Complex phase structures of AB/CD diblock copolymer blends were investigated. Two pairs of poly(isoprene-block-2-vinylpyridine) (IP) and poly(styrene-block-4-hydroxystyrene) (SH) diblock copolymers with different molar ratios, 5:5 and 9:1, were used; they were coded IP-55, IP-91, SH-55, and SH-91, respectively. Two polymer species, that is, poly(2-vinylpyridine) and poly(4-hydroxystyrene), out of four components are miscible with each other via hydrogen bonding interactions in bulk as well as in solution. From morphological observation using TEM and SAXS, it was confirmed that IP/SH blends were self-assembled to form the complex three-phase microphase-separated structures. Namely, two kinds of hierarchical structures were observed, that is, one is a unique lamellar structure from a IP-55/SH-55 ) 50/50 blend, where mixed one phases from P and H form alternating lamellae, whereas I and S phases are arranged alternatingly in every second lamellar phase. The other one from IP-91/SH-91 ) 50/50 consists of another peculiar lamellar structure, where the P/H mixed phase forms isolated cylinders between lamellar S and I layers.

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.

A two-component multiblock copolymer with undecablockstwo of them on both chain ends are long and nine of them are shortswas successfully prepared by anionic polymerization using the sixstep sequential monomer addition technique. Polymer components are polystyrene (S) and polyisoprene (I), its total molecular weight is 275K, and the overall S/I volume ratio is 0.70/0.30. Microphase-separated structure of the copolymer was observed by transmission electron microscopy and small-angle X-ray scattering, and it was confirmed that the copolymer forms a complex lamellar structure; its long period is 45 nm, which is composed of one thick lamellar domain formed by long polystyrene chains and I-S-I three thin lamellar domains, the length of the short period for I-S lamellae being about one-third of the longer period. This fact shows short block chains at the center favorably adopt a loop conformation over a bridge one. This unique lamellar structure having two length scales must be the first experimentally observed simple hierarchical structure for the block copolymer where the component polymers are connected by covalent bonds.

The effect of molecular weight distribution on microphase-separated structures for both AB diblock and BAB triblock copolymers was investigated in comparison with that of composition distribution. Monodisperse poly(styrene-b-2-vinylpyridine) (SP) and poly(2-vinylpyridine-b-styrene-b-2vinylpyridine) (PSP) parent block copolymers were synthesized by living anionic polymerizations whose volume ratios were all designed to be 0.5/0.5. Three parent copolymers were blended variously with both number-average molecular weight and composition kept constant but having different molecular weight distribution. Microphase-separated structures of sample films obtained by solvent-casting followed by annealing were observed by transmission electron microscopy and small-angle X-ray scattering. It has been found that both SP and PSP block copolymers show simple lamellar structures even when the molecular weight distribution is relatively wide and that lamellar domain spacing increases with increase in polydispersity index of the blend system. Furthermore, the increment for PSP triblock is larger than that for SP diblock as was the result for the study on composition distribution. The microdomain expansion can be caused by the localization of polydisperse block chains in both phases, which was commonly observed for both the composition distribution system and the molecular weight distribution system.

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