Cylinder <-> sphere epitaxial transitions in block copolymer melts

We examine the Landau free energy surface of a diblock copolymer melt using self-consistent field theory. The topography of its surface is found to ex hibit a low-energy pathway connecting the local minima associated with the cylindrical (C) and spherical (S) morphologies. The pathway corresponds to an epitaxial transition where the periodicity of the two phases is matched with the cylinder axis oriented in one of the [111] directions of the bcc s pheres. The energy barriers and stability limits are evaluated, and from th em we conclude that the C <---->S transition should normally occur by a nuc leation and growth mechanism. Based on the generally small energy barriers, we suggest that fluctuation effects are much stronger than previously anti cipated. Furthermore, we examine the way the morphology evolves during a tr ansition and discuss its signature in a small-angle scattering experiment. (C) 2001 American Institute of Physics.