The effect of oxygen on the irradiation process of ethylene-butene copolymer

A model ethylene-butene copolymer of narrow molecular weight distribution was irradiated with γ-rays under different oxygen-containing atmospheres. Gel fraction, carbonyl content and molecular weights were measured. Weight average molecular weights have been measured for all pregel samples and for the sol fraction of the postgel samples. Two different regimes seem to be present. Early on, molecular weight decreases with irradiation dose. At a later stage, further irradiation leads to a molecular weight increase. The dose at which this change of regime takes place varies with the oxygen concentration in the processing atmosphere. We have modeled this process assuming that oxygen is consumed at the early stages of irradiation, and that the variable that triggers the difference in response is the presence or absence of oxygen. Good agreement with measured values is found using either a probability model or a kinetic model.