Compared to the systems which are composed of simple atoms or molecules, the structures of polymer systems in equilibrium are richer. The main reason is that a polymer molecule is composed of many monomers with the same or different chemical property. Take AB diblock copolymers as an example. Each molecule has a form AA• • •ABB• ••B where the monomer A and B have different chemical properties. The equilibrium structures for diblock copolymer, e.g., lamellar, hexagonal cylinder, bicontinuous gyroid and bcc sphere, etc, have been found for different composition of monomer B through computation  and experiments . This equilibrium structures (A-rich and B-rich phase domain) with a length scale of 10-100 nm, is the result of competition of two factors: the repulsion of two chemically distinct monomer A and B, and the conformal entropy penalty. Since monomers A and B are connected in each molecule, monomer A and B are only be separated microscopically. Thus it is called microseparation.