Phase separation mechanism

This is the kinetic mechanism when the system phase separates within the metastable region. Phase separation is rather slow and will only occur in the presence of nucleating sites or finite temperature perturbations. With nucleating sites, the concentration profile and its evolution is shown in Fig. 1.4.2 Here, it is evident that there is downhill movement of material, as seen in the direction of the arrows. Also, the spacing of domains is dictated by the composition distribution of the nucleating sites. For a rigorous mathematical treatment of nucleation and growth, one can look into the works of De Fontaine (1975), Cahn (1956, 1957), Cahn and Hilliard (1958, 1959), Hillert (1961), Coriell and Sekerka (1983), Rasmussen (1982) and Rasmussen et al., (1982). 

As we have seen, binary polymer mixtures can vary in structure with temperature, forming either a homogeneous phase or in a miscibility gap a two-phase structure. We now have to discuss the processes which are effective during a change, i.e. the mechanisms of phase transition. 

we have introduced the free energy density, i.e. the Gibbs free energy per unit volume, denoted p(0). Series expansion of p(0) up to the second order in 0 yields for 6Q the expression 

We calculate d g/d(fp with the aid of the Flory-Huggins equation, i.e. write 

This is a most interesting result. It tells us that, depending on the sign of the curvature the fluctuation may either lead to an increase, 

Equation (3.123) emanates from the view that is set up by two contributions, one being related to the gain in the bulk Gibbs free energy of the precipitate, the other to the effect of the interface between particle and ma  

It is exactly the latter situation which results in a spinodal decomposition. The process is sketched at the bottom of Fig. 4.13. The drawing indicates 

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Phase Separation Mechanism in Hexane-Epoxy Systems. . Influence of Reaction Parameters on the Morphology of Cyclohexane-Modified Epoxy Networks Prepared via CIPS... 

Within the following section it is intended to identify the phase separation mechanism and morphology development and to investigate the effect of reaction parameters on pore size and distribution. 

Phase Separation Mechanism in Hexane-Epoxy Systems... 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

The observation of a bimodal size distribution is the key to unravel the phase separation mechanism with respect to the kinetics of the two types of phase separation processes. 

The collage of electron micrographs shown in Figure 19 Illustrates a probable transition from a nucleation and growth phase separation mechanism to a spinodal decomposition. The 7% PS sample shows more or less spherical PS domains, characteristics of nucleation and growth. However, a few domains appear to have some elliptical characteristics. When the PS content increases up to midrange, the shape of the PS domains becomes more obviously elliptical, suggestive of cross-sections of cylinders or other worm-like structures. As the PS content increases further, the... 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

Relatively few papers have reported the phase separation mechanism during curing of toughened thermosets [1-3, 7-9, 32-40], and the phase separation mechanisms is not well understood. 

Figure 3.1 Phase diagram illustrating the phase separation mechanisms of thermoplastic/thermoset blends at constant cure temperature...

In their paper, Inoue et al. [36] proposed a new approach about phase separation mechanism under successive increases in quench depth. Figure 3.3 shows schematic... 

From the experimental viewpoint it may be said that the resolution in the separation with respect to molecular weight, which is attainable by the molecular sieving mechanism in TLC, is not quite as high in comparison with that by the phase-separation mechanism. An attempt has been made to improve the resolution by increasing the thickness of thin layer from 0.25 mm to 1.0 mm29. An improvement of the resolution was really observed with increase in the thickness. Because of the technical difficulty, however, the thickness of thin layer was limited up to some 1.0 mm so that the best resolution was lower than that observed by GPC which was performed under analogous conditions to the TLC experiment. 

As described before, TLC separation based on the phase-separation mechanism causes fractionation primarily by molecular-weight differences. The resolution attained by this mechanism in TLC will be discussed below by comparing it with those attained by GPC and velocity ultracentrifugation. To this end, some findings obtained by Miyamoto etal. during the course of a study on TLC separation of star-shaped polystyrenes96 97) are referred to. 

Fig. 11. Chromatograms obtained by phase-separation mechanism in TLC for sample S-A, its precursor (linear) polystyrene (S-AP), and reference polystyrene samples (L-02 and L-05). For details, see text...

Fig. 12. Chromatograms obtained by phase-separation mechanism for starshaped (S-series) and linear (L-series) polystyrene samples...

A wider choice of solvents to be used as developer is possible, and in certain cases, the TLC method is much more sensitive to a small amount of satellite component (cf. Section V.l.). This section is devoted to a summary of some results on the determination of molecular-weight distributions by the phase-separation mechanism in TLC. 

At exactly the same hydrostatic pressure an anomaly has been measured for the transition temperature and it has been attributed to a deformation of the Cu05 pyramids [31], This structural modification affects several phonons but mainly the double peak at -458-465 cm 1. The splitting of this mode must again be related with a phase separation mechanism, similar to the ones discussed above for the other cuprates. 

One weakness of the dominant reverse phase separations mechanism has been the poor retention of highly polar analytes, and hydrophilic interaction liquid chromatography (HILIC) has emerged as an alternative. In HILIC, a polar stationary phase such as silica gel is used to retain highly polar analytes. Mobile phases components similar to those described above for reverse phase separations are used, but the proportions of aqueous vs. organic are changed. Analytes are retained under conditions of relatively low water content, and eluted using increased water content. 

Assuming that different polymorphisms can be found in the extractant systems, a better understanding also comes from other phase-separation mechanisms studied in classical amphiphilic systems such as soaps and lipids. The first, largely described here, is the phase separation resulting from increased attractive interactions. The second occurs when a sphere-to-rod transition is observed for the shape of the aggregates. The attraction between cylinders is higher than between spheres when attraction is dominated by van der Walls (VdW) forces between polar cores (119). For micellar solutions (reverse or not), the liquid-liquid phase transition cannot be unambiguously attributed to either shape or attractive interactions only, as it appears that these two effects coexist in nonionic surfactants solutions (91, 120-123). 

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