ISSN 0965-545X, Polymer Science, Ser. A, 2014, Vol. 56, No. 3, pp. 383–392. © Pleiades Publishing, Ltd., 2014. Original Russian Text © L.V. Zherenkova, P.V. Komarov, 2014, published in Vysokomolekulyarnye Soedineniya. Ser. A, 2014, Vol. 56, No. 3, pp. 346–356. 383 INTRODUCTION Copolymer macromolecules consisting of units of various chemical types can form ordered spatial struc- tures with various morphologies [1]. The self-organiz- ing ability is determined, on the one hand, by different characters of interaction between copolymer units and, on the other hand, by the covalent connection of units within the same macromolecule. The second factor hampers separation of the system into homoge- neous macroscopic phases; under certain conditions, this circumstance may stabilize one or another type of microdomain structure. The segregation of block copolymers accompanied by the formation of micro- structures of various morphologies has been much studied in recent decades [1–3]. Structure formation depends on the composition of the copolymer and the degree of segregation, χN, where χ is the Flory–Hug- gins parameter and N is the degree of polymerization. When a solvent is introduced into a block copoly- mer, the effective degree of segregation should be esti- mated to allow for interaction not only between copol- ymer blocks but also between the blocks and the sol- vent. An increase in the concentration of the solvent at a specific temperature entails diverse lyotropic phase transitions because the effective degree of segregation corresponds now to other stable structures different from that of the pure block copolymer. The tempera- ture dependence of effective interactions between copolymer blocks leads to an additional number of thermotropic phase transitions. Both lyotropic and thermotropic phase behaviors of block copolymers in selective and nonselective aqueous and organic sol- vents have been much studied theoretically and exper- imentally [4–8]. Ionic liquids (ILs) belong to a new class of solvents composed of ions solely: bulky organic cations and inorganic or organic anions [9, 10]. In fact, these are low-temperature melts of organic salts. The supramo- lecular structure of an IL is characterized by a high level of self-organization and the presence of a three- dimensional network composed of anions and cations. An ionic liquid features amphiphilic behavior because the cation is composed of polar and nonpolar groups. The combination of block copolymers with ILs made it possible to obtain a new class of functional materials that is characterized by structure formation on the nanometer level [11]. Functionality is provided by the unique physicochemical properties of ILs, whereas structure formation is related to the self-orga- nization of the block copolymers. A wide spectrum of technological applications has stimulated theoretical investigation of the structural features of IL–block- copolymer systems; the mechanical properties of such a system may be optimized independently. Recent experimental studies of the phase behavior of block-copolymer–IL mixtures [12–22] demon- strated that, in many respects, it differs from the phase behavior of block copolymers in organic and aqueous solvents. Note that, under conditions where an IL is a selective solvent for a block copolymer, lyotropic and thermotropic transitions qualitatively correspond in many respects to the character of the phase behavior of melts of block copolymers and their mixtures with organic solvents. For example, small-angle-scattering data [13, 15] indicate the presence of lamellar, cylin- drical, and disordered phases. At high concentrations of a block copolymer, an IL behaves as a salt, thereby increasing the glass-transition temperature T g of the Study of the Phase Behavior of a Diblock Copolymer in an Ionic Liquid: Outlook for Use of the Integral-Equation Theory L. V. Zherenkova a, * and P. V. Komarov b a Tver State University, Sadovyi per. 35, Tver, 170002 Russia b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia *e-mail: zherenkova@mail.ru Received July 31, 2013; Revised Manuscript Received October 2, 2013 Abstract—It is shown that the integral-equation theory may be used to study the phase behavior of a diblock copolymer in an ionic liquid with allowance made for the solvent structure. Features of microphase separa- tion are exemplified via calculation of the mean-field spinodal temperature and order–disorder transition temperature as functions of the copolymer concentration at two different lengths of the cationic tail of the ionic liquid. The need to allow for the solvent structure during construction of the theory of the phase behav- ior of block copolymers in ionic liquids is substantiated. DOI: 10.1134/S0965545X14030201 THEORY AND SIMULATION