Some Aspects of Free Volume Studies in Molecular Substances Using Positron Annihilation Experiments

The first problem in the application of positron annihilation lifetime (PAL) spectroscopy for the study of elementary free volumes (EFV) in solids comes from inhibition of positronium formation, i.e. essential influence of small admixtures on the long-lived component intensity of the PAL spectrum. The nature of this effect and correlation of the phenomena with thermo-stimulated luminescence (TSL) are considered here along with calculation of the number density of free volume holes using characteristics of the PAL spectra in polyimides and systems with a highly developed specific surface, such as cross-linked polystyrenes (polymer sorbents) and silica based glasses. Finally, a special attention is paid to comparison of this value calculated on the bases of positron annihilation data with that found from sorption isotherms and Brunauer–Emmett–Teller (BET) theory.