Phase separation, polymer mixtures

When a colloid-polymer mixture phase separates into a coUoid-rich and polymer-rich phase an interface appears in between. For a colloidal gas-liquid... 

An additional complexity is the existence of two critical temperatures. When the temperature of an initially homogeneous mixture is lowered below the phase boundary, phase separation begins to occ-ur. The temperature of phase separation is dependent on composition and the critical point associated with the cloud point curve is called upper critical solution temperature. (UCST) For some polymer-solvent and polymer-polymer mixtures phase separation is also olDserved when the temperature of the mixture is raised.The corresponding critical point is called lower critical solution temperature (LCST),... 

Process in which one component of a polymer mixture, usually not a polymer, undergoes phase separation and migration to an external surface of the mixture. 

This mirrors the experience of everyday life, where if you try to get one thing to mix with another you often heat them (Figure 11-15). However, as we will see later, polymers can also give counter-intuitive behavior, where mixtures phase-separate upon heating ... 

Many investigators have opted to study polymer compatibility in solution in mutual solvents, because of uncertainty as to whether a bulk mixture is actually in an equilibrium stale. Compatible components form a single, transparent phase in mutual solution, while incompatible polymers exhibit phase separation if the solution is not extremely dilute. 

In Figure 20.17, we compare a polymer-surfactant mixture, a nonionic polyoxyethylene surfactant and dextran, with a related polymer-polymer mixture, poly(ethylene glycol) and dextran. We can see that the two mixtures phase-separate in qualitatively the same way. 

The segregative phase separation is not the only one observed for mixtures of a nonionic polymer and a nonionic surfactant. For the case of a less polar polymer, the phase separation can be associative, especially at higher temperatures, due to hydrophobic association. 

Figure 4.30. Schematic phase diagrams illustrating the different paths which may lead to phase separation. In (a), a mixture of two polymers undergoes phase separation on cooling. In (b), two polymers are dissolved in a common solvent and the solvent is then removed, leading to phase separation. In (c), one or both of the components polymerises, causing phase separation to oceur when a certain critical degree of polymerisation has been reached.

For the immiscible blended polymer type, phase-separated mixtures are obtained when one tries to mix most polymers. However, strangely enough, the phase-separated materials also turn out to be sometimes useful. The examples of immiscible polymer are polystyrene and polybutadiene. When polystyrene is mixed with a small amount of polybutadiene, the two polymers do not blend. Polybutadiene separates from the polystyrene into Uttle spherical blobs. 

Due to a difference in the state equations of two polymers, a phase separation region with a LCST should bo expected in their mixtures, which drastically shifts along the T axis when the differences (oi — 02) and (fli — P2) increase (l- igure 3.91). 

By the addition of non-adsorbing polymers to colloidal suspensions the mixture phase separates into a coUoid-rich and a polymer-rich phase, as discussed above. The understanding of this polymer-induced phase separation is very important, not only for colloid science but also for industrial systems, such as food dispersions [74, 77, 236, 237]) and paint [238]. In these systems colloids and polymers (or surfactants) are jointly present and influence the stability and hence related processing issues. 

Similar to the fluorous biphasic concept, a system was developed that formed a homogenous phase at elevated temperature, but phase-separated at room temperature [49c]. It was found that a mixture of functionalized PNIPAM polymer, ethanol, heptane and water exhibited these properties. The hydrogenation of 1-octadecene and 1-dodecene using a phosphine functionalized PNIPAM with a rhodium precursor were taken as test reactions and the high activity was foimd was similar to that of RhCl(PPh3)3. At room temperature the mixture phase separated and the catalysis stopped since the catalyst is completely insoluble in heptane. The substrate is dissolved in the heptane allowing a facile catalyst/product separation without the loss of activity. The concept is obviously limited to substrates that show... 

The dependence of AGm on composition cpi and x 12 for a binary blend consisting of two polymers of equal molar mass can be determined from equation 21, and it is similar to that reported for polymer solutions. As temperature increases, the specific interactions between the two components weaken owing to molecular motion and the mixture phase separates. A large number of polymer blends show LCS behavior represented in Figure fib. However, a UCST may still be observed in some polymeric binary systems, particularly low molecular weight nonpolar mixtures. In this case, even if the entropic term is small it is still larger than Aifm. and ASm governs the miscibility. 

Blending of two or more polymers to improve properties is becoming increasingly important. It can reduce the cost of an expensive thermoplastic, ease the processibility of a heat-sensitive plastic or improve impact resistance. It can be a homogeneous mixture, phase-separated systems or a combination of both. 

In the second section we shall briefly review more recent trends in molecular rheology, including attempts to deepen the understanding of entanglements on a molecular level, and the use of the reptation ideas to describe various practical problems such as polymer/polymer welding, phase separation of polymer mixture, flow behaviour of a polymer melt close to a solid wall... 

ABSTRACT. Many commercial materials contain mixtures of incompatible polymers. During synthesis or processing, these polymers undergo phase separation to produce structures which presumably play an important role in the properties and applications of these materials. One would like to have analytical tools capable of elaborating these structures and exploring the interfaces and interphases within the system. Tools based upon fluorescence and phosphorescence spectroscopy show considerable promise here, particularly those based upon emission quenching processes. In this chapter some of these applications to a prototype material will be described. 

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