Confinement effects on the miscibility of block copolymer blends

Abstract

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 nu of monolayers. For neat melts, confinement causes the lamellar period, D, to deviate from its bulk value, D_b, in order to be commensurate with the film thickness, i.e., L=nu D/2. For blends, however, the melt also has the option of macrophase separating into nu^(l) large and nu^(s) small monolayers so as to provide a better fit, where L = nu^(l) D^(l)/2 + nu^(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.