ABSTRACT
In some situations, the viscosity of polymer liquids is too high to be characterized by gravity-driven flow in a viscometer. In many other situations treated in Chapter 7, the polymer viscosity is not a material property and may be a complex function of deformation rate, imposed stress, and time. In yet other circumstances, polymeric fluids exhibit solid-like elasticity and even in the simple shear flow device depicted in Figure 7.1 might manifest net forces in directions orthogonal to the direction of the applied deformation. These so-called normal stress differences are responsible for some of the most spectacular flow phenomena such as rod climbing, tubeless syphons, and turbulent drag reduction associated with high molar mass polymer solutions. Altogether these features imply that the response one obtains when a polymer melt or solution is subjected to an applied stress or flow field can be quite complex. Polymer liquids are for this reason given the collective designation complex fluids. The task of characterizing and understanding the flow properties of a complex fluid cannot be handled using viscometers. These tasks require, instead by sophisticated mechanically driven flow measurement devices, or rheometers, equipped with precise temperature measurement and control capabilities and specialized sensors for carrying out time-dependent measurements of shear rate, shear strain, shear stress, and normal stress differences.
