Processing Dependence of Polyelectrolyte Complex Microstructure

Nonequilibrium behavior of aqueous polyelectrolytes is critical to many applications, yet engineering control is largely limited to equilibrium phase behavior. Here, we investigate the mechanism of polyelectrolyte complex (PEC) growth and its sensitivity to flow processing. Microfluidic mixing of poly(acrylic acid) and poly(allylamine) salt solutions reveals that PECs grow by coalescence at high salt and low polyelectrolyte concentration, and elsewhere by diffusion-limited droplet aggregation to form a rough microstructure. This transition is flow rate-dependent, consistent with macroscale observations that aggregates form at some concentrations only when the polyelectrolytes are vortex- rather than pipette-mixed. Aggregate roughness can also be time-dependent and relaxes via internal coalescence. Importantly, coacervate-like viscoelasticity does not preclude an aggregated, precipitate-like microstructure. These results suggest that colloidal and mass transport effects underly sensitivity of PEC formation and relaxation to various mixing parameters, replacing the conventional concept of a state diagram with disparate precipitate and coacervate regions, and opening the door to controlling PEC formation and nonequilibrium structure.