A positron annihilation lifetime study of isotactic polypropylene

Abstract

AbstractPositron lifetime studies were performed on well‐characterized annealed and quenched samples of isotactic polypropylene. The positron experiments were conducted from −20 to 110°C as a function of both heating and cooling. Of the three decaying exponential components resolved from the lifetime spectra, only the long‐lifetime ortho‐positronium (o‐Ps) pickoff component was affected by the changes in temperature. The behavior of both the lifetime and intensity of the o‐Ps component was interpreted with the aid of x‐ray diffraction, densitometry, and optical microscopy examinations and results from previously reported investigations of the thermal transition behavior of polypropylene. The present experiments demonstrate that o‐Ps lifetimes were similar for both the annealed and quenched samples, independent of thermal cycling, while the o‐Ps component intensity was significantly larger for the quenched material during heating, with both sample types exhibiting a significant hysteresis upon cooling. These results suggest that the mean free‐volume cavity size is independent of prior thermal treatment, while the density of free‐volume sites is a sensitive function of structure and prior thermal history. The variations of lifetime and of intensity with temperature have provided insight into polypropylene's glass transition phenomena.