The role of Fe in the thermal stabilization of ormosils

The thermal stability of organically modified silicates (Ormosils) is limited by that of their organic constituent. In the case of the polydimethylsiloxane (PDMS)-silica hybrids, degradation of the PDMS begins around 250°C, which restricts their range of applications. Several strategies have been used to stabilize PDMS, such as substitution of the methyl groups by phenyl groups. Another strategy is the addition of very small amounts (typically about 1 wt%) of iron. This technique has been used successfully in the stabilization of liquid silicones. In the case of PDMS-SiO2 hybrids, this small dopant concentration has a very significant effect on the thermal stability, increasing it by up to 200°C. However, very little is known of the mechanism of stabilization. In the present work we carry out an investigation of the materials in order to explain the mechanisms involved. The materials were investigated by liquid and solid state 29Si NMR, BET, SEM, TGA and DMA. The data indicates that Fe plays a most significant role at the solution stage already. The structures of the hybrids with and without Fe are very different. In other words, the thermal stabilization mechanism appears to be due not so much a direct redox process taking place in the solid state (as in the case of liquid silicones) but rather, to differences in chemical structures induced by Fe in the liquid. Specifically, PDMS chain cleavage and increased cross-linking to SiO2 appear to be the cause of the thermal stabilization.