Universal aspects of macromolecules in polymer blends, solutions, and supercritical mixtures.

We demonstrate that macromolecules in miscible polymer blends may behave as good, Theta, and poor polymeric solvents for each other. We construct a conceptual phase diagram, delineating the range of validity of the random-phase approximation, outside of which polymers contract or expand beyond their unperturbed dimensions, contrary to common assumptions. Remarkably, the correlation length for polymer blends, solutions, and supercritical mixtures collapses onto a master curve, reflecting universal behavior for macromolecules in polymeric and small-molecule Theta solvents.