Thermal analysis of polydimethylsiloxanes. I. Thermal degradation in controlled atmospheres

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Abstract
Thermogravimetric analyses (TGA) of catalyst‐free polydimethylsiloxanes (PDMS) have been carried out in controlled atmospheres and a kinetic analysis of the results has enabled the various decomposition processes to be separated and identified. The calculated activation energy for thermal depolymerization is 42 ± 3 kcal/mole, while thermo‐oxidation has an apparent activation energy of 30 ± 2 kcal/mole. Quantitative analyses of the major degradation products and molecular weight distribution studies of the residues from degradation studies under isothermal conditions have shown that in vacuo, PDMS fractions depolymerize to cyclic dimethylsiloxanes and low molecular weight linear residues by a randomly initiated mechanism which, it is postulated, involves the formation of an intramolecular, cyclic, four‐centered transition state followed by siloxane bond rearrangement. This mechanism is a basic property of linear PDMS fractions and is independent of molecular weight. Molecular weight distribution (MWD) changes observed from further isothermal investigations on hydroxy endblocked PDMS fractions, have shown the presence of a chain‐lengthening process in vacuo below the depolymerization temperature. This process, with an apparent activation energy of 8.6 ± 1 kcal/mole, is attributed to the intermolecular condensation of terminal hydroxyl groups.

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