Thermodynamic Stability of Copolymer Blends

Abstract

Liquid-liquid phase diagrams of polymer blends show a large variety in shape and depend very sensitively on average molar mass and molar-mass distribution. If one or both constituents are statistical copolymers phase behavior is still more complex and of surprising subtlety. A mixture of two statistical copolymers based on the same pair of monomers but differing in chemical composition is usually characterized by a maximum tolerable composition difference for the system to remain homogeneous. Mixtures of homopolymers and copolymeys, whether sharing a common repeat unit or not, may show unexpected composition ranges of miscibility within vast areas of virtual immiscibility. Molecular modelling of such phenomena is possible on different levels of sophlstlcation. We restrict the discussion to the rigid lattice model and explore its capability of predicting phase relations with a minimum of a priori knowledge. The Flory-Huggins-Staverman-Scott model represents the simplest version and has a considerable predictive power in locating regions of (im)miscibility in a semiquantitative fashion. For more precise descriptions the model must be amended, either accounting for minute differences In molecular volume or chain length, or using Staverman’s contact statistical treatment.