymer film containing cinnamoyl side group is induced mainly by [2+2] photodimerization rather than trans-cis photoisomerization. As another clue for the [2+2] photodimerization, the formation of cyclobutyl group was spectrocopically detected from the films exposed to UV light. In addition, LC alignment on the films was found to be controlled mainly by the rubbing process, regardless of the sequence of LPUVL exposure and rubbing in the surface treatment process.
