ABSTRACT
The flow of polymer melts and concentrated solutions is a complex process in which degradation of energy and memory effects embodied in the visc osity and the equilibrium recovery compliance function, respectively, play a determinant role. While the viscosity of monomers is perfectly defined at a given temperature, the answer to the question “What is the value of the viscosity of a polymer?” is not a simple one, because the value may vary within several orders of magnitude. Actually, the viscosity of polymers depends on molecular weight, molecular weight distribution, topology of the chains (linear, comb-like, and star structures; irregular branching; etc.), and temperature. In addition, the viscosity of molecular chains, unlike that of monomers or low molecular weight compounds, shows a strong depen dence on the shear rate. In most cases the viscosity decreases with increasing shear rate. Normal stress effects are also detected in these fluids. For exam ple, in steady Couette flow, high molecular weight polymers exert a greater normal thrust on the rotating inner cylinder than on the steady outer cylin der, in opposition to what one would expect on the basis of considering only inertial forces. In cone-plate flow, a greater normal thrust is exerted on the plate near the center than at the edge. In general, normal stress effects increase with shear rate.
