Phase diagram of microphase-separated multiblock copolymers

The behavior of a random AB multiblock copolymer melt with Markovian sequence of monomers has been studied. We demonstrate that, in contrast to ordinary disordered solids, the quenched randomness does not destroy the long-range periodic order: In the Brazovskiı̆ approximation it is shown that thermodynamic fluctuations in the critical point give rise to an incommensurate lamellar structure whose period is greater than the average size of a block. This structure appears as a result of the first-order phase transition from the homogeneous phase, and these two phases coexist with one another in the finite temperature range. We calculate the phase diagram of such a multiblock copolymer near the critical point and show that, depending on the average block size, it can include the regions with different symmetry of the superlattice (lamellar, hexagonal and bcc) and the regions of coexistence of the corresponding microphase-separated phases. The parameters of these phases are calculated both in the mean field approximation and taking into account the fluctuation effects. We demonstrate that the period of the superstructures varies continuously with the temperature variation.