Demixing processes

In this application the reader will examine the influence of the interrelation between the two liquids on the extent and rate of the demixing process. In Example 5.1, the two liquids have a modest affinity for each other, characterized by rules describing a relatively low breaking probability rule between them. These setup data are found in Parameter setup 5.1. 

Figure 5.2. A cellular automata model of the interface between two immiscible liquids, after the demixing process has reached an equilibrium...

To evaluate the demixing process under the nonisoquench depth condition, they carried out computer simulations of the time dependent concentration fluctuation using the Cahn-Hilliard nonlinear diffusion equation. 

At high polymer concentrations, one may also have what is known as depletion stabilization. The polymer-depleted regions between the particles can only be created by demixing the polymer chains and solvent. In good solvents the demixing process is thermodynamically unfavorable, and under such conditions one can have depletion stabilization. 

The physical reason for the (A, B)0 demixing process is always the difference in the mobilities of the cations. This is reflected in Eqn, (8.11) where p is the only kinetic parameter, and dNAO = 0 for p= 1. Let us emphasize this fact by a somewhat different argument. The cation flux equation j2A = -cA-bA-Vt]A2+ can be rewritten as... 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

Khidirov, I. (2001) Ordering and Demixing Processes in a Solid Solution of Nitrogen and hydrogen in a-Titanium, Zh. Neorg. Khimii, 46 (3), 499 p. 

The demixing of the immiscible components in an initially one-phase melt proceeds rapidly due to a thermodynamic instability, this is spinodal decomposition, or a thermodynamic metastability (i.e., nucleation and growth of a new phase). However, this distinction of the character of the liquid-liquid phase transition is largely academic for the purposes of this work, because in either case the system rapidly produces droplets of a new phase (i.e., for off-critical mixtures). The demixing process itself was not studied in detail in this work. However, further details of this process may be found elsewhere (21). The production of droplets of a new phase in these immiscible systems is extremely rapid compared with the subsequent coarsening of the system to the final morphology. 

In most cases, the "demixing" process was rapid, but its rate and completeness diminished with the complexity and concentration of the solutes. For instance, the acidic system C (no TBP) evolved to two well separated phases in less than O.S ns while, for systems like I or J, no full phase separation was achieved after S ns. Anyway these events remain extremely fast at the extraction experiment time scale, and we thus mainly focus on the systems at the end of the simulation. More detailed analysis can be found in ref (4J). The main features are illustrated by final snapshots, while density curves of the different species provide a statistical description during the last 0.2 ns. 

FIGURE 19.2 Composition profile in the liquid-liquid demixing process (a) instantaneous and (b) delayed. 

From the morphology and mechanical properties of these as-spun PANI fibers, it is obvious that the size and number of macrovoids must be reduced. Therefore, the effects of the temperature and composition of the coagulation bath were subsequently investigated. The coagulation bath temperature should affect the morphology of the as-spun PANI fibers. From theoretical calculations, a high bath temperature is expected to accelerate the diffusion and the demixing processes. From the above study, we... 

When two polymers are intimately mixed, a polyblend (polymer alloy) is produced. Polyblends are usually thermodynamically unstable (see Section 6.6.6). Because of the very low diffusion coefficient, however, the demixing process is extremely slow, so that polybends appear kinet-ically stable. 

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