Microphase-separated structures of diblock copolymers with narrow molecular weight distributions but with various composition distributions were investigated. Monodisperse nine parent block copolymers composed of polystyrene and poly(2-vinylpyridine) having almost the same molecular weight but with different polystyrene volume fraction, φs, covering 0.1-0.9, were prepared. Successively they were blended variously to produce samples with various composition distributions but with constant average composition; that is, φ s was kept constant at approximately 0.5. Structures from solvent-cast and well-annealed films were observed through transmission electron microscopy and small-angle X-ray scattering. It has been found that the blends with wide composition distributions show periodic alternating lamellar structure up to 1.7 in terms of M w(S)/Mn(S), where Mw(S) and Mn(S) denote weight-average and number-average molecular weights of polystyrene block, respectively, and that the microdomain spacing increases with increase of polydispersity of each block. Further, it was clarified that the blend shows macrophase separation in between two kinds of regular microphase-separated structures if M w(S)/ Mn(S) reaches approximately 1.8.

The effects of composition distribution on microphase-separated structures formed by monodisperse BAB triblock copolymers were investigated. Monodisperse nine parent BAB triblock copolymers consisting of polystyrene for A and poly(2-vinylpyridine) for B were prepared by anionic living polymerization. These nine copolymers were designed such that polystyrene volume fraction, φs, ranged from 0.1 to 0.9, and they were blended to produce samples with various composition distributions but with constant average composition at φ s of 0.5. Periodic simple alternating lamellar structures were observed for solvent cast and well-annealed blend films as long as composition distribution is relatively low. It has been found that microdomain spacing increases with increasing composition distribution of copolymer samples up to 1.25 in terms of M w(S)/Mn(S), where Mw(S) and Mn(S) are weight-average and number-average molecular weights of polystyrene blocks, respectively, as was the case of the AB diblock copolymer system, though the increment is larger for triblock than diblock. Furthermore, it was clarified that triblock copolymer tends to macrophase separate into several regular microphase-separated structures more easily than diblock copolymer.

Morphological variations of ABC triblock copolymers through the blending of B or A/C homopolymers, all with short chain lengths, were studied experimentally. The samples were symmetric ISP triblock copolymers, where I, S, and P denote polyisoprene, polystyrene, and poly(2‐vinylpyridine), and component homopolymers. Microphase‐separated structures of the solvent‐cast films were observed with transmission electron microscopy and small‐angle X‐ray scattering. For an ISP/S system, the lattice constant of the tricontinuous gyroid structure (G‐structure) increased with an increase in the volume fraction of S (ϕs) if the amount of added homopolymer was small, but it reached a certain limit, reflecting the fact that the midblock chain had a limit for chain stretching. For I/ISP/P blends, on the contrary, the lattice constant of the G‐structure continued increasing with decreasing ϕs. This result shows that the I and P domains did not have a limit for chain stretching because the two end blocks had free ends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1135–1141, 2002