Entropic self-assembly of diblock copolymers into disordered and ordered micellar phases

We investigate the self-assembly of an athermal model of AB diblock copolymers into disordered and ordered micellar microphases. The original microscopic lattice model with ideal A strands and self-avoiding B strands is mapped onto a system of ultrasoft dumbbells, with monomer-averaged effective interactions between the centers of mass (CMs) of the two blocks. Extensive Monte Carlo simulations of this coarse-grained model are reported for several length ratios f ) L A/( L A + L B) of the two strands of lengths L A and L B. Clear-cut evidence is found for clustering and self-assembly into micelles with a mean aggregation number of n ≃ 100 beyond a critical micellar concentration (cmc) in the semidilute regime. The cmc is found to decrease with increasing f , as predicted by an analytic calculation based on the random phase approximation. The initially disordered dispersion of polydisperse spherical micelles undergoes a disorder-order transition to a micellar crystal phase at higher copolymer concentrations. The effective pair potential between the micellar CMs is determined by inverting the measured CM-CM pair distribution function and is found to become steeper with increasing density. © 2009 American Chemical Society