ORIENTATION-DEPENDENT MECHANICAL-PROPERTIES AND DEFORMATION MORPHOLOGIES FOR UNIAXIALLY MELT-EXTRUDED HIGH-DENSITY POLYETHYLENE FILMS HAVING AN INITIAL STACKED LAMELLAR TEXTURE

The mechanical properties and the associated plastically deformed morp hologies of high density polyethylene films were investigated by tensi le testing, wide-angle X-ray scattering and transmission electron micr oscopy. Uniaxially oriented films having a well-defined stacked lamell ar morphology, both with and without row-nucleated structure were defo rmed at three angles, 0 degrees, 45 degrees and 90 degrees, with respe ct to the original machine (extrusion) direction. A distinct orientati on dependence of the mechanical properties was observed and this depen dence has been related to the different morphologies developed during the plastic deformation processes. It was shown that lamellar separati on, lamellar shear and lamellar break-up were the dominant initial def ormation mechanisms for the respective 0 degrees, 45 degrees and 90 de grees deformations. As a result, the 45 degrees and 90 degrees deforma tions generated a final microfibril morphology oriented along the stre tch direction, while the 0 degrees deformation resulted in broken bloc ks of crystalline lamellae. The presence of distinct row-nucleated cry stalline fibrils in the initial structure stiffens the material in the 0 degrees deformation; however, it significantly limits the ability o f the materials to cold draw at the 90 degrees deformation. Morphologi cal models were proposed to explain the plastic deformation process fo r the different deformation angles, as well as for the deformation beh aviour of semicrystalline polymers with an isotropic spherulitic morph ology.