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: polysty rene, PAN: polyacrylonitrile, SAN: P(S-ran-AN)] was studied in terms of sec ond virial coefficients and phase separation in the range of moderate polym er concentrations. The most remarkable finding consists in fundamental disc repancies between the Flory-Huggins parameters obtained for the interaction between the segments of polymer X and polymer Y either straightforwardly f rom 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 A(2)(XY), the second osmotic virial coefficients referring to the opening o f intermolecular contacts between X and Y by the insertion of solvent and i n terms of W-2, the normalized excess Gibbs energy of contact formation bet ween the two types of solute in the concentration region of pair interactio n according to X-X+Y-Y --> 2X-Y. The pair interaction coefficient W-2 resul ts negative for PS/SAN, but positive for PAN/SAN and for PAN/PS. A theoreti cal explanation of this unexpected result is offered. (C) 2001 American Ins titute of Physics.