COEXISTENCE CURVE OF DILUTE POLYMER-SOLUTION IN A MIXED-SOLVENT HAVING CRITICAL DEMIXING POINT

The coexistence curves of dilute solutions of Poly(vinyl methyl ether) (PVME) in a solvent mixture methanol/cyclohexane which has a critical demixing point were measured. We found that in the methanol-rich region the presence of a small amount of PVME does not cause any appreciable change in the phase boundary of binary methanol/cyclohexane mixture, whereas in the cyclohexane-rich region it gives rise to a marked shift of phase boundary toward higher temperature region. It is shown that this characteristic phase behavior can be well explained qualitatively on the basis of Flory-Huggins solution theory, together with the interaction parameters deduced from small angle neutron and X-ray scattering experiments, as due to the smallness of the interaction parameter, chi13, between methanol and PVME. The smallness of chi13 can be interpreted as a manifestation of the association of methanol molecules.