Polar ordering at an interface between a liquid crystal monolayer and a rubbed polyimide

Optical second harmonic generation (SHG) was used to study how mechanical r ubbing modifies the surface structure of a polyimide (poly[4,4'-oxydiphenil enepyromellitimide], PMDA-ODA) film and how the rubbed polyimide surface al igns a liquid crystal monolayer (4'-n-octyl-4-cyanobiphenyl, 8CB) adsorbed on it. It was found that rubbing would align the polymer chains and make th e polymer surface more polar and hence more SHG active. The rubbed polymer surface would then align the adsorbed 8CB monolayer via molecule-molecule i nteraction in an orientational epitaxy manner. Approximate orientational di stributions for molecules at the rubbed polymer surface and in the aligned 8CB monolayer were deduced from the measurements.