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 (SiO 2 - 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 PEO 45 MA). Adsorption of SiO 2 - g -PDMAEMA and Star PEO 45 MA 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 SiO 2 - g -PDMAEMA and hydrogen-bonding-driven adsorption of Star PEO 45 MA. 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 SiO 2 - g -PDMAEMA, while Star PEO 45 MA adsorption produces purely repulsive steric interactions. Bridging between SiO 2 - 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 PEO 45 MA adsorption. Sequential adsorption of SiO 2 - g -PDMAEMA and Star PEO 45 MA generates high-coverage mixed nanoparticulate brush layers with uniformly repulsive normal forces and reduced friction forces. Adsorption and force measurements reveal that Star PEO 45 MA not only adsorbs to silica but also binds to SiO 2 - g -PDMAEMA. The latter allows sequential adsorption of the two components to produce mixed multilayers. The mixed SiO 2 - g -PDMAEMA/Star PEO 45 MA multilayers exhibit larger layer thicknesses, no bridging, and sustained smooth friction, highlighting their potential usefulness as aqueous boundary lubricant layers.