On correlations of primary normal stresses in polymer solutions

Abstract

AbstractPrimary normal stress difference data of some 32 different polymer solutions have been examined and correlated to the shear stress by a simple power‐law model. The normal stress at constant shear stress is shown to increase with the molecular weight of the polymer, but decreases with the polymer concentration. The method proposed by Abdel‐Khalik, et al., for predicting the primary normal stress difference from viscosity data for these polymer solutions is inadequate. The FENE‐P dumbbell model is used to obtain master curves combining normal stress and viscosity data as a function of a dimensionless shear rate. It is found that this presents several drawbacks. It predicts shear‐thinning effects with a power‐law slope of −2/3, which is found to be too restrictive. The model fails to predict shear‐thinning effects for dimensionless shear rates λEγ lower than approximately 0.5. It also fails to predict satisfactorily the low shear rate value of the dimensionless combination of viscosity and first normal stress coefficient Ψ1/λE(η − ηs). An empirically modified correlation is proposed. The fit for a series of polyacrylamide (Dow Separan AP‐30) solutions is very good. In the case of a series of monodisperse polystyrene solutions, the fit is not as good, as the data show a more pronounced effect of polymer concentration at high shear rates.