On the pyrolysis of a methyl-silsesquioxane in reactive CO2 atmosphere: A TG/MS and FT-IR study

Polymer-derived SiOC-C composites are typically obtained through pyrolysis of a polysiloxane precursor in inert atmosphere. Recent studies have shown that novel SiOC microstructures and compositions can be obtained when the pyrolysis is carried out in a reactive environment, as CO2, which leads to a selective oxidation of the Si─C bonds leaving a microstructure constituted by a nano-dispersed sp2 carbon phase within an SiO2 matrix. However, little is known about the reaction mechanisms between CO2 and the preceramic polymer to date. In this work, we investigated the pyrolysis of a methyl-silsesquioxane in reactive (CO2) and inert (Ar or He) atmosphere by combining TG/MS and FT-IR analysis. The results showed that CO2 starts to react with the preceramic polymer from ≈750°C when the Si─CH3 groups start to form Si─CHx-Si units. The reaction breaks the Si─C bond increasing the amount of the free carbon phase and releasing water vapor, detected by MS, even at temperatures exceeding 900°C. At higher temperatures (≈950°C), CO2 reacts with the free carbon phase leading to a weight loss and the formation of CO.