Polymer–polymer interaction parameters for homopolymers and copolymers from light scattering and phase separation experiments in a common solvent

The thermodynamic behavior of the quaternary system DMAc/PS/PAN/SAN and of all its solvent containing subsystems [DMAc: dimethylacetamide, PS: polystyrene, PAN: polyacrylonitrile, SAN: P(S–ran–AN)] was studied in terms of second virial coefficients and phase separation in the range of moderate polymer concentrations. The most remarkable finding consists in fundamental discrepancies between the Flory–Huggins parameters obtained for the interaction between the segments of polymer X and polymer Y either straightforwardly from the light scattering experiments (ranging from −0.88 to −0.14) and that resulting from the modeling of the measured phase diagrams (+0.01 to +0.33) or from solubility parameter theory (+0.03 to +1.92). The reasons for the disagreement even in the sign of the parameters are discussed in terms of A2XY, the second osmotic virial coefficients referring to the opening of intermolecular contacts between X and Y by the insertion of solvent and in terms of W2, the normalized excess Gibbs energy of contact formation between the two types of solute in the concentration region of pair interaction according to X–X+Y–Y→2X–Y. The pair interaction coefficient W2 results negative for PS/SAN, but positive for PAN/SAN and for PAN/PS. A theoretical explanation of this unexpected result is offered.