Rheology of Flexible Homopolymers

Abstract Numerous flexible homopolymers and flexible random copolymers are commercially available. Thus, understandably, a number of research groups have reported on the rheological behavior of flexible homopolymers and flexible random copolymers in the bulk and solution states. There are too many studies to cite them all here. In Chapters 3 to 5 we presented the rheological behavior, in general terms, of linear flexible homopolymers in steady-state shear flow, elongational flow, and/or oscillatory shear flow. In this chapter, we present the effects of temperature, molecular weight (although in Chapter 4 we presented theoretical predictions of the effect of molecular weight), and molecular weight distribution on the rheological behavior of linear flexible homopolymers, and also flexible homopolymers with long-chain branching. The rheological behavior of much more complex polymer systems is presented in other chapters of this volume. From the point of view of polymer processing, temperature is one of the most important variables that greatly affect the rheological behavior of polymeric liquids. Therefore, it is very important to present the effect of temperature on rheological behavior, placing emphasis on the methods that enable one to obtain temperature-independent correlations for rheological properties. Such correlations, when available, will help one to estimate the rheological properties of the same polymer without conducting additional experiments. With respect to polymer synthesis and polymer processing, a better understanding of the effects of molecular weight and molecular weight distribution on the rheological behavior of a polymer is of fundamental importance. In Chapter 4 we have presented molecular theory, demonstrating that the molecular weight of a linear flexible homopolymer has a profound influence on its rheological properties. Thus, information on the relationships between molecular weight and rheology, when available, will help one to choose, with little waste of time and effort, optimum processing conditions. One of the common features of all commercial homopolymers is that they are polydisperse and, therefore, it is not difficult to surmise that the molecular weight distribution of a polymer also has a profound influence on its rheological properties.