Phase separation arrested

There seems to be a sort of analogy here with the arrested phase separation of a protein-stabilized depletion-flocculated emulsion containing a thermodynamically incompatible hydrocolloid like xanthan gum (Moschakis et al., 2005 Dickinson, 2006b). 

Foffi, G., C. de Michele, F. Sciortino, and P. Tartaglia. 2005. Arrested phase separation in a short-ranged attractive colloidal system A numerical study. Journal of Chemical Physics. 122, 224903. 

FIGURE 7.10 Snapshots of the AO system with arrested phase separation, at volume fraction

[Pg.149]

In practice, colloidal systems do not always reach tlie predicted equilibrium state, which is observed here for tlie case of narrow attractions. On increasing tlie polymer concentration, a fluid-crystal phase separation may be induced, but at higher concentration crystallization is arrested and amorjihous gels have been found to fonn instead [101, 102]. Close to the phase boundary, transient gels were observed, in which phase separation proceeded after a lag time. 

In the micellar region the trend to decreasing colloid stability is arrested and a partial improvement, in line with the enhanced level of polymer adsorption, is noted until the conditions for gross phase separation are reached. Only the intermediate block copolymer BC 42 shows indications of discontinuities in behavior at the solvent composition for micelle formation. The results presented here do not show the sharp transition from stability to instability found experimentally (4,8,17) by Napper and generally expected on theoretical grounds. However, there are important differences in experimental methodology that must be emphasised. 

A secondary phase separation may take place inside the (3-phase, and due to diffusion effects it leads to a sub-structure of thermoset-rich domains. Secondary phase separation may be arrested by gelation or vitrification of the (3-phase. 

Blend solutions. Solutions of blends comprising immiscible polymers Pj and P2 in a nonselective solvent have miscibility gaps as shown schematically in Fig. 14. When the polymer concentration increases by solvent evaporation the polymer coils start to interpenetrate above a certain concentration. As a consequence, interactions between the polymers become operative and phase separation must start above a critical polymer concentration p. The composition of the new phases will be situated on the branches of the coexistence curve. Finally, the unmixing process is arrested owing to enhanced viscosity. This simple scheme reveals the factors directing morphology evolution in blend solutions ... 

Cure of Plates. In an oven, -240 g of blend in a 6-mm-thick mold was precured at 180, 150, or 120 X for a time necessary to arrest the phase separation, and then it was postcured with the same cycle at nigher temperatures until completion of the polycondensation of CE monomer without any degradation. The evolution of the real temperature of the plate was followed by use of a thermocouple. 

In the copolymers described in Sect. 3.4, the multiple components of the system are joined by chemical bonds and demixing, needed for complete phase separation of the components, is strongly hindered and may lead to partial or complete decoupling from crystallization. The resulting product is then a metastable micro- or nanophase-separated system with arrested, local equilibria. In some cases, however, it is possible to change the copolymer composition during the crystallization or melting by chemical reactions, such as trans-esterification or -amidation. In this case, the chemical and physical equilibrium must both be considered and a phase separation of the copolymer into either crystalline homopolymers or block copolymers is possible. 

LL phase separation may play a key role in the gelation of some noncrystalline polymers such as atactic polystyrene (a-PS) [34-36]. For a-PS, the physical junctions are not crystals and there are no specific interchain interactions, yet a-PS can form gels. Amauts and Berghmans were the first to explain the gelation of a-PS in terms of LL phase separation combined with vitrification [36]. In this case, vitrification acts as the agency responsible for arrest of the LL phase separation, and locks the system in a metastable state. 

Fig. 4 Schematic phase diagrams of a polymer solution showing LL phase separation with UCST behavior. Curve s is the spinodal, curve b is the binodal, and curve g is the glass transition temperature as a function of polymer concentration. BP indicates the Berghmans point, (a) LL phase separation is the only thermodynamic transformation of the system [17,25, 36]. (b) Curve c shows the crystallization temperature of a polymer fully miscible in a solvent as a function of concentration in the solution [17, 25], The LL phase coexistence curve (combined with vitrification) is a (classical) metastable process that lies beneath the crystallization curve c. In route 1, a polymer solution is supercooled at ALj, and the only active process is polymer crystallization. In route 2, the initially homogeneous solution is supercooled to a larger undercooling than namely AL2. Crystallization may compete either with LL phase separation when reaching point C, or LL phase separation coupled with vitrification when reaching point D. At C, crystallization may take place in the polymer-rich phase. At D, both LL phase separation and crystallization may become arrested by vitrification...

A second consequence of vitrification is that morphological development is arrested and all further compositional changes cease, and the glass transition becomes invariant with composition. If, at the moment of the arrest of LL phase separation, the vitrified phase has become connected throughout the macroscopic sample volume, the solution converts into a gel [17, 25]. Possible limiting morphologies of these gels are illustrated as an example in Fig. 5. 

The microstructure of the modified epoxy networks largely depends on the cure condition. For a multistep cured system, the final microstructure depends on initial cure temperature, and post-curing condition has no role in microstructure development because phase separation gets arrested at gelation. An increase in initial cure temperature causes increase in particle size due to a decrease in system viscosity. 

These observations are similar to those observed in a rubber-toughened epoxy system. Yamanka and co-workers [114] observed a phase-inverted co-continuous structure by decreasing the cure temperature in their PES-modified epoxy system. They reported that a decrease in the cure temperature slowed down the rate of phase separation (based on spinodal decomposition process) without significantly reducing the rate of the chemical reaction. This arrested the phase separation at an early stage of phase separation and finally resulted in an interconnected globular epoxy particle in a co-continuous modifier-rich matrix. 

Fig. 4.24 Sketch of a gelation by arrested spinodal phase separation. A space spanning network of colloidal spheres is aggregated through depletion of non-adsorbing polymer chains...

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