Polymer Rheology

On December 9^th of 1929, a little over a month after the Wall Street crash, and seven years after he published his book Fluidity and Plasticity [1], Eugene Bingham (Fig. 1.1), a chemistry professor at Lafayette College in Easton, Pennsylvania, and a group of chemists, engineers, and physicists met for the first time in Washington D.C.; they called themselves the Society of Rheology. Hence, for the first time the word rheology, coined by Markus Reiner and Eugene Bingham in 1920, was officially used¹

Because of their molecular structure, polymers are by far the most complex fluids engineers will encounter. Therefore, what seems trivial when dealing with Newtonian fluids, is a complex experimental and mathematical exercise for a rheologist dealing with polymeric melts. An important aspect of any rheologist’s work is to find relations between deformation and stresses for various well defined conditions, such as transient shear flows, step strain, creep, and oscillatory shear flow, to name a few. These relations, also called material functions, are determined using different types of rheometric techniques. This chapter will introduce the reader to the causes of the various phenomena only observed with plastics and the relation of these effects to the molecular architecture of polymer melts.

The field of transport phenomena provides the basis for modeling in plastics rheology and processing. Modeling of a system, whether it is a rheometer or an actual process, often begins with a dimensional analysis of the system, which provides insight into the meaningful parameters that govern the system or process. The resulting dimensionless groups or numbers, in conjunction with experiments and models, can help the engineer determine significant conditions or effects, such as inertia, viscous heating, and if dealing with a process, scale a pilot or model of the process to industrial dimensions.

Although polymers have their distinct transitions and may be considered liquid when above the glass transition or melting temperatures, or solid when below those temperatures, in reality they are neither liquid nor solid, but viscoelastic. In fact, at any temperature, a polymer can be either a liquid or a solid, depending on the time scale or the speed at which its molecules are being deformed.

The measurement of rheological properties and the evaluation of fluid models require specific devices that can be summarized as rheometers. These devices are needed to achieve different objectives. Research needs them for complex measurements such as the investigation of viscosity and normal stress differences as well as deriving and evaluating flow models. In industry they are needed to design machines and fixtures such as mixers, extruders, injection molding machines, and molds. They are needed in product and process design for the selection of materials and for processing simulation, but also for the development of completely new materials. In addition, quality control during production becomes more and more important and requires simple analyses to check material consistency. Rheometers are also used to understand complex behavior of polymers in the large and relatively unexplored field of non-linear viscoelasticity.