Ordering in stretch-tunable polymeric opal fibers

Dial-In Synthesis of 'Polymer Opal' Core-Interlayer-Shell Composite Nanoparticles

The emulsion polymerization process via which core-interlayer-shell polymer nanoparticles are synthesized is engineered to offer a crucial control of the eventual size and monodispersity of the polystyrene (PS) cores. We examine the role of key experimental parameters, optimizing the temperature, reactant purity, and agitation (stirring) rate. The subsequent addition of a poly(methyl-methacrylate) (PMMA) grafting layer and a poly(ethyl-acrylate) (PEA) shell layer produces composite particles, which are shear-orderable into opaline films, known as 'polymer opals'. We thus demonstrate pathways toward a 'dial-in' process, where the time taken to obtain the target core size is mapped to the expected resultant structural color. At reaction temperatures of 60 and 70 °C, viable conditions are found where all syntheses give an excellent level of monodispersity (polydispersity index < 0.02), suitable for interlayer and shell growth. These reports may be readily applied to a wider industrial scale fabrication pipeline for these polymeric photonic materials.

Scalable optical manufacture of dynamic structural colour in stretchable materials

Structurally coloured materials that change their colour in response to mechanical stimuli are uniquely suited for optical sensing and visual communication^1-4. The main barrier to their widespread adoption is a lack of manufacturing techniques that offer spatial control of the materials' nanoscale structures across macroscale areas. Here, by adapting Lippmann photography⁵, we report an approach for producing large-area, structurally coloured sheets with a rich and easily controlled design space of colour patterns, spectral properties, angular scattering characteristics and responses to mechanical stimuli. Relying on just a digital projector and commercially available photosensitive elastomers, our approach is fast, scalable, affordable and relevant for a wide range of manufacturing settings. We also demonstrate prototypes for mechanosensitive healthcare materials and colorimetric strain and stress sensing for human-computer interaction and robotics.

An Experimental and Theoretical Determination of Oscillatory Shear-Induced Crystallization Processes in Viscoelastic Photonic Crystal Media

A study is presented of the oscillatory shear-ordering dynamics of viscoelastic photonic crystal media, using an optical shear cell. The hard-sphere/“sticky”-shell design of these polymeric composite particles produces athermal, quasi-solid rubbery media, with a characteristic viscoelastic ensemble response to applied shear. Monotonic crystallization processes, as directly measured by the photonic stopband transmission, are tracked as a function of strain amplitude, oscillation frequency, and temperature. A complementary generic spatio-temporal model is developed of crystallization due to shear-dependent interlayer viscosity, giving propagating crystalline fronts with increasing applied strain, and a gradual transition from interparticle disorder to order. The introduction of a competing shear-induced flow degradation process, dependent on the global shear rate, gives solutions with both amplitude and frequency dependence. The extracted crystallization timescales show parametric trends which are in good qualitative agreement with experimental observations.

Structural color from solid-state polymerization-induced phase separation

Structural colors are produced by wavelength-dependent scattering of light from nanostructures. While living organisms often exploit phase separation to directly assemble structurally colored materials from macromolecules, synthetic structural colors are typically produced in a two-step process involving the sequential synthesis and assembly of building blocks. Phase separation is attractive for its simplicity, but applications are limited due to a lack of robust methods for its control. A central challenge is to arrest phase separation at the desired length scale. Here, we show that solid-state polymerization-induced phase separation can produce stable structures at optical length scales. In this process, a polymeric solid is swollen and softened with a second monomer. During its polymerization, the two polymers become immiscible and phase separate. As free monomer is depleted, the host matrix resolidifies and arrests coarsening. The resulting polymeric composites have a blue or white appearance. We compare these biomimetic nanostructures to those in structurally-colored feather barbs, and demonstrate the flexibility of this approach by producing structural color in filaments and large sheets.

Mechanochromism in Structurally Colored Polymeric Materials

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 field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

Thermally Responsive Photonic Fibers Consisting of Chained Nanoparticles

Fibers that can reversibly and passively change colors along with body temperatures are highly desired for potential applications including temperature sensors, smart wearables, and photonic devices. Here, we develop a facile strategy to fabricate thermochromic photonic fibers, which could exhibit tunable structural colors as a function of temperatures. The thermochromic fibers are prepared by aligning superparamagnetic, carbon-encapsulated Fe₃O₄ colloidal nanocrystal clusters (Fe₃O₄@C CNCs) in a thermoresponsive hydrogel, poly(N-isopropylacrylamide) (PNIPAM), forming chain-like structures under an external magnetic field before gelation. When the fiber is transferred from air to water at room temperature, it changes color from dark green to red as it swells. The red color can be reversibly changed back to green as the temperature is raised to 36 °C, while the fiber shrinks and the reflection peak shifts from 642 to 494 nm. The swelling of the fiber is anisotropic: by 60% in the diameter direction but 45% in the length direction. Therefore, the fiber can act as a thermochromic actuator.

Structural Color Fibers Directly Drawn from Colloidal Suspensions with Controllable Optical Properties

Fibers with structural colors are of great interest due to their unique dye-free optical properties and show great potential in the textile industry. However, the preparation of structural color fibers with controllable optical properties in a simple way is still a challenge. In this paper, we prepared structural color fibers by simply drawing bare fibers from colloid suspensions. The obtained fibers displayed brilliant colors due to the assembled photonic crystal structures on the surface. The layer numbers of colloid coatings were tunable by varying the drawing speeds, concentration of colloid suspension, and diameters of core fibers. The optical properties of the obtained structural color fibers varied by layer numbers, viewing angles, and structure defects and were systematically studied both by experimental measurements and by computer simulations. Furthermore, noncrack blue fibers were demonstrated by coating "soft" poly[styrene- co-(butyl acrylate)- co-(acrylic acid)] (P(St-BA-AA)) polymer spheres on PET fibers. The coating was mechanically robust and made the fiber bendable with weaving ability, which means this method has versatile applicability and could be potentially used for green textile dyeing.

Tuning of Structural Colors Like a Chameleon Enabled by Shape-Memory Polymers

Nature often uses structuring of materials for coloration rather than incorporating dye molecules, since single-construction materials are capable of producing any vivid visible color in plants and insects. By precisely engineering features that diffract or scatter light, more recently, humans have created similarly intense non-fading colors. Stretchable polymer opals have emerged as a single material which can dynamically shift across the whole visible spectrum using structural colors, by temporary stretching or compression. For energy efficiency and practical considerations, however, it is necessary to fix semi-permanently desired colors without continuous stretching or application of other stimuli or energy. Here, a polymer opal incorporating a shape-memory polymer embedded in its matrix can keep a particular color fixed without the application of external forces, yet can be reprogrammed to a different fixed color on demand. The influence of the material composition on its optical appearance, shape-fixity, and shape recovery abilities in controlled stretch experiments is quantified. High-speed printing-compatible localized compression pattern imprinting is shown to generate stable but easily erasable color patterns. This opens up the potential for durable and energy-efficient yet reusable and reconfigurable displays, wearables, or packaging and security labeling based on such polymeric film materials.

Chameleon-Inspired Mechanochromic Photonic Films Composed of Non-Close-Packed Colloidal Arrays

Chameleons use a non-close-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in the full visible range. Inspired by the biological process uncovered in panther chameleons, we designed photonic films containing a non-close-packed face-centered-cubic array of silica particles embedded in an elastomer. The non-close-packed array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photocuring of the dispersion medium. The artificial skin exhibits a structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables tuning without experiencing significant rearrangement of particles, providing elastic deformation and reversible color change, as chameleons do. The simple fabrication procedure consists of film casting and UV irradiation, potentially enabling the continuous high-throughput production. The mechanochromic property of the photonic films enables the visualization of deformation or stress with colors, which is potentially beneficial for various applications, including mechanical sensors, sound-vision transformers, and color display.

The structural coloration of textile materials using self-assembled silica nanoparticles

The work presented investigates how to produce structural colours on textile materials by applying a surface coating of silica nanoparticles (SNPs). Uniform SNPs with particle diameters in a controlled micron size range (207-350 nm) were synthesized using a Stöber-based solvent varying (SV) method which has been reported previously. Photonic crystals (PCs) were formed on the surface of a piece of textile fabric through a process of natural sedimentation self-assembly of the colloidal suspension containing uniform SNPs. Due to the uniformity and a particular diameter range of the prepared SNPs, structural colours were observed from the fabric surface due to the Bragg diffraction of white light with the ordered structure of the silica PCs. By varying the mean particle diameter, a wide range of spectral colours from red to blue were obtained. The comparison of structural colours on fabrics and on glasses suggests that a smooth substrate is critical when producing materials with high colour intensity and spatial uniformity. This work suggested a promising approach to colour textile materials without the need for traditional dyes and/or pigments. Graphical abstract.

Generating Bulk-Scale Ordered Optical Materials Using Shear-Assembly in Viscoelastic Media

We review recent advances in the generation of photonics materials over large areas and volumes, using the paradigm of shear-induced ordering of composite polymer nanoparticles. The hard-core/soft-shell design of these particles produces quasi-solid “gum-like” media, with a viscoelastic ensemble response to applied shear, in marked contrast to the behavior seen in colloidal and granular systems. Applying an oscillatory shearing method to sub-micron spherical nanoparticles gives elastomeric photonic crystals (or “polymer opals”) with intense tunable structural color. The further engineering of this shear-ordering using a controllable “roll-to-roll” process known as Bending Induced Oscillatory Shear (BIOS), together with the interchangeable nature of the base composite particles, opens potentially transformative possibilities for mass manufacture of nano-ordered materials, including advances in optical materials, photonics, and metamaterials/plasmonics.

Visibly vapor-responsive structurally colored carbon fibers prepared by an electrophoretic deposition method

A new type of photonic crystal carbon fiber exhibits tunable structural colors upon exposure to organic vapors.

Real-time measurements of crystallization processes in viscoelastic polymeric photonic crystals

We present a study of the dynamic shear ordering of viscoelastic photonic crystals, based on core-shell polymeric composite particles. Using an adapted shear-cell arrangement, the crystalline ordering of the material under conditions of oscillatory shear is interrogated in real time, through both video imaging and from the optical transmission spectra of the cell. In order to gain a deeper understanding of the macroscopic influences of shear on the crystallization process in this solvent-free system, the development of bulk ordering is studied as a function of the key parameters including duty cycle and shear-strain magnitude. In particular, optimal ordering is observed from a prerandomized sample at shear strains of around 160%, for 1-Hz oscillations. This ordering reaches completion over time scales of order 10 s. These observations suggest significant local strains are needed to drive nanoparticles through energy barriers, and that local creep is needed to break temporal symmetry in such high-viscosity nanoassemblies. Crystal shear-melting effects are also characterized under conditions of constant shear rate. These quantitative experiments aim to stimulate the development of theoretical models which can deal with the strong local particle interactions in this system.

Structural Coloration of Colloidal Fiber by Photonic Band Gap and Resonant Mie Scattering

Because structural color is fadeless and dye-free, structurally colored materials have attracted great attention in a wide variety of research fields. In this work, we report the use of a novel structural coloration strategy applied to the fabrication of colorful colloidal fibers. The nanostructured fibers with tunable structural colors were massively produced by colloidal electrospinning. Experimental results and theoretical modeling reveal that the homogeneous and noniridescent structural colors of the electrospun fibers are caused by two phenomena: reflection due to the band gap of photonic structure and Mie scattering of the colloidal spheres. Our unprecedented findings show promise in paving way for the development of revolutionary dye-free technology for the coloration of various fibers.

Revealing Invisible Photonic Inscriptions: Images from Strain

Photonic structural materials have received intensive interest and have been strongly developed over the past few years for image displays, sensing, and anticounterfeit materials. Their "smartness" arises from their color responsivity to changes of environment, strain, or external fields. Here, we introduce a novel invisible photonic system that reveals encrypted images or characters by simply stretching, or immersing in solvents. This type of intriguing photonic material is composed of regularly arranged core-shell particles that are selectively cross-linked by UV irradiation, giving different strain response compared to un-cross-linked regions. The images reversibly appear and disappear when cycling the strain and releasing it. The unique advantages of this soft polymer opal system compared with other types of photonic gels are that it can be produced in roll to roll quantities, can be vigorously deformed to achieve strong color changes, and has no solvent evaporation issues because it is a photonic rubber system. We demonstrate potential applications together with a fabrication procedure which is straightforward and scalable, vital for user take-up. Our work deepens understanding of this rubbery photonic system based on core-shell nanospheres.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.

Structurally colored carbon fibers with controlled optical properties prepared by a fast and continuous electrophoretic deposition method

Structurally colored fiber was fabricated by an electrophoretic deposition method under a circinate electric field. These fibers exhibit structural color, based on the external field-assembly of charged PMMA microspheres on the surface of the electroconductive carbon fiber, with reflectance spectra stretch-tunable in the 430-608 nm, which are determined by the lattice constants of the photonic crystals. Also, the influence of applied voltage, deposition time and electroconductivity on the number of deposited layers and efficiency were studied. In addition, we further developed a horizontal and continuous process to fabricate a long range structurally colored fiber. And the method is a drastic acceleration in comparison with the gravity sedimentation technique that needs weeks or even months, and it would be fast and facile for the further study of structural color on the surface of the fiber. The process may be used to simulate the conventional fiber coloration process. Such elastically tuned structurally colored fibers are of interest for many applications.

Cylindrical Bragg mirrors on leg segments of the male Bolivian blueleg tarantula Pamphobeteus antinous (Theraphosidae)

The large male tarantula Pamphobeteus antinous is easily recognized at the presence of blue-violet iridescent bristles on some of the segments of its legs and pedipalps. The optical properties of these colored appendages have been measured and the internal geometrical structure of the bristles have been investigated. The coloration is shown to be caused by a curved coaxial multilayer which acts as a "cylindrical Bragg mirror".

Fabrication of Structurally-Colored Fibers with Axial Core-Shell Structure via Electrophoretic Deposition and Their Optical Properties

Structurally colored fibers were fabricated using different-sized polystyrene (PS) nanospheres via electrophoretic deposition on conductive carbon fiber surfaces. The reflective spectra corresponding to different colors were taken by microzone and angle-resolved spectrometers from a single colloidal fiber. As confirmed by structural analysis, the outer layer of the core-shell colloidal fibers consisted of face-centered cubic (f.c.c.) domains without long-range order. It is revealed that the absence of long-range order in the colloidal assembly caused isotropic reflection in radial and longitudinal directions on the colloidal fibers. Furthermore, due to the incorporation of random defects during growth process, the experimental spectra are blue-shifted and broad compared to reflective spectra calculations based on the curved f.c.c. structure. This technique is speculated to have potential application in structural coloration and radiation-proof fabrics.

Structural evolution of electrospun composite fibers from the blend of polyvinyl alcohol and polymer nanoparticles

Electrospinning provides a versatile method for generating fibrous materials from a large variety of substances, including polymers, composites, proteins, and nano/microcolloids. In particular, the incorporation of nano/microparticles with polymeric materials is beneficial to many of electrospun fibers with multiple functionalities. This report evaluates the spinnability of a polymer solution containing polymer nanoparticles obtained through electrospinning. Tunable structures of electrospun composite fibers were obtained from a blended solution of polyvinyl alcohol (PVA) and polystyrene nanospheres (PSNs). The in-fiber arrangements of polymer nanoparticle fibers, influenced by the PVA:PSN weight ratio, and the viscosity of the blended solution and the size of PSNs were systemically studied. Once PVA was determined to dominate the solution, the diameter of the electrospun PVA fibers was comparable to the diameters of the colloidal particles, which confined the nanospheres into string-on-bead and necklace-like structures. When PSNs occupied a large portion of the solution, PVA wrapped the PSNs, forming a blackberry-like aggregate and a uniform colloidal fiber. The results from the colloid electrospinning serve as references in the creation of novel composite fibers involving various polymer nanoparticles via electrospinning. The obtained composite fibers of the polymers and colloids are expected to have potential application in various areas.