STOICHIOMETRY CONTROL OF SIOC CERAMICS BY SILOXANE POLYMER FUNCTIONALITY

The guidelines, or empirical rules, previously described in the litera ture to estimate ceramic compositions from preceramic polymer composit ions have been refined and quantified. Thermogravimetric and residual gas analyses of the pyrolysis of organosilsesquioxane polymers have id entified the organic degradation products at various temperatures and indicated that essentially all of the silicon and oxygen atoms of thes e highly branched polymers are retained in the 1200 degrees C ceramic residue. Series of organosilsesquioxanes with systematically varied am ounts of organosilsesquioxane and endcapping components were synthesiz ed, cured, and pyrolyzed to 1200 degrees C under an inert atmosphere. Multiple linear regression analysis was used to quantify the relations hips between the amount of carbon retained in the ceramic residues and the mole fractions of the various organic components of the precerami c polymer, allowing for retention of the silicon and oxygen of the sil sesquioxane. Specifically, a phenylsilsesquioxane fragment contributes an average of 3.94 carbons to the resulting ceramic material, vinylsi lsesquioxane, 1.52 carbons, methylsilsesquioxane, 0.59 carbons and a v inyldimethylsilyl endcapping group, 2.75 carbons. The utility of the m odel was shown by employing this information to predict a select set o f candidate precursors to an SiOC with a carbon content near a desired 18 wt.% level. One of the candidate precursors (MeSiO1.5)(0.84)(Me(2) ViSiO(0.5))(0.16) (predicted to afford an SiOC at 18.1 wt.% carbon) wa s then prepared, cured, pyrolyzed and analyzed to test the accuracy of the model. The 1200 degrees C ceramic was found to have 18.4 wt.% car bon, indicating good agreement between the actual and predicted values .