Dose effects in the crosslinking of irradiated polysiloxane

Crosslink density calculations based on swelling, ball bearing indentation and extension measurements of γ-irradiated polydimethyl-siloxane as a function of irradiation dose up to 500 Mrad are reported. The crosslink density dependence on dose is linear up to approximately 160 Mrad and exponential for higher doses, tending to a constant value for extremely high doses. A mechanism involving free radical production and taking into account the network mobility-segmental motions and translational chain motion -is proposed. G values have been calculated as a function of dose and a value of 2.8 + = 0.3 has been found for the relatively low dose range (< 160 Mrad). Thereafter the initial constant rate of crosslink formation with radiation dose decreases to a value of 0.5 at 500 Mrad. This behaviour is mainly ascribed to the hindrance of translational chain motion due to the crosslinks. Measurements of the volume of gases evolved during irradiation have confirmed the above-mentioned mechanism. A radiation dependent cross-linking probability, p, per chain and per unit of radiation dose, has been used in empirical expressions which describe quite adequately the crosslinking process for this silicone system. (author)