Nonequilibrium Molecular Dynamics of Flowing Polyelectrolyte Complex Coacervates

Molecular dynamics simulations are applied to model the nonequilibrium dynamics of concentrated complex coacervates during nonlinear processing flows. We generate solvated "wet" and solvent-free "dry" bead-spring model coacervate phases with varying concentrations of polymer and salt ions and simulate them during startup shear and extensional flows. Molecular weights are varied from unentangled to entangled regimes and strain rates span from linear response to highly nonlinear flow. We characterize the nonequilibrium chain dynamics of polyelectrolytes during flow and relate them to the macroscopic shear and extensional stresses. Short chain systems display Rouse-like stress relaxation and chain dynamics in linear response. Sheared systems display power-law shear thinning of the viscosity with a slope independent of salinity. We observe the time-salt superposition of nonlinear shear viscosity data over a wide range of salt concentrations. In extension, coacervates can display power-law rate thickening with a salt-independent exponent that is qualitatively different from the force-extension response of uncharged polymers. We relate trends in nonlinear stresses to the nonequilibrium electrostatic interaction energies of the nonequilibrium chain conformations.