Influence of high molecular weight component on the hierarchical crystalline structures of injection-molded bars of polyethylene

A small amount of high molecular weight molecules can have a dramatic influence on the flow-induced crystallization kinetics and orientation of polymers. To elucidate the effects of the high molecular weight component under a real processing process, we prepared model blends in which high density polyethylene with a high molecular weight and wide molecular weight distribution was blended with a metallocene polyethylene with a low molecular weight and very narrow molecular weight distribution. To enhance the shear strength, gas-assisted injection molding was utilized in producing the molded bars. The hierarchical structures and orientation behavior of the molded bars were intensively explored by using scanning electron microscopy and two-dimensional wide-angle X-ray diffraction, focusing on effects of the high molecular weight component on the formation of the shish kebab structure. It was found that there exists a critical concentration of high molecular weight component for the formation of a shish kebab structure. The threshold was about 5.5-7.0 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. Moreover, the rheological properties of molten polyethylene melts were studied by dynamic rheological measurements and a critical characteristic relaxation time for shish kebab formation was obtained under the processing conditions adopted in this research. (C) 2013 Society of Chemical Industry