Reversible Diels Research Articles

Amine functionalized maleimide grafted MWCNT–CFRP hybrid composites interfacial/matrix healing and mechanical performance through thermally reversible cycloaddition

AbstractRestoration of the structural integrity of damaged CFRP composite through thermally reversible Diels–Alder bonds with the exact interfacial healing between matrix/fiber is extremely desirable for manufacturing of high‐performance self‐healing laminates. This article demonstrates the fabrication of amine‐functionalized maleimide‐grafted MWCNT (with different weight ratios 0.5 wt%, 1.0 wt%, and 1.5 wt%) reinforced thermally reversible self‐healing CFRP composite. The morphological changes, functional elements, elemental groups, and thermal degradation of functionalized‐MWCNTs were characterized by FESEM/HR‐TEM, FTIR, XRD, and TG‐DTA analysis. The results demonstrated that CFRP reinforced with silane/maleimide grafted MWCNT enhanced the dispersion, exhibited 82% healing efficiency and improvement in flexural strength‐67.11%, tensile strength‐64.75% concerning pure‐CFRP. FE‐SEM fractography of composites indicated that nanotube pullout was predominant failure‐criteria in CFRP‐pristine‐MWCNTs whereas strong interfacial adhesion dominated in CFRP‐maleimide‐MWCNTs. The purpose of this article is to investigate matrix/fiber/nanofillers interface healing properties of CFRP‐composite before and after damage. Furthermore, it is intended to reform structural integrity and examines average healing efficiency.Highlights Amine‐functionalized maleimide‐grafted MWCNT, CFRP composite fabrication. Characterized, morphological changes functional elements thermal degradation. Improvement in healing efficiency (82%), flexural strength‐(67.11%). The enthalpy of 0.5 wt% MWCNTBm enhanced to 10.91 J/g. Fractographic examinations show extensive intralaminar/interlaminar deformation.

Self‐healing reversible network from furfuryl‐functionalized copoly(triazine‐r‐polypropylene glycol‐r‐polydimethylsiloxane)

AbstractNew shape‐memory networks composed of both reversible Diels–Alder covalent crosslinks and the hydrogen and π–π stacking bonds of triazine functional groups, with self‐healing properties as a result of the reversibility of these dynamic bonds were achieved from a furfuryl‐functionalized copoly(triazine‐r‐polypropylene glycol‐r‐polydimethylsiloxane), crosslinked with a maleimide end‐capped polycaprolactone and a long polycaprolactone‐based chain extender. The presence of multiple dynamic bonds, including the Diels–Alder and triazine‐derived π–π stacking and H‐bond interactions as well as the enhanced network mobility arising from polypropylene glycol, polydimethylsiloxane and polycaprolactone segments upon triggering the shape memory effect resulted in a material with high toughness (~200 MPa J−1) and efficient healing ability (with a recoveries of tensile strength and toughness after being cut and healed of 87% and 94%, respectively).Highlights Furfuryl dichlorotriazine was coupled with PPG and PDMS forming a copolymer. The copolymer was crosslinked with PCL‐dimaleimide and PCL‐difuran extender. Multiple dynamic bonds (Diels–Alder, π–π stacking and H‐bond) were present. The network showed high toughness and good healing performance.

Synthesis of High Mechanical Strength and Thermally Recyclable and Reversible Polyurethane Adhesive by Diels–Alder Reaction

AbstractRecyclability of polyurethane materials is significant to relieve environmental problems caused by damaged polymers. Inspired by plenty of self‐healing properties based on dynamic covalent bonds. A high mechanical strength and thermally reversible polyurethane adhesive are acquired through co‐polymerization of poly‐1,4‐butylene adipate glycol (PBA), soybean oil‐based polyol (MESO), and toluene diisocyanate (TDI) whose linear polymer chains are constructed by Diels–Alder reaction between furfuryl alcohol (FA) and bismaleimide (BMI), named DAPU. Further, the obtained polyurethane adhesives show great recyclability, mechanical performance (Whose tensile strength can reach 91.7 MPa), and appropriate self‐healing ability through the thermally reversible Diels–Alder covalent bonds and hydrogen bonds between urethane groups, which may pave a way for further development of recyclable materials.

Evaluation of optical and self-healing properties of Titania/ionic liquid/poly(butyl methacrylate) hybrid material based on thermally reversible Diels–Alder chemistry

ABSTRACT In this study, we report the development of a hybrid polymer material comprising poly(butyl methacrylate) and titania, which exhibits the three key elements of visible-light transparency, UV shielding, and self-healing. Nanodispersion of titania in the polymers was achieved using the sol – gel method using titanium propoxide as the raw material. Furthermore, by adding tetrabutylphosphonium chloride, an ionic liquid, we suppressed visible light absorption due to charge transfer between poly(butyl methacrylate) and titania, resulting in enhanced visible light transmittance. An approach involving crosslinking points comprising Diels – Alder functional groups was used to heal scratches in the hybrid material at 75°C for 30 min. Dynamic viscoelastic measurements and microscopic observations were conducted to investigate the self-healing mechanism. These findings suggested that this phenomenon occurred through the fluidization of the polymer matrix and the recombination reaction of the crosslinking points, which comprise Diels – Alder and titania. The development of intelligent materials with combined transparency, UV protection, and self-healing capabilities provides potential avenues for future research to enhance and broaden their versatile characteristics for widespread applications.

Structure–Property Relationships of Self-Healing Polymer Networks Based on Reversible Diels–Alder Chemistry

Structure–Property Relationships of Self-Healing Polymer Networks Based on Reversible Diels–Alder Chemistry

Bio-based recyclable Form-Stable phase change material based on thermally reversible Diels–Alder reaction for sustainable thermal energy storage

Polymer-based form-stable phase change materials (FPCMs) have attracted much attention due to their excellent shape stability and facileness, low-energy-consumption preparation. However, bio-based recyclable phase change energy storage materials (PCMs), crucial for reducing pollution and sustainable energy storage, have not yet been prepared. Herein, we prepared a novel bio-based recyclable PCM (DGEM-18/FA/MA/xBW), in which a novel epoxy resin (DGEM) was synthesized by using bio-based magnolol and epichlorohydrin, bio-based 2-Furanmethanamine (FA) and C36-alkylenediamines bismaleimide (MA) were used to construct a reversible crosslinked network based on epoxy-amine ring opening and Diels-Alder (D-A) reactions as the supporting frame. And 1-Octadecanethiol (ODT) and bio-based beeswax (BW) with phase change property were introduced into this supporting frame by grafting on the C=C of Magnolol via “thiol-ene” click reaction and physical blending, respectively. The D-A and rD-A reactions proceed at the temperature below 80 ℃ and above 120 °C, respectively, which provides recyclability of the crosslinked network. The ODT and beeswax provide phase change latent heat up to 119.1 J/g for the sample DGEM-18/FA/MA/1.1BW. Furthermore, taking DGEM-18/FA/MA/0.4BW as an example, its shape memory performance has been explored, as well as its potential applications in recyclable thermal management coatings. To the best of our knowledge, this is the first report on the preparation of bio-based recyclable FPCMs.

Interfacial and matrix healing of thermally reversible bismaleimide infused Graphene Nano Platelets reinforced polymer nanocomposite through Diels-Alder bonding

The interfacial and matrix healing capability have been studied in this work for thermally reversible self-healing furan maleimide Diels Alder (FMDA) based hybrid polymer composite. FMDA composite with various weight ratios (0.5, 1.0, and 1.5 wt%) of bismaleimide infused Graphene Nano Platelets (GNPBm) was fabricated and characterized. FMDA with varying epoxy, GNPBm wt% were considered for mechanical and morphological features. Amine functionalization was used to address GNPBm aggregation, precise interface management. Thermal stability of functionalized GNPs were revealed from TG-DTA data. FE-SEM images attributes homogenous distribution, bismaleimide moieties infusion on GNPs. Successful integration of DA and retro-DA bonds of FMDA composite was confirmed with TG-DSC through exo-endothermic peaks. TDCB specimens were fabricated and tested to evaluate FMDA's self-healing efficiency. An improvement in Breaking load, Tensile strength, Elastic modulus, and Self-healing efficiency was observed in 45FMDA(Gr-1.0), these being 30 %, 162 %, 42 %, and 90 % respectively, when compared to pristine sample.

Reversible Diels-Alder Addition to Fullerenes: A Study of Dimethylanthracene with H2@C60.

The study of isolated atoms or molecules inside a fullerene cavity provides a unique environment. It is likely to control the outer carbon cage and study the isolated species when molecules or atoms are trapped inside a fullerene. We report the Diels–Alder addition reaction of 9,10-dimethyl anthracene (DMA) to H2@C60 while 1H NMR spectroscopy is utilized to characterize the Diels–Alder reaction of the DMA with the fullerene. Through 1H NMR spectroscopy, a series of isomeric adducts are identified. The obtained peaks are sharp, precise, and straightforward. Moreover, in this paper, H2@C60 and its isomers are described for the first time.

Selective Laser Sintering 4D Printing of Dynamic Cross-linked Polyurethane Containing Diels–Alder Bonds

Selective laser sintering (SLS) four-dimensional (4D) printing of smart materials remains a big challenge and largely unexplored due to the lack of suitable materials. Here, we developed one kind of polyurethane containing reversible Diels–Alder bond (PUDA) covalent adaptable network (CAN) with self-healing and shape memory functions for SLS 4D printing. The dynamic mechanism of the Diels–Alder bonds was confirmed by 1H NMR and in situ FTIR. Furthermore, the kg-scale PUDA materials as well as their carbon nanotube (CNT) composite powders were prepared by the freeze-grinding method. The SLS printed PUDA materials show nearly isotropic mechanical properties due to the improved interlayer interaction by dynamic cross-linking. The SLS printed PUDA/CNT composites possess NIR-triggered self-healing and shape memory functions. This study provides a paradigm for designing dynamic polymers/composites for SLS 4D printing.

Combination of permanent hydrosilylation and reversible Diels–Alder reactions for self-healing poly(dimethylsiloxane) materials with enhanced ageing properties

Poly (dimethylsiloxane)s (PDMS) are widely used in space applications thanks to their transparency, thermal and UV resistance, inter alia. The prolonged exposure of these materials to the geostationary environment leads to the apparition of cracks and changes in their optical properties, this being detrimental to their function. To enhance their lifespan, self-healing PDMS based on a dual network featuring both permanent and reversible units thanks to hydrosilylation and Diels–Alder reactions were designed. The tunable chemical composition of the networks was characterized by proton Nuclear Magnetic Resonance spectroscopy (1H NMR) and Fourier-transformed infrared-attenuated total reflection (FTIR-ATR) measurements. The thermal and mechanical behaviors of pristine and healed materials were mainly studied by differential scanning calorimetry, dynamic mechanical analysis, and tensiometry. Ageing under proton irradiations was also performed, simulating part of the radiations encountered in geostationary environment. Ultraviolet-visible-near infrared analyses were carried out to compare the materials optical properties both before and after irradiations. Finally, chemical degradation mechanisms were studied by FTIR-ATR analyses and discussed.

Octagonal-siloxane based transparent and robust crack-healing surface for optical-scar recovery

Siloxane derivatives are a class of emerging hybrid materials that provide optical transparency and mechanically flexible characteristics owing to the low rotational barrier of the framework. To devise a robust and flexible self-healing platform, we designed an octagonal-siloxane derivative with four furan units and applied bifunctional maleimides as dienophiles to implement the reversible Diels–Alder (DA) reaction for crack healing. The feasibility of the DA reaction was confirmed by both the appearance of a cyclohexyl moiety in 1H-nuclear magnetic resonance spectra and heat transition behaviors during heating-cooling cycles in differential scanning calorimetry. The crack-healing efficiency of the designed octagonal-siloxane was sensitively affected by the heat-treatment temperature, and the crack recovery rate reached over 95% within 60 s upon heating at 100 °C. In addition, the crack recovery tendency was almost intact during repetitive heating-cooling cycles. The devised crack-healing film exhibited greater than 95% transparency in the entire visible region, and the effective modulus reached 7.8 GPa. In particular, when CdSe/ZnS quantum dots (QDs) were embedded into the octagonal-siloxane based film, it was confirmed that optical scars exhibiting luminescence defects were effectively recovered, and the emission characteristics of the QDs were not critically compromised even under solvent-exposed circumstances.

Limitations of Diels–Alder Dynamic Covalent Networks as Thermal Conductivity Switches

Creating polymers that exhibit a switchable thermal conductivity requires a fundamental understanding of how polymer structure and inter- and intramolecular interactions influence thermal transport. To develop this fundamental understanding, we synthesize a series of precursor un-cross-linked statistical copolymers with varying mole fractions of furan functional groups via reversible-addition fragmentation chain transfer (RAFT) polymerization. These precursor polymers, containing the furan diene, were then cross-linked using a reversible Diels–Alder cycloaddition to form dynamic covalent networks of varying cross-link densities. Furthermore, two separate dienophile cross-linkers were used to investigate how the molecular weight and length of the cross-linker influence the mechanical and thermal properties of the network. Counter to some recent theoretical studies, we found that both the elastic modulus and thermal conductivity were largely unaffected by the extent of cross-linking and the structure of the cross-linker. For this system, the Diels–Alder reaction was also found to be slow in the solid state, further limiting its utility as a thermal conductivity switch.

In Situ Visualization of Reversible Diels-Alder Reactions with Self-Reporting Aggregation-Induced Emission Luminogens.

The dynamic reversible Diels-Alder (DA) reactions play essential roles in both academic and applied fields. Currently, in situ visualization and direct monitoring of the formation and cleavage of covalent bonds in DA reactions are hampered by finite compatibility and expensive precise instruments, especially limited in solid reactions. We herein report a fluorescence system capable of in situ visualization by naked eyes and monitoring DA/retro-DA reactions. With the fluorescence quenching effect, the synthesized TPEMI could work as an innovative self-indicator for both DA termination and retro-DA occurrence. The fluorescence increases during DA reactions, and the mechanism is investigated to establish qualitative and quantitative relations. Besides rapid screening of reaction conditions and monitoring of DA exchange processes, the TPEMI fluorescence system can visualize heterogeneous and solid-state reactions with the AIE character. The TPEMI platform is expected to offer novel insights into reversible DA processes and dynamic covalent chemistry.

Thermally driven self‐healing PEDOT conductive films relying on reversible and multiple Diels–Alder interaction

AbstractThermally healing capability of cracks and defects is important and urgent for the safe operation and life extending of electric materials and devices. Here, by the combination of thermally driven reversible Diels–Alder (DA) interaction and in‐situ chemical oxidative polymerization of 3,4‐ethylenedioxythiophene (EDOT), a series of intrinsically conductive poly(3,4‐ethylenedioxythiophene) (PEDOT)/DA composites possess intrinsically self‐healing property under low‐temperature (reverse DA reaction at 100°C; DA crosslinking at 60°C) stimulus were achieved. The crosslinking DA bonding reactions are multiple from the co‐existence of pre‐synthesized macromolecular polyurethane attached DA units (PU‐DA) and 2,4‐hexadiyne‐1,6‐diol (DADOL) in the films. PU‐DA involved in the polymerization process of EDOT to endow PEDOT with outstanding solution‐processability, uniform film making, and structural self‐healing capability, while DADOL was added to enhance the cross bonding between polymer chains. This work will accelerate the research and application development of intrinsically self‐healing conducting polymers for commercial capacitors, antistatic coatings, implantable, printable electronics, and so on.

Electroactive Self-Healing Shape Memory Polymer Composites Based on Diels–Alder Chemistry

Both shape memory and self-healing polymers have received significant attention from the materials science community. The former, for their application as actuators, self-deployable structures, and medical devices; and the latter, for extending the lifetime of polymeric products. Both effects can be stimulated by heat, which makes resistive heating a practical approach to trigger these effects. Here we show a conductive polyketone polymer and carbon nanotube composite with cross-links based on the thermo-reversible furan/maleimide Diels–Alder chemistry. This approach resulted in products with efficient electroactive shape memory effect, shape reprogrammability, and self-healing. They exhibit electroactive shape memory behavior with recovery ratios of about 0.9; requiring less than a minute for shape recovery; electroactive self-healing behavior able to repair microcracks and almost fully recover their mechanical properties; requiring a voltage in the order of tens of volts for both shape memory and self-healing effects. To the best of our knowledge, this is the first report of electroactive self-healing shape memory polymer composites that use covalent reversible Diels–Alder linkages, which yield robust solvent-resistant polymer networks without jeopardizing their reprocessability. These responsive polymers may be ideal for soft robotics and actuators. They are also a step toward sustainable materials by allowing an increased lifetime of use and reprocessability.

Processable and recyclable polyurethane/HNTs@Fe3O4 solid–solid phase change materials with excellent thermal conductivity for thermal energy storage

AbstractThe permanently chemically cross‐linking solid–solid phase change materials (SSPCMs) were designed to solve the problem of leakage and poor shape stability during the whole process of phase transformation. However, these materials lead to environment pollution and resources waste because of the non‐recyclability. Therefore, a SSPCMs was fabricated using dynamic thermal reversible Diels–Alder bonds. The prepared SSPCMs exhibited excellent shape, heat storage stability, and reliability and reprocessed ability, resulting from dynamic Diels–Alder bonds. Moreover, the Halloysite nanotubes decorated with Fe3O4 nanoparticles (HNTs@Fe3O4) were used to enhance the thermal conductivity of SSPCMs. When the filler content was 0.5 wt%, the best integrated properties (phase change properties and thermal conductivity) of the SSPCMs were obtained, it had the highest melting and freezing phase change enthalpies about 116.8 and 127.2 J/g, and thermal conductivity value, 0.223 W/m K, which was 101% higher than pure SSPCMs. Hence, the prepared SSPCMs can be considered as promising save‐energy, friendly‐environment thermal management materials.

Polyhedral oligomeric silsesquioxane (POSS)-based epoxy nanocomposite involving a reversible Diels–Alder-type network as a self-healing material

Epoxy-polyhedral oligomeric silsesquioxane (POSS) nanocomposites were prepared by incorporating POSS nanofiller into the epoxy-amine network. The monofunctional epoxy-POSS monomer or nonfunctional octaphenyl POSS unit were applied to reinforce the epoxy network by producing the nanocomposite with chemically bound pendant POSS or physically admixed POSS. The combination of reactive and unreactive POSS, leads to a synergistic effect and to the most efficient reinforcement of the epoxy network. Both polymer-POSS and POSS–POSS interactions are present in the system and lead to a strong physical crosslinking through the path polymer-bound POSS-unbound POSS domain. The nanocomposite was modified by incorporation of the thermoreversible Diels–Alder (DA) type network. The produced interpenetrating network composed of the irreversible reinforced epoxy and the reversible DA networks exhibits an efficient self-healing capability. This strong nanocomposite, showing a modulus of 800 MPa at room temperature, was healed with an efficiency of 61%. The developed stiff nanocomposite is shown to be a promising material because self-healing of strong thermosets is still a big challenge. With the increasing content of POSS nanofiller and heterogeneity of the nanocomposite, however, the healing efficiency diminishes. The insufficient contact of crack surfaces due to involved inhomogeneities results in a worse sealing of the material crack.

Recyclable Epoxy Resin via Simultaneous Dual Permanent/Reversible Crosslinking Based on Diels–Alder Chemistry

AbstractTaking advantage of the reversible Diels–Alder (DA) reaction, a simple strategy to obtain recyclable epoxy resins is presented. For this purpose, blends of furan‐functionalized and nonfunctionalized epoxy resin are prepared. After the addition of diamine and bismaleimide, blends are heated at 150 °C for 5 min, where the permanent amine/epoxy reaction has taken place and upon cooling to room temperature the reversible DA reaction has happened, giving rise to a dual permanent/nonpermanent network. Both reactions are confirmed by Fourier‐transform infrared (FTIR) and13C‐crosspolarization, magic‐angle spinning (CP/MAS) nuclear magnetic resonance (NMR). Modulated differential scanning calorimetry (MDSC) shows that the epoxy/amine and bismaleimide/amine curing reaction take place, after the DA reaction, simultaneously with the retroDA reaction and before the bismaleimide homopolymerization. Therefore, under the appropriate curing conditions, the Michael's addition and the bismaleimide homopolymerization do not avoid the formation of a hybrid network, as stated in other reports. The reversibility of the DA reaction in three consecutive cycles is confirmed by DSC. Finally, the dual‐cured sample is reprocessed three times without significant loss of mechanical properties.

Thermally Remendable Polyurethane Network Cross-Linked via Reversible Diels-Alder Reaction.

We prepared a series of thermally remendable and recyclable polyurethanes crosslinked via reversible furan-maleimide Diels–Alder reaction based on TDI end-caped branched Voranol 3138 terminated with difurfurylamine and 4,4′-bis(maleimido)diphenylmethane (BMI). We showed that Young modulus strongly depends on BMI content (from 8 to 250 MPa) that allows us to obtain materials of different elasticity as simple as varying BMI content. The ability of DA and retro-DA reactions between furan and maleimide to reversibly bind material components was investigated by NMR spectroscopy, differential scanning calorimetry, and recycle testing. All polymers obtained demonstrated high strengths and could be recovering without significant loss in mechanical properties for at least five reprocessing cycles.

Self‐healing interface of carbon fiber reinforced composites using a thermally reversible sizing agent

AbstractA thermally reversible sizing agent was prepared by directly mixing the furfurylamine‐modified epoxy resin (FAE) and the maleimide‐functionalized Jeffamine T403 (MAT), and it was used to impart self‐healing ability at interface based on the reversible Diels–Alder reaction for potential application in carbon fiber composites. The surface characteristics of FAE/MAT‐modified carbon fibers were investigated using scanning electron microscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and dynamic contact angle meter and tensiometer. The initial interfacial shear strength of modified carbon fiber composites and the multiple healings of the interface were investigated via a microbond test. The results demonstrated that the healing efficiency of the continuous and uniform FAE–MAT sizing layer functionalized carbon fiber composites was up to 93.8% for the first healing cycle. Furthermore, it is expected that the interface self‐healing of carbon fiber reinforced composites can be achieved from research laboratory to industrial applications by this strategy.