Noncentrosymmetric Lamellar Phase in Blends of ABC Triblock and ac Diblock Copolymers

Abstract

The phase behavior of blends of ABC triblock and ac diblock copolymers is examined using self-consistent mean-field theory. Several equilibrium lamellar structures are observed, depending on the volume fraction of the diblocks, φ2, the monomer interactions, and the degrees of polymerization of the copolymers. For segregations just above the order−disorder transition the triblocks and diblocks mix together to form centrosymmetric lamellae. As the segregation is increased, the triblocks and diblocks spatially separate either by macrophase-separating or by forming a noncentrosymmetric (NCS) phase of alternating layers of triblock and diblock (...ABCcaABCca...). The NCS phase is stable over a narrow region near φ2 = 0.4. This region is widest near the critical point on the phase coexistence curve and narrows to terminate at a triple point at higher segregation. Above the triple point there is two-phase coexistence between almost pure triblock and diblock phases. The theoretical phase diagram is consistent with experiments.