Melt rheology of polypropylene containing small amounts of high-molecular-weight chain. 2. Uniaxial and biaxial extensional flow

Uniaxial extensional flow behavior was measured for the high-melt-strength polypropylene using a Meissner-type rheometer under transient extensional f low with constant tensile strain rate. The equibiaxial extensional flow beh avior of the high-melt-strength polypropylene was measured via the lubricat ed squeezing flow method under constant strain rate. The high-melt-strength polypropylene consists of polypropylene (PP) as a main component and high- molecular-weight polyethylene (PE) component as a long relaxation time comp onent (see part 1). This system is generally believed to be an immiscible s ystem, at least under the quiescent state. Nevertheless, in part 1, we have found that the high-melt-strength PP with very high-molecular-weight PE sh ows distinctly different shear flow behaviors from conventional PP, e.g., h igh elasticity and two-step viscosity at low shear rates and strong and wea k strain-dependent nonlinear damping functions characterizing fast and slow relaxation processes. In this study, the transient uniaxial viscosity of t he high-melt-strength PP melts first increased gradually with time, followi ng the linear viscoelastic rule in which the uniaxial extensional viscosity is 3 times the shear viscosity development. Beyond a certain critical stra in, the uniaxial extensional viscosity showed rapid increase, which was ref erred to as strain hardening. Furthermore, the transient biaxial extensiona l viscosity showed also the strain hardening behavior over a critical strai n. These prominent behaviors are unexpected for conventional PP. The nonlin ear upturn behavior was discussed from a high-molecular weight chain stretc hing point of view via molecular constitutive equations given by Osaki et a l. for bimodal polymer blends.