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

摘要

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