Free volume fraction and nanoholes shapes in polyvinyl acetate

Positron annihilation lifetime and specific volume measurements were carried out in polyvinyl acetate, with the aim to evaluate the free volume fraction in a range of temperatures above the glass transition. Comparison with the analogous quantity calculated on the basis of the Simha–Somcynsky theory shows that the usual assumption of spherical intermolecular spaces (nanoholes) forming the free volume does not give the correct quantitative result. Although irregularly shaped, nevertheless nanoholes can be better framed in terms of flattened discs.