Positron annihilation in free volume elements of polymer structures

Free volume effects are important for positronium formation, pick-off annihilation and chemical reactions. In order to make the method of positron annihilation an effective technique for quantitative studies of molecular solids, for example polymers, the relation between the intensity of the long-lived component of lifetime distribution of positron annihilation and the number of defects (elementary free volumes) was considered. The analysis assumes selective trapping of positrons and positronium in the defects of ordered and disordered (crystalline and amorphous) sites, respectively, the sizes of nonhomogeneities are assumed to be lower than positron diffusion length. The results on positronium annihilation in porous poly(phenylene oxide) allow one to estimate nontrapped positronium diffusion coefficient which is equal to 0.5 x 10(-4) < D-Ps < 1.5 X 10(-4) cm(2)/s. The relation between the positronium lifetime and effective size of free volume for large pores (effective radius greater than or equal to 1 nm) is considered. Experimental results were obtained using the CONTIN program; some comments on its application for calculations of size distribution of elementary free volumes in polymers are discussed.