Equilibrium behavior of asymmetric ABA triblock copolymer melts

Melts of ABA triblock copolymers with two different sized end blocks are examined using self-consistent field theory. At small asymmetries, the A block bidispersity reduces the stretching energy of the A domains. This effect causes a slight increase in the domain spacing and shifts the order-order transitions (OOTs) toward higher A volume fractions. At large asymmetries, a significant number of the short A blocks pull free of their domains allowing their B blocks to relax. This effect produces a much greater increase in domain spacing and shifts the OOTs toward lower A volume fractions. It also leads to a sizable region of close-packed spheres. At the crossover between these two effects, the ordered morphologies are shifted from their normal compositions by about 10%. The displacement of the complex gyroid region is found to have a strong influence on the metastability of the perforated-lamellar phase. (C) 2000 American Institute of Physics. [S0021-9606(00)51237-9].