Dynamic Crosslinking Research Articles

Activated Phenyl Ester Vitrimers.

Aromatic esters are amongst the oldest known chemical motifs that allow for thermal (re)processing of thermosetting polymers. Moreover, phenyl esters are generally known as activated esters that do not require a catalyst to undergo acyl transfer reactions. Even though dynamic aromatic esters find applications in commercialized thermoset formulations, all-aromatic esters have found limited use so far in the design of covalent adaptable networks (CAN) as a result of their high glass transition temperature (Tg)and specific curing process. Here, a strategy to include partly aromatic esters as dynamic cross-links inside low Tg (-40°C) thermosetting formulations, using aliphatic esters derived from para-hydroxybenzoic acid, which serves as a highly activated phenol or as a reactive "phenylogous anhydride" is reported. A small molecule study shows that the activated phenyl ester bonds can readily exchange with free phenol moieties at 200°C under catalyst-free conditions, while the addition of a catalyst allows for a faster exchange. Robust and hydrophobic polymer networks are conveniently prepared via rapid thiol-ene UV-curing of unsaturated phenol esters. The obtained networks show high thermal stability (350°C), fast processability, good water resistance, and low creep up to 120°C, thus showing good promise as a platform for CAN.

Self-healing electronic skin with high fracture strength and toughness.

Human skin is essential for perception, encompassing haptic, thermal, proprioceptive, and pain-sensing functions through ion movement. Additionally, it is mechanically resilient and self-healing for protection. Inspired by these unique properties, researchers have attempted to develop stretchable, self-healing sensors based on ion dynamics. However, most self-healing sensors reported to date suffer from low fracture strength and toughness. In this work, we present an ion-based self-healing electronic skin with exceptionally high fracture strength and toughness. We enhanced self-healing polymers and ionic conductors by introducing two types of orthogonal dynamic crosslinking bonds: dynamic aromatic disulfide bonds and 2-ureido-4-pyrimidone moieties. These dynamic bonds provide autonomous self-healing and high mechanical toughness even in the presence of ionic liquids. As a result, our self-healing polymer and self-healing ionic conductor exhibit remarkable stretchability (700%, 850%), fracture strength (34 MPa, 30 MPa), and toughness (78.5MJ/m3, 87.3MJ/m3), the highest values reported among self-healing ionic conductors to date. Using our materials, we developed various fully self-healing sensors and a soft gripper capable of autonomously recovering from mechanical damage. By integrating these components, we created a comprehensive self-healing electronic skin suitable for soft robotics applications.

Use of a hydrosilylation reaction for the preparation of structure-controlled boroxine-based polyborosiloxanes

The structure of organosilicon vinyl-containing boroxine was confirmed by 1H nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Its thermal properties were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) methods. The ability of boroxine to undergo hydrosilylation using the Karstedt's catalyst was investigated on various organosilicon substrates. Boroxine was shown to be involved in these reactions as a cross-linking agent. The formation of dynamic boroxine cross-links allowed the analysis of the reaction products by NMR spectroscopy, including the use of polyfunctional hydride-containing reagents. In all cases the addition proceeds selectively to the β-position and with complete conversion. The structure of the resulting polyborosiloxane containing 1.4 mol% of modified units was also confirmed by 1H NMR spectroscopy, and its thermal and rheological properties were studied.

A concise strategy for enhancing the performance of bio-based polyurethane aerogels using melamine-modified sodium alginate via Aza-Michael addition reaction

Bio-based waterborne polyurethane (PU) has garnered significant attention as a sustainable alternative petroleum-derived counterpart, offering environmentally friendly alternatives. In this work, a new approach is proposed to fabricate two kinds of composite aerogels of lowly/highly melamine-modified sodium alginate (SAML/SAMH) with methacrylate-terminated PU (SAML-PU and SAMH-PU) through dynamic covalent cross-linking via Aza-Michael addition reactions. We aim to improve the poor compatibility of polysaccharide and PU associated with traditional blending methods and impart the resulting aerogels with good biodegradability, compressive strength, thermal insulation, flame-retardant, and self-healing. Comprehensive characterizations, including Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy collectively demonstrate the successful derivatization of alginate and the effective synthesis of the Aza-Michael addition copolymer involving derivate alginate and PU. Substantial improvements in compressive strength and thermal properties are realized at comparable densities. Higher compressive strength, higher decomposition temperature and lower thermal conductivity are significantly achieved for SAML-PU (0.35 MPa, 410 °C and 0.024 W/m/K at 0.13 g/cm3) and SAMH-PU (0.58 MPa, 427 °C and 0.022 W/m/K at 0.12 g/cm3) compared to pristine PU foams (0.15 MPa, 330 °C and 0.031 W/m/K at 0.15 g/cm3). Additionally, molecular simulations of the Aza-Michael addition reaction and finite element simulations of aerogel thermal-insulation properties are conducted to corroborate experimental results. Notably, the dynamic polymers of SAML-PU and SAMH-PU exhibited excellent self-healing, recycling, biodegradability, and flame-retardant attributes. The elucidated preparation methodology and the exceptional performance and versatile applications of sodium alginate-based PU aerogels suggest a promising trajectory for future research endeavors and industrial implementation.

Environmentally stable and multi-functional conductive gelatin/PVA/black wattle bark tannin based organogel as strain, temperature and bioelectric sensor for multi-mode sensing

Conductive hydrogels are regarded as ideal candidates for the application of flexible sensors owing to their excellent flexibility, portability and conductivity. However, it is still challenging and meaningful to prepare multifunctional (self-healing, adhesion, anti-freezing, biocompatibility, antibacterial and conductivity properties) and multi-mode sensing hydrogel-based sensors. Herein, we developed an environmentally stable and multi-functional conductive organogel via dynamic crosslinks based on biomass materials gelatin, black wattle bark tannin and PVA in the propylene glycol/water binary solvent system. Thanks to the dynamic interactions in the system, the good mechanical strength and self-healing performance of the obtained organogel are simultaneously realized. Meanwhile, the organogel integrates many crucial properties such as adhesion, environmental stability (anti-freezing and water retention), biocompatibility, antibacterial behavior and conductivity capacity. Significantly, the organogel can be assembled as three-mode sensors for strain, bioelectricity and temperature sensing. This three-mode sensor can effectively monitor human health data, resulting in providing supplement human health information and conditions. This work displays an interesting approach to construct an intelligent multi-functional conductive biomass organogel based multi-mode flexible sensors.

Fabrication of Zinc(II) Mediated Poly(Acrylamide Co Acrylic Acid) Hydrogel with Thixotropic and Tribological Properties.

Hydrogels have emerged as promising candidates for biomedical applications, such as replacing natural articular cartilage, owing to their unique viscoelastic properties. However, sufficient mechanical properties, self-healing ability, and adhesive nature are some issues limiting its application window. Here, a facile one-pot synthesis of dual cross-linked zinc-coordinated copolymer hydrogels is presented. The network structure of the copolymer hydrogels is strategically developed via dynamic and reversible physical cross-linking by Zn2+ ions and simultaneous covalent cross-linking through a covalent cross-linker viz methylene bisacrylamide. Fourier-transform infrared (FTIR), X-ray diffraction (XRD) scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analysis have thoroughly characterized the structure of the synthesized hydrogels. The introduction of Zn2+ offers dynamic and reversible complexation, leading to excellent mechanical properties and self-healing features. Moreover, the percentage of the equilibrium water content of zinc-coordinated copolymer hydrogel samples is comparable with that of natural articular cartilage. The Shear sliding study shows the dominant adhesive behavior of HGel-Zn(NO3)2 sample compared to the parent HGel sample. This facile dual cross-linked hydrogel, HGel-Zn(NO3)2, with a combination of good mechanical properties, efficient self-recovery, adequate water content, and favorable adhesive nature, seems very promising to mimic the articular cartilage.

Host-guest-induced electronic state triggers two-electron oxygen reduction electrocatalysis

Supramolecular polymers possess great potential in catalysis owing to their distinctive molecular recognition and dynamic crosslinking features. However, investigating supramolecular electrocatalysts with high efficiency in oxygen reduction reaction to hydrogen peroxide (ORHP) remains an unexplored frontier. Herein, we present organic polymers for ORHP by introducing cyclodextrin-containing noncovalent building blocks, affording these supramolecules with abundant dynamic bonds. The electronic states and reaction kinetics are further well-modulated via a host-guest strategy, resulting in appropriate regional electron binding force and controllable chemical activity. Notably, integrating supramolecular units into phenyl group-containing model covalent polymer achieves a production rate of 9.14 mol g−1cat h−1, with 98.01% Faraday efficiency, surpassing most reported metal-free electrocatalysts. Moreover, the dynamic bonds in supramolecular catalysts can effectively regulate the binding ability of oxygen intermediates, leading to high reactivity and selectivity for the 2e− pathway. Supported by theory calculation and in situ experiment, C atoms (site–1) adjacent to the –C = N (N) group are potential active sites. This work pioneers host-guest strategy and provides inspiring ideas for the ORHP process.

The dynamic crosslinking between gut microbiota and inflammation during aging: reviewing the nutritional and hormetic approaches against dysbiosis and inflammaging.

The early-life gut microbiota (GM) is increasingly recognized for its contributions to human health and disease over time. Microbiota composition, influenced by factors like race, geography, lifestyle, and individual differences, is subject to change. The GM serves dual roles, defending against pathogens and shaping the host immune system. Disruptions in microbial composition can lead to immune dysregulation, impacting defense mechanisms. Additionally, GM aids digestion, releasing nutrients and influencing physiological systems like the liver, brain, and endocrine system through microbial metabolites. Dysbiosis disrupts intestinal homeostasis, contributing to age-related diseases. Recent studies are elucidating the bacterial species that characterize a healthy microbiota, defining what constitutes a 'healthy' colonic microbiota. The present review article focuses on the importance of microbiome composition for the development of homeostasis and the roles of GM during aging and the age-related diseases caused by the alteration in gut microbial communities. This article might also help the readers to find treatments targeting GM for the prevention of various diseases linked to it effectively.

Reprocessible, Reusable, and Self-Healing Polymeric Adsorbent for Removing Perfluorinated Pollutants.

Here, we report a reprocessible, reusable, self-healing, and form-switching polymeric adsorbent for remediating fluorinated pollutants in water. The copolymer hydrogel is designed to contain fluorophilic segments and cationic segments to induce strong binding with perfluorinated pollutants. The sorption performance reveals rapid and quantitative removal of these pollutants, driven by the synergistic effect of fluorophilic and electrostatic interaction. Importantly, a disulfide-containing dynamic crosslinker plays a crucial role in imparting multifunctionality. This enables self-healing by the restoration of crosslinks at the cut surfaces by disulfide exchange reactions and allows for the repeated use of the adsorbent via multiple adsorption-desorption cycles. Furthermore, the adsorbent is reprocessible by cleaving the crosslinks to afford linear copolymers, which can be repolymerized into a hydrogel network on demand. Also, form-switching capability is showcased through the aqueous self-assembly of linear copolymers into a fluorinated micelle, serving as another form of adsorbent for pollutant removal.

Engineered Protein Fibers with Reinforced Mechanical Properties Via β-Sheet High-Order Assembly.

Protein fibers are ideal alternatives to synthetic polymers due to their unique mechanical properties, biocompatibility, and sustainability. However, engineering biomimetic protein fibers with high mechanical properties remains challenging, particularly in mimicking the high molecular weight of natural proteins and regulating their complex hierarchical structures. Here, a modular design and multi-scale assembly strategy is developed to manufacture robust protein fibers using low- or medium-molecular-weight proteins. The distinct functional and structural properties of flexible, rigid, and cross-linked domains in modular proteins are skillfully harnessed. By regulating the ratio of rigid to flexible domains, the formation of high-order β-sheet crystals aligned along the fiber axis is promoted, enhancing both strength and toughness. Furthermore, the dynamic imine cross-linking network, formed by the aldehyde-amine condensation reaction of the cross-linked domains, further reinforces the protein fibers. Remarkably, fibers spun from modular proteins significantly smaller than natural spidroin exhibit outstanding mechanical properties, surpassing those of protein fibers with same or even higher molecular weights. This strategy offers a promising pathway for fabricating protein fibers suitable for diverse applications.

Strong, Spontaneous, and Self-Healing Poly(Ionic Liquid) Elastomer Underwater Adhesive with Borate Ester Dynamic Crosslinking.

Adhesion in aqueous environments is often hindered by the water layer on the surface of the substrate due to the water sensitivity of the adhesive, greatly limiting the application environment. Here, a borate ester dynamically crosslinked poly(ionic liquid) elastomer adhesive (PIEA) with high strength, toughness, self-healing abilities, and ionic conductivity is synthesized by copolymerizing hydrophobic ionic liquid monomer ([HPVIm][TFSI]) and 2-methoxyethyl acrylate (MEA). The adhesion strength of PIEA can increase spontaneously from almost no adhesion to 314 kPa after 12 h without any external preloading due to the dissociation of the borate ester in water, leading to noncovalent interactions between the hydroxyl groups of PIEA and the substrate. Additionally, PIEA can be developed for soft sensors or ion electrodes to enable underwater detection and communication. This strategy offers broad application potential for the development of novel underwater smart adhesives.

Recyclable Membranes through Reversible and Dynamic Crosslinking

Recyclable Membranes through Reversible and Dynamic Crosslinking

ROS-responsive injectable hydrogels loaded with exosomes carrying miR-4500 reverse liver fibrosis

The reversal of liver fibrosis requires effective strategies to reduce oxidative stress and inhibition of hepatic stellate cell (HSC) activation. MiR-4500 regulates pathological angiogenesis and collagen mRNA stability, with the potential to inhibit fibrosis. Herein, we explored the inhibition of HSC activation in vitro by exosomes (Exos) carrying miR-4500 and encapsulated ExosmiR−4500 in an intelligent injectable hydrogel with biological activity and reactive oxygen species (ROS) responsiveness for application in oxidative stress environments. Briefly, reversible boronic ester bonds were integrated into gelatin-based hydrogels through dynamic crosslinking of quaternized chitosan (QCS) and 4-carboxyphenylboronic acid (CPBA)-modified gelatin. The QCS-CPBA-Gelatin (QCG) hydrogel scavenged excess ROS from the local microenvironment and released ExosmiR−4500 through the dissociation of boronic ester bonds, providing a favorable microenvironment and in situ sustained-release drug delivery system for ExosmiR−4500. The results showed that QCG@ExosmiR−4500 hydrogel has biocompatibility, biodegradability, and slow-release ability, which could effectively clear ROS and inhibit HSC activation and pathological angiogenesis in vitro and in vivo. Furthermore, transcriptome analysis suggests that the pharmacological mechanism of the QCG@ExosmiR−4500 hydrogel is mainly related to anti-oxidation, anti-angiogenesis, anti-fibrosis processes, and signaling pathways. Thus, our study demonstrates that an intelligently responsive ExosmiR−4500 delivery system based on injectable hydrogels is a promising strategy for the treatment of liver fibrosis.

Reversible Vitrimisation of Single‐Use Plastics and their Mixtures

AbstractVitrimers are promising next‐generation materials, often exhibiting superior properties such as mechanical strength and solvent resistance compared to their thermoplastic counterparts, while still featuring recyclability due to their dynamic covalent crosslinks. The most common strategy to recycle vitrimers is via mechanical reprocessing at high temperatures, while others utilize chemical degradation to reobtain the constituent monomers. This work presents a new approach toward reprocessing by selectively breaking of the vitrimer's crosslinks, turning it back into a thermoplastic. This allows for reprocessing to be achieved in a more sustainable manner such as at lower temperatures and shorter times. After the desired shape has been obtained, facile re‐crosslinking turns the material into a vitrimer, with full restoration of thermal stability, mechanical properties, and gel fraction. To achieve this, a carbene C─H insertion strategy is utilized to introduce an imine‐based crosslinker into various thermoplastics and plastic mixtures to convert them into vitrimers. Properties typical of vitrimers such as a retained polymer network at high temperatures, and an Arrhenius‐type relationship between stress relaxation rate and temperature are observed, while its mechanical properties such as elongation at break and tensile toughness, particularly for plastic mixtures are significantly enhanced, first by crosslinking, then further enhanced up to 8 folds by the reversible crosslinking process.

Vitrimer-like elastomers with rapid stress-relaxation by high-speed carboxy exchange through conjugate substitution reaction

We report vitrimer-like elastomers that exhibit significantly fast stress relaxation using carboxy exchange via the conjugate substitution reaction of α-(acyloxymethyl) acrylate skeletons. This network design is inspired by a small-molecule model that shows the carboxy exchange reaction even at ambient temperature in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO). The acrylate and acrylic acid copolymers are cross-linked using bis[α-(bromomethyl)acrylates] and doped with 10 wt% DABCO, exhibiting processability to obtain a transparent film by hot pressing. The high-speed bond exchange in the network, validated by stress-relaxation tests, allows quick molding with household iron. In addition, the material is applied as an adhesion sheet for plastic and metal substrates. Because dynamic cross-linking with the proposed bond exchange mechanism can be implemented for any polymer bearing carboxyl pendants, our approach can be applied to versatile backbones, which must thus be meaningful in the practical sense.

Dynamic imine crosslinking for waterproof starch plastic with tunable mechanical properties

Starch-based plastics offer a potential solution to environmental pollution and can reduce the dependence on petroleum-based plastics. However, effectively preparing starch-based plastics with moisture resistance and high strength remains challenging. Herein, dialdehyde starch (DAS) with different degrees of oxidation was first synthesised. The molecular weight and crystallinity of DAS decreased depending on the aldehyde content, as determined through scanning electron microscopy, X-ray diffraction and molecular weight analyses. Subsequently, a sustainable starch plastic (DAS-DA) was prepared via the dynamic imine crosslinking of DAS with diamines. The influence of the degree of oxidation of DAS on the structure and properties of DAS-DA was systematically investigated. No linear relationship was observed between the tensile strength of DAS-DA and the aldehyde content of DAS. When the aldehyde content was 41.1 %, the resulting DAS41-DA exhibited optimal mechanical strength (27.8 MPa), exceeding that of most reported starch-based plastics. In addition, owing to the presence of imine crosslinking networks, DAS41-DA exhibited excellent water and solvent resistance (>60 days), good thermal stability (456 °C–462 °C) and excellent reprocessability and biodegradability. These findings provide a theoretical basis for the preparation of high-performance bioplastics from aldehyde-containing polysaccharides.

Dynamic Covalent Bonds Enabled Carbon Fiber Reinforced Polymers Recyclability and Material Circularity.

Due to their remarkable features of lightweight, high strength, stiffness, high-temperature resistance, and corrosion resistance, carbon fiber reinforced polymers (CFRPs) are extensively used in sports equipment, vehicles, aircraft, windmill blades, and other sectors. The urging need to develop a resource-saving and environmentally responsible society requires the recycling of CFRPs. Traditional CFRPs, on the other hand, are difficult to recycle due to the permanent covalent crosslinking of polymer matrices. The combination of covalent adaptable networks (CANs) with carbon fibers (CFs) marks a new development path for closed-loop recyclable CFRPs and polymer resins. In this review, we summarize the most recent developments of closed-loop recyclable CFRPs from the unique paradigm of dynamic crosslinking polymers, CANs. These sophisticated materials with diverse functions, oriented towards CFs recycling and resin sustainability, are further categorized into several active domains of dynamic covalent bonds, including ester bonds, imine bonds, disulfide bonds, boronic ester bonds, and acetal linkages, etc. Finally, the possible strategies for the future design of recyclable CFPRs by combining dynamic covalent chemistry innovation with materials interface science are proposed.

Jellyfish-Inspired Polyurea Ionogel with Mechanical Robustness, Self-Healing, and Fluorescence Enabled by Hyperbranched Cluster Aggregates.

Ionogels are promising for soft iontronics, with their network structure playing a pivotal role in determining their performance and potential applications. However, simultaneously achieving mechanical toughness, low hysteresis, self-healing, and fluorescence using existing network structures is challenging. Drawing inspiration from jellyfish, we propose a novel hierarchical crosslinking network structure design for in situ formation of hyperbranched cluster aggregates (HCA) to fabricate polyurea ionogels to overcome these challenges. Leveraging the disparate reactivity of isocyanate groups, we induce the in situ formation of HCA through competing reactions, enhancing toughness and imparting the clustering-triggered emission of ionogel. This synergy between supramolecular interactions in the network and plasticizing effect in ionic liquid leads to reduced hysteresis of the ionogel. Furthermore, the incorporation of NCO-terminated prepolymer with dynamic oxime-urethane bonds (NPU) enables self-healing and enhances stretchability. Our investigations highlight the significant influence of HCA on ionogel performance, showcasing mechanical robustness including high strength (3.5 MPa), exceptional toughness (5.5 MJ m-3), resistance to puncture, and low hysteresis, self-healing, as well as fluorescence, surpassing conventional dynamic crosslinking approaches. This network design strategy is versatile and can meet the various demands of flexible electronics applications.

A strain rate response and self-healing organohydrogel for use in impact-protective materials

Impact-protective materials are used in many fields owing to their excellent mechanical properties, impact resistance, and rapid responses to external stimuli. In this study, a new organohydrogel (PAIG) was designed and prepared through emulsion copolymerization by combining polymer chain segment as a skeleton and oil phase containing IHP as dynamic crosslinking points. The PAIG gel system provided a stable structure and overcame the cold flow property of impact-hardening polymers (IHP) with a tensile stress of 0.48 MPa. In addition, the introduced flexible IHP chains provided a large number of boron–oxygen dynamic bonds and hydrogen bonds, which improved both the impact resistance (the peak force reduction rate of 74 %), speed- response and the self-healing ability (with an efficiency of ca. 90 %). Impressively, PAIG can withstand cyclic twisting and bending even at −20 °C, showing its excellent freezing resistance. These properties make the PAIG promising for intelligent impact-protection applications.

A multifunctional hydrogel dressing loaded with antibiotics for healing of infected wound

Wound bacterial infections can significantly delay the healing process and even lead to fetal sepsis. There is a need for multifunctional dressings that possess antibacterial property, tissue adhesive property, self-healing capability, and biocompatibility to effectively treat bacteria-infected wound. In this study, we report a dual dynamically crosslinked hydrogel, OHA-PBA/PVA/Gen, which incorporates the antibiotic gentamicin (Gen) as a dynamic crosslinker. The hydrogel is formed through the formation of Schiff base bonds between phenylboronic acid-grafted oxidized hyaluronic acid (OHA-PBA) and Gen, as well as boronic acid ester bonds between OHA-PBA and polyvinyl alcohol (PVA). This unique composition imparts tissue adhesiveness, injectability and self-healing property to the hydrogel. The hydrogel also exhibits pH-responsive antibiotic release behavior due to the acid-responsive dissociation of Schiff base bonds. As a result, it demonstrates strong antibacterial activity against both Gram-positive bacteria S. aureus and Gram-negative bacteria E. coli through contact killing and diffusion killing mechanisms. Importantly, the OHA-PBA/PVA/Gen hydrogel avoids incorporation of toxic small molecular crosslinking agents, and all the components of the hydrogel are biocompatible, ensuring its biosafety. In a S. aureus-infected wound mouse model, this hydrogel effectively eradicated bacteria and promoted angiogenesis, leading to significantly accelerated wound healing. These results highlight the potential of the dual dynamically crosslinking hydrogel OHA-PBA/PVA/Gen as a multifunctional wound dressing for the treatment of bacteria-infected wound.