Role of Precursor Molecular Structure on the Microstructure and High Temperature Stability of Silicon Oxycarbide Glasses Derived from Methylene-Bridged Polycarbosilanes

Cross-linked polycarbosilane/siloxane hybrid polymers were synthesized by sol−gel processing of both the linear and the hyperbranched alkoxy-substituted polycarbosilanes of the type “[Si(OR) 2 CH 2 ] n ”, where R = Me or Et. The molecular structure of these gels and their pyrolysis to silicon oxycarbide ceramics were investigated by elemental analysis, thermogravimetric analysis, FT-IR, and solid-state (SS)NMR spectroscopies. The microstructure of the polymer gels and their pyrolysis products were investigated by nitrogen adsorption−desorption experiments (the BET test). SSNMR and IR spectra show that the initial gels after drying have a complex structure involving siloxy linkages and pendant Si−OR (R = H, Me or Et) groups in addition to the initial Si−CH 2 −Si bonding in the starting carbosilanes. After heating to 600 °C, the gels become nearly fully condensed with an approximate “[Si(O)CH 2 ] n ” average formula, except in the case of the gel derived from the hyperbranched “[Si(OMe) 2 CH 2 ] n ” polycarbosilane, which still contains appreciable Si−OH groups. Between ca. 600 and 1000 °C, conversion to an inorganic structure occurs, which is accompanied by extensive redistribution reactions involving the exchange of Si−O and Si−C bonds. Particularly in the case of the [Si(OMe) 2 CH 2 ] n -derived gel, this is likely facilitated by a Si−OH-induced attack on the Si−CH 2 −Si linkages, leading to additional siloxy linkages and terminal Si−CH 3 bonds. All of the investigated gels and their pyrolysis products were found to have relatively high surface areas and a microporous structure. Moreover, the results of this work indicate that the molecular structure of the starting carbosilane, as well as the degree of hydrolysis and condensation in the initial gel, has an significant effect on both the pyrolysis chemistry of the gel and the composition and the microstructure of the ceramic product which, in turn, strongly affects its high-temperature stability.