Sequential Adsorption of Nanoparticulate Polymer Brushes as a Strategy To Control Adhesion and Friction

This work investigates surface forces that result from adsorbed layers of silica nanoparticles with grafted pH-responsive, cationic poly(2-(dimethylamino)ethyl methacrylate) brushes (SiO₂-g-PDMAEMA) and how adhesive bridging forces can be suppressed and friction forces reduced by "backfilling" these heterogeneous adsorbed layers with nonionic poly(ethylene oxide) star copolymers (Star PEO₄₅MA). Adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA to silica is measured as a function of pH by quartz crystal microbalance with dissipation (QCM-D) in order to evaluate the electrostatically driven adsorption of SiO₂-g-PDMAEMA and hydrogen-bonding-driven adsorption of Star PEO₄₅MA. Force measurements performed using colloidal probe force microscopy show the strong role that limited surface coverage plays in adhesive bridging forces between silica with adsorbed SiO₂-g-PDMAEMA, while Star PEO₄₅MA adsorption produces purely repulsive steric interactions. Bridging between SiO₂-g-PDMAEMA-coated surfaces produces frictional forces that tend to be larger than those acting between bare surfaces at similar normal loads, while friction is consistently decreased by Star PEO₄₅MA adsorption. Sequential adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA generates high-coverage mixed nanoparticulate brush layers with uniformly repulsive normal forces and reduced friction forces. Adsorption and force measurements reveal that Star PEO₄₅MA not only adsorbs to silica but also binds to SiO₂-g-PDMAEMA. The latter allows sequential adsorption of the two components to produce mixed multilayers. The mixed SiO₂-g-PDMAEMA/Star PEO₄₅MA multilayers exhibit larger layer thicknesses, no bridging, and sustained smooth friction, highlighting their potential usefulness as aqueous boundary lubricant layers.