Processing‐Induced Heterogeneous Structure and Mechanical Anisotropy in Extruded Polyethylene Sheets

ABSTRACT

Processing‐induced hierarchical structures are central to the mechanical performance of extruded polyethylene sheets. Polyethylene sheets prepared by extrusion‐calendering are studied to clarify how roller temperature and sheet thickness regulate orientation distribution, skin‐core heterogeneity, and mechanical anisotropy. Tensile tests reveal that extrusion‐direction specimens possess higher strength, modulus, and toughness, whereas transverse specimens show larger elongation at break. SEM observations identify layered morphologies near the surface and a uniform core. Synchrotron microfocus WAXD mapping resolves the crystal orientation: the width direction remains weakly oriented, while the thickness direction develops a pronounced skin‐core gradient. Strong orientation and azimuthal peak shifts appear in near‐surface regions, indicating chain alignment and orientation‐direction deviation under coupled shear and cooling conditions. Decreasing roller temperature enhances the retention of non‐equilibrium orientation by limiting molecular relaxation after calendering. Reducing sheet thickness further strengthens surface orientation by increasing high‐shear regions and shortening the cooling distance. Quantitatively, for the sheet prepared at an extrusion thickness of 2 mm and a roller temperature of 80°C, the affected surface‐layer depth decreases from 300 μm near the water inlet to 60 μm near the outlet, while the width‐direction orientation factor remains around −0.03. These results provide guidance for regulating the performance of polyolefin sheets.