29Si MAS NMR investigation of the pyrolysis process of cross-linked polysiloxanes prepared from polymethylhydrosiloxane

Abstract

Polymethylhydrosiloxanes (PMHSs) crosslinked with hexa-l,5-diene or hexane-l,6-diol have been considered as potential precursors for Si–C–O systems. Their pyrolysis under an argon flow has been investigated mainly by ²⁹Si MAS NMR spectroscopy. The two systems, with similar C/Si but different O/Si ratios, show different pyrolytic behaviour. This is essentially due to different graftings between the siloxane backbone and the alkyl chains, Si–C vs. Si–O–C bonds. In both cases, degradation of the organic chains occurred at 600 °C; but in the case of PMHS cross-linked with hexanediol (DO6), all the C groups are lost, while for PMHS cross-linked with hexadiene (DE6), the C groups bonded to Si remain. As a consequence, the oxycarbide phase obtained at 1000 °C is much richer in C for DE6, compared to DO6, but the amount of free C is also higher. These differences in composition strongly influence the nature of the samples obtained at 1500 °C: crystalline silicon carbide for DE6 and mixture of amorphous silica and silicon carbide for DO6.