High transparent soluble polyimide/silica hybrid optical thin films

Polyimide-silica hybrid thin films (6FDA-6FpDA-4ABA/SiO2, PS) were prepared from soluble fluorine-containing polyimide and 12 nm monodispersed colloidal silica. The soluble polyimide with carboxylic acid end groups (6FDA-6FpDA-4ABA-COOH) was synthesized from 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4'-(hexafluoroisopropylidene) dianiline (6FpDA), and 4-aminobenzoic acid (4ABA). Such end groups could undergo a condensation reaction with colloidal silica to provide organic-inorganic bonding and, thus, prevent macrophase separation. The silica content in the hybrid films was varied from 0 to 50 wt% (PS0-PS50). TGA analysis showed that the hybrid films had high thermal stability and the thermal decomposition temperature (T-d) increased as the silica content increased. The UV-vis spectra indicated the cutoff wavelength of hybrid films was less than 272 nm and could be tunable through the silica content. The n&k analysis illustrated that the refractive index (n) of the hybrid films was in the range of 1.575-1.479, which could be controlled by the silica content. The extinction coefficients (k) were almost zero in the 300-900 nm wavelength range, indicating the prepared hybrid films had excellent optical transparency in both the UV and visible regions. TEM images showed that the particle size of silica in the hybrid thin films was similar to the original size of pure silica and could be effectively controlled. The results of SEM and AFM showed that all the prepared hybrid films had a good film formability and planarity. Based on this study, it would be possible to prepare polyimide-silica hybrid thin films using monodispersed colloidal silica instead of preparing silica networks from alkoxysilanes, with no additional coupling agents in the preparation of the hybrids. (C) 2009 Elsevier Ltd. All rights reserved.