Odile Fichet

Polysiloxane networks with or without phenyl groups have been subjected to isothermal, thermal sweep and cycling thermal treatments to investigate the phenyl effect on evolution of their properties during thermal aging. Polydimethyldiphenylsiloxanes (PDMDPS) containing between 5 and 30 mol% phenyl groups were more stable than polydimethylsiloxanes (PDMS) under isothermal aging, and no effect of the phenyl content was noted. Hence, only PDMS and PDMDPS with 5 mol% phenyl groups were then retained for the monitoring of their weight loss, tensile stress, elongation at break and Young's modulus during isothermal treatments between 150 and 300°C. Weight losses as well as the cracks appearance time during cycling aging were recorded simultaneously. The activation energies under the different aging conditions were determined, and found equal to 175 kJ/mol from the weight loss (characteristic of the depolymerization reaction) and 145–165 kJ/mol from the mechanical properties (characteristic of the post‐cross‐linking reaction) for PDMDPS, and 110 kJ/mol and 125–145 kJ/mol for PDMS, respectively. These activation energies confirm that silicone containing phenyl moieties are thermally more stable than those without. Finally, all these results empirically allow connecting weight loss and changes in mechanical properties to the appearance of cracks during thermal cycle aging.