Using a combinatorial screening method based on the self-consistent-field theory for polymers, we study the bulk morphology and the phase behavior of π-shaped ABC block copolymers, in which A is the backbone and B and C are the two grafts. By systematically varying the positions of the graft points, the π-shaped block copolymer can change from a star block copolymer to a linear ABC block copolymer. Thus, the corresponding order−order phase transition due to the architecture variation can be investigated. At two given compositions, we find seven different morphologies (“three-color” lamellar phase, “three-color” hexagonal honeycomb phase, lamellae with beads inside, dodecagon−hexagon−tetragon, hexagon−hexagon, lamellae with alternating beads, and octagon−octagon−tetragon). The hexagon−hexagon morphology has not been reported previously for linear and star triblock copolymers in the bulk state. The phase diagram of the π-shaped ABC block copolymer with symmetric interactions among the three species is constructed. When the volume fractions of block B and block C are equal, the triangle phase diagram shows reflection symmetry. When the shorter block is fixed at the backbone end and the other block moves to the other end along the backbone, the resulting morphology reaches to the same as that of a linear triblock copolymer rapidly. These results may help the design of the microstructures of complex block copolymers.

Introduction.Phase separation in polymer blends can occur due to the immiscibility between blend components, resulting in a variety of structures, which are important to many applications ranging from biomedical (e.g., gas separating membrane) to microelectronic device fabrication (e.g., lithography). 1,2 It is known that, after a blend is quenched into a metastable or unstable state, phase separation may proceed either by nucleation and growth (NG) or by spinodal decomposition (SD). The time evolution of SD phase separation can be divided into three stages, namely, the early, intermediate, and late stages. In the early stage, the behavior is well described by Cahn's linearized theory. 3,4 In the intermediate and the late stages, time evolution of phase separating domains is traditionally characterized by the power law q*(t) ∼ t -n , where q*(t) and n are the peak wavenumber and the power exponent characterizing the time evolution. After Lifshitz and Slyozov 5 obtained a scaling exponent n ) 1 / 3 considering diffusion effects and Siggia 6 proposed tube hydrodynamic instability with n ) 1 for the bulk system, various theories and simulations [7][8][9][10][11][12][13][14][15][16][17][18][19][20] and many experiments (especially scattering experiments) [21][22][23][24][25][26][27][28][29][30] have been focused on the power law describing the phase separation of polymer