Relaxation of Free Volume in Polycarbonate and Polystyrene Studied by Positron Annihilation Lifetime Spectroscopy

We present the time evolution of polycarbonate and polystyrene free volume probed by positron annihilation lifetime spectroscopy, which occurs after thermal rejuvenation above the glass transition temperature (T$\text{}_{g}$) and after plastic deformation via cold rolling. The ortho-positronium (τ$\text{}_{o-Ps}$) lifetime and its intensity (I$\text{}_{o-Ps}$) are used to characterise, respectively, size and concentration of the free volume holes. Our results suggest that the effect of plastic deformation on the polymer microstructure is far different from that of a thermally rejuvenated sample contrarily to common beliefs. In particular, plastic deformation results in an increase inτ$\text{}_{o-Ps}$ and a decrease in I$\text{}_{o-Ps}$, whereas rejuvenation above T$\text{}_{g}$ results in an increase in I$\text{}_{o-Ps}$ whileτ$\text{}_{o-Ps}$ remains unchanged. In addition, physical aging below T$\text{}_{g}$ after plastic deformation manifests as a reduction of τ$\text{}_{o-Ps}$, whereas a decrease in I$\text{}_{o-Ps}$ is observed after thermal rejuvenation. These results corroborate the idea of an amorphous-amorphous transition induced by plastic deformation