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

Previous work has shown that under mechanical deformation, three grades of polyethylene which differ mainly in terms of branch content exhibit two yield points. The first yield point occurs at low applied strains and marks the onset of temporary plastic deformation; the second yield point occurs at higher strains and marks the onset of permanent plastic deformation, and is associated with the development of a neck in tension. The present work describes structural measurements carried out on these three polyethylene samples before and subsequent to different levels of tensile deformation. X-ray diffraction, transmission electron microscopy, and optical microscopy were carried out on the isotropic materials and after the first and second yield points under tensile deformation. The results show that the first yield point marks the onset of a recoverable reorientation process of the lamellae within the spherulites, with little or no destruction of the lamellae themselves. This reorientation is explained in terms of the relative lamellar orientation to the draw direction due to the initial spherulitic structure. The reorientation leads to the lamellae oriented predominantly at approximately 45 degrees to the draw direction. The second yield point is associated with the destruction of the lamellae lying at 45 degrees to the draw direction by c shear.