Coexistence curve of polystyrene in methylcyclohexane. IV. Three-phase coexistence curve of ternary system

Three-phase equilibria have been studied for the ternary system polystyrene (Mw=1.73×104)+polystyrene (Mw=7.19×105)+methylcyclohexane by numerical analyses of Gibbs free energy and experimental measurements. The calculation showed that the three-phase equilibrium appears at the temperature range from 21.4 to 23.4 °C. At these two temperatures two phases of the three coexisting phases coalesce into one phase and phase equilibria are represented by a tie line with one end being a critical point. The intersection of the two tie lines gives compositions the solution of which can give rise to the entire three-phase coexistence curve. The coexistence curve measurements were made for two solutions with different compositions near the calculated intersection. By a refractive index method coexistence curves were obtained in a temperature-total polymer composition diagram. The three-phase region was observed in a very narrow temperature range from 24.1 to 24.7 °C. The two solutions could give rise to almost entire three-phase coexistence curve and two critical points were predicted to be just above and below the three-phase region from strong opalescence. The mechanism of the three-phase separation was explained as due to the heterogeneous double plait point.