Surface vibrational spectroscopic studies of rubbed polyvinyl cinnamate for liquid crystal alignment

The critical role of buffing strength of poly(vinyl cinnamate) (PVCi) surface for liquid crystal (LC) alignment is reported. It has been found that weakly and strongly rubbed PVCi films provide LC alignment which is perpendicular and parallel to the rubbing direction, respectively. Surface-specific sum-frequency vibrational spectroscopy have been used to investigate the structure of poly(vinyl cinnamate) (PVCi) surface on a molecular level. Quantitative analysis provides an approximate orientation distribution function (ODF) for the aligned cinnamate moieties and the polymer backbones. Weak rubbing aligns the phenyl ring of the cinnamate side-chains perpendicular to the rubbing direction, with the main axis along the surface normal. Strong rubbing orients the phenyl rings toward the plane normal to the surface along the rubbing direction.