DOUBLE YIELD POINTS IN POLYETHYLENE - STRUCTURAL-CHANGES UNDER TENSILE DEFORMATION

Previous work has shown that under mechanical deformation, three grade s of polyethylene which differ mainly in terms of branch content exhib it two yield points. The first yield point occurs at low applied strai ns and marks the onset of temporary plastic deformation; the second yi eld point occurs at higher strains and marks the onset of permanent pl astic deformation, and is associated with the development of a neck in tension. The present work describes structural measurements carried o ut on these three polyethylene samples before and subsequent to differ ent levels of tensile deformation. X-ray diffraction, transmission ele ctron microscopy, and optical microscopy were carried out on the isotr opic materials and after the first and second yield points under tensi le deformation. The results show that the first yield point marks the onset of a recoverable reorientation process of the lamellae within th e spherulites, with little or no destruction of the lamellae themselve s. This reorientation is explained in terms of the relative lamellar o rientation to the draw direction due to the initial spherulitic struct ure. The reorientation leads to the lamellae oriented predominantly at approximately 45 degrees to the draw direction. The second yield poin t is associated with the destruction of the lamellae lying at 45 degre es to the draw direction by c shear.