Thermally induced microstructural changes in undoped and doped polyacrylonitrile: A positron-annihilation study

Abstract

Positron-lifetime measurements have been made in undoped and polymers 0.8 wt %--iodine-doped polyacrylonitrile (PAN) under annealing from room temperature to 160 \ifmmode^\circ\else\textdegree\fi{}C. From the measured lifetimes, the free-volume size is calculated following the treatment of Nakanishi, Jean, and collaborators. The variation in free-volume size as function of temperature seems to suggest molecular-chain-folding processes in amorphous regions of the polymer, positron trapping at kink-type defects, and at the interface between crystalline and amorphous regions at lower temperatures. This study indicates that in the undoped state, the polymer has a glass transition temperature at 90 \ifmmode^\circ\else\textdegree\fi{}C which is shifted toward lower temperature in the doped state by 30 \ifmmode^\circ\else\textdegree\fi{}C. The results also suggest the degradation of the polymer in the two states after 100 \ifmmode^\circ\else\textdegree\fi{}C and the effect of annealing is only on the size but not on the density of free-volume sites.