Interfacial Adhesion Mechanism of Polyelectrolyte Brushes Induced by Oppositely Charged Polycations

Polyelectrolyte brushes are widely used as model systems to study electrostatic interactions at soft interfaces and to achieve exceptional lubrication properties. Previous studies have focused on exposing these brushes to multivalent counterions, revealing brush collapse, ionic crosslinking, and significant changes in interfacial structure. However, the interactions between polyelectrolyte brushes and oppositely charged macromolecular counterions, such as polycations, are less understood. We use surface-initiated grafting to prepare well-defined polystyrene sulfonate brushes and Surface Force Apparatus measurements to quantify their interaction with polycations. Our results show that, in contrast to multivalent ions, polycations do not induce noticeable brush collapse. Instead, adhesion increases with contact time and applied load, eventually reaching a steady plateau, indicating steric contributions from the large polymer size. In addition, we confirm that introducing monovalent Na⁺ competitively disrupts the pre-formed polycation-brush interactions, leading to a marked decrease in adhesion. Friction measurements further support this effect, showing a partial recovery of the original lubrication behavior upon Na⁺ addition. These results reveal a distinct interaction mechanism between polyelectrolyte brushes and macromolecular counterions and provide new insight into the molecular principles governing interfacial properties in brush-based lubrication systems.