THERMAL-REACTIONS OCCURRING DURING PYROLYSIS OF CROSS-LINKED POLYSILOXANE GELS, PRECURSORS TO SILICON OXYCARBIDE GLASSES

The pyrolysis under argon of three polysiloxane gels, MeSiO1.5, PhSiO1.5 and crosslinked oligovinylsiloxane, was investigated by Si-29 solid-state NMR and thermogravimetric analysis coupled to mass spectrometry. Above 500-600-degrees-C, redistribution reactions involving the exchange of Si-C and Si-O bonds take place, which lead to a random environment of the silicon atom at 1200-degrees-C. The use of oligovinylsiloxanes crosslinked by hydrosilation leads to high yields of silicon oxycarbide glasses with an O/Si ratio close to 1.