Coexistence Curves of Polystyrene in Cyclohexane near the Critical Double Point in Composition-Pressure Space : Condensed Matter: Structure, etc.

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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×l0^4 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)^'lt;βp> 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 light of the molecular weight dependence of _p calculated from the free energy, the observed values of _p Was found to reveal the crossover from the usual critical point to CDP.
社団法人日本物理学会の論文
2000-08-15