Morphology of undeformed and deformed polyethylene lamellar crystals

Morphology of undeformed polyethylene crystals obtained by high pressure crystallization was investigated by SEM. It was revealed by exposing the interior of the samples by microtoming followed by permanganic etching. The etching procedure was refined to reveal defected sites of lamellae in addition to differentiation of crystalline and amorphous phases. From 1 to 3 screw dislocations with large Burgers vector per 1 μm2 of lamellae basal planes were detected. Lamellae, when viewed edge-on give an impression of a “blocky architecture”, while their real shape, as seen on SEM images of flat-on and oblique lamellae, resembles platelets with a few defects in the form of screw dislocations protruding a platelet.High pressure-crystallized polyethylene samples were deformed plastically by uniaxial compression and were studied by SEM and AFM. When the deformation is interrupted dislocations are arrested within the crystals. It was observed that in contrast to undeformed samples, the side faces of deformed lamellae were not any longer smooth and a large number of screw dislocations with low Burgers vectors crossing the lamellae thickness could be distinguished. These observations are in accordance with polymer crystal plasticity theory that relies on the rate controlled nucleation and propagation of screw dislocations across polymer crystals. An existence of numerous screw dislocations arrested in lamellae is a direct proof of action of fine crystallographic slips along the macromolecular chains in PE crystals during plastic deformation. The kinking of lamellae due to plastic deformation was also observed. Large sections of lamellae between kinks rotated towards the plane of compression while the chain stems in lamellae rotated in the opposite direction, away from the compression direction, which is a signature of the fine crystallographic slip.Plastically deformed polyethylene crystals are highly defected due to many dislocations incorporated within them - the density of dislocations was approximated as 1016 m−2. However, deformed crystal melting temperature is nearly unaffected while the heat of melting is slightly reduced, yet only in thin crystals. It suggests that the arrested dislocations contribute more to the surface energy of lamellae basal planes rather than to a bulk energy of polyethylene crystals.