Molecular Scale Characterization of Sheared Interfaces by Combined Time-Resolved Neutron Reflectometry and Polarized Infrared Spectroscopy: From polymer brushes to lipid multilayers

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Shear is a widespread phenomenon in nature and technology, and can significantly alter the structure, function and performance of biological and artificial systems. E.g. joint movement is accompanied by shear and friction forces which can lead to joint wear and osteoarthritis. Biological cells respond to mechanical stress exerted by liquid flow in vessels by changing their morphology and metabolism, and reduction of friction by lubricants and surface coatings is important to reduce energy consumption and increase the lifetime of almost any machine with moving parts. To understand, control and utilize the effects of shear, it is highly desirable to analyze its impact on the structure and function of the sheared interfaces on a molecular scale. For this purpose, we will combine in situ neutron reflectometry (NR) and infrared (IR) spectroscopy to follow shear-induced interfacial processes with time resolution down to about 10 ms. NR is an excellent tool for structural investigation of buried interfaces, allowing extraction of isotope-sensitive density profiles at interfaces down to atomic resolution. However, it does not provide chemical and molecular information, which will, thus, be obtained by IR spectroscopy. Using polarization analysis, IR spectroscopy will also provide insight into the molecular orientation of sheared interfacial films. Interfacial films are often composed of different types of molecules. Here, the problem arises to differentiate between their individual behavior in response to shear, in particular, if they are chemically similar (e.g. different biomolecules with similar backbone or chains) or even identical (e.g. entangled polymer chains of the same type but surface-anchored or free). A major goal of the studies is, therefore, to discriminate the shear-induced behavior of such complex interfaces by deliberately deuterating one of the species involved. In case of NR, specific molecules will be highlighted due to altered scattering length density. In IR spectroscopy, deuteration will shift the respective bands to lower wavenumbers allowing discriminating between deuterated and protonated species. Based on model systems, the French and German partners will jointly investigate two shear phenomena of high technical and biomedical relevance, respectively: i) the molecular origin of shear-dependent friction of entangled polymeric liquids at solid walls, which has been proposed to arise from surface-adsorbed chains and a coil-to-stretch transition of the latter, resulting in a subsequent disentanglement with the free flowing chains beyond a critical stress, and ii) the response of lubricating films in mammalian joints to shear forces in order to elucidate the mechanisms of osteoarthritis and related medical cures. For these studies, a specialized shear cell will be designed which allows for parallel time-resolved NR and IR measurements in situ, and the generation of complex shear patterns (start-stop, oscillatory, etc.).