Orientation-dependent mechanical properties and deformation morphologies for uniaxially melt-extruded high-density polyethylene films having an initial stacked lamellar texture

Abstract

The mechanical properties and the associated plastically deformed morphologies of high density polyethylene films were investigated by tensile testing, wide-angle X-ray scattering and transmission electron microscopy. Uniaxially oriented films having a well-defined stacked lamellar morphology, both with and without row-nucleated structure were deformed at three angles, 0°, 45° and 90°, with respect to the original machine (extrusion) direction. A distinct orientation dependence of the mechanical properties was observed and this dependence has been related to the different morphologies developed during the plastic deformation processes. It was shown that lamellar separation, lamellar shear and lamellar break-up were the dominant initial deformation mechanisms for the respective 0°, 45° and 90° deformations. As a result, the 45° and 90° deformations generated a final microfibril morphology oriented along the stretch direction, while the 0° deformation resulted in broken blocks of crystalline lamellae. The presence of distinct row-nucleated crystalline fibrils in the initial structure stiffens the material in the 0° deformation; however, it significantly limits the ability of the materials to cold draw at the 90° deformation. Morphological models were proposed to explain the plastic deformation process for the different deformation angles, as well as for the deformation behaviour of semicrystalline polymers with an isotropic spherulitic morphology.