Mechanisms of phase separation

The mechanism of phase separation proposed here (and also observed experimentally) involves the formation in the first stage of polymer blanks1, the globules size depends on the initial comonomers and the copolymerization conditions. In the case of slow phase separation proceeding near the thermodynamic equilibrium... 

The gels contain inhomogeneities with a characteristic size of about 1 um Under favorable circumstances, this structure can be visualized in the wet gel by epi-illumination microscopy. When it can be visualized, the structure has a "spongy appearance, consistent with what one might expect of a system that has undergone spinodal decomposition. So far, however, there is no concrete evidence to support this mechanism of phase separation. 

Important aspects of polymer I/monomer Il/polymer II ternary phase diagrams were determined for the system cross-Dolvbutadiene-inter-cross-polystyrene as monomer II, styrene, is polymerized. Information on the mechanisms of phase separation suggest first nucleation and growth, followed by a modified spinodal... 

Relying heavily on their established expertise In the field of polymer solution thermodynamics, Ramlde and Manabe (12) take a more detailed look at the mechanism of phase-separation which occurs during membrane formation. In this context, phase separation Is the process which takes place during phase Inversion. A theory Is developed which allows for the prediction of pore characteristics... 

Most of the research conducted with aqueous two-phase systems has been experimental and empirical few studies of the fundamental thermodynamic mechanisms of phase separation and partitioning have been conducted (5, 23, 24). Furthermore, the systems which have been described use highly purified, expensive polymers, for model laboratory-scale applications. Novel bioseparation research based on aqueous two-phase systems needs to focus more on fundamental aspects needed to design phase diagrams and calculate partition coefficients. This... 

Spinodal demixing was excluded as a possible mechanism of phase separation during polymerization in solutions containing less than 12% rubber by volume [250]. 

On the other hand, near the critical composition, the binodal and spinodal merge so that the metastable region reduces considerably in size. In this case, it is easier to pass directly into the unstable region by a thermal excursion before phase separation begins. Once in the unstable region, spinodal decomposition is the mechanism of phase separation. 

Figures 7 and 8 were obtained with a 25% SAN composition near the critical composition. At the critical composition, roughly equal phase volumes are expected. The SAN-rich phase constitutes roughly 35% of the phase volume in the micrographs of Figure 8. However, the key to the mechanism lies not in the phase volume ratio rather, it lies in the observed kinetics of the phase separation process. Figure 7 shows clearly that phase separation proceeds by a gradual change in composition over fairly well-defined regions in space. The scale of the phase separation (500-1000 A) is quite consistent with the data in Table II for spinodal decomposition in polymer-polymer systems. These observations, along with the observed high level of phase interconnectivity, are all consistent with theoretical predictions based on the Cahn theory, and they confirm semiquantitatively that spinodal decomposition is indeed the mechanism of phase separation.

The mechanism of phase separation is analyzed from the R vs. q dependence. The dynamics of phase separation within the SD domain starts with balance between the thermodynamics and material flux. The mean field theory of phase separation leads to the following simple form of the virtual structure function, S(q) [Cahn and Hilliard, 1958] ... 

In Fig. 16.13, the simulated structures are shown for the 70/30 sPP/EPDM mixture under the same conditions as described in the preceding case. The asymmetry in the composition results in the change of mechanism of phase separation from the spinodal to the nucleation-growth mechanism. Now the minority EPDM-rich phase forms the droplets in the continuum of sPP-rich phase. As can be anticipated, the PP crystallization lags behind the liquid-liquid phase separation because phase separation must occur first in order for the sPP phase to reach or exceed its critical... 

Phase separators may be classified according to the different mechanisms of phase separation. The earlier separator designs were all based on differences in densities of the immiscible phases. These are designated here as gravitational separators. Later, separator designs were introduced which were based on different affinities of the immiscible phases to a separation membrane, hence the name, membrane phase separators. The latter come closer to the requirements of an ideal phase separator, and are used almost exclusively nowadays under a variety of different designs, including sandwich and tubular membrane separators. 

Since the simplest oil-in-water (O/W) and water-in-oil (W/O) microemulsions are ternary systems in which the particles are swollen direct and reverse micelles, respectively, the examples given for the application of electrical birefringence will include both microemulsions and micelles. As the studies reveal, the experiments are usually carried out to find answers to specific questions instead of the complete physical characterization of the particles. Often, however, interesting additional information is derived such as the mechanism of phase separation or the elasticity constant of the monolayer in W/O microemulsions. 

General Mechanism of Phase Separation Induced by Chemical... 

Fig. 3.3 Mechanism of phase separation during membrane formation.

Phase separation of polymer blends and block copolymers. Confining polymer blends and block copolymers between surfaces may influence the phase separation process, as a consequence of the preferential affinity of one of the components for the interface. Since the pioneer works of Reich and Cohen [26] and later by Nesterov et al. [27], Ball and Essery [28], and Jones [29] amongst others much work has been done to understand the mechanisms of phase separation in polymer thin films. The presence of substrate-film and/or film-air interfaces introduces an additional complexity compared to bulk phase separation processes [30-35]. Complex structures can be produced by slight differences on parameters... 

Within the spinodal line, any small fluctuation in composition will lead to a lowering of the free energy under these conditions phase separation will proceed immediately via a mechanism of amplification of random composition fluctuations called spinodal decomposition (Binder 1991). In the metastable part of the phase diagram a small composition fluctuation actually raises the free energy and, in order to begin the phase-separation process, a droplet of the minority phase, of a size greater than a critical size, has to be nucleated. Thus this mechanism of phase separation is known as nucleation and growth. 

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