Influence of pH, salt ions, and binary mixtures of different molecular weights on the extensional rheology of polyethylene oxide

In the context of modifying the extensional rheology of agricultural sprays to improve retention of sprays on plants, here we characterize the extensional rheology of dilute solutions of polyethylene oxide (PEO). Specifically, we examine the influence of pH and ionic strength, which vary significantly among agricultural sprays, as well as binary mixtures of two different nominal molecular weights of the polymer additive. Because PEO is nonionic, common intuition would predict that varying the pH and/or adding salt ions would have a minimal or negligible effect. However, the results presented here show a significant, complex, nonmonotonic, and ion-specific trend that is systematically documented for the first time. The role of shear degradation when mixing the polymer solutions is ruled out, and the data suggest that specific ion-polymer interactions appear to be more likely than changes in the solvent quality for producing this unexpected trend. We discuss some possible mechanistic explanations for the trend and highlight the potential impacts on product formulation as well as the need for improved theory of polymer physics.