Second Newtonian region of polymer solutions

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Polymer solutions are a class of fundamental rheological systems with countless applications. Their viscosity typically depends strongly on the shear load: At low shear rates, in the 1st Newtonian branch, the polymer solutions have a constant viscosity. Beyond this branch, the viscosity diminishes considerably. Finally, at very high shear rates another regime of constant viscosity, the 2nd Newtonian branch is expected. Apart from shear rate, the viscosity depends substantially on the type of polymer and its structure, the molecular weight and its distribution, the polymer concentration, and on the solvent. In the 1st Newtonian branch, different regimes of polymer solutions can be distinguished depending particularly on concentration and on molecular weight. While the 1st Newtonian branch and the onset of shear thinning are well studied, this is not the case for the 2nd Newtonian branch. Here, detailed knowledge about viscosity, normal-stress differences and the orientation of the polymers, their dependencies on the above-mentioned parameters as well as studies about the onset of the 2nd Newtonian branch are lacking. Proper knowledge about this branch, however, is not only necessary for a fundamental understanding of polymer solutions but is also needed in a number of industrial applications. Essential reason for the lack of knowledge about the 2nd Newtonian branch is the deficiency of adequate measuring technique: The shear-rate range is typically 1-2 decimal orders beyond the parameter range covered by commercial rheometers. Besides, very low viscosities need to be detected. We have improved the geometrical precision of rotational rheometers by about a factor of 30 to 100. This permits measurements at considerably narrower gaps that enable reaching the necessary high shear rates and precision. Furthermore, to the best of the authors knowledge, the improved rheometer is the only one that allows for detecting normal forces in polymer solutions and for carrying out rheo-optical studies at shear rates up to 105 s-1. It is aimed for further enhancing the precision within the project. Central aim of the project is to characterize the 2nd Newtonian branch of well-defined as well as industrially relevant polymer solutions. Simultaneously with the viscosity measurements, shear-induced normal forces due to the solution viscoelasticity and the orientation of the polymers are to be detected. The latter is to study whether the microstructure changes in the 2nd Newtonian branch. The main issue is to clarify if there are different regimes as a function of polymer concentration in the 2nd Newtonian branch similar to those in the 1st Newtonian branch. Furthermore, the impact of molecular weight, changes in polymer structure and solvent should be elucidated. To avoid shear-induced degradation, the study is planned to be carried out with semi-flexible polymers.