Nonlinear Optical Spectroscopic Studies of Polymer Surfaces for Liquid Crystal Alignment: Photo-Irradiated Polyimide and Rubbed Polystyrene Surfaces

Abstract

Surface specific sum-frequency vibrational spectroscopy (SFVS) and secondharmonic generation (SHG) were used to study the structures of polymer surfaces modified by linearly polarized UV irradiation or mechanical rubbing. The spectroscopic results show that the surface anisotropy of a polyimide surface resulting from bond breaking by linearly polarized UV irradiation is extremely small compared to a rubbed surface. On a rubbed polystyrene surface, the phenyl sidegroups are well aligned by rubbing in the direction perpendicular to rubbing but tilt from the surface normal with a broad distribution.