Dynamic mechanical properties of irradiated polyethylene

Dynamic mechanical properties of eight samples of polyethylene irradiated to various extents have been determined over a temperature range from −70 to +280°C. and a frequency range from 103 to 2 × 104 cycles/sec. At room temperature, minimum values in Young's modulus and maximum values in damping factor are observed for a critical radiation dose. Three transition points Tg, Ts, and Tm are exhibited by the sound velocity‐temperature curves; these may be associated with the transition from the glass state, the softening of the amorphous regions, and the melting of the crystallites, respectively. With increasing dose, Tm, decreases and Tg, increases, while Ts shows a minimum for a radiation dose equal to the critical value. At temperature above the melting point of the crystallites Tm, lightly crosslinked samples show rubber‐like elasticity, but highly crosslinked samples show the same dynamic mechanical behavior as highly crosslinked amorphous materials. Results are discussed and explained on the basis of the partial crystallinity of polyethylene and of the changes that irradiation causes on the molecular structure.