Studies on miscibility in homopolymer/ random copolymer blends by equation of state theory

Abstract

The miscibility of poly(methyl methacrylate) (PMMA) and styrene-acrylonitrile random copolymers (SAN) blends was investigated on the basis of the Flory—Orwoll—Vrij equation of state theory. To obtain the equation of state parameters ( P∗, V∗ sp , T∗: characteristic parameters ), the pressure—volume—temperature ( PVT) behaviour was measured for PMMA and a series of SANs with various acrylonitrile contents. The exchange energy parameter X ij was also calculated by fitting the theory to some phase diagrams of PMMA/SAN blends. The Flory—Huggins interaction parameter χ was separated into two contributions based on the equation of state theory for mixtures: the exchange energy term χ inter and the free volume term χ free. Both the temperature and copolymer composition dependences of χ inter and χ free were estimated by calculations using the equation of state parameters. There exists a region in which χ inter is negative, leading to a miscibility window in PMMA/SAN blends. However, the immiscibility at high temperatures in the blends cannot be explained only by χ inter; it is caused by the free volume contribution, χ free. The miscibility window behaviour in PMMA/SAN blends may be explained within the framework of the equation of state theory.