(452f) Suppression of Dynamics in Concentrated Polyelectrolyte Solutions of Varying Polymer and Counterion Concentrations

Polyelectrolytes are an easily tunable class of charge-containing species that are commonly used in drug and gene delivery systems and other applications that rely on their dynamic behavior. In concentrated polyelectrolyte systems, this dynamic behavior is modulated by the balance of electrostatic interactions that are introduced by the charge-bearing groups and the physical interactions that occur between neighboring chains. Because of this balance of forces, polyelectrolyte dynamics trends depart from those derived for neutral polymers. Diffusivity measurements can be used to determine the macroscopic dynamic properties for polyelectrolyte systems. Using single particle tracking experiments, we probe the diffusivity of polylysine (PL) chains through concentrated matrices of various polymer and counterion concentrations. These experiments uncover a strikingly strong polymer concentration dependence for diffusivity. When the diffusion of PL is measured for different counterion concentrations across similar polymer concentrations this strong polymer concentration dependence is still observed. Notably, this trend even exists in the high salt limit where the charged groups on the polyelectrolytes are well shielded, and dynamics would be expected to be closer to neutral polymer behavior. The strong dependence on polymer concentration is similarly seen in the viscosity trend over the same concentrations found using rheological measurements. Overall, these trends show that the solutions stiffen and suppress dynamics at a higher-than-expected rate. The most likely explanations for the suppression of dynamics involve frictional contributions restricting polymer movement and a disruption of solvent dynamics. As the solution becomes more concentrated, interchain interactions become more prevalent and disrupt the system dynamics.