UCST-type phase diagram

The effect of a simple shear flow on the phase behavior and morphology was investigated with the use of a parallel-plate apparatus (Fig. 8.4, Madbouly et al. 1999a) for some polymer mixtures poly(methyl methacrylate) (PMMA)/ poly(styrene-co-acrylonitrile) (SAN-29.5) and polystyrene (PS)/poly(vinyl methyl ether) (PVME), which have an LCST-type phase diagram PS/PMMA, which has a UCST-type phase diagram and polycarbonate (PC)/SAN and nylon4, 6(PA4,6)/ poly(phenylene sulfide) (PPS), which are immiscible in the whole measurable region under the quiescent state. 

Experimental Methods The cloud-point method is commonly used to determine the phase diagram. Let us consider a solution that has a UCST-type phase diagram. We prepare solutions at different concentrations and bring them into a single phase by heating. The solutions are then cooled slowly. In Figure 2.21, the... 

Figure 2.21. Cloud point is defined as the temperature at which the solution becomes turbid as the solution in a single phase is brought into the two-phase regime. Illustration is given for the UCST-type phase diagram.

Figure 2 3. Relationship between the theta temperature Tj with the critical temperature T. a UCST-type phase diagram, b LCST-type phase diagram. The second virial coefficient A2 changes its sign at T = Tj.

A binary mixture of deuterated polybutadiene (DPB) and polybutadiene (PB) was used to investigate the phase-separation processes of SD. Detailed information about the DPB/PB mixture, e.g., molecular characteristics, phase diagram, and sample preparation, can be found elsewhere [72]. Note that the DPB/PB mixture used here has an upper critical solution temperature (UCST)-type phase diagram and it requires homogenization by mechanical mixing [73] in order to bring the system to the single-phase state. The thickness of the mixture, D, was 200 fim. 

A unique situation can be observed with a UCST type phase diagram. In this particular case the blend is miscible above Tn, but displays a miscibility gap be-low.(69,70) There is no equilibrium basis for liquid-liquid phase separation to take place below the melting temperature-composition boundary. However, it is possible for the melt to be sufficiently supercooled below Tn that liquid-liquid phase separation does in fact occur. Therefore, in a practical sense crystallization occurs in competition with liquid-liquid phase separation. 

When the iateraction energy density is positive, equation 5 defines a critical temperature of the UCST type (Fig. la) that is a function of component molecular weights. The LCST-type phase diagram, quite common for polymer blends, is not predicted by this simple theory unless B is... 

If available molecular weight combinations do not lead to observable phase-diagram boundaries of either the UCST or LCST type, then the interaction energy can only be estimated to He within upper and lower bounds using this technique (93). 

Phase equilibrium resulting in a UCST is the most common type of binary (liquid + liquid) equilibrium, but other types are also observed. For example, Figure 14.5 shows the (liquid + liquid) phase diagram for (xiH20 + jc2(C3H7)2NH. 7 A lower critical solution temperature (LCST) occurs in this system/ That is, at temperatures below the LCST, the liquids are totally miscible, but with heating, the mixture separates into two phases. 

In a UCST system, when the temperature is reduced to a final value 7/ that is below the critical temperature Tc, a mixture with a concentration 0 not too far from the critical composition phase separate into two phases whose compositions lie on the opposite sides of the binodal envelope line of Fig. 9-1. The dynamics of the separation process of a single phase into these two phases is controlled by Tf, the composition , the rate of the quench dT/dt, the viscous (or viscoelastic) properties of the phases formed, and the interfacial tension F between the two phases. Although a variety of different kinds of behavior can occur, there are two generic types of phase separation, namely, spinodal decomposition (SD) and nucleation and growth (NG). SD occurs when the mixture is quenched into a part of the phase diagram where the mixture is unstable to small variations in composition, leading to immediate growth of phase-separated domains. When the quenched... 

This figure clearly shows the temperature and composition windows where it is either a two-phase system or a single-phase system. The characteristic features of an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) corresponding to the phase transition are identified. For a particular composition of two immiscible polymers, if the temperature is increased, the UCST is the highest temperature at which two phases may co-exist in the blend. There is then a window of miscibility as the temperature is increased further, followed by phase separation again at the LCST. This type of diagram is often seen for polymer solutions, e.g. polystyrene in cyclohexane. Often polymer blends show... 

A third type of phase diagram, such as the one shown for molecular weight equal to 19,800, is called hourglass type and has been observed for only a few systems. Most polymer solutions only exhibit the UCST and LCST behavior. As the temperature increases for UCST and as the temperature decreases for LCST, the difference between... 

There is another type of phase diagram enconntered in some polymer solutions, where the LCST lies below the UCST. This is called closed-loop and appears for polymer solutions where hydrogen bonding effects are dominant snch as polyethylene gly-col/water and polyvinyl alcohol/water (Figure 16.4). 

Figure 7.4. Phase diagrams for type I (top) and type II (bottom) binary mixtures with carbon dioxide as one component (L = liquid and v = vapor). The UCST line indicates the temperature at which the two immiscible liquids merge to form a single liquid phase. The critical mixture curve is the locus of critical mixture points spanning the entire composition range. (From ref. [44] American Chemical Society).

According to the type of T versus q> diagram (Fig. 25.4), the binary solution can exhibit an upper critical solution temperature (UCST), a lower critical solution temperature (LCST), or both (close-loop phase behavior). Above the UCST or below the LCST the system is completely miscible in all proportions [82], Below the UCST and above LCST a two-phase liquid can be observed between cp and cp". The two-phase liquid can be subdivided into unstable (spontaneous phase separation) and metastable (phase separation takes some time). These two kinds of mixtures are separated by a spinodal, which is outlined by joining the inflexion points (d AGIdcp ) of successive AG versus cp phase diagrams, obtained at different temperatures (Fig. 25.3b). Thus, the binodal and spinodal touch each other at the critical points cp and T. ... 

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