Block Copolymer Photonic Microparticles as Modular Optical Units Beyond the Lattice

Stretchable photonic composites typically generate color through the compression or expansion of a continuous photonic lattice, which couples the optical response directly to the mechanical properties of the host matrix. We present an alternative design strategy based on discrete, anisotropic block copolymer (BCP) photonic microparticles whose orientation, rather than lattice spacing, determines the structural color. These particles, formed through confined emulsion self-assembly with internally ordered lamellar nanostructures, can be magnetically aligned at a defined angle within an elastic matrix before curing. Mechanical stretching then reorients the embedded particles, producing a reversible and predictable shift in reflected color that depends on the initial alignment rather than on changes in the matrix itself. This orientation-based mechanism allows the strain response, color range, and spatial pattern of a composite to be programmed independently of its mechanical formulation, enabling latent, direction-selective optical signatures suited for anticounterfeiting and strain sensing. Beyond this one-dimensional lamellar design, the same emulsion-confinement strategy can be extended to block copolymer particles with two-dimensional hexagonal cylinder and three-dimensional gyroid internal periodicities, in both cases representing the first demonstration of structural color from such discrete colloidal architectures. Solvent-driven swelling expands the characteristic domain spacing of each lattice into the visible range, with the cylinder morphology offering an additional in-plane tuning axis and the gyroid network producing a three-dimensionally periodic photonic bandgap. These distinct internal architectures expand the design space of BCP colloidal particles as modular photonic building blocks, pointing toward a broader class of processable polymeric composites with programmable structural color.