Alignment models for coumarin-containing polymers for liquid crystal displays

In this paper, a range of polymethacrylate derivatives of hydroxycoumarin are investigated as photoinduced alignment layers for liquid crystals, and their performance is compared with polyvinyl cinnamate. For all the coumarin-containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpendicular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnamate. The azimuthal anchoring energies of the alignment layers are measured and values greater than 6 x 10(-5) J m(-2) are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1 degrees.