Coexistence Curves of Polystyrene in Cyclohexane near the Critical Double Point in Composition-Pressure Space

Abstract

Phase equilibrium near the critical double point (CDP) was studied in pressure p -temperature T -volume fraction φ space for the system polystyrene (PS) in cyclohexane (CH). The p-T critical lines measured for the molecular weight M w ×10 -5 =1.15, 7.19 and 15.6 had a minimum, i.e., CDP at elevated pressures and were used to determine the free energy of the system on the basis of the Flory-Huggins equation. An analysis of the free energy predicted that CDP appeared at atmosphere for M w =7.93×10 4 and disappeared for larger molecular weights. For M w ×10 -4 =7.5 and 3.7 the coexistence curve in p-φ space were measured by increasing pressure from atmosphere near to 6 MPa and the critical exponent β p for the volume fraction difference φ + -φ - ∼( p - p c ) β p were determined to be β p =0.62±0.03 and 0.39±0.02, respectively. The former value is close to the doubly enhanced critical exponent at CDP and the latter value is considerably larger than the theoretical value β=0.327 at the usual critical point. On...