Mechanisms of Shear Thinning in Dilute Polymer Solutions

The origin of shear thinning in dilute polymer solutions has been attributed to four different molecular mechanisms: hydrodynamic interaction between monomers, finite extensibility of the polymer chain, polymer excluded volume, and internal viscosity. However, the four explanations differ in the predicted effect of solvent viscosity and polymer molecular weight upon the amount of shear thinning. Furthermore, of the four mechanisms, only hydrodynamic interaction is consistent with the stress-optical rule for flow birefringence. We report the shear rate dependence of the viscometric functions for several model systems, in which a monodisperse high molecular weight polymer is dissolved in either a low molecular weight liquid or a highly viscous solvent of the type used in "Boger" fluids.