Synthesis of Polycarbosilane/Siloxane Hybrid Polymers and Their Pyrolytic Conversion to Silicon Oxycarbide Ceramics

Polycarbosilane/siloxane hybrid polymers were synthesized by sol−gel processing of the cyclic organosilicon compound 1,1,3,3-tetraethoxy-1,3-disilacyclobutane (CBS) and the linear polymer poly(diethoxysilylenemethylene) (L-EPCS) derived from CBS by ring-opening polymerization. The structure of these polymers and their pyrolysis to silicon oxycarbide ceramics were investigated by elemental analysis, thermogravimetric analysis, FT-IR, and solid-state NMR spectroscopy. The results obtained indicate that the gel, obtained by hydrolysis/condensation of CBS with acid catalyst, underwent a Si−OH-induced, ring-opening reaction before the organic-to-inorganic conversion took place during pyrolysis, leading to the formation of intermolecular Si−O−Si(Me) linkages. In the case of the linear polycarbosilane (L-EPCS), sol−gel processing under acid catalysis leads to a gel of the type [SiOCH 2 ] x [Si(OR)O 0.5 CH 2 ] y (R = H or Et). 29 Si SSNMR spectra show that this gel, which has a nearly pure SiC 2 O 2 microenvironment at the outset, converts into a silicon oxycarbide that contains a statistical distribution of the five possible SiC 4 - x O x environments between 600 and 1000 °C. This rearrangement of the Si environments was attributed to redistribution reactions involving the exchange of Si−O and Si−C bonds during the latter stages of the pyrolysis, likely facilitated by an analogous Si−OH-induced attack on the Si−CH 2 −Si linkages. Both SSNMR and elemental analysis indicated that bridging CH 2 groups have the effect of increasing the carbidic carbon content and decreasing the free carbon content in the resulting silicon oxycarbide ceramics relative to polymers with pendent Si−CH 3 groups.