The Mechanism of Phase Separation

Thermally induced phase separation (TIPS) an increased reaction temperature, or a high concentration, produces a high-connectivity phase structure so that the polymerization surpasses the phase separation using a low-temperature stage, a moderate decrease in phase separation is observed at a lower nucleation temperature (Kim et al. 2016 Padilla et al. 2011 Stieger etal.2003). 

Polymerization-induced phase separation (PIPS) (Yuhong 2013). 

The increasing size of the growing polymer molecules and the resulting morphology (stabilized by gelation in crosslinking polymerization) are the main factors affecting PIPS. The PIPS processes are influenced by thermodynamic and kinetic factors that can determine stable systems (where no phase separation is produced), metastable systems (when the phase separation may take place), or unstable systems (for which the phase separation does not take place). 

The phase separation mechanism during the porous crosslinked polymer synthesis assumes 

The sudden appearance of heterogeneity when the threshold concentrations of crosslinker and diluent are achieved 

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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... 

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... 

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] ... 

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. 

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... 

The determination of particle sizes in the process of phase separation gives information about the mechanism of phase separation kinetics. 

It should be noted that measuring particle sizes using a turbidity spectrum in the course of phase. separation gives additional and very important information about fine details of separation. In particular, the character of the time dependence of new-phase particle sizes allows one to judge the mechanism of phase. separation, but these questions concern phase. separation kinetics which is not the subject of the present book. 

The inhomogeneous character of CTosslinking polymerization is not a sufficient condition for the formation of the two-phase structure (the second phase refers to diluent or voids after drying). Therefore, the mechanism of phase separation (Hsiao et al. 1995) that appears near the limit of the thermodynamic stability is very important. 

AND Anderson, V.J. and Jones, R.A.L., The influence of gelation on the mechanism of phase separation of a biopolymer mixture. Polymer, 42, 9601, 2001. 

The compatibility measurement, Cj, of the morphology at the solution-substrate interface, is plotted with the respect to time in Figure 15.29, and with respect to the thickness direction in Figure 15.30. It can be seen that there exists a critical time during the evolution of Cj, after which the mechanism of phase separation... 

The ternary model was established to investigate the effects of a solvent in polymer blends during phase separation. In cases of constant solvent concentration it emerged that, the less solvent that was in solution, the slower was the evolution of the morphology in phase separation. This effect was due to the polymers being immiscible with each other, but both being miscible with the solvent. The addition of a solvent decreased the free energy level in the blend, which in turn slowed down the evolution of phase separation. The mechanism of phase separation with solvent evaporation was further complicated by dynamic solvent evaporation from the ternary system. 

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 that are effective during a change, i.e., the mechanisms of phase separation. 

Phase Separation Kinetics. There currently is considerable interest by the polymer physics community in the mechanism of phase separation in polymer blends. In an early study, Kwei, Wang and Nishi [4] employed pulsed NMR and found that the Cahn-Hilliard linearized non-statistical theory of spinodal decomposition [82] adequately described the process at short times the average composition of the two phases changed exponentially with time while their volume fractions remained constant. In addition, it was observed that no change in phase volume fraction took place at longer times when the linearized model no longer was valid. 

Various types of IPNs may also be classified by the mechanism of phase separation proceeding during IPN formation. These mechanisms are nucle-ation and growth, and spinodal decomposition. Differences in the conditions of phase separation predetermine the physical and morphological features of IPNs. As a rule, simultaneous IPNs are phase-separated by a spinodal mechanism, and sequential ones via a mechanism of nucleation and growth. Another approach to IPN nomenclature was proposed by Sperling [1], who used the differences in morphological features of IPNs. 

The comparison of the phase behavior of semi-IPNs based on almost miscible polymers—linear PS and poly-a-methylstyrene cross-linked by DVB and mixtures of the corresponding homopolymers—has shown that for homopolymer blends only one glass transition is observed, its position obeying the Fox equation. Simultaneously, for semi-IPNs there were two glass transitions, far from the glass transition temperatures of the components, for the same compositions where linear blends are miscible. A difference in phase behavior between blends and semi-IPNs seems to be evident. However, no phase diagrams allow one to determine the mechanism of phase separation. 

For simultaneous semi-IPNs made from PU and PS, the kinetics of phase separation was studied using optical microscopy completed by image analysis [90]. The development of a nodular structure was observed. A thermodynamic approach has allowed us to establish that the diameter of the phase-separated species was the result of the competition between the kinetics of network formation and the kinetics of phase separation. The mechanism of phase separation was not discussed. 

Generally, the following conclusion can be drawn from the analysis of the mechanisms of phase separation in IPNs. The structures arising at different... 

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