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

剪切是自然界和技术中普遍存在的现象，可以显著改变生物和人工系统的结构、功能和性能。例如，关节运动伴随着剪力和摩擦力，这可能导致关节磨损和骨关节炎。生物细胞通过改变其形态和代谢来响应容器中液体流动施加的机械应力，并且通过润滑剂和表面涂层减少摩擦对于减少能量消耗和增加几乎任何具有运动部件的机器的使用寿命都是重要的。为了理解、控制和利用剪切效应，在分子尺度上分析剪切对剪切界面结构和功能的影响是非常必要的。为此，我们将结合原位中子反射（NR）和红外（IR）光谱来跟踪剪切诱导的界面过程，时间分辨率低至约10 ms。NR是研究埋藏界面结构的优秀工具，可以提取界面上的同位素敏感密度曲线，直至原子分辨率。然而，它不提供化学和分子信息，因此，这将通过红外光谱获得。利用偏振分析，红外光谱也将提供洞察剪切界面膜的分子取向。界面膜通常由不同类型的分子组成。在这里，问题是如何区分它们对剪切的个体行为，特别是如果它们在化学上相似（例如，具有相似主链或链的不同生物分子）或甚至相同（例如，相同类型的纠缠聚合物链，但表面锚定或自由）。因此，研究的一个主要目标是，通过故意氘化所涉及的一种物质，来区分这种复杂界面的剪切诱导行为。在NR的情况下，由于散射长度密度的改变，特定的分子将被突出显示。在红外光谱中，氘化会将各自的波段移到较低的波数，从而可以区分氘化和质子化的物质。基于模型系统，法国和德国合作伙伴将共同研究两种具有高技术和生物医学相关性的剪切现象：I)在固体壁上纠缠的聚合物液体的剪切依赖摩擦的分子起源，这被认为是由表面吸附链和后者的卷曲到拉伸过渡引起的，导致随后与自由流动的链在超过临界应力时解缠；ii)哺乳动物关节中的润滑膜对剪切力的响应，以阐明骨关节炎的机制和相关的医学治疗。对于这些研究，将设计一个专门的剪切单元，允许并行时间分辨NR和IR原位测量，并生成复杂的剪切模式（启停，振荡等）。

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.).