Lower Critical Solution Temperature Behavior in Polymer Blends: Compressibility and Directional-Specific Interactions

Lower critical solution temperature (LCST) behavior in polymer blends is shown to be the result of the compressible nature of the system, the directional-specific character of the intermolecular interactions, or a combination of both. Certain random copolymer-homopolymer blends are examples of the first possibility. To illustrate the possibility of LCST behavior due to the directional-specific character of the intermolecular interactions, a recently introduced incompressible lattice-gas model by Vause and Walker is adapted to the polymer problem. Homopolymers are shown to be miscible if there is an acid-base interaction of sufficient strength and if the difference between the respective solubility parameters is not too large. The amount of entropy lost in the formation of a specific interaction is shown to be an additional important factor in determining polymer miscibility. Specific heat measurements can, in principle, decide whether the behavior of a polymer mixture at a given temperature is dominated by its compressibility or by the directional-specific interactions.