Complex 3D-Printed Mechanochromic Materials with Iridescent Structural Colors Based on Core-Shell Particles

A scalable protocol for design and subsequent 3D-printing of polymeric core-shell-particles is reported. The particle synthesis by emulsion polymerization in starved-feed mode is used for tailoring particle architecture and composition. Control of size, mechanical properties, and chemical functionalities allow to achieve the specific requirement profile for subsequent extrusion-based additive manufacturing. The core-shell particles consist of hard polystyrene cores and a comparably soft polyalkylacrylate-based shell. Size and monodispersity, as well as core-to-shell ratio, are determined by means of dynamic light scattering and transmission electron microscopy. Thermal and rheological properties are investigated by means of dynamic scanning calorimetry and thermogravimetric analysis as well as oscillation and capillary rheometry. During 3D-printing, the monodisperse particles self-assemble into an ordered close packed lattice structure, leading to visible reflection colors according to Bragg's law of diffraction. Distinct and angle-dependent reflection colors are recorded via UV-vis spectroscopy. As the structural color depends, inter alia, on the underlying particle sizes, resulting colors are easily tunable by adjusting the applied synthesis parameters. Under mechanical deformation, the color changes due to controlled lattice deformation, which enables mechanochromic sensing with the printed objects. They are also promising candidates for decorative ornaments, smart optical coatings, or advanced security devices.