AbstractPolyisobutylene was irradiated by γ‐rays from a Co60source in vacuum and in nitrous oxide atmosphere. The irradiated specimens in N2O were damaged to a lesser extent than those in vacuum. Irradiated specimens were dissolved in CCl4and their viscosity was measured at 30°C. A plot of reciprocal of viscosity‐average molecular weight against dose gives a straight line in each case for specimens irradiated in vacuum and in N2O. Moreover, the presence of nitrous oxide reduces the extent of main‐chain fracture to 63. The degree of branching is nearly the same for all of the specimens, since Huggins' constantk'is constant (k'= 0.40) throughout our experiments. It can be concluded that the N2O has a protective effect for main‐chain fracture of polyis
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