Polydispersity and block copolymer self-assembly

Block copolymers consist of two or more chemically distinct polymers that are covalently bound. These materials self-assemble into fascinating mesostructures with features on the nanometer length scale and have been the subject of intense research interest for about four decades. These efforts have generally focused on model block copolymer systems where the molecular weight distributions of all blocks are very narrow. Traditionally, many block copolymer systems have been prepared by living anionic polymerization and thus usually exhibit narrow molecular weight distributions in all blocks. Therefore, the assumption of monodisperse blocks that greatly simplifies theoretical work is on solid experimental ground. Preparation of block copolymers with relatively broad molecular weight distributions in one or more block has become increasingly common, however, as use of synthetic techniques such as controlled radical polymerization has proliferated. Advances in these techniques have increased the number of monomers readily incorporated into block copolymers and potentially will drive commercial costs down. These polymerization strategies often, however, result in broader molecular weight distributions than are typically obtained using living anionic, cationic, or metal-catalyzed techniques; understanding polydispersity effects should aid deployment of these block copolymers in advanced materials applications. This review describes both theoretical and experimental investigations of the effects of polydispersity on the melt-phase morphological behavior of block copolymers. The summary includes research efforts focused on both continuous molecular weight distributions and multicomponent blends. The review concludes with a summary and outlook on the potential utility of polydispersity as a tool to tune the morphological behavior of block copolymers.