Coexistence curve of polystyrene in methylcyclohexane. V. Critical behavior of the three-phase coexistence curve

The three-phase coexistence curve for the ternary system polystyrene (Mw=1.73×104)+polystyrene (Mw=7.19×105)+methylcyclohexane was analyzed from the viewpoint of the critical phenomena. The critical exponent at the upper critical end point was determined as βf=0.381±0.012, which is close to 0.384±0.004 at the plait point for the same ternary system and compared with the fully renormalized critical exponent. The critical exponent at the lower critical end point was found to be smaller as βf=0.342±0.031. According to the numerical analyses of the empirical Gibbs free energy, this small value was ascribed to an improper behavior of the tie lines in a composition triangle. The tie lines near the upper critical end point were parallel with each other and contained the critical end point. The observed three-phase coexistence curve in a temperature-total polystyrene volume fraction diagram was much more symmetrical than the calculated one and suggested a tricritical phenomenon.