PHASE-BEHAVIOR OF TERNARY POLYMER BLENDS COMPOSED OF 3 MISCIBLE BINARIES

The phase behavior of ternary polymer blends consisting of three miscible binary pairs is examined in terms of thermodynamic theories and by experiment. The local stability of these mixtures is analyzed by using the second variation of Gibbs free energy constrained by mass balance. This treatment considers copolymer systems by using a simple binary interaction model combined with the Flory-Huggins model having temperature-dependent interaction parameters and equation of state effects by using the lattice fluid theory. The predictions indicate that asymmetry in the binary interactions may lead to ternary-phase instability manifested as large decreases in the temperatures at which phase separation occurs on heating (LCST behavior) ternary compositions relative to the binaries and/or the appearance of ternary UCST behavior when none exist in the binaries. The experimentally determined phase behavior of a series of ternary blends based on sty rene/maleic anhydride (SMA) copolymers, styrene/acrylonitrile (SAN) copolymers, and either poly-(methyl methacrylate), poly(ethyl methacrylate), or a methyl methacrylate/acrylonitrile (MAN) copolymer is reported. In every case, each binary pair formed miscible blends because of favorable polymer-polymer interactions. Asymmetry of binary interactions can be produced by variation of copolymer mer content. This was found to lead to a large drop in the ternary cloud point associated with LCST behavior relative to that of the binaries for three of the ternary mixtures. UCST behavior was not observed for any of the reported blends. Ternary spinodal calculations based on previously determined Flory-Huggins interaction parameters are grossly inaccurate when the asymmetry of the binary interactions is a dominant effect, and explanations for this are given. © 1990, American Chemical Society. All rights reserved.