ABSTRACT
In this chapter, the authors begin by considering the origin and effect of a non-zero energy of mixing. E. A. Guggenheim classified mixtures primarily into two types, "those in which molecular orientation is unimportant and those in which is it all important." In the most commonly employed treatments of the thermodynamics of mixing, it is assumed that interactions of this type are pairwise additive. The authors describe relatively weak interactions as "physical," while those interactions that result in the formation of identifiable species as "chemical" interactions. They consider the extent to which they can obtain a qualitative or semi-quantitative understanding of miscibility in polymer blends through the use of simple models, where factors such as free volume are neglected. Accordingly free volume factors are important in determining the phase behavior of non-polar polymer solutions. Certain aqueous solutions of polar polymers also have closed loop coexistence curves.
