Interactions between dissimilar materials

Explain how Hamaker constants for interaction between identical materials in vacuum can be used to determine Hamaker constants for interaction between dissimilar materials immersed in an arbitrary medium. 

The London dispersion forces are present and important in most adsorption processes and in adhesive interactions between dissimilar materials. The free energy of interaction per unit area between materials 1 and 2 in contact is where W 2 -s... 

Polymer blends and compatibilizing agents historically have been the subject of a wide variety of studies and an extensive body of literature on these materials exists. Without specific chenucal interactions between dissimilar polymers, most polymer mixtures tend to phase separate due to the unfavorable entropy of mixing between the polymer chains. Efforts to control or retard the phase separation process have led to the research and development of compatibilizing agents for polymer blends. For a variety of systems the dispersed phase particle size has been found to decrease with increasing copolymer concentration. Above a critical concentration of copolymer, the size of the dispersed phase remains constant. 

More often than not one deals with colloidal objects immersed in a liquid or other such media, and therefore interactions between similar or dissimilar materials in an arbitrary medium are of importance in colloid science. Moreover, it is very useful to relate such dissimilar interactions to those between identical particles in vacuum. In the last section (Section 10.8) we present what are known as combining relations for accomplishing this. The van der Waals forces between macroscopic objects are usually attractive, but under certain circumstances they (and, as a consequence, the Hamaker constant) can be negative, as noted in Vignette X. A brief discussion of this completes Section 10.8. 

The assumption of forces of interaction between solvent and solute led to the century old principle that like dissolves like . In many cases the presence of similar functional groups in the molecules suffices. This rule of thumb has only limited validity since there are many examples of solutions of chemically dissimilar compounds. For example, for small molecules methanol and benzene, water and N,N-dimethylformamide, aniline and diethyl ether, and for macromolecules, polystyrene and chloroform, are completely miscible at room temperature. On the other hand, insolubility can occur in spite of similarity of the two partners. Thus, polyvinylal-cohol does not dissolve in ethanol, acetyl cellulose is insoluble in ethyl acetate, and polyacrylonitrile in acrylonitrile [12], Between these two extremes there is a whole range of possibilities where the two materials dissolve each other to a limited extent. 

Diffusion Theory. The diffusion theory of adhesion is mostly applied to polymers. It assumes mutual solubility of the adherend and adhesive to form a true interpliase. The solubility parameter, the square root of the cohesive energy density of a material, provides a measure of the intemiolecular interactions occurring witliin the material. Thermodynamically, solutions of two materials are most likely to occur when the solubility parameter of one material is equal to that of the other. Thus, the observation that "like dissolves like." In other words, the adhesion between two polymeric materials, one an adherend, the other an adhesive, is maximized when the solubility parameters of the two are matched ie, the best practical adhesion is obtained when there is mutual solubility between adhesive and adherend. The diffusion theory is not applicable to substantially dissimilar materials, such as polymers on metals, and is normally not applicable to adhesion between substantially dissimilar polymers. 

Nanocomposites encompass a large variety of systems composed of dissimilar components that are mixed at the nanometer scale. These systems can be one-, two-, or three-dimensional organic or inorganic crystalline or amorphous. A critical issue in nanocomposite research centers on the ability to control their nanoscale stmcture via their synthesis. The behavior of nanocomposites is dependent on not only the properties of the components, but also morphology and interactions between the individual components, which can give rise to novel properties not exhibited by the parent materials. Most important, the size rednction from microcomposites to nanocomposites yields an increase in snrface area that is important in applications such as mechanically reinforced components, nonlinear optics, batteries, sensors, and catalysts. 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a... 

Proton spin-temperature equilibration between the hard- and soft-segment-rich domains of the polyurethane elastomer on the order of 10-100 ms might be considered fast relative to a macroscopically phase-separated blend [26] or copolymer, but slow relative to a strongly interacting mixture [25]. This is reasonable for a microphase-separated material whose solid state morphology has been the subject of considerable theoretical and experimental research. Under fortuitous circumstances, intimate (near-neighbor) contact between dissimilar molecules in a mixture can be studied via direct measurement of proton spin diffusion in a two-dimensional application of the 1H-CRAM PS experiment (Combined Rotation And Multiple Pulse Spectroscopy). Belfiore et al. [17,25,31] have detected intermolecular dipolar communication in a hydrogen-bonded cocrystallized solid solution of poly(ethylene oxide) and resorcinol on the f00-/xs time scale, whereas Ernst and coworkers [26] report the absence of proton spin diffusion on the 100-ms time scale for an immiscible blend of polystyrene and poly(vinyl methyl ether), cast from chloroform. 

The London-VW interactions between similar particles of surfaces (identical material) is always attractive A positive), whereas for dissimilar materials it can be positive or negative, depending on the values of the e, and n that is, the interactions can be repulsive. 

A new interface is formed when two dissimilar materials are brought into intimate contact at the expense of the two free surfaces. The strength of the bond that forms between the coating and the substrate is determined by the nature of the interaction at the interface. The extent of these interactions is greatly determined by the wettabUity of one phase by the other. A criterion necessary for adhesion is wetting. Only with the presence of effective wetting between the substrate and the coating can the mechanisms of adhesion previously discussed become operational. 

In this case, cracks run along the interface between two materials due to interactions between the stress field in the adhesive layer and spatial variations in fracture properties. The cracks are not generally free to evolve as mode I cracks, as was the case for cohesive cracks, and mixed-mode fracture concepts (combinations of tension and shear) have to be considered. Mode II or shear components are induced, even in what appear to be nominally mode I loadings, due to differences in moduli about the interface. Again, if the presence of the adhesive layer is being ignored and the adherends are dissimilar, then a crack appears to be adhesive (i.e. an adhesion failure) on the macroscopic scale. 

Another important feature of some random copolymers is the abihty to achieve miscibility in either a homopolymer or a second random copolymer. This "copolymer effect" has been shown empirically for quite some time, eg, PVC is miscible with random copolymers of ethylene and vinyl acetate (52). Such systems are effective because repulsions between the dissimilar segments in the copolymer are enough to overcome the repulsions between these segments and those of the second component in the mixture. In other words, in the above example, the ethylene units "hate" vinyl acetate units more than either of them "hate" PVC. Thus there is a net negative interaction energy and the two materials are miscible (53). 

The dissimilarity found between the homogeneous and heterogeneous reduction processes can be attributed to a specific interaction of the sulfur-containing groups with the material of the electrode. 

When analyzing data from a dissimilar system there are two potentials involved. In Fig. 3 we show theoretical force-separation curves for different pairs of potentials that when multiplied together give the same number. For constant charge systems there is very little difference between the curves produced by the different pairs of potentials. At large separations, where theory is lined to the experimental data to determine the diffuse layer potentials, there is little difference between the constant potential systems. Clearly, there is not a unique pair of diffuse layer potentials that fits the individual experimental force curves. Even when the constant potential interaction fits are considered, any differences between different potential pairs at small separations may be obscured if there is an extra non-DLVO short-range repulsion. For this reason it is necessary to have independently obtained values of the potentials of the materials for comparison. 

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