Engineering Photonic Pigments From Titania–Block Copolymer Composites

Abstract

Structural color pigments based on block copolymer self-assembly offer non-fading color. However, their brightness and saturation are often limited by a low-refractive index contrast and incoherent scattering arising from disorder. Here, we engineer non-iridescent photonic pigments by incorporating ligand-functionalized TiO₂ nanoparticles into the poly(2-vinylpyridine) domains of onion-like polystyrene-block-poly(2-vinylpyridine) microspheres fabricated by emulsion-confined self-assembly. Thermally assisted solvent evaporation improves long-range lamellar order at high nanoparticle loadings, suppressing disorder and enabling strong Bragg reflections in individual microspheres. Quantitative loading analysis reveals high incorporation efficiency up to large selective TiO₂ fractions. Ultra-small angle X-ray scattering and electron microscopy demonstrate that increasing the nanoparticle content reduces lamellar periodicity and couples refractive index enhancement to a blue-shift. By combining selective swelling with molecular-weight tuning, we shift the Bragg peak across the visible spectrum and fabricate pigments with enhanced saturation and luminance compared to nanoparticle-free references. We use optical modeling with a transfer-matrix method and Ewald-sphere analysis to interpret the measurement geometry, guide the design process, and establish links between the objective numerical aperture, mode collection, and perceived coloration. These results provide practical design rules for high-performance structural pigments by coupling control over ordering, scattering, and composite refractive index contrast.