Covalent Adaptable Networks Research Articles

Direct restoration of photocurable cross-linkers for repeated light-based 3D printing of covalent adaptable networks.

Light-based processing of thermosets has gained increasing attention because of its broad application field including its use in digital light processing (DLP) 3D printing. This technique offers efficient design and fabrication of complex structures but typically results in non-recyclable thermoset-based products. To address this issue, we describe here a photocurable, dynamic β-amino ester (BAE) based cross-linker that is not only suitable for DLP printing but can also be chemically degraded via transesterification upon the addition of 2-hydroxyethyl methacrylate (HEMA) as a decross-linker. This conceptually new protocol allows for efficient depolymerization with the direct restoration of curable monomers in a single step without the addition of external catalysts or solvents. By implementing this protocol, we have established a chemical recycling loop for multiple cycles of photo-cross-linking and restoration of cross-linkers, facilitating repeatable DLP 3D printing without generating any waste. The recycled materials exhibit full recovery of thermal properties and Young's modulus while maintaining 75% of their tensile strength for at least three cycles. Simultaneously, the presence of BAE moieties enables the (re)processability of these materials through compression molding.

“Polymer to polymer” recycling and body-temperature triggered shape memory of a carvone-based epoxy thermoset system achieved using a thiol-Michael covalent adaptable network

Closed-loop chemical recycling and body temperature-triggered shape memory of an epoxy thermoset system were achieved by using reversible thiol-Michael chemistry.

Design of remoldable, shape-memory, welded biomass composites based on the acetal bond of the cellulose chain

The growing environmental concern over petrochemical-based plastics continuously promotes the exploration of green and sustainable biomass-based covalent adaptable networks composites (CANs). Compared with petrochemical products, cellulose and its derivatives have overwhelming superiority in terms of availability, cost and biodegradability; nevertheless, CANs using the abundant acetal bonds in the cellulose chain rather than elaborately designing other types of dynamic covalent bonds still have not received sufficient attention. Herein, we report acetal-based cellulose covalent adaptable networks (ACC) synthesized with poly(ethylene glycol) diglycidyl ether as the crosslinker for hydroxypropyl cellulose and tetrabutylammonium bromide (TBAB) as an accelerant. For the first time, exchangeability of the cellulose chain acetal reversible bonds at elevated temperature have been demonstrated to endow ACC reprocessability, shape memory and welding. The TBAB has been used as an accelerant of dynamic exchange for acetal bonds first and its accelerating mechanism and advantages have also been investigated. In addition, the thermal, mechanical and hydrophilic properties of ACC can be adjusted by simply adjusting the raw material ratio and the ACC has excellent chemical and biodegradability. Hence, ACC will provide new ideas for solving white pollution and building new CANs.

Exploiting β-amino ester chemistry to obtain methacrylate-based covalent adaptable networks

In this study, β-amino ester-based covalent adaptable networks (CANs) were synthesised through aza-Michael addition of amines to methacrylates, with the aim of developing industrial products. Typically, CANs based on β-amino esters employ acrylate moieties, which are known to be toxic and hazardous to human health and the environment. Therefore, methacrylate-based networks were explored as a safer alternative to the chemistry currently in use. Network formation was monitored by following the evolution of the glass transition temperature over time and conducting soluble fraction tests. Subsequently, industrial methacrylic monomers were employed to develop an industrially relevant material, utilizing previously established curing conditions for methacrylate systems. The dynamic behaviour of the network was assessed through rheological characterisation. The material demonstrated the ability to relax applied stress at various temperatures displaying typical viscoelastic fluid behaviour, indicative of exchangeable linkages within the macromolecular architecture. Shape memory and reprocessing capability were successfully tested under appropriate thermo-mechanical stimulation. Both thermal analyses and FT-IR spectra confirmed the retention of chemical composition and material properties throughout recycling process.

A designer Schiff based motif offered dual dynamic exchangeable bonds, faster curing and closed‐loop circularity in epoxy vitrimers

AbstractA fast‐curing epoxy with improved mechanical properties, self‐healing and re‐processability was designed using a tailored Schiff based motif that offered dual covalent adaptable network (CAN) in epoxy. The designer motif is a dynamic hardener with both acetal linkage and a Schiff base motif in the same molecule which allowed us to gain a mechanistic insight into the evolving transient network in epoxy vitrimers. The dynamic hardener besides offering a cross‐linked network in epoxy can facilitate rapid exchange reactions, resulting in a material with high tensile strength (74 MPa), high glass transition temperature (121°C), and that can be re‐purposed through solvent induced degradation. The proposed dynamic curing agent is suitable to be employed as a coating material on substrates to ensure excellent self‐healing and re‐processability ensuring chemical recyclability. Few carbon fiber‐based laminates were prepared using the epoxy vitrimer and the recovery of CF post laminate preparation clearly demonstrates the closed‐loop circularity in epoxy vitrimers.Highlights A unique dynamic curing agent is designed consisting of an aldehyde (salicylaldehyde) and an amine and acetal containing species (amino acetaldehyde dimethyl acetal). It facilitates imine and acetal exchange that can bond at temperatures <60°C and break at temperatures >105°C as evidenced by the vitrimer transition temperature. Epoxies show high tensile strength (74 MPa), exceptional Tg (121°C), fast stress relaxation and an activation energy of 96.2 KJ/mol. The imine‐acetal exchange system rendered the epoxies re‐processable, malleable and re‐usable.

Synthesis and Characterization of Vitrimer-like Self-Healing Polymer Electrolytes for Lithium Metal Batteries

Solid-polymer electrolytes have become crucially significant with the fast-expanding energy requirements in both the academic and industrial spheres due to their better safety and energy density compared to liquid electrolytes. Despite these encouraging possibilities, a few obstacles are associated with these electrolytes, chief among them being the limited charge transport via the solid electrode-electrolyte interface. Due to the difference in electrochemical potential between the electrode and the electrolyte, this obstruction initiates electrode-electrolyte interactions through electrode volumetric changes, interface reactions, and space charge layers.One possible way to enhance the LIB performance could be by integrating a new class of covalent adaptable networks called vitrimers.The self-healing, shape memory, recyclability, and reprocessibility properties of vitrimers, a class of covalent adaptable networks (CANs), are derived from a covalent molecular network that can change its topology through molecular rearrangements while maintaining the overall number of network bonds. In this manner, Li dendrites that might develop during battery cycling could be eliminated and the electrolyte-electrode contact could be restored by vitrimers. Figure 1

Solvent-Triggered Chemical Recycling of Ion-Conductive and Self-Healable Polyurethane Covalent Adaptive Networks.

Given the substantial environmental challenge posed by global plastic waste, recycling technology for thermosetting polymers has become a huge research topic in the polymer industry. Covalent adaptive networks (CANs), which can reversibly dissociate and reconstruct their network structure, represent a key technology for the self-healing, reprocessing, and recycling of thermosetting polymers. In the present study, we introduce a new series of polyurethane CANs whose network structure can dissociate via the self-catalyzed formation of dithiolane from the CANs' polydisulfide linkages when the CANs are treated in N,N-dimethylformamide (DMF) or dimethyl sulfoxide at 60 °C for 1 h. More interestingly, we found that this network dissociation even occurs in tetrahydrofuran-DMF solvent mixtures with low DMF concentrations. This feature enables a reduction in the use of high-boiling, toxic polar aprotic solvents. The dissociated network structure of the CANs was reconstructed under UV light at 365 nm with a high yield via ring-opening polydisulfide linkage formation from dithiolane pendant groups. These CAN films, which were prepared by a sequential organic synthesis and polymerization process, exhibited high thermal stability and good mechanical properties, recyclability, and self-healing performance. When lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt was added to the CAN films, the films exhibited a maximum ion conductivity of 7.48 × 10-4 S cm-1 because of the contribution of the high concentration of the pendant ethylene carbonate group in the CANs. The ion-conducting CAN films also showed excellent recyclability and a self-healing performance.

Polyurethane Adhesives with Chemically Debondable Properties via Diels-Alder Bonds.

Covalent adaptable networks (CANs) represent a pioneering advance in polymer science, offering unprecedented versatility in materials design. Unlike conventional adhesives with irreversible bonds, CAN-based polyurethane adhesives have the unique ability to undergo chemical restructuring through reversible bonds. One of the strategies for incorporating these types of reactions in polyurethanes is by functionalisation with Diels-Alder (DA) adducts. By taking advantage of the reversible nature of the DA chemistry, the adhesive undergoes controlled crosslinking and decrosslinking processes, allowing for precise modulation of bond strength. This adaptability is critical in applications requiring reworkability or recyclability, as it allows for easy disassembly and reassembly of bonded components without compromising the integrity of the material. This study focuses on the sustainable synthesis and characterisation of a solvent-based polyurethane adhesive, obtained by functionalising a polyurethane prepolymer with DA diene and dienophiles. The characterisation of the adhesives was carried out using different experimental techniques: nuclear magnetic resonance spectroscopy (NMR), Brookfield viscosity, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and T-peel strength testing of leather/adhesive/rubber joints to determine the adhesive properties, both before and after the application of external stimuli. The conversion of both the DA and retro-Diels-Alder (r-DA) reactions was confirmed by 1H-NMR. The adhesive properties were not altered by the functionalisation of the adhesive prepolymer, showing similar thermal resistance and good rheological and adhesive properties, even exceeding the most demanding technical requirements for upper-to-sole joints in footwear. After the application of an external thermal stimuli, the bonded materials separated without difficulty and without damage, thus facilitating their separation, recovery and recycling.

Probing the Solubility of Imine-Based Covalent Adaptable Networks.

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that-under certain conditions-both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.

Curing and thermal degradation behavior of epoxy‐based vitrimers

AbstractEpoxy with associative covalent adaptable networks (CANs), called vitrimer, is reported to be promising to realize the recyclable utilization. The typical epoxy vitrimers are synthesized from diglycidyl ether of bisphenol A and dicarboxylic catalyzed by triazabicyclodecene (TBD). The study on the curing and thermal degradation behavior facilitates in‐depth understanding of the associative CANs. In this work, the effect of TBD on the curing and thermal degradation behavior is investigated via curing kinetics and thermal degradation performance analysis. The results reveal that the activation energy of curing reaction is increased as the amount of TBD is increased. EST‐5 has an average activation energy of 70.1 kJ/mol, while EST‐15 reaches as high as 75.1 kJ/mol. Especially, when the conversion is up to 50%, the activation energy shows an obvious increment. In addition, it has been demonstrated that TBD accelerates the thermal degradation of cured networks at lower temperature and increases the amount of volatiles during thermal decomposition.

Covalent Adaptable Network with High Topology Freezing Temperatures and Tunable Mechanical Properties: Polyureas Containing Dynamic Aniline–Urea Bonds

Covalent Adaptable Network with High Topology Freezing Temperatures and Tunable Mechanical Properties: Polyureas Containing Dynamic Aniline–Urea Bonds

Active Materials for Functional Origami.

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

Covalent adaptable network offers “sustainable” closed‐loop circularity in epoxy vitrimers

AbstractTraditional Epoxies take hours to cure before their properties can be realized and this restricts its use in applications where quick turn‐around time is imperative. Herein, we developed fast curing Epoxies (which cure within 2 h) using Vitrimer chemistries. A unique dynamic curing agent (bio‐based Schiff motif) consisting of an aldehyde (here, cinnamaldehyde) and an amine (here, mussel inspired dopamine) is used to facilitate imine exchange during the curing process and maintains the network integrity‐ a key requirement to achieve excellent structural properties in epoxy based components. By incorporating this covalent adaptable network, various epoxies were designed which can cure within 2 h. The resulting epoxy vitrimers exhibited high tensile strength (68 MPa), exceptionally high Tg (128°C), fast stress relaxation (a relaxation time of 15.2 s at 180°C) and an activation energy of 105 kJ/mol. This associative covalent adaptable network (imine exchange) also rendered the epoxies re‐processable and re‐usable and offers closed‐loop circularity in plastics. The resulting epoxy also exhibited self‐healing property triggered by the exchangeable bonds. The self‐healing behavior of this epoxy was manifested from the surface conductivity measurements wherein the conducting pathway (facilitated by spraying conducting particles on the vitrimer epoxy substrate) was restored post self‐healing in substrates containing epoxy vitrimers. In addition, the epoxies cured using this Schiff based motif dissolves in a specific solvent and can be recovered and re‐purposed. The excellent mechanical properties achieved by retaining the network integrity together with the self‐healing properties offers myriad applications for the designed epoxy cured using bio‐based Schiff motif.Highlights Developed fast curing Epoxies that cures in 2 h using Vitrimer chemistries. Fully bio‐based hardener was used to realize the network topology. The dynamic exchange reaction results in re‐processability, reusable, recyclable and self‐healable that is not possible in the case of traditional epoxy. It exhibited high tensile strength, exceptional Tg, fast stress relaxation and an activation energy of 105 KJ/mol.

Covalent adaptable networks and thermosets of multi-block ethylene/1-octene copolymers made by free-radical processing: Effects of melt flow index and crystallinity on thermomechanical properties and reprocessability

Chain shuttling polymerization has facilitated the rapid development of olefin block copolymers (OBCs) such as ethylene/1-octene multi-block copolymers. The properties of OBCs stem from various parameters such as average block length, number of blocks per chain, etc., which lead to complexity associated with establishing structure–property relationships between precursor OBCs and properties of cross-linked OBCs. Additionally, permanent cross-linking of OBCs sacrifices their recyclability, and cross-linked OBCs capable of being reprocessed have seldom been reported. Here, permanently cross-linked OBCs (OBCXs) and dynamically cross-linked OBCs, or OBC covalent adaptable networks (CANs), were synthesized by melt-state reactive processing of neat OBCs with various crystallinities and melt flow indices (MFIs). The OBC CANs are capable of dialkylamino disulfide dynamic chemistry via the grafting of bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, or BiTEMPS methacrylate, cross-linkers onto OBC backbone chains. Thermomechanical characterizations indicate that increasing crystallinity and decreasing MFI in precursor OBCs lead to higher cross-link densities in OBCXs and OBC CANs. Dynamically cross-linking OBCs into CANs also significantly improves their elevated-temperature creep resistance. Stress relaxation results of OBC CANs indicate an increased average relaxation time with greater cross-link density. Distinct from other CANs capable of dialkylamino disulfide chemistry, the stress relaxation behaviors of OBC CANs of higher cross-link density show evident dependence on their network viscoelasticities. In OBC CANs of lower cross-link density, the dynamic chemistry more strongly governs their stress relaxation behaviors. Finally, the OBC CANs exhibit full cross-link density and thermomechanical property recovery after reprocessing, whereas the OBCXs cannot be reprocessed due to their irreversible cross-links.

Fully biomass-derived polyurethane based on dynamic imine with self-healing, rapid degradability, and editable shape memory capabilities

In this study, a novel bio-based diol containing imine dynamic bonds (Vanp2) were synthesized using vanillin and bio-based 1,5-pentanediamine. Vanp2 was then introduced into the cross-linking network of betulin-based polyurethanes to obtain betulin-based polyurethanes containing covalent adaptive networks (CANs). Imine dynamic bonds within CAN endowed these betulin-based polyurethanes with self-healing, re-processability, degradability, and editable shape memory functionalities. Meanwhile, the mechanical and thermal properties of these fully bio-based polyurethane materials were characterized. The maximum tensile strength reached 9.5 MPa, while the maximum strain at break was 248 % and the maximum toughness was 13.2 MJ/m3. Thermal decomposition temperature was greater than 300 °C. Since the imine structure could be dissociated under acidic conditions, these polyurethanes could be rapidly degraded in a mixed acid solution at 50 °C in 4 h. This study demonstrated a strategy for synthesizing betulin-based polyurethane elastomers containing CAN using only bio-based feedstock.

Review on the Synthesis, Recyclability, Degradability, Self-Healability and Potential Applications of Reversible Imine Bond Containing Biobased Epoxy Thermosets

Epoxy thermosets need to be designed for simple recycling and biomass resource utilization in order to be fully sustainable building materials. The development of covalent adaptive networks (CANs) using adaptive covalent chemistry (ACC) may be helpful in this regard. Several reversible covalent bonds are incorporated into the epoxy polymer to overcome the challenge of reprocessability or recyclability, degradability and self-healability. The imine bond, also referred to as the Schiff base, is one of the reversible covalent bonds that can participate in both associative and dissociative reactions. This opens up possibilities for mechanical and chemical recycling as well as self-healing. This review summarises the progress related to the synthesis and mechanical and thermal properties of epoxy thermosets based on reversible imine bonds derived from different sustainable resources over the past few decades. The feedstocks, physical and thermal properties, recycling conditions, degradability and self-healability of the biomass epoxy thermosets are addressed along with the main obstacles, prospective improvements and potential applications.

Elastomeric vitrimers from designer polyhydroxyalkanoates with recyclability and biodegradability.

Cross-linked elastomers are stretchable materials that typically are not recyclable or biodegradable. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are soft and ductile, making these bio-based polymers good candidates for biodegradable elastomers. Elasticity is commonly imparted by a cross-linked network structure, and covalent adaptable networks have emerged as a solution to prepare recyclable thermosets via triggered rearrangement of dynamic covalent bonds. Here, we develop biodegradable and recyclable elastomers by chemically installing the covalent adaptable network within biologically produced mcl-PHAs. Specifically, an engineered strain of Pseudomonas putida was used to produce mcl-PHAs containing pendent terminal alkenes as chemical handles for postfunctionalization. Thiol-ene chemistry was used to incorporate boronic ester (BE) cross-links, resulting in PHA-based vitrimers. mcl-PHAs cross-linked with BE at low density (<6 mole %) affords a soft, elastomeric material that demonstrates thermal reprocessability, biodegradability, and denetworking at end of life. The mechanical properties show potential for applications including adhesives and soft, biodegradable robotics and electronics.

Unraveling the rheology of inverse vulcanized polymers

Multiple relaxation times are used to capture the numerous stress relaxation modes found in bulk polymer melts. Herein, inverse vulcanization is used to synthesize high sulfur content (≥50 wt%) polymers that only need a single relaxation time to describe their stress relaxation. The S-S bonds in these organopolysulfides undergo dissociative bond exchange when exposed to elevated temperatures, making the bond exchange dominate the stress relaxation. Through the introduction of a dimeric norbornadiene crosslinker that improves thermomechanical properties, we show that it is possible for the Maxwell model of viscoelasticity to describe both dissociative covalent adaptable networks and living polymers, which is one of the few experimental realizations of a Maxwellian material. Rheological master curves utilizing time-temperature superposition were constructed using relaxation times as nonarbitrary horizontal shift factors. Despite advances in inverse vulcanization, this is the first complete characterization of the rheological properties of this class of unique polymeric material.

Synthesis of dynamic polymers by amino-yne click reaction using multifunctional amine

Covalent adaptable networks (CANs) have become sustainable alternatives to traditional thermosetting materials due to their powerful reprocessing and rearrangement capabilities. Therefore, developing and investigating reversible reactions that can be used in the construction of CANs is a popular research topic. Amino-yne click chemistry is known for its green characteristics—it is a catalyst-free and efficient reaction—and is widely applied in the synthesis of bio-organic molecules, but there have been few reports on CANs, especially multifunctional amine-based amino-yne click reactions. In this study, various CANs were synthesized via the amino-yne click reaction using multifunctional amines (diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA)), and they were compared with CANs that were prepared from a primary amine (tris(2-aminoethyl) amine (TREN) and a secondary amine (tris[2-(methylamino)ethyl]amine (TMEN)) under the same conditions. The results showed that H-DETA CANs exhibited the smallest glass transition temperature Tg (15 °C) and Young's modulus (105 MPa) because the crosslinker (DETA) contained a low content of primary amine and secondary amine, but its crosslinking density followed the trend of H-TMEN < H-DETA < H-TREN. In addition, the relaxation time, activation energy, and creep rate results for the CANs followed the same trend as the crosslinking density. With the increase in the secondary amine content of the multifunctional amines, the Tg of the CANs increased from 15 °C to 57 °C, the Young's modulus increased from 105 to 666 MPa, and the crosslinking density increased from 955 to 2690 mol/m3. Notably, the increase in the secondary amine content led to the weaker dynamics of the material, namely, prolonged relaxation time, increased activation energy, and decreased creep rate. This work also verified the dynamic exchange mechanism of the materials by the molecular model reaction and the amine solvent dissolution tests. Finally, hot-pressing experiments demonstrated the good plasticity and recovery ability of prepared CANs. This work provides an elegant path for preparing materials in energy and chemical fields such as coatings and rubber.

Covalent Adaptable Networks with Rapid UV Response Based on Reversible Thiol–Ene Reactions in Silicone Elastomers

Covalent Adaptable Networks with Rapid UV Response Based on Reversible Thiol–Ene Reactions in Silicone Elastomers