Liquid-liquid phase separation in multicomponent polymer systems, 12. Molecular weight dependence of the pair-interaction parameter in the system polystyrene/cyclohexane

Consistent data have been obtained from measurements of the thermodynamic stability limit (spinodal locus) and of the critical point in the system polystyrene/cyclohexane by Pulse-Induced Critical Scattering and phase-volume-ratio techniques which involve direct measurement of the second and third derivatives of the free enthalpy (Gibbs free energy) of mixing with respect to concentration. These data indicate that the pair-interaction parameter in the system polystyrene/cyclohexane cannot depend on temperature and overall polymer concentration alone. The observed pattern can be described with a rather arbitrary empirical function for the molecular weight dependence of the interaction parameter. Arguments are advanced suggesting why Schultz-Flory plots are usually found to be linear, although the underlying assumptions are seldom, if ever, fulfilled.