Free volumes and holes near the polymer surface studied by positron annihilation

Positron annihilation lifetime and Doppler broadening of annihilation radia tion experiments have been performed in a polystyrene film using the mono-e nergetic slow positron probe as a function of implantation energy. Signific ant variations of positron annihilation signals are observed at a short dis tance from the surface(less than or equal to 200 Angstrom). The ortho-posit ronium lifetime in the polymer increases near the surface, while its intens ity decreases. The intensity results are interpreted in terms of a free-vol ume hole model for positronium formation. The lifetime results are interpre ted as an expansion of local hole volume near the polymer surface. The free -volume and hole distribution near the surface is found to be broader than in the bulk. Applications of slow positrons to industrial problems, such as degradation of polymer coatings and membrane separations, are promising. ( C) 1999 Elsevier Science B.V. All rights reserved.