THERMODYNAMIC ANALYSIS ON THE PHASE-BEHAVIOR OF COPOLYMER BLENDS - AN EQUATION OF STATE APPROACH

A model of copolymer blends based on the Prigogine-Flory-Patterson equation of state theory is presented to understand quantitatively the effect of comonomer on the LCST behavior of copolymer blends. According to Patterson's treatment, the free volume effect was introduced to the Flory-Huggins interaction parameter chi(AR) of copolymer blends. The model explains that the intramolecular interaction within copolymers contributes not only to the interactional term but also to the free volume term. Unlike its contribution to the interactional term, the repulsion is not always favorable to the free volume term which depends on the difference in the characteristic temperatures between blend components. Since the characteristic parameters P* and T* of the copolymers are a function of those of the copolymer components, the difference depends on the magnitudes of the characteristic parameters of selected comonomer as compared with those of base polymer. The applicability of the proposed model was examined for poly(vinyl methyl ether) (PVME)/styrene copolymer blends. The calculated variation of chi(AR) with the copolymer composition at a given temperature was in agreement with the variation of the LCST with the copolymer composition and the type of comonomer.