Thermodynamic Stability of Immiscible Polymer Blends

Abstract

Calculations based on an extended Flory−Huggins−Staverman model demonstrate that miscibility gaps in virtually immiscible blends may encompass a complex pattern of metastable and unstable equilibria that differ little in Gibbs free energy. Examples are given that demonstrate the development of near immiscibility by sideways coalescence of upper-critical and lower-critical gaps upon an increase of the chain lengths of the constituents. The mechanisms of merging are discussed, as well as the effect of chain-length distribution and copolymer composition. The conceivable occurrence of nonstable equilibria may lead to a change in physical properties of immiscible blends with time, here called thermodynamic aging of the second kind.