Transparent epoxy/TiO₂ optical hybrid films with tunable refractive index prepared via a simple and efficient way

A simple and efficient method without any coupling/chelating agents was developed to prepare the transparent epoxy/TiO₂ optical hybrid films with tunable refractive index. Instead of adding any coupling agents, the study found out that the lateral hydroxyl groups of epoxy resin E51 could serve as an intermediary to form organic–inorganic bonding between the organic matrix and the hydrolyzed titanium butoxide (TBOT). A series of homogeneous hybrid films were obtained without phase separation via adjusting the mole ratio of TBOT/hydroxyl groups, mass ratio of acid/water and the mixed solvent system. FTIR, XRD and TEM results proved the successful formation of nanoscale crystal-titania domains with the size of 3–5 nm and homogeneously dispersed in organic matrix. Furthermore, SEM and AFM results indicated the relatively good surface smoothness of the resultant optical hybrid films. More importantly, the refractive index of epoxy resin E51/TiO₂ optical hybrid films were tunable in the range of 1.512–1.731 at 633 nm, which linearly increased with the content of TiO₂ from 0 to 20 wt.%. These hybrid films with continuously tunable refractive index were expected to have great potential application in advanced optical devices.