Application of Flory-Huggins theory to ternary polymer-solvents equilibria: A case study

The ternary equilibrium data of the following systems: benzene/heptane/NBR at 60 degrees C (system 1), heptane/isooctane/PE at 25 degrees C (system 2), ethanol/water/Ca at 20 degrees C (system 3) and ethanol/water/P(E-co-VAc) at 32 degrees C (system 4), already reported in the literature, have been used and experimental results compared to predictions offered by the Flory-Huggins theory applied to ternary mixtures (one polymer and two liquids), with constant interaction parameters and negligible elastic contribution. Polymer-solvent interaction parameters have been determined from swelling in pure liquid, while liquid-liquid interaction parameters have been estimated from liquid-vapour equilibrium data curve fitting. It is shown that the Flory-Huggins theory offers reasonable prediction in the case of apolar liquids in an elastomeric matrix (system 1), while approximate isotherm patterns with significant discrepancies, are obtained with apolar liquids in a thermoplastic (system 2). The equilibrium data of polar liquids in either homopolymer or copolymer (systems 3 and 4) can hardly be achieved by Flory-Huggins theory, even when a variable liquid-liquid interaction parameter is used. Implications in ternary diagram simulations and possible prediction improvements are discussed.