ABSTRACT
The phase diagrams of polymer-polymer blend systems and polymer-solvent mixtures can be constructed as a function of temperature versus composition. The mixture will try to reach a state where the free energy of mixing of the blend will be minimized. From this point, when the temperature is increased the free energy will change. Often, when the free energy of the blend is less than or equal to zero, the blend would be a miscible one-phase mixture. As the temperature is increased, the free energy of the blend changes in a monotonic and continuous manner. At some point, the free energy of the blend will be greater than zero. This is the onset of phase separation. The loci of all such points can be seen to be the lower critical solution temperature (LCST). Further increase in temperature again may change the free energy in such a fashion that it may become negative at a third temperature. The loci of such points can be seen to be the upper critical solution temperature (UCST).
