Аннотация:Modem physical concepts of microphase separation leading to thermodynamically equilibrium domain structure are reviewed. The relationship between the morphology and chemical structure of various polymer systems is discussed. Basic views on the influence of chemical structure and fluctuation effects on phase diagrams of polymer systems (e.g., melts and block copolymers of different architecture) are treated in detail. Fundamental differences are demonstrated between evolution of long-living, but unstable structures emerging during spinodal decomposition of low- and high-molecular-mass blends and establishment of thermodynamically equilibrium domain structure (which is examined in connection with the theory of random copolymers). New types of phase diagrams, which were recently predicted for solutions and blends of weakly charged polyelectrolytes, are described; the multiplet structure of ionomers is treated as a particular case of the domain structure; some data relevant to microphase separation of interpenetrating polymer networks and cross-linked homopolymer melts are discussed. A principally new class of polymer systems capable of microphase separation is suggested, which includes blends and melts of homopolymers with different glass transition temperatures of individual components and nonlocal entropy of mixing.