Insight into the thermal degradation behaviour and degradation products of cross-linked polydimethylsiloxanes

Excellent thermal stability is one of the key features of polydimethylsiloxanes (PDMS), which make them suitable for a wide range of applications. Thermal resistance of silicones at high temperatures is due to the inherent strength and flexibility of the polymer backbone. In particular, the Si-O bond has high bond dissociation energy (108 kcal/mol) and low energy barrier to bond rotation. While the thermal degradation mechanism of linear PDMS is well understood[1,2], the mechanism of the thermal degradation process of cross-linked PDMS networks has not been unveiled yet. Thus the goal of this study is the elucidation of the thermal degradation mechanism of cross-linked silicone elastomers (Figure 1) through the investigation of their thermal degradation behaviour and degradation products. Thermogravimetric analysis (TGA) performed in inert atmosphere was carried out on the PDMS networks synthesised with different stoichiometric ratios. In order to determine to which extent the thermal degradation is influenced by the sol fraction of the silicone elastomers (Figure 2), extraction of the samples in heptane was exploited in order to remove unreacted PDMS chains. Furthermore, long-term thermogravimetric measurements at constant temperatures were performed with the aim of recovering the volatile and soluble degradation products of the thermally treated elastomers. Thermal degradation products were analysed through the combination of different techniques such as size exclusion chromatography, nuclear magnetic resonance and infrared spectroscopy.