Mechanochromism in Structurally Colored Polymeric Materials

Abstract

of color in these materials has fascinated scientists for centuries, with early breakthroughs in understanding these phenomena made by Hooke, Newton [6] and Lord Rayleigh. [7] Another type of structural color arises from surface plasmon resonance, i.e., the resonant coupling between light and metallic nanostructures, which craftspeople have used since the Bronze age to impart color to ceramics and glass. [8,9] Over the past 30 years, fabricating structurally colored materials has been greatly facilitated by the rapid development of lithography-based technologies and directed self-assembly methodologies, which permit precise control of structural ordering at the nano-and microscale. Photonic and plasmonic materials can now be used to control and manipulate the flow of light in a plethora of colorful applications in the modern world, ranging from microoptical components [10] and lasing materials [11,12] to household products, such as nontoxic and photostable pigments for paints and cosmetics. [13-15] Artificial, structurally colored materials have been made predominantly from hard, rigid components. Their colors are stable as long as the nanostructure remains intact, but the application of excessive forces to these relatively brittle structures typically leads to an irreversible structural change and thus a change or loss of color. By contrast, many natural photonic coloration strategies are dynamic and responsive, such as in chameleons, [16] cephalopods, [17-20] tetra fish, [21] and the tortoise beetle. [22] For example, chameleon skin contains photonic arrays of high-refractive-index guanine nanocrystals that are embedded in a softer, low-refractive-index cytoplasmic matrix. [16] The reversible deformation of such composite structures enables the animal to reflect wavelengths of light in a spatially and spectrally selective manner for dazzling camouflage displays. Recent progress in synthetic approaches toward precisely ordered soft nanostructures has opened up a new class of structurally colored materials that also respond dynamically and reversibly to stimuli, such as pH, heat, light, and deformation. The design of responsive photonic structures has often been informed and inspired by natural coloration strategies. [3,23-25] To mimic natural materials and access responsive structural color, polymers are frequently employed as a component on account of their mechanical (low moduli and optionally elastic behavior) and optical (high transparency, low refractive Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this fi...

Shape‐Memory Photonic Thermoplastics from Cellulose Nanocrystals

“…[ 16 ] In a SMPC, the microstructure of the photonic crystal is altered when the SMP matrix changes between its permanent and temporary shapes, ultimately changing the color of the material. This structure‐dependent color is extremely beneficial as it can be tuned, will not fade, can be mechanoresponsive [ 17 ] and does not require the use of toxic or photodegradable dyes. [ 18,19 ] SMPCs have been prepared that can respond to heat, [ 20–23 ] electric fields, [ 24 ] light, [ 25 ] mechanical deformation, [ 26 ] organic solvents [ 20,27 ] and vapors.…”

Mechano‐Optical Response of Novel Polymer Composites Based on Elastic Polyurethane Matrix Filled with Low‐Molar‐Mass Cholesteric Droplets

“…Such materials are also so‐called «structurally colored». [ 22 ] Structurally colored mechanochromic polymers exhibit deformation‐induced changes in their optical properties [ 23–25 ] due to a change in the distance between layers or their thickness, or violation of the periodicity of the structure, a change in the angle of incidence of light, etc. In the case of photonic materials, deformation changes the position of the bandgap, or stop‐band (1D, [ 26–28 ] 2D, [ 29,30 ] or 3D [ 31–33 ] structures, diffraction gratings [ 34,35 ] ); for plasmonic materials, changes in the localized surface plasmon resonance frequency is observed.…”

Elastomer-immobilized tunable colloidal photonic crystal films with high optical qualities and high maximum strain

“…17,18 If elastomer-immobilized colloidal crystals with even lower particle volume fractions are prepared, they may exhibit a higher maximum strain. 22 However, this preparation is a challenge because strong electrorepulsive interactions are not generally generated in elastomer precursor solutions. In addition, although optical qualities, especially the uniformity of Bragg reflection colour over a large area, are essential for practical applications, those of colloidal crystals prepared using the common methods are not high because they provide polycrystalline structures.…”