A study of the miscibility of polycarbonate and poly(α-methylstyrene-co-acrylonitrile)

In this paper we report on five phase diagrams of blends of bisphenol A polycarbonate (PC) and poly(α-methylstyrene-co-acrylonitrile) (αSAN) with varying molecular weight and polydispersity of αSAN. Temperature-composition phase diagram covering the temperature range from 125°C to 180°C were determined by examining the phase behavior - either phase separation or persistence of a single phase - of compositions purposely prepared as intimate mixtures, at any temperature. Intimate mixing of the constituent polymers was achieved by dissolution in a common solvent from which the polymer mixture was recovered via freeze-drying. Homogeneous mixtures were obtained in all cases, as revealed by a single Tg intermediate between the Tg's of the two pure components. The dependence of the Tg's of these mixtures on their composition was subsequently used as a calibration curve for phase composition analysis. The phase behavior of each composition was determined by annealing portions of the freeze-dried, homogeneous samples at any temperature in the relevant range for time periods sufficient to reach phase equilibrium. After the thermal treatment, the samples were quenched and again analyzed via DSC. The appearance of either one or two Tg's was taken as evidence for one or two phases, respectively. The composition of each phase was determined from the value of its Tg, using the calibration curve previously determined. All phase diagrams exhibited LCST-type miscibility gaps over the temperature range examined. Samples of different starting composition demixed into two phases belonging to different coexistence curves, one for each starting composition. The effect of αSAN molecular weight and polydispersity on the LCST behavior was examined. An attempt was made to calculate two of the phase diagrams following a scheme based on the Flory-Huggins theory.