Rheological Properties of Polymeric Materials

Rheological studies explore the flow of a material as an external force acts upon it. This flow depends not only on the magnitude and directionality of the external force, but also on the molecular composition and structure of the material that experiences the force. In this chapter, we vill focus on the flow behavior of molten polymers, as it relates to their molecular structure. It is important to note that the molecular characteristics that determine a molten polymer s behavior also define the polymer s solid state behavior. Therefore, many of the concepts introduced in this chapter will reappear in Chapter 8, Solid State Properties of Polymers. 

---

Techniques for measuring rheological properties of polymeric materials have been well described previously by others (e.g., Whorlow, 1980 Macosko, 1994). The text by Van Wazer et al. (1963) is still a valuable reference that explains in detail many facets of earlier attempts to measure rheological properties of polymeric materials as well as basic equations of viscometric flows. The unique nature of fluid foods prompted this author to review both the rheological properties of fluid foods and their measurement about 30 years ago (Rao, 1977a, 1977b). Subsequent efforts on rheology of foods include those of Rao (1992, 2005) and Steffe (1996). 

The rheological properties of polymeric materials vary with their chemical structures. Therefore, it is highly desirable to be able to relate the rheological properties of polymeric materials to their chemical structures. For instance, one may ask Why are the rheological properties of polystyrene so different from the rheological properties of polyethylene under an identical flow condition Unfortunately, at present there is no comprehensive molecular theory that can answer such a seemingly simple and fundamental question. 

The fabrication of an article from a polymeric material in the bulk state, whether it be the molding of a thermosetting plastic or the spinning of a fiber from the melt, involves deformation of the material by applied forces. Afterward, the fiiushed article is inevitably subjected to stresses hence, it is important to be aware of the mechanical and rheological properties of each material, as well as understand the basic principles imderlying their response to such forces. 

Figure 24 Shear-dependent viscosity of polyacrylamide solutions. (From KC Tam and C Tiu, Water-soluble polymers (rheological properties) in Polymeric Materials Encyclopedia, JC Salamone, ed. Boca Raton, FL CRC Press, 1996, p. 8655.)...

The transport properties of polymeric materials which distinguish them most from other materials are their flow properties or rheological behavior. There are many differences between the flow properties of a polymeric fluid and typical low molecular weight fluids such as water, benzene, sulfuric acid, and other fluids, which we classify as Newtonian. Newtonian fluids can be characterized by a single flow property called viscosity (/r) and their density (p). Polymeric fluids, on the other hand, exhibit a viscosity function that depends on shear rate or shear stress, time-dependent rheological properties, viscoelastic behavior such as elastic recoil (memory), additional normal stresses in shear flow, and an extensional viscosity that is not simply related to the shear viscosity, to name a few differences. 

The model contains four parameters a, Ai, A2, and r]o and a = al(l - A,2/A,i). The model is capable of describing many of the observed rheological properties of polymeric fluids. Show that the steady shear material functions are... 

Rohn LR, "Analytical Polymer Rheology Structure-Processing-Property Relationships", Carl Hanser, Munich, 1995. Rosen SL, "Fundamental Principles of Polymeric Materials", Wiley, New York, 2nd Ed, 1993. 

Thin solid films of polymeric materials used in various microelectronic applications are usually commercially produced the spin coating deposition (SCD) process. This paper reports on a comprehensive theoretical study of the fundamental physical mechanisms of polymer thin film formation onto substrates by the SCD process. A mathematical model was used to predict the film thickness and film thickness uniformity as well as the effects of rheological properties, solvent evaporation, substrate surface topography and planarization phenomena. A theoretical expression is shown to provide a universal dimensionless correlation of dry film thickness data in terms of initial viscosity, angular speed, initial volume dispensed, time and two solvent evaporation parameters. 

---