No abstract

A positron annihilation lifetime spectroscopic (PALS) study was carried out on a typical representative of simple hydrogen-bonded glass formers-glycerolover 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 highfrequency 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.

A free-volume theory is developed based on the defect diffusion model (DDM). In addition, positronium annihilation lifetime spectroscopy (PALS) ortho-positronium free-volume and intensity data are presented for poly(propylene glycol) with a molecular weight of 4000 (PPG 4000) in both the glassy and liquid states and dielectric relaxation and electrical conductivity data are reported for PPG 4000 in the liquid state. The DDM is used to interpret all of the data for PPG 4000 and previously reported PALS and dielectric relaxation data for glycerol. It is shown that while the PPG 4000 data exhibit a preference for the three-halves power law, the data for glycerol favor the first power (standard Vogel-Fulcher-Tammann) law. Good agreement between the DDM and the experimental results is found for all of the electrical data and the PALS free-volume data. While reasonable agreement is also found for the PALS intensity data for PPG 4000, a discrepancy exists between the experimental PALS intensity data and theory for glycerol. For the electrical conductivity for PPG 4000, a transition is observed at the same temperature (about 1.4 T(g) where T(g) is the glass transition temperature) where the PALS free volume changes from steeply rising with temperature to approximately independent of temperature. The same behavior is observed at about 1.5 T(g) for previously reported dielectric relaxation and PALS data for glycerol. Model parameters are presented that show the dominance of mobile single defects above (1.4-1.5) T(g) and the dominance of immobile clustered single defects below T(g) . Finally, a coherent picture of glasses and glass-forming liquids is presented based on the theory and results of the experiments.

Amorphous polycarbonate has been investigated from the point of view of free-volume microstructure in the temperature range 50-350 K by positron-annihilation-lifetime spectroscopy. Lifetime~3 and relative intensity I3 of 0-Ps show two regions of different behavior with the transition at T& =130 K. The temperature dependence of~3 has been analyzed using the model of a particle in a spherical potential well. The correlations between the dilatometric measurements and the~3{T) and I3{T) dependences have been interpreted in the framework of the free-volume model, which allowed us to estimate the fraction of free volume accessible to 0-Ps as well as the concentration of free-volume entities. We have determined that the polycarbonate is a relatively defect-full material. On the basis of a simple thermodynamical approach allowing for simultaneous change of the size and number of free-volume entities, an attempt has been made to specify the molecular mechanism responsible for the free-volume changes. The comparison of the results of such a thermodynamical model with the simulation of the structure of amorphous polycarbonate and the dynamics of the motion of phenyl groups suggests that partial flips of this group might be the potential generators of the increased free volume at T & Tb.