AbstractThe control of microphase-separated structures in a poly(styrene-block-butadiene-block-methyl methacrylate) (SBM) was investigated in three dimensions by transmission electron microtomography. Neat SBM self-assembled into a double-helical structure of polybutadiene (PB) domains around hexagonally packed core polystyrene (PS) cylinders in a poly(methyl methacrylate) matrix. When PS homopolymer with a lower molecular weight than that of the PS block in SBM was added to the SBM, the PB domains transformed from double-helical structures to spherical domains, while maintaining the helical trajectories. In contrast, adding a higher molecular weight PS to the SBM changed the helical structures from double- to triple-stranded structures and even to four-stranded structures. The helical structures of the PB domains were strongly affected by the distribution of the blended polystyrenes in the core cylindrical PS domains.