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Structurally coloured responsive materials provide an interesting avenue for the development of autonomous temperature regulating window films. One interesting class of such thermochromic materials is cholesteric liquid crystals. However, cholesteric liquid crystals have rarely been applied in coatings for smart window applications. In this work, we report the synthesis of endcapped cholesteric liquid crystal oligomers and its application as near-infrared thermochromic coatings for windows. Two isomerically pure monoacrylate endcapping molecules and its isomeric mixture are synthesised. The molecules are used to synthesise a variety of endcapped cholesteric liquid crystal oligomers to study the effect of the isomeric purity on the thermochromic properties of the coatings. It is found that while the oligomers are almost identical in composition and phase behaviour, only one isomer produces a clear transparent coating, highlighting the significance of minute isomeric differences. Remarkably, the thermochromic behaviour of the coatings for all oligomers is the same. The best performing oligomer is able to reversibly blueshift by 250 nanometres when heated from room temperature to 100°C, opening the way of cholesteric liquid crystals for use in temperature regulating window films. [ABSTRACT FROM AUTHOR]

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]

Untethered soft fiber actuators are advancing toward next‐generation artificial muscles, with rotating polymer fibers allowing controlled rotational deformations and contractions accompanied by torque and longitudinal forces. Current approaches, however, are based either on non‐recyclable and non‐reprogrammable thermosets, exhibit rotational deformations and torques with inadequate actuation performance, or involve intricate multistep processing and photopolymerization impeding scalable fabrication and manufacturing of millimeter‐thick fibers. Here, the melt‐extrusion and drawing of a 50 m long thermoplastic liquid crystal elastomer fiber with a ≈1.3 mm diameter on a large scale is reported. With the responsive thermoplastic material, rotating actuators are fabricated via easily exploited programming freedom resulting in large, reversible rotational deformations and torques. The actuation performance of the twisted fibers may be controlled by the programmed twisting density without complicated preparation steps or photocuring being required. The thermoplastic behavior enables fabrication of plied fibers, demonstrated as a triple helical twisted rope constructed from individual rotating fibers delivering up to three times as great rotational and longitudinal forces capable of reversibly opening and lifting a screw cap vial. Besides the programmability, the thermoplastic material employed lends itself to be completely reprocessed into other configurations with self‐healing properties in contrast to thermosets. [ABSTRACT FROM AUTHOR]

The extracellular environment defines a physical boundary condition with which cells interact. However, to date, cell response to geometrical environmental cues is largely studied in static settings, which fails to capture the spatiotemporally varying cues cells receive in native tissues. Here, a photoresponsive spiropyran‐based hydrogel is presented as a dynamic, cell‐compatible, and reconfigurable substrate. Local stimulation with blue light (455 nm) alters hydrogel swelling, resulting in on‐demand reversible micrometer‐scale changes in surface topography within 15 min, allowing investigation into cell response to controlled geometry actuations. At short term (1 h after actuation), fibroblasts respond to multiple rounds of recurring topographical changes by reorganizing their nucleus and focal adhesions (FA). FAs form primarily at the dynamic regions of the hydrogel; however, this propensity is abolished when the topography is reconfigured from grooves to pits, demonstrating that topographical changes dynamically condition fibroblasts. Further, this dynamic conditioning is found to be associated with long‐term (72 h) maintenance of focal adhesions and epigenetic modifications. Overall, this study offers a new approach to dissect the dynamic interplay between cells and their microenvironment and shines a new light on the cell's ability to adapt to topographical changes through FA‐based mechanotransduction. [ABSTRACT FROM AUTHOR]

The fabrication of a foldable, compact liquid crystal actuator based on oxetane reactive monomers is reported. The free-standing liquid crystal network films are optically clear, flexible, and have easily tunable anisotropic thermal and mechanical properties. The oxetane liquid crystal networks demonstrate excellent one-way shape memory properties, with good temporary shape fixation and complete shape recovery. In addition, the flexible polyether backbone of the oxetane liquid crystal networks results in large two-way bending actuation, with the actuators showing rotation inversion and making multiple rotations. Both actuation mechanisms could be combined to form actuators that can be temporarily folded before fully recovering to their original shape prior to use. As a result, the oxetane liquid crystal networks have great potential as foldable, compact actuators. [ABSTRACT FROM AUTHOR]

Multi‐stimuli responsivity in 3D‐printed objects is receiving much attention. However, the simultaneous interplay between different environmental stimuli is largely unexplored. In this work, we demonstrate direct ink writing of an oligomeric ink containing an azobenzene photo‐switch with an accessible hydrogen bond allowing triple responsivity to light, heat, and water. The resulting printed liquid crystal elastomer performs multiple actuations, the specific response depending on the environmental conditions. Bilayer films formed by printing on a static substrate can rapidly change shape, bending almost 80 degrees if irradiated in air or undergoing a shrinkage of about 50 % of its length when heated. The bilayer film assumes dramatically different shapes in water depending on combined environmental temperature and lighting conditions. [ABSTRACT FROM AUTHOR]

Soft and mechanically responsive actuators hold the promise to revolutionize the design and manufacturing of devices in the areas of microfluidics, soft robotics and biomedical engineering. In many of these applications, the actuators need to operate in a wet environment that can strongly affect their performance. In this paper, we report on the photomechanical response in a biological buffer of azobenzene-containing liquid crystal elastomer (LCE)-based actuators, prepared by four-dimensional (4D) printing. Although the photothermal contribution to the photoresponse is largely cancelled by the heat withdrawing capacity of the employed buffer, a significant photoinduced reversible contraction, in the range of 7% of its initial length, has been achieved under load, taking just a few seconds to reach half of the maximum contraction. Effective photomechanical work performance under physiological conditions has, therefore, been demonstrated in the 4D-printed actuators. Advantageously, the photomechanical response is not sensitive to salts present in the buffer differently to hydrogels with responses highly dependent on the fluid composition. Our work highlights the capabilities of photomechanical actuators, created using 4D printing, when operating under physiological conditions, thus showing their potential for application in the microfluidics and biomedical fields. [ABSTRACT FROM AUTHOR]

Recent years have seen major advances in the additive manufacturing of stimuli‐responsive materials, also known as "4D printing," among which liquid crystal elastomers (LCEs) play an important role. However, during fabrication photo‐crosslinking of the LCEs is required, but this step is time‐consuming and efficient polymerization is challenging, especially in the case of light‐responsive materials. In this work, the first light‐fueled supramolecular LCEs suitable for the direct ink writing (DIW) of soft actuators are synthesized in which a photopolymerization step is not needed. With the responsive supramolecular material, 3D‐printed objects are fabricated by exploiting the thermoreversible interplay of the hydrogen‐bonding physical cross‐links. After printing, the supramolecular LCE shows reversible shape changes in response to light and is capable of bending and lifting a load. Through the combined photothermal and photochemical contributions of the incorporated azobenzene, the actuators can be triggered both in air and water. The freedom provided by DIW allows for the printing of complex responsive objects, as demonstrated by fabricating re‐entrant honeycomb and spiral director structures. This approach of printing light‐responsive supramolecular soft actuators opens avenues toward the application of "smart" and sustainable materials for additive manufacturing without the requirement of photo‐crosslinking. [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]

The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO2, CH4 and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH4 and He over CO2 decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO2). [ABSTRACT FROM AUTHOR]

For preserving food packed in environmentally friendly and recyclable paperboard packages, it is important to have sufficient gas barrier performance of the paperboard container. Paperboard has poor intrinsic barrier properties and to overcome this deficiency, so a barrier coating is needed that does not hinder the recycling of the paperboard substrate. However, the gas barrier properties and the recyclability of such coatings have been rarely studied. Here, both the gas barrier performance and the removal of an alkali-soluble resin (ASR)-stabilized waterborne barrier coatings from paperboard are investigated. For barriers for gases, such as nitrogen, carbon dioxide, and oxygen, defect-free coatings are needed which is achieved by applying three coating layers. The oxygen transmission rate (OTR) of the three-layered coating on paperboard was 920 cm3/(m2∙day). For water vapor barriers, two coating layers already show a strong improvement, as water follows a different penetration mechanism than the other tested gases. The water vapor transmission rate WVTR of double coated paperboard was 240 g/(m2∙day). Preliminary results show that the coating is removed by immersion of the coated paperboard in an aqueous alkaline solution at room temperature. This causes de-protonation of the carboxylic acids of the ASR and subsequent re-dispersion of the coating in water. Removing double-layer coatings from the paperboard is more challenging, possibly due to the coating/coating interface between the two coating layers and enhanced adhesion between coating and paperboard. [ABSTRACT FROM AUTHOR]

Low molecular weight cholesteric liquid crystals (CLCs) can be trapped as droplets or particles inside a polymer binder, forming polymer dispersed cholesteric liquid crystal (PDCLC) systems, which are typically used for smart windows, displays, and optical sensors. While dispersing a single CLC mixture via emulsification, rendering reflective PDCLC films upon drying with a stable optical response, is well studied, the incorporation of distinct microdroplets inside a PDCLC coating is barely explored. Here, structural colored PDCLC coatings are prepared by mixing distinctive thermosensitive photonic microdroplets, featuring an optical time–temperature response that is visualized as a reflective color loss or shift dependent on the composition of the CLC mixtures. The presented optical time–temperature response is induced by diffusion of CLCs between adjacent liquid crystal domains via interconnecting passageways, causing an irreversible optical response until homogeneous mixing is attained. Hence, blending distinct CLC microdroplets in a polymer matrix provides a facile and scalable method for fabricating optical time–temperature integrators; moreover, alternative optical time–temperature responses can be realized when adjusting the composition of the photonic mixtures, demonstrating the potential of this method to acquire novel photonic applications based on low molecular weight CLCs. [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]

Self-assembly provides an attractive route to functional organic materials, with properties and hence performance depending sensitively on the organization of the molecular building blocks (1-5). Molecular organization is a direct consequence of the pathways involved in the supramolecular assembly process, which is more amenable to detailed study when using one-dimensional systems. In the case of protein fibrils, formation and growth have been attributed to complex aggregation pathways (6-8) that go beyond traditional concepts of homogeneous (9-11) and secondary (12-14) nucleation events. The self-assembly of synthetic supramolecular polymers has also been studied and even modulated (15-18), but our quantitative understanding of the processes involved remains limited. Here we report time-resolved observations of the formation of supramolecular polymers from π-conjugated oligomers. Our kinetic experiments show the presence of a kinetically favoured metastable assembly that forms quickly but then transforms into the thermodynamically favoured form. Quantitative insight into the kinetic experiments was obtained from kinetic model calculations, which revealed two parallel and competing pathways leading to assemblies with opposite helicity. These insights prompt us to use a chiral tartaric acid as an auxiliary to change the thermodynamic preference of the assembly process (19). We find that we can force aggregation completely down the kinetically favoured pathway so that, on removal of the auxiliary, we obtain only metastable assemblies., The π-conjugated oligomer S-chiral oligo(p-phenylenevinylene) (SOPV, Fig 1a) serves as a functional material in a variety of organic electronic devices, with performance critically dependent on the material's morphology (20). This [...]

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]

The replacement of plastic packaging by paperboard can significantly contribute to meeting sustainability goals in important application fields, such as in food packaging. The (water) barrier properties of paperboard‐based products need, however, to be improved. Waterborne coatings offer an environmentally friendly possibility to enhance these barrier properties, which in turn highly depend on the coating chemistry and film formation. Here, the relation between the characteristics of a waterborne coating and its water barrier performance is investigated. A waterborne polymer dispersion is prepared by emulsion polymerization, using an alkali‐soluble resin (ASR) as stabilizing agent. The ASR‐stabilized dispersion is applied as a coating on paperboard. During this application, the carboxylate groups of the ASR are converted to uncharged carboxylic acids. The coatings are characterized by Fourier‐transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and scanning electron microscopy (SEM), and the water barrier properties are evaluated by dynamic vapor sorption (DVS) and the Cobb method. It is found that the water barrier performance is dominated by the carboxylate concentration remaining in the film, which is dependent on the moisture content. High drying temperatures improve particle deformation and polymer flow during coating formation, but do not influence the water barrier performance. [ABSTRACT FROM AUTHOR]

We report on the synthesis of monodisperse, flower‐like, liquid crystalline (LC) polymer particles by precipitation polymerization of a LC mixture consisting of benzoic acid‐functionalized acrylates and disulfide‐functionalized diacrylates. Introduction of a minor amount of redox‐responsive disulfide‐functionalized diacrylates (≤10 wt %) induced the formation of flower‐like shapes. The shape of the particles can be tuned from flower‐ to disk‐like to spherical by elevating the polymerization temperature. The solvent environment also has a pronounced effect on the particle size. Time‐resolved TEM reveals that the final particle morphology was formed in the early stages of the polymerization and that subsequent polymerization resulted in continued particle growth without affecting the morphology. Finally, the degradation of the particles under reducing conditions was much faster for flower‐like particles than for spherical particles, likely a result of their higher surface‐to‐volume ratio. [ABSTRACT FROM AUTHOR]

Liquid‐crystalline polymer particles prepared by classical polymerization techniques are receiving increased attention as promising candidates for use in a variety of applications including micro‐actuators, structurally colored objects, and absorbents. These particles have anisotropic molecular order and liquid‐crystalline phases that distinguish them from conventional polymer particles. In this minireview, the preparation of liquid‐crystalline polymer particles from classical suspension, (mini‐)emulsion, dispersion, and precipitation polymerization reactions are discussed. The particle sizes, molecular orientations, and liquid‐crystalline phases produced by each technique are summarized and compared. We conclude with a discussion of the challenges and prospects of the preparation of liquid‐crystalline polymer particles by classical polymerization techniques. [ABSTRACT FROM AUTHOR]

Untethered, light‐responsive, high‐stress‐generating actuators based on widely‐used commercial polymers are appealing for applications in soft robotics. However, the construction of actuators that are stable and reversibly responsive to low‐intensity ultraviolet, visible, and infrared lights remains challenging. Here, transparent, stress‐generating actuators are reported based on ultradrawn, ultrahigh molecular weight polyethylene films. The composite films have different draw ratios (30, 70, and 100) and contain a small amount of graphene in combination with ultraviolet and near‐infrared‐absorbing dyes. The composite actuators respond rapidly (t0.9 < 0.8 s) to different wavelengths of light (i.e., 780, 455, and 365 nm). A maximum photoinduced stress of 35 MPa is achieved at a draw ratio of 70 under near‐infrared light irradiation. The photoinduced stress increases linearly with the light intensity, indicating the transfer of light into thermally induced mechanical contraction. Moreover, the addition of additives lead to a reduction in the plastic creep rate of the drawn films compared to their nonmodified counterparts. [ABSTRACT FROM AUTHOR]