A Fresh Look at Solutions of Polymer Mixtures

Abstract

Analysis of phase equilibria in ternary systems of the type solvent — polymer(l) — polymer(2) is facilitated by introduction of two new parameters replacing traditional separation factors σi, namely ŋ = (σ1 σ2)1/2 and ξ = (σ2/σ1)1/2. The projection of the latter one into the complex plane of the reduced interaction term creates a one-dimensional global space of all ternary systems, ordered according to the tendency of their polymeric components to separate into two different phases. Surprisingly, ξ changes very little over the entire span of the cloud-point curve of a given polymer mixture. The geometrical average ŋ is a measure of the distance of a cloud-point from the critical point, just as σ was for quasibinary systems. It can be utilized to derive criteria for existence of multiple critical points. Conditions for single, double and triple critical points are established. The approximate plait point criterion derived years ago by Scott is shown to arise, with a minor modification, as an exact solution for a special case.