Dynamics of polycarbonate far below the glass transition temperature: A positron annihilation lifetime study

In this study, we have extended the knowledge of the dynamics of polycarbonate ~PC! far below Tg by means of positron annihilation lifetime spectroscopy ~PALS!, which is able to detect the isothermal relaxation of the free volume after rejuvenation above Tg . The free volume relaxation patterns at different temperatures below Tg were modeled according to a phenomenological model presented by Struik to obtain the equilibrium relaxation times of the process at each aging temperature. The dynamics of supercooled liquids above the glass transition temperature ( Tg) is usually described by the Williams-Landel-Ferry ~WLF! or Vogel-FulcherTammann ~VFT! equation. Recently, deviations from this behavior were found below Tg . In contrast, an Arrhenius-type temperature dependence was found. Our results suggest that, far below Tg , two possible scenarios can hold: ~a! the relaxation process of PC is dominated by the b process, though the fast modes of the a process still manifest even at the lowest investigated temperature; ~b! an Arrhenius-type process, deriving from the coupling of few b events, drives aging far below Tg . Finally, the aging process was related to a diffusion mechanism and a coupling parameter was introduced as a measure of the efficiency of theb process in producing appreciable changes in the density. The magnitude of the coupling parameter suggests a weak coupling between the b relaxation and the physical aging process.