Strain-induced changes of free volume measured by positron lifetime spectroscopy in ultrahigh molecular weight polyethylene

Positron lifetime spectroscopy and wide-angle X-ray scattering have been applied to investigate the structural changes induced by tensile deformation in ultrahigh molecular weight linear polyethylene. The results reveal a correlation between the crystallinity, the positron annihilation characteristics, and the strain of the polymer. The probability of positronium formation increases and the crystallinity of the material decreases with increasing tensile strain. The size distribution of the free-volume holes, where positronium atoms form, is determined from their lifetime distribution. The positron lifetime measurements are interpreted in terms of a free-volume approach assuming that the free volume associated with each liquidlike cell of the amorphous phase is divided into free-volume holes whose size distribution is given by a normal frequency function. The cumulative distribution functions determined from the positron annihilation measurements agrees with those predicted by the free-volume model. This indicates that the positrons contributing to the lifetime component associated with orthopositronium annihilation really probe the free volume in the amorphous phase of the polymer. The experiments evidence free-volume changes induced by tensile strain which are unrecoverable after relieving the stress.