Adhesion understanding molecular-scale

Understanding molecular-scale adhesion, friction, lubrication, and wear is crucial to modern technologies, such as microelectromechanical systems (MEMS) and hard disk drives.With atomic force/friction force microscopy (AFM/FFM),several studies have shown the correlation of frictional properties of self-assembled monolayers (SAMs) of alkanethiols with their chain lengths and terminal groups. The long chain monolayers (more than 12 carbon units) have lower friction coefficients compared with their short chain counterparts. Recently, Kim et al investigated the effect of the size of chain termination on frictional properties and found that the difference in friction arises predominantly from the difference in the size of the terminal groups. The AFM/FFM has also been applied to study frictional... 

Once formed, crystals can interact with or adhere to other components. Adhesion can occur directly between crystals to form larger structures which may act as the nidus for further mineralization. Crystals can also interact with metabolites and/or tissues to form such structures. These interactions can and do alter the stereochemical course of crystal assembly, and molecular recognition probably accounts for the striations and intricate geometries adopted by biomineralized structures. The role and composition of metabolites comprising the matrix may be useful to understanding the local and large-scale structure of biominerals. 

While the macroscopic concepts of hardness, adhesion, friction, and slide have evolved over the last two centuries, atomic level understanding of the mechanical properties of surfaces eluded researchers. The discovery of the atomic force microscope in recent years promises to change this state of affairs. Being able to measure forces as small as 10 newton or as large as 10 newton [5] over a very small surface area (few atoms) and by simultaneously providing atomic spatial resolution, this technique permits the study of deformation (elastic and plastic), hardness, and friction on the atomic scale. The buried interface between moving solid surfaces can be studied with spectroscopic techniques on the molecular level. Study of the mechanical properties of interfaces is, again, a frontier research area of surface chemistry. 

There is no way that we can model every atom or molecule in a block of material. It is far better to understand the overall macroscopic behavior of a material in terms of the continuum mechanics of elasticity theory, for example. However, it is equally evident that such continuum theories must fail as we approach molecular dimensions where adhesive failure is occurring. It is far too simplistic to suggest that a single variable in the equation of state (e.g. stress or pressure) should dictate adhesion. This might have worked for the Magdeburg hemispheres but does not work for molecular forces. Instead, consider the situation deincted in Fig. 7.14. This shows that, at large scales we should treat... 

An important facet of adhesion bonds is the locus of the proposed action or the scale to which the adhesive and adherend interact. Table 1.1 shows a scale of action for each mechanism, which is intended to aid in the understanding of these mechanisms. Of course, adhesive-adherend interactions always take place at the molecular level, which is discussed later in this chapter. 

Industrial use of thin films has increased for several reasons including the development of ever-smaller electronic devices. As applications of polymers become smaller and thinner, the behavior of polymer chains in these confined geometries needs to be understood. Many aspects need to be probed such as the effect of molecular weight, thermal degradation, and the adhesion properties. In the first study, one characterization scheme, cooperativity, was chosen to summarize the influence of the small scale on polymer behavior. The theory of cooperativity focuses on polymer chain interactions and relates those interactions to macroscopic behavior. This research looks specifically at the well-defined system of polymethyl methacrylate and silicon to understand better how cooperativity reveals polymeric behavior in thin films. 

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