Polyelectrolytes comprise multiple facets of our daily lives, including common household adhesives, coatings, cosmetics, and the biopolymers in our bodies’ tissues such as proteins and DNA. More recently, efforts are addressing the importance of hydrodynamic interactions (i.e., solvent-mediated dynamics) for polyelectrolytes versus neutral polymers. Less is known, in particular, of how stress relaxation and polymer chain tumbling (i.e., cycles of polymer chain stretching and collapse during shear flow) of polyelectrolytes differ in the dilute versus semi-dilute regimes. As polyelectrolytes are commonly used as viscosity modifiers in commercial products such as foods and shampoos, understanding the molecular basis for the viscoelastic properties of polyelectrolyte solutions will inform the design of novel, polyelectrolytes with tunable structure and rheological properties. In this work, we have used coarse-grained molecular dynamics simulations to examine the impact of solution conditions on the structure and tumbling of a prototypical polyelectrolyte, sodium polystyrene sulfonate (NaPSS), in aqueous solutions. Our simulations show that NaPSS chains exhibit a higher propensity for tumbling with increasing chain length, and that tumbling times show a weak dependence on NaPSS concentration in the semi-dilute regime. We hypothesize that the lack of concentration dependence of polymer tumbling stems from a balance between reduced tumbling due to crowding effects, and an enhanced propensity for tumbling due to collective tumbling dynamics of neighboring polyelectrolyte chains. Simulations also reveal a power-law dependence of tumbling times on shear rate with a power law exponent of approximately -2/3, in quantitative agreement with previous experiments on DNA and other polymer systems. In the semi-dilute regime, simulations do not show counterion release upon exposure to shear. Overall, our simulations demonstrate that subtle changes in polymer chain length and solution conditions can significantly impact the chain conformations, stress, and dynamics of viscoelastic, polyelectrolyte solutions.
