COEXISTENCE CURVE OF POLYSTYRENE IN METHYLCYCLOHEXANE .7. COEXISTENCE SURFACE AND CRITICAL DOUBLE POINT OF BINARY-SYSTEM IN T-P-PHI SPACE

Cloud-point measurements were made for binary systems or 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 X 10(4) and at the critical composition for M(w) = 1.02 x 10(4) and 3.49 X 10(4), decrease first, reach a minimum and increase with increasing pressure. For M(w) = 1.61 x 10(4) the coexistence surface was investigated in a T-p-phi space, where phi denotes the volume fraction of PS. On a T-phi plane the coexistence curves give a critical composition independent of pressure. On a p-phi 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-degrees-C, p = 48.6 MPa, and phi = 0.1719. The shape of the coexistence curves near the CDP is very sensitive to temperature and is characterized by the critical exponent 2beta. The Gibbs free energy of the present system was evaluated as a function of T, p, and phi from the molecular weight dependence of the observed critical lines.