Toughness and Friction of Model Polyelectrolyte Gels

Overview The proposed project is designed to understand the molecular-scale factors responsible for the fracture toughness and frictional response of model soft solids. Block copolymer gels are used as model materials because of their well-defined and tunable structure at the molecular level. Energy dissipation mechanisms responsible for the bulk and interfacial mechanical properties of the materials are controlled by the addition of multivalent ions. The proposed model materials have well-defined molecular features that will enable the existing models of the mechanics of soft solids to be tested and refined. Intellectual Merit Hybrid gels consisting of a charged, polyelectrolyte network to which multivalent cations have been added have extraordinary mechanical properties arising from the existence of both weak and strong crosslinks in the same material. The proposed work will explore the limits of these mechanical properties, and develop design strategies producing self-assembling materials with the desired mechanical toughness and frictional response. The work is based on the use of gels formed from the controlled assembly of high molecular weight triblock copolymers with a poly(methacrylic acid) (PMAA) midblock and poly(methyl methacrylate) (PMMA) end blocks. Assembly of these polymers results in the formation of polymer gels with the ’strong’ bonds consisting of glassy PMMA aggregates. The weak bonds are introduced either be complexation with small-molecule or polymeric cations, or by the introduction of a secondary, ionically-crosslinked network. Broader Impacts The project addresses issues that are relevant to a range of communities. Particular attention will be paid to the issues pertaining to medical implant technology and to art conservation, where gels of the sort described here can play an important role. Results will be communicated to collaborators at the Art Institute of Chicago and at Rush University Medical Center. In the context of Art Conservation, we will investigate the potential for the gels developed here to be used for the cleaning and restoration of paintings of great historic and cultural value. The relevance to medical technology is through a more detailed understanding of interfacial gels formed when metal ions are released from a metal surface into a surrounding fluid medium that is rich in proteins. These interfacial gels can substantially impact the wear behavior of the alloys, enhancing the performance of biomedical implants such as artificial hip or knee joints. Information from the project will be incorporated into curricula at Northwestern University and will be made available to the general public through a variety of mechanisms.