Abstract
Linearly polarized light (LPL) irradiation of coumarin-substituted polymer films induces in-plane liquid crystal (LC) alignment. The direction of LC alignment depends on the methylene spacer length in the polymer side chains. For a polymer with a coumarin directly attached to polymer backbones, the initial direction of LC alignment is in parallel with the LPL electhc vector, it is reversed to perpendicular alignment when exposure energy doses increase. The reversion of LC alignment was influenced by the characters of LC, the level of the coumarin and pre-exposure with non-polarized light. Particularly, the pre-exposure of non-polarized light before LPL irradiation showed an attractive result. After irradiation with non-polarized light of 500 mJ cm^<-2>, perpendicular alignment was generated without the emergence of parallel alignment. These results suggest that initial anisotropic dimerization of coumarins induces the parallel alignment and further dimerization causes the perpendicular alignment.
