A high-temperature aromatic polyimide film displaying non-linear optical second-harmonic generation based on the approach of the poled guest–host system

Abstract

A segmented rigid-rod polyimide has been synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2′-trifluoromethyl-4,4′-diaminobiphenyl (PFMB) in m-cresol at elevated temperature in a one-step Polymerization. This aromatic polyimide (6FDA-PFMB) is soluble in a variety of organic solvents. 6FDA-PFMB has a glass-transition temperature (T_g) of 344 °C and a high optical transmission above 375 nm. The refractive indices along the in-plane and the out-of-plane directions are different and show a negative birefringence. Both of the refractive indices decrease with increasing wavelength. When the non-linear optical guest molecule, 2-(p-nitrophenyl)-4,5-bis(p-hydroxyphenyl) oxazole (TNON) is doped into the 6FDA-PFMB matrix, phase separation does not occur below a 25% w/w concentration of TNON. The T_g of the guest-host system decreases with increasing TNON concentration. After corona poling, second-harmonic generation is retained up to 300 h at 80 °C. Even after the film is annealed at 200 °C for 0.5 h, a measurable fraction of the second-harmonic non-linearity is retained.