Plane Stress Fracture Toughness of Partially Miscible, High-Density Polyethylene/Poly (Ethylene-CO-1-Octene) Blends

Abstract

: Blends of high-density polyethylene (PE) with poly(ethylene-co-1-octene) (EOC) were found to be partially miscible in the melt-state. Rapid crystallization of the sheared melts led to the formation of a physically interpenetrating, solid-state morphology comprising PE lamellar crystals and EOC-rich regions. Samples were extruded through a coat-hanger die at 170°C to produce thin sheet specimens for fracture toughness testing under plane stress conditions. While crystallinity and crystal size of the PE, determined through differential scanning calorimetry, were not significantly affected by the presence of the copolymer, X-ray diffraction experiments showed that there was a preferred crystal orientation that was the result of crystallization of the oriented PE melt. Moreover, a bimodal crystal orientation was observed in the pure PE compounds. The level of bimodality was reduced with increasing concentration of copolymer in the blends. For the sheet extrusion material, plane-stress fracture toughness tests revealed that the presence of the branched copolymer in the blends did not affect the work of fracture when crack propagation took place in the transverse direction of the extruded sheet, in comparison with the pure PE. The branched copolymer increased the overall work of fracture for crack propagation in the machine direction due to an increase in the nonessential work of fracture, but had no significant effect upon the essential work of fracture.