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-condensation of diethoxydimethylsilane and zirconium n-propoxide mixtures with different Si/Zr ratio, has been studied using TG-GC-MS coupled techniques, FTIR spectroscopy, and N-2 adsorption analysis. The starting gels have been described as nanocomposites formed with poly(dimethylsiloxane) chains and oxide particles. The pyrolysis pathway appears to be strongly dependent on the chemical composition of the xerogels. The evolution of methane is observed throughout the whole thermal process and increases with increasing zirconia content. The presence of zirconium atoms causes a decrease in thermal stability of the Si-C bonds in the SiMe2O units with the evolution of methane starting at temperature 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 released through rearrangement reactions based on Si-O/Si-O and Si-C/Si-O bond exchanges along the poly(dimethylsiloxane) chains. A different pyrolytic behavior is observed for the sample containing 50% ZrO2 which exhibits the release of siloxane dimers and tetramethylsilane. From the pyrolysis pathways, different structural arrangements between zirconia particles and the organic-modified counterpart are proposed as a function of chemical composition.