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 s ol-gel processing of both the linear and the hyperbranched alkoxy-substitut ed polycarbosilanes of the type "[Si(OR)(2)CH2](n)", where R = Me or Et. Th e molecular structure of these gels and their pyrolysis to silicon oxycarbi de ceramics were investigated by elemental analysis, thermogravimetric anal ysis, 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 sho w that the initial gels after drying have a complex structure involving sil oxy linkages and pendant Si-OR (R = H, Me or Et) groups in addition to the initial Si-CH2-Si bonding in the starting carbosilanes. After heating to 60 0 degrees C, the gels become nearly fully condensed with an approximate "[S i(O)CH2](n)" average formula, except in the case of the gel derived from th e hyperbranched "[Si(OMe)(2)CH2](n)" polycarbosilane, which still contains appreciable Si-OH groups. Between ca. 600 and 1000 degrees C, conversion to an inorganic structure occurs, which is accompanied by extensive redistrib ution reactions involving the exchange of Si-O and Si-C bonds. Particularly in the case of the [Si(OMe)(2)CH2](n)-derived gel, this is likely facilita ted by a Si-OH-induced attack on the Si-CH2-Si linkages, leading to additio nal siloxy linkages and terminal Si-CH3 bonds. All of the investigated gels and their pyrolysis products were found to have relatively high surface ar eas and a microporous structure. Moreover, the results of this work indicat e that the molecular structure of the starting carbosilane, as well as the degree of hydrolysis and condensation in the initial gel, has an significan t 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.