Interaction energies for blends of poly(methyl methacrylate), polystyrene, and poly(.alpha.-methylstyrene) by the critical molecular weight method

Interaction energies for each of the binary pairs PS/Pα-MS, PS/PMMA, and PMMA/Pα-MS were calculated by fitting spinodal curves predicted by the Flory-Huggins theory and the Sanchez-Lacombe equation of state theory to experimental cloud point data for blends of low molecular weight polymers. For blends of PS and Pα-MS, where cloud points could not be observed, the molecular weight limit of miscibility was used to bracket the interaction energy. Phase separation boundaries for this system were predicted to be caused by UCST-type behavior. Oligomeric blends of PMMA with PS showed UCST-type phase boundaries while blends of PMMA with Pα-MS showed LCST-type boundaries. The mechanism of phase separation boundaries was found to stem from the differences between the characteristic temperatures for the pure components (ΔT * )