Extension of the Flory-Huggins theory to study incompatible polymer blends in solution from phase separation data

Abstract

Abstract A method is presented to evaluate the Koningsveld g -functions for quasi-ternary polymer solutions and blends, involving binary and ternary interactions. A robust set of 12 equations derived from the Flory—Huggins lattice theory, dealing with liquid—liquid phase separation conditions, have been solved using as input data the experimental volume fractions of each component in each coexisting phase. These values were found by means of a liquid microextraction procedure followed by size-exclusion chromatography analysis. Several approximations are proposed and discussed in order to select the best option to predict thermodynamic properties of binary polymer blends and blends in solution. The dimethylformamide/poly(vinylidene fluoride)/polystyrene ternary solution was chosen to test the validity of our proposal. In general, the analytical form of the g -function is adequately described by a second order polynomial, the inclusion of the ternary interaction parameter also being recommended. From the values of the PVDF/PS interaction function it can be inferred that this blend behaves as slightly incompatible under environmental conditions, in clear agreement with data previously reported. In contrast, the incompatibility is suppressed when a low molar mass component, such as dimethylformamide, is added, reaching the semidilute regime (total polymer volume fraction p ≈ 0.35). Values of the Gibbs free-energy of mixing as a function of the blend composition were also evaluated for both ternary solution and dry blend and discussed in terms of their stability.