Surface second-harmonic generation from two interfaces of polyimide film coated on a substrate

Side chain orientation of polyimide (PI) films for liquid crystal alignment has been studied by surface second-harmonic generation (SHG). SHG from two interfaces of the film was separately observed using thick films, in which SH light from the rear interface was almost absorbed in the film. For as-coated films, the average tilt angle of the side chain at the air-PI interface is smaller than that of the other interface. Rubbing action caused in-plane anisotropy only for the air-PI interface. The distribution function of the side chain of the rubbed PI film was also discussed.