Coexistence curve of polystyrene in methylcyclohexane. IX. Pressure dependence of tricritical point

Abstract

The effect of pressure on the tricritical point (TCP) of a ternary system, polystyrene (PS) I+PS II+methylcyclohexane, was studied by numerical analysis of the generalized Flory–Huggins equation for the Gibbs free energy. For the ternary system, the molecular weight of PS II M2t, total volume fraction of PS φst, volume fraction of PS II with respect to total volume of PS ξ2t, and temperature Tt at TCP were calculated for various values of the molecular weight of PS I M1 in the pressure range from 0 to 100 MPa. As M1 increases, the ratio rt=M2t/M1 and φst decrease, while Tt and ξ2t increase monotonically irrespective of p. The curves of rt and φst vs p have a maximum, while the curves of Tt and ξ2t vs p have a minimum, irrespective of M1. For small M1 these curves are nearly flat and roughly symmetrical with respect to the extrema. From these calculated results it is predicted that the tricritical solution at atmospheric pressure remains very near the tricritical state along the cloud-point curve in the temperature vs pressure diagram.