Free-volume microstructure of amorphous polycarbonate at low temperatures determined by positron-annihilation-lifetime spectroscopy.

Abstract

Amorphous polycarbonate has been investigated from the point of view of free-volume microstructure in the temperature range 50--350 K by positron-annihilation-lifetime spectroscopy. Lifetime ${\mathrm{\ensuremath{\tau}}}_{3}$ and relative intensity ${\mathit{I}}_{3}$ of o-Ps show two regions of different behavior with the transition at ${\mathit{T}}_{\mathit{b}}$=130 K. The temperature dependence of ${\mathrm{\ensuremath{\tau}}}_{3}$ has been analyzed using the model of a particle in a spherical potential well. The correlations between the dilatometric measurements and the ${\mathrm{\ensuremath{\tau}}}_{3}$(T) and ${\mathit{I}}_{3}$(T) dependences have been interpreted in the framework of the free-volume model, which allowed us to estimate the fraction of free volume accessible to o-Ps as well as the concentration of free-volume entities. We have determined that the polycarbonate is a relatively defect-full material. On the basis of a simple thermodynamical approach allowing for simultaneous change of the size and number of free-volume entities, an attempt has been made to specify the molecular mechanism responsible for the free-volume changes. The comparison of the results of such a thermodynamical model with the simulation of the structure of amorphous polycarbonate and the dynamics of the motion of phenyl groups suggests that partial flips of this group might be the potential generators of the increased free volume at Tg${\mathit{T}}_{\mathit{b}}$.