Geometry-controlled interface localization-delocalization transition in block copolymers.

Lamellar copolymers confined into a film of thickness D by two stripe-patterned surfaces, which are rotated against each other by a twist angle α, form lamellar domains that register and align with the respective chemical surface patterns. The two domains of thickness x and D-x are separated by an interface that resembles a twist grain boundary. At small twist angles α or strong selectivity of the surface patterns, this interface fluctuates around the middle of the film, x≈D/2, while the interface is localized at one of the surfaces, x≈0 or x≈D, in the opposite limit. These two morphologies are separated by an interface localization-delocalization transition (ILDT) that can be controlled by the twist angle α. For thin films, we find a second-order ILDT while the ILDT is first-order for large D values. A phenomenological interface Hamiltonian is used to relate the findings to the ILDT of symmetric mixtures, and the predictions are confirmed by molecular simulation.