Dynamic Mechanical Behavior of Irradiated Polyethylene

Samples of a ``high-density'' polyethylene (ρ=0.964 g/cc) irradiated in a nuclear reactor to thermal neutron doses as high as 2.9×1018 nvt and of ``low-density'' polyethylene (ρ=0.915 g/cc) subjected to gammaray bombardment from a Co60 source up to a dosage of 1×109 rep have been investigated by dynamic mechanical methods from 80°K to 450°K at audio-frequencies. Degrees of cross-linking were estimated from the observed changes in resonance frequency at temperatures where the specimens were ``rubber-like.'' Densities and x-ray crystallinities at 25°C were also measured. For the pile-irradiated ``high-density'' polyethylene the γ peak found around 165°K shows a marked increase in height and area at dosages above 1018 nvt. The α peak at about 390°K decreases markedly upon receiving a thermal neutron dose of 0.6×1018 nvt. Further changes in this absorption with higher dosages are obscured by the appearance of a β peak around 330°K which is almost absent in the unirradiated material; the appearance of the β region is attributed to the creation of branch points and cross-link points in the essentially linear polymer by the irradiation. The irradiated ``low-density'' specimens exhibit three damping peaks. Over the dosage range studied the γ peak (165°K) increases in height and area, the β peak (270°K) shifts to higher temperatures, and the α peak (360°K) shifts to lower temperatures with increasing dose. The cross-linking degree is not proportional to dose and to the highest dosages studied, there is very little, if any, reduction in x-ray crystallinity.