Stimuli‐Responsive Materials Based on Interpenetrating Polymer Liquid Crystal Hydrogels

Stimuli‐responsive materials based on interpenetrating liquid crystal‐hydrogel polymer networks are fabricated. These materials consist of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid) network that provides a humidity and pH response. The volume change in the cross‐linked hydrogel polymer results in a dimensional alteration in the cholesteric network as well, which, in turn, leads to a color change yielding a dual‐responsive photonic material. Furthermore a patterned coating having responsive and static interpenetrating polymer network areas is produced that changes both its surface topography and color.     

Ionic Liquid–Polymer Composites: A New Platform for Multifunctional Applications

Ionic liquids (ILs) have emerged as a novel class of chemical compounds for the development of advanced (multi)functional materials with outstanding potential in applications of several areas due to their unique properties and functionalities. The combination of ILs with polymers, in a composite, allows for developing smart materials, which synergistically combine the features of specific polymers and ILs. Moreover, ILs can be extensively modified by the incorporation of functional groups with specific properties into the cation, anion, or both. Thus, it is possible to tune the IL, the polymer, or both to obtain a broad spectrum of multifunctional composites and address the specific requirements of many applications. This work focusses on advanced materials and strategies concerning ILs and polymers for the development of smart IL/polymer‐based materials for applications including responsive and sensitive sensors, actuators, environment, batteries, fuel cells, and biomedical applications.     

Modulating the Properties of Azulene‐Containing Polymers through Controlled Incorporation of Regioisomers

Two libraries of random conjugated polymers are presented that incorporate varying ratios of regioisomeric azulene units connected via the 5‐membered or 7‐membered ring in combination with bithiophene or fluorene comonomers. It is demonstrated that the optoelectronic and stimuli‐responsive properties of the materials can be systematically modulated by tuning the relative percentage of each azulene building block in the polymer backbone. Significantly, these materials exhibit stimuli‐responsive behavior in the solid state with spin‐coated thin films undergoing rapid and reversible color switching. Ultimately, this work introduces a new design strategy in which the optoelectronic properties of conjugated polymers can be modulated by varying only the regiochemistry of the constituent building blocks along a polymer chain.     

Pressure/Temperature Dual-Responsive Cellulose Nanocrystal Hydrogels for On-Demand Schemochrome Patterning

Cellulose nanocrystal (CNC) based optical devices with adjustable schemochrome have attracted immense interest. However, most of the previously reported structural colored CNC-based materials can only achieve simple stress-induced color change, which have difficulty achieving multimode control of complex patterning that can be accurately identified. Here, inspired by the nanostructure-based color-changing mechanism of neon tetra, this study presents a pressure/temperature dual-responsive CNC-based schemochrome hydrogel with adjustable dynamic chiral nematic structure. By incorporating abundant interfacial noncovalent interactions, dynamic correlations between adjustable helical pitch of the vertically stacked cholesteric liquid crystalline (LC) phase and responsiveness of flexible thermosensitive substrate are established, which further enable wide-range optical characteristic (12°–213° in HSV color model and 421–734 nm in the UV–Vis spectra) and identifiable visualized patterning. The resultant hydrogels are applied in proof-of-concept demonstrations of on-demand schemochrome patterning, including customizable patterned dual-encryption label, smart digital display, temperature monitor, and intelligent recognition/control system. This study envisages that the bioinspired construction of structural colored nanomaterials will have promising applications in smart responsive photonic equipment including smart display, anticounterfeiting, and intelligent control systems.     

Advanced AAO Templating of Nanostructured Stimuli‐Responsive Polymers: Hype or Hope?

Nanostructures play a significant role in introducing distinctive functionality to materials. A synergistic combination of nanofabrication techniques with material properties holds great promise in creating smart biomimetic structures. An advanced preparation technique to fabricate complex and sophisticated hierarchical polymeric nanostructure templates via anodized alumina oxide membranes is highlighted. Moreover, nanostructures made of responsive polymers activated by environmental stimuli offer a huge potential in a wide range of applications by enhancing their responsiveness. The current state of research on novel nanostructures fabrication by integrating anodic aluminum oxide with stimuli‐responsive polymers is presented, with an emphasis on the underlying actuation mechanism in terms of application. Furthermore, the potential direction for future research is discussed.     

Stimuli‐Responsive Hybridized Nanostructures

Through innovative nanosynthesis techniques and advanced surface‐passivation methods, diversified luminescent nanocrystals, like quantum dots, metal nanoclusters, carbon dots, and upconversion nanoparticles, are produced successfully to exhibit greatly improved performance in various applications, due to their color tunability and resistance to photobleaching. Their further hybridization with stimuli‐responsive polymers endows the resultant nanohybrids with unique smart functions, which can reversibly respond to external stimuli or environmental changes via alternation in luminescence. Due to their multifunctional properties, these responsive luminescent nanohybrids are attracting more and more interest in foundation research and promising applications recently. Here, important developments and achievements made in this emerging field are summarized to highlight the integration concepts and fabrication methods for luminescent nanohybrids, and their special responsive functions to temperature, pH, fields, and analytes. At the same time, their smart applications are also overviewed for demonstrating novel actions of responsive nanohybrids via various intelligent operations. The aim is to understand and accelerate more advanced developments in creating varied and intelligent nanosystems, and provide perspectives to promote a further revolution of smart materials and technology.     

An Adaptable Tough Elastomer with Moisture‐Triggered Switchable Mechanical and Fluorescent Properties

Smart materials with coupled optical and mechanical responsiveness to external stimuli, as inspired by nature, are of interest for the biomimetic design of the next generation of soft machines and wearable electronics. A tough polymer that shows adaptable and switchable mechanical and fluorescent properties is designed using a fluorescent lanthanide, europium (Eu). The dynamic Eu‐iminodiacetate (IDA) coordination is incorporated to build up the physical cross‐linking network in the polymer film consisting of two interpenetrated networks. Reversible disruption and reformation of Eu‐IDA complexation endow high stiffness, toughness, and stretchability to the polymer elastomer through energy dissipation of dynamic coordination. Water that binds to Eu³⁺ ions shows an interesting impact simultaneously on the mechanical strength and fluorescent emission of the Eu‐containing polymer elastomer. The mechanical states of the polymer, along with the visually optical response through the emission color change of the polymer film, are reversibly switchable with moisture as a stimulus. The coupled response in the mechanical strength and emissive color in one single material is potentially applicable for smart materials requiring an optical readout of their mechanical properties.     

太赫兹复合材料无损检测技术及其应用

随着材料科学的迅速发展,复合材料、高分子材料在航空航天领域得到了广泛应用。由于这些材料的特殊性质,现有的较为成熟的探伤手段都不能有效对其进行检测。但对于太赫兹波来说,许多非极性、非金属材料都是半透明,可以有效探测到这些材料的内部缺陷。本文简要介绍了太赫兹的性质及太赫兹无损探伤原理,并以航天领域应用较广的几种复合材料为例对太赫兹无损检测应用做了简介。     

Responsive Smart Windows from Nanoparticle–Polymer Composites

Windows play significant roles in commercial and residential buildings and automobiles, which direct and control light illumination, thermal insulation, natural ventilation, and aesthetics. Various approaches are attempted to make windows “smart” by tailoring their transparency and thermal insulation in response to environmental changes. Hence, there has been much effort to develop smart windows that can dynamically modulate the transmission and reflectance of the visible light and solar radiance into buildings according to weather conditions or personal preferences. Development of smart window materials is also beneficial to applications including wearable sensors, energy harvesting and storage, and medical devices. By carefully matching the refractive indices of nanoparticle (NPs) and polymer matrix, surface chemistry, and their mechanical properties, particle‐embedded polymer composites can exhibit synergistic effects with improved chemical and mechanical stability, enhanced dispersion of NPs, and optimized and stimuli‐responsive optical properties. Here, an overview of recent progresses in the development of smart windows based on electro‐, thermo‐, and mechanoactuations is provided. Additional functionalities, e.g., flexibility, stretchability, and mechanical/chemical stability, can also be achieved by careful choices of NPs and polymers.     

Going with the Flow: Tunable Flow‐Induced Polymer Mechanochemistry

Mechanical forces can drive chemical transformations in polymers, directing reactions along otherwise inaccessible pathways, providing exciting possibilities for developing smart, responsive materials. The state‐of‐the‐art test for solution‐based polymer mechanochemistry development is ultrasonication. However, this does not accurately model the forces that will be applied during device fabrication using processes such as 3D printing or spray coating. Here, a step is taken toward predictably translating mechanochemistry from molecular design to manufacturing by demonstrating a highly controlled nozzle flow setup in which the shear forces being delivered are precisely tuned. The results show that solvent viscosity, fluid strain rate, and the nature of the breaking bond can be individually studied. Importantly, it is shown that the influence of each is different to that suggested by ultrasonication (altered quantity of chain breakage and critical polymer chain length). Significant development is presented in the understanding of polymer bond breakage during manufacturing flows to help guide design of active components that trigger on demand. Using an anthracene‐based mechanophore, the triggering of a fluorescence turn‐on is demonstrated through careful selection of the flow parameters. This work opens the avenue for programmed chemical transformations during inline manufacturing processes leading to tunable, heterogeneous final products from a single source material.     

Spatial and Orientational Templating of Semiconducting Polymers in a Cholesteric Liquid Crystal

A one‐dimensional pattern‐forming state of a cholesteric liquid crystal (CLC) is used as a template for the self‐organization of ordered, spatially orientated, acetylene‐based semiconducting polymers. The polymers are formed by metathesis reaction with all chemical components contained in an ordinary electro‐optic cell. The polymer morphology consists of parallel ～ 1 μm thick bundles, uniformly spaced at ～ 10 μm over the full macroscopic active area of the cell substrates. The polymer templating can be explained by a model that predicts a corrugation in polymer density determined by the spatially periodic profile of the orientational energy density associated with the pattern‐forming CLC state.     

Thermally Recordable Cholesteric Liquid Crystalline Copolymers Containing Pendant Menthyl and Cholesteryl Groups

In order to investigate the effect of spacer length and linkages between the rigid mesogenic core and the terminal group on the molecular interaction and physical properties of polymers, a series of novel side chain liquid crystalline polymers were synthesized. These were composed of liquid crystalline monomers with six or eleven methylene segments as spacers, and chiral monomers end capped with menthyl or cholesteryl groups. Liquid crystalline phases of the polymers were investigated using differential scanning calorimetry and polarized optical microscopy, and confirmed with X-ray diffractometry. Color image recording of the synthesized polymer films was achieved using a thermal treatment, and then fixed by quenching. This investigation demonstrates that the introduction of carbonate linking groups between the rigid mesogenic core and terminal group decreases both the lateral molecular interaction and thermal stability of the liquid crystalline polymers. The RGB reflection colors of the cholesteric composite film could be tuned by varying the film temperature and applying an external field. The results of this investigation present significant scientific and practical contributions with respect to the development of unique cholesteric polymer materials.      

胆甾相液晶在彩色显示技术中的应用

胆甾相液晶分子呈螺旋结构,并且其螺距可以用加热冷却、光照和施加电场的方法进行可逆调整。这些性质使得其在彩色显示上具有巨大的应用前景,目前已经能够用胆甾相液晶做成各种彩色图案。为了探索实用的胆甾相液晶材料,人们制备并研究了各种结构和组成的胆甾相液晶,以及控制这些材料颜色的方法。     

Smart Polymers for Microscale Machines

Microscale machines are able to perform a number of tasks like micromanipulation, drug-delivery, and noninvasive surgery. In particular, microscale polymer machines that can perform intelligent work for manipulation or transport, adaptive locomotion, or sensing are in-demand. To achieve this goal, shape-morphing smart polymers like hydrogels, liquid crystalline polymers, and other smart polymers are of great interest. Structures fabricated by these materials undergo mechanical motion under stimulation such as temperature, pH, light, and so on. The use of these materials renders microscale machines that undergo complex stimuli-responsive transformation such as from planar to 3D by combining spatial design like introducing in-plane or out-plane differences. During the past decade, many techniques have been developed or adopted for fabricating structures with smart polymers including microfabrication methods and the well-known milestone of 4D printing, starting in 2013. In this review, the existing or potential active smart polymers that could be used to fabricate active microscale machines to accomplish complex tasks are summarized.     

Tunable Photonic Materials via Monitoring Step‐Growth Polymerization Kinetics by Structural Colors

The functional and responsive properties of elastomeric materials highly depend on crosslink density and molecular weight between crosslinks. However, tedious analytical steps are needed to obtain polymer network structure–property relationships. In this article, an in situ structure–property characterization method is reported by monitoring the structural color change in a photonic elastomeric material. The photonic materials are prepared in a two‐step polymerization process. First, linear chain extension occurs via Michael addition. Second, photopolymerization ensures crosslinking, resulting in the formation of an elastomeric photonic network. During the first step, the step‐growth polymer process can be monitored by following the photonic reflection band redshift, allowing to program the molecular weight between the crosslinks. During network formation, the crosslink density, chain length between crosslinks, and the colors are “frozen in.” These processes can be locally controlled creating both single‐layered multicolor patterned and broadband reflective coatings at room temperature. The scalability of the coating process is further demonstrated by using a gravure printing technique. Additionally, the final coatings are made responsive toward specific solvents and temperature. Here the modulus, response, and color of the coating are controlled by tuning the crosslink density and molecular weight between crosslinks of the elastomeric material.     

Programmable 3D Shape Changes in Liquid Crystal Polymer Networks of Uniaxial Orientation

3D programmable materials are highly interesting and have a great potential to enable smart robotic devices. Stimuli‐responsive liquid crystal polymer networks (LCNs) offer an attractive platform for the design and fabrication of 3D programmable materials. To date, extensive efforts have been devoted to the design of 3D programmable LCNs by spatially modulating the orientation of liquid crystals. However, the practical application of LCN actuators has been elusive, partly due to tedious orientation technology and monotonous geometry. To resolve this issue, programmable 3D shape changes achieved in LCNs with uniaxial orientation and homogenous composition using a mechanical programming process inspired by the “programming process” of shape‐memory polymers are reported. The mechanical programming process is suitable for LCNs with distinct geometries, for example, the film and fiber, suggesting a promising way for the design of 3D programmable LCN actuators with complex geometries, and deformation profiles (buckle, helix, horn).     

3D Printing of a Thermo‐ and Solvatochromic Composite Material Based on a Cu(II)–Thymine Coordination Polymer with Moisture Sensing Capabilities

This work presents the fabrication of 3D‐printed composite objects based on copper(II) 1D coordination polymer ( CP1 ) decorated with thymine along its chains with potential utility as an environmental humidity sensor and as a water sensor in organic solvents. This new composite object has a remarkable sensitivity, ranging from 0.3% to 4% of water in organic solvents. The sensing capacity is related to the structural transformation due to the loss of water molecules that CP1 undergoes with temperature or by solvent molecules' competition, which induces significant change in color simultaneously. The CP1 and 3D printed materials are stable in air over 1 year and also at biological pHs (5–7), therefore suggesting potential applications as robust colorimetric sensors. These results open the door to generate a family of new 3D printed materials based on the integration of multifunctional coordination polymers with organic polymers.     

Single‐Thread‐Based Wearable and Highly Stretchable Triboelectric Nanogenerators and Their Applications in Cloth‐Based Self‐Powered Human‐Interactive and Biomedical Sensing

The development of wearable and large‐area fabric energy harvester and sensor has received great attention due to their promising applications in next‐generation autonomous and wearable healthcare technologies. Here, a new type of “single” thread‐based triboelectric nanogenerator (TENG) and its uses in elastically textile‐based energy harvesting and sensing have been demonstrated. The energy‐harvesting thread composed by one silicone‐rubber‐coated stainless‐steel thread can extract energy during contact with skin. With sewing the energy‐harvesting thread into a serpentine shape on an elastic textile, a highly stretchable and scalable TENG textile is realized to scavenge various kinds of human‐motion energy. The collected energy is capable to sustainably power a commercial smart watch. Moreover, the simplified single triboelectric thread can be applied in a wide range of thread‐based self‐powered and active sensing uses, including gesture sensing, human‐interactive interfaces, and human physiological signal monitoring. After integration with microcontrollers, more complicated systems, such as wireless wearable keyboards and smart beds, are demonstrated. These results show that the newly designed single‐thread‐based TENG, with the advantage of interactive, responsive, sewable, and conformal features, can meet application needs of a vast variety of fields, ranging from wearable and stretchable energy harvesters to smart cloth‐based articles.     

Printable,Down/Up-Conversion Triple-Mode Fluorescence Responsive and Colorless Self-Healing Elastomers with Superior Toughness

Expanding the practical application range of self-healing materials has become an important challenge for developing smart materials. Herein, the synthesis of printable, triple-mode fluorescence responsive, and colorless self-healing elastomers, formed by the combination of disulfide cross-linked polyurethane (PU) polymers, carbon dots (CDs) down-conversion fluorescence materials, and lanthanide ions doped upconversion fluorescence materials. The PU elastomers with optimal mechanical properties and good restorability (recover large strain of 500% after relaxation at room temperature (R.T.) for 2 h) are selected to incorporate CDs for fabricating fluorescence responsive elastomers (denoted as PU-CDs). Significantly, the prepared PU-CDs exhibit not only superior tensile strength and toughness (20.95 MPa and 85.13MJ m^–3, respectively) than the previously reported R.T. self-healing elastomers but also show good self-healing properties and achieve blue fluorescence emissions under the ultraviolet excitation. Furthermore, a series of dual-mode fluorescence patterns based on formulated core@shell structural upconversion inks are prepared on the transparent PU-CDs elastomers by a directly screen-printing method. Self-healing and integrating a series of fluorescence patterns and damaged electrical patterns can also be accomplished successfully. The designed self-healing materials provide new ideas and important guidance for developing and applying the next generation of smart materials.     

Printable All‐Organic Electrochromic Active‐Matrix Displays

All‐organic active matrix addressed displays based on electrochemical smart pixels made on flexible substrates are reported. Each individual smart pixel device combines an electrochemical transistor with an electrochromic display cell, thus resulting in a low‐voltage operating and robust display technology. Poly(3,4‐ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) served as the active material in the electrochemical smart pixels, as well as the conducting lines, of the monolithically integrated active‐matrix display. Different active‐matrix display addressing schemes have been investigated and a matrix display fill factor of 65 % was reached. This is achieved by combining a three‐terminal electrochemical transistor with an electrochromic display cell architecture, in which an additional layer of PEDOT:PSS was placed on top of the display cell counter electrode. In addition, we have evaluated different kinds of electrochromic polymer materials aiming at reaching a high color switch contrast. This work has been carried out in the light of achieving a robust display technology that is easily manufactured using a standard label printing press, which forced us to use the fewest different materials as well as avoiding exotic and complex device architectures. Together, this yields a manufacturing process of only five discrete patterning steps, which in turn promise for that the active matrix addressed displays can be manufactured on paper or plastic substrates in a roll‐to‐roll production procedure.