MORPHOLOGICAL-CHANGES IN PRESSURE ANNEALED POLYETHYLENE

Polyethylene samples have been annealed under high pressures in order to transform the morphology and thereby make specimens, which on subse quent orientation by hydrostatic extrusion, produce high-modulus mater ials for comparatively low deformation ratios. The pre-extrusion morph ology, as revealed by permanganic etching followed by transmission ele ctron microscopy, is shown to be highly sensitive to the annealing con ditions, especially in the vicinity of the orthorhombic/hexagonal phas e boundary. The morphological changes have also been studied by gel pe rmeation chromatography of nitrated samples and by differential scanni ng calorimetry. The moduli of samples, extruded to a standard extrusio n ratio of 7, reflect differences in the morphology of the starting ma terial. It is shown that the highest modulus is not obtained by anneal ing totally within the hexagonal phase but rather in the changeover re gion between it and the orthorhombic phase. Although the lamellar size can be dramatically increased by annealing within the hexagonal phase , this development occurs at the expense of reducing the integrity of the molecular network so that for high lamellar size the modulus falls and the samples become increasingly incoherent.