(529c) Surface Forces Between Adsorbed Polymer Brush Nanoparticles

ABSTRACT: It is well established that dense polymer brushes under good solvent conditions produce strong repulsive forces and promote low friction sliding between surfaces in contact. Brushes are most frequently formed by methods such as adsorption of block copolymers or covalent grafting of polymers from surfaces. As an alternative, we propose the adsorption of polymer brush nanoparticles. Polymer brush nanoparticles (BNPs) are a class of surface active materials consisting of a nanoscale core surrounded by a dense, covalently attached polymer brush. This type of nanostructure can be designed to adsorb strongly to interfaces with multi-chain contact while simultaneously extending a large number of chains outward into solution. This offers a flexible route to non-covalently establish brush coatings on surfaces. However, lateral repulsions between particles at the surface ultimately limit the maximum attainable coverage, resulting in heterogeneous “pseudo-brush” layers whose surface forces are governed by the type, coverage, and structure of BNP layer. Here, we evaluate the surface forces between silica surfaces coated with spherical BNPs. We utilize high molecular weight poly(ethylene oxide) star polymers (Star PEO) as neutral BNPs and silica nanoparticles grafted with the weak polybase poly(dimethylaminoethyl methacrylate)   (SiO₂-g-PDMAEMA) as charged BNPs. Mixed layers of these BNPs are also studied as a route for enhanced coverage and hybrid performance in controlling surface forces. First, the adsorption of neutral, charged, and mixed BNPs from aqueous suspensions to the solid-liquid interface is studied using ellipsometry, quartz-crystal microbalance with dissipation (QCM-D), and streaming potential measurements. Changes in the solution pH and ionic strength are used to probe the competing adsorption mechanisms of electrostatics (SiO₂-g-PDMAEMA/silica) and hydrogen bonding (Star PEO/silica). Colloidal probe atomic force microscopy is then used to evaluate the steric, electrostatic, and/or electrosteric forces between the neutral, charged, and mixed BNP layers. Regimes are identified where these BNPs can be useful as boundary lubrication modifiers.