Olefin Metathesis for Effective Polymer Healing via Dynamic Exchange of Strong Carbon–Carbon Double Bonds

Harnessing the Day-Night Rhythm of Humidity and Sunlight into Mechanical Work Using Recyclable and Reprogrammable Soft Actuators

Toward a sustainable society, soft actuators driven by environmentally friendly energy from nature are of great social and economic significance. Meanwhile, recyclability, repeated reconfiguration for other use, and complex three-dimensional (3D) geometries are also essential for mitigating the energy crisis and practical application demands. Here, we integrate all of the above features in one actuator using vitrimers with exchangeable disulfide links. By reconfiguration, welding, patterning, and kirigami techniques, complex 3D actuators can be easily fabricated, which can be repeatedly reconfigured for other applications to save cost in new material preparation. These actuators operate synergistically with the day-night rhythm of humidity and sunlight without the need of extra energy input.

Detecting topology freezing transition temperature of vitrimers by AIE luminogens

Vitrimers are one kind of covalently crosslinked polymers that can be reprocessed. Topology freezing transition temperature (Tv) is vitrimer's upper limit temperature for service and lower temperature for recycle. However, there has been no proper method to detect the intrinsic Tv till now. Even worse, current testing methods may lead to a misunderstanding of vitrimers. Here we provide a sensitive and universal method by doping or swelling aggregation-induced-emission (AIE) luminogens into vitrimers. The fluorescence of AIE-luminogens changes dramatically below and over Tv, providing an accurate method to measure Tv without the interference of external force. Moreover, according to this method, Tv is independent of catalyst loading. The opposite idea has been kept for a long time. This method not only is helpful for the practical application of vitrimers so as to reduce white wastes, but also may facilitate deep understanding of vitrimers and further development of functional polymer materials.

Reversible Self-Healing Carbon-Based Nanocomposites for Structural Applications

Reversible Hydrogen Bonds (RHB) have been explored to confer self-healing function to multifunctional nanocomposites. This study has been carried out through a sequence of different steps. Hydrogen bonding moieties, with the intrinsic ability to simultaneously perform the functions of both hydrogen donors and acceptors, have been covalently attached to the walls of carbon nanotubes. The epoxy matrix has been modified to adapt the formulation for hosting self-healing mechanisms. It has been toughened with different percentages of rubber phase covalently linked to the epoxy precursor. The most performant matrix, from the mechanical point of view, has been chosen for the incorporation of MWCNTs. Self-healing performance and electrical conductivities have been studied. The comparison of data related to the properties of nanocomposites containing incorporated functionalized and nonfunctionalized MWCNTs has been performed. The values of the electrical conductivity of the self-healing nanocomposites, containing 2.0% by weight of functionalized multiwalled carbon nanotubes (MWCNTs), range between 6.76 × 10-3 S/m and 3.77 × 10-2 S/m, depending on the nature of the functional group. Curing degrees, glass transition temperatures, and storage moduli of the formulated multifunctional nanocomposites prove their potential for application as functional structural materials.

Recyclable Self-Healing Polyurethane Cross-Linked by Alkyl Diselenide with Enhanced Mechanical Properties

Dynamic structures containing polymers can behave as thermosets at room temperature while maintaining good mechanical properties, showing good reprocessability, repairability, and recyclability. In this work, alkyl diselenide is effectively used as a dynamic cross-linker for the design of self-healing poly(urea-urethane) elastomers, which show quantitative healing efficiency at room temperature, without the need for any catalysts or external interventions. Due to the combined action of the urea bond and amide bond, the material has better mechanical properties. We also compared the self-healing effect of alkyl diselenide-based polyurethanes and alkyl disulfide-based polyurethanes. The alkyl diselenide has been incorporated into polyurethane networks using a para-substituted amine diphenyl alkyl diselenide. The resulting materials not only exhibit faster self-healing properties than the corresponding disulfide-based materials, but also show the ability to be processed at temperatures as low as 60 °C.

Sustainable, Naringenin-Based Thermosets Show Reversible Macroscopic Shape Changes and Enable Modular Recycling

A sustainable biobased thermoset exhibiting shape-memory behavior and modular recycling capabilities has been developed herein. The prepared thermoset consists of naringenin and biocompatible polymer components. Naringenin, which has three phenolic moieties, has been converted to a multifunctional monomer containing glycidyl groups and readily formed a thermosetting network via epoxide ring opening reaction with a poly(ethylene glycol) diacid under solvent-free conditions. The resulting material is malleable yet as strong as articular cartilage and selectively absorbs water when compared with n-dodecane oil. Moreover, the thermoset can be physically reused. After being crumpled, stretched, or coiled, the initial shape of the material is restored in response to heat or water. Furthermore, the material is amenable to chemical recycling in a bulk state via transesterification, and its components can be recovered on a molecular level after degradation under benign conditions, as was confirmed using a model compound.

Rapid Stress Relaxation and Moderate Temperature of Malleability Enabled by the Synergy of Disulfide Metathesis and Carboxylate Transesterification in Epoxy Vitrimers

Vitrimers make up a class of polymeric materials combining the advantages of thermosets and thermoplastics, because they can be reprocessed while being at the same time permanently cross-linked. However, a long heating duration or an elevated temperature is necessary for most vitrimers to relax the stress from deformation and exhibit malleability. Herein, a disulfide-containing carboxylic acid is applied as a curing agent to synthesize epoxy vitrimers with simultaneous disulfide metathesis and carboxylate transesterification. The insoluble networks exhibit rapid stress relaxation and have relaxation times ranging from 1.5 s (200 °C) to 5500 s (60 °C), while the temperature of malleability is as low as 65 °C. Moreover, this vitrimer can be efficiently reprocessed at 100 °C in 1 h with full recovery of mechanical strength for at least four cycles. Additionally, such a material is simply synthesized from commercially available chemicals and may have potential applications in the electronics industry where a high temperature is not allowed.

A High-Lignin-Content, Removable, and Glycol-Assisted Repairable Coating Based on Dynamic Covalent Bonds

Conventional thermoset coatings cannot be easily repaired and removed owing to their highly crosslinked structure. The investigation of repairable or removable coatings has been receiving extensive attention, but few reported coatings possess both features. In this work, a repairable and removable coating was developed through the curing of a modified Kraft lignin (L-COOH) with poly(ethylene glycol) diglycidyl ether (PEG-epoxy) in the presence of zinc catalyst. The L-COOH was prepared by functionalization of Kraft lignin with carboxylic acid groups. The cured material had a high lignin content (>47 wt %). At elevated temperatures (>140 °C), dynamic transesterification in the cured network was activated, which resulted in fast stress relaxation and imparted excellent repairability. If the vitrimer system was used as a coating for tin plates, it provided adequate hardness and adhesion properties. In addition, the lignin-PEG coating could be easily removed from the tin plate by using a mild (0.01-0.1 m) NaOH aqueous solution owing to the unique swelling ability of the coating in alkaline aqueous solution. With the assistance of ethylene glycol, this coating could achieve stress-free repairability in 15 min. This work demonstrates the first lignin-based repair- and removable epoxy coating based on vitrimer chemistry.

Integrating Sacrificial Bonds into Dynamic Covalent Networks toward Mechanically Robust and Malleable Elastomers

Vitrimers are a class of covalently cross-linked polymers that have drawn great attention due to their fascinating properties such as malleability and reprocessability. The state of art approach to improve their mechanical properties is the addition of fillers, which, however, greatly restricts the chain mobility and impedes network topology rearrangement, thereby deteriorating the dynamic properties of vitrimer composites. Here, we demonstrate that the integration of sacrificial bonds into a vitrimeric network can remarkably enhance the overall mechanical properties while facilitating network rearrangement. Specifically, commercially available epoxidized natural rubber is covalently cross-linked with sebacic acid and simultaneously grafted with N-acetylglycine (NAg) through the chemical reaction between epoxy and carboxyl groups, generating exchangeable β-hydroxyl esters and introducing amide functionalities into the networks. The hydrogen bonds arising from amide functionalities act in a sacrificial and reversible manner, that is, preferentially break prior to the covalent framework and undergo reversible breaking and reforming to dissipate mechanical energy under external load, which leads to a rarely achieved combination of high strength, modulus, and toughness. The topology rearrangement of the cross-linked networks can be accomplished through transesterification reactions at high temperatures, which is accelerated with the increase of grafting NAg amount due to the dissociation of transient hydrogen bonds and increase of the ester concentration in the system.

Thermal Healing, Reshaping and Ecofriendly Recycling of Epoxy Resin Crosslinked with Schiff Base of Vanillin and Hexane-1,6-Diamine

A bio-derived dihydroxylimine hardener, Van2HMDA, for the curing of epoxy resin was prepared from vanillin (Van) and hexamethylene-1,6-diamine (HMDA) by Schiff base formation. The epoxy resin of diglycidyl ether of bisphenol A was cured with Van2HMDA in the presence of the catalyst, 2-ethyl-4-methylimidazole (EMI). The crosslinked epoxy resin showed thermal-healing properties at elevated temperatures. Moreover, the crosslinked epoxy resin can be reshaped by heating via imine metathesis of the hardener units. The imine metathesis of Van2HMDA was confirmed experimentally. Stress-relaxation properties of the epoxy resin crosslinked with Van2HMDA were investigated, and the activation energy obtained from Arrhenius plots of the relaxation times was 44 kJ/mol. The imine bonds in the epoxy polymer matrix did not undergo hydrolysis after immersing in water at room temperature for one week. However, in the presence of acid, the crosslinked polymer was easily decomposed due to the hydrolysis of imine bonds. The hydrolysis of imine bonds was used for the ecofriendly recycling of crosslinked polymer. It is inferred that thermal-healing, reshaping, and reprocessing properties can be implemented in the various crosslinked epoxy resins with the bio-derived dihydroxylimine hardener, albeit the recycled epoxy resin is of inevitably lower quality than the original material.

Mechanistic Study of Stress Relaxation in Urethane-Containing Polymer Networks

Cross-linked polymers are used in many commercial products and are traditionally incapable of recycling via melt reprocessing. Recently, tough and reprocessable cross-linked polymers have been realized by incorporating cross-links that undergo associative exchange reactions, such as transesterification, at elevated temperatures. Here we investigate how cross-linked polymers containing urethane linkages relax stress under similar conditions, which enables their reprocessing. Materials based on hydroxyl-terminated star-shaped poly(ethylene oxide) and poly((±)-lactide) were cross-linked with methylene diphenyldiisocyanate in the presence of stannous octoate catalyst. Polymers with lower plateau moduli exhibit faster rates of relaxation. Reactions of model urethanes suggest that exchange occurs through the tin-mediated exchange of the urethanes that does not require free hydroxyl groups. Furthermore, samples were incapable of elevated-temperature dissolution in a low-polarity solvent (1,2,4-trichlorobenzene) but readily dissolved in a high-polarity aprotic solvent (DMSO, 24 to 48 h). These findings indicate that urethane linkages, which are straightforward to incorporate, impart dynamic character to polymer networks of diverse chemical composition, likely through a urethane reversion mechanism.

Reprocessable Polyhydroxyurethane Network Composites: Effect of Filler Surface Functionality on Cross-link Density Recovery and Stress Relaxation

Conventional polymer network composites cannot be recycled for high-value applications because of the presence of permanent covalent cross-links. We have developed reprocessable polyhydroxyurethane network nanocomposites using silica nanoparticles with different surface functionalities as reinforcing fillers. The property recovery after reprocessing is a function of the interaction between the filler surface and the network matrix during the network rearrangement process. When nonreactive silica nanoparticles lacking significant levels of surface functional groups are used at 4 wt % (2 vol %) loading, the resulting network composite exhibits substantial enhancement in mechanical properties relative to the neat network and based on values of rubbery plateau modulus is able to fully recover its cross-link density after a reprocessing step. When nanoparticles have surface functional groups that can participate in dynamic chemistries with the reprocessable network matrix, reprocessing leads to losses in mechanical properties associated with cross-link density at potential use temperatures, along with faster rates and lower apparent activation energies of stress relaxation at elevated temperature. This work reveals the importance of appropriate filler selection when polymer network composites are designed with dynamic covalent bonds to achieve both mechanical reinforcement and excellent reprocessability, which are needed for the development of recyclable polymer network composites for advanced applications.

Preparation and property investigation of chain end functionalized cis-1,4 polybutadienes via de-polymerization and cross metathesis of cis-1,4 polybutadienes

The end-functional cis-1,4 polybutadiene displayed improved thermal stability and mechanistic properties.

Optically healable polyurethanes with tunable mechanical properties

The mechanical properties of polyurethanes can be nicely tuned by UV irradiation in a reversible way, endowing the polyurethanes with optical healing properties.

Reprocessable vinylogous urethane cross-linked polyethylene via reactive extrusion

Reactive extrusion of a precursor polymer and a diamine cross-linker leads to reprocessable vinylogous urethane polyethylene vitrimers with desired rheological properties.

Recycling of vitrimer blends with tunable thermomechanical properties

A composite-based strategy to prepare vitrimer blends with tunable thermomechanical properties utilizing the good weldability of vitrimers.

Versatile Approach to Building Dynamic Covalent Polymer Networks by Stimulating the Dormant Groups

Despite many efforts, there is no versatile way to realize reversible cross-linking for most polymers. Inspired by the abstraction of hydrogen and the iniferter polymerization of benzophenone (BP), we report a versatile approach for building dynamic covalent networks for polymers containing C-H bonds. Under ultraviolet irradiation, BP can effectively abstract the hydrogen from polymers to form dormant diarylsemipinacol (DASP) groups on the polymer chains. Then, the dormant DASP-based linkages can be homolytically cleaved upon heating, after which they generate carbon-centered aliphatic radicals and DASP-based radicals. Therefore, the cross-linked polymer network can rearrange its topology through the dissociation and association of DASP-based linkages, which endow polymer networks with remodeling and self-healing abilities. Given that most commercially available polymers contain aliphatic C-H bonds, this provides a general method for forming thermal reversible cross-linked networks.

Colorless, Transparent, Robust, and Fast Scratch-Self-Healing Elastomers via a Phase-Locked Dynamic Bonds Design

Robust self-healing thermoplastic elastomers are expected to have repeated healing capability, remarkable mechanical properties, transparency, and superior toughness. The phase-locked design in this work provides excellent tensile mechanical properties and efficient healability at a moderate temperature due to the dynamic disulfide bonds embedded in the hard segments and mainly being locked in the viscoelastic hard microphase region. The self-healing elastomers exhibit a maximum tensile stress of 25 MPa and a fracture strain of over 1600%, which are quite prominent compared to previous reports. The nanoscale domains of the elastomer are smaller than the wavelength of visible light by microphase separation control resulting in colorless, nearly 100% transparency, and are as good as quartz glasses. The high dynamics of the phase-locked disulfide bonds renders a high healing efficiency of scratches on the surface within 60 s at 70 °C. The rapid scratch healing and complete transparency recovery of the elastomers provide new avenues in the highly transparent surface or protective films which finds potential applications for precision optical lenses, flexible display screens, and automobile or aircraft lighting finishes.

Dynamic Remodeling of Covalent Networks via Ring-Opening Metathesis Polymerization

Reversible transformations in bulk polymers offer numerous possibilities for materials remodeling and reprocessing. While reversible systems based on dynamic covalent chemistry such as the Diels-Alder reaction and transesterification have been intensively studied to enable local bond dissociation and formation, reports regarding the reversion from bulk network polymers to monomers are rare. Herein, we report a reversibly polymerizable system based on ring-opening metathesis polymerization of cyclopentene derivatives in the bulk state. The network polymer is thermodynamically stable and mechanically robust at room temperature and readily depolymerizes at elevated temperatures to yield liquid monomers that are repolymerized to cross-linked polymers by simply cooling to room temperature. This reversible process was characterized by differential scanning calorimetry and rheological tests.

Dynamics of Vitrimers: Defects as a Highway to Stress Relaxation

We propose a coarse-grained model to investigate stress relaxation in star-polymer networks induced by dynamic bond-exchange processes. We show how the swapping mechanism, once activated, allows the network to reconfigure, exploring distinct topological configurations, all of them characterized by complete extent of reaction. Our results reveal the important role played by topological defects in mediating the exchange reaction and speeding up stress relaxation. The model provides a representation of the dynamics in vitrimers, a new class of polymers characterized by bond-swap mechanisms which preserve the total number of bonds, as well as in other bond-exchange materials.

Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect

Three dimensional chemically cross-linked polymer networks present a great challenge for recycling and reutilization of waste tire rubber. In this work, the covalently cross-linked networks of ground tire rubber (GTR) were degraded heterogeneously under 150 °C due to the synergistic effects of the soybean oil and controlled oxidation. The degradation mechanism was discussed using Horikx theory and Fourier transformation infrared spectroscopy (FTIR). The results showed that the structural evolution of sol and gel parts, which indicated that the sols consisted of degraded GTR chains with low molecular weight, while the gels were mainly composed of bound rubber coated carbon black, which are separated from the cross-linked network of GTR in a high degradation degree. The degraded GTR compound demonstrated an excellent reinforcing effect on solution styrene-butadiene rubber (SSBR), due to the presence of core-shell structured carbon black. This work provide an efficient and economic approach to degrade GTR and transform it into useful products.

In Situ Network Formation in PBT Vitrimers via Processing-Induced Deprotection Chemistry

Although the network dynamics and mechanical properties of poly(butylene terephthalate) vitrimers can to some extent be controlled via chemical and physical approaches, it remains a challenge to be able to process PBT vitrimers with the same processing conditions via, for example, injection molding as neat PBT. Here, it is shown that the use of protected pentaerythritol as a latent cross-linker and the use of a Zn(II) transesterification catalyst allows for the in situ dynamic network formation in PBT during processing, with a delayed onset of gelation. This process can be controlled by adjusting the processing temperature, (protected) cross-linker content, and the type of protection group. This solvent-free deprotection strategy opens the way to high production rates of PBT vitrimer products via injection molding with the combination of low viscosity during processing and vitrimer characteristics in the final product.

Covalently Cross-Linked Elastomers with Self-Healing and Malleable Abilities Enabled by Boronic Ester Bonds

Covalently cross-linked rubbers are renowned for their high elasticity that play an indispensable role in various applications including tires, seals, and medical implants. Development of self-healing and malleable rubbers is highly desirable as it allows for damage repair and reprocessability to extend the lifetime and alleviate environmental pollution. Herein, we propose a facile approach to prepare permanently cross-linked yet self-healing and recyclable diene-rubber by programming dynamic boronic ester linkages into the network. The network is synthesized through one-pot thermally initiated thiol-ene "click" reaction between a novel dithiol-containing boronic ester cross-linker and commonly used styrene-butadiene rubber without modifying the macromolecular structure. The resulted samples are covalently cross-linked and possess relatively high mechanical strength which can be readily tailored by varying boronic ester content. Owing to the transesterification of boronic ester bonds, the samples can alter network topologies, endowing the materials with self-healing ability and malleability.

Brønsted-Acid-Catalyzed Exchange in Polyester Dynamic Covalent Networks

The effect of catalyst strength on polyester-alcohol dynamic covalent exchange was systematically studied using Brønsted acids and a low-T_g poly(4-methylcaprolactone) vitrimer formulation. Relaxation times, activation energies, and Arrhenius prefactors are correlated with pK_a. Strong protic acids induce facile network relaxation at 25 °C on the order of 10⁴-10⁵ s, significantly faster than Lewis acid alternatives that function only above 100 °C. Activation energies span 49-67 kJ/mol and increase as pK_a decreases. The opposite trend is observed with the Arrhenius prefactor. We anticipate that the quantitative understanding of Brønsted acid effects disclosed herein will be of utility in future studies that exploit acid-catalyzed dynamic covalent bond exchange.

Dynamic Ureas with Fast and pH-Independent Hydrolytic Kinetics

Low cost, high performance hydrolysable polymers are of great importance in biomedical applications and materials industries. While many applications require materials to have a degradation profile insensitive to external pH to achieve consistent release profiles under varying conditions, hydrolysable chemistry techniques developed so far have pH-dependent hydrolytic kinetics. This work reports the design and synthesis of a new type of hydrolysable polymer that has identical hydrolysis kinetics from pH 3 to 11. The unprecedented pH independent hydrolytic kinetics of the aryl ureas were shown to be related to the dynamic bond dissociation controlled hydrolysis mechanism; the resulting hindered poly(aryl urea) can be degraded with a hydrolysis half-life of 10 min in solution. More importantly, these fast degradable hindered aromatic polyureas can be easily prepared by addition polymerization from commercially available monomers and are resistant to hydrolysis in solid form for months under ambient storage conditions. The combined features of good stability in solid state and fast hydrolysis at various pH values is unprecedented in polyurea material, and will have implications for materials design and applications, such as sacrificial coatings and biomaterials.