Macroscopic composites

Copolymerizations. The uniform chemical environment of a CSTR makes it ideally suited for the production of copolymers. If the assumption of perfect mixing is justified, there will be no macroscopic composition distribution due to monomer drift, but the mixing time must remain short upon scaleup. See Sections 1.5 and 4.4. A real stirred tank or loop reactor will more closely... 

Internal boundaries in a crystal, when disordered, form extended defects. However, if the boundaries become ordered, they simply extend the unit cell of the structure and hence are no longer regarded either as boundaries or defects (Fig. 3.20c). In addition, some boundaries can change the composition of a solid locally and, if present in large numbers, can change the macroscopic composition noticeably. When these are ordered, new series of compounds form. Boundaries that do cause significant composition changes are described in Chapter 4. 

The earliest equations for Gibbs excess energy, like Margules and van Laar, were largely empirical. More recent equations and NRTL and UNIQUAC are based on a semiempirical physical model, called the two-liquid theory, based on local composition. The molecules do not mix in a random way, but because of different bonding effects, the molecules prefer a certain surrounding. This results in a composition at the molecular level, the local composition, which differs from the macroscopic composition. 

Tabic 1. Macroscopic composition o) BA constituents i/i comparison to a study from ten years ago (Lichtensteiyer Zellner IW.i)... 

We shall conclude with some remarks on the structure of glassy polymers. If one frequently speaks of glass structures, this does not mean that there exists one definite glass structure similar to a crystal. In a macromolecular solid-e.g., the polystyrene-plasticizer system, entirely different glasses are obtainable, the macroscopic composition of which is always the same (8). In Figure 10 the full... 

The deterministic methods in Section 2.2.1 numerically solve the differential equations, thus determining the macroscopic composition of the system at a given time. On the contrary, the KMC method determines the time of change (reaction) of a species with the use of a random number generated with an appropriate distribution.89 The result is the system s composition, as well. 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

We know well that mechanical and other properties of polymer-based materials can be improved by introduction of reinforcements, for instance of chopped glass fibers into an epoxy. The resulting class of materials has been called macroscopic composites (1) or else heterogeneous composites (2.) The use of such composites causes on occasions serious problems - due to insufficient... 

Membranes play an important role in natural science and for many technical applications. Depending on their purpose, their shape can be very different. For instance, membranes include porous or non-porous films, either supported or non-supported, with two interfaces surrounded by a gas or by a liquid. Important properties of non-porous membranes are their permeability for certain compounds and their stability. In biological cells their main task is to stabilize the cell and to separate the cell plasma from the environment. Furthermore, different cells and cell compartments have to communicate with each other which requires selective permeability of the membranes. For industrial applications membranes are often used for separation of gases, liquids, or ions. Foams and emulsions for instance are macroscopic composite systems consisting of many membranes. They contain the continuous liquid phase surrounded by the dispersed gas phase (foams) or by another liquid (emulsions). Beside these application possibilities membranes give the opportunity to investigate many questions related to basic research, e.g. finite size effects. 

The change in polymer composition as a result of monomer drift gives a macroscopic composition distribution. It can be eliminated by polymerizing at the azeotrope, by polymerizing to low conversion, by continuously adding the more reactive monomer in a semibatch reactor, or by polymerizing in a CSTR, the last method... 

When a mixture is brought into the two-phase region, it splits into two phases, each with its own composition. The situations is shown schematically in Figure lo-i. On a microscopic basis, molecules of both components pass continuously from one phase to the other. It is this molecular transfer that allows the system to find and maintain its equilibrium composition. When this composition is reached, the net transfer between phases is zero. Molecules continue to cross the interface in such way that the rate of transfer in one direction matches the rate in the reverse direction, so that the average (macroscopic) composition of each phase remains constant. We analyze this situation as follows. We form a mixture by mixing ru moles of component i with n mols of component 2, and fix the temperature and pressure of the... 

These structures differ from homogeneous sequential IPN s in that the macroscopic composition varies in a systematic way. For example, the ratio... 

Pure water, regardless of its source, consists of 11% hydrogen and 89% oxygen by mass. This macroscopic composition corresponds to the molecular composition, which consists of two hydrogen atoms combined with one oxygen atom ... 

Because of the very similar interacting forces, the local composition is identical with the macroscopic composition (random mixture, almost ideal behavior... 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

Gradient IPN an IPN of nonuniform macroscopic composition, usually by nonequilibrium swelling in monomer II, and polymerizing rapidly. 

Li, V.C., Wang, S., 2006. Microstructure variability and macroscopic composite properties of high performance fibre reinforced cementitious composites. Probabilistic Engineering Mechanics, 21(3), pp. 201-206. 

We shall outline two different models used to interpret the elastic behaviour of highly oriented linear polyethylene. Both models take macroscopic composite theory as their starting point but diverge in the way in which they account for the evidence from morphological studies for crystalline regions that can extend for more than 100 nm in the draw direction. 

The macroscopic composites are those in which one or both phases tend to have structures large enough to be easily seen with the naked eye. These include paint hlms (paint itself is usually a particulate composite), adhesive joints, and foams (e.g., the familiar polystyrene foams used for coffee cups because of their low heat transfer coefficient). The fabric laminates alternate layers of inorganic cloth and plastic. An important subclass usually has multiple layers of glass cloth or chopped fiber interspersed with layers of unsaturated polyester-styrene AB-cross-linked materials see Section 2.7.3. These sheet molding compounds are widely used for the hulls of pleasure boats, because they are light weight and very puncture resistant, as well as for automotive bodies see Section 13.9.1. 

It is difficult to determine the maximum matrix strain before cracks are open because it may vary considerably. In a more developed material structure, composed of inclusions, pores, fibres, etc., the value of is higher than in homogeneous ones like cement paste. As to the heterogeneity of the matrix, not only are the differences between hard inclusions, weaker paste and voids considered, but also the fact that certain macroscopic composite regions have considerably different mechanical properties than others. 

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