A dielectric study of molecular relaxations in irradiated high density polyethylene

The molecular relaxation behaviour of high density polyethylene (HDPE), exposed to gamma radiation in air to various absorbed doses (up to 700 kGy), has been investigated by dielectric loss (tanδ) analysis. All relaxation zones (γ, β and α, in order of increasing temperature), between 25 K and the melt temperature, were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the samples to radiation. Radiation-induced changes in the crystal fraction, oxidative degradation and degree of network formation, observed by wide angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), infrared spectroscopy (IR) and gel measurements, were well connected with the changes in intensity, position and activation energy of α and γ relaxations. Complete disappearance of the already weak β relaxation with irradiation is attributed to the more restricted chain segment mobility in the net structure, but the contribution due to radiation-induced increase in crystallinity should also be taken into account.