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

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 (tau(3) = 0.9-1.2 ns), and in free-volume holes in the amorphous phase (tau(4) = 1.5-2.8 (PE); tau(4) = 2.5-4.4 ns (PTFE)). The values of tau(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 nm3 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 14, From this and from the intensity ratio of the LT spectra, 3I(1)/(I-3 + I-4) approximate to 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 3I(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. (C) 1998 Elsevier Science B,V. All rights reserved.