Microdomain Structures of Triblock Copolymers of the ABC Type

To understand microdomain structures of triblock copolymers of the ABC type at the molecular level, we studied the variation of morphology of microdomain with composition, the molecular weight dependence of lamellar domain spacing and the chain conformation of middle-block polymer in lamellae by transmission electron microscopy(TEM), small-angle X-ray scattering(SAXS) and small-angle neutron scattering(SANS), in comparison with microdomain structures of diblock copolymers of the AB type, because the middle-block chain of the triblock copolymers has a bridge conformation, while the one end of block chain of diblock copolymers is always free.Samples used here were isoprene(I)-styrene(S)-2-vinylpyridine(P) triblock copolymers of which the two end-block polymers have the same chain lengths and the corresponding triblock copolymers with deuterated middle-block polymers, prepared by a succesive anionic polymerization technique. Film specimens for TEM, SAXS and SANS were prepared by solvent-casting and annealing.TEM and SAXS studies revealed that the morphologies of microdomains change in the order of spherical, cylindrical, ordered tricontinuous double-diamond and lameller structures as the S block polymer content decreases. The order is similar to that of diblock copolymers, but the composition ranges and the types of packing in lattice are different. SAXS studies revealed that the lamellar domain spacings of the triblock copolymers are larger than those of SP diblock copolymers at least for high molecular weight samples in accordance with the theories of microphase separation in the strong segregation limit. Moreover, SANS studies suggested that the middle-block polymer of the triblok copolymers is contracted along the direction parallel to lamellae at the almost same magnitude as the block chain of the diblock copolymers.