Stoichiometry control of SiOC ceramics by siloxane polymer functionality

Abstract

The guidelines, or empirical rules, previously described in the literature to estimate ceramic compositions from preceramic polymer compositions have been refined and quantified. Thermogravimetric and residual gas analyses of the pyrolysis of organosilsesquioxane polymers have identified the organic degradation products at various temperatures and indicated that essentially all of the silicon and oxygen atoms of these highly branched polymers are retained in the 1200 °C ceramic residue. Series of organosilsesquioxanes with systematically varied amounts of organosilsesquioxane and endcapping components were synthesized, cured, and pyrolyzed to 1200 °C under an inert atmosphere. Multiple linear regression analysis was used to quantify the relationships between the amount of carbon retained in the ceramic residues and the mole fractions of the various organic components of the preceramic polymer, allowing for retention of the silicon and oxygen of the silsesquioxane. Specifically, a phenylsilsesquioxane fragment contributes an average of 3.94 carbons to the resulting ceramic material, vinylsilsesquioxane, 1.52 carbons, methylsilsesquioxane, 0.59 carbons and a vinyldimethylsilyl endcapping group, 2.75 carbons. The utility of the model was shown by employing this information to predict a select set of candidate precursors to an SiOC with a carbon content near a desired 18 wt.% level. One of the candidate precursors (MeSiO_1.5)_0.84(Me₂ViSiO_0.5)_0.16 (predicted to afford an SiOC at 18.1 wt.% carbon) was then prepared, cured, pyrolyzed and analyzed to test the accuracy of the model. The 1200 °C ceramic was found to have 18.4 wt.% carbon, indicating good agreement between the actual and predicted values.