(161o) Processing Dependence and Aging of the Coacervate-Precipitate Transition in Mixed Polyelectrolytes

Solid-like aggregates form from many synthetic and natural polyelectrolyte systems that are otherwise expected to undergo equilibrium liquid-liquid phase separation in aqueous solution. Such “precipitates” can render materials incongruent with traditional processing, and their existence is inaccessible to equilibrium theories. Identified by either a rougher surface structure in microscopy, opacity, and/or solid-like rheology, precipitates are generally recognized to be kinetically arrested states that occur at low ionic strength and high polymer fraction. Previous studies have considered precipitates to appear only in specific salt-polymer concentration regions of experimental phase diagrams. Thus, changing the identities of the chemical components has been the only reported way to control the salt and polymer concentrations at which precipitation occurs.

Here, we show that the formation and aging of precipitates from oppositely-charged polyelectrolytes is highly sensitive to the mechanics of mixing. Using a series of mixing protocols spanning several orders of magnitude of Reynolds number, we show that the compositions where precipitates are observed shifts significantly with mixing type. Furthermore, by examining their aging and stability, we show that precipitates are not permanently arrested, but rather can be transient states that coexist with and age to form coacervate droplets over a broad range of conditions. This aging is also dependent on the mixing protocol, suggesting that mass transport effects dominate the early formation of precipitates. Video microscopy experiments used to track the initial growth and morphology of precipitates suggest that a diffusion-limited aggregation process, similar to flocculation in unstable colloidal dispersions, underlies the sensitivity of precipitate formation and relaxation to various mixing parameters. Our studies highlight the transient nature of the coacervate-precipitate transition, and open new opportunities for using mixing and flow to control material processing in mixed polyelectrolytes.