Free-volume hole relaxation in molecularly oriented glassy polymers

The free-volume hole relaxation in polycarbonate and poly(methyl methacrylate) with different levels of molecular orientation was studied by positron annihilation lifetime spectroscopy at variable pressures. The molecular orientation was achieved through a simple shear process performed at different temperatures and extrusion rates. It has been demonstrated that the β relaxation is largely responsible for the free-volume hole anisotropy after simple shear orientation. Upon the removal of mechanical force, the deformation of the free volume is mostly reversible at temperatures much lower than the glass transition. No strong correlation between macroscopic deformation and the free-volume hole deformation was found regardless of molecular orientation.