Molecular basis of polymer rheology

It was noted earlier in this chapter that the flow of both polyacrylamide and xanthan solutions in a capillary is similar in character, i.e. it is shear thinning 

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

Another interpretation of the viscoelastic behaviour of random coil molecules has been proposed by de Gennes (1974). In this work, he presents an analysis which suggests that an elastic response will be seen based on a coil-stretch transition which will occur within the molecule in strong elongational flows. This would not be seen for more rigid molecules such as xanthan. Chauveteau (1981) used this viewpoint in his interpretation of experiments in stepped capillaries. 

The relationship between polymer structure and the rheological properties of polymer solutions is very wide and complex. This brief account is simply intended to indicate that the observed differences between the rheological behaviour of polyacrylamide and xanthan are based on their molecular structures. Although these two polymers may be superseded by improved polymers, both synthetics and biopolymers, as discussed in Chapter 2, the 

Measure pressure drop, AP, for a fixed flow rate, Q (or vice versa) 

---