Macromolecules of block copolymers and molecules of surfactants contain segments (blocks) of different chemical nature. In blends of homopolymers, even if they belong to the same homologous series, a slight incompatibility of components may result in phase separation of the solution or melt. Chemical bond between sections of molecules of different chemical nature contained in block copolymers and surfactants impedes such macroscopic phase separation to enable occurrence in the solution or melt of these compounds the so-called microphase separation, i.e. spatial separation into regions containing predominantly one type of functional group, caused by intermolecular interaction forces. The dimensions of such microregions are comparable to the size of macromolecules.

Microphase separation results in the development of micellar or other types of organised structures. For example, microphase separation in a melt of diblock copolymer whose macromolecules are made up of two bonded polymer chains (types A and B) may result in the development of aligned lamels (layers), cylinders or spherical regions, as well as two-bond structures (see figure). The formation by block copolymers of aligned structures with a period of approximately the wavelength of light (photonic crystals) shows promise for application in various optics devices.