Multiple Glass Transitions in Butadiene—Acrylonitrile Copolymers. II. Formation of Incompatible Phases during Copolymerization

Abstract Differential Thermal Analysis and Dynamic Mechanical measurements show that the presence of two glass transition temperatures in nitrile rubbers is not unusual. Two glass transition temperatures occur in all commercial non-crosslinked butadiene-acrylonitrile rubbers of less than 35 per cent acrylonitrile even though they have gone previously undetected. We have investigated the changes in composition and glass transition temperatures during the course of the copolymerization. The composition versus conversion results were in agreement with previously published data. Fractionation and polymer-polymer compatibility experiments were used to demonstrate the presence of two incompatible phases of different acrylonitrile content, corresponding to the glass transitions of the original co-polymer. The mechanism for the formation of these two phases is discussed.