Free-volume microstructure of glycerol and its supercooled liquid-state dynamics

A positron annihilation lifetime spectroscopic (PALS) study was carried out on a typical representative of simple hydrogen-bonded glass formers - glycerol - over a wide temperature region from 100 K up to room temperature. Several crossover temperatures, T_bi, were identified in the temperature dependences of the ortho-positronium (o-Ps) lifetime and the relative o-Ps intensity. The onset temperature of free-volume change, 137 K, was compared with the value of 135±3 K obtained from extrapolating thermodynamic data and with the temperatures from relaxation data. In order to determine the influence of the free-volume evolution on the relaxation dynamics above the glass transition temperature T_g, the free-volume fraction from the PALS data was related to the behaviour of the dielectric loss of glycerol. A new relationship between the temperature parameters of the onset of free-volume change and of the high-frequency wing of the dielectric loss is presented. Taking the PALS free-volume data as input for a free-volume model (Doolittle ansatz), it was found that the relaxation times τ_α can be reproduced above the bend temperature T_b2 = 241 K; the latter lies in the vicinity of crossover temperatures reported from mode coupling theory analyses of dynamic data. In the lower-temperature region T_b2>T>T_b1≅T_g, a contribution from thermally activated mobility has to be included to reproduce the dielectric relaxation time data.