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To fulfill the current circular economy concept, the academic and industrial communities are devoting significant efforts to plastic materials' end-of-life. Unlike thermoplastics, which are easy to recover and re-valorize, recycling thermosets is still difficult and challenging. Conversely, because of their network structure, thermosetting polymer systems exhibit peculiar features that make these materials preferable for several applications where high mechanical properties, chemical inertness, and thermal stability, among others, are demanded. In this view, vitrimers have quite recently attracted the attention of the scientific community, as they can form dynamic covalent adaptive networks that provide the properties typical of thermosets while keeping the possibility of being processed (and, therefore, mechanically recycled) beyond a certain temperature. This review aims to provide an overview of vitrimers, elucidating their most recent advances and applications and posing some perspectives for the forthcoming years. [ABSTRACT FROM AUTHOR]

Carbon-fiber-reinforced polymers (CFRPs) with epoxy matrices are widely applied in high-performance structural applications and represent one of the biggest classes of materials with urgent need for end-of-life management. Available waste management methodologies for conventional thermoset composites with a focus on CFRPs are briefly reviewed and their limitations are highlighted. In the quest to obtain materials with mechanical performance, thermal stability, and sustainability, the research community has turned its interest to develop polymer composites with adaptable and dynamic networks in their matrix, and lately also at an interface/interphase level. The current review focuses on the life extension/waste management options that are opened through the introduction of covalent adaptable networks in the epoxy matrix of CFRPs. The processing conditions that are applied for the healing/repairing, welding/reshaping, and/or recycling of CFRPs are presented in detail, and compared based on the most common dynamic exchange reactions. [ABSTRACT FROM AUTHOR]

Vitrimers 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. [ABSTRACT FROM AUTHOR]

Vitrimers are an innovative class of polymers that boast a remarkable fusion of mechanical and dynamic features, complemented by the added benefit of end-of-life recyclability. This extraordinary blend of properties makes them highly attractive for a variety of applications, such as the automotive sector, soft robotics, and the aerospace industry. At their core, vitrimer materials consist of crosslinked covalent networks that have the ability to dynamically reorganize in response to external factors, including temperature changes, pressure variations, or shifts in pH levels. In this review, the aim is to delve into the latest advancements in the theoretical understanding and computational design of vitrimers. The review begins by offering an overview of the fundamental principles that underlie the behavior of these materials, encompassing their structures, dynamic behavior, and reaction mechanisms. Subsequently, recent progress in the computational design of vitrimers is explored, with a focus on the employment of molecular dynamics (MD)/Monte Carlo (MC) simulations and density functional theory (DFT) calculations. Last, the existing challenges and prospective directions for this field are critically analyzed, emphasizing the necessity for additional theoretical and computational advancements, coupled with experimental validation.

Recently, the growth of the recyclability of carbon fiber reinforced polymer (CFRP) composites has been driven by environmental and circular economic aspects. The main aim of this research work is to investigate the strength retention of a bio-based vitrimer composite reinforced with carbon fibers, which offers both recyclability and material reusability. The composite formulation consisted of an epoxy resin composed of diglycidyl ether of bioshpenol A (DGEBA) combined with tricarboxylic acid (citric acid, CA) and cardanol, which was then reinforced with carbon fibers to enhance its performance. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were performed to analyze the chemical composition and curing behavior of the vitrimer. Mechanical testing under tensile loading at room temperature was carried out on epoxy, vitrimer, and associated carbon fiber reinforced composite materials. The results demonstrated that the DGEBA/CA/cardanol vitrimer exhibited thermomechanical properties comparable to those of an epoxy cured with petroleum-based curing agents. It was observed that the maximum tensile strength of vitrimer is about 50 MPa, which is very close to the range of epoxy resins cured with petroleum-based curing agents. Notably, the ability of the vitrimer composite to be effectively dissolved in a dimethylformamide (DMF) solvent is a significant advantage, as it enables the recovery of the fibers. The recovered carbon fiber retained comparable tensile strength to that of the fresh carbon composites. More than 95% strength was retained after the first recovery, which confirms the use of fibers for primary and secondary applications. These research results open up new avenues for efficient recycling and contribute to the overall sustainability of the composite material at an economic level.

The flexible electronics market is rapidly growing, driven by demand in the healthcare, automotive, and consumer industries. While these technologies enhance the daily lives, the electronic waste (e‐waste) generated in tandem poses significant environmental challenges. Covalent adaptable networks (CANs) are crosslinked polymers bearing dynamic covalent bonds (DCBs). These exchangeable bonds endow CANs not only with the high mechanical and chemical stability of thermosets but also with the stimuli‐activated reprocessability of thermoplastics. CAN‐derived electronics offer a solution to alleviate e‐waste generation since the DCBs allow for the self‐healing and recycling of these devices. Additionally, the ability of CANs to respond to stimuli is highly desirable for smart materials, especially flexible electronics. In this review, how CANs play the role of conformable substrates and dielectrics, and also as flexible electronic connectors, sensors, and actuators is comprehensively cataloged. Furthermore, the advantages gained by utilizing CANs in electronic sensors and actuators is highlighted, as well as provide the insights into design strategies to address the challenges of creating high‐performance sustainable soft electronics. [ABSTRACT FROM AUTHOR]

The welding behavior of prototype vitrimer composites with respect to adjustable parameters and protocols is investigated, and a method for resistance welding of vitrimer composites directly adapted from the welding of thermoplastic composites is described. Adherend laminates are positioned on either side of a matrix-saturated carbon fiber heating element, through which current is driven, and resistance heating welds the adherends and heating element together, forming a single lap joint. Weld strengths matched or exceeded the strength of composite parts produced using the manufacturer-recommended consolidation method (12.0 ± 2.6 MPa vs. 8.4 ± 0.6 MPa). Furthermore, repeating the welding process yielded greater shear strength, withstanding up to five weld–break–reweld cycles with an average increase of 4.6 ± 1.5 MPa or 65% compared to the first weld. The findings from resistance weld experiments highlight the suitability of vitrimer matrix composites for repair. Finally, a process for reversing a welded joint was shown, demonstrating the potential for vitrimers for temporary joining and rejoining. [ABSTRACT FROM AUTHOR]

Recently, the growth of the recyclability of carbon fiber reinforced polymer (CFRP) composites has been driven by environmental and circular economic aspects. The main aim of this research work is to investigate the strength retention of a bio-based vitrimer composite reinforced with carbon fibers, which offers both recyclability and material reusability. The composite formulation consisted of an epoxy resin composed of diglycidyl ether of bioshpenol A (DGEBA) combined with tricarboxylic acid (citric acid, CA) and cardanol, which was then reinforced with carbon fibers to enhance its performance. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were performed to analyze the chemical composition and curing behavior of the vitrimer. Mechanical testing under tensile loading at room temperature was carried out on epoxy, vitrimer, and associated carbon fiber reinforced composite materials. The results demonstrated that the DGEBA/CA/cardanol vitrimer exhibited thermomechanical properties comparable to those of an epoxy cured with petroleum-based curing agents. It was observed that the maximum tensile strength of vitrimer is about 50 MPa, which is very close to the range of epoxy resins cured with petroleum-based curing agents. Notably, the ability of the vitrimer composite to be effectively dissolved in a dimethylformamide (DMF) solvent is a significant advantage, as it enables the recovery of the fibers. The recovered carbon fiber retained comparable tensile strength to that of the fresh carbon composites. More than 95% strength was retained after the first recovery, which confirms the use of fibers for primary and secondary applications. These research results open up new avenues for efficient recycling and contribute to the overall sustainability of the composite material at an economic level. [ABSTRACT FROM AUTHOR]

Epoxy resins, ubiquitous and indispensable thermosetting material in various industries, suffer from the resistance to crack initiation, poor ductility, and irreparable permanent cross‐linked network, which hinder their utilization in high‐performance applications and are detrimental to the development of a sustainable economy. However, achieving the trade‐off between toughness, robustness, and reprocessability of epoxy resins remains a formidable challenge. Herein, an epoxy resin that combines ultratoughness, robustness, and reprocessability by incorporating a hierarchical crosslink structure embedded in a transesterification network is reported. The construction of hierarchical coordination structures facilitates formation of dense nano‐hard domains, enabling enhancement of both strength and toughness at multilength scales. As a result, the epoxy resin, integrating covalent adaptable networks with sacrificial non‐covalent networks, exhibits exceptional elongation at break (280%), modulus (1.8 GPa), stress at break (56.3 MPa), and tensile toughness (142 MJ m−3), showcasing its excellent endurance and ability to undergo multiple reprocessability. The application of this epoxy resin in carbon fiber‐reinforced polymer composites further underscores its potential as the resulting HCEV‐CFRPs exhibit unprecedented tensile properties and facilitate multiple non‐destructive recycling of high‐value carbon fiber under mild conditions. This research provides novel design principles for recyclable and high‐performance materials that combine strength and toughness. [ABSTRACT FROM AUTHOR]

The incorporation of self‐healing properties to repair scratches (or other minor damage) has revolutionized the coating industry by increasing service life, sustainability, and optical appearance. This work addresses challenges with the robustness of self‐healing coatings through the inclusion of surface‐tethered covalently adaptable networks (CANs). Surface‐initiated polymerization is combined with spray‐coating to deposit polymers to produce coatings with reversible crosslinks to the tethered chains. These robust coatings are based on reversible vinylogous urethane bonds using 2‐(acetoacetoxy)ethyl methacrylate‐based polymers and tris(2‐aminoethyl) amine (TREN). Here, TREN enables reversible covalent bonding between the spray‐coated and surface‐tethered polymers. Without this polymer brush layer, the physisorbed CAN coatings fail to self‐heal completely, are labile to solvent, and exhibit shear delamination upon scratching. The utility of this tethered coating approach is highlighted through its ability to autonomously self‐heal incisions within seconds at elevated temperatures, or more steadily under ambient conditions. The key to these advancements is the use of polymer brushes as a primer layer to attach the CAN to enhance healing and improve the environmental robustness of the coating. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Dynamic polymer networks combine the noteworthy (thermo)mechanical features of thermosets with the processability of thermoplastics. They rely on externally triggered bond exchange reactions, which induce topological rearrangements and, at a sufficiently high rate, a macroscopic reflow of the polymer network. Due to this controlled change in viscosity, dynamic polymers are repairable, malleable, and reprocessable. Herein, several dynamic polyurethane networks were synthetized as model compounds, which were able to undergo thermo-activated transcarbamoylation for the use in rebondable adhesives. Ethylenediamine-N,N,N′,N′-tetra-2-propanol (EDTP) was applied as a transcarbamoylation catalyst, which participates in the curing reaction across its four -OH groups and thus, is covalently attached within the polyurethane network. Both bond exchange rate and (thermo)mechanical properties of the dynamic networks were readily adjusted by the crosslink density and availability of -OH groups. In a last step, the most promising model compound was optimized to prepare an adhesive formulation more suitable for a real case application. Single-lap shear tests were carried out to evaluate the bond strength of this final formulation in adhesively bonded carbon fiber reinforced polymers (CFRP). Exploiting the dynamic nature of the adhesive layer, the debonded CFRP test specimens were rebonded at elevated temperature. The results clearly show that thermally triggered rebonding was feasible by recovering up to 79% of the original bond strength. [ABSTRACT FROM AUTHOR]

Reversible adhesives are crucial for a circular economy of composites as they play a key role for rework, repair, and recycling of adhesively bonded components. Herein, electrically debondable adhesives are prepared by introducing ionic liquids in dynamic thiol–epoxy networks. The function of the ionic liquid in the networks is threefold as it accelerates the curing reaction between thiol and epoxy monomers, facilitates electrical debonding, and catalyzes thermoactivated transesterification reactions, required for rebonding at elevated temperature. A library of 1,3‐dibutylimidazolium‐based ionic liquids with varying anions is synthetized and it is found that 1,3‐dibutyl‐1H‐imidazol‐3‐ium dicyanamide (DiButIm─N(CN)2) is superior in accelerating bond‐exchange reactions between hydroxy and ester moieties at elevated temperature. Thus, a thiol–epoxy resin containing 20 wt% of DiButIm─N(CN)2 is used to impregnate glass fiber mats yielding adhesive connections for aluminum substrates with 10.2 MPa pull‐off strength. The adhesive connections are successfully debonded at the metal–adhesive interface by applying 120 V. The samples are then rebonded via the thermoactivated change in the networks' viscoelastic properties and ≈80% (8.1 MPa) of their original bond strength can be regained. By providing a simple strategy to synthetize reversible adhesives, this approach paves a way toward improved recyclability and repairability of adhesively bonded structures. [ABSTRACT FROM AUTHOR]

Fiber‐reinforced polymer composites (FRPCs) are versatile materials with applications in diverse fields such as transportation, construction, and electronics. With the composites market expected to reach 15.5 Mt by 2026, increasing the sustainability of FRPCs is imperative. The main factors driving the sustainability of FRPCs, namely end‐of‐life management and recyclability, the use of natural, bio‐based, and sustainable materials, as well as biodegradability and product simplification are presented and discussed. [ABSTRACT FROM AUTHOR]

The scientific strategies reported in the literature for developing biobased epoxy vitrimers are summarized. Biomass resources such as lignin, cellulose, or different vegetable oils can be exploited as biobased building blocks for epoxy thermosets as an alternative to the bisphenol‐A‐based ones. Biorenewable resources have been synthesized introducing dynamic covalent bonds in the cross‐linked networks. This combination allows to achieve cross‐linked biobased epoxy networks with thermoset‐like properties at a temperature of use, but showing (re)processability, recyclability, and self‐healing properties above a well‐defined temperature named topology freezing transition temperature (Tv). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vitrimers exhibit unique properties, such as thermal recyclability akin to thermoplastics, while structurally mirroring thermosets in terms of strength, durability, and chemical resistance. However, a significant limitation of these materials is their dependence on an external catalyst. Consequently, this research aims to develop vitrimer materials that incorporate an intrinsic catalyst, thus maintaining excellent thermomechanical properties and recyclability. Polyaddition polymerization is employed to synthesize the desired polymer, incorporating a self‐synthesized tertiary amine unit, (bis(2‐hydroxyethyl)‐3,3′‐((2‐(dimethylamino)ethyl)azanediyl)dipropanoate) (N‐diol), as an internal catalyst for transcarbamoylation and potential transesterification reactions. The resulting polymer, with a gel content of 97% and a glass transition temperature of 29 °C, is fabricated into test samples for comprehensive thermal and mechanical evaluations. The material demonstrates an initial Young's modulus of 555 MPa, retaining 81% of this value after two recycling processes. Additionally, using stress relaxation analysis (SRA), a topology freezing temperature of 82 °C, indicative of the characteristic Arrhenius‐like relaxation behavior, is identified with a bond exchange activation energy of 163 kJ mol−1. [ABSTRACT FROM AUTHOR]

Unsaturated polyester resins (UPR) are widely in composite manufacturing, particularly in the energy, wind, and construction sectors. When these composites sustain damage, defects or cracks, several methodologies are employed to heal and repair them to extend their lifespan while reducing environmental impacts. In this work, the use of vitrimeric UPR (UPRv) as a matrix for carbon fiber composite, was investigated in terms of healing efficiency by mechanical and thermal properties. Flexural and interlaminar shear strength tests revealed healing efficiencies exceeding 40% and 90%, respectively. These results were attributed to the transesterification exchange occurring within the crosslinked networks of UPRv. As described in our previous work, the incorporation of "Covalent Adaptable Networks (CANs)" enables the conversion of commercial thermoset (UPR) into vitrimeric resins, thereby providing reprocessing and healing capabilities. Furthermore, a comparison between pristine and healed composites with Mode I DCB and Mode II ENF tests, confirm how the introduction of vitrimer in composite facilitates the manufacturing of a self‐healing product, with a significant recovery of the original properties. Highlights: Application of a previous synthetized vitrimeric unsaturated polyester resin (UPRv)—fabrication of CF's composites reinforced with UPRv—investigation of the self‐healing property of the developed laminates—potential application as green and sustainable composites. [ABSTRACT FROM AUTHOR]

Vitrimer materials have machinability and stability because of their shape remodeling and self‐healing properties at high temperatures. This article reports the synthesis of epoxy resin using vanillin, p‐aminophenol, and epichlorohydrin as raw materials, and polyetheramine‐400 as a curing agent. To study the effect of free amine on the mechanical and self‐healing properties, two materials with epoxy and amine stoichiometric ratios of 1:1 and 1:1.2 were studied, respectively. The experimental results show that when the molar ratio of epoxy to amine stoichiometric is 1:1.2 (V1:1.2), the tensile strength of vitrimer materials can reach 5.4 MPa and the self‐healing efficiency can reach 99.5% after 6 h of healing at 120°C. When the molar ratio is 1:1 (V1:1), the obtained epoxy material has a higher tensile strength of 9.5 MPa. After 24 h of healing at 120°C, the self‐healing efficiency can reach 83.4%. In addition, epoxy vitrimer materials can be remodeled at 120°C, and the self‐healing and mechanical properties are not influenced after remodeling. This work proposes a novel preparation method for epoxy vitrimer materials, which is beneficial to the development of high‐strength remodelable self‐healing materials. [ABSTRACT FROM AUTHOR]

Epoxy asphalt has been extensively utilized in the construction of long-span steel deck bridges. Due to its thermosetting nature, however, the end-of-life epoxy asphalt cannot be reclaimed as thermoplastic polymer modified asphalts. In this paper, recyclable epoxy asphalt vitrimers (EAVs) were developed by introducing bond-exchange reactions into epoxy resin with fatty acids as curing agents. The existence of asphalt accelerated the bond-exchange reaction of the epoxy vitrimer (EV). Furthermore, the relaxation time of EAVs decreased with the asphalt concentration. The recycling enhanced the storage modulus of EAVs at the glassy stage. However, the glass transition temperature remained after recycling. The recycling lowered the mechanical properties of EAVs. However, both tensile strength and elongation at break of EAVs increased with the recycling time. For EAV containing 45% asphalt binder, the tensile strength and elongation at break after three-time recycling were onefold and twofold higher than those after only one-time recycling. The use of renewable epoxy resin in the development of epoxy asphalt not only reduces the dependence on petroleum resources but also makes the recycling of epoxy asphalt feasible. [ABSTRACT FROM AUTHOR]

Vitrimer materials have attained great interest owing to their sustainable functions and distinctive physical properties. Compared with conventional cross‐linked materials, the thermoplastic‐like nature, such as the stress relaxation and creep at high temperatures, is outstanding, which is owing to the activation of the bond exchange in the cross‐linked network. The activation temperature of the bond exchange, so‐called topology freezing temperature (Tv), are treated as the important material parameters, and non‐isothermal (i.e., temperature‐ramp) creep test is one of the method to determine the Tv. This study investigates effects of network defects, that is, the breaking point of network connectivity, on the creep and Tv. The transesterification vitrimer samples with varying fraction of the network defects are designed using thiol‐epoxy click reaction, where the overall fraction of the bond exchange units, that is, the ester bond and hydroxyl group, is kept constant. Non‐isothermal creep tests are conducted with varying temperature‐ramp rate and applied stress levels, and the difference of Tv and creep behaviors at T > Tv are discussed in terms of network strand diffusion during bond exchange and chain elongation under stress. [ABSTRACT FROM AUTHOR]

Recent research is shifting towards adopting recyclable and sustainable plastics instead of traditionally used thermosetting materials. Accordingly, vitrimers behave both as thermoplastics as well as thermosetting polymers. Inimitable vitrimer polymers have been developed with dynamically crosslinked structures enhancing the durability, reprocessing, and mechanical and chemical stability features. This state-of-the-art review discusses vitrimer nanocomposites reinforced with carbonaceous (carbon nanotube, graphene, and graphene oxide) and inorganic (silica, halloysite nanotube) nanofillers has been particularly emphasized. Beneficial structural, microstructural, mechanical strength, modulus, thermal, conducting, reprocessing, engineering, shape memory, self-healing, kinetic, and theoretical topographies have been observed with vitrimers or modified vitrimers (disulfide or ether functional) matrices along with nano-reinforcements towards multifunctional architectures. High-tech vitrimeric nanocomposites show potential regarding electromagnetic shielding, anticorrosion coatings, sensors, joints welding, adhesives, and other sectors. Versatile design compensations, degradability, reprocessability, and methodological features/applications point towards next-generation sustainable materials for modern industries. Hence, this article presents advancements in the field of high performance vitrimeric nanocomposites. [ABSTRACT FROM AUTHOR]

Over the past decade, rapid developments and advancements in the electronic industry have led to a sharp increase in electronic waste (e-waste) disposed in landfills and not reusable. A wide and diversified range of advanced materials are present in e-waste. Among them, polymer nanocomposites represent a valuable example of high-added-value materials that can have however negative environmental consequences. Although thermoset polymer nanocomposites have demonstrated various beneficial properties such as high chemical resistance, and good mechanical strength, owing to their covalently crosslinked network, their potential damage could shorten the lifespan of electronic devices, increasing the amount of e-waste. To extend their service life, polymer matrices with self-healing abilities, and a good recovery of mechanical properties after healing have been developed. In this context, polymer matrices with dynamic covalent bonds activated by a thermal stimulus stand out as an attractive choice because they combine the re-processability of thermoplastics with the high mechanical performance of thermoset polymers. This chapter will discuss two emerging categories of polymer matrices with thermally reversible covalent bonds: Diels-Alder (DA)- and vitrimer-based systems. Within these two classes of dynamic polymer nanocomposites, the focus will be on the reaction mechanisms enabling self-healing, the types of nanofillers used, the recovery of mechanical and functional properties after damage, and the potential applications in devices and sensors. This work will therefore foster new studies and discussions in the field of repairable electronic and functional devices to prolong their lifetime and reduce their environmental impact. [ABSTRACT FROM AUTHOR]

In this contribution, we reported the crosslinking of polyethylene (PE) with dynamic silyl ether bonds. First, polycyclooctene copolymers bearing hydroxyl groups were synthesized via the ring‐opening metathesis copolymerization of cyclooctene with 5‐norbornene‐2‐methanol. The hydroxy‐functionalized polycyclooctene copolymers were further hydrogenated into the corresponding hydroxy‐functionalized PE copolymers. The PE copolymers were allowed to react with 3‐isocyanatopropyltriethoxysilane to obtain the PE copolymers bearing triethoxysilane groups. By taking advantage of hydrolysis and condensation of triethoxysilane groups grafted onto PE chains, the PE copolymers were well crosslinked; polysilsesquioxane (PSSQ) segments behaved as the crosslinking linkages in the networks. Notably, the PE networks were reprocessable (or recyclable) while zinc trifluoromethanesulfonate [Zn(OTf)2] was incorporated. The reprocessing properties of the PE networks were quite dependent on the crosslinking densities. The reprocessing behavior of the networks is attributable to the introduction of two dynamic chemistries: (i) the metathesis of silyl ether bonds which was catalyzed with Zn(OTf)2 and (ii) transcarbamoylation of carbamates which was catalyzed with dibutyltin dilaurate. Owing to the crosslinking, the PE networks displayed excellent shape memory properties. For the shape memory networks, notably, the original shapes can be reprogrammed with the aid of the exchange of the dynamic covalent bonds introduced into the systems. [ABSTRACT FROM AUTHOR]

Composite laminates were produced by RTM using similar glass and flax fabrics and both vitrimer epoxy and aerospace-grade epoxy, both formulated for liquid molding. Tensile and flexural properties were measured and compared, revealing that the vitrimer composites exhibited equivalent performance in flexural strength and tensile modulus, but slightly lower performance in tensile strength relative to reference epoxy composites. In general, glass–fiber composites outperformed flax–fiber composites in tension. However, both glass and flax–fiber composites yielded roughly equivalent flexural strength and tensile modulus-to-weight ratios. Flax fabrics were recovered from composites by matrix dissolution, and a second-life laminate showed full retention of the mechanical properties relative to those produced from fresh flax. Finally, a demonstration of re-forming was undertaken, showing that simple press-forming can be used to modify the composite shape. However, re-forming to a flat configuration resulted in local fiber damage and a decrease in mechanical properties. An alternative forming method was demonstrated that resulted in less fiber damage, indicating that further refinements might lead to a viable forming and re-forming process. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) composites are renowned for their exceptional mechanical properties, with applications in industries such as automotive, aerospace, medical, civil, and beyond. Despite these merits, a significant challenge in CFRPs lies in their repairability and maintenance. This study, for the first time, delves into the processing and self-healing capability of aromatic thermosetting co-polyester vitrimer-based carbon fiber composites through mechanical testing. Vitrimers are an emerging class of thermosetting polymers, which, owing to their exchangeable covalent bonds, enable the re-formation of bonds across cracks. The specific vitrimer chosen for this study is an aromatic thermosetting co-polyester (ATSP). The mechanical properties of samples were analyzed initially through three-point bending (3PB) testing at room temperature before and after healing (by curing samples for 2 h at 280 °C). Samples were also 3PB tested at 100 °C to analyze their mechanical properties at an elevated temperature for comparison to the samples tested at room temperature. To investigate the fracture properties, optical microscopy images of samples were taken after 3PB tests, which were analyzed to observe crack initiation and crack growth behavior. Through load–displacement curves from double cantilever beam (DCB) mechanical testing, the Mode I crack initiation fracture toughness values of self-healed composites and control composites were calculated to evaluate healing efficiency in ATSP CFRP composites cured at 280 °C for 2 h. Scanning electron microscopy (SEM) showed a similar surface morphology of cracks before and after self-healing. Micro-computed tomography (CT) X-ray imaging confirmed that the healed samples closely resembled the as-fabricated ones, with the exception of some manufacturing voids, caused by outgassing in the initial healing cycle. This research demonstrated the ability for the in situ repair of ATSP CFRPs by restoring the fracture toughness to values comparable to the pristine composite (~289 J/m2). [ABSTRACT FROM AUTHOR]

Vitrimers offer a unique combination of mechanical performance, reprocessability, and recyclability that makes them highly promising for a wide range of applications. However, achieving dynamic behavior in vitrimeric materials at their intended usage temperatures, thus combining reprocessability with adaptivity through associative dynamic covalent bonds, represents an attractive but formidable objective. Herein, we couple boron‐nitrogen (B−N) dative bonds and B−O covalent bonds to generate a new class of vitrimers, boron‐nitrogen vitrimers (BNVs), to endow them with dynamic features at usage temperatures. Compared with boron‐ester vitrimers (BEVs) without B−N dative bonds, the BNVs with B−N dative bonds showcase enhanced mechanical performance. The excellent mechanical properties come from the synergistic effect of the dative B−N supramolecular polymer and covalent boron‐ester networks. Moreover, benefiting from the associative exchange of B−O dynamic covalent bonds above their topological freezing temperature (Tv), the resultant BNVs also possess the processability. This study leveraged the structural characteristics of a boron‐based vitrimer to achieve material reinforcement and toughness enhancement, simultaneously providing novel design concepts for the construction of new vitrimeric materials. [ABSTRACT FROM AUTHOR]

To facilitate the ongoing transition toward a circular economy, renewable 3D print materials that are both sustainable and competitive must be accessible. However, the growing demand for bio‐based thermosetting resins, which are used as ink for vat photopolymerization, gives rise to environmental concerns in terms of plastic waste management. Therefore, photocurable materials that are renewable and recyclable at the same time are needed. In this work, a mechanically robust and reprocessable 3D printed photopolymer is developed from renewable feedstock. Reaction of malic acid with glycidyl methacrylate introduces both methacrylate moieties that can undergo photopolymerization in the 3D printer, and β‐hydroxyester linkages that can act as dynamic crosslinks via bond exchange reactions. By combining modified malic acid with reactive diluents, a photoinitiator, and phosphate catalyst, three distinct resins are formulated, resulting in bio‐based contents ranging from 43% to 49%. The formulations demonstrate good layer fusion and accurate print quality, while the 3D printed specimens are robust and thermally stable. Notably, the printed object with shortest relaxation time displayed Arrhenius flow behavior with an activation energy of 36.0 kJ mol−1, and its mechanical performance is maintained after being recycled three times. This contributes to the end‐of‐life perspective of photocurable resins in additive manufacturing. [ABSTRACT FROM AUTHOR]

Covalent adaptable networks and vitrimers are novel polymers with dynamic reversible bond exchange reactions for crosslinks, enabling them to modulate their properties between those of thermoplastics and thermosets. They have been gathering interest as materials for their recycling and self-healing properties. In this review, we discuss different molecular simulation efforts that have been used over the last decade to investigate and understand the nanoscale and molecular behaviors of covalent adaptable networks and vitrimers. In particular, molecular dynamics, Monte Carlo, and a hybrid of molecular dynamics and Monte Carlo approaches have been used to model the dynamic bond exchange reaction, which is the main mechanism of interest since it controls both the mechanical and rheological behaviors. The molecular simulation techniques presented yield sufficient results to investigate the structure and dynamics as well as the mechanical and rheological responses of such dynamic networks. The benefits of each method have been highlighted. The use of other tools such as theoretical models and machine learning has been included. We noticed, amongst the most prominent results, that stress relaxes as the bond exchange reaction happens, and that at temperatures higher than the glass transition temperature, the self-healing properties are better since more bond BERs are observed. The lifetime of dynamic covalent crosslinks follows, at moderate to high temperatures, an Arrhenius-like temperature dependence. We note the modeling of certain properties like the melt viscosity with glass transition temperature and the topology freezing transition temperature according to a behavior ruled by either the Williams–Landel–Ferry equation or the Arrhenius equation. Discrepancies between the behavior in dissociative and associative covalent adaptable networks are discussed. We conclude by stating which material parameters and atomistic factors, at the nanoscale, have not yet been taken into account and are lacking in the current literature. [ABSTRACT FROM AUTHOR]

Vitrimers represent an emerging class of polymeric materials that combine the desirable characteristics of both thermoplastics and thermosets achieved through the design of dynamic covalent bonds within the polymer networks. However, these materials are prone to creep due to the inherent instability of dynamic covalent bonds. Consequently, there are pressing demands for the development of robust and stable dynamic covalent chemistries. Here, we report a catalyst‐free α‐acetyl cinnamate/acetoacetate (α‐AC/A) exchange reaction to develop vitrimers with remarkable creep resistance. Small‐molecule model studies revealed that the α‐AC/A exchange occurred at temperatures above 140 °C in bulk, whereas at 120 °C, this reaction was absent. For demonstration in the case of polymers, copolymers derived from common vinyl monomers were crosslinked with terephthalaldehyde to produce α‐AC/A vitrimers with tunable thermal and mechanical performance. All resulting α‐AC/A vitrimers exhibited high stability, especially in terms of creep resistance at 120 °C, while retaining commendable reprocessability when subjected to high temperatures. This work showcases the α‐AC/A exchange reaction as a novel and robust dynamic covalent chemistry capable of imparting both reprocessability and high stability to cross‐linked networks. [ABSTRACT FROM AUTHOR]

Dynamic covalent crosslinking of commodity thermoplastics is a desirable target in material development, as it promises to combine the enhanced mechanical properties and thermal/solvent stability of thermosets with reprocessability and plastic flow under certain conditions activating the bond exchange. Many attempts of this development suffer from the same two problems: enhanced cost due to complex and often toxic chemicals, and the effective melt‐flow index being too low for practical use. Here we return to the origins of polymer networks, and mimic the vulcanization of natural rubber in the commodity polypropylene using elemental sulfur initiated by peroxide. Forming sulfur bridges allows easy catalyst‐free reprocessability based on the disulfide bond exchange. We study a broad range of compositions and reaction conditions, finding optimal balance between the crosslinking and chain scission in the melt compounder, and demonstrating much enhanced characteristics of the resulting materials. We specifically discuss and evaluate the balance between the rubber‐elastic network response at high temperatures and the plastic flow enabled by disulfide exchange, responsible for the reprocessing of our vitrimers. [ABSTRACT FROM AUTHOR]

Dynamic BO bonds are often applied to fabricate reversible‐cross‐linking and reprocessable polymer networks due to their high thermodynamic stability and kinetic tunability. However, it is still difficult to tailor boronic ester polymers, which have both excellent mechanical and self‐healing properties in the absence of external stimuli. To address the above challenge, a range of room‐temperature self‐healable vitrimers are prepared by the reactions of various poly(vinyl hydroxyalkyl ether‐alt‐dialkyl maleate) copolymers and 1,4‐phenyldiboronic acid. The dynamic cross‐linking networks offer the as‐prepared vitrimers superior mechanical properties and thermostability. Because of the presence of dynamic boronic ester bonds, the networks also demonstrated self‐healing (the tensile strength recovered to 83% after 3 days at room temperature) and reprocessing capabilities (no significant change in tensile strength after thrice process recycles). Moreover, the vitrimers have potential as promising damping materials with wide temperature windows around room temperature. For example, the vitrimer of poly(ethylene glycol monovinyl ether‐alt‐dibutyl maleate) and 1,4‐phenyldiboronic acid demonstrates its tanδmax 1.18, Tg = 6.2°C and effective damping temperature range is from −16.4 to 48.5°C. [ABSTRACT FROM AUTHOR]

Vitrimers are a novel class of polymers known for their dynamic and recyclable properties. They are gaining significant attention as potential alternatives to traditional thermosets in composite manufacturing due to their repairability and reprocessability. This study explores the environmental sustainability of vitrimers in composite materials through a comprehensive Life Cycle Assessment (LCA) approach. The research focuses on vitrimer composite parts production and their recycling process, with a "from cradle to grave approach". Given the scientific and industrial interest, vitrimer containing disulfide bonds obtained reacting bisphenol A and 4-aminophenyl disulfide (4-AFD or APD) was considered as matrix for the investigated composite part; a traditional epoxy resin matrix composite reinforced with carbon fibers was selected as reference scenario. The recycling process of vitrimer based composites was evaluated, comparing it with conventional thermoset composite recycling method (mechanical recycling). The effects on the environment of these alternatives were evaluated considering different impact categories to provide a comprehensive view of the scenarios. The findings of this study demonstrate that vitrimers can represent a sustainable alternative to traditional thermoset, especially thanks to the possibility of extending the life cycle of a component and recovering continuous filaments from the parts. By shedding light on the potential benefits of vitrimers in reducing the ecological footprint of composite materials, this study contributes valuable insights to sustainable materials development. [ABSTRACT FROM AUTHOR]

The increased use of plastics and the associated environmental impact has catalyzed research on the development of bio-derived polymers. Bio-based polyesters have gained increased attention due to the abundance of their starting materials and ease of processing. Lignin is naturally occurring in biomass with rich carbon content, whose functionality and rigidity make it an ideal bio-derived candidate for bio-based polyesters. Herein, a lignin-based polyester with good thermal stability and self-repairability was synthesized from carboxylated lignin and epoxidized soybean oil. The synthesized lignin/epoxidized soybean oil (ESO) vitrimer was brittle such that its mechanical performance could not be recorded. However, when polyethylene glycol (PEG) was incorporated as a plasticizer, polymer samples exhibited acceptable ductility. From thermomechanical analysis of the synthesized polyesters, the plasticizer did not impair thermal stability of polymers, but greatly enhanced mechanical properties. Notably, all samples exhibited stability at high temperatures, and good glass transition temperatures (51.0 ± 0.9–78.0 ± 1.2 °C). The highest tensile strength (3.983 ± 0.1 MPa) and storage modulus (1463.67 ± 12.6 MPa) were recorded for the polyester containing 6 % w/w PEG. Moreover, the polymer samples exhibited self-healing capability at 180 °C. This work expands on valorization of lignin through the synthesis of bio-derived materials.

Two different self-healing approaches are studied in this work to analyze the possible contribution of each of the self-healing mechanisms at different temperatures and the possible synergetic effects between them. Thermal and mechanical properties were differently affected by the addition of each of the thermoplastic polymers. This was caused by different phase separations induced during the curing reaction in terms of size and number of the thermoplastic phase domains dispersed within the epoxy matrix and due to the different amounts of thermoplastic polymer that remain solved in the matrix. When using the vitrimeric matrix, phase separation only occurred at the nanometer scale when utilizing poly(bisphenol-A-co-epichlorohydrin) (PBAE) as a thermoplastic agent or higher contents of polycaprolactone (PCL).Self-healing capabilities showed a strong dependence on the temperature used and the type of crack. Low temperatures allowed the thermoplastic phase to flow and fill partially the cracks with moderate levels of self-healing which were only available when phases were separated. Higher temperatures allowed dynamic bonds to induce material healing reaching very high efficiencies but, more importantly, a synergic effect was observed when material was removed from the cracks. In these cases, the flow of the thermoplastic phase filled better the crack and there was an enhanced cooperation between the two healing mechanisms. At higher self-healing temperatures, materials with nanometric size phase separation enabled greater self-healing efficiencies (above 90 %) due to the vitrimeric self-healing capability while the thermoplastic phase helped fill the gaps due to material removal.

A hafnium‐based coordination vitrimer (Hf−CooAN‐10) has been prepared by carboxylate ligand exchange from [Hf6O4(OH)4(O2CMe)12]2 and poly‐((ethylhexyl methacylate)‐co‐(carboxyethyl acrylate)) (Mn = 14.4 kg mol−1, Đ = 2.2, 90 : 10 comonomer molar ratio), using a 10 % molar Hf loading with respect to the polymer maximum crosslinking capacity. The material shows high insoluble fraction and expected properties for a 3D network and has been reshaped three times at 50 °C under a 3‐ton pressure without significant property alteration. The Hf−CooAN‐10 thermomechanical and rheological properties are very similar to those previously reported for the analogous Zr−CooAN‐10. The associative nature of carboxylate exchange between the [Hf6O4(OH)4(O2CR)12] crosslinks and the polymer matrix free −COOH functions is suggested by parallel 1H NMR line‐broadening kinetic investigations of acetate exchange in the [Hf6O4(OH)4(O2CMe)12]2 precursor with free MeCOOH, the rate of which has a zero‐order dependence on [MeCOOH], and by DFT calculations on a [Hf(μ‐O2CMe)(O2CMe)2(OH)(H2O)]2 model. The results of these investigations, including the activation parameters for the exchange of both chelating and bridging ligands, are very similar to those of the previously investigated zirconium system and suggest partial acetate dissociation assisted by H‐bond formation with a (μ3‐OH) ligand as the rate‐determining step, followed by acid coordination and intramolecular proton transfer. [ABSTRACT FROM AUTHOR]

Herein, a new lightweight syntactic foam is reported with strong mechanical properties, unique multifunctionalities, and recyclability. Multifunctionality of materials and structures has gained ever‐increasing interest as an excellent approach to designing minimalistic systems. Inspired by nature, these materials can perform multiple functions besides bearing a load. Due to their shape‐changing and damage‐healing property, shape memory vitrimers (SMVs) are a great example of multifunctional materials readily exploited for many applications. Using nickel and silver‐plated hollow glass microbubbles (HGMs), an SMV‐based syntactic foam is introduced here that supplements the multifunctionality of SMVs with electrical conductivity and ferromagnetism, which enables a series of additional potentials such as strain sensing, damage monitoring, Joule heating, and electromagnetic interference shielding. Despite its low density and outstanding mechanical properties, this foam exhibits shape memory behavior, which can be triggered by an electrical current, and damage healing capability due to its reversible dynamic covalent bonds. Especially its recyclability makes recycling the expensive silver‐coated and nickel‐coated HGMs feasible, making this foam cost‐effective and environmentally sustainable. With its many features and economical manufacturability, this syntactic foam has a potential to be utilized in many applications, ranging from aerospace structures to biomedical devices to household items. [ABSTRACT FROM AUTHOR]

Vitrimers are dynamic covalent networks, showing considerable promise in self‐healing and reprocessability applications. In this work, molecular simulations are used to study the shear stress‐induced dynamics of vitrimers in the glassy regime and compare their response with that of permanent networks, providing valuable insights into the macroscopic properties of these networks. Results show that vitrimers undergo bond exchange reactions induced by the application of the shear stress, and these events result in accelerated dynamics and subsequently, higher shear compliance of vitrimer compared to the thermoset. When the applied shear stress is high and deformation enters the nonlinear regime, the vitrimer network shows large shear creep compliance and enhanced non‐affine dynamics. Furthermore, the analysis of the self‐particle dynamics demonstrates that the dynamical behavior of vitrimers is highly heterogeneous at short observation times under low and intermediate stress values, while they become homogeneous in the regime of the shear compliance where it shows significantly higher compliance than the permanent network. The significance of the rate of deformation on the dynamics of glassy vitrimers is successfully demonstrated. [ABSTRACT FROM AUTHOR]

Bio‐based polymer building blocks derived from abundant biomass represent a promising class of monomers for the synthesis of sustainable high‐performance polymers. Lignin‐derived vanillin is used as a bio‐based, aromatic molecular platform for chemical modifications. The use of vanillin aldehyde derivatives as monomers with different alkyl chain length, cured with bio‐based and less‐toxic di‐ and triamines, leads to covalent adaptable Schiff base networks and thus enables sustainable and thermally reprocessable high‐performance materials without using highly toxic amines. A process is presented to prepare homogeneous films of crosslinked materials that are thermally reprocessable while maintaining their mechanical performance. The network structures, mechanical properties, and thermal stability of the obtained polymeric sheets are characterized in detail. By systematically adjusting the composition of the network building blocks, the mechanical properties could be varied from tough materials with a high elastic modulus of 1.6 GPa to materials with high flexibility and elastomeric behavior with an elongation at break of 400%. Furthermore, the stress–relaxation behavior of stoichiometric and nonstoichiometric Schiff base vitrimers is investigated. The combination of bio‐based building blocks and the degradability of Schiff base networks under acidic conditions resulted in sustainable, environmentally friendly, chemically and thermomechanically recyclable vitrimers with self‐healing and shape‐memory properties. [ABSTRACT FROM AUTHOR]

Vinylogous urethane (VUO) based polymer networks are widely used as catalyst‐free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VUN) undergo much faster bond exchange than VUO and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VUO and VUN vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VUO and VUN linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VUO vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VUN, these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea ‐ urethane (VU) network of strong non‐covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks. [ABSTRACT FROM AUTHOR]