Abstract
Using Monte Carlo simulations we find that a monodisperse symmetric ABCD star-block copolymer melt undergoes a microphase separation, in which the three space is divided into cellular domains. The domain shape is a cubooctahedron known as the Wigner-Seitz cell of the body-centered cubic lattice. To be precise, the shape is a Lord Kelvin's minimal tetrakaidecahedron proposed in 1887 for the space-filling problem of equal-sized foam bubbles.