Overview of Viscoelasticity and Rheology

There are several terms that apply to time-dependent behavior. Thixotropic fluids possess a structure that breaks down as a function of time and shear rate. Tlius the viscosity is lowered. A famous example of this reversible phenomenon is the ubiquitous catsup bottle, yielding its contents only after sharp blows. The opposite effect, although rarely observed, is called antithixotropic, or rheopectic behavior, where materials set up as a function of time and shear rate that is, the viscosity increases or the material gels. 

Viscoelastic fluids exhibit elastic recovery from deformations that occur during flow. The Weissenberg effect and die swell have been discussed previously. Another very simple example is the stirring of concentrated polymer solutions rapidly, then stopping. Elastic effects make the fluid move backward for a time. 

Several flow equations are summarized in Table 10.8 (58) for various models. Here,/o is the yield stress, rjo the viscosity at low shear rates and t] at high shear rates, and a and n are constants. 

This chapter has illustrated how stress relaxation, creep, and rheology in polymers depend on the rate of molecular motion of the chains and on the presence of entanglements. It must be remembered that aU macroscopic deformations of matter depend ultimately on molecular motion. In the case of high polymers, the chain s radius of gyration is changed during initial deformation or flow. Thermal motions tend to return the polymer to its initial conformation, thus raising its entropy. Clearly, there is a direct relationship between the mechanical or viscous behavior of polymeric materials and their molecular behavior. 

Tobolsky, Properties and Structure of Polymers, Wiley, New York, 1960. 

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