Microphase Separation in Correlated Random Copolymers

ABSTRACT: In the Leibler description of copolymer systems containing only two kinds of monomers, the state of the system is described by one order parameter, ª(r), representing the deviation of the local composition from the space-averaged value. The Landau free energy is expanded up to fourth order in powers of ª(r) and subsequently minimized with respect to the symmetry, the amplitude, and the period of the microstructure. However, if the system contains some kind of polydispersity, so that the copolymers are no longer all identical, the description in terms of only one order parameter is an oversimplification. Recently, Erukhimovich and Dobrynin refined the theory by introducing separate order parameters for each type of molecule (Erukhimovich, I.; Dobrynin, A. V.Macromol. Symp. 1994, 81, 253). Their procedure eventually leads to a Leibler-like free energy, with various additional terms. One of these terms, called the nonlocal term, is of great importance for the system under consideration, since it is responsible for microphase separation instead of the macrophase separation predicted by previous theories. In our article, the results of the application of this theory to a random copolymer, containing two types of monomers A and B, are presented. The resulting microstructure will be given for various values of the ł-parameter and the A-monomer fraction. 1