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Cholesteric liquid crystal oligomers are an interesting class of temperature responsive structurally colored materials. However, the role of endcap molecules in these oligomers is rather unexplored. In this work, we demonstrate the role of endcap molecules on structural color stability and hypsochromic shift in temperature‐responsive cholesteric liquid crystal oligomers. First, new liquid crystal monoacrylate endcap molecules are synthesized, which are then used to synthesize various cholesteric liquid crystal oligomers. In addition, cholesteric oligomers using commercial monoacrylate endcap molecules are also prepared. It is found that the molecular weight and the polydispersity of the oligomers can be tuned by the endcapping molecules. The oligomers are used to produce reflective, structurally colored coatings. It was found that the coatings using the commercial monoacrylate lose their color and crystallize over time, most likely due to the presence of crystalline dimers. The coatings containing the newly synthesized monoacrylate endcap molecules did not exhibit this crystallization, resulting in structurally colored coatings that remained stable over time. These latter coatings possessed temperature responsive hypochromic behavior, which makes them interesting for advanced optical applications. [ABSTRACT FROM AUTHOR]

Envisioning robotic devices with functionality resulting from processing rather than exclusively material composition, multimodal responsive devices produced from a single azobenzene‐functionalized cholesteric liquid crystal elastomer ink are demonstrated. The resulting device displays simultaneous structural color and actuation in response to both ultraviolet and blue light exposures. Through direct ink writing, a microextrusion‐based additive manufacturing technique, the liquid crystalline ink is deposited into different effective mesophases—planar and slanted cholesteric, and a uniaxial pseudo‐nematic state—by altering only two printing conditions: deposition speed and time before photopolymerization. Comparing the three different printed mesophases, marked differences in optical properties and actuation behaviors are found. Demonstrators based on bilayer architectures present concepts for interactive soft multiactuators deposited in a single printing step from a single ink. [ABSTRACT FROM AUTHOR]

Hydrogen‐bonded cyclohexanoic derivatives are applied in a structural colored cholesteric liquid crystal (CLC) helical polymer that demonstrates an extraordinary dual thermal response. Integration of the dynamic hydrogen‐bonded cyclohexanoic acid derivatives in the CLC polymer as supramolecular crosslinks yields an immediate and irreversible loss of the green structural color upon exposure to temperatures above the cholesteric‐to‐isotropic transition temperature (TNI) caused by the helical order loss. Exposure to temperatures below the TNI yields a gradual irreversible red color shift of the reflected wavelength over broad temperature and time ranges. Most likely due to polymerization confinement in combination with the dynamics of the carboxylic acid hydrogen bonds, helical unwinding takes place over time causing the red shift of the reflected wavelength. The temperature response can be programmed by altering the chemical composition of the liquid crystal mixture. Furthermore, the green, structural color of the polymer can also be easily changed making it possible to print different colored indicators combined with different respective temperature responses on a single substrate. The printed optical sensors can be easily delaminated by dissolving the non‐covalent crosslinked polymer. Such optical polymer materials are attractive for easy processable time–temperature integrators and thermal paper. [ABSTRACT FROM AUTHOR]

Additive manufacturing with stimuli‐responsive materials—4D printing—is a rapidly growing field, with direct ink writing allowing deposition of a wide variety of materials. The synthesis of a humidity‐sensitive cholesteric liquid crystal oligomer ink is reported. With the responsive cholesteric ink, demonstrator devices exhibiting the ink's "four dimensionality" are printed in disparate fashions: as a structural color change or as a preprogrammed deformation mode. After printing, the photonic ink changes color in response to atmospheric humidity, demonstrated as a hydrochromic coating precisely deposited atop a 3D‐printed beetle. After activation in aqueous acid, the beetle exhibits vibrant color shifts across the visible spectrum. Alternatively, a scallop‐inspired actuator with a 3D‐programmed structural color is selectively treated with acid, to allow reversible "opening" and "closing" when exposed to humid and dry air, respectively. The ink enables additive manufacturing of both monolithic and multimaterial stimuli‐responsive, shape‐changing, structurally colored objects, toward broad application of cholesterics in future "smart," 4D structurally colored devices. [ABSTRACT FROM AUTHOR]

Polymer indicators are autonomous responsive materials that provide an optical signal of a specific exposure in time. This review describes the different polymer systems utilized to obtain indicators based on structural color. Structural color originates from the interaction of light with a periodic nanostructured polymer which causes a specific wavelength to be reflected. This reflected light can be used for fabricating battery-free indicators that show visible structural color changes upon exposure to a stimulus or analyte. In this review, the typical structural color response types categorized by stimulus are discussed and compared. Furthermore, the steps toward possible applications of optical indicators based on structural colored polymers are outlined. [ABSTRACT FROM AUTHOR]

We report on the wavelength‐selective photopolymerization of a hybrid acrylate‐oxetane cholesteric liquid crystal monomer mixture. By controlling the sequence and rate of the orthogonal free‐radical and cationic photopolymerization reactions, it is possible to control the degree of phase separation in the resulting liquid crystal interpenetrating networks. We show that this can be used to tune the reflective color of the structurally colored coatings produced. Conversely, the structural color can be used to monitor the degree of phase separation. Our new photopolymerization procedure allows for structuring liquid crystal networks in three dimensions, which has great potential for fabricating liquid crystal polymer materials with programmable functional properties. [ABSTRACT FROM AUTHOR]

Cholesteric liquid crystals (CLCs) are chiral photonic materials reflecting only circularly polarized light with the same handedness as the helical polymer structure. Concurrent shape and color changes can be achieved using CLCs, but the fabrication of CLCs with switchable 3D shape, structural color, and hyper‐reflectivity, that is, reflecting both left‐ and right‐handed circularly polarized light simultaneously, has not yet been achieved. Here, CLC elastomer (CLCE) actuators are reported to reflect equal amounts of left‐ and right‐handed circularly polarized light. Hyper‐reflectivity is achieved by uniaxially stretching the partially crosslinked film to induce helix deformation which is then fully crosslinked to fix the deformed helical structure. The shape, structural color, and hyper‐reflectivity of the polymer film are switchable with temperature. At high temperatures, only right‐handed circularly polarized light is reflected and the color is redshifted. The film can be shaped in three dimensions: a structural colored 3D shaped beetle is fabricated using molding, which reflects both left‐ and right‐handed circularly polarized light and shows reversible, temperature responsive structural color and 3D shape changes. Hence, 4D engineered bioinspired multifunctional materials are fabricated, which are interesting for applications ranging from sensing actuators to switchable hyper‐reflective films and objects. [ABSTRACT FROM AUTHOR]

Mechanochromic photonic polymers, which can change structural color upon mechanical deformation, are promising for many applications including strain/stress sensors and security features. Here, a patterned poly(dimethylsiloxane) (PDMS)/cholesteric liquid crystal elastomer (CLCE) photonic mechanochromic bilayer film for reversible image reveal is reported, in which the PDMS component determines the mechanical properties and the CLCE determines the mechanochromic properties. The structural color pattern is achieved by locally crosslinking the CLCE layer at different temperatures using a photomask and utilizing the temperature response of the cholesteric liquid crystal main‐chain oligomers. The resulting PDMS/CLCE bilayer is able to reversibly reveal the predesigned invisible pattern upon stretching, showing that this method can be used to convey more advanced information to a potential user than would be possible with a simple homogeneous color change. It is also demonstrated that the bilayer film can be used as a stimuli‐responsive sticker able to detect bending deformations. [ABSTRACT FROM AUTHOR]