HYBRID SILOXANE-OXIDE MATERIALS VIA SOL-GEL PROCESSING - STRUCTURAL CHARACTERIZATION

Hybrid siloxane-oxide gels can be prepared by sol-gel techniques, by using either monomeric species, such as dimethyldiethoxysilane or by OH-terminated poly(dimethylsiloxanes) (PDMS) crosslinked by metallic alkoxides, M(OR)n with M = Si, Ti or Zr. Investigations performed by Si-29 solution NMR, have shown that reactions occur between the silanes and the metallic alkoxides during the hydrolysis process. The structure of the gels for various siloxane/oxide compositions was characterized by Si-29 and H-1 MAS-NMR, as well as Ti-K edge X-ray absorption for titanium-containing gels. The influence of the alkoxide nature on the development of the siloxane network was clearly demonstrated: tetraethoxysilane appears as a crosslinking agent which prevents the growth of PDMS chains, while titanium or zirconium alkoxides seem to behave mainly as catalysts for condensation of siloxane units and thus lead to the formation of long PDMS chains, together with oxide-based particles.