A general scheme derived from video-controlled stretching tests and WAXS for describing tensile deformations of polyethylenes

Abstract

When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastid deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. The dependencies of the relative weights of these processes on crystallinity were analyzed in a comprehensive study of a series of samples: high-density polyethylene (HDPE), low-density polyethylene (LDPE), and ethylene-vinylacetate copolymers. The comparison was based on measured true stress-strain curves at constant strain rates, on the recovery properties of the samples studied in tensile tests with included unloading-reloading loops, and on wide-angle X-ray spectroscopy (WAXS) measurements carried out to determine the related texture changes. It was found that, in spite of the large changes in the gross mechanical properties from solid-like to rubberlike behavior, there always exist four characteristic points at which the deformation behavior changes. These may be associated with (1) the onset of isolated slip processes, (2) a change into a collective activity of the slips, (3) the beginning of crystallite fragmentation, and(4) chain disentanglement. Increasing crystallinity leads to increasing stresses at these points; the related strains, however, remain essentially constant. The crystal texture is a function of the imposed strain only. Experiments support the novel picture of a granular substructure of the crystalline lamellae as a basic structural feature.