Mechanism of three phase separation in the ternary system of a polymer in a mixed solvent

•Ternary system of polymer in a mixed solvent in which each binary system has UCST.•Accurate and rapid measurement of cloud point curves by use of a novel cell.•Appearance and coalescence of binodals on a composition triangle.•Three phase separation caused by contact between binodals.•Binodal surface with a col at which a critical line terminates.

The three phase separation due to contact between binodals was observed for the ternary system nitroethane(NE) + methylcyclohexane(MCH) + polystyrene(PS) with the molecular weight (Mw = 9.64 × 104 g/mol). A phase equilibrium experiment was made to determine binodals and three phase triangles on a composition triangle. With decreasing temperature a binodal spreads out from the MCH-PS side of a composition triangle at 48.4 °C and reaches the NE-MCH side at 23.6 °C. Another binodal spreads out from the NE-PS side at 43.6 °C and makes contact with the binodal running between the MCH-PS and NE-MCH sides at 3.3 °C. Simultaneously with the contact the three phase separation appears. The contact occurs at the critical point of the former binodal and at a noncritical point of the latter. The three phase separation usually occurs due to projection of a new binodal from the two phase region.

Graphical abstractTie lines of the binodal near the three phase region in ternary system nitroethane + methylcyclohexane + polystyrene.Download high-res image (100KB)Download full-size image