Synthesis and properties of photosensitive polyimide–nanocrystalline titania optical thin films

In this study, synthesis, morphology, and properties of high refractive index photosensitive polyimide–nanocrystalline titania hybrid materials are reported. A soluble polyimide grafted with carboxylic acid or methacrylate groups (P1) was first synthesized from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,5-diaminobenzoic acid (DABA), 4-aminobenzoic acid (4ABA), and 2-hydroxyethyl methacrylate (2HEMA). The residual carboxylic acid groups could undergo an esterification reaction with titanium butoxide to provide an organic–inorganic bonding. On the other hand, the grafted methacrylate groups rendered photosensitive property for photopatterning. A homogeneous hybrid solution was obtained through the formulation on different mole ratios of titanium butoxide/carboxylic acid, water/acid content in a mixed solvent. It was followed by spin-coating, photocuring and post-baking. The titania domain size in the hybrid materials analyzed by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) was around 4–7 nm. The prepared optically transparent films had tunable refractive index (1.583 < n < 2.029), relatively good surface planarity and high thermal stability. A fine pattern with a line width of 50 μm was produced by direct lithographic process on the hybrid films. The present study demonstrates a novel approach for preparing high refractive index hybrid photosensitive materials with patternability.