EXTENSION OF THE FLORY-HUGGINS THEORY TO STUDY INCOMPATIBLE POLYMER BLENDS IN SOLUTION FROM PHASE-SEPARATION DATA

A method is presented to evaluate the Koningsveld g-functions for quas i-ternary polymer solutions and blends, involving binary and ternary i nteractions. A robust set of 12 equations derived from the Flory-Huggi ns lattice theory, dealing with liquid-liquid phase separation conditi ons, have been solved using as input data the experimental volume frac tions of each component in each coexisting phase. These values were fo und by means of a liquid microextraction procedure followed by size-ex clusion chromatography analysis. Several approximations are proposed a nd discussed in order to select the best option to predict thermodynam ic properties of binary polymer blends and blends in solution. The dim ethylformamide/poly(vinylidene fluoride)/polystyrene ternary solution was chosen to test the validity of our proposal. In general, the analy tical form of the g-function is adequately described by a second order polynomial, the inclusion of the ternary interaction parameter also b eing recommended. From the values of the PVDF/PS interaction function it can be inferred that this blend behaves as slightly incompatible un der environmental conditions, in clear agreement with data previously reported. In contrast, the incompatibility is suppressed when a low mo lar mass component, such as dimethylformamide, is added, reaching the semidilute regime (total polymer volume fraction phi(p) approximate to 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 d ry blend and discussed in terms of their stability. Copyright (C) 1996 Elsevier Science Ltd.