Effects of polydispersity on phase behavior of diblock copolymers

The influence of polydispersity on the phase behavior of diblock copolymer melts is studied using a self-consistent field theoretic model. A perturbation theory is developed for the polydisperse system, in which the polydispersity index, kappa = M-w/M-n - 1, is taken as a perturbation parameter. Phase diagrams are constructed using the reciprocal space method. It is found that the order-disorder transition boundary shifts to higher temperatures, as predicted by a random phase approximation analysis. For asymmetric polymer distributions, polydispersity favors structures with curved interfaces. The domain spacing of the ordered structures increases with increasing polydispersity index. An analysis of the free energy provides an understanding of the mechanism of these effects.