Pulsed slow-positron beam for polymer films

A pulsed slow-positron beam using 22Na as the positron source has been constructed for characterizing thin polymer films, coating polymer materials and their interfaces. The construction is very simple and the time resolution achieved is about 500 ps, which is sufficient for measuring long-lived positron lifetime components in thin polymer films. The positron lifetime in a 45 nm thick epoxy resin thin film held on a single-crystal silicon substrate has been measured using this pulsed slow-positron beam. Formation of o-positronium (ortho-Ps) is not observed when the incident positron energy is more than about 1 keV. The surface effect of the glass-transition temperature of a 0.3 mm thick polystyrene (PS) film has been also studied. It has been found that the glass-transition temperature of the sub-surface region of PS is lower than that of the PS bulk. # 2003 Elsevier B.V. All rights reserved.