Temperature effect on free volume of polymethylpentene studied by positron annihilation technique

Polymethylpentene (PMP) possesses high mechanical, thermal and chemical stability coupled with high conductivity and ionic selectivity which is used for autoclavable medical and laboratory equipment. The Positron annihilation Lifetime (PAL) measurements were performed at temperatures from 300 K down to 165 K (1st cooling run), from 175 K up to 425 K (1st heating run), and from 425 K to 165 K (2nd cooling run) under vacuum. The temperature dependence of the free volume and its contents, which were deduced from the ortho‐ positronium (o ‐Ps) lifetime, exhibited a little change between the cooling and heating runs, so that the behavior appears to be reversible. The variation of the free volume with temperature reflects the glass transition temperature of PMP at about 300 K. Based on the free volume theory of Cohen‐Turnbull, the ionic conductivity σ is correlated with the mean hole volume Vh·. A linear relation between ln(σT0.5) and 1/Vh was observed and the critical hole volume was also estimated to be around 811.9 Å3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)