PYROLYSIS CHEMISTRY OF SOL-GEL-DERIVED POLY(DIMETHYLSILOXANE)-ZIRCONIA NANOCOMPOSITES - INFLUENCE OF ZIRCONIUM ON POLYMER-TO-CERAMIC CONVERSION

The pyrolytic transformation of hybrid gels, obtained by hydrolysis-co ndensation of diethoxydimethylsilane and zirconium n-propoxide mixture s with different Si/Zr ratio, has been studied using TG-GC-MS coupled techniques, FTIR spectroscopy, and N-2 adsorption analysis. The starti ng gels have been described as nanocomposites formed with poly(dimethy lsiloxane) chains and oxide particles. The pyrolysis pathway appears t o be strongly dependent on the chemical composition of the xerogels. T he evolution of methane is observed throughout the whole thermal proce ss and increases with increasing zirconia content. The presence of zir conium atoms causes a decrease in thermal stability of the Si-C bonds in the SiMe2O units with the evolution of methane starting at temperat ure as low as 275 degrees C. For zirconium percentages up to 30%, the thermal treatment leads mainly to the loss of cyclic oligomers either entrapped in the siloxane matrix during the gelling process or release d through rearrangement reactions based on Si-O/Si-O and Si-C/Si-O bon d exchanges along the poly(dimethylsiloxane) chains. A different pyrol ytic behavior is observed for the sample containing 50% ZrO2 which exh ibits the release of siloxane dimers and tetramethylsilane. From the p yrolysis pathways, different structural arrangements between zirconia particles and the organic-modified counterpart are proposed as a funct ion of chemical composition.