Rheological properties of some polyolefine melts in transient uniaxial elongational flow, described with a special type of constitutive equation

Abstract

A single integral constitutive equation with an integral kernel factorable into a material-specific, time-dependent function and a material-specific strain tensor gives an excellent description of the viscoelastic behaviour of polymer melts in simple shear flow. The applicability of this equation to simple extensional flow of polymer melts has been investigated by analyzing the data obtained by subjecting a low-density polyethylene material and two high-density polyethylene samples to constant stretching-rate experiments, creep experiments under constant tensile stress, and stress-relaxation experiments after a sudden step in extensional strain. The low-density polyethylene melt responds to the uniaxial extensional flow in the way predicted by the constitutive model proposed, but experimental results on the two high-density polyethylenes seem to indicate that an extra function dependent on the invariants of the rate of strain tensor should be introduced into the constitutive equation.