Dispersity-Driven Stabilization of Coexisting Morphologies\nin Asymmetric Diblock Copolymer Thin Films

Abstract

Despite\ndecades of research, characterization of the effects of\npolymer chain dispersity on the structural properties of block copolymer\nthin films remains challenging. We present an integrated experimental\nand modeling approach to characterize the morphology of thin films\ncontaining asymmetric diblock copolymers. Specifically, we used synergistic\nneutron reflectivity (NR) and self-consistent field theory (SCFT)-based\nmodeling to realize unexpected morphology of thin films containing\nasymmetric copolymers. Using NR, a highly stable and reproducible\nmixed phase of coexisting cylinders and lamellar domains was discovered\nin asymmetric poly­(deuterated-styrene-<i>b</i>-<i>n</i> butyl methacrylate) (dPS-PBMA) copolymer thin films containing 34%\nvolume fraction of dPS. SCFT reveals how to obtain such a thermodynamically\nstable morphology in the presence of disperse majority block and asymmetric\ninteractions of polymer species with surfaces. Stabilization of the\ncoexisting domains is a consequence of the depth segregation based\non chain-length distribution. The asymmetric chains microphase-separate\ninto cylindrical domains close to the substrate and near-symmetric\nchains form lamellar domains at the air interface. In the absence\nof dispersity, the coexistence of cylindrical and lamellar domains\nis thermodynamically unstable because of the absence of depth segregation.\nOverall, such an effect of dispersity on diblock copolymer thin-film\nmorphology reveals a unique and powerful strategy to create coexisting\nnanoscale domains and tailor properties of thin films.