Polar ordering at an interface between an LC monolayer and a rubbed polyimide

Abstract

Optical second harmonic generation (SHG) was used to study how mechanical rubbing modifies the surface structure of a polyimide (poly[4,4'-oxydiphenilenepyromellitimide], PMDA-ODA) film and how the rubbed polyimide surface aligns 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 the polymer surface more polar and hence more SHG-active. The rubbed polymer surface would then align the adsorbed 8CB monolayer via molecule-molecule interaction in an orientational epitaxy manner. Approximate orientational distributions for molecules at the rubbed polymer surface and in the aligned 8CB monolayer were deduced from the measurements.