Strengthening of mille-feuille structured high-density polyethylene by heat elongation

Abstract

We found the strengthening of the extruded high-density polyethylene (HDPE) film by heat elongation perpendicularly to the longer direction of the lamellae, i.e., the yield point disappeared and the tensile strength at break increased from 35 MPa to 170 MPa by the elongation up to 400%. Tear-drop pattern was observed in the small-angle X-ray scattering of the extruded HDPE and it changed to the layer one by the heat elongation, suggesting that mille-feuille structure consisting of hard crystalline lamella layers and soft amorphous ones is formed in the extruded HDPE and the lamellar length becomes shorter by the elongation. DSC and wide-angle X-ray diffraction measurements revealed that irregularly tilted lamellae of the extruded film were arranged to regular ones by elongation and the chain orientation increased with increasing the elongation ratio without change of the crystallinity and lamellar thickness. The strengthening by the elongation might be attributed to the chain orientation and regular arrangement of the short lamellae tied in the mille-feuille structure, which suppress the yielding behavior by preventing the fracture of lamellae and void formation.