Ex situ synthesis of high-refractive-index polyimide hybrid films containing TiO2 chelated by 4-aminobenzoic acid

In this study we used the ex situ synthesis method to fabricate high-refractive-index polyimide-TiO2 hybrid films at low temperature. We incorporated pre-made TiO2 nanoparticles (NPs) chelated with 4-aminobenzoic acid into a soluble polyimide synthesized from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride by one-step polymerization. The refractive index of the hybrid film with theoretic 90 wt% (experimental 79.04 wt%) TiO2 can reach as high as 1.941—close to that of the PI-TiO2 hybrid film fabricated at high temperature. Compared with epoxy-TiO2 hybrids, the polyimide-TiO2 hybrid films maintain their integrity without cracking in the thick-film condition and reveal extraordinarily high refractive indices. Infrared spectra and thermogravimetric analysis suggested that the enhanced properties in refractive indices and structure might result from the organic–inorganic bonding between 4-aminobenzoic-acid-chelated TiO2 and polyimide featuring hydroxyl groups.