Rheological flows elongational, extensional

Three kinds of viscometric flows are used by rheologists to obtain rheological polymer melt functions and to study the rheological phenomena that are characteristic of these materials steady simple shear flows, dynamic (sinusoidally varying) simple shear flows, and extensional, elongational, or shear-free flows. 

The rheology of polymer melts in shear flow is considered. It is included with limited data for rheological properties of some PO melts in uniaxial elongational (extensional) flow. Table 1 systematizes the following. 

Demonstrations are given of the importance of extensional or elongational viscosity in the foam process. New polypropylenes are compared in extensional flow and it is shown how rheological differences allow the prodnction of low density foam on tandem extrnsion equipment. 6 refs. 

We have illustrated the many ways in which elongational flow behavior can provide novel information about the behavior of macromolecules. Special emphasis has been given to rheological changes associated with extensional flow fields. 

In order to characterize polymeric fluids and to test rheological equations of state it is customary to use simple, well defined flows. The two main flows are simple shear and simple elongational. These are shown schematically in Figure 1. In shear flow, material planes (see Figure 1) move relative to each other without being stretched, whereas in extensional flow the material elements are stretched. These two different flow histories generate different responses in not only flexible chain polymers but in liquid crystalline polymers. When these flows are carried... 

Rheological properties of the same polymeric solution measured in shear and elongational flows can be very different as discussed in earlier section. However, a cursory examination of current textbooks on rheology (e.g., Bird et al. [2], Ferry [3], and Tanner [14]) shows that shear rheology dominates and research in extensional rheology is comparatively much more recent. The measurements of the shear properties of polymeric fluids are well established and a number of rheometers are available for both melts and dilute polymeric solutions. Lately, more efforts have been directed in measuring the extensional properties of fluids [8, 9,15-19]. 

Dilute solutions of polymer coils have special rheological properties they tend to thread. Microscopically, the chains become highly elongated in certain shear flows. The most spectacular effects occur in extensional flow fields such as... 

Although most viscometric and rheological studies are carried out in simple shear flows such as rotational viscometers, real flows experienced by real liquids are very often extensional (stretching or elongational) in nahire, and for some liquids there can be a very large difference between their shear and extensional viscosities. 

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