Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by In Situ Positron Annihilation Lifetime Experiments

Using the newly developed positron annihilation lifetimespectroscopy(PALS) facility with a high count rate up to 3000 cps, insitu PALS experiments were performed for the first time onthe continuous stretching process of polymers to quantitatively analyzethe minute-scale evolution of free-volume holes. According to thestress-strain relationship and PALS results of four types ofpolyethylenes with different crystallinities, the tensile processcould be divided into four distinct stages: elastic, initial nonlinear(until yield point), postyield, and strain hardening stages. The increaseof o-Ps (orthopositronium) lifetime in the first three stages exhibitsan enlargement of free-volume hole size with increasing strain. Thedecrease of the o-Ps lifetime in the last stage is most probably dueto the increasing anisotropy of free-volume holes. The relative fractionalfree volume FFVr (derived from hole radius R (calculated from the Tao-Eldrup model) and o-Ps intensity)generally increases in the first two stages but remains nearly unchangedin the other two stages. This work demonstrates a new feasibilityto disclose minute-scale evolution of microstructure of materialsthrough in situ PALS experiments in the future.