Polymer solutions display a typical phase diagram as illustrated in Fig.8.1a, which
exhibits a highest critical phase separation temperature, calledupper 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 themonotectic triple
point. At this point, a dilute solution, a concentrated solution and a crystalline phase
can coexist.
A single-component system often has three basic phases such as gas, liquid and solid.
If the conventional temperature-versus-pressure phase diagrams of the single compo-
nent are redrawn according to temperature-versus-density, as illustrated in Fig.8.2, one
may recognize that the phase diagrams are quite similar with those drawn in Fig.8.1c.
The UCST-type critical point and the triple point locate at parallel positions. Therefore,
we can make an analogue between single-component systems and polymer solutions.
The condensation process for the single component from gas to liquid resembles to the
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
148 8 Statistical Thermodynamics of Polymer Solutions