The mixing enthalpies of blends of polymethylmethacrylate (PMMA) with poly(styrene-co-acrylonitrile) (SAN) were investigated
by analogue calorimetry through the determination of the excess enthalpies of pseudobinary model mixtures corresponding to
the addition of methyl-i-butyrate to a binary mixture 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 mixtures was calculated.
The results show a qualitative agreement with the experimental miscibility data and indicate that both the analogue calorimetry
and the group contribution procedures yield correct results when acetonitrile, and not propionitrile, is chosen as the model
for the polyacrylonitrile repeat unit of the copolymer.