THE TEMPERATURE-DEPENDENCE OF THE LOCAL FREE-VOLUME IN POLYETHYLENE AND POLYTETRAFLUOROETHYLENE - A POSITRON LIFETIME STUDY

Positron lifetime measurements, performed in the temperature range 80- 300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data process ing routines LIFSPECFIT and MELT. Two long-lived components appear, wh ich are attributed to pick-off annihi lation of ortho-positronium in c rystalline regions and at holes in the amorphous phase. The ortho-posi tronium lifetimes, tau(3) and tau(4), are used to estimate the crystal line packing density and the size of local free volumes in the crystal line and amorphous phases. The interstitial free volume in the crystal s exhibits a weak linear increase with the temperature which is attrib uted to thermal expansion of the crystal unit cell. In the amorphous p hase, the hole volume varies between 0.053 and 0.188 nm(3) (PE) and be tween 0.152 and 0.372 nm(3) (PTFE). Its temperature variation may be f itted by two straight lines, the intersection of which is used to esti mate a glass transition temperature of T-g = 195 K for both PE and PTF E. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macrosco pic volume change below and above the glass transition. From this comp arison a fractional hole volume at T-g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) X 10(27) holes/m(3) is estimate d. Finally, it is found that the intensity of o-Ps annihilation in cry stals shows a different temperature dependence to that in the amorphou s phase. (C) 1998 John Wiley & Sons, Inc.