Positron annihilation lifetime study of high and low free volume glassy polymers: Effects of free volume sizes on the permeability and permselectivity

Free volume in a big group of glassy polymers distinguished by a wide range of gas permeability (P in the range 1-10(4) Barrer) was studied using the positron annihilation lifetime (PAL) method. To take into account an additi onal channel of decay of positronium (Ps), caused by the reaction with sorb ed O-2, measurements were performed both under ambient conditions and under a nitrogen atmosphere. The finite-term and continuous lifetime analyses (P ATFIT and CONTIN programs) were used for the treatment of the positron anni hilation decay curves. It was shown that better statistical fitting paramet ers can be obtained for four component PAL spectra of a vast majority of th e glassy polymers studied. The presence of two Ps lifetimes (tau(3) = 1.7-3 .3 ns and tau(4) = 2.7-10.9 ns) indicates that size distribution of free vo lume elements (FVE) in glassy polymers is typically bimodal. The presence o f smaller and larger FVE (R-3 = 2.5-3.5 Angstrom, R-4 = 3.5-7.0 Angstrom) w as proved by PATFIT analysis of the data. Continuous analysis clearly indic ated two broad peaks, especially for more permeable polymers. As permeabili ty and free volume of the polymers decrease, the two peaks start to approac h and overlap each other, ending with one broader or asymmetric peak for th e less permeable polymers. In the presence of oxygen both lifetimes tau(3) and tau(4) are shifted to shorter values; this shift is more pronounced for the polymers with larger radii R-4 Of FVE. A novel correlation of diffusio n selectivity of glassy polymers with the size of FVE is demonstrated.