Positron annihilation response and broadband dielectric spectroscopy: Propylene carbonate

A phenomenological analysis of the ortho-positronium (o-Ps) lifetimes from positron annihilation lifetime spectroscopy (PALS) and the dielectric α-relaxation times from broadband dielectric spectroscopy (BDS) on glass-forming propylene carbonate (PC) is presented. As known from other glass-formers, the temperature dependence of the o-Ps lifetime, τ3, shows a sigmoidal increase with increasing temperature, which can be ascribed to an increase of free volume. The onset of the high-temperature plateau in the τ3-T plot at Tb2L ∼200 K occurs when τ3 matches the mean relaxation time of the primary α-process. In addition, it coincides with the crossovers in the dielectric parameters, i.e., relaxation strength and spectral width, of the main process and is approximately related to a crossover of the α-relaxation time τα(T) from non-Arrhenius to Arrhenius behavior. We demonstrate that in PC the changes of slope seen in τ3 can be traced back to corresponding anomalies in the temperature-dependent loss due to transitions between distinct spectral regions as the relaxation and the excess wing. Overall, all the empirical coincidences indicate a close connection between the PALS response and the dielectric relaxation for PC.