COMPARISON OF CONSTITUTIVE-EQUATIONS FOR POLYMER-SOLUTIONS IN UNIAXIAL EXTENSION

The Oldroyd-B, Giesekus, FENE-P, and White-Metzner models are used to predict the uniaxial extensional stress growth results of two ideal el astic fluids and a shear-thinning fluid. The model predictions are com pared with the data obtained, at constant deformation rates, using the filament stretching technique. The linear viscoelastic parameters for the models were obtained by fitting the Oldroyd-B equations to the dy namic data. For the Giesekus and FENE-P models, the extra nonlinear pa rameters were obtained by fitting the model predictions to the steady shear data. The parameters for the White-Metzner model (using Carreau- type equation) were also obtained by directly fitting the steady shear data. For the ideal elastic fluids, Oldroyd-B, Giesekus, and FENE-P m odels with multiple modes of relaxation times generally provide good p redictions of the extensional stress growth at small strain, but not p articularly well at high strain. While the Oldroyd-B model predicts in finite extensional viscosity at finite extension rates, the Giesekus a nd FENE-P models predict steady extensional viscosities which are of t he correct order of magnitude. The FENE-P equation also shows a more a brupt approach to steady state, similar to that observed experimentall y. For the shear-thinning fluid the Giesekus and FENE-P models fail to predict the large stresses found in extensional flow. On the other ha nd, the White-Metzner model, while unable to predict the extensional s tress growth for the ideal elastic fluids, is an improvement over the Oldroyd-B model for the shear-thinning fluid. (C) 1995 Society of Rheo logy.