MICROPHASE SEPARATION OF ABC-TYPE TRIBLOCK COPOLYMERS

We have studied microphase separation of ABC-type triblock copolymers in the strong segregation limit by generalizing our previous theory for diblock copolymers. The free energy functional in terms of local monomer concentrations is derived via a mean field approximation and a local approximation for higher order coupling. In the special case where the monomer number of the A block is the same as that of the C block, we calculate the critical values of the volume fraction for morphological transitions of ordered domains from lamellar to cylinder and from cylinder to sphere. It is emphasized that in the ABC-tpye copolymer, a square lattice is predicted to be more stable than a hexagonal one for cylindrical domains. The possibility of an ordered tricontinuous diamond structure is also examined. The results are compared with the recent experiments by Matsushita et al.