PHASE-STABILITY OF HYDROGEN-BONDED POLYMER MIXTURES - APPLICATION OF THE QUASI-CHEMICAL THEORY

Expressions for the second derivative of Gibbs energy of mixing with r espect to composition and/or the corresponding Flory-Huggins interacti on parameter are derived using the Barker-Guggenheim quasichemical the ory. The calculation is performed first for a system of compounds with a homogeneous molecular surface, then for a binary mixture of molecul es the surface of which consists of various kinds of contact sites. Th e result is expressed in terms of ''indexes of nonrandomness'', the fa ctors which characterize deviations of individual types of contact pai rs from random mixing at a given composition. Main attention is paid t o a mixture of substances, each of which contains a different type of polar groups and a nonpolar residue which is similar for both componen ts. Translucent relations are obtained in two limit cases, namely for small deviations from random mixing and for small attraction between p olar and nonpolar groups. The effect of the relative size of the polar groups and of the affinity to formation of various heterocontacts is illustrated using the limit relations derived, as well as by means of model calculations.