Effect of large degrees of polydispersity on strongly segregated block copolymers.

We investigate the effect of polydispersity on the lamellar phase of a diblock copolymer melt using self-consistent field theory (SCFT). A previous SCFT calculation predicted that polydispersity increases the domain spacing consistent with experiment, but it also suggested that the effect vanishes with increasing segregation contrary to experiment. We attribute this disagreement to a problem of slow convergence of the Gaussian-quadrature technique used to integrate over the molecular-weight distribution when either the segregation or polydispersity index is large. Here the problem is overcome by a new efficient algorithm that allows high-order quadratures for relatively little computational cost. When implemented, we find that the elevated domain spacing does indeed persist into the strong-segregation regime consistent with experiment. This conclusion is also substantiated by the analytical strong-segregation theory (SST).