Phase Separation Phenomena

Phase separation in bulk mixtures commonly leads to an isotropic, disordered morphology of the coexisting phases [34]. The presence of a surface can significantly alter the phase-separation process, however [35,36]. 

Phase separation can emerge spontaneously in thin films of polymer blend, and it is this thermodynamically driven phase-separation process that produces the patterning of thin polymer blend films. It can be induced by several parameters, each with critical values, including (i) the ratio between the components [10,22] (ii) the demixing temperature [12] and (iii) the chemical nature of the solvent. 

The phase separation of thin polymer blend films can be also driven by substrate effects, including the effects of (i) the substrate composition (ii) the substrate functionalization and/or (iii) the substrate structures. In particular, the self-assembly of polymer molecules on a substrate can be controlled by selectively modifying the chemistry of the substrate surface and/or the polymers to create patterned polymer films with tailored length scales. 

F re 18.3 ToF-SIMS chemical maps of the sum of the intensities of the most typical signals polystyrene (PS) (a) and the sum of the most typical signals of poly(2-vinylpyridine) (P2VP) (b) (c) Overlay image between the sum of the intensities of the most typical signals of 

PS (blue) and the sum of the most typical intensities signals of P2VP (red). TIC, total ion current (d) ToF-SIMS chemical maps refer to the film obtained by the 1/15 weight ratio P2VP-PS mixture. Reproduced with permission from Ref [22]. 

In spinodal decomposition, phase separation does not arise due to nucleation and growth of spherical domains, but rather arises from fluctuations in concentration such that phase separation arises in intercoimected regions. Sperling (2001) has highlighted the differences between the two methods of phase separation in terms of 

Criteria for miscibility have been developed using various experimental techniques that have specific sensitivities to the scale of miscibility (or phase separation). An example is the glass-transition temperature (Section 1.1.4). Only a single glass-transition temperature should be detected for a single-phase, fully-miscible blend. As phase separation occurs 

Phase-separated polymers are of practical importance. An example is the way that a brittle polymer such as polystyrene may be toughened by the addition of an incompatible rubber that undergoes phase separation. Provided that this is done with attention to the interface between the two phases, a rubber-toughened high-impact polystyrene (HIPS) will result. 

On changing the length of the two blocks, the morphology changes from spheres to cylinders to lamellae as follows  

In addition to these morphologies, perforated layers, bicontinuous layers and other unstable morphologies have been described (Sperling, 2001). It has also been noted that, unlike polymer blends that become less miscible at higher temperatures, block copolymers become more miscible due to the temperature dependence of the Flory-Huggins parameter, x- 

A negative free energy change is necessary but not a sufficient condition for homogeneity between two polymers. More appropriately the shape of AG as a function of concentration of one of its constituents at a temperature T describes homogeneity or heterogeneity of the mixture as discussed in Section 8.2. 

As mentioned earlier, temperature becomes a very important factor for complete miscibility of two liquids when 0. Partial miscibility of binary liquid systems is a well-known phenomenon. For example, hexane and aniline are completely miscible at a temperature above 60 C [2]. But below this temperature complete miscibility of the two liquids depends on the composition of the mixture. Any system having a composition given by a point under the bellshaped binodal curve shown in Fig. 1.2 will separate into two conjugated liquid 

Amorphous polymers have a thermodynamic behavior that is very similar to that of highly viscous liquids [19]. Their miscibility with poor liquid solvents resembles that of partially miscible liquids. Obviously, there is a strong molecular-weight effect and a binodal curve exists for each homologue of a given polymer, as shown in Fig. 1.3 [20]. As molecular weight increases, the UCST increases and the maximum of the binodal curve is shifted to low polymer concentration. A relation between UCST (TJ and molecular weight (M) has been derived [8] and is expressed as 

Phase diagram for four fractions of a polymer in a poor solvent. CP , are UCST points for bi-nodals corresponding to 

The preceding discussion on phase equilibria may explain the limited use of some plastic materials at extreme temperature conditions. A perfect indoor building material made of a plasticized polymer may become a complete mess when used as outdoor building material in countries where temperature goes from -60°C in winter to 35°C in summer, as in some northern parts of Canada. 

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This type of analysis is useful for studying the phase separation phenomena, also in detecting influences of external fields [218] (strain, etc.). In particular, by investigating speckles on scattering patterns, a certain information about interface fluctuations can be obtained [214]. 

Phase separation (precipitation) of a polymer strongly depends on all its molecular characteristics. On the one hand, this allows very efficient separations in polymer HPLC utilizing phase separation and re-dissolution processes [20]. On the other hand, due to complexity of phase separation phenomena, the resulting retention volumes of complex polymers may simultaneously depend on several molecular characteristics of separated macromolecules. This may complicate interpretation of the separation results. Both precipitation and redissolution of most polymers is a slow process. It may be affected by the presence of otherwise inactive surface of the column packing. Therefore, the applicability and quantitative control of the phase separation phenomena may be limited to some specific systems of polymer HPLC. 

When two different polymer solutions are mixed, they frequently undergo one or several distinct types of interaction, which in each case can lead to phase separation at polymer concentrations above a certain critical level [12]. In one case, two solution phases of approximately equal volume are formed, consisting of polymer A- and polymer B-rich solutions, respectively. This phase separation is called incompatibility, or simple coacervation. In the second case, two phases are formed but both polymers are concentrated in one of the phases (the precipitate ) while the other phase (the supernatant ) may be essentially polymer free. This separation is called complex coacervation. The two phase separation phenomena are shown in Fig. 2. 

ORBITAL ORDERING AND PHASE SEPARATION PHENOMENA IN LIGHTLY DOPED MANGANITES... 

Orbital ordering and phase separation phenomena in lightly doped manganites... 

In the case of polymers which are miscible due to specific interactions the free energies involved are an order of magnitude greater, their change with temperature is also greater, and phase separation phenomena are much more well defined processes. 

On surveying the above results it is obvious that the very simple model introduced here provides a semiquantitative framework for understanding the origin of phase separation phenomena in real binary solutions. 

A theoretical approach to the formation of porous polymeric membranes is demonstrated through the phase separation phenomena of polymer solutions. 

Segregated polymer brushes were fabricated utilizing phase-separation phenomena in ultrathin polymer blend films (PS-COOH/PMMA) and the combination of grafting-to and grafting-from methods. 

We may draw a close analogy here between the behaviour of colloidal dispersions and molecular systems. Thus the first case discussed above is analogous to the presence of clusters of molecules in a vapour approaching its condensation point, or in a solution close to saturation. The limiting concentration at which flocculation occurs corresponds to the saturation vapour pressure, or to the solubility of a solid in solution. More complex colloidal systems often exhibit phase behaviour which is paralleled by various phase separation phenomena in molecular systems. Detailed discussion ofthese matters is outside the scope of this book. However, pursuit of these analogies and their interpretation is a currently active area of research. 

In this study, we have concluded that both the crystallite growth and the phase separation of CeosZro502 have been inhibited by doping A1 ions. The lattice strain is another key factor to induce the phase separation of Ceo.sZro 5O2 beside the particle size effect. The phase separation phenomena of Ceo.5Zro.. 02 will be further systematically studied based on the crystalline structure, defect chemistry and thermodynamics to clarify this issue. 

Addition of an anti-solvent to a polymer solution causes the polymer solution to split into a polymer-rich phase and a solvent-rich phase. When a non-solvent is added the overall density of the original solvent becomes lower, which decreases the Lower Critical Solution Temperature (LCST) of the solution. A liquid-liquid phase-split thus occurs without raising the temperature. A low-molecular weight anti-solvent like CO2, propane or ethane can effectively decrease the LCST of the polymer solution (6) and thus induce a liquid-liquid phase-split. It is due to this effect that Gas Anti-Solvent precipitation of polymers has focused on the production of polymer particles with a specific size, structure or shape such as micro tubes (7) or micro balloons (8). Phase separation phenomena in PPE solutions during the formation of polymer membranes by the addition of a conventional anti-solvent have been described by (9). 

Wijmans J.G., Rutten H.J.J., Smolders C.A., Phase separation phenomena in solutions of Poly(2,6-dimethyl-l,4-phenylene Oxide) in mixtures of trichloroethylene, 1-octanol and methanol Relationship to membrane formation. Journal of Polymer Science polymer physics edition, 1985, (23), 1941-1955... 

Fractionation of polymers according to molar mass represents the most significant analytical application of phase-separation phenomena. The polymers of highest molar mass separate out first on lowering the temperature of a quasibinary endothermic dilute solution system. Of course, this precipitation represents the formation of a highly concentrated gel phase and a dilute sol phase. Successive decreases in temperature lead to further... 

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