Fecha de publicación: -- Fuente: Journal of applied polymer Lugar: RESEARCH ARTICLE This work investigates how the roller temperature and sheet thickness regulate the hierarchical structure and mechanical anisotropy in extruded polyethylene (PE) sheets. A theoretical model based on the planar Poiseuille flow and heat conduction is further developed to rationalize the formation of the heterogeneous skin-core structure, providing quantitative correlations between processing parameters and final structural features.

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.