Dilatometric Behavior and Glass Transition in a Styrene-Acrylonitrile Copolymer

Equilibrium and glass transition behavior of a styrene-acrylonitrile copolymer (SAN) under different thermobaric histories were studied by means of a PVT dilatometer. Equilibrium behavior, as determined by isothermal and isobaric measurements, could be satisfactorily described using the Simha-Somcynsky and Tait equations of state. Glass transition behavior depended upon the applied transformation path from the liquid-equilibrium state to the glassy state. From isobaric cooling ramps performed at constant rate and at several pressures, it was possible to determine the glass transition temperature and its dependence upon pressure; whereas from isothermal compressions at various temperatures, it was possible to determine a glass transition pressure and its temperature dependence. Both the dependences were linear, and a correlation was observed between the slopes of the fitting lines. A possible interpretation of this correlation is provided in terms of free volume determined at the glass transition point by applying the Simha-Somcynsky theory.