Abstract
AbstractAnnihilation of slow positrons at a polymeric surface has been discussed in terms of positron diffusion and trapping of positronium into free volume holes. The above model has been used to calculate the ortho‐positronium lifetime (τ3) in polystyrene (PS), epoxy resin film, polyurethane (PU) and polytetrafluroethylene (PTFE) as a function of temperature, incident positron energy and mean implantation depth. The results have been compared with the experimental observations of other authors. The variation in τ3 with respect to temperature clearly demonstrates a discontinuity in the curve at Tg corresponding to the glass transition temperature. The variation of calculated τ3 shows that the lifetime increases significantly above Tg however, below Tg it increases only slowly. This is a direct consequence of the change in the size of free volume holes. The Tg has been found to be dependent both on positron energy and density of the polymer. Large variation in τ3 has been observed at low energies suggesting a significant structure of free volume holes near the surface. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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