Thermodynamics of plasticized triblock copolymers. I. Theory

The thermodynamic theory of bulk ABA copolymers developed by Leary and Williams is extended to copolymer–solvent systems. Free energy expressions are derived for five hypothetical phase‐separated morphologies and evaluated specifically for a polymer with approximately 25% of the A component. The separation temperature, T s , at which a given morphology will be in equilibrium with a homogeneous mixture, is also evaluated. The major result is the prediction of the T s (ϕS) depression, where ϕs is the solvent fraction. Depression is maximized when δ S is equidistant between δ A and δ B , but becomes rapidly less when δ S is outside the δ A –δ B range. Morphological favoritism is independent of ϕ S and δ S (model does not apply to preferential precipitation), with a planar microstructure being favored along with microstructures containing domains of B in continuous A for the 25% A polymer.