Tuning Shear Banding and Wall Slip in Entangled Polyelectrolytes by Intermolecular Interactions
The study of flow instabilities such as shear banding and wall slip in entangled polymers is of great significance for both fundamental science and industry. There is, however, an ongoing debate in the polymer rheology community whether shear banding is generic for polymers. This discussion is involved as the molecular mechanism for shear banding in entangled polymers is poorly known and the interplay of shear banding with wall slip makes the problem even more complicated. Therefore we try to answer three questions in this thesis: 1) Is shear banding a generic phenomenon for entangled polymers or a more system-specific phenomenon? 2) Will the shear banded systems always show classic shear banded velocity profiles, with a constant shear rate within both bands, independent of the applied shear rate? 3) What is the molecular mechanism for shear banding in entangled polymers?
In order to address these questions, we performed in this thesis standard rheology measurements, birefringence and velocity profile experiments in order to characterize the linear and non-linear flow behavior of polymers for which the interactions and properties can be tuned. Polyelectrolytes are a logical choice, as both the interactions and particle properties can be tuned by varying the ionic strength. To access the generic character, we used two different polyelectrolytes, namely the semi-flexible xanthan and DNA. In addition, we synthesized an attractive polymer system namely DNA grafted with PNIPAm, which becomes hydrophobic above a critical temperature. This allowed us to also access the effect of attraction.
We demonstrate that shear banding and wall slip in entangled xanthan and DNA can be finely tuned by intermolecular interactions such as the electrostatic repulsions and intermolecular attraction. For xanthan and DNA we observe sharp shear banding at low ionic strength, though for DNA there is a strong competition with wall slip. The sharp interfaces broaden upon addition of salt, up to a point at high salt where no shear bands are observed and the velocity profile becomes linear. We introduced a new analysis method to quantify this broadening and also the difference between a shear thinning factor as obtained from the rheological flow curve and the velocity profile. For the attractive PNIPAm-DNA we find a reentrant behavior as upon increasing temperature first wall slip is suppressed in favor of shear band formation, while shear banding is suppressed when the system tends to gel. These observations suggest that shear band formation is generic for polymers when they are sufficiently stiff and when the friction between the polymers is low. Shear banding is suppressed when the disentangled polymers can easily collapse either by screening the charges along its backbone or increasing attraction. The mechanism of suppression is related with the widening of the interface. We claim that generally polymeric systems that display a curvature in the velocity profile which is stronger than expected from the rheology, do display shear banding, but with an interface which is too broad for the gap of the shear cell geometry.