Kinetic Study of Thermal Degradationof Polydimethylsiloxane: The Effect ofMolecular Weight on Thermal Stability inInert Atmosphere

Thermal degradation of polydimethylsiloxane (PDMS) of different molecular weights in inert and oxygen atmosphere is studied and discussed in detail. PDMS samples of different molecular weights were prepared by anionically initiated ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion. The molecular weight of PDMS drastically affected the shape of TGA curve at higher temperatures when thermogravimetric (TGA) measurements were performed in the inert atmosphere. The thermal stability of PDMS decreased with increasing the average molecular weight of the sample, which could be explained by prevailing unzipping mechanism at lower molecular weights over intramolecular and intermolecular redistribution, which were more important at higher molecular weights. On the other hand, experiments in oxygen atmosphere showed zero effect of polydimethylsiloxane molecular weight on its thermal stability. In addition, a kinetic model describing the thermal degradation of PDMS in inert atmosphere as a function of PDMS molecular weight was proposed. The proposed kinetic model was composed of diffusion-limited kinetics step related to evaporation rates of the degradation products and of rate-determined step related to the formation of cyclic degradation products. As a result of mathematical modeling by fitting the calculated data to experimental data kinetic parameters were derived properly.