Nitrogen-Coordinated Boroxines Enable the Fabrication of Mechanically Robust Supramolecular Thermosets Capable of Healing and Recycling under Mild Conditions

Ultra-Fast-Healing Glassy Hyperbranched Plastics Capable of Restoring 26.4 MPa Tensile Strength within One Minute at Room Temperature

The growing concern regarding widespread plastic pollution has propelled the development of sustainable self-healing plastics. Although considerable efforts have been dedicated to fabricating self-healing plastics, achieving rapid healing at room temperature is extremely challenging. Herein, we have developed an ultra-fast-healing glassy polyurethane (UGPU) by designing a hyperbranched molecular structure with a high density of multiple hydrogen bonds (H-bonds) on compliant acyclic heterochains and introducing trace water to form water bridge across the fractured surfaces. The compliant acyclic heterochains allow the dense multiple hydrogen bonds to form a frozen network, enabling tensile strength of up to 70 MPa and storage modulus of 2.5 GPa. The hyperbranched structure can drive the reorganization of the H-bonding network through the high mobility of the branched chains and terminals, thereby leading to self-healing ability at room temperature. Intriguingly, the presence of trace water vapor facilitates the formation of activated layers and the rearrangement of networks across the fractured UGPU sections, thereby enabling ultra-fast self-healing at room temperature. Consequently, the restored tensile strength after healing for 1 minute achieves a historic-record of 26.4 MPa. Furthermore, the high transparency (>90 %) and ultra-fast healing property of UGPU make it an excellent candidate for advanced optical and structural materials.

Recycling of Polydicyclopentadiene Enabled with N-Coordinated Boronic Ester Bonds

In this contribution, the transformation of polydicyclopentadiene (PDCPD) from thermoset into vitrimer is introduced. First, two N-coordinated diboronic diols are successfully synthesized via the reaction of N,N,N-tri(2-hydroxyethyl)amine and/or N,N,N",N"-tetrakis(2-hydroxyethyl)ethylene diamine with 4-(hydroxymethyl) phenylboronic acid and then they are transformed into two N-coordinated cyclic boronic diacrylates. The latter two dienes carrying electron-withdrawing substituents are used for the ring opening insertion metathesis copolymerization (ROIMP) of dicyclopentadiene to afford the crosslinked PDCPD. In the crosslinked PDCPD networks, N-coordinated cyclic boronic ester bonds are integrated. It is found that the as-obtained PDCPD networks displayed the excellent reprocessing properties. In the meantime, the fracture toughness is significantly improved. Owing to the inclusion of N-coordinated cyclic boronic ester bonds, the modified PDCPDs have the thermal stability much superior to plain PDCPD. The results reported in this work demonstrate that PDCPD can successfully be transformed into the vitrimers via the introduction of N-coordinated cyclic boronic ester bonds.

Water-stable boroxine structure with dynamic covalent bonds

Boroxines are significant structures in the production of covalent organic frameworks, anion receptors, self-healing materials, and others. However, their utilization in aqueous media is a formidable task due to hydrolytic instability. Here we report a water-stable boroxine structure discovered from 2-hydroxyphenylboronic acid. We find that, under ambient environments, 2-hydroxyphenylboronic acid undergoes spontaneous dehydration to form a dimer with dynamic covalent bonds and aggregation-induced enhanced emission activity. Intriguingly, upon exposure to water, the dimer rapidly transforms into a boroxine structure with excellent pH stability and water-compatible dynamic covalent bonds. Building upon these discoveries, we report the strong binding capacity of boroxines toward fluoride ions in aqueous media, and develop a boroxine-based hydrogel with high acid-base stability and reversible gel-sol transition. This discovery of the water-stable boroxine structure breaks the constraint of boroxines not being applicable in aqueous environments, opening a new era of researches in boroxine chemistry.

Ultrastrong bonding, on-demand debonding, and easy re-bonding of non-sticking materials enabled by reversibly interlocked macromolecular networks-based Janus-like adhesive

Simultaneously gluing hydrophobic and hydrophilic materials is a highly desired but intractable task. Herein, we developed a facile strategy using reversibly interlocked macromolecular networks (ILNs) as an adhesive. As shown by the proof-of-concept assembly of glass/ILNs/fluoropolymer (i.e., a simplified version of a photovoltaic module), the sandwiched ILNs were stratified after hot-pressing owing to temporary decrosslinking enabled by the built-in reversible covalent bonds. The fragmented component networks were enriched near their respective thermodynamically favored substrates to form a Janus-like structure. Strong elaborate interfacial bespoke chemical bonds and mechanical interlocking were thus established accompanied by the reconstruction of ILNs after cooling, which cooperated with the robust cohesion of the core part of the ILNs resulting from topological entanglements and led to a record-high peeling strength of 64.86 N cm-1. Also, the ILN-based Janus-like adhesive possessed reversible recyclability, adhesivity and on-demand de-bondability. The molecular design detailed in this study serves as a guide for developing a high-performance smart adhesive that firmly bonds non-sticking materials. Compared with existing Janus adhesives, our ILNs-based adhesive not only shows extremely useful reversibility but also greatly simplifies the adhesion process with no surface treatment required.

Tough adhesion enhancing strategies for injectable hydrogel adhesives in biomedical applications

Injectable hydrogel adhesives have gained widespread attention due to their ease of use, fast application time, and suitability for minimally invasive procedures. Several biomedical applications depend on tough adhesion between hydrogel adhesives and tissues, including wound closure and healing, hemostasis, tissue regeneration, drug delivery, and wearable electronic devices. Compared with bulk hydrogel adhesives formed ex situ, injectable hydrogel adhesives are more difficult to achieve strong adhesion strength due to a further balance of cohesion and adhesion while maintaining their flowability. In this review, the critical principles in designing tough adhesion of injectable hydrogel adhesives are summarized, including simultaneously enhancing their intrinsic interfacial toughness (Γ0inter) and mechanical dissipation (ΓDinter). Thereafter, various design strategies to enhance the Γ0inter and ΓDinter are discussed and evaluated respectively, involving multiple noncovalent/covalent interactions, topological connections, and polymer network structures. Furthermore, targeted biomedical applications of injectable hydrogel adhesives for specific tissue needs are systematically highlighted. In the end, this review outlines the challenges and trends in producing next-generation multifunctional injectable hydrogels for both practical and translational applications.

Design of Intelligent Protective Composite Material with Stress Rate Sensitivity, Strong Interface Adhesion, and Recyclability

Poly(dimethyl siloxane) (PDMS) elastomers play a significant role in smart materials, actuators, and flexible electronics. However, current PDMS lacks adhesion abilities and intelligent responsive properties, which limit its further application. In this study, the polydimethylsiloxane-ureidopyrimidinone impact hardening polymer (PDMS-UI) composites are manufactured by a dual cross-linking compositing tactic. PDMS, a chemically stable cross-linked network, acts as a framework owing to its excellent mechanical strength, whereas UI, a reversible dynamic physically cross-linked network with quadruple hydrogen bonding, endows the PDMS-UI with excellent self-healing ability (efficiency > 90%) and energy absorption (75.23%). Impressively, owing to multivalent hydrogen bonds, the PDMS-UI exhibits superior adhesion performance: the adhesion strength on various substrates exceed 150 kPa and that on the Ferrum substrate reaches 570 kPa. These outstanding properties make the PDMS-UI a potential candidate for application in both well-developed fields, such as, wearable protective materials, artificial skin and soft robotics.

Mechanical property-enhanced thermally conductive self-healing composites: preparation using designed self-healing matrix phase and hyBNNSs

Polymer composites with good thermal conductivity are gaining more and more attention in the current electronics sector, due to their superior heat management capabilities. However, conventional thermally conductive polymer composites are usually subject to interruptions in heat transfer because of physical damage. The present study prepared mechanical property-enhanced thermally-conductive self-healing composites through compositing a self-healing polyurethane matrix with hydroxylated boron nitride (hyBNNSs). The self-healing polyurethane was obtained by incorporating ligands and cerium(III) triflate [Ce(SO3CF3)3] as the metal center into the polyurethane elastomer. An optimal sample (PUp2C) with high tensile strength (6.8 MPa) and stretchability (1053%), ideal toughness (49.2 MJ m-3), and remarkable healing efficiency (97% healing after 48 h at 35 °C) was obtained. An increase in the content of hyBNNSs from 10% to 30% led to a significant increase in the mechanical performance of hyBNNSs20%/PUp2C, which manifested as the increase in the elongation at break (from 1053% to 1302.5%) and stress (from 6.8 MPa to 16.4 MPa). The XRD results revealed that combining PU with hyBNNSs through coordination bonds could significantly promote the crystallization of PUp2C, which was beneficial to enhancing the mechanical properties of the composites. The through-plane (λ⊥) and the in-plane (λ∥) values of the BNNSs30%/PUp2C composite reached 0.41 and 1.42 W mK-1, respectively, which were 195.2% and 507.1% higher than those of the original PUp2C, respectively.

Structural Interconversion Based on Intramolecular Boroxine Formation

A novel structural interconversion unit based on intramolecular boroxine formation has been developed. A macrocyclic triboronic acid consisting of three phenylboronic acid units linked by covalent linkers preferentially underwent intramolecular rather than intermolecular boroxine formation, resulting in a quantitative formation of tricyclic boroxine. This structural transformation was accompanied by changes in the polarity, flexibility, and size of the molecule. Dynamic interconversion between the macrocyclic triboronic acid and the tricyclic boroxine was achieved by simple heating/cooling, whereas no boroxine formation occurred upon heating when three boronic acid units were not connected by linkers. Thermodynamic analysis revealed that the entropic advantage of the intramolecular boroxine formation process resulted in these unique features. The entropically stabilized tricyclic boroxine also shows high stability with respect to hydrolysis.

Mechanically Strong and Tough Poly(urea-urethane) Thermosets Capable of Being Degraded under Mild Condition

The development of degradable polymeric materials such as degradable polyurethane or polyurea has been much highlighted for resource conservation and environmental protection. Herein, a facile strategy of constructing mechanically strong and tough poly(urea-urethane) (PUU) thermosets that can be degraded under mild conditions by using triple boron-urethane bonds (TBUB) as cross-linkers is demonstrated. By tailoring the molecular weight of the soft segment of the prepolymers, the mechanical performance can be finely controlled. Based on the cross-linking of TBUB units and hydrogen-binding interactions between TBUB linkages, the as-prepared PUU thermosets have excellent mechanical strength of ≈40.2 MPa and toughness of ≈304.9 MJ m^-3 . Typically, the PBUU900 strip can lift a barbell with 60 000 times its own weight, showing excellent load-bearing capacity. Meanwhile, owing to the covalent cross-linking of TBUB units, all the PUU thermosets show initial decomposition temperatures over 290 °C, which are comparable to those of the traditional thermosets. Moreover, the TBUB cross-linked PUU thermosets can be easily degraded in a mild acid solution. The small pieces of the PBUU sample can be fully decomposed in 1 m HCl/THF solution for 3.5 h at room temperature.

Boronic Acid Esters and Anhydrates as Dynamic Cross-Links in Vitrimers

Growing environmental awareness imposes on polymer scientists the development of novel materials that show a longer lifetime and that can be easily recycled. These challenges were largely met by vitrimers, a new class of polymers that merges properties of thermoplastics and thermosets. This is achieved by the incorporation of dynamic covalent bonds into the polymer structure, which provides high stability at the service temperature, but enables the processing at elevated temperatures. Numerous types of dynamic covalent bonds have been utilized for the synthesis of vitrimers. Amongst them, boronic acid-based linkages, namely boronic acid esters and boroxines, are distinguished by their quick exchange kinetics and the possibility of easy application in various polymer systems, from commercial thermoplastics to low molecular weight thermosetting resins. This review covers the development of dynamic cross-links. This review is aimed at providing the state of the art in the utilization of boronic species for the synthesis of covalent adaptable networks. We mainly focus on the synthetic aspects of boronic linkages-based vitrimers construction. Finally, the challenges and future perspectives are provided.

Synthesis and optical properties of N→Ga coordinated gallium boroxines

Reactions of the N,C,N-chelated organogallium amide LGa(NEt₂)₂ (1), where L is {2,6-(Me₂NCH₂)₂C₆H₃}^-, with organoboronic acids RB(OH)₂ yielded molecular gallium boroxines LGa(O₃B₂R₂) (2: R = OH, 3: R = Ph, 4: R = 4-MeO-C₆H₄, 5: R = 4-CHO-C₆H₄, 6: R = Fc), neutral analogues of gallaborates. The molecular structures revealed the presence of a six-membered central GaB₂O₃ ring. The film forming properties of 5 allowed the deposition of transparent thin films by a spin coating method. The thicknesses, refractive index, energy of the optical gap (Eoptg), activation energy of surface electrical conductivity (Esa) and pre-exponential factor (σ₀) of the thin layers of 5 were measured and they are close to those found for related oxygen glass. Finally, GBO 5 was also used as an additive to printing ink and a thin film of 5 was prepared by the gravure printing technique.

Laser-Induced Remote Healing of Stretchable Diselenide-Containing Conductive Composites

Remotely controlled on-demand functional healing is vital to components that are difficult to access and repair in distance such as satellites and unmanned cruising aircrafts. Compared with other stimuli, a blue laser is a better choice to input energy to the damaged area in distance because of its high energy density and low dissipation through the air. Herein, diselenide-containing polyurethane (PUSe) is first employed to fabricate visible light-responsive stretchable conductive composites with multiwalled carbon nanotubes (MWCNTs). Then, laser-induced remote healing was realized based on the characteristics of long-distance propagation of lasers and the dynamic properties of diselenide bonds. Moreover, the PUSe/MWCNT composite film can be used to transfer an electrical signal in the circuit containing a signal generator. This laser-induced remote healing of conductivity paves the way for developing healing conductors which are difficult to access and repair.

Stretchable dual cross-linked silicon elastomer with a superhydrophobic surface and fast triple self-healing ability at room temperature

Stretchable elastomers with superhydrophobic surfaces have potential applications in wearable electronics. However, various types of damage inevitably occur on these elastomers in actual application, resulting in the deterioration of the superhydrophobic properties. In this work, superhydrophobic elastomers (HB-imine-BZn-PDMS), was fabricated by employing a dual-layered structure. The bottom layer was a silicon elastomer (imine-BZn-PDMS) with an imine/coordination dual cross-linked structure and room temperature self-healing efficiency of 94%. The top layer was imine-BZn-PDMS/silica nanocomposites to provide superhydrophobic properties. The HB-imine-BZn-PDMS elastomer exhibited fast triple self-healing ability at room temperature toward surface oxidation/decomposition, ruptures, or pinholes, and high durability under abrasion and stretching. The dual dynamic bonds of imine-BZn-PDMS enabled fast recovery of superhydrophobicity in 20 min at room temperature via bond exchange, after generating pinholes across the elastomer. Following surface chemical damage, the HB-imine-BZn-PDMS elastomer also exhibited fast (40 min) room-temperature self-healing ability, which is superior to that of most current self-healing superhydrophobic materials.

Engineering Hydrogel Adhesion for Biomedical Applications via Chemical Design of the Junction

Hydrogel adhesion inherently relies on engineering the contact surface at soft and hydrated interfaces. Upon contact, adhesion normally occurs through the formation of chemical or physical interactions between the disparate surfaces. The ability to form these adhesion junctions is challenging for hydrogels as the interfaces are wet and deformable and often contain low densities of functional groups. In this Review, we link the design of the binding chemistries or adhesion junctions, whether covalent, dynamic covalent, supramolecular, or physical, to the emergent adhesive properties of soft and hydrated interfaces. Wet adhesion is useful for bonding to or between tissues and implants for a range of biomedical applications. We highlight several recent and emerging adhesive hydrogels for use in biomedicine in the context of efficient junction design. The main focus is on engineering hydrogel adhesion through molecular design of the junctions to tailor the adhesion strength, reversibility, stability, and response to environmental stimuli.

Biomimetic Boroxine-Based Multifunctional Thermosets via One-Pot Synthesis

Boroxine-based thermosets with remarkable mechanical tunability, self-healing ability, recyclability, and adhesive strength are of significant importance in various applications. However, complex multistep reactions are often required to prepare such thermosets. Herein, a facile one-pot approach to synthesize boroxine-based malleable thermosets is proposed. Random copolymers with pendant boronic acid groups were synthesized from alkenyl monomers containing boronic acids [4-vinylphenylboronic acid (4-VPBA), 3-vinylphenylboronic acid, or 3-acrylamidophenylboronic acid] and octadecanoxy polyethylene glycol methacrylate. Then, the as-prepared copolymers were cured to form thermosets with boroxine bonds. The tensile strengths of the thermosets were tailored to range from 9.3 to 27.5 MPa by increasing the concentration of 4-VPBA. Moreover, because of the reversible nature of dynamic boroxine bonds (transformation between boroxines and boronic acids) induced by water, the thermosets exhibit remarkable self-healing efficiency (up to 99%), tunable mechanical properties, and excellent recyclability. Additionally, the thermosets also demonstrate superior adhesive strength (as high as 73.9 MPa) on different substrates.

Assembling of Reprocessable Polybutadiene-Based Vitrimers with High Strength and Shape Memory via Catalyst-Free Imine-Coordinated Boroxine

Vitrimers endow cross-linked polymers with malleability and reprocessability via exchange reactions. However, designing of reprocessable, shape-memory polymer materials with high strength via a catalyst-free method remains a challenge under mild conditions. Here, we propose a facile strategy to address this dilemma by introducing the exchangeable imine bond and N-coordinated boroxine into a polybutadiene (PB)-based network. Specifically, PB grafted with 2-aminoethanethiol is reacted with the formyl group of phenylboronic acid and dehydrated to form a dual-dynamic covalently cross-linked network at room temperature. The dynamic network draws on the advantage of imine (toughness) and N-coordinated boroxine (strength), making the PB-based materials exhibit favorable malleability, mechanical property, reprocessability, and thermal-induced shape-memory behavior. We can obtain customized high mechanical properties by tuning the cross-linking density, and the tensile strength reaches a high value (12.35 MPa) without fillers or any other additives. Meanwhile, the unique network framework makes the material recycle over several times without sacrificing its property. This work presents a facile and effective approach to achieve a multifunctional polymer with customized attributes. Besides, this strategy can recycle end-of-life rubber to alleviate environmental pollution and provide inspiration for fabricating targeted materials by uniting the dynamic covalent or noncovalent bonds.

Rediscovering Surlyn: A Supramolecular Thermoset Capable of Healing and Recycling

Surlyn consists of ionomers of poly(ethylene-co-methacrylic acid) (PEMA) partially neutralized with metal ions. Considering its huge consumption every year, it is highly desirable to realize efficient healing and recycling of Surlyn through an easily available method. Herein, healable and recyclable Surlyn materials are fabricated by complexation of PEMA with Zn²⁺ ions followed by a hot-pressing process. The PEMA/Zn composites exhibit a tensile strength of ≈37 MPa, Young's modulus of ≈343 MPa, and toughness of ≈95 MJ m^-3 . Structural analysis discloses that the PEMA/Zn composites are dynamically cross-linked with coordination interactions and reinforced with polyethylene nanocrystals, and have the typical structure of supramolecular thermosets. As supramolecular thermosets, the reversibility of coordination interactions endows the PEMA/Zn composites with good healing and recycling capacities. The PEMA/Zn composites can fully heal mechanical damage to restore their original mechanical strength when heated at 90 °C. Under a pressure of 3 MPa at 100 °C, the PEMA/Zn composites can be recycled multiple times to regain their structural integrity and mechanical properties.

Room-temperature autonomous self-healing glassy polymers with hyperbranched structure

Glassy polymers are extremely difficult to self-heal below their glass transition temperature (T _g) due to the frozen molecules. Here, we fabricate a series of randomly hyperbranched polymers (RHP) with high density of multiple hydrogen bonds, which show T _g up to 49 °C and storage modulus up to 2.7 GPa. We reveal that the hyperbranched structure not only allows the external branch units and terminals of the molecules to have a high degree of mobility in the glassy state, but also leads to the coexistence of "free" and associated complementary moieties of hydrogen bonds. The free complementary moieties can exchange with the associated hydrogen bonds, enabling network reconfiguration in the glassy polymer. As a result, the RHP shows amazing instantaneous self-healing with recovered tensile strength up to 5.5 MPa within 1 min, and the self-healing efficiency increases with contacting time at room temperature without the intervention of external stimuli.

Functionalization of amine-cured epoxy resins by boronic acids based on dynamic dioxazaborocane formation

Functionalization of epoxy resins after curing was performed based on dynamic dioxazaborocane formation between intrinsic diethanolamine units in amine-cured epoxy resins and boronic acid modifiers.

Thermally healable polyurethane with tailored mechanical performance using dynamic crosslinking motifs

A tunable dynamic cross-linked polyurethane that makes a tradeoff between superior mechanical performance and excellent self-healing ability.