Shapes of the free volume holes in amorphous polymers as estimated by positron annihilation lifetime spectroscopy

Intermolecular spaces in polymer chains form the free volume, a useful concept to understand mechanical and transport properties of polymers. Quantification of the free volume can be obtained theoretically, using appropriate lattice models, as well as experimentally, through suitable probes. Among these, positronium (Ps) has become widespread due to the nondestructive character of the technique and the correlation between the Ps lifetime and the size of the holes. In most of the investigations, the cavity is approximated to a sphere. However, this may bias the evaluation of the free volume fraction. We show that by coupling results from Ps lifetime and specific volume measurement for amorphous polymers at equilibrium and the predictions of the Simha–Somcynsky equation of state, it is possible to shed light on dimensions of the holes as well as on their morphology.