High-Density Spherical Packing Phases from A(AB₃)₃ Dendron-Shaped Miktoarm Star Copolymer

Block copolymers show various nanostructures depending on the volume fraction (f) of the one block. Among them, spherical microdomains are important in understanding various 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 copolymer offers an efficient strategy for shifting the phase boundaries towards a higher volume fraction since the curved interface towards a major block is energetically more preferred despite of 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, thereby stabilizing the large interfacial curvature of the spherical phase. The experimental results are consistent with the predictions based on self-consistent field theory (SCFT).