Nonequilibrium Self-Assembly of Ultrahigh-Molecular-Weight Block Copolymers into an Asymmetric Nanostructure

Self-assembly of block copolymers (BCPs) provides a unique platform for producing periodic and orderly structured soft materials of nanometer scale. The kinetics of the assembly process defines the accessible range of morphologies and allows for the formation of asymmetric hierarchies. Here, self-assembly of ultrahigh-molecular-weight BCPs with relatively slow molecular chain dynamics is used to fabricate a metastable asymmetric structure. More specifically, a kinetically trapped solvent vapor annealing process is applied to poly(styrene-block-2-vinylpyridine) thin films, wherein the concentration ratio of trichloroethylene to tetrahydrofuran of the mixed solvent vapor gradually increases throughout the annealing process, pushing the system away from equilibrium. Under such a dynamic process, poly(styrene-block-2-vinylpyridine) micelles rearrange into vertical protuberances that mimic moth-eye structures enhancing the light transmission. Sequential infiltration synthesis is used to convert the poly-2-vinylpyridine domain into alumina in order to verify the formation mechanism of the asymmetric protuberances. It is determined that vertically packed and merged micelles are only formed under a gradually built solvent vapor environment.