Polydimethylsiloxane thermal degradation Part 1. Kinetic aspects

A combination of traditional kinetic formal treatments and computer simulation has been made to analyze polydimethylsiloxane (PDMS) thermal degradation. It was shown that PDMS thermally decomposes to cyclic oligomers through Si–O bond scission in a chain-folded cyclic conformation energetically favored by overlapping of empty silicon d-orbitals with orbitals of oxygen and carbon atoms. Kinetic treatment shows that PDMS thermal volatilization, as rate of heating increases, becomes dominated by rate of diffusion and evaporation of oligomers produced on its decomposition. At high heating rate (e.g. 100°C min−1.) thermal decomposition in nitrogen and in air tend to overlap because the rate of reaction between the material and oxygen is strongly reduced by low-oxygen solubility and high-thermal degradation rate. In nitrogen a small black residue is formed (silicon oxycarbide) which is produced by an alternative decomposition path leading to cyclic oligomers, made possible at high heating rate.