This focus review introduces recent progress made in microphase-separated structures under three-dimensional (3D) confinement. Block copolymers spontaneously form unique structures when the polymer molecules are assembled in confined spaces. The polymers are frustrated because of the limited space for phase separation, resulting in morphologies that are more complex than those of bulk films. In addition to conventional parameters such as block ratio, molecular weight and interactions of constituent polymers, the confinement effect is a significant parameter for controlling the morphologies. Here I give an overview of experimental and theoretical results for spherical 3D confinement and discuss the prospects for this area of research.
