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.