Thermodynamic stability of immiscible polymer blends

Calculations based on an extended Flory-Huggins-Staverman model demonstrate that miscibility gaps in virtually immiscible blends may encompass a compl ex pattern of metastable and unstable equilibria that differ little in Gibb s 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 mechani sms of merging are discussed, as well as the effect of chain-length distrib ution and copolymer composition. The conceivable occurrence of nonstable eq uilibria may lead to a change in physical properties of immiscible blends w ith time, here called thermodynamic aging of the second kind.