Microscopic origin of liquid crystal alignment on rubbed polymer surfaces

Surface-sensitive and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) measurements clearly reveal a preferred in-plane and out-of-plane orientation of phenyl and C=O groups at the surface of rubbed polyimide films. The unidirectional molecular alignment at the surface is argued to provide the template for liquid crystal (LC) alignment of the films. Both the LC orientation along the rubbing direction as well as the direction of the out-of-plane LC pretilt are explained in a simple model. In this model the LC direction follows the preferential orientation of the phenyl rings at the surface. The preferred phenyl orientation is explained in terms of preferential chain segment alignment through a pulling action of the rubbing cloth fibers. The proposed LC alignment model is based on the existence of a statistically significant unidirectional bond asymmetry at the polymer surface, and it does not require the existence of crystalline order.