Synthesis and Characterization of Aromatic Polyimides Bearing Nonlinear Optical Chromophores

Abstract

The syntheses and characterizations of two aromatic polyimides with nonlinear optical (NLO) chromophore side chains were investigated through a two-step synthetic route. These two polymers were prepared by polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) with 4-(4-amino, 2-hydroxy) phenoxyaniline (HODA) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP), respectively. First the poly(hydroxy)imides were prepared by direct thermal imidization. The resulting polyimides bearing hydroxy groups were found to react easily with the terminal hydroxy group of the chromophore (Disperse Red1 (DR1)) via the Mitsunobu reaction. The high content of chromophore for the polyimide with 6FAP diamine was confirmed by UV spectroscopy and ellipsometry spectroscopy measures. The resulting NLO polyimides possess high glass transition temperatures (T_g > 185°C), excellent solubilities and processabilities even though the extent of chromophore grafting is up to 95 mol%. The second harmonic coefficient (d ₃₃) at the wavelength of 1320 nm is close to 30 pm V⁻¹. The thermal and temporal stabilities of the NLO properties were investigated. The relaxation temperature of the SHG signal measured at the 50% decay of the initial SHG signal (I _2ω /2 at 2°C min⁻¹) was 168°C for the polyimide based on HODA and 157°C for the polyimide based on 6FAP.