Poly(etherimide)s and poly(esterimide)s containing azobenzene units: Characterization and study of photoinduced optical anisotropy

In this paper novel processable, photoresponsible aromatic polymers with imide rings and covalently boned azobenzene units are presented. Prepolymerization strategy based on polycondensation of two diamines with azobenzene group, that is, 2,4-diamino-4′-cyanoazobenzne and 2,4-diamino-4′-nitroazobenzene and two dianhydrides, was utilized for preparation of the new photochromic polyimides. The obtained polymers differ in the chemical structure of polymer backbone and the kind of substituents on azobenzene moieties. Polymers were characterized and evaluated by FT-IR, ¹H NMR, X-ray, UV-vis, DSC, and TGA methods. The synthesized azopolymers exhibited glass transition temperatures in the range of 140-170 °C, thermal stability with initial decomposition temperatures (T_d) in the range of 266-290 °C, and excellent solubilities in common organic solvents, providing optical-quality films. The light-induced optical anisotropy was studied in obtained polymers by photoinduced birefringence measurements. The birefringence was generated by the femtosecond pulses at 400 nm via a linear absorption of the trans-azobenzene. A large light-induced birefringence (Δn = 0.02) was achieved in poly(esterimide)s contrary to poly(etherimide)s. The birefringence exhibited a very stable behavior after switching off the writing light indicating a permanent storage capacity.