High-Density Packing of Spherical Phases in A(AB₃)₃ Dendron-like Miktoarm Star Copolymer

Block copolymers show various nanostructures depending on the volume fraction (f) of the one block. Among them, spherical phases are important in understanding multiple crystalline orders. However, the expansion of the spherical region beyond f = 0.5 remains a great challenge due to their large interfacial curvature. Miktoarm star copolymers provide an efficient strategy for shifting phase boundaries towards higher volume fractions, as their complex chain architecture favors the formation of a curved interface towards the major block despite the volume ratio.

In this study, we synthesized A(AB₃)₃ miktoarm star copolymer using polystyrene (A) and poly(2-vinylpyridine) (B) through a combination of anionic polymerization, atom transfer radical polymerization (ATRP), and click reaction. We observed the body-centered cubic (BCC) phase even at f_PS = 0.51, where PS blocks became spheres rather than a matrix. This remarkable expansion of the spherical region is attributed to the unique chain architecture of A(AB₃)₃. In this structure, three generations of arms provide a gradual radial distribution towards the A domain, stabilizing the spherical phase's large interfacial curvature. The experimental results are also consistent with predictions based on self-consistent field theory (SCFT).