Photonic gaps in cholesteric elastomers under deformation

Significant anisotropic deformation and optical shifts in stretched cholesteric liquid crystal elastomers

This study explores the opto-mechanical response of cholesteric liquid crystal elastomers (CLCEs) subjected to uniaxial stretching along the x-axis, perpendicular to their helical z-axis. A definitive crossover is observed in the strain (εx) dependencies of various optical and mechanical properties, such as the transmission spectra, degree of mesogen orientation, Poisson's ratios, and tensile stress. At low strains, CLCEs exhibit a blue shift in the selective reflection band due to a reduction in the helical pitch, accompanied by a decrease in reflection selectivity for circularly polarized light. Beyond a certain critical strain further pitch alterations halt. This strain regime is marked by substantial anisotropic lateral contractions without any z-axis contraction, as indicated by a Poisson's ratio (μxz) of zero. Within this intermediate strain regime, local directors predominantly reorient towards the x-direction within the xy-plane, resulting in a quasi-plateau of tensile stress. Approaching a higher critical strain a complete loss of reflective selectivity occurs. Past this threshold, while the mechanical responses resemble those of isotropic conventional rubber, they retain a periodic structure albeit without phase chirality. These observed features are accounted for by the Mao-Terentjev-Warner model, especially when the network anisotropy parameter is adjusted to match the critical strain magnitude associated with the cessation of selective reflection.

3D-Printed Biomimetic Structural Colors

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

Recent Advances in 4D Printing of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are renowned for their large, reversible, and anisotropic shape change in response to various external stimuli due to their lightly cross-linked polymer networks with an oriented mesogen direction, thus showing great potential for applications in robotics, bio-medics, electronics, optics, and energy. To fully take advantage of the anisotropic stimuli-responsive behaviors of LCEs, it is preferable to achieve a locally controlled mesogen alignment into monodomain orientations. In recent years, the application of 4D printing to LCEs opens new doors for simultaneously programming the mesogen alignment and the 3D geometry, offering more opportunities and higher feasibility for the fabrication of 4D-printed LCE objects with desirable stimuli-responsive properties. Here, the state-of-the-art advances in 4D printing of LCEs are reviewed, with emphasis on both the mechanisms and potential applications. First, the fundamental properties of LCEs and the working principles of the representative 4D printing techniques are briefly introduced. Then, the fabrication of LCEs by 4D printing techniques and the advantages over conventional manufacturing methods are demonstrated. Finally, perspectives on the current challenges and potential development trends toward the 4D printing of LCEs are discussed, which may shed light on future research directions in this new field.

Multiresponsive Cylindrically Symmetric Cholesteric Liquid Crystal Elastomer Fibers Templated by Tubular Confinement

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The synthesis inside a sacrificial tube secures radial orientation of the cholesteric helix, and the ground state retroreflection wavelength is easily tuned throughout the visible spectrum or into the near-infrared by varying the concentration of a chiral dopant. The fibers display continuous, repeatable, and quantitatively predictable mechanochromic response, reaching a blue shift of more than -220 nm for 180% elongation. The cylindrical symmetry renders the response identical in all directions perpendicular to the fiber axis, making them exceptionally useful for monitoring complex strains, as demonstrated in revealing local strain during tying of different knots. The CLCE reflection color can be revealed with high contrast against any background by taking advantage of the circularly polarized reflection. Upon heating, the fibers respond-fully reversibly-with red shift and radial expansion/axial contraction. However, there is no transition to an isotropic state, confirming a largely forgotten theoretical prediction by de Gennes. These fibers and the easy way of making them may open new windows for large-scale application in advanced wearable technology and beyond.

Robust cholesteric liquid crystal elastomer fibres for mechanochromic textiles

Mechanically responsive textiles have transformative potential in many areas from fashion to healthcare. Cholesteric liquid crystal elastomers have strong mechanochromic responses that offer attractive opportunities for such applications. Nonetheless, making liquid crystalline elastomer fibres suitable for textiles is challenging since the Plateau-Rayleigh instability tends to break up precursor solutions into droplets. Here, we report a simple approach that balances the viscoelastic properties of the precursor solution to avoid this outcome and achieve long and mechanically robust cholesteric liquid crystal elastomer filaments. These filaments have fast, progressive and reversible mechanochromic responses, from red to blue (wavelength shift of 155 nm), when stretched up to 200%. Moreover, the fibres can be sewed into garments and withstand repeated stretching and regular machine washing. This approach and resulting fibres may be useful for applications in wearable technology and other areas benefiting from autonomous strain sensing or detection of critically strong deformations.

Liquid crystal-based structural color actuators

Animals can modify their body shape and/or color for protection, camouflage and communication. This adaptability has inspired fabrication of actuators with structural color changes to endow soft robots with additional functionalities. Using liquid crystal-based materials for actuators with structural color changes is a promising approach. In this review, we discuss the current state of liquid crystal-based actuators with structural color changes and the potential applications of these structural color actuators in soft robotic devices.

Photo-Rewritable Glaring Patterns Composed of Stripe Domains in Nematic Elastomers

Dynamic ordered micropatterns in polymeric materials provide an effective approach for the on-demand tuning of optical properties toward a smart optical material. In this study, we show that glaring patterns exhibiting strong anisotropic light diffusion can be developed at specific locations in nematic liquid-crystal elastomers with light-sensitive azobenzene units. Glaring originates from the stripe domains of the nematic directors that self-organize in light-irradiated regions after a simple uniaxial stretching and releasing process without any complicated lithographic technique. The nematic order transiently reduced by the photo-induced cis azobenzene isomers unlocks entropic elasticity, which induces local uniaxial shrinkage that causes buckling of the directors forming stripe domains. The written pattern on the film is tangibly visible with the backlight owing to the difference in anisotropic light diffusion. Furthermore, this pattern can be erased by light irradiation or thermal annealing. These films can be applied to optical elements for achieving augmented luminaries, security labeling, and sign-sheeting applications. This article is protected by copyright. All rights reserved.

Retention and deformation of the blue phases in liquid crystalline elastomers

The blue phases are observed in highly chiral liquid crystalline compositions that nascently organize into a three-dimensional, crystalline nanostructure. The periodicity of the unit cell lattice spacing is on the order of the wavelength of visible light and accordingly, the blue phases exhibit a selective reflection as a photonic crystal. Here, we detail the synthesis of liquid crystalline elastomers that retain blue phase I, blue phase II, and blue phase III. The mechanical properties and optical reconfiguration via deformation of retained blue phases are contrasted to the cholesteric phase in fully solid elastomers with glass transition temperatures below room temperature. Mechanical deformation and chemical swelling of the lightly crosslinked polymer networks induces lattice asymmetry in the blue phase evident in the tuning of the selective reflection. The lattice periodicity of the blue phase elastomer is minimally affected by temperature. The oblique lattice planes of the blue phase tilt and red-shift in response to mechanical deformation. The retention of the blue phases in fully solid, elastomeric films could enable functional implementations in photonics, sensing, and energy applications.

Permanently retaining liquid crystalline blue phases in a solid matrix warrants many prospective applications in photonics. Schlafmann and White are able to make a blue phase rubber that retains a crystalline structure upon deformation and responds with angle-dependent color changes.

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.

Cellulose Nanocrystal Elastomers with Reversible Visible Color

Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC-elastomer composite. By increasing the percentage elongation of the material (ca. 50-300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D-X-ray diffraction. Finally, by either water-patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC-elastomers.

Dispersion relation for electromagnetic propagation in stochastic dielectric and magnetic helical photonic crystals

We theoretically study the dispersion relation for axially propagating electromagnetic waves throughout a one-dimensional helical structure whose pitch and dielectric and magnetic properties are spatial random functions with specific statistical characteristics. In the system of coordinates rotating with the helix, by using a matrix formalism, we write the set of differential equations that governs the expected value of the electromagnetic field amplitudes and we obtain the corresponding dispersion relation. We show that the dispersion relation depends strongly on the noise intensity introduced in the system and the autocorrelation length. When the autocorrelation length increases at fixed fluctuation and when the fluctuation augments at fixed autocorrelation length, the band gap widens and the attenuation coefficient of electromagnetic waves propagating in the random medium gets larger. By virtue of the degeneracy in the imaginary part of the eigenvalues associated with the propagating modes, the random medium acts as a filter for circularly polarized electromagnetic waves, in which only the propagating backward circularly polarized wave can propagate with no attenuation. Our results are valid for any kind of dielectric and magnetic structures which possess a helical-like symmetry such as cholesteric and chiral smectic-C liquid crystals, structurally chiral materials, and stressed cholesteric elastomers.

Novel cholesteric liquid crystalline elastomers containing dimer type nematic and chiral liquid crystalline side-chains

A new set of cholesteric side chain liquid crystalline elastomers (ChLCEs) E₁–E₇ were graft copolymerized by hydrosilylation reaction with poly(methylhydrogeno)siloxane, nematic monomer (M₁), chiral monomer (M₂), and crosslinking agent (CL).

Tunable lasing in cholesteric liquid crystal elastomers with accurate measurements of strain

We report wide range and reversible tuning of the selective reflection band of a single crystal cholesteric liquid crystal elastomer (CLCE). The tuning is the result of mechanical shortening of the helical pitch achieved by imposing a uniform uniaxial strain along the helical axis. On doping the CLCE sample with a laser dye, we observe lasing from the CLCE in both glassy and rubbery states. By changing the cholesteric pitch, mechanical compression provides tuning of the laser emission from the dye doped CLCE over a significant part of the fluorescence band of the laser dye. In this work we demonstrate for the first time that both the CLCE pitch and the lasing wavelength are linearly dependent on the strain imposed on the CLCE film.

Thermal response of cholesteric liquid crystal elastomers

The effects of temperature variation on photonic properties of cholesteric liquid crystal elastomers (CLCEs) are investigated in mechanically unconstrained and constrained geometries. In the unconstrained geometry, cooling in the cholesteric state induces both a considerable shift of the selective reflection band to shorter wavelengths and a finite degree of macroscopic expansion in the two directions normal to the axis of the helical director configuration. The thermal deformation is driven by a change in orientational order of the underlying nematic structure S and the relation between the macroscopic strain and S is explained on the basis of the anisotropic Gaussian chain network model. The helical pitch varies with the film thickness in an affine manner under temperature variation. The CLCEs under the constrained geometry where thermal deformation is strictly prohibited show no shift of the reflection bands when subjected to temperature variation. This also reveals the strong correlation between the macroscopic dimensions and the pitch of the helical director configuration.

Stretch-tuneable dielectric mirrors and optical microcavities

We demonstrate how tuneable Distributed Bragg Reflectors (DBRs) and resonant micro-cavities can be built by a scalable layer assembly of the transparent utility rubbers polydimethylsiloxane and polystyrene-polyisoprene. Stretching the devices by more than 60% leads to an affine contraction of the layer thicknesses thereby tuning both DBR and cavity modes across the entire visible spectrum. Such rapidly- and reversibly- stretch-tuneable cavities can be used in tuneable micro-lasers and for quantitative optical strain sensing applications.

Deformation of cholesteric elastomers by uniaxial stress along the helix axis

The deformation of cholesteric elastomers by mechanical stress applied parallel to the helix axis is studied by calculation of the free-energy density. The Frank-elasticity contribution is taken into account. A chiral solvent, present at cross-linking time, is in general considered to be replaced after cross-linking by a solvent with different chirality. Two special cases considered are zero and unchanged solvent chirality, the first known as that of imprinted cholesteric elastomers, the latter equivalent to intrinsic cholesteric elastomers with chemically attached chiral groups. Depending on material parameters and imposed strain, the director can show a tilt towards the helix axis up to the maximum tilt, corresponding to a nematic state. In case of intrinsic elastomers with low conformation anisotropy, direct transitions from untilted to nematic states can be induced by straining. The helix structure of the director field is coarsened with an average wave number different to that of the information inscribed in the network at cross-linking time, if this lowers the average free-energy density. Switching between different states can be achieved with electric fields of reasonable values applied parallel to the helix axis. Spectra of the reflection of polarized light are calculated.

The role of Frank elasticity in cholesteric elastomers

The effect of Frank elasticity on deformations of cholesteric elastomers by mechanical stress applied perpendicular to the helix axis is studied by numerical minimization of the free energy. Above a critical strain, a solution with an only oscillating director is found to be stable in comparison to a distorted helix. At the critical strain, the contractions perpendicular to stress change discontinuously. The critical strain is found to increase with increasing Frank elasticity contribution to the free energy density, and to diverge when the conformation anisotropy of the polymer backbone vanishes. The results are compared with recent experiments which indicated that, in case of weak conformation anisotropies, the Frank elasticity contribution to the free energy cannot be neglected.

Mechanical manipulation of molecular lattice parameters in smectic elastomers

Smectic liquid crystalline elastomers (SLCE) represent unique materials that combine a 1-D molecular lattice arrangement and orientational order with rubber-elasticity mediated by a polymer network. Such materials may exhibit large thermo-mechanical, opto-mechanical and electro-mechanical effects, due to the coupling of macroscopic sample geometry and microscopic structural features. It is shown that the molecular layer dimensions in the smectic phases can be influenced reversibly by macroscopic strain of the material. We present a microscopic model on the basis of experimental results obtained by mechanical dilatation measurements, optical interferometry, X-ray scattering, (13)C NMR, FTIR and polarizing microscopy data. The model gives an explanation of the controversial results obtained in different types of smectic elastomers.

Resonantly absorbing one-dimensional photonic crystals

A compact theoretical description of the effects of dissipation on the propagation of light waves through a multilayer periodic mirror built from resonant absorbing atoms is presented. Depending on the lattice periodicity, ultranarrow photonic gaps, weak polaritonic gaps, as well as rather atypical gap structures may be observed. Because of the atom's absorption line shape Bloch gap modes may acquire quite a cumbersome structure which is thoroughly studied here or which may even disappear when dissipation becomes sufficiently strong. The same approach well applies also to resonantly absorbing photonic crystals based on excitonic resonances.