Semicrystalline blends of polyethylene and isotactic polypropylene: Improving mechanical performance by enhancing the interfacial structure

The low-temperature mechanical behavior of semicrystalline polymer blends i s investigated. Isotactic polypropylene (iPP) is blended with both Zeigler- Natta polyethylene (PE) and metallocene PE. Transmission electron microscop y (TEM) on failed tensile bars reveals that the predominate failure mode in the Zeigler-Natta blend is interfacial, while that in the metallocene blen d is failure of the iPP matrix. The observed change in failure mode is acco mpanied by a 40% increase in both tensile toughness and elongation at -10 d egrees C. We argue that crystallite anchoring of interfacially entangled ch ains is responsible for this dramatic property improvement in the metalloce ne blend. The interfacial width between PE and iPP melts is approximately 4 0 Angstrom allowing significant interfacial entanglement in both blends. TE M micrographs illustrate that the segregation of low molecular weight amorp hous material in the Zeigler-Natta blend reduces the number and quality of crystallite anchors as compared with the metallocene blend. The contributio n of anchored interfacial structure was further explored by introducing a b lock copolymer at the PE/iPP interface in the metallocene blend. Small-angl e X-ray scattering (SAXS) experiments show the black copolymer dilutes the number of crystalline anchors, decoupling the interface. Increasing the int erfacial coverage of the block copolymer reduces the number of anchored int erfacial chains. At 2% block copolymer loading, the low-temperature failure mode of the metallocene blend changes from iPP failure to interfacial fail ure, reducing the blend toughness and elongation to that of the Zeigler-Nat ta blend. This work demonstrates that anchored interfacial entanglements ar e a critical factor in designing semicrystalline blends with improved law-t emperature properties. (C) 2000 John Wiley & Sons, Inc.