It is generally held that the incompatibility of polymers in solution arises from an unfavorable interaction (X23) between segments of the polymers and is independent of the polymer-solvent interactions (Xi2 and Xi3). This is correct when the polymer-solvent interactions are of equal magnitude, as assumed in Scott's original treatment of such ternary systems. We reexamine the theory, calculating spinodals which show that a small difference in polymer-solvent interactions has a marked effect on polymer incompatibility. This is in agreement with experimental findings of Dondos and others. At high polymer concentration, polymer incompatibility is associated with X23, but at low concentrations it should be mainly due to the difference between Xi2 and Xi3. Setting X23 = 0 but keeping a sufficient difference between Xi2 and 33 leads to a closed region of incomplete miscibility within the triangular phase diagram, although the binaries show complete miscibility.

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Values of the polymer-solvent interaction parameter, , are predicted using the solubility parameter theory and the corresponding states theory in the form given by Flory and collaborators. The theories predict

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