Interaction of Water with Polymer Surfaces: Consequence on Wetting, Adhesion, and Friction

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NON-TECHNICAL SUMMARY:The interaction of water with polymer surfaces has significant implications in diverse technological areas. The impacts are far-reaching: from energy-harvesting devices, biomedical implants, corrosion-resistant coatings, and extending to any innovation with surfaces that is influenced by wettability in aqueous environments. The goal of this project is to understand how polymer surfaces restructure themselves on a molecular level after coming in contact with water. Polymers are widely used as adhesives and lubricants and the knowledge of how these materials interact with water is of great importance. This understanding will be obtained using advanced specialized surface and spectroscopic techniques, some developed in the PI's laboratory. The fundamental understanding developed by the planned research will help in designing better materials that either adapt or resist changes upon contact with water. The PI will be training two graduate students in the area of surface science, which is a key requirement of industries developing or manufacturing adhesives or coatings. In addition to these directly integrated educational aspects, the PI will also be involved in broader outreach activities to middle and high school students as well as to community colleges. TECHNICAL SUMMARY:It is well accepted that when we bring water in contact with surfaces, polar groups can migrate or reorient at the contact interface and reduce the interfacial energy. This surface rearrangement results in contact angle hysteresis. For example, when two solid surfaces come in contact, this interfacial rearrangement leads to adhesion hysteresis and higher friction. But the direct observations of these structural rearrangements are elusive. Surface-sensitive sum frequency generation spectroscopy (SFG) has been used to study surface rearrangements upon contact with water, changes during solid-solid contact, and during sliding contact. However, the kinetics of the surface rearrangements in contact with water are not well understood, particularly how dynamical effects are related to the Tg of the polymer or even surface Tg. The PI will conduct SFG experiments to study the kinetics of surface rearrangements for poly(alkyl methacrylates) and poly(alpha-hydroxymethyl substituted acrylate) as a function of time and temperature after bringing the surfaces in contact with water, humidity, and steam. This will allow the PI to relate these surface changes with contact-angle hysteresis and adhesion hysteresis underwater, and to study the differences between wet and dry friction coefficients. Recently, the PI and his collaborators have shown that molecular dynamics (MD) simulations are able to reproduce the SFG spectra of the poly(methyl methacrylate) surface. Here, the PI will complement the SFG experiments with MD simulations for polymer-water interfaces. The proposed research will be a concerted effort at the molecular-level to understand the interaction of water with polymer surfaces.