Polymer-poor phase

Figure 8.3b shows that phase separation in polymer mixtures results in two solution phases which are both dilute with respect to solute. Even the relatively more concentrated phase is only 10-20% by volume in polymer, while the more dilute phase is nearly pure solvent. The important thing to remember from both the theoretical and experimental curves of Fig. 8.3 is that both of the phases which separate contain some polymer. If it is the polymer-rich or precipitated phase that is subjected to further work-up, the method is called fractional precipitation. If the polymer-poor phase is the focus of attention, the method... 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Macroscopically, the solvent and precipitant are no longer discontinuous at the polymer surface, but diffuse through it. The polymer film is a continuum with a surface rich in precipitant and poor in solvent. Microscopically, as the precipitant concentration increases, the polymer solution separates into two interspersed Hquid phases one rich in polymer and the other poor. The polymer concentration must be high enough to allow a continuous polymer-rich phase but not so high as to preclude a continuous polymer-poor phase. 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

Process in which a polymeric material, consisting of macromolecules differing in some characteristic affecting their solubility, is separated from a polymer-rich phase into fractions by successively increasing the solution power of the solvent, resulting in the repeated formation of a two phase system in which the more soluble components concentrate in the polymer-poor phase. 

Although PEG is water soluble, upon varying the temperature of a solution it can form distinct polymer-rich and polymer-poor phases. This is due to the hydrophobic methylene groups along the backbone of the polymer (Figure 8.1), interspersed with the hydrophilic ether groups and alcohol end groups. This... 

In coacervation by Polymer 2-Polymer 3 repulsion, the addition of Polymer 3 causes phase separation between the two polymer species dissolved in a common solvent 1. This phase separation produces a viscous, liquid phase of Polymer 2, i.e., the coacervate, and a low-viscous phase of Polymer 3, often called continuous or polymer-poor phase. Under stirring, coacervate droplets are formed and dispersed in the continuous phase. The solubility of Polymer 3 in solvent 1 should be superior to that of Polymer 2 in this common solvent. For particle production, the Polymer 3 should also function as stabilizer for the coacervate droplets to prevent their aggregation. Further, for the entrapment of a biologically active material, the coacervate must have a certain degree of fluidity and a high affinity to the core material, whereas the affinity between core material and continuous phase should be low... 

When the solution becomes unstable it splits into two phases, which are immiscible between them. In one phase, the polymer concentration is high (polymer-rich phase) and in the other it is low (polymer-poor phase). Phase separation process is known as liquid-liquid demixing. 

The time required to start and the rate at which decomposition occurs are crucial in the morphology of the membrane as during demixing, nuclei of the polymer poor phase originate the pores. 

There are several ways to prepare porous polymer scaffolds by controlled phase separation of polymer solutions in a polymer-rich phase and a polymer-poor phase. 

The growth of a polymer-poor phase by SD or NG is an isotropical process, which takes place as soon as the solvent-nonsolvent contents supply the thermodynamic condition for dembdng. To understand the macrovoid formation, a quite interesting explanation was provided by McKelvey and Koros [28] as de-... 

Kim et al. discussed surface structure and the phase separation mechaiusm of polysulfone membranes by AFM [43]. A membrane formed by immersion in a pure water coagulation bath showed a nodular structure with a nodule size of about 25 nm, which was believed to be the result of spinodal decomposition. A membrane formed by immersion in a coagulation bath mixture (water/NMP 20/80 by weight) had the porous structure with a mean radius of 146 nm, which was the result of nucleation and growth of the polymer-poor phase. 

When a cast film is immersed in a coagulation hath, the casting solution at the surface that is in contact with the coagulation media will spht into two phases, i.e. polymer-poor phase and polymer-rich phase [36]. After solidification, the polymer-poor phase will become pores, while the polymer-rich... 

In thermal concentrated solutions, polymer chains in a good solvent behave similarly to those in an athermal solvent, while in a poor solvent, polymer chains experience a phase separation with the coexistence of polymer-rich and polymer-poor phases. We will give more descriptions about the phase separation behaviors in Chap. 9. 

The gelation mechanism as a result of phase separation is schematically illustrated in Fig. 84 for solutions of monodisperse polymer. Upon cooling a diluted solution, as presented, for example, by point A, phase sq>aTation will take place as soon as the temperature crosses the binodal at point B. The rate of the phase separation process depends on the presence of nuclei as long as the spinodal curve (not illustrated in Fig. 84) is not reached. When the mint C is reached, two phases will develop Ci, the polymer poor phase and C2, the polymer rich phase. In principle, in the long run two clearly distinct pha wiU be created the solvent rich phase is clear, whereas, in neral, the polymer rich phase will be turbid due to the presence of solvent rich droplets which, even after months, are not phase parated due to the high viscosity of the polymer rich phase. 

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