Monotectic triple point

Fig. 8.1 Illustration of typical phase diagrams of polymer solutions for (a) UCST phase separation, (b) crystallization, and (c) monotectic triple point formed by the interception of two phase diagrams...

Polymer solutions display a typical phase diagram as illustrated in Fig. 8.1a, which exhibits a highest critical phase separation temperature, called upper critical solution temperature (UCST). Within the same temperature window, polymer solutions may also crystallize below the solution-crystal coexistence line, as illustrated in Fig. 8.1b. Two kinds of phase transitions will interplay with each other, so that an interception point is observed in the corresponding phase diagrams. The interception point is a three-phase-coexisting point, as illustrated in Fig. 8.1c, called the monotectic triple point. At this point, a dilute solution, a concentrated solution and a crystalline phase can coexist. 

When the L-S curve intersects with the L-L curve in the overlapping temperature windows, both curves will be terminated at the intersection point, which is referred to the monotectic triple point. The typical phase diagram in polymer solutions is shown in Fig. 4.5. 

In polymer solutions, liquid-liquid (L-L) demixing is another common phase transition besides crystallization. The thermodynamic boundary conditions for both of them behave as the functions of polymer concentrations and temperatures, demonstrated as phase diagrams. The schematic L-L binodal and liquid-solid (L-S) coexistence curves in polymer solutions and their interception are shown in Figure 13.2. The illustrated L-L binodal contains an upper critical solution temperature. Some other solutions also contain binodals with a lower critical solution temperature. When the L-S curve intersects with the L-L curve in the overlapping temperature windows, both curves are terminated at the intersection point, which is referred to as the monotectic triple point. 

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