Abstract
We report and discuss the change in free volume in high density polyethylene due to inelastic deformation under plain strain conditions as a function of the draw ratio λd, studied via positron lifetime spectroscopy. Two long-lived ortho-positronium pick-off lifetimes are observed and attributed to the (interstitial) free volume in crystallites (τ3 ∼ 1.4 ns) and to local free volumes in the amorphous phase (τ4 ∼ 2.7 ns). From this we estimate the crystalline packing density and the average size of the free volume holes, r = 0.343 nm and υ = 0.170 nm3. From the continuous positron lifetime intensity distribution derived via Laplace inversion of the lifetime spectra, we obtained free volume hole size distributions in the samples. The hole size distributions are bimodal with maxima corresponding to τ3 and τ4. We observe an increase in τ3 but a decrease in τ4 at small draw ratios. The increase in τ3 indicates increasing free volume in the crystalline region, which may be attributed to an increase in the paracrystalline disorder. The decrease in τ4 is attributed to the decrease in free volume hole sizes in the amorphous phase or to a change in the shape of the holes from ‚spheres’︁ to ‚ellipsoids’︁.
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