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 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′-;z-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.