POSITRON STATES IN POLYETHYLENE AND POLYTETRAFLUOROETHYLENE - A POSITRON LIFETIME AND DOPPLER-BROADENING STUDY

Simultaneous positron lifetime (LT) and Doppler-broadening (DB) measur ements on semi-crystalline polyethylene (PE) and polytetrafluoroethyle ne (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-positr oniums (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) obtain ed 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 narrowe st of which is assumed to represent self-annihilation of para-positron iums (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 import ant Ps quenching reaction. Therefore, the variation in the LT and DB i ntensities 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 averag e positron lifetime is dominated by the o-Ps component, while the beha viour of the DB peak height is mainly affected by p-Ps annihilation. F urther, 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 li mited, physical relevance. (C) 1998 Elsevier Science B,V. All rights r eserved.