Preparation, structure, growth mechanisms and properties of siloxane composites containing silica, titania or mixed silica-titania phases

Poly(dimethylsiloxane) (PDMS) [-Si(CH3)(2)O-] networks were prepared by tetrafunctionally endlinking hydroxyl-terminated chains with tetraethoxysilane. Composites were prepared by in situ base-catalysed hydrolysis of an organosilicate and an organotitanate, resulting in the formation of reinforcing silica, titania or silica-titania mixed-oxide fillers within the networks. The growth processes and resulting structures of the reinforcing fillers were studied by small-angle X-ray scattering. Both systems were found to yield dense particles with fractally rough surfaces. Some of the scattering results were used to interpret previously reported mechanical properties obtained on such composites using equilibrium stress-strain measurements in elongation. Of particular interest were mixed silica-titania fillers, since in at least some cases these are known to reinforce PDMS networks better than either silica or titania alone. The scattering results suggest that the corresponding growth processes proceed by the formation of relatively uniform titania particles, followed by the formation of significantly larger silica particles. (C) 1998 Elsevier Science Ltd. All rights reserved.