SOL-GEL SYNTHESIS OF A SILOXYPOLYCARBOSILANE GEL AND ITS PYROLYTIC CONVERSION TO SILICON OXYCARBIDE

A new ethoxy-substituted polycarbosilane (I) has been prepared from a highly branched chloropolycarbosilane [Si(OEt)x(Et)0.15ClyCH2]n. After hydrolysis, condensation, and drying, a gel having the general formul a [CH2Si(OH)a(OEt)b(Et)0.15Oc]n (II) was obtained from I. The dried ge l was pyrolyzed under argon to give a silicon oxycarbide glass. The ge l and its pyrolysis chemistry were characterized by infrared and Si-29 , C-13, and H-1 magic angle spinning nuclear magnetic resonance (MAS N MR). By 600-degrees-C the gel has developed a fully condensed structur e and can be represented by the approximate formula [CH2SiO]n, but wit h a distribution of Si microenvironments ranging from X (SiC4) to Q (S iO4, minor component). The conversion of the polymeric network into a silicon oxycarbide glass occurs between 600 and 1000-degrees-C. A comp arison between the chemical analysis data and the Si-29 NMR spectra si mulations leads to an estimation of the carbon content and its distrib ution in the silicon oxycarbide. The results were compared with those obtained from a methylsilsesquioxane gel, CH3SiO1.5 (III). These resul ts show that a higher proportion of the carbon in the initial precurso r is retained in the oxycarbide product if the carbon atoms are initia lly bonded to two silicon atoms rather than one. Moreover, the amount of carbon that is incorporated into carbidic sites (CSi4) in the case of the ''CH2SiO'' precursor is found to be approximately twice that ob tained by using the CH3SiO1.5 gel.