Interphase Constituents

Vapor/liquid equilibrium (XT E) relationships (as well as other interphase equihbrium relationships) are needed in the solution of many engineering problems. The required data can be found by experiment, but such measurements are seldom easy, even for binaiy systems, and they become rapidly more difficult as the number of constituent species increases. This is the incentive for application of thermodynamics to the calculation of phase-equilibrium relationships. 

Figure 3 a presents the variation of the thermal expansion coefficients a for the inclusions (f), the matrix (m) and the composite (c) and the derived values for a s at the interphase (a.). Similarly, Fig. 3b gives the variation of the normalized to the unit-lengths thermal expansions of the constituents versus temperature T. 

In the previous sections of this book, we focused on the nature of contaminants and the geochemical reactions that can occur in the subsurface environment. Chemical compounds introduced into infiltrating water or in contact with soil or rock surfaces are subject to chemically and biologically induced transformations. Other compounds are retained by the soil constituents as sorbed or bound residues. Thus, in terms of geochemical interactions and reactions among dissolved chemical species, interphase transfer occurs in the form of dissolution, precipitation, volatilization, and various forms of physicochemical retention on the solid surfaces. 

Previous studies of the interphase/interlayer have mainly focused on the coefficient of thermal expansion (CTE) and residual thermal stresses. The importance of residual thermal stresses cannot be overemphasized in composites technology because the combination of dissimilar materials in a composite creates inevitably an interphase across which residual stresses are generated during fabrication and in service due to the difference in thermo-mechanical characteristics. The importance of an interlayer is clearly realized through its effects in altering the residual stress fields within the composite constituents. 

The chemical composition of the composite constituents and the interphase is not limited to any particular material class. There are metal-matrix, ceramic-matrix, and polymer-matrix composites, all of which find industrially relevant applications. Similarly, reinforcements in important commercial composites are made of such materials as steel. E-glass, and Kevlar . Many times a bonding agent is added to the fibers prior to compounding to create an interphase of a specified chemistry. We will describe specific component chemistries in subsequent sections. 

Despite the emphasis on favorable interactions between the matrix and reinforcement and compound formation between them, it may be beneficial in certain circumstances for the interaction between the two primary constituents to be relatively weak. This is especially true in ceramic-ceramic composites, where both constituents are brittle, and the only way to impart some ductility on the composite is for the interphase to fail gracefully —that is, for the fibers to actually pull out of the matrix in a controlled manner. Optimization of the interphase properties in advanced composites is currently the focus of much research. 

Much of what we need to know abont the thermodynamics of composites has been described in the previous sections. For example, if the composite matrix is composed of a metal, ceramic, or polymer, its phase stability behavior will be dictated by the free energy considerations of the preceding sections. Unary, binary, ternary, and even higher-order phase diagrams can be employed as appropriate to describe the phase behavior of both the reinforcement or matrix component of the composite system. At this level of discussion on composites, there is really only one topic that needs some further elaboration a thermodynamic description of the interphase. As we did back in Chapter 1, we will reserve the term interphase for a phase consisting of three-dimensional structure (e.g., with a characteristic thickness) and will use the term interface for a two-dimensional surface. Once this topic has been addressed, we will briefly describe how composite phase diagrams differ from those of the metal, ceramic, and polymer constituents that we have studied so far. 

It is clear that other components quite different chemically from the main constituents of the epoxy resin system may be present in the starting material. The structure of the cured epoxy may or may not incorporate these components. To the extent that these other species are not part of the crosslinked epoxy network, they can be concentrated at the interphase or they may be able to migrate to the interphase during the curing process. 

However, the chemical bonding theory cannot account for the increase in adhesion experienced between non-reactive matrices such as polyolefins and inorganic reinforcements in which chemical bonds will not be formed [4], This observation, among others, leads to an alternative proposal that an interphase composed of various constituents forms surrounding the reinforcement. This third phase in the composite is possibly formed through interdiffusion of physisorbed silane and matrix molecules in the interphase and perhaps via preferential adsorption of both matrix components as well as silane coupling agents on the reinforcement surface [5],... 

These constituents of typical sizing systems are compatible and can interact with each other. The resulting distribution of these components in the interphase region is not well understood, with the formulation of effective sizing systems... 

The basic issue confronting the designer of polymer blend systems is how to guarantee good stress transfer between the components of the multicomponent system. Only in this way can the component s physical properties be efficiently used to give blends with the desired properties. One approach is to find blend systems that form miscible amorphous phases. In polyblends of this type, the various components have the thermodynamic potential for being mixed at the molecular level and the interactions between unlike components are quite strong. Since these systems form only one miscible amorphous phase, interphase stress transfer is not an issue and the physical properties of miscible blends approach and frequently exceed those expected for a random copolymer comprised of the same chemical constituents. 

Here, Ms and Ms,ads are the electrochemical potentials of S in the bulk solution and in the adsorbed state. Let us apply the Gibbs adsorption equation to the interphase between a pure metal M and an aqueous solution containing molecular and ionic species denoted by the subscript j, in addition to water w and the species S. Choosing the neutral metal atoms M and the electrons e in excess with respect to metal atoms as the constituents of the metal phase, we may formally write ... 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

In the description of the interphase mass transfer process, a variety of measures for constituent concentrations, mixture reference velocities, and diffusion fluxes (with respect to the arbitrarily defined mixture velocity) are used. Table 1.1 summarizes the most commonly used concentration measures together with a number of other quantities that will be needed from time to time. 

Figure 2.4 shows a comparison of the results obtained from the PCA and real chemical models for a Raman emulsion image [30]. The latter image is formed by four constituents related to the drop phase, the interphase, an additive and the off-drop phase. The two models (real and PCA) resemble each other when considering general trends for example, the score maps are reminiscent of the real distribution maps, although the information seems to be more mixed, and the most salient spectral features in the real spectra can also be found in the different... 

The absence of an interphase compositional differential makes the separation of azeotropes into their constituent components impossible by conventional vapor-liquid separation processes. The azeotropic pattern may be altered by adding an external component—an entrainer—that breaks the original azeotrope while forming new ones with the feed components. The process may be designed so that the... 

The purpose of the equipment used for mass-transfer operations is to provide intimate contact of the immiscible phases in order to permit interphase diffusion of the constituents. The rate of mass transfer is directly dependent upon the interfacial area exposed between the phases, and the nature and degree of dispersion of one phase into the other are therefore of prime importance. 

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