Melt rheology of polypropylene containing small amounts of high molecular weight chain. I. Shear flow

Large enhancements of the melt strength of polypropylene (PP) were achieved by the introduction of high molecular weight polyethylene (PE) into PP. Th e viscoelastic properties of the high-melt-strength PP melts under shear fl ow were investigated. It was found that the rheological properties of the h igh-melt-strength PP were distinctly different from those of conventional P P. The elastic response at low frequencies was significantly enhanced in co mparison with the conventional PP, implying a presence of a long relaxation time mode that was not revealed in conventional PP, In step-shear measurem ents, the fast and slow relaxation processes that characterized the linear viscoelastic properties were observed also for nonlinear relaxation moduli. The dependence of the damping for the slow process of the high-melt-streng th PP on shear strain was much weaker than that of the fast process. These rheological behaviors characterizing the long relaxation time mode were fur ther enhanced with the increasing concentration of high molecular mass PE. The unusual shear rheological behaviors were discussed in view of the role of high molecular weight PE as a long relaxation time mode within PP. (C) 2 001 John Wiley & Sons, Inc.