Positronium Diffusion in Polystyrene at Low Temperatures

Abstract

Diffusion of ortho-positronium (o-Ps) in polystyrene (PS) at low temperatures (20-300K) was studied by means of the positron annihilation lifetime technique. 2,2'-Dinitrobiphenyl was added to PS as a Ps quencher, and the diffusion coefficients were determined from the measured Ps quenching rate constants, assuming that the reaction between Ps and the quencher is entirely diffusion-controlled. The determined diffusion coefficient increases with increasing temperature, and is significantly influenced by such structural relaxation of PS as δ and β transitions. The estimated diffusion length of o-Ps, 1.5 2.5 nm. is only a few times larger than the Ps cavity size, indicating that the effect of the Ps diffusion on the Ps lifetime distribution is almost insignificant in PS at low temperatures. The size distributions of free volume in pure PS were obtained from the lifetime distributions of o-Ps without taking an effect of the diffusivity of o-Ps into consideration. Peculiar broadening of the size distribution of free volume for PS, which is induced by an inhomogeneous local structure. is observed at the transition temperatures, T δ and T β .