The mechanical anisotropy of strain-crystallized linear polyethylenes

Abstract

Abstract Linear polyethylenes have been cross-linked both chemically and by irradiation. In each case oriented samples have been produced both by cold drawing followed by annealing and by crystallization under strain. Elastic moduli have been measured in different directions and over a wide range of temperatures. Structural investigations have shown that all the samples have lamellar morphology, but the lamellae are differently oriented, being perpendicular to the draw direction in strain-crystallized materials but inclined at 45° to this direction in the cold-drawn materials. The mechanical properties have been analyzed and it has been shown that the patterns of anisotropy observed are consistent with simple representations of the materials as composite solids, with the difference in lamellar orientation playing a key role in determining the differences in properties between strain-crystallized and cold-drawn and annealed materials. There is no evidence from the mechanical behavior for the presence of extended chain crystallites whose existence in strain-crystallized polyethylene has been suggested.