The effect of interphase coupling on the structure and mechanical properties of silica-siloxane composites

Silica-reinforced siloxane composites are synthesized with heat labile azo groups at the polymer-filler interface. The azo group serves as a smart interface in that it is responsive to heat. This system is used to assess the influence of interfacial chemical bonds on mechanical properties. By thermally severing the azo linkage, we can determine the influence of interfacial bonding on properties independent of other morphological factors. The virgin filler powders, azo-modified powders, silica-filled composites and azo-modified-silica-filled composites are characterized using small-angle x-ray scattering, IR spectroscopy, and scanning electron microscopy. Mechanical properties are correlated with the presence or absence of interface coupling, filler loading and thermal exposure. The data show that, at loadings above 5 wt%, elongation is enhanced in the azo-linked composites compared to the composites with bare fillers; modulus is enhanced at low silica loadings (<5wt%) but is reduced at high loadings. On thermal clipping of the coupling agent, we find a slight enhancement of the ultimate elongation, but only at loadings above 5 wt%. We conclude that although interfacial bonding has a positive effect on reinforcement, the influence of physical interactions at the interface overrides that of chemical bonds.