A spinodal equation based on the modified Prigogine Flory′s equation of state theory was derived and applied to predicting the phase behavior of a ternary polymer blend comprising styrene methyl methacrylate copolymer(SMMA), styrene acrylonitrile copolymer(SAN)and poly methyl methacrylate(PMMA). According to Flory′s combining rules, the parameters of state equation for copolymers were evaluated from those of the constituent monomer segments, and binary segmental contact energy parameters X ij between copolymer and copolymer were computed. The numerical solution of spinodals of SMMA PMMA SAN at various temperatures was derived by computer programming, and confirmed by the corresponding experimental results. It was found that Flory′s equation of state theory could be successfully extended to predict the phase behavior of ternary copolymer blends by adding the term of interaction entropy parameters Q ij evaluated from the phase diagrams of the corresponding binary blends.
