Coexistence curve of polystyrene in methylcyclohexane. VII. Coexistence surface and critical double point of binary system in T–p–φ space

Abstract

Cloud‐point measurements were made for binary systems of polystyrene (PS) +methylcyclohexane (MCH) with three PS samples in the pressure range up to 90 MPa. On a temperature (T)–pressure (p) plane, observed cloud‐point curves, which were determined at various compositions for the molecular weight M w =1.61×104 and at the critical composition for M w =1.02×104 and 3.49×104, decrease first, reach a minimum and increase with increasing pressure. For M w =1.61×104 the coexistence surface was investigated in a T–p–φ space, where φ denotes the volume fraction of PS. On a T–φ plane the coexistence curves give a critical composition independent of pressure. On a p–φ plane one phase region appears between two coexistence curves. The top and the bottom of the coexistence curves give an upper critical solution pressure (UCSP) and a lower critical solution pressure (LCSP), respectively. The two critical points coincide forming a critical double point (CDP) at T=19.28 °C, p=48.6 MPa, and φ=0.1719. The shape of the coexistence curves near the CDP is very sensitive to temperature and is characterized by the critical exponent 2β. The Gibbs free energy of the present system was evaluated as a function of T, p, and φ from the molecular weight dependence of the observed critical lines.