Lamellar separation during the deformation of high-density polyethylene

Abstract

Upon tensile straining at low deformation rates (ε=2×10⁻⁵ sec⁻¹), spherulitic linear polyethylene behaves reversibly for extensions up to 40%. In stress relaxation experiments on unloaded specimens the stress increases with time. Samples kept under constant strain (ε = 40%) over four months show macroscopic cracking. Microstructural investigation was performed using low- and wide-angle X-ray diffraction and transmission electron microscopy. These investigations reveal a very inhomogeneous deformation within those lamellar stacks for which the crystalline lamellae lie normal to the tensile axis. The deformation in that case is similar to what has been observed for “elastic hard fibres”. A two-mechanism model to explain the macroscopic observation on the basis of the microscopic observations is developed.