The positron lifetimes in a source have been measured. The fraction of positrons annihilated in salt and in the Kapton supporting foils has been determined as a function of the atomic number Z of the sample. All of the experimental results have been compared with calculated values obtained from a computer program based on a Monte Carlo algorithm. An algorithm has been proposed for the determination of the source correction in lifetime spectra in positron annihilation lifetime spectroscopy.

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Positron lifetime measurements are reported for a linear polyethylene and several ethylene-1-hexene copolymers synthesized with metallocene catalysts. The lifetimes, their intensity, the relaxation time, and the relative free volume fraction obtained are correlated with comonomer content, crystallinity, location of relaxation, and some mechanical parameters, such as elastic modulus and microhardness. A structural discontinuity is found with comonomer molar fraction that evidences the existence of two different regimes: the first one including the homopolymer and copolymers with contents up to around 5-7 mol % and the second regime for higher compositions. Possible reasons for this behavior are discussed. In addition, a linear relationship is found between the location of the relaxation, which is associated with large-scale cooperative motions in the amorphous environments, and the free volume fraction: the higher the relaxation temperature is, the lower free volume value is obtained.

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