Influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene

In the present study, the influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross-linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross-linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission.