Radiation-induced modification of dielectric relaxation spectra of polyolefins: polyethylenes vs. polypropylene

Abstract

The molecular relaxation behaviour of polyolefins exposed to high-energy radiation has been investigated by dielectric loss (tan δ) analysis. Therefore, low-density polyethylene, high-density polyethylene (HDPE), and isotactic polypropylene (iPP) were gamma-irradiated in air to various absorbed doses (up to 700 kGy). All relaxation zones (α, β, γ, and δ in the order of decreasing temperature), between 25 K and melting temperature, were studied. The radiation-induced changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the polyolefins to radiation. Wide-angle X-ray diffraction, infrared spectroscopy, and gel measurements were used to determine the radiation-induced changes in the crystalline structure, oxidative degradation, and the degree of network formation, respectively. The present study reveals high dielectric and/or relaxation sensitivity of polyolefins to gamma radiation. Disappearance of some relaxations (such as β relaxation in HDPE and low temperature γ and δ relaxations in iPP) is clearly observed with irradiation. For the other relaxations, besides the large changes in the relaxation intensity, radiation also induces smaller/larger changes in the distribution of relaxation times, peak position, and activation energy.