Ortho-positronium lifetime distribution analyzed with MELT and LT and free volume in poly(ε-caprolactone) during glass transition, melting, and crystallization

Positron annihilation lifetime spectroscopy (PALS) was used to study the free volume behavior in the temperature range between 100 and 370 K in semi-crystalline poly(epsilon-caprolactone) (PCL). For the analysis of the spectra we used the well-known routine MELT as well as the new program LT8.0, which allows both discrete and log-normal distributed annihilation rates. From experiments, confirmed by the analysis of simulated spectra, we found that MELT returns too large values for the o-Ps lifetime tau(3) associated with too small intensities I-3. This is due to the underestimation of the width of o-Ps lifetime distribution in MELT (the spectra analyzed contained 3 million counts). The same effects were observed in the parameters obtained from the discrete term analysis. LT, however, returns, when allowing the o-Ps lifetime to be distributed, rather accurate values for tau(3), I-3 and the width (standard deviation sigma(3)) of the o-Ps lifetime distribution. The effect of the glass transition, melting, and crystallization on the annihilation parameters was observed. These results were compared with differential scanning calorimetry (DSC) and pressure-volume-temperature (PVT) experiments. From this comparison, the number density of holes and the fractional free (hole) volume have been estimated. At a "knee" temperature T-k approximate to 1.5 T-g, a leveling off of the o-Ps lifetime tau(3) and a distinct decrease in the width, sigma(3), of its distribution was observed; the latter effect was detected for the first time. Fast motional processes and/or the disappearance of the dynamic heterogeneity of the glass and the transition to a homogeneous liquid are discussed as possible reasons for these effects. (C) 2003 Wiley Periodicals, Inc.