Monomer effective charge and cationic specificity play a critical role in determining the polyelectrolyte (PE) brush responsiveness. Herein, we report a systematic simulation study to investigate how trivalent salt cation size and monomer effective charge cooperatively regulate surface morphology, chain dynamics, and internal ionic states of a planar PE brush system. Our simulations reveal a rich phase behavior governed by the competition between electrostatic bridging and excluded volume interactions. At fixed monomer charge, increasing cation size drives a nonmonotonic evolution in brush height and lateral structure, transitioning from a homogeneous collapsed state to heterogeneous pinned micelles and, ultimately, to a loosely reswollen layer. Specifically, the monomer effective charge is shown to modulate ionic correlations, reducing the critical cation size required for microphase separation and inducing the transition between intrachain and interchain bridging. Additionally, monomer lateral mobility exhibits a nonmonotonic dependence on cation size at high monomer charge, reflecting the dynamical consequences of this structural evolution. These findings provide a theoretical framework for understanding multifactor coupling in PE brush systems and offer parametric guidance for designing smart interfaces with ion-responsive properties.

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• Phase behavior
• Polymer brushes
• Brownian dynamics
• Langevin algorithm