Surface Morphologies of Planar Ring Polyelectrolyte Brushes Induced by Trivalent Salts

The morphology of a polymer brush is of great importance in determining surface properties, such as lubrication. In light of this, we systematically study the morphologies of ring polyelectrolyte brushes grafted onto planar substrates using molecular dynamics simulations. Through varying concentration of trivalent salts, grafting density, and solvent property, the effects of electrostatic correlation and excluded volume are focused and both lateral and vertical phase separations are observed in the polyelectrolyte brush system, resulting in the formation of a variety of surface morphologies, including the pinned micelles at low grafting density and the “carpet + brush” double-layer morphology in poor solvents at high grafting density. To pinpoint the mechanism of morphology formation, we carefully analyze the distribution of monomers, the orientation properties of bonds, and the monomer–monomer pair correlation function. Furthermore, electrostatic correlations, manifested as the bridging through trivalent salts, are examined by identifying four states of trivalent ions. Our study provides both insights into the understanding of electrostatic correlation in polyelectrolytes and a guide for the experimental design of smart materials based on surface polyelectrolyte brushes.