Closed-Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room-Temperature Programmability and Reconfigurable Functionalities.

Dynamic covalent liquid crystal networks (DCv-LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv-LCNs are still limited to heat-assisted programming and polymer-to-polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene-based DCv-LCN with room-temperature programmability and polymer-to-monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene-containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water-activated dynamics of the imine bonds, the resultant DCv-LCN can be simply programmed, upon water-soaking at room temperature, to yield a UV/Vis light-driven actuator. Importantly, the reported DCv-LCN undergoes depolymerization in an acid-solvent medium at room temperature because of the acid-catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv-LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed-loop polymer-monomers-polymer recycling.

Ultrastable, Superrobust, and Recyclable Supramolecular Polymer Networks.

Supramolecular polymer networks (SPNs), crosslinked by noncovalent bonds, have emerged as reorganizable and recyclable polymeric materials with unique functionality. However, poor stability is an imperative challenge faced by SPNs, because SPNs are susceptible to heat, water, and/or solvents due to the dynamic and reversible nature of noncovalent bonds. Herein, the design of a noncovalent cooperative network (NCoN) to simultaneously stabilize and reinforce SPNs is reported, resulting in an ultrastable, superrobust, and recyclable SPN. The NCoN is constructed by multiplying the H-bonding sites and tuning the conformation/geometry of the H-bonding segment to optimize the multivalence cooperativity of H-bonds. The rationally designed H-bonding segment with high conformational compliance favors the formation of tightly packed H-bond arrays comprising higher-density and stronger H-bonds. Consequently, the H-bonded crosslinks in the NCoN display a covalent crosslinking effect but retain on-demand dynamics and reversibility. The resultant ultrastable SPN not only displays remarkable resistance to heat up to 120 °C, water soaking, and a broad spectrum of solvents, but also possesses a superhigh true stress at break (1.1 GPa) and an ultrahigh toughness (406 MJ m-3 ). Despite the covalent-network-like stability, the SPN is recyclable through activating its reversibility in a high-polarity solvent heated to a threshold temperature.

Stretchable Ionic Conductors for Soft Electronics.

With the rapid development of soft electronics in the era of Internet of Everything (IoE), electrical conductors with stretchability, the indispensable components of soft electronics, have gained new opportunities and also faced increasing challenges. According to the principles of electrical conductivity, stretchable electrical conductors can be divided into electronic conductors and ionic conductors. Different from the stretchable electronic conductors derived from stretchable polymeric matrices integrated with electronically conductive fillers, stretchable ionic conductors are constructed by embedding mobile ions into the crosslinked polymer networks. Therefore, stretchable ionic conductors have received extensive attention and in-depth research in the past decade, thanks to their intrinsic stretchability and electrical conductivity. This review systematically summarizes the achievements on the different categories of stretchable ionic conductors (e.g., hydrogels, ionogels, and liquid-free ion-conductive elastomers), in terms of their design, fabrication, properties, and applications. The advantages and limitations of the different types of stretchable ionic conductors are discussed. Outlooks are also provided to envision the remaining challenges for the further development and practical applications of stretchable ionic conductors. It is expected to arouse inspirations for the design and fabrication of new and high-performance stretchable ionic conductors and advanced soft electronics for the IoE era.

Thermal Dynamic Self-Healing Supramolecular Dopant Towards Efficient and Stable Flexible Perovskite Solar Cells.

Flexible perovskite solar cells (FPSCs) have attracted great attention due to their advantageous traits such as low cost, portability, light-weight, etc. However, mechanical stability is still the weak point in their practical application. Herein, we prepared efficient FPSCs with remarkable mechanical stability by a dynamic thermal self-healing effect, which can be realized by the usage of a supramolecular adhesive. The supramolecular adhesive, which was obtained by random copolymerization of acrylamide and n-butyl acrylate, is amphiphilic, has a proper glass transition temperature and a high density of hydrogen-bond donors and receptors, providing the possibility of thermal dynamic repair of mechanical damage in FPSCs. The adhesive also greatly improves the leveling property of the precursor solution on the hydrophobic poly[bis(4-phenyl)(2,4,6-trimethylphenyl)]amine (PTAA) surface. PSCs containing this adhesive achieve more than a 20 % power conversion efficiency (PCE) on flexible substrates and a 21.99 % PCE on rigid substrates (certified PCE of 21.27 %), with improved electron mobility and reduced defect concentration.

Hybrid Elastic Organic Crystals that Respond to Aerial Humidity.

Reshaping of elongated organic crystals that can be used as semiconductors, waveguides or soft robotic grippers by application of force or light is now a commonplace, however mechanical response of organic crystals to changes in humidity has not been accomplished yet. Here, we report a universal approach to instigating a humidity response into elastically bendable organic crystals that elicits controllable deformation with linear response to aerial humidity while retaining their physical integrity entirely intact. Hygroresponsive bilayer elements are designed by mechanically coupling a humidity-responsive polymer with elastic molecular crystals that have been mechanically reinforced by a polymer coating. As an illustration of the application of these cladded crystalline actuators, they are tested as active optical transducers of visible light where the position of light output can be precisely controlled by variations in aerial humidity. Within a broader context, the approach described here provides access to a vast range of mechanically robust, lightweight hybrid hygroresponsive crystalline materials.

Polymer-Coated Organic Crystals with Solvent-Resistant Capacity and Optical Waveguiding Function.

Recently, luminescent organic crystals have been widely studied as new optoelectronic materials. However, corrosion and dissolution of organic crystals by solvents have always been a great challenge for the application of organic crystals in various fields. In this work, we propose a general method of fabricating a solvent-resistant coating to prevent organic crystals from being corroded or dissolved by organic solvents. The coatings involved layer-by-layer assembly of poly(diallyldimethylammonium) (PDDA) and poly(styrenesulfonate) (PSS) onto crystals, followed by immersing the coated crystals into polyvinyl alcohol (PVA) aqueous solutions for 2 minutes. The coated crystals can remain intact over 24 h in common organic solvents without being damaged and even insoluble in dichloromethane for 5 days. Moreover, the thin and transparent coatings have little effect on the optical properties of crystals which still have excellent optical waveguide performance with the coatings.

Substrate-Independent, Reversible, and Easy-Release Ionogel Adhesives with High Bonding Strength.

It is highly desirable to develop reversible and easy-release adhesives with high bonding strength for a broad range of substrates, while the adhesion of low-surface-energy materials (e.g., polytetrafluoretyhylene, PTFE) is challenging. Herein, a substrate-independent ionogel adhesive is developed by blending an ionic liquid with the copolymer bearing charged segments. By regulating the viscoelastic properties of the ionogel, the adhesive and cohesive strength of the ionogel can be well balanced to maximize the bonding strength for different substrates. The as-developed ionogel exhibits high bonding strength (>0.3 MPa) for PTFE, plastics, metal, wood, and glass, because the variety of functional groups in the ionogel can form various supramolecular interactions with different substrates. The ionogel also exhibits reversible, easy-release, and reusable properties for multiple times of bonding and on-demand debonding without leaving obvious residues on the substrates. The ionogel has high potential for practical applications as temporal adhesives with high bonding strength.

Effect of the Self-Assembled Structures of Hydrated Polyzwitterionic and Polyanionic Brushes on Their Self-Cleaning Capabilities.

The capability of polyelectrolyte brushes to spontaneously clean oil fouling via water is determined by factors including water wettability and the self-assembled structures of hydrated polyelectrolytes. Although the charged groups of polyelectrolytes provide the original source of water wettability, the self-assembled structures play a significant role in the self-cleaning performances. Here, we employ coarse-grained molecular dynamics simulations to study the general self-cleaning characteristics of two types of surface-grafted polyelectrolyte brushes (i.e., zwitterionic and anionic polyelectrolytes). It has been found that the high grafting density is favorable to fouling reduction for both polyzwitterions and polyanions. To be specific, the hydrated polyzwitterions form an intermolecular cross-linked network via zwitterionic complexes, resulting in better self-cleaning capabilities than the polyanions at lower grafting densities. However, polyanions form bundles with each consisting of several chains via hydrophobic interactions and electrostatic repulsions presenting better self-cleaning performances than the polyzwitterions at higher grafting densities.

Fast Modulation of Surface Amphiphobicity/Amphiphilicity via Bidirectional Substitution between Perfluorinated Surfactants and Polyanions throughout Pre-Assembled Polyelectrolyte Multilayers.

In the present work, we demonstrate a bidirectional substitution between perfluorooctanoate (PFO) surfactants and polyanions throughout the pre-assembled polyelectrolyte multilayers (PEMs) for a rapid modulation of surface wettability between amphiphobicity and amphiphilicity. Upon incubation of the PEMs made of alternating deposition of poly(diallyldimethylammonium) (PDDA) and poly(styrenesulfonate) (PSS) in PFO solutions at concentrations above or around its critical micelle concentration, the majority (ca. >75%) of PSS molecules throughout the PDDA/PSS PEMs can be substituted by PFO anions within 10 s, generating PFO-substituted PDDA/PSS (PFO-PDDA/PSS) films. This effective substitution of PSS polyanions in PDDA/PSS PEMs by PFO anions is suggested by the mechanism that the stability of PDDA/PFO complexes is higher than that of PDDA/PSS PEMs. Furthermore, PFO anions all the way through the PFO-PDDA/PSS films can be reversibly substituted by PSS polyanions, while the substitution efficiency depends on the ionic strength of the PSS solutions. The processes of bidirectional and reversible substitution between PFO anions and PSS polyanions throughout the PDDA/PSS films can be repeated at least 10 times accompanied with a negligible change in the film thickness and surface morphology. The surface wettability study reveals that the PFO-PDDA/PSS films are amphiphobic with water and oil contact angles (CAs) of 114 ± 2 and 64 ± 2°, respectively, while PSS-substituted PFO-(PDDA/PSS) films are amphiphilic with water and oil CAs of 6 ± 1 and 0°, respectively. These novelties of the films enable switchable surface wettability simply by dipping the PDDA/PSS film-coated objects into PFO solutions for 2 s or PSS solutions for 30 s.

Confined Flocculation of Ionic Pollutants by Poly(L-dopa)-Based Polyelectrolyte Complexes in Hydrogel Beads for Three-Dimensional, Quantitative, Efficient Water Decontamination.

The development of simple and recyclable adsorbents with high adsorption capacity is a technical imperative for water treatment. In this work, we have successfully developed new adsorbents for the removal of ionic pollutants from water via encapsulation of polyelectrolyte complexes (PECs) made from positively charged poly(allylamine hydrochloride) (PAH) and negatively charged poly(l-3,4-dihydroxyphenylalanine) (PDopa), obtained via the self-polymerization of l-3,4-dihydroxyphenylalanine (l-Dopa). Given the outstanding mass transport through the hydrogel host matrixes, the PDopa-PAH PEC guests loaded inside can effectively and efficiently remove various ionic pollutants, including heavy metal ions and ionic organic dyes, from water. The adsorption efficiency of the PDopa-PAH PECs can be quantitatively correlated to and tailored by the PDopa-to-PAH molar ratio. Because PDopa embodies one catechol group, one carboxyl group, and one amino group in each repeating unit, the resulting PDopa-PAH PECs exhibit the largest capacity of adsorption of heavy metal ions compared to available adsorbents. Because both PDopa and PAH are pH-sensitive, the PDopa-PAH PEC-loaded agarose hydrogel beads can be easily and completely recovered after the adsorption of ionic pollutants by adjusting the pH of the surrounding media. The present strategy is similar to the conventional process of using PECs to flocculate ionic pollutants from water, while in our system flocculation is confined to the agarose hydrogel beads, thus allowing easy separation of the resulting adsorbents from water.

Ion-specific oil repellency of polyelectrolyte multilayers in water: molecular insights into the hydrophilicity of charged surfaces.

Surface wetting on polyelectrolyte multilayers (PEMs), prepared by alternating deposition of polydiallyldimethylammonium chloride (PDDA) and poly(styrene sulfonate) (PSS), was investigated mainly in water-solid-oil systems. The surface-wetting behavior of as-prepared PEMs was well correlated to the molecular structures of the uncompensated ionic groups on the PEMs as revealed by sum frequency generation vibrational and X-ray photoelectron spectroscopies. The orientation change of the benzenesulfonate groups on the PSS-capped surfaces causes poor water wetting in oil or air and negligible oil wetting in water, while the orientation change of the quaternized pyrrolidine rings on the PDDA-capped surfaces hardly affects their wetting behavior. The underwater oil repellency of PSS-capped PEMs was successfully harnessed to manufacture highly efficient filters for oil-water separation at high flux.

One-Pot Preparation of Skin-Inspired Multifunctional Hybrid Hydrogel with Robust Wound Healing Capacity.

Bioinspired hydrogels have demonstrated multiple superiorities over traditional wound dressings for wound healing applications. However, the fabrication of bioinspired hydrogel-based wound dressings with desired functionalities always requires multiple successive steps, time-consuming processes, and/or sophisticated protocols, plaguing their clinical applications. Here, a facile one-pot strategy is developed to prepare a skin-inspired multifunctional hydrogel within 30 min by incorporating elastin (an essential functional component of the dermal extracellular matrix), tannic acid, and chitosan into the covalently cross-linked poly(acrylamide) network through noncovalent interactions. The resulting hydrogel exhibits a Young's modulus (ca. 36 kPa) comparable to that of human skin, a high elongation-at-break (ca. 1550%), a satisfactory tensile strength (ca. 61 kPa), and excellent elastic self-restorability, enabling the hydrogel to synchronously and conformally deform with human skin when used as wound dressings. Importantly, the hydrogel displays a self-adhesive property to skin tissues with an appropriate bonding strength (ca. 55 kPa measured on intact porcine skin), endowing the hydrogel with the ability to rapidly self-adhere to intact human skin, sealing the wound surface and also easily being removed without residue left or trauma caused to the skin. The hydrogel also possesses remarkable antibacterial activity, antioxidant capability, and hemocompatibility. All of these collective beneficial properties enable the hydrogel to significantly accelerate the wound healing process, outperforming the commercial wound dressings.

Solvent-Free and Skin-Like Supramolecular Ion-Conductive Elastomers with Versatile Processability for Multifunctional Ionic Tattoos and On-Skin Bioelectronics.

The development of stable and biocompatible soft ionic conductors, alternatives to hydrogels and ionogels, will open up new avenues for the construction of stretchable electronics. Here, a brand-new design, encapsulating a naturally occurring ionizable compound by a biocompatible polymer via high-density hydrogen bonds, resulting in a solvent-free supramolecular ion-conductive elastomer (SF-supra-ICE) that eliminates the dehydration problem of hydrogels and possesses excellent biocompatibility, is reported. The SF-supra-ICE with high ionic conductivity (>3.3 × 10-2  S m-1 ) exhibits skin-like softness and strain-stiffening behaviors, excellent elasticity, breathability, and self-adhesiveness. Importantly, the SF-supra-ICE can be obtained by a simple water evaporation step to solidify the aqueous precursor into a solvent-free nature. Therefore, the aqueous precursor can act as inks to be painted and printed into customized ionic tattoos (I-tattoos) for the construction of multifunctional on-skin bioelectronics. The painted I-tattoos exhibit ultraconformal and seamless contact with human skin, enabling long-term and high-fidelity recording of various electrophysiological signals with extraordinary immunity to motion artifacts. Human-machine interactions are achieved by exploiting the painted I-tattoos to transmit the electrophysiological signals of human beings. Stretchable I-tattoo electrode arrays, manufactured by the printing method, are demonstrated for multichannel digital diagnosis of the health condition of human back muscles and spine.

Strong and Tough Supramolecular Covalent Adaptable Networks with Room-Temperature Closed-Loop Recyclability.

Development of closed-loop chemically recyclable plastics (CCRPs) that can be widely used in daily life can be a fundamental solution to the global plastic waste crisis. Hence, it is of great significance to develop easy-to-recycle CCRPs that possess superior or comparable material properties to the commodity plastics. Here, a novel dual crosslinked CCRP, namely, supramolecular covalent adaptable networks (supra-CANs), is reported, which not only displays mechanical properties higher than the strong and tough commodity polycarbonate, but also exhibits excellent solvent resistance as thermosets. The supra-CANs are constructed by introducing reversible noncovalent crosslinks into the dynamic covalent polymer networks, resulting in highly stiff and strong thermosets that also exhibit thermoplastic-like ductile and tough behaviors as well as reprocessability and rehealability. In great contrast, the analogs that do not have noncovalent crosslinks (CANs) show elastomeric properties with significantly decreased mechanical strength. Importantly, the developed supra-CANs and CANs can be converted back into the initial monomers in high yields and purity at room temperature, even with additives, which enables the sustainable polymer-monomer-polymer circulation. This work provides new design principles for high-performance chemically recyclable polymers as sustainable substitutes for the conventional plastics.

Bioinspired soft robots based on organic polymer-crystal hybrid materials with response to temperature and humidity.

The capability of stimulated response by mechanical deformation to induce motion or actuation is the foundation of lightweight organic, dynamic materials for designing light and soft robots. Various biomimetic soft robots are constructed to demonstrate the vast versatility of responses and flexibility in shape-shifting. We now report that the integration of organic molecular crystals and polymers brings about synergistic improvement in the performance of both materials as a hybrid materials class, with the polymers adding hygroresponsive and thermally responsive functionalities to the crystals. The resulting hybrid dynamic elements respond within milliseconds, which represents several orders of magnitude of improvement in the time response relative to some other type of common actuators. Combining molecular crystals with polymers brings crystals as largely overlooked materials much closer to specific applications in soft (micro)robotics and related fields.

Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays.

The performance of any engineering material is naturally limited by its structure, and while each material suffers from one or multiple shortcomings when considered for a particular application, these can be potentially circumvented by hybridization with other materials. By combining organic crystals with MXenes as thermal absorbers and charged polymers as adhesive counter-ionic components, we propose a simple access to flexible hybrid organic crystal materials that have the ability to mechanically respond to infrared light. The ensuing hybrid organic crystals are durable, respond fast, and can be cycled between straight and deformed state repeatedly without fatigue. The point of flexure and the curvature of the crystals can be precisely controlled by modulating the position, duration, and power of thermal excitation, and this control can be extended from individual hybrid crystals to motion of ordered two-dimensional arrays of such crystals. We also demonstrate that excitation can be achieved over very long distances (>3 m). The ability to control the shape with infrared light adds to the versatility in the anticipated applications of organic crystals, most immediately in their application as thermally controllable flexible optical waveguides for signal transmission in flexible organic electronics.

Rapid seeded growth of monodisperse, quasi-spherical, citrate-stabilized gold nanoparticles via H2O2 reduction.

In this report, we demonstrate a rapid and simple seeded growth method for synthesizing monodisperse, quasi-spherical, citrate-stabilized Au nanoparticles (Au NPs) via H(2)O(2) reduction of HAuCl(4). Au NPs with diameter ranging from 30 to 230 nm can be synthesized by simply adding 12 nm citrate stabilized Au NP seeds to an aqueous solution of H(2)O(2) and HAuCl(4) under ambient conditions. The diameter of the resulting Au NPs can be quantitatively controlled by the molar ratio of HAuCl(4) to the Au seeds. The standard deviation of the Au NP sizes is less than 10%, and the ellipticity (ratio of major to minor axes) of the NPs is less than 1.1. Compared to existing ones, the present seeded growth approach is implemented within 1 min under ambient condition, and no unfavorable additives are involved because H(2)O(2) can readily decompose into H(2)O during storage or via boiling.

Electrostatic repulsion-controlled formation of polydopamine-gold Janus particles.

Polydopamine (PDA)-Au Janus particles were obtained by simply adding HAuCl(4) to a PDA particle suspension, prepared via self-polymerization of dopamine in basic solution at room temperature. The structures of the PDA-Au particles are readily controlled by electrostatic repulsion between the constituent particles, which can be realized simply via adjusting the environmental pH. PDA-Au Janus particles are formed only in a narrow pH range of 2.5-3.0 due to the properly enhanced electrostatic repulsion between the Au particles growing on as-prepared PDA particles and between the Au and PDA particles. The obtained PDA-Au Janus particles can become interfacially active and self-assemble at oil/water interfaces as a result of spatially well-separated hydrophilic (PDA) and hydrophobic (Au) domains on the surfaces, reminiscent of amphiphilic molecules.

Highly Transparent and Self-Healable Solar Thermal Anti-/Deicing Surfaces: When Ultrathin MXene Multilayers Marry a Solid Slippery Self-Cleaning Coating.

Solar anti-/deicing can solve icing problems by converting sunlight into heat. One of the biggest problems, which has long been plaguing the design of solar anti-/deicing surfaces, is that photothermal materials are always lightproof and appear black, because of the mutual exclusiveness between generating heat and retaining transparency. Herein, a highly transparent and scalable solar anti-/deicing surface is reported, which enables the coated glass to exhibit high transparency (>77% transmittance at 550 nm) and meanwhile causes a >30 °C surface temperature increase relative to the ambient environment under 1.0 sun illumination. Such a transparent anti-/deicing surface can be fabricated onto a large class of substrates (e.g., glass, ceramics, metals, plastics), by applying a solid omniphobic slippery coating onto layer-by-layer-assembled ultrathin MXene multilayers. Hence, the surface possesses a self-cleaning ability to shed waterborne and oil-based liquids thanks to residue-free slipping motion. Passive anti-icing and active deicing capabilities are, respectively, obtained on the solar thermal surface, which effectively prevents water from freezing and simultaneously melts pre-formed ice and thick frost. The self-cleaning effect enables residue-free removal of unfrozen water and interfacially melted ice/frost to boost the anti-/deicing efficiency. Importantly, the surface is capable of self-healing under illumination to repair physical damage and chemical degradation.

Remote and precise control over morphology and motion of organic crystals by using magnetic field.

Elastic organic crystals are the materials foundation of future lightweight flexible electronic, optical and sensing devices, yet precise control over their deformation has not been accomplished. Here, we report a general non-destructive approach to remote bending of organic crystals. Flexible organic crystals are coupled to magnetic nanoparticles to prepare hybrid actuating elements whose shape can be arbitrarily and precisely controlled simply by using magnetic field. The crystals are mechanically and chemically robust, and can be flexed precisely to a predetermined curvature with complete retention of their macroscopic integrity at least several thousand times in contactless mode, in air or in a liquid medium. These crystals are used as optical waveguides whose light output can be precisely and remotely controlled by using a permanent magnet. This approach expands the range of applications of flexible organic crystals beyond the known limitations with other methods for control of their shape, and opens prospects for their direct implementation in flexible devices such as sensors, emitters, and other (opto)electronics.