Interface-engineered transparent and mechanochromic non-close-packed photonic crystals

Abstract

Mechanochromic photonic crystals are promising for smart optical materials due to their tunable photonic stop band. Here, we report interface-engineered transparent and mechanochromic non-close-packed photonic crystals (NPCs) by incorporating polystyrene@vinyl-modified SiO₂ (PS@V-SiO₂) nanospheres (n_PS = 1.59, \(n_{\mathrm{V}-\text{SiO}_{2}}=1.46\)) into a photocurable phenoxypolyethylene glycol acrylate (PEGPEA) matrix (n_PEGPEA = 1.52). The nanospheres formed solvation-mediated liquid NPCs in the precursor. Ultraviolet (UV) curing promoted copolymerization between surface C=C bonds of nanospheres and the matrix, which reduced interfacial scattering and enabled highly transparent NPC films. Meanwhile, non-uniform polymer shrinkage led to variations in the ordering of nanospheres, especially in structures with a low volume fraction (φ ⩽ 0.23). Under external strain (ε: 0–64%), the film exhibited a dynamic color response. Initially, stretching enhanced the ordering of the nanospheres and the reflection intensity of NPCs, thereby activating the structural color. Further deformation, however, introduced defects and reduced the reflectivity. A blue shift of ∼213 nm was achieved in an NPC (φ = 0.23) fabricated by 170 nm of PS@V-SiO₂ nanospheres, accompanied by a color gradient from red to blue. Comparisons across SiO₂-poly(ethylene glycol) diacrylate (PEGDA, n_PEGDA = 1.45), SiO₂-PEGPEA, and PS@V-SiO₂-PEGDA NPC systems highlighted the key role of interfacial scattering, which is affected by the synergistic effects of interfacial covalent polymerization, refractive index matching between the elastomer matrix and nanospheres, and the crosslinking density. This work demonstrates spectrally tunable mechanochromism via size control and patterned anti-counterfeiting labels, thereby providing insights for designing advanced anti-counterfeiting materials applicable in flexible electronics and displays.