Feature article. Preparation of new polyimide–silica hybrid materials via the sol–gel process
After a description of the preparation of polymer-metal oxide hybrid materials via the sol-gel method, recent results on the preparation of new polyimide-silica hybrid materials are reviewed. Tetraethoxysilane (TEOS) can be hydrolysed and polycondensed to form a three-dimensional silica-gel network in a solution of polyamic acids, the precursor polymers of polyimides, and N,N-dimethylacetamide (DMAc). Films of the polyamic acid-silica gel mixtures can be cast onto glass plates, and can then be heated to 300 °C to convert the component matrix polymers to polyimides. Five kinds of polyamic acid have been designed and prepared. One of them was the usual polyamic acid, having no connection site with silica, whereas others had ethoxysilyl groups in the polymer backbone. The polyimide-silica hybrid films containing 70 wt.% of silica were self-standing and tough. The transparent-opaque changing points shifted to higher silica content with increasing ethoxysilyl-group content of the matrix polyimides. The silica particles were dispersed homogeneously in the matrix polyimides, as observed by scanning electron microscopy. The size of the silica particles increased with increasing silica content, and decreased with increasing ethoxysilyl content. Results of the dynamic-mechanical analysis indicate that movement of the polyimide chain in the matrix is restricted, especially when the polyimides contain higher amounts of ethoxysilyl. The tensile strength of the hybrid films showed no decrease with increasing silica content, whereas the tensile modulus increased in all cases except that where the matrix polyimide did not contain any ethoxysilyl groups.