Bottlebrush Block Copolymer Photonic Pigments: Interfacial Self-Assembly Dependent on Grafting Density

Structural colored microparticles, or photonic pigments, can be readily prepared via interfacial self-assembly of amphiphilic bottlebrush block copolymers (BBCPs) in a water-in-oil-in-water multiple emulsion system formed by organized spontaneous emulsification. However, how chain conformation, governed by the grafting density, affects self-assembly, porous morphology, and optical properties has remained unclear. Here, we synthesize a family of amphiphilic BBCPs with precisely tuned grafting density Z of the hydrophilic side chains by randomly incorporating a small-molecule diluent into the hydrophilic block. As Z decreases, the internal nanostructures within the microparticles evolve from ordered, to less ordered, and ultimately to disordered or fragmented states. We identify a transitional Z window (0.18-0.35) that bridges well-controlled self-assembly (Z > 0.35) and an uncontrolled regime (Z < 0.18). Dynamic interfacial tension measurements further confirm that grafting density greatly influences the arrangement of BBCPs at the water/oil interface. This work provides design guidelines for tailoring BBCP architecture to achieve precise control over microparticle morphology and the resulting optical properties.