High refractive index polyimide-nanocrystalline-titania hybrid optical materials

In this study, a new synthetic route was developed to prepare polyimide-nanocrystalline-titania hybrid materials with a relatively high titania content (up to 90 wt%). A soluble polyimide with carboxylic acid end groups (6FDA-6FpDA-COOH) was first synthesized from 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4'-(hexafluoroisopropylidene) dianiline (6FpDA), and 4-aminobenzoic acid (4ABA). Such end groups could undergo an esterification reaction with titanium butoxide and provide organic-inorganic bonding. A homogeneous hybrid solution was obtained through the mole ratio of titanium butoxide/carboxylic acid, water/acid content, and a mixed solvent system. HRTEM and XRD results indicated the formation of nanocrystalline-titania domains of around 3-4 nm in the hybrid films. The spin-coated hybrid films had relatively good surface planarity, high thermal stability, tunable refractive index (1.571 < n < 1.993), and optical transparency in the visible range. The achieved refractive index of 1.943 is the highest among the polymer-titania based hybrid materials as far as we know. A three-layer anti-reflection coating based on the prepared hybrid films was designed and possessed a reflectance of less than 0.5% in the visible range. Our study demonstrates a new route for preparing high refractive index polyimide-nanocrystalline-titania hybrid materials for potential optical applications.