Theory of microphase separation for block copolymers in nonselective solvents.

A theory to describe microphase separation of block copolymers dissolved in a good, nonselective solvent is presented. We demonstrate that the addition of a solvent component to a copolymer melt leads to regions of two-phase coexistence between a solvent-rich disordered phase and solvent-poor ordered phases. These two-phase regions, however are shown to be extremely narrow and for all practical purposes can be ignored. On the basis of this observation we demonstrate how a reasonable approximation for the phase diagram of a block copolymer solution can be obtained from previous theoretical calculations for pure copolymer melts. Both semidilute and concentrated regimes are considered, and excluded volume effects and nonclassical corrections arising from composition fluctuations are taken into account. Our results are consistent with existing experiments on copolymers in neutral solvents and suggest new experiments.