An Examination of the Shear-Thickening Behavior of High-Molecular-Weight Polymers Dissolved in Low-Viscosity Newtonian Solvents

The anomaly of shear thickening at high shear rates can be observed under certain conditions for high molecular weight polymers dissolved in low-viscosity Newtonian solvents despite the fact that shear-thinning behavior is considered the norm for these fluids. The nature of the shear-thickening region of the flow curve is examined herein through the application of a recent rheological model that has the capability of quantifying not only the rheological properties of the material, but its internal microstructural state as well. The results of this examination provide a self-consistent explanation of the full flow characterization of this anomalous behavior, including both rheological and optical experimental measurements. The results presented herein suggest that the shear-thickening behavior is actually caused by the destruction of structures formed during shear at lower shear rates, not by their formation, as previously assumed. The linear birefringence and linear dichroism observed experimentally in correlation with the shear-thickening behavior are well described by the rheological model and give predictions in line with experimental measurements. Furthermore, quantitative predictions are made for rheological characteristic functions, such as the first and second normal-stress coefficients, for which experimental measurements for these solutions have not yet been made.