Tunable Angle‐Independent Structural Color from a Phase‐Separated Porous Gel

Abstract

In the natural world, there exist many sources of inspiration for the creation of new high-performance materials. Structural color observed in various creatures, plants, and minerals has received a great amount of attention in recent years because of its eye-catching, non-photobleachable, and energy-saving properties, which are important for developing unprecedented coloring materials. One of the most popular methods to obtain artificial structurally colored materials is the elaboration of periodic dielectric structures on a length scale that is comparable to the wavelength of light in materials and thus causes multiple Bragg reflections. However, the fabrication of such mesoscale structures requires a great amount of time, effort, and, in some cases, cost when using existing top-down or bottom-up approaches. Herein, we demonstrate a facile self-assembly approach to obtain a porous gel exhibiting angle-independent structural color using a principle different from that of traditional dielectric periodically structured materials. The channel-like porous structure in the gel might be self-assembled from the phaseseparated polymers that result from spinodal decomposition and it might be locked in by kinetic arrest, likely through entanglement or cross-linking. The porous swollen gel changes color reversibly with solvent, covering the whole visible region in response to solvent composition and temperature by means of changes in the wavelength dispersions of the refractive indices of the gel portion and the solvent portion in the pore.Structural color is generally defined as coloration caused by complicated and diverse interactions between light and materials, such as interference effects, diffraction grating, light scattering, and dispersion of refractive index, and it essentially does not lose the energy of the light. [1][2][3] For the reason, structural color has been the subject of extensive studies for applications in energy saving mobile devices with a reflective full-color paperlike display. Though structural coloration can be caused by not only periodic dielectric structures but also randomly dispersed particles and a simple prism, recent studies of artificial structurally colored materials have focused mainly on the fabrication of periodical microstructures, such as multilayer films [4][5][6] and photonic crystals. [7][8][9] This trend is in part due to the relative ease to understand the structures, and the simple principle of Bragg diffraction, which generates structural color from periodic dielectric structures. However, to design a reflective full-color paperlike display with a wide viewing angle, the angle dependence of the structural color based on Bragg diffraction becomes a major issue.Recently, we demonstrated for the first time that amorphous arrays with a short-range order prepared from submicrometer spherical colloidal gel particles [10] or silica colloidal particles [11] are capable of displaying angle-independent structural color and can be applied to nonfading color materials and full-color pape...

25th Anniversary Article: Ordered Polymer Structures for the Engineering of Photons and Phonons

“…Building blocks such as tetrahedrally‐connected rods can be packed in arbitrary orientation or can be dispersed in a matrix without losing the omni‐directional structural color. Especially since the non‐crystalline keratin structure is a result of self assembly induced phase separation of β ‐keratin from the cellular cytoplasm, researchers are trying to create interconnected amorphous photonic structures via self‐assembly based on the phase‐separation of polymers . Other systems include an aqueous solution of aggregated gel particles and disordered arrays of PS microspheres with interstitial air .…”

“…Especially since the non-crystalline keratin structure is a result of self assembly induced phase separation of β -keratin from the cellular cytoplasm, 209 researchers are trying to create interconnected amorphous photonic structures via self-assembly based on the phase-separation of polymers. 213 Other systems include an aqueous solution of aggregated gel particles 214 and disordered arrays of PS microspheres with interstitial air. 215 The PS/air system displays structural colors with a very low angle dependence.…”

Absence of red structural color in photonic glasses, bird feathers, and certain beetles

“…However, to our knowledge there are no photonic glasses with saturated yellow, orange, or red color. While systems with angle-independent structural red have been reported [12,15,17,18,21], the color saturation for longwavelength hues is poor compared to that for blue. Interestingly, red angle-independent structural color also appears to be rare in nature.…”

Structural color from solid-state polymerization-induced phase separation

“…33 Structural colors have also been observed in gels of a single polymer, formed by polymerization-induced phase separation. 34 Here, we produce structural colors by polymerizationinduced phase separation (PIPS) in the solid state. Starting with a solid block of uncrosslinked polystyrene (PS) swollen with methyl methacrylate monomer (MMA), we initiate free-radical polymerization and subsequent phase separation.…”