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 flow with constant tensile strain rate. The equibiaxial extensional flow behavior of the high-melt-strength polypropylene was measured via the lubricated 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 component (see part 1). This system is generally believed to be an immiscible system, 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 shows distinctly different shear flow behaviors from conventional PP, e.g., high elasticity and two-step viscosity at low shear rates and strong and weak strain-dependent nonlinear damping functions characterizing fast and slow relaxation processes. In this study, the transient uniaxial viscosity of the high-melt-strength PP melts first increased gradually with time, following the linear viscoelastic rule in which the uniaxial extensional viscosity is 3 times the shear viscosity development. Beyond a certain critical strain, the uniaxial extensional viscosity showed rapid increase, which was referred to as strain hardening. Furthermore, the transient biaxial extensional viscosity showed also the strain hardening behavior over a critical strain. These prominent behaviors are unexpected for conventional PP. The nonlinear upturn behavior was discussed from a high-molecular weight chain stretching point of view via molecular constitutive equations given by Osaki et al. for bimodal polymer blends.