Template‐Free Synthesis and Selective Filling of Janus Nanocups

We report on the formation of shape‐ and surface‐anisotropic Janus nanocups (JNCs) by evaporation‐induced confinement assembly (EICA) of ABC triblock terpolymers. During microphase separation in spherical confinement, the triblock terpolymer spontaneously adopted a hemispherical shape with an inner concentric lamella–lamella (ll) morphology. Cross‐linking and disassembly of the microparticles resulted in well‐defined JNCs with different chemistry on the inside and outside. By synthesizing polymers with increasing length of the cross‐linkable block, we tuned the mechanical stability of the nanocups, which is relevant to control opening and closing of the cup cavity. We utilize the Janus properties for selective uptake of cargo exemplified by the filling of JNCs with polymer or gold nanoparticles. The directional properties of JNCs suggest applications in locomotion, oil‐spill recovery, storage and release, templating, and as nanoreactors with attoliter volume. Filled to the brim: Two types of Janus nanocups were prepared from ABC triblock terpolymers through cross‐linking and disassembly of shape‐anisotropic “tulip‐bulb” microparticles. Both nanocups feature mirrored Janus properties that allow selective filling of the cup interior or the exterior with compatible materials.