Compatibility range in polymer mixtures. An approach using analogue calorimetry and group contribution procedures

The mixing enthalpies of blends of polymethylmethacrylate (PMMA) with poly( styrene-co-acrylonitrile) (SAN) were investigated by analogue calorimetry t hrough the determination of the excess enthalpies of pseudobinary model mix tures corresponding to the addition of methyl-i-butyrate to a binary mixtur e of acetonitrile or propionitrile plus toluene or ethylbenzene. A group contribution procedure, based on UNIQUAC equation, was also devised and the polymeric mixing enthalpies were calculated from properly defined group contributions. Enthalpies for polymeric interactions were introduced into the Flory-Huggins equation and the miscibility window of PMMA-SAN mixt ures was calculated. The results show a qualitative agreement with the experimental miscibility data and indicate that both the analogue calorimetry and the group contribu tion procedures yield correct results when acetonitrile, and not propionitr ile, is chosen as the model for the polyacrylonitrile repeat unit of the co polymer.