Thin films of long and short symmetric AB diblock copolymers are examined using self-consistent field theory (SCFT). We focus on hard confining walls with a preference for the A component, such that the lamellar domains orient parallel to the film with an even number ν of monolayers. For neat melts, confinement causes the lamellar period, D, to deviate from its bulk value, Db, in order to be commensurate with the film thickness, i.e., L = νD/2. For blends, however, the melt also has the option of macrophase separating into ν(l) large and ν((s)) small monolayers so as to provide a better fit, where L = ν(l)D(l)/2 + ν(s)D((s))/2. In addition to performing full SCFT calculations of the entire film, we develop a semi-analytical calculation for the coexistence of thick and thin monolayers that helps explain the complicated interplay between miscibility and commensurability.
Keywords: Soft Matter: Polymers and Polyelectrolytes.