Pretilt angle of liquid crystals on polyimide films photo-aligned by single oblique angle irradiation with un-polarized light

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photo-aligned films of polyimide containing azobenzene in the backbone structure (Azo-PI). The photo-alignment treatment was single oblique-angle irradiation with un-polarized light (UP-L) at an incidence angle of 45°. It was performed on films of polyamic acid (Azo-PAA) using a light source of wavelength 340-500 nm. The photo-treated films of Azo-PAA were converted into Azo-PI by thermal imidization. The pretilt angle of LC molecules was determined by a crystal rotation method. The pretilt angle increased with the UP-L exposure, reaching ∼3° at 880J/cm². For UP-L exposures above 440J/cm² the LC alignment was uniform and defect-free, while marbled textures were observed below 220J/cm², This is due to the small in-plane anisotropy of the photo-aligned Azo-PI films.