Positron annihilation in free volume elements of polymer structures

Free volume effects are important for positronium formation, pick-off annih ilation and chemical reactions. In order to make the method of positron ann ihilation an effective technique for quantitative studies of molecular soli ds, for example polymers, the relation between the intensity of the long-li ved component of lifetime distribution of positron annihilation and the num ber of defects (elementary free volumes) was considered. The analysis assum es selective trapping of positrons and positronium in the defects of ordere d 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) a llow 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 t he 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 appli cation for calculations of size distribution of elementary free volumes in polymers are discussed.