CHARACTERIZATION OF THE PYROLYTIC CONVERSION OF POLYSILSESQUIOXANES TO SILICON OXYCARBIDES

A series of silsesquioxane copolymers synthesized by hydrolysis and co ndensation of phenyl- and methyltrimethoxysilanes have been studied as preceramic polymers. The pyrolytic conversion to ceramics was charact erized by thermogravimetric analysis, Si-29 and C-13 nuclear magnetic resonance and Raman spectroscopy. The pyrolysed materials were further characterized by differential thermal analysis, X-ray diffractometry and transmission electron microscopy. The ratio of phenyl to methyl gr oups in the copolymer was found to control polymer structure and rheol ogy, as well as ceramic composition and char yield. On pyrolysis to 10 00-degrees-C under inert conditions, silicon oxycarbides were formed, along with glassy carbon. On heating from 1200-degrees-C to 1400-degre es-C, the oxycarbide structure diminished, and the materials were comp rised primarily of amorphous silica, amorphous Si-C, some small crysta llite SiC and graphitic carbon. The carbon content increased, and char yield decreased, with increasing concentration of phenyl groups in th e copolymer. The presence of free carbon appears to inhibit the crysta llization of silica. Significant carbothermal reduction was observed o nly above 1500-degrees-C. Oxidation studies of the pyrolysed materials indicated the presence of at least two forms of carbon.