Confined Sphere-Forming Block Copolymers: Phase Behavior and the Role of Chain Architecture

Abstract

We have considered the phase behavior of sphere-forming block copolymers in confinement by means of dynamic density functional theory (DDFT). We have in particular focused on the role of the chain architecture by choosing two equivalent AB and ABA block copolymers. The role of surface fields (EM) and film thickness (H) on the phase behavior is elucidated. For both systems, we identified surface reconstructions such as a wetting layer, parallel cylinders, perforated lamellae and lamellae, that were previously also found in confined cylinder-forming systems. In contrast to cylinder-forming systems, we observed that the part of the film that is affected by the surface field depends on the nature of the surface layer. In particular, cylindrical and lamellar surface reconstructions connect to the bulk structure in the interior of the film via cylindrical layers. The origin of this extended surface field range lies in the restricted chain conformations. As a direct consequence, confinement plays a role also in relatively thick films for stronger surface fields. We show in detail the complex interplay between the surface field and confinement, and how this introduces a certain amount of asymmetry even for this setup with symmetric wetting conditions. We identified the role of the chain architecture, which is only striking in neutral surfaces, where a small repulsive interaction balances the entropie gain of positioning the shortest block next to the surface. The AB diblock forms a layered spheres structure for most H, while for the ABA triblock a new structure, perpendicular cylinders that span the film, is observed for all considered H. © 2009 American Chemical Society.