Polymerization-induced highly brilliant and color-recordable mechanochromic photonic gels for ink-free patterning.

Mechanochromic photonic crystals (MPCs) are extremely attractive since they can adjust their structural color by forces. However, the poor color saturation and color-recordability of conventional MPCs significantly limit their practical applications. Herein, a highly brilliant and color-recordable MPC gel (MPCG) has been fabricated by photopolymerizing the liquid photonic crystals with silica particles non-closely packed in acrylate, dichlorobenzene, and oleylamine. Photopolymerization induces elastic gradient non-close-packing structures and thus broad photonic bandgaps (>100 nm), resulting in 1) high color saturation despite possessing a small refractive index contrast (0.06), 2) remarkable mechanochromic properties, including a large wavelength tuning range (228 nm), fast responsiveness (8.8-10.3 nm/ms), and high sensitivity (4.4 nm/kPa), and 3) unconventional color-recordable properties. MPCGs were experimentally proved to be ideal rewritable papers for constructing multicolor and high-resolution patterns in an ink-free way, difficult for traditional MPC-based units. The unique working mechanism of polymerization-induced phase separation and thus continuous swelling and gelation, and precise design of materials and structures are the keys to MPCGs' characteristics. This study paves a new way for constructing advanced stimulus-responsive photonic structures and will promote their applications in printing, display, anti-counterfeiting, etc.