Chemorheological Studies of a γ‐Irradiated Dimethylsiloxane Elastomer

The degradation of a peroxide‐cured polydimethylsiloxane (PDMS) elastomer subjected to γ‐irradiation at various temperatures has been studied by chemorheological techniques. Preliminary investigations of the thermal degradation of mechanisms operative in the elastomer utilising continuous and intermittent stress relaxation measurements revealed that degradation took place by a chain interchange reaction. The cleavage reaction occurred randomly along the main chain and was a hydrolytic scission reaction catalyzed by the presence of peroxide decomposition products in the material—namely, 2, 4‐dichlorobenzoic acid. Subsequent chain reformation took place by the condensation of silanol chain end groups. The reaction was found to have an activation energy of $15±4 kJ mol−1.$ Intermittent stress relaxation techniques revealed that the net crosslink density of the elastomer increased as thermal degradation proceeded. This phenomenon has not been reported in any previous studies of the thermal degradation of PDMS elastomers. The effect has been attributed to the reformation of network chains cleaved during the post‐cure operation prior to the commencement of the thermal degradation studies. This reaction which is also believed to take place by the condensation of silanol groups, occurs simultaneously with the conventional interchange reaction. Continuous stress relaxation studies during γ‐irradiation indicated that the rate of the radiation induced scission reaction increased with increasing temperature. The temperature dependence of this rate did not obey the Arrhenius relationship suggesting a possible change in the scission kinetics at higher temperatures. For the particular sample geometry in the investigations, it was discovered that there was no significant difference between the response of elastomers tested in air and in an inert atmosphere.