Noncentrosymmetric lamellar phase in blends of ABC triblock and ac diblockcopolymers

The phase behavior of blends of ABC triblock and ac diblock copolymers is e xamined using self-consistent mean-field theory. Several equilibrium lamell ar structures are observed, depending on the volume fraction of the diblock s, <(<phi>)over bar>(2), the monomer interactions, and the degrees of polym erization 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 spat ially separate either by macrophase-separating or by forming a noncentrosym metric (NCS) phase of alternating layers of triblock and diblock (...ABCcaA BCca...). The, NCS phase is stable over a narrow region near <(<phi>)over b ar>(2) = 0.4. This region is widest near the critical point on the phase co existence curve and narrows to terminate at a triple point at higher segreg ation. Above the triple point there is two-phase coexistence between almost pure triblock and diblock phases. The theoretical phase diagram is consist ent with experiments.