Free volume evolution in polyoxymethylene studied by positron annihilation spectroscopy

The free volume hole and its distribution in polyoxymethylene have been studied over the temperature range (5-90 °C) by positron annihilation lifetime spectroscopy. At a certain temperature (20 °C) the variation of ortho-positronium lifetime shows a distinct increase in its slope. The hole volume shows a small linear increase with temperature below glass transition temperatue and a steeper increase above it. A linear relation between ln(σT0.5) and 1/Vh was observed and the critical hole volume was estimated.