Free volume changes in mechanically milled PS and PC studied by positron annihilation lifetime spectroscopy (PALS)

The effect of mechanical milling on free volume was studied by means of positron annihilation lifetime spectroscopy (PALS) in polystyrene (PS) as a typical brittle polymer and in polycarbonate (PC) as a tough representative. Long-time milling increases the free volume, while a decrease is observed for short milling times. The changes are mostly irreversible in PS. The irreversible fraction is much smaller for PC. Gel permeation chromatography (GPC) measurements show a decrease of the molecular weight, which is much more pronounced in PS. The milling-induced irreversible changes in free volume are attributed to chain-end defects resulting from chain scission. In PC, other deformation-induced defects that anneal upon heat treatment above the glass transition temperature dominate.