Bond Exchange Reactions Research Articles

Internal hydrogen bonding of imines to control and enhance the dynamic mechanical properties of covalent adaptable networks

Covalent Adaptable Networks (CANs) have the potential to replace current thermoset materials, as the dynamic covalent bonds in CANs enable reprocessability and recycling of crosslinked polymer networks. A current limitation of CANs is, however, that they are generally susceptible towards creep, as the bond exchange reactions facilitate stress relaxation. In this work, we propose the use of internal hydrogen bonding in polyimine CANs as an efficient tool to enhance creep resistance (at operating temperature) and further control the dynamic mechanical properties, while still enabling malleability at elevated temperatures. We are able to show on a small-molecule level that ortho-substituted hydroxy groups on aromatic imines stabilise the dynamic covalent imine bonds as a result of the internal hydrogen bond that is formed between the hydroxy and imine group. Furthermore, we show that polyimine CANs with incorporated ortho hydroxy groups have significantly enhanced material properties, as can be seen in the glass transition temperature (Tg), elastic modulus (G’), creep resistance and solvent resistance. While we also consider additional steric and electronic effects that might have arisen due to installation of the hydroxy groups, we find that the stabilising effect of the internal hydrogen bond is primarily dictating the material performance.

A Hamiltonian graph model for the cooperative toughening of crystalline phases and covalent adaptable networks in semi-crystalline thermoset epoxy

The existence of bond exchange reactions and covalent adaptable networks (CANs) in thermoset epoxy has facilitated its self-healing and reversible mechanical capabilities. However, the toughening mechanisms and cooperative coupling of these crystal phases and CANs in a semi-crystalline thermoset epoxy have not been well understood. In this study, a Hamiltonian graph model is formulated to examine toughening mechanisms in the semi-crystalline thermoset epoxy based on the vertices and paths, both of which are employed to describe the crystalline phases and CANs, respectively. A free-energy equation is also developed based on the tail and tie free energy functions to investigate the cooperative coupling of crystal phases and CANs. The crystal phases increase the cross-linking density of the CANs, which helps the crystal phases with a homogeneous dispersion. Moreover, an extended Maxwell model is developed along with the Hamiltonian graph to explore the coupling effect of crystal phase and CAN on the mechanical behaviors of semi-crystalline thermoset epoxy. A constitutive stress–strain relationship is then proposed to describe the self-healing and toughening behaviors of semi-crystalline thermoset epoxy. The stress–strain relationship of semi-crystalline polymers, which incorporates crystal phases and CANs, has been thoroughly investigated using the analytical results obtained from the proposed Hamiltonian graph model. Finally, the effectiveness of the proposed model is verified using the finite element analysis method and a set of experimental data.

Isotopic Fractionation as a Mechanistic Probe in Light-Driven C-H Bond Exchange Reactions.

Here, we report a diagnostic framework for elucidating the mechanisms of photoredox-based hydrogen isotope exchange (HIE) reactions based on hydrogen/deuterium (H/D) fractionation. Traditional thermal HIE methods generally proceed by reversible bond cleavage and bond reformation steps that share a common transition state. However, bond cleavage and bond reformation in light-driven HIE reactions can proceed via multiple, non-degenerate sets of elementary steps, complicating both mechanistic analysis and attendant optimization efforts. Building on classical treatments of equilibrium isotope effects, the fractionation method presented here extracts information regarding the nature of the key bond-forming and bond-breaking steps by comparing the extent of deuterium incorporation into an exchangeable C-H bond in the substrate relative to the H/D isotopic ratio of a solvent reservoir. We show that the extent of fractionation is sensitive to the mechanism of the exchange process and provides a means to distinguish between degenerate and non-degenerate mechanisms for isotopic exchange. In model systems, the mechanisms implied by the fractionation method align with those predicted by thermochemical considerations. We then employed the method to study HIE reactions whose mechanisms are ambiguous on thermodynamic grounds.

Tough Epoxy Vitrimer with Hybrid Networks and Its Applications in Reprocessable Composites

Progress in the ever‐growing vitrimer chemistry opens up great opportunities for a sustainable society by producing reprocessable thermosets. However, most vitrimers suffer from the conflict between mechanical performance and dynamic reactivity, hindering the development of industrial applications. Herein, a facile but universal material design strategy is proposed to yield tough epoxy vitrimers while preserving excellent reprocessability. Copolymerization and bond exchange reactions can build covalent bridges between heterogeneous polymer networks, thus leading to a hybrid dynamic covalent polymer network containing hard and soft segments. Tailorable mechanical and thermal properties are obtained by controlling the ratio of hard‐to‐soft components. The resulting epoxy vitrimer can balance strength and stretchability while exhibiting brittle‐to‐ductile transformation in tensile tests. Enhanced fracture toughness is achieved compared to both pristine hard and soft vitrimers. The resulting vitrimers exhibit excellent processability and potential applications in composite fabrication, welding and reshaping, and multiple shape memory functions. This work presents a feasible approach to satisfying the demands of mechanical performance and dynamic reactivity in the design of epoxy vitrimers. The material design principle will be applicable to a wide range of vitrimer systems to pursue desirable properties.

Mechanism of Reduced Glutathione Induced Lysozyme Defolding and Molecular Self-Assembly.

The distinctive assembly behaviors of lysozyme (Lys) feature prominently in food, materials, biomedicine, and other fields and have intrigued many scholars. Although our previous work suggested that reduced glutathione (GSH) could induce lysozyme to form interfacial films at the air/water interface, the underlying mechanism is still obscure. In the present study, the effects of GSH on the disulfide bond and protein conformation of lysozyme were investigated by fluorescence spectroscopy, circular dichroism spectroscopy, and infrared spectroscopy. The findings demonstrated that GSH was able to break the disulfide bond in lysozyme molecules through the sulfhydryl/disulfide bond exchange reaction, thereby unraveling the lysozyme. The β-sheet structure of lysozyme expanded significantly, while the contents of α-helix and β-turn decreased. Furthermore, the interfacial tension and morphology analysis supported that the unfolded lysozyme tended to arrange macroscopic interfacial films at the air/water interface. It was found that pH and GSH concentrations had an impact on the aforementioned processes, with higher pH or GSH levels having a positive effect. This paper on the exploration of the mechanism of GSH-induced lysozyme interface assembly and the development of lysozyme-based green coatings has better instructive significance.

Low-temperature reprogrammable dual light-responsive liquid crystalline elastomer films

The liquid crystal elastomers (LCEs) possessing azobenzene moieties and boron ester bonds were prepared using Michael addition reaction and photopolymerisation. The azobenzene moieties imparted the LCEs with photoresponsiveness, whereas the boron ester bonds enabled the LCEs to undergo director alignment, reprogramming, and self-healing through a dynamic bond exchange reaction (DBER). This combined strategy overcame the competition between actuation and a DBER at high temperatures, which has been a drawback in the previously developed LCE vitrimers. Interestingly, the developed monodomain LCEs (MLCEs) showed the bending actuation in responsive to both UV and blue lights and unbending motion in responsive to green light, which paves the way for the design of new actuators for biomedical applications. Furthermore, taking advantage of the dynamic nature of boron ester chemistry, bilayer-structured actuators capable of thermo- or photo-controllable bending/unbending motions were fabricated by welding the MLCE film with a poly(ethylene terephthalate) substrate or itself below their nematic-to-isotropic transition temperatures via a ‘glue-free’ method. The MLCEs developed in this study can undergo multidirectional movements in response to different stimuli and can be used in applications such as microrobots, untethered biomimetic grippers, and surgical instruments.

Fluorine-free, robust and self-healing superhydrophobic cotton fabric based on the disulfide bond exchange reaction

Fluorine-free, robust and self-healing superhydrophobic cotton fabric based on the disulfide bond exchange reaction

Tough and Thermostable Polybutylene Terephthalate (PBT)/Vitrimer Blend with Enhanced Interfacial Compatibility

Front Cover: Article 2200972 by Tiejun Wang and co-workers proposes a reactive blending strategy to develop thermoplastic-thermoset blends with enhanced compatibility based on dynamic bond exchange reactions at the interface. This work offers an innovative way to design and fabricate new polymeric materials by blending thermoplastics and thermosets. It also suggests a facile direction towards upcycling thermoplastics and thermosets.

Dynamic elastomers based on bio‐derived crosslinker

AbstractPetroleum‐derived monomers are the most common building blocks for ester‐based thermosets. Bio‐derived thermoset elastomers are becoming viable alternatives to conventional thermosets. Herein, we developed a biobased vitrimer‐type thermoset elastomers using abundant and sustainable raspberry ketone as feedstock. We utilize raspberry ketone to create building blocks for dynamic oxime chemistry and crosslinked these through free radical polymerization with poly(ethylene glycol) methyl ether methacrylate as a comonomer. In contrast to other dynamic networks based on ester bonds, which need catalysts, this is undesirable since catalyst deactivation or leaching lowers its effect over time and may impair reuse. This network incorporates catalyst‐free bond exchange reactions in catalyst‐dependent polyester networks by substituting oxime‐esters for typical ester linkages. The elastomer exhibits stress relaxation, a low glass transition temperature (Tg) (−55 to −40.2°C) and tensile strength up to 5.2 ± 3.0 kPa. Furthermore, the dynamic oxime transesterification exchange mechanism allows elastomers to be reprocessed using a hot press at 160°C and 8 × 103 kPa pressure. After reprocessing, the tensile strength of elastomers can be recovered up to 78.1 ± 10.9%. This work integrates the principles of catalyst‐free dynamic exchange process and mechanical recycling coupled with biobased components to provide a rational solution towards conventional elastomers. In the future, these elastomers can be exploited for the development of hydrogels, recyclable elastomers, and commodity plastics.

3D printing of dynamic covalent polymer network with on-demand geometric and mechanical reprogrammability

Delicate geometries and suitable mechanical properties are essential for device applications of polymer materials. 3D printing offers unprecedented versatility, but the geometries and mechanical properties are typically fixed after printing. Here, we report a 3D photo-printable dynamic covalent network that can undergo two independently controllable bond exchange reactions, allowing reprogramming the geometry and mechanical properties after printing. Specifically, the network is designed to contain hindered urea bonds and pendant hydroxyl groups. The homolytic exchange between hindered urea bonds allows reconfiguring the printed shape without affecting the network topology and mechanical properties. Under different conditions, the hindered urea bonds are transformed into urethane bonds via exchange reactions with hydroxyl groups, which permits tailoring of the mechanical properties. The freedom to reprogram the shape and properties in an on-demand fashion offers the opportunity to produce multiple 3D printed products from one single printing step.

Scalable micropatterned epoxy vitrimer films by thermo-triggered bond exchange for repairable and recyclable triboelectric nanogenerators

Triboelectric nanogenerators are believed to be a promising solution to energy crisis by mechanical-to-electrical energy conversion through the contact electrification between constituent tribo-materials. However, most of the current tribo-materials are difficult to be repaired if damaged, and recycled for their reuse or disposal, limiting the development for reusable and sustainable energy-harvesting devices. Therefore, we explore the development of durable, repairable and recyclable triboelectric nanogenerators based on an epoxy-derived vitrimer (EV) with thermo-reversible cross-linked networks. Rather than polymerizing the EVs by adding thermoplastic components, they are generated by a solvent-free, quasi-thermoforming and scalable process. Thanks to the flowability of cross-linked networks realized by thermo-triggered hydroxyl/ester bond-exchange reactions, surface-micropatterned EV film based tribo-materials are successfully fabricated by a thermal micro-imprinting process, showing 3D shape-adaptive thermoforming ability. The EV films exhibit excellent flexibility and ductility with the smaller elastic hysteresis under external deformations, whose molecular origin is revealed to be the achievable macromolecular segment relaxation and energy dissipation of networks. The favorable mechanical robustness and recoverability endow the EV-based nanogenerators (EV-TENG) with significantly durable electricity-generating performances under repeated compressions (≥10,000 cycles) and larger mechanical loads. More importantly, by taking the triboelectric signal as a probe for network evolution, repairability and recyclability of EV-TENGs are further evaluated quantitively. Interestingly, the introduction of EVs makes the device self-repairable simply by heating treatment with a higher healing efficiency of 98%. Moreover, the device could be recycled by both physical and chemical approaches, without losing the original triboelectric performance and structural integrity. Our results demonstrate the potential applications of vitrimers derived from industrial raw materials in developing repairable, recyclable and scalable energy-harvesting electronics with prolonged lifetimes and environmental sustainability.

Synthesis of Renewable, Recyclable, Degradable Thermosets Endowed with Highly Branched Polymeric Structures and Reinforced with Carbon Fibers

This paper demonstrates the molecular design of renewable polymer thermosets that are chemically recyclable yet degradable on demand. In this design, the thermosets comprise a limonene-based highly branched polymer, which has been synthesized via the regioselective thiol–ene reaction between natural limonene and multifunctional thiol, and a Meldrum’s acid derivative for the click/declick reaction of thiol groups that are deliberately positioned on the outer surface of the branched polymer. Thus, the accessible thiols expedite the covalent bond exchange reaction and enable the formation of a network and bulk recycling of the entire network; the labile cross-linkers enable molecular degradation or repurposing of the materials under benign conditions. The thermosets can be also reinforced with the addition of carbon fibers. Thus, reversible deformation or self-lamination of the resultant carbon composites, which are degradable and recyclable, was achieved owing to the thermosets, which act as a matrix or binder. We envision that the renewable materials presented here would be advanced by introducing diverse biomolecules or reversible chemistry to develop disposable bio-based platforms that can be further upcycled at the end of their lifecycles.

Sustainable Bio-Based UV-Cured Epoxy Vitrimer from Castor Oil.

Vitrimers brought new properties in thermosets by allowing their reshaping, self-healing, reprocessing, and network rearrangement without changing structural integrity. In this study, epoxidized castor oil (ECO) was successfully used for the straightforward synthesis of a bio-based solvent-free vitrimer. The synthesis was based on a UV-curing process, which proceeded at low temperatures in the absence of any solvents, and within a short time. Real time Fourier-transformed infrared spectroscopy and photo-DSC were exploited to monitor the cationic photocurable process. The UV-cured polymer networks were able to efficiently undergo thermo-activated bond exchange reactions due to the presence of dibutyl phosphate as a transesterification catalyst. Mechanical properties, thermal resistance, glass transition temperature, and stress relaxation were investigated as a function of the amount of transesterification catalyst. Mechanical properties were determined by both DMTA and tensile tests. Glass transition temperature (Tg) was evaluated by DMTA. Thermal stability was assessed by thermogravimetric analysis, whilst vitrimeric properties were studied by stress relaxation experiments. Overall, the ECO-based vitrimer showed high thermal resistance (up to 200 °C) and good mechanical properties (elastic modulus of about 10 MPa) and can therefore be considered as a promising starting point for obtaining more sustainable vitrimers.

Enhanced corrosion resistance of self-healing waterborne polyurethane coating based on tannic acid modified cerium-montmorillonites composite fillers

Nanofiller composite coatings with synergistic properties for long-term corrosion protection of metals. In this work, a new composite waterborne polyurethane coating with both self-healing and active corrosion protection was developed for the corrosion protection of carbon steel. The composite coating demonstrated remarkable long-term corrosion resistance thanks to the labyrinth effect of MMT and the synergistic action of corrosion inhibitors Ce3+ and TA. The impedance modulus was still above 108 Ω·cm2 after 50 days of immersion, which was nearly three orders of magnitude higher than that of the pure PU coating. Moreover, the self-healing experiments confirmed that the physical damage was effectively repaired after heat treatment, which was attributed to the disulfide bond exchange reaction. Importantly, the healed coating also exhibited durable corrosion resistance. This kind of double functional anti-corrosive coating shows great potential in the field of long-term anti-corrosion.

Flexible Vitrimers for Self‐healable Triboelectric Nanogenerators

AbstractDespite rapid advancements in the development of triboelectric nanogenerators (TENG), mechanical defects generated during their working condition limit the service lifetime of these devices. Polyester‐based vitrimers can offer a solution to solve this issue via their dynamic bond exchange reactions. However, they often require high temperatures and a long time to crosslink, rendering them not useful for large‐scale TENG fabrication. To solve these issues, a self‐healable vitrimer‐based triboelectric nanogenerator (VITENG) is developed based on diacrylate poly(dimethylsiloxane) (AA‐PDMS) via a fast thiol‐Michael reaction. By including 2,3‐dihydroxypropyl methacrylate (DHPMA), hydroxy end dangling side chains are generated in the matrix, which increase the PDMS network's flexibility and transesterification efficiency. Efficient transesterification occurs on the abundant dynamic β‐hydroxy ester bonds in the PDMS network, resulting in 100% efficient mechanical damage repair. The VITENG demonstrates a good performance with an output voltage of 135 V under a load of 10 N, demonstrating the potential for mechanical energy harvesters with long lifetimes and high design flexibility.

Analysis of Poly(thiourethane) Covalent Adaptable Network through Broadband Dielectric Spectroscopy.

The dielectric spectra of the poly(thiourethane) network, HDI-S3, have been analyzed to know the nature and the cooperativity of each of the six dielectric processes observed. At low temperatures, γ1, γ2, and β dielectric relaxations were attributed to noncooperative local motions in the glassy state, in which apparent activation energies are 30, 36, and 60 kJ·mol-1, respectively. At higher temperatures, three dielectric relaxations are observed (αTg, α*, ρ). The αTg relaxation is attributed to the glass transition, and it is overlapped with the α* relaxation. The molecular origin of α* relaxation is associated with the bond exchange reaction. Finally, the ρ relaxation is ascribed to the heterogeneity of the sample although its origin is uncertain. The DC conductivity (σDC) is found to be an appropriate variable to analyze the bond exchange reaction. Accordingly, the HDI-S3 has a molecular exchange mechanism of dissociative nature.

Digital light processing 3D printing of dynamic magneto-responsive thiol-acrylate composites.

Additive manufacturing is one of the most promising processing techniques for fabricating customized 3D objects. For the 3D printing of functional and stimuli-triggered devices, interest is steadily growing in processing materials with magnetic properties. Synthesis routes for magneto-responsive soft materials typically involve the dispersion of (nano)particles into a non-magnetic polymer matrix. Above their glass transition temperature, the shape of such composites can be conveniently adjusted by applying an external magnetic field. With their rapid response time, facile controllability, and reversible actuation, magnetically responsive soft materials can be used in the biomedical field (e.g. drug delivery, minimally invasive surgery), soft robotics or in electronic applications. Herein, we combine the magnetic response with thermo-activated healability by introducing magnetic Fe3O4 nanoparticles into a dynamic photopolymer network, which undergoes thermo-activated bond exchange reactions. The resin is based on a radically curable thiol-acrylate system, whose composition is optimized towards processability via digital light processing 3D printing. A mono-functional methacrylate phosphate is applied as a stabilizer to increase the resins' shelf life by preventing thiol-Michael reactions. Once photocured, the organic phosphate further acts as a transesterification catalyst and activates bond exchange reactions at elevated temperature, which render the magneto-active composites mendable and malleable. The healing performance is demonstrated by recovering magnetic and mechanical properties after the thermally triggered mending of 3D-printed structures. We further demonstrate the magnetically driven movement of 3D-printed samples, which gives rise to the potential use of these materials in healable soft devices activated by external magnetic fields.

Structure and dynamics of dynamic covalent cross-linked PEOs and PEO/LiPF6 electrolytes: a coarse-grained simulation study.

The incorporation of dynamic covalent bonds has been an attractive strategy to synthesize adaptive solid polymer electrolytes (SPEs). Here, we present molecular dynamics results concerning the relationship between ion transport and segmental dynamics for dynamic covalent cross-linked PEO-Li+ SPEs. To dissolve LiPF6 into PEO, a 1/r4-form approximation of ion-dipole interactions is employed as the solvation potential. Its parameters are estimated with the assistance of the Bayesian optimization algorithm and validated by comparing the resulting behaviors of PEO/LiPF6 with experimental observations. The dynamic associations of EO with Li+ and PF6- significantly reduce the segmental mobility of PEO, verifying the coupling of PEO segmental dynamics with ion transport. In order to reproduce the unique behaviors of associative covalent adaptive networks (CANs), the bond-exchange reaction is controlled by the collision probability and the user-defined activation energy (Ea ≥ 0) based on a hybrid of molecular dynamics and Monte Carlo methods. The dynamics of network topology, facilitated by the reshuffling of dynamic covalent bonds, is analyzed using graph theory. The network mesh size varies with time, which can be considered as one of the characteristics for associative CANs. The reshuffling of dynamic bonds releases the constraint from cross-linked structures, and enhances the long-range segmental mobility as well as the mobilities of Li+ and PF6-. By drawing comparisons with its conventionally cross-linked counterpart, the effect of dynamic-bond reshuffling on ion transport is studied for the dynamic covalent cross-linked PEO16-LiPF6 electrolyte in terms of self-diffusivities, cation transference number, and ionic conductivity.

3D Printing of continuous fiber composites using two-stage UV curable resin.

3D printing allows for moldless fabrication of continuous fiber composites with high design freedom and low manufacturing cost per part, which makes it particularly well-suited for rapid prototyping and composite product development. Compared to thermal-curable resins, UV-curable resins enable the 3D printing of composites with high fiber content and faster manufacturing speeds. However, the printed composites exhibit low mechanical strength and weak interfacial bonding for high-performance engineering applications. In addition, they are typically not reprocessable or repairable; if they could be, it would dramatically benefit the rapid prototyping of composite products with improved durability, reliability, cost savings, and streamlined workflow. In this study, we demonstrate that the recently emerged two-stage UV-curable resin is an ideal material candidate to tackle these grand challenges in 3D printing of thermoset composites with continuous carbon fiber. The resin consists primarily of acrylate monomers and crosslinkers with exchangeable covalent bonds. During the printing process, composite filaments containing up to 30.9% carbon fiber can be rapidly deposited and solidified through UV irradiation. After printing, the printed composites are subjected to post-heating. Their mechanical stiffness, strength, and inter-filament bonding are significantly enhanced due to the bond exchange reactions within the thermoset matrix. Furthermore, the utilization of the two-stage curable resin enables the repair, reshaping, and recycling of 3D printed thermosetting composites. This study represents the first detailed study to explore the benefits of using two-stage UV curable resins for composite printing. The fundamental understanding could potentially be extended to other types of two-stage curable resins with different molecular mechanisms.

The effect of photolatent catalysts on the exchange kinetics of dual-wavelength 3D printable and photopatternable thiol-click vitrimers

The use of photolatent transesterification catalysts provides a unique way to locally control dynamic bond exchange reactions in vitrimers.