COMPATIBILITY AND THERMODYNAMIC INTERACTION IN RANDOM COPOLYMER BLENDS

Some random copolymer blends have been found to be miscible in a certain range of copolymer composition even though any combinations of their corresponding homopolymers are not miscible. The opposite case may exist. These two types of miscibility behaviors have been called miscibility and immiscibility windows, respectively. Such two miscibility behaviors were discussed by application of the equation‐of‐state theory to copolymer systems. The equation‐of‐state theory gives two kinds of temperature dependences of the interaction parameter X: (a) a U‐shaped curve which is always positive regardless of temperature and (b) a function increasing monotonically from negative to positive values. Infinite molecular weight polymer blends are immiscible over all the temperature in the case (a), while in the case (b) two polymers are miscible below a temperature at which X=0. Applying the equation‐of‐state theory to random copolymer blends in which miscibility changes with the copolymer composition at a certain temperature to be immiscible → miscible → immiscible, two types of dependences of the temperature‐X curve can be obtained: (1) (a) → (b) → (a) dependent on the copolymer composition and (2) (b) regardless of the copolymer composition. For the blends in which miscibility changes with the copolymer composition to be miscible → immiscible → miscible, there can be two types: (3) (b) → (a) → (b) and (4) (b) regardless of the copolymer composition. It may be concluded that socalled miscibility and immiscibility windows should be defined by the types (1) and (3), respectively. The equation‐of‐state theory for random copolymer systems was applied to the real systems. The blends of poly(vinyl acetate‐co‐vinyl chloride) and poly(ethylene‐co‐vinyl acetate) were of the type (1), while it was suggested that the blends of poly(vinyl acetate‐co‐vinyl chloride) and poly(isobutyl methacrylate‐co‐ butyl methacrylate) may be of the type (4) though this system behaved like an immiscibility window at a certain temperature. Copyright © 1990 Hüthig & Wepf Verlag