Positron annihilation at polymeric surfaces

The mechanism of slow positron annihilation at polymeric surfaces has been discussed in terms of positron diffusion at the surface and trapping of positrons and positronium in free volume holes. The one-dimensional diffusion equation has been solved and the rate equations have been set up to describe the various processes supposed to occur when a thermalized positron encounters the polymeric surface. The model has been used to calculate the Doppler broadening of the line shape parameter (S parameter) in polyurethane and polystyrene as a function of incident positron energy and temperature. The results have been compared with the available experimental data. The S parameter vs temperature curves show a remarkable discontinuity at the glass transition temperature (T-g). Large variation in the S parameter has been observed at low energies, suggesting a significant structure of free volume holes near the surface.