Variation of Free Volume in Polyethylene upon Extension Probed by Positron Annihilation Lifetime Technique

Variations of an average size, size distribution, and numerical concentration of a free volume in polyethylene upon the extension up to 8 times were studied by a positron annihilation lifetime measurement. The average free volume size is not affected with an increase of the extension up to 6 times of a polyethylene film. This result suggested that the free volume holes in the amorphous phase of polyethylene are unchanged by the mechanical strain. The numerical concentration of the free volume decreases with an increase of the extension rate of polyethylene, suggesting that the total percentage of the amorphous phase of polyethylene decreases through the conversion to the crystalline phase to adapt the mechanical strain. A further extension of the film over 6 times, i. e., after saturation of crystallization, renders a decrease of the free volume size.