Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by <i>In Situ</i> Positron Annihilation Lifetime Experiments
Dongmei Huang, Yun Dong, Hang Guo, Erjie Yang, Yiwen Chen, Mi Luo, Ziwen Pan, Jiandang Liu, Liangbin Li, Hongjun Zhang, Bangjiao Ye
Abstract
Using the newly developed positron annihilation lifetime spectroscopy (PALS) facility with a high count rate up to 3000 cps, in situ PALS experiments were performed for the first time on the continuous stretching process of polymers to quantitatively analyze the minute-scale evolution of free-volume holes. According to the stress–strain relationship and PALS results of four types of polyethylenes with different crystallinities, the tensile process could be divided into four distinct stages: elastic, initial nonlinear (until yield point), postyield, and strain hardening stages. The increase of o-Ps (orthopositronium) lifetime in the first three stages exhibits an enlargement of free-volume hole size with increasing strain. The decrease of the o-Ps lifetime in the last stage is most probably due to the increasing anisotropy of free-volume holes. The relative fractional free volume FFV r (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 unchanged in the other two stages. This work demonstrates a new feasibility to disclose minute-scale evolution of microstructure of materials through in situ PALS experiments in the future.