Examining Polymer Structure and Tumbling in Semi-Dilute Solutions of Polyelectrolytes under Shear Flow

Polyelectrolytes comprise many facets of our daily lives, including our foods, cosmetics, common household plastics, and the biopolymers in our bodies' tissues such as proteins and DNA. Recently, there has been significant progress in addressing the importance of hydrodynamic interactions for polyelectrolytes versus neutral polymers. Less is known, however, of how stress relaxation and polymer chain tumbling (i.e., cycles of polymer chain stretching and collapse during flow) of polyelectrolytes differ in the dilute versus semi-dilute regimes. 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 salt-free solutions under simple shear flow conditions. 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. The lack of concentration dependence of polymer tumbling highlights that hydrodynamic interactions are not screened at reasonably high polyelectrolyte concentrations, in contrast with results for neutral polymers. Overall, our simulations demonstrate that subtle changes in polymer chain length and solution conditions can significantly impact the chain conformations, stress, and dynamics of polyelectrolyte solutions.