Positron annihilation and broadband dielectric spectroscopy: A series of propylene glycols

We report a thorough joint analysis of the behavior of the ortho-positronium lifetime as obtained from positron annihilation lifetime spectroscopy and of the dipolar relaxation spectra investigated by broadband dielectric relaxation spectroscopy in a series of glass-forming propylene glycols including propylene glycol, dipropylene glycol and tripropylene glycol. A number of empirical correlations between the temperature dependence of the ortho-positronium annihilation lifetime, τ3(T), and the various spectral and relaxational quantities have been found. The phenomenological evaluation of the quasi-sigmoidal τ3(T) dependence reveals three characteristic temperatures: TgPALS, Tb1 = (1.23 − 1.27)TgPALS and Tb2 = (1.46 − 1.53)TgPALS, which are found to decrease with increasing fragility. The slighter change of slope in the PALS response at Tb1 in this series of propylene glycols appears to be related to the crossover from the α-process to the excess wing or secondary relaxation, found in the dielectric spectra. The onset of the high-temperature plateau in the τ3(T) plot at Tb2 occurs when τ3 matches the average relaxation time of the primary α process. Moreover, the plateau region lies in the vicinity of the crossover in the dielectric parameters of the structural relaxation in all the samples, i.e. spectral width and relaxation strength. In addition it is approximately related to a crossover of the α relaxation time τα(T) from non-Arrhenius to Arrhenius regime. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in the series of propylene glycols.