Restrictions in Polymer Dynamics in Concentrated Oppositely-Charged Polyelectrolyte Solutions

Understanding the dynamic behavior of mixtures of oppositely-charged polyelectrolytes is imperative to design of new drug delivery technologies and encapsulation of industrial chemicals, yet thus far the properties of highly concentrated solutions are poorly understood. While more dilute solutions are well-described using single chain models such as Rouse and reptation, these predictive models break down in more concentrated solutions where collective phenomena dominate system dynamics. Using single particle tracking experiments, we directly observe the diffusion of polylysine chains through concentrated matrices of polylysine and polyglutamic acid. These studies reveal that diffusivity is a strong function of concentration in both single-phase systems and polymer-rich coacervates obtained from phase-separating conditions. This strong concentration-dependent trend appears to be independent of charge and extends to the dynamics of solely polylysine as well. We hypothesize that these trends represent the onset of glassy dynamics within concentrated, charge-laden systems driven by a decrease in plasticizing water and contributions from the large excluded volumes associated with charged monomers. As the solution becomes crowded, movement is constrained by associative interactions and a lack of available free volume. This behavior appears to be universal to other polyelectrolyte systems and underlies the need for additional, targeted theories to describe concentrated solutions.