Phase equilibrium and conformation of a polymer in a binary solvent

The phase equilibrium behavior of a dilute high molecular weight polymer (poly-ethylene oxide PEO, molecular weight 9 × 105, concentration 0.001 g/cc) solution in a two-component liquid mixture (isobutyric acid + water, IBAW) is studied experimentally. Far above the critical temperature Tc of IBAW, we found that although PEO dissolved in pure isobutyric acid (IBA) and pure water, mixtures of IBA and water with 0.1% PEO became cloudy within a certain composition range. Far below Tc, when the binary mixture phase separates in two coexisting phases, the presence of PEO which dissolved in both phases did not affect the coexistence curve of the binary mixture. Around Tc, three phases in coexistence were obsrrved in a narrow composition range near the critical composition of IBAW. At high temperature (≈ 24°C above Tc) the size of PEO in IBAW was measured by light scattering method at different IBAW compositions. We observed expansion of PEO molecules in the mixed solvent, which can be explained using the single liquid approximation by Scott. Such an expansion caused the macromolecules to overlap and separate from the solvent.