Epoxy Vitrimers Research Articles

An investigation on the performance of epoxy vitrimers based on disulfide bond

AbstractThe research for self‐healing epoxy vitrimers has attracted a lot of attention over the years. However, due to the special structure of the epoxy molecular chain, which limits its movement ability, epoxy vitrimers usually need to be healed under high temperature and pressure. So it is relatively difficult to healing. Here, a new type of cross‐linked network is obtained by cross‐linking hydrogenated bisphenol A epoxy resin and polypropylene glycol diglycidyl ether (PPGDGE) as hard and soft segments with an amine curing agent containing disulfide bonds. The addition of PPGDGE can improve the mobility of the molecular chains in the cross‐linked network we designed, thereby obtaining a novel epoxy vitrimers with better healing properties. The effect of PPGDGE content on the mechanical properties and self‐healing properties of the material was studied. The ideal sample HEP‐3 has a good tensile strength (33.4 MPa) and a completely broken sample can achieve a good self‐healing rate (71.5%) at 100°C for 3 h without high pressure. In addition, the ideal sample also has excellent shape memory properties (shape memory fixity ratio Rf = 97.1%, shape recovery ratio Rr = 94.9%). Moreover, due to the existence of the dynamic disulfide bond introduced by the curing agent 2‐aminophenyl disulfide, compared with traditional thermosetting epoxy resins, epoxy vitrimers prepared by us can be reshaped or postprocessed like thermoplastic polymers under specific conditions. These characteristics will become the guarantee for expanding the application range of epoxy materials.

Polyaddition Synthesis Using Alkyne Esters for the Design of Vinylogous Urethane Vitrimers

Vitrimers are a subclass of covalent adaptable networks which introduce reshapeability and recyclability in thermoset materials while maintaining a high degree of chemical resistance and dimensional stability. Vitrimer materials based on vinylogous urethane (VU) chemistry have drawn a lot of attention in this area. Classically, these are obtained by the polycondensation polymerization of acetoacetate and amine monomers. Unfortunately, this also releases water, often leading to porosity defects in the initially obtained non-reprocessed cross-linked materials. Here, we demonstrate that alkyne esters (AE) can be used as alternative building blocks for VU vitrimers by a polyaddition polymerization with amines, leading to water-free formulations and straightforward access to defect-free cured VU vitrimer materials. The bond formation and dynamic bond exchange was also studied by small molecule reactions, further rationalized by a computational (DFT) approach. The resulting water-free VU vitrimers display similar material properties compared to vitrimers based on acetoacetates, although also some differences are seen, which can be related to a minor amide-bond forming side reaction. Furthermore, by use of this novel AE approach, polyaddition curing of VU epoxy vitrimers can easily be prepared in a one-pot three-component method, combining AE, amine, and epoxy monomers. This study shows that AE monomers can be used as an easy drop-in method to obtain processable epoxy materials with tunable viscoelastic properties, where the viscous flow behavior can in principle be fully tuned by varying the monomers’ ratios and compositions.

Carbon fiber/epoxy vitrimer composite patch cured with bio‐based curing agents for one‐step repair metallic sheet and its recyclability

AbstractVitrimers have gained a great deal of attention from researchers, yet research on its application is still lacking. This study, a novel bio‐based vitrimer was developed from epoxy (EP) and bio‐based curing agents, that is, cashew nut shell liquid (CNSL) and citric acid (CA), and then reinforced by carbon fiber. The vitrimers with different ratios of acid to epoxy (R ratio) at 0.30–0.40 contained ester and ether linkages. All EP/CA/CNSL vitrimers showed the stress relaxation over 70–100°C due to transesterification. The vitrimers were applied as polymer matrices for the carbon fiber composites and then used as repair patches. By using the carbon fiber‐reinforced vitrimer with the R ratio of 0.30, patch repair on a damaged alloy sheet revealed that approximately 98% of the tensile strength of the damaged alloy sheet was recovered. The vitrimer can be dissolved from carbon fiber composite to recover carbon fiber. The recovered carbon fiber retained good tensile strength compared to the pristine composite. Based on this study, the EP/CA/CNSL vitrimers showed the comparable thermomechanical properties with the epoxy vitrimer cured by the petroleum‐based curing agent. The vitrimer composite patch could therefore be an alternative new repair method to extend the service life of damaged structures.

Enhancing toughness, healing and reprocessability of sustainable epoxy vitrimer composites by PEG-assisted regenerated cellulose

Sustainable and high-performance thermoset composites have recently drawn extensive attention in industry and academia. Herein, we report an epoxy vitrimer composite with excellent recyclability and mechanical properties reinforced with regenerated cellulose (RC). The RC filler was prepared from natural softwood pulp and integrated into the composites based on a PEG-assisted strategy. The innovative introduction of polyethylene glycol (PEG) effectively realizes the refinement and dispersion of filler. Compared with RC (labeled as RC-SP) prepared by direct precipitation, the average particle size of RC (labeled as RC-P) obtained by co-precipitation with PEG as a template decreases from 66.9 μm to 12.6 μm. An appropriate amount of RC-P significantly improves the mechanical properties of epoxy vitrimer composites in terms of tensile strength, flexural strength, and impact strength, indicating that the toughness has been well improved. In addition, the synergistic addition of PEG and RC-P also endows epoxy composites with improved scratch healing and reprocessability. The reason may be that the abundant hydroxyls of RC-P provide active sites for interface exchange in the epoxy-anhydride system based on transesterification. The effective application of regenerated polysaccharides in vitrimer provides new opportunities for the industrial application of sustainable vitrimer composites.

Degradable bio-based epoxy vitrimers based on imine chemistry and their application in recyclable carbon fiber composites

Degradable bio-based epoxy vitrimers based on imine chemistry and their application in recyclable carbon fiber composites

Self-healing Ability of Epoxy Vitrimer Nanocomposites Containing Bio-Based Curing Agents and Carbon Nanotubes for Corrosion Protection

Epoxy is extensively used for anti-corrosion coatings on metallic materials. Conventional epoxy coatings have a permanent crosslink network that is unable to repair itself when cracks and damages occur on the coating layer. This study aims to develop self-healing epoxy vitrimer/carbon nanotube (CNTs) nanocomposite for coating. Two bio-based curing agents viz., cashew nut shell liquid (CNSL) and citric acid (CA) were employed to create covalent adaptable networks. The 0–0.5 wt% CNTs were also incorporated into epoxy/CNSL/CA matrix (V-CNT0-0.5). Based on the results of our study, thermomechanical properties of V-CNT nanocomposites increased with increasing CNTs content. The bond exchange reaction of esterification was thermally activated by near infrared (NIR) light. The V-CNT0.5 showed the highest self-healing efficiency in Shore D hardness of 97.34%. The corrosion resistance of coated steel with V-CNT0 and V-CNT0.5 were observed after immersing the samples in 3.5 wt% NaCl for 7 days. The corrosion rate of coated steel with V-CNT0.5 decreased from 9.53 × 102 MPY to 3.12 × 10–5 MPY whereas an increase in protection efficiency of 99.99% was observed. By taking advantages of the superior self-healing and anti-corrosion properties, V-CNT0.5 could prove to be a desirable organic anti-corrosion coating material.

Sugarcane Bagasse-Derived Activated Carbon- (AC-) Epoxy Vitrimer Biocomposite: Thermomechanical and Self-Healing Performance

Vitrimeric materials have emerged as fascinating and sustainable materials owing to their malleability, reprocessability, and recyclability. Sustainable vitrimeric materials can be prepared by reinforcing polymeric matrix with bioderived fillers. In the current work, a sustainable vitrimer is prepared by incorporating biomass-derived activated carbon (AC) filler into the epoxy matrix to achieve enhanced thermal and mechanical properties. Thus, prepared biocomposite vitrimers demonstrate a lower-temperature self-healing (70°C for 5 min) via disulfide exchanges, compared to the pristine epoxy vitrimers (80°C for 5 min). Significantly, the self-healing performances have been studied extensively with the flexural studies; and changes in material healing efficiency have been demonstrated based on the observed changes in modulus.

Sustainable alternative for bisphenol A epoxy resin high-performance and recyclable lignin-based epoxy vitrimers

Epoxy vitrimers based on dynamic covalent bond chemistry have been developed in recent years as they can solve the problem of recycling and reuse for traditional thermosetting materials. Lignin as an aromatic raw material has recently used to partially replace commercial diglycidyl ether bisphenol A (DGEBA) in the epoxy vitrimers, however, it is difficult to achieve high lignin replacement rate, excellent mechanical property and reprocessability in a facile method at the same time. In this work, high mechanical strength and lignin replacement rate of lignin-based vitrimers were successfully synthesized by gradually replacing DGEBA with glycidyl ether of lignin. The mechanical properties of the lignin-based epoxy vitrimers can be tuned by different ratios of glycidyl ether of lignin and DGEBA, the Young’s modulus and tensile strength of those vitrimers can reach up to 2.00 GPa and 46.8 MPa, respectively. It's worth noting that the tensile strength and elongation at break were 39.5 MPa and 5.81 % when the lignin replacement rate was 100 %. All the lignin-based epoxy vitrimers exhibit excellent self-healing, reprocessing and shape memory property. The crack widths of the vitrimers were repaired over 80 % in just 5 min at 160 ℃, the tensile strength can reach up to 97 % healing efficiency after being hot reprocessed. The great properties of lignin-based epoxy vitrimers indicate that they can be used as renewable alternatives for bisphenol A epoxy vitrimers.

Shape memory epoxy vitrimers based on waste frying sunflower oil

AbstractThermoset polyesters are prepared from epoxidized waste frying sunflower oil (ESO), commercially available epoxy compounds and glutaric acid. Influence of the nature and concentration of bi‐ and trifunctionnal epoxy compounds on mechanical properties is studied. Static and dynamic mechanical tests are performed. The molar amount of commercial epoxy compounds used ranges from 20% to 80% regarding the molar amount of ESO. It enables to obtain thermosetting polyesters with glass transition temperatures ranging from 6°C to 102°C, as well as storage modulus ranging from 8 GPa to 14 GPa. 40% of trifunctional epoxy compound and 60% of commercial epoxy compound are found to be the best compromises between bio‐based content and mechanical properties. Furthermore, shape memory and vitrimer behavior of those epoxy‐acid based networks with 60% of commercial epoxy compounds are evaluated both qualitatively and quantitatively by cyclic thermo‐mechanical and stress relaxation tests. Excellent shape memory behavior with fixity ratios above 94% and recovery ratios above 98% is demonstrated. A transesterification catalyst is needed to obtain good vitrimer behavior. Overall, thanks to the previously mentioned properties of those partially biobased thermosets polyesters, industrial applications such as protective coatings, foams and temperature‐memory polymer actuators might be considered.

Polyurethane modified epoxy vitrimer and its stress relaxation behavior

Abstract Anhydride cured epoxy vitrimers usually exhibit desired mechanical strength but poor toughness and slow transesterification rate. Therefore, the repairing property of the material was restricted. In this paper, polyurethane modified epoxy vitrimer (PU-Epv) was prepared. PU was introduced into the vitrimer system of tetrahydrophthalic anhydride cured epoxy to improve the toughness of the material. Meanwhile, because of the presence of amino ester, the transesterification reaction was promoted and the activation energy of the transesterification was only 33.59 kJ/mol. In the thermal welding experiment, the material could be welded at least five times, and scratches on the surface of the samples could be efficiently repaired within 30 min. The toughness of the material was improved without damaging the strength. Meanwhile, the hard thermosetting epoxy was endowed with excellent repairing properties to increase the service life of the material.

Biobased High-Performance Epoxy Vitrimer with UV Shielding for Recyclable Carbon Fiber Reinforced Composites

A novel biobased epoxy vitrimer (Gte-VA) with desirable mechanical properties was synthesized from glycerol triglycidyl ether (Gte) and an imine-containing hardener (VA), which was also a biobased ...

Vitrimer chemistry enables epoxy nanocomposites with mechanical robustness and integrated conductive segregated structure for high performance electromagnetic interference shielding

Constructing segregated pathways in the conductive polymer composites (CPCs) is efficient to achieve satisfactory electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) with low content of nanofillers. Conventionally, most of segregated CPCs were based on thermoplastics and precisely controlled processing conditions were essential to achieve structural integrity and mechanical robustness. Herein, by taking the advantage of the associative dynamic bonding rearrangement in the epoxy vitrimer, we fabricated segregated MWCNTs/epoxy composites in a wide range of compression temperature and pressure. The segregated CPCs show a low conductive percolation threshold at 0.066 wt% and an EMI SE of 22 dB when only 2 wt% of MWCNTs were loaded. More promisingly, the mechanical robustness was achieved by the volatile ethylene glycol (EG) involved decomposition and repolymerization of β-hydroxyl ester at the interface even when numerous MWCNTs were in between. The segregated structure and macroscopic properties can be reserved after reprocessing and the nanofillers can be recovered with excessive EG at elevated temperature.

The effect of matrix on shape properties of aromatic disulfide based epoxy vitrimers

Aromatic disulfide based vitrimers show elasticity driven shape-memory and plastic reprocessability via associative rearrangement of dynamic covalent crosslinks. Those processess represent the two sides of a coin: the storage and relaxation of the strain energy caused by a deformation load. The key temperatures that trigger the underlying mechanisms, i.e. phase transition and disulfide exchange reaction, are extremely sensitive to the molecular structure of the polymer and under certain condition overlap. To gain insight on the relationship between the structure, dynamic and shape-changing properties, five aromatic disulfide-based epoxy networks with a range of Tg values (32–142 °C), molecular structure and crosslink densities (2252–462 mol m−3) were synthesized. The epoxy matrices were formulated combining different ratios of rigid bisphenol A diglycidyl ether (DGEBA) and flexible poly(propylene glycol) diglycidic ether (DGEPPG) epoxy monomers crosslinked by 4-aminophenyldisulfide hardener.

Dynamic Crosslinking: An Efficient Approach to Fabricate Epoxy Vitrimer.

Epoxy vitrimers with reprocessability, recyclability, and a self-healing performance have attracted increasingly attention, but are usually fabricated through static curing procedures with a low production efficiency. Herein, we report a new approach to fabricate an epoxy vitrimer by dynamic crosslinking in a torque rheometer, using diglycidyl ether of bisphenol A and sebacic acid as the epoxy resin and curing agent, respectively, in the presence of zinc acetylacetonate as the transesterification catalyst. The optimal condition for fabricating the epoxy vitrimer (EVD) was dynamic crosslinking at 180 °C for ~11 min. A control epoxy vitrimer (EVS) was prepared by static curing at 180 °C for ~11 min. The structure, properties, and stress relaxation of the EVD and EVS were comparatively investigated in detail. The EVS did not cure completely during static curing, as evidenced by the continuously increasing gel fraction when subjected to compression molding. The gel fraction of the EVD did not change with compression molding at the same condition. The physical, mechanical, and stress relaxation properties of the EVD prepared by dynamic crosslinking were comparable to those of the EVS fabricated by static curing, despite small differences in the specific property parameters. This study demonstrated that dynamic crosslinking provides a new technique to efficiently fabricate an epoxy vitrimer.

Transparent wood developed by introducing epoxy vitrimers into a delignified wood template

Transparent wood (TW) has shown great potential in various functional applications, including light, thermal, electromagnetic, and energy management fields. Despite significant progress in functional TW, programmable shape-memory TW (PSMTW) has not yet been reported. Epoxy vitrimers with exchangeable dynamic covalent crosslinking networks exhibit excellent stimuli-responsive, shape-memory, reprocessing, and self-healing properties, which are suitable for fabricating smart TW. In this study, a TW with programmable shape-memory ability was developed by introducing transparent, refractive index-matching, and intelligent epoxy-based vitrimers into delignified wood. The transmittance and haze of the resultant TW with a thickness of 2 mm were approximately 60% and 95%, respectively. Vitrimers with dynamic covalent crosslinking networks perfectly combine the glass transition temperature (Tg)-inducted phase change and topology freezing transition temperature (Tv)-inducted rearrangement under thermal stimulus, which endow TW with excellent programmable shape-memory properties, such as shape-recovery, shape-programming, shape-erasing, and re-programming. The programmable shape-memory properties of TW have not been investigated thus far, and development of PSMTW would be a multi-functionalized and efficient application of TW-based materials.

Mechanically Strong, Thermally Healable, and Recyclable Epoxy Vitrimers Enabled by ZnAl-Layer Double Hydroxides

To meet the demand of sustainable development, epoxy vitrimers based on the dynamic transesterification reaction (DTER) have received considerable attention recently due to their reprocessability a...

Imidazolium-catalyzed dynamic ester cross-links towards reprocessable epoxy vitrimers

Ionic liquids (ILs), in particular imidazolium salts, have raised huge interest as organocatalysts in recent years but have never been considered as activators in associative covalently adaptable networks, also referred to as vitrimers. In this work, we described the first imidazolium-catalyzed vitrimers. The latter consist in epoxy-acid resins that can be reshaped and recycled through catalyzed associative transesterification reactions. A series of cross-linked epoxy-acid networks were produced in the presence of different imidazolium-acetate salts prepared by the straightforward Radziszewski multicomponent reaction. The mechanical properties and dynamics of the resulting materials were studied. Stress-relaxation experiments carried out on resins formulated with few percent of imidazolium salts highlighted the ability of the latter to promote transesterification reactions within the networks and to confer them with vitrimers’ features. These imidazolium-catalyzed epoxy vitrimers were also successfully recycled several times while preserving their excellent mechanical properties. Overall, this work demonstrates the potential of ILs as catalysts in the field of adaptable networks and, given the large variety of ILs and of the IL-catalyzed reactions, this approach is likely to broaden the scope of vitrimers in the future.

Catalyst-free malleable, degradable, bio-based epoxy thermosets and its application in recyclable carbon fiber composites

Carbon fiber reinforced composites (CFRCs) are a kind of most potential materials due to the high strength-to-weight ratio, excellent fatigue and corrosion resistance. However, the vast majority of CFRCs prepared from carbon fiber and thermosetting matrices are difficult to be recycled and depend on limited fossil resources. Here, for the first time, we reported fully bio-based catalyst-free epoxy vitrimers for CFRCs. The epoxy vitrimers exhibited excellent malleability and could be reprocessed by compression molding. Outstanding degradability was also achieved, which enabled multiple recovery of CF from their CFRCs without damage. In addition, CFRCs regenerated from multiple recovered CF exhibited insignificant differences from the original CFRCs. This work will provide an effective method for the sound development of CFRCs from both economic and environmental aspects.

Cellulose Nanocrystals: Accelerator and Reinforcing Filler for Epoxy Vitrimerization.

The novel vitrimerization concept of converting permanently cross-linked networks of thermoset polymers into dynamic exchangeable networks often relies on transesterification reactions. Transesterification exchange reactions, for example, in epoxy vitrimers, are usually contingent on equivalent ratios of hydroxyl to ester groups and large amounts of catalysts to achieve proper dynamic exchange capability. In general, postconsumer epoxy cured with anhydrides contains very few hydroxyl groups in the network, and it is challenging to convert it into efficient dynamic networks by vitrimerization. Here, we demonstrate that introducing cellulose nanocrystals as feedstock of external hydroxyl groups in the mechanochemical vitrimerization process could improve the exchange reaction rate as well as the thermomechanical properties of the vitrimerized epoxy. This work offers an effective way to recycle and reprocess postconsumer epoxy/anhydride waste with inherent low ratios of hydroxyl to ester groups into higher value-added vitrimer nanocomposites.

A Triple Crosslinking Design toward Epoxy Vitrimers and Carbon Fiber Composites of High Performance and Multi-shape Memory

It remains a challenge to use a simple approach to fabricate a multi-shape memory material with high mechanical performances. Here, we report a triple crosslinking design to construct a multi-shape memory epoxy vitrimer (MSMEV), which exhibits high mechanical properties, multi-shape memory property and malleability. The triple crosslinking network is formed by reacting diglycidyl ether of bisphenol F (DGEBF) with 4-aminophenyl disulfide, γ-aminopropyltriethoxysilane (APTS) and poly(propylene glycol) bis(2-aminopropyl ether) (D2000). The triple crosslinking manifests triple functions: the disulfide bonds and the silyl ether linkages enable malleability of the epoxy network; the silyl ether linkages impart the network with high heterogeneity and broaden the glass transition region, leading to multi-shape memory property; a small amount of D2000 increases the modulus difference between the glassy and rubbery states, thereby improving the shape fixity ratio. Meanwhile, the high crosslinking density and rigid structure provide the MSMEV with high tensile strength and Young’s modulus. Moreover, integrating carbon fibers and MSMEV results in shape memory composites. The superior mechanical properties of the composites and the recyclability of carbon fiber derived from the dissolvability of MSMEV make the composites hold great promise as structural materials in varied applications.