Measurements of microscopic voids in polymers using positron annihilation lifetime spectroscopy.

Abstract

This work has confirmed that Positron Annihilation Lifetime Spectroscopy (PALS) techniques can be used as sensitive probes of the microscopic voids in polymers. The o-Ps lifetime is found to be closely related to the size of the microscopic voids from the thermal expansion and mechanical loading experiments. We have also found that craze-initiation by fatigue is a very localized process. By using a mono-energetic positron beam, we showed that the surface of polystyrene has the same glass transition temperature as that of the bulk. This result contradicts the result obtained by using ellipsometry, and therefore carries significant importance to the theoretical research work on the polymer surface dynamics and properties. It is found in this work that the o-Ps intensity is proportional to not only the density of microscopic holes but also the fraction of positronium formed in polymers. Having explored the process of the positron-polymer solid interaction and the mechanism of positronium formation in polymer solids, we concluded that the spur-electron capture is the dominant process in positronium formation. The size of the terminal spur is found to be about 200 to 2000 A depending on the material properties. We have shown that it is possible to determine the absolute value of the hole-volume fraction of a polymer by using PALS and the specific volume change indicated by either thermal expansion or static stress. The hole-volume fractions of the polymers studied in this work are about 3-7% at room temperature. The dynamic nature of the microscopic voids becomes evident in the analysis of the hole volume fraction. The voids that fluctuate with frequencies higher than 10$\sp9$ Hz are not distinguishable from the bulk when probed by o-Ps.