Differential hydration lubrication performance of polyelectrolyte-modified UHMWPE promoted by diverse charge characteristics

Hydrated ions can achieve exceptional hydration lubrication through their adsorption onto oppositely charged surfaces. Similarly, the charge characteristics of polyelectrolytes are expected to significantly impact the hydration lubrication performance of polyelectrolyte-modified materials through the adsorption of counterions with surrounding hydration layers of different strength. To verify this hypothesis, a comprehensive polyelectrolyte-embedded modification on ultra-high molecular weight polyethylene (UHMWPE) employing polyanionic, polyzwitterionic, and polycationic brushes was performed for the construction of modified materials with diverse surface charge characteristics. Subsequently, the polyelectrolyte-modified UHMWPE were subjected to systematic investigations to understand the effect of polyelectrolyte charges on the surface hydration and lubrication performance under varying electrolyte conditions, including concentration and types of counterions. All polyelectrolyte-modified UHMWPE displayed more effective hydration lubrication with increasing ion concentrations, showcasing the contribution of hydrated counterions in the load-bearing and friction reduction of charged polyelectrolytes. A vertical comparison among different polyelectrolytes revealed that, polyanionic poly(3-sulfopropyl methacrylate potassium) (PSPMK), characteristic of the highest surface charge density, exhibited the strongest hydration lubrication that enables macroscale superlubricity. At the same time, a horizontal comparison of varying counterions in the solutions within each polyelectrolyte-modified UHMWPE displayed a sequence of hydration lubrication performance with more strongly hydrated ions resulting in lower friction and wear. These findings elucidate the impact of polyelectrolyte charge characteristics on hydration lubrication, highlighting the combined influence of ion adsorption density, determined by intrinsic surface potential, and the ionic hydration strength of surrounding counterions in determining the overall hydration lubrication performance of modified UHMWPE.