Insights into positron annihilation lifetime spectroscopy by molecular dynamics simulations. Free-volume calculations for liquid and glassy glycerol

The relationship between free-volume properties measured from positron annihilation lifetime spectroscopy ( PALS) and calculated from molecular dynamics simulations has been investigated for glassy and liquid glycerol in the temperature range 150 - 400 K. A virtual probing procedure has been developed to retrieve information on the basic free-volume properties of the simulated microstructures, i.e. mean cavity volume and free-volume cavity fractions. Our data leads us to infer on the occurrence of experimentally non-detectable small cavities with mean equivalent radius of 1.8 - 1.9 Angstrom between 250 and 275 K. The size of these limiting cavities is found to be temperature dependent, being smaller at low temperatures. At high temperatures, above a characteristic PALS temperature T(b2)(L), the formation of very large cavities is predicted. This finding suggests that, when the dimension of the holes in the system exceeds a given value, the PALS measurements become unable to catch the complete structural information and phenomena of dynamical origin enter into play in the PALS signal decay. The calculated number of cavities is found to be almost independent on the temperature from the glassy up to the liquid phase, thus furnishing a certain support to theoretical models proposed to evaluate the free-volume cavity fractions.