THE LOCAL FREE-VOLUME IN DEFORMED POLYETHYLENE - A POSITRON LIFETIME STUDY

Rr,,,port and discuss the change in free volume in high density polyet hylene due to inelastic deformation under plain strain conditions as a function of the draw ratio lambda(d), studied via positron lifetime s pectroscopy. Two long-lived ortho-positronium pick-off lifetimes are o bserved and attributed to the (interstitial) free volume in crystallit es (tau(3) similar to 1.4 ns) and to local free volumes in the amorpho us phase (tau(4) similar to 2.7 ns). From this we estimate the crystal line packing density and the average size of the free volume holes, r = 0.343 nm and nu = 0.170 nm(3). From the continuous positron lifetime intensity distribution derived via Laplace inversion of the lifetime spectra, we obtained free volume hole size distributions in the sample s. The hole size distributions are bimodal with maxima corresponding t o tau(3) and tau(4). We observe an increase in tau(3) but a decrease i n tau(4) at small draw ratios. The increase in tau(3) indicates increa sing free volume in the crystalline region, which may be attributed to an increase in the paracrystalline disorder. The decrease in tau(4) i s attributed to the decrease in free volume hole sizes in the amorphou s phase or to a change in the shape of the holes from 'spheres' to 'el lipsoids'.