Demixing concentration

Comments. The solvent mixture is used at its binary critical demixing concentration, i.e.,... 

BCA2 Kaddour, L.O., Anasagasti, M.S., and Strazielle, C., Molecular weight dependence of interaction parameter and demixing concentration in polymer-good solvent systems. Comparison with theory, MaAro/wo/. Chem., 188, 2223,1987. 

Recently, renormalization group calculations have been used to derive new scaling laws for the molecular weight dependence of the interaction parameter, Xab between unlike polymers (A and B) in a good solvent and the critical demixing concentration. The purpose of this paper is to present some experimental results which verify this theory for several mixtures of two polymers polystyrene (PS) - poly(dimethyl-siloxane) (PDMS), poly(methyl-methacrylate) (PMMA)-PDMS, PS-PMMA and PS-poly (vinylacetate) (PVAc). 

For low molecular weight mixtures, the compositions of coexisting phases can be immediately read from the temperature dependence of the demixing concentrations. Because polymer samples normally contain many unlike species differing in chain length or/and with respect to other properties like molecular architecture, this procedure is usually not permissible for polymer solutions and... 

Studies described in earlier chapters used cellular automata dynamics to model the hydrophobic effect and other solution phenomena such as dissolution, diffusion, micelle formation, and immiscible solvent demixing. In this section we describe several cellular automata models of the influence of the hydropathic state of a surface on water and on solute concentration in an aqueous solution. We first examine the effect of the surface hydropathic state on the accumulation of water near the surface. A second example models the effect of surface hydropathic state on the rate and accumulation of water flowing through a tube. A final example shows the effect of the surface on the concentration of solute molecules within an aqueous solution. 

Fig. 17 B/E-p dependence of the critical temperatures of liquid-liquid demixing (dashed line) and the equilibrium melting temperatures of polymer crystals (solid line) for 512-mers at the critical concentrations, predicted by the mean-field lattice theory of polymer solutions. The triangles denote Tcol and the circles denote T cry both are obtained from the onset of phase transitions in the simulations of the dynamic cooling processes of a single 512-mer. The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...

Another characteristic concentration in Fig. 17 is point , which gives the solubility limit of the solvent in the unreacted precursor mixture. If the solvent concentration exceeds this solubility hmit, demixing occurs in the initial state. 

An increase in the water concentration of the eluent cause the adsorbent ttuiivity lu ilecicuae and an a result Iho elution of the more polar components is accelerated. When a dry eluent is replaced by a wet one demixing occurs on the adsorbent, i.e., more water is taken by the adsorbent at the/ront of the column than by that at the end. the transition zone from the less active to the more active adsorbent is moi or less con-tinuuub and inuvoa gradually duwn the column as more an more water is added. Because of this, the sample components are coitipressed into... 

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. 

Figure 3.4 Time dependence of the concentration fluctuation during demixing with successive increases in the quench depth (quench rate = 0.6K/s). (Reproduced from [36])...

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. 

Eqn. (8.6) describes the steady state concentration profile of an (A, B) alloy which has been exposed to the stationary vacancy flux j°. The result is particularly simple if the mobilities, b are independent of composition, that is, if P = constant. From Eqn. (8.6), we infer that, depending on the ratio of the mobilities P, demixing can occur in two directions (either A or B can concentrate at the surface acting as the vacancy source). The demixing strength is proportional toy°-(l-p)/RT, and thus directly proportional to the vacancy flux density j°, and to the reciprocal of the absolute temperature, 1/71 For p = 1, there is no demixing. 

In concluding, let us comment on the time needed to attain the steady state after establishing the surface activities. Two transient processes having different relaxation times occur I) the steady state vacancy concentration profile builds up and 2) the component demixing profile builds up until eventually the system becomes truly stationary. Even if the vacancies have attained a (quasi-) steady state, their drift flux is not stationary until the demixing profile has also reached its steady state. This time dependence of the vacancy drift is responsible for the difficulties that arise when the transient transport problem must be solved explicitly, see, for example, [G. Petot-Er-vas, et al. (1992)]. 

Figure 8-11. Results of a demixing experiment for (Mn,Fe)On. a) Theoretical steady state concentration profile and b) phase sequence photograph. p Q = 3.3xlO 6bar p o = 4,6x 10 bar T = 120O°C sp = spinel (Fe,Mn)304, w = wiistite (Fe, Mnjo [Y. Ueshima, et at. (1989)].

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