A positron lifetime and Doppler-broadening study of positron states and free volume in polyethylene

Abstract

Positron lifetime (LT) and Doppler-broadening (DB) studies of polyethylene have been performed simultaneously in the temperature range between 80 and 300 K. The LT spectra have been analysed assuming four exponential components. Two long-lived components appear, which were attributed too-Ps pick-off annihilation in crystalline regions (τ3 = 0.9 to 1.2 ns) and at free-volume holes in the amorphous phase (\(\tau _{\Delta _3 } = 1.5\) to 2.8), The variation in τ4 correponds to an increase of the mean hole size from 0.053 nm3 at 80 K to 0.188 nm3 at 300 K. From the data the glass transition temperature (Tg=195 K), the coefficient of thermal expansion of holes in the glassy and rubbery phase (α h, g = 14.5 · 10−4 K−1 and α h, r = 189 · 10−4 K−1) and the fractional free volume (2.8% to 10.4%) were estimated. The DB curves were fitted by a sum of three Gaussians, the narrowest of which is assumed to represent the self-annihilation ofp-Ps localised at holes. The intensity of the narrow component,In, varies between 0 and 7.3% in a similar way as the LT intensityI4/3 varies. From this it was concluded that other Ps reactions beside pick-off are not important. Further, it was shown that the average positron lifetime is dominated by theo-Ps component,T4τg, while the behaviour of the DB peak height is mainly affected by thep-Ps narrow componentI n .