Changes in the microstructure of polyvinyl chloride polymer during the process of stress relaxation probed by positron annihilation

Abstract

Positron lifetime measurements were performed on poly(vinyl chloride) (PVC) polymer as a function of stress relaxation time (t=0-120 h). According to the variations in the orthopositronium (o-Ps) lifetime tau ₃ and intensity I₃, the relaxation process can be divided into three periods. In the first period (t approximately=0-30 h), the decrease in tau ₃ and the increase in I₃ indicate the segmental orientation and the creation of free-volume holes in the amorphous regions. In the second period (t approximately=30-90 h), the changes in the o-Ps parameters imply the coalescence of holes. Above t=90 h, both I₃ and tau ₃ reach constant levels, which suggests that the structure of PCV approaches the equilibrium state. These results show that the positron annihilation lifetime is sensitive to the microstructure changes of glassy polymers during the process of stress relaxation.