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(610d) Normal and Lateral Forces Between Adsorbed Star Polymer Layers

Riley, J. K., Carnegie Mellon University
Tilton, R. D., Carnegie Mellon University

Polymer brushes have the ability to greatly reduce friction between surfaces. Initial studies on planar non-ionic and polyelectrolyte brushes provided great insight into the lubrication mechanism and thus aided in the design of high-density, high load-bearing, low coefficient of friction coating strategies. Recently attention has shifted toward the study of more structurally complex, biologically-inspired branched and brush-like polymeric structures and how they adsorb to and mediate friction between surfaces in aqueous tribosystems. This talk will discuss our research efforts using multi-arm star polymers as boundary lubricants. Homo-star polymers and hetero-block-star polymers based on cationic poly(dimethylamino ethyl methacrylate) (PDMAEMA), non-ionic poly(oligoethylene oxide methacrylate) (POEOMA), and zwitterionic poly(methacrylolyloxyethyl phosphorylcholine) (PMPC) are synthesized using atom-transfer radical polymerization (ATRP). ATRP provides precise control over the number of arms, degree of polymerization of the arms, and composition of the arms. Hetero-block-star polymers are designed with a surface active anchor block (PDMAEMA or POEOMA) and a zwitterionic block (PMPC), as zwitterionic polymers have recently been shown to remain strongly hydrated and lubricious under compression. Adsorption and conformation of the star polymers on silica substrates is investigated using quartz crystal microbalance with dissipation (QCM-D), ellipsometry, and atomic force microscopy (AFM) imaging. Individual, single-component adsorption of the star polymers is first investigated as a function of pH and ionic strength to probe the competing effects of electrostatics and hydrogen bonding on mono- and sub-monolayer formation for the homo-star and hetero-block-star polymers. Sequential, layer-by-layer adsorption is also utilized to generate thicker, extended layers, including the incorporation of zwitterion-containing star polymers into multi-layer coatings.  Colloidal probe atomic force microscopy measurements are used to evaluate polymer-mediated steric and electrosteric forces. This approach enables the structure of the adsorbed layers to be correlated with the range and nature of the surface forces, providing valuable insight into how star polymers and star polymer multilayers can be used to improve tribological performance.