Ortho-positronium lifetime and intensity in pressure-densified amorphous polymers

We report the dependence of the ortho-positronium (o-Ps) lifetime τ3 and intensity I3 on the thermodynamic history of a number of amorphous homopolymer and copolymer glasses that were pressure-densified at pressures up to 200 MPa. Small macroscopic volume changes up to 1.85% yield large microscopic volume changes in the pressure-densified glasses, i.e., τ3 decreases by a maximum of 9%, while I3 remains constant, and the Simha-Somcynsky free-volume fraction decreases by a maximum of 31% and correlates with the free-volume size Vτ3.