Pretilt angle control of liquid crystal molecules by photoaligned films of azobenzene-containing polyimide with a different content of side-chain

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photoaligned films of a series of polyimides. The polyimides were random copolymers synthesized from pyromellitic dianhydride and a mixture of 4, 4′ -diaminoazobenzene and 4- [4′ -propylbi(cyclohexan)-4-yl]phenyl 3,5-diaminobenzoate (PBCP-DABA). PBCP-DABA is a diamine to introduce a side-chain structure into polyimide. We found that the pretilt angle of LC molecules can be controlled from 0° to 90° by varying the molar fraction (x) of PBCP-DABA from 0 to 0.5. Defect-free uniform LC alignment was observed for x0.125 and x0.3, but threadlike textures appeared for 0.15x0.25. Since the interaction between the polyimide backbone structure and the LC molecule may be blocked by relatively dense side-chains, the appearance of threadlike texture is tentatively attributed to weak azimuthal anchoring strength of the photoaligned polyimide films with x0.15.