ABSTRACT
This chapter presents the classical Flory–Huggins theory, which can explain a large number of observations regarding the phase behavior of concentrated polymer solutions. The classical Flory–Huggins theory assumes at the outset that there is neither a change in volume nor a change in enthalpy on mixing a polymer with a low molecular weight solvent; the influence of nonathermal behavior is accounted for at a later stage. The Flory–Huggins theory is remarkably successful in explaining most observations concerning the phase behavior of polymer–solvent systems. As D. Patterson explains in his very readable review, the free volume of the polymer differs markedly from the solvent free volume. As the free-volume dissimilarity between the polymer and the solvent increases with increasing temperature, the free-volume effect is likely to be more important at elevated temperatures. The solubility parameter is used to estimate the endothermic heat of mixing that accompanies the dissolution of an amorphous polymer by a low molecular weight solvent.
