Temperature Dependence of Positron Mean Lives in Polymers

The mean lives and intensities of positrons decaying by the pickoff mechanism in polystyrene, Lucite, and polyethylene (Marlex 50) have been measured in the temperature interval -200 to +150^∘{}C. The meanlife variation with temperature in both polystyrene and Lucite has the same general shape as the specific-volume variation of these materials and may be interpreted as a free-volume effect. However, an observed sharp drop of τ₂ with decreasing temperature below -20^∘{}C in Marlex 50 does not correspond to the smooth change in volume of this material in that temperature range.