Stress relaxation in high density polyethylene. Effects of orientation and gamma radiation

The stress relaxation of unoriented (lambda = 1) and oriented (lambda = 9) high density polyethylene (HDPE) was studied for different levels of radiation (0, 100, 300, and 500 kGy). The relaxation behavior is analyzed in terms of a formerly established two-process model. In that model, one process is related to the crystal and the other to the amorphous fraction of a polyethylene sample. The modified viscoelastic properties of HDPE, due to effects of orientation and irradiation, are reflected in changes in viscosity coefficients and elasticity moduli of these two fractions. This made possible better comprehension of the nature of changes in the relaxation behavior of HDPE induced by irradiation and orientation.