Positron states in polyethylene and polytetrafluoroethylene: A positron lifetime and Doppler-broadening study

Abstract

Simultaneous positron lifetime (LT) and Doppler-broadening (DB) measurements on semi-crystalline polyethylene (PE) and polytetrafluoroethylene (PTFE) polymers have been performed in the temperature range 80–300 K. The LT spectra are analysed assuming four exponential components. Two long-lived components are found and are attributed to ortho-positroniums (o-Ps) pick-off annihilation in crystalline regions ( τ 3=0.9–1.2 ns), and in free-volume holes in the amorphous phase ( τ 4=1.5–2.8 (PE); τ 4=2.5–4.4 ns (PTFE)). The values of τ 4 obtained correspond to an increase of the mean hole size from 0.053 (at 80 K) to 0.188 nm 3 (at 300 K) in PE, and from 0.152 to 0.372 nm 3 in PTFE. The DB curves were fitted with a sum of three Gaussians, the narrowest of which is assumed to represent self-annihilation of para-positroniums (p-Ps) localised at holes. The intensity of the narrow component, I n, varies between 0% and 5% in a similar way to the LT intensity I 4. From this and from the intensity ratio of the LT spectra, 3 I 1/( I 3 + I 4) ≈ 1, we conclude that pick off is the only important Ps quenching reaction. Therefore, the variation in the LT and DB intensities is attributed exclusively to inhibition or anti-inhibition of Ps formation. The intensity ratio of the narrow DB component 3 I n/ I 4=0.55 to 0.9 is smaller than the anticipated value of 1. Possible reasons of this effect will be discussed. It is shown that the average positron lifetime is dominated by the o-Ps component, while the behaviour of the DB peak height is mainly affected by p-Ps annihilation. Further, the results of the four-component decomposition of LT spectra were compared with those obtained from a three-component analysis, and from the comparison it was concluded that a three-component analysis of the o-Ps lifetime in semi-crystalline polymers, has some, albeit limited, physical relevance.