Friction Control by Adsorption of Polyelectrolyte-Grafted Nanoparticles

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CBET-1133175P.I.: Tilton, Robert D.Intellectual Merit:This work provides a new boundary lubricating system together with new understanding of how molecular structure and intermolecular forces combine to minimize friction in nanostructured organic/inorganic composite systems. It therefore embodies both technological and scientific novelty. Prior fundamental research has shown that polyelectrolyte brushes grafted on solid surfaces greatly diminish adhesion and friction between those surfaces. Brushes are descriptively named layers of densely packed polymer chain molecules attached by one end to a surface. The chains stretch tens of nanometers away from the surface to form a protective coating that resists impact by other surfaces. Polyelectrolytes are polymers that have electrical charge, and this charge strengthens chain stretching and surface protection characteristics. A critical challenge is that existing methods for creating brushes have shortcomings that limit their uses in engineering applications. In order to realize the potential of boundary lubrication by polyelectrolyte brushes, this work will provide a fundamentally new way to deploy such brushes in practical systems. Poly(2-(dimethylamino)ethyl methacrylate) polyelectrolyte brushes will be grafted from silica nanoparticles at high, controlled grafting densities and controlled degrees of polymerization. These grafted nanoparticles will serve as pre-formed brush elements. By suspending them in the liquid phase bathing the surfaces to be lubricated, they will adsorb onto those surfaces and create stable brush coatings, piece by piece, to minimize friction. Unlike other methods for creating dense polyelectrolyte brushes, this approach can be applied to many different material types and geometries and also offers the novelty of self-healing after surface damage. Optimizing this new lubrication system motivates specific research aims that represent novel engineering science. Since there is no precedent for this approach, experiments are designed to reveal its mechanisms for friction control. The research plan will provide a multi-scale, quantitative structure versus activity relationship spanning from the conformation and ionization of nanoparticle-grafted polyelectrolyte chains to the multiparticle organization of surface layers and finally to the friction coefficient between coated surfaces. The work uses state of the art methods atom transfer radical polymerization to create grafted polyelectrolyte brushes and colloidal probe force microscopy to measure friction as a function of load.Broader Impacts:One Ph.D., one M.S. and several undergraduate students will receive research education through this project. The Ph.D. student will benefit from an international collaboration experience with researchers in Stockholm. Discovery-based science lessons on friction and lubrication will be developed with an elementary science teacher and disseminated through teacher training networks. The research project will produce effective water-based lubrication agents that exploit the low friction properties of polyelectrolyte brushes in a practical manner that cannot be achieved with current methods of brush formation. This will lead to new water-based lubricants to be used as machining fluids, for example. Machining fluids tend to be atomized into the workplace air. Because they would replace hydrocarbons with water, new water-based machining fluids would have lower occupational health risks and environmental impacts. Because of its inherently flexible mode of application, this lubrication strategy may also enable new microelectromechanical system (MEMS) technologies that currently are limited by friction and adhesion between ultraminiaturized moving parts. With further work, the basic principles developed here can be translated in the future to new oil-based lubricants to improve engine wear and fuel efficiency.