Epoxy Vitrimers Research Articles

An Effective Approach to Improve the Thermal Conductivity, Strength, and Stress Relaxation of Carbon Nanotubes/Epoxy Composites Based on Vitrimer Chemistry

An effective method was developed to improve the interfacial interaction between Mutiwalled carbon nanotubes (MWCNTs) and epoxy matrix. The performance of thermal conductivity and strength of the epoxy vitrimer were enhanced by polydopamine (PDA) coating. Polydopamine is a commonly used photothermal agent, which of course, was effective in modifying MWCNTs used in photoresponsive epoxy resin. The surface temperature of the epoxy composite with 3% MWCNTs@PDA fillers added increased from room temperature to 215 °C in 48 s. The metal–catechol coordination interactions formed between the catechol groups of PDA and Zn2+ accelerated the stress relaxation of epoxy vitrimer. Moreover, the shape memory, repairing, and recycling of epoxy vitrimer were investigated. Therefore, dopamine coating is a multifunctional approach to enhance the performance of epoxy vitrimer.

Interlaminar fracture of structural fibre/epoxy composites integrating damage sensing and healing

Technologies to control and reduce delamination damage in fibre reinforced polymer composites involve the use of fibre optic sensors and self-healing polymers, respectively. While the former technology can monitor the evolution of damage by measuring microscopic variations in the material's strain field, the latter can repair delamination damage by recreating broken interfaces. To evaluate the combination of these technologies, a high-performance epoxy vitrimer was used as the matrix phase of a fibre-glass composites incorporating Fibre Bragg Grating (FBG) sensors. Different laminates, manufactured with varying ratios a dynamic crosslinker and epoxy prepolymer, were compared. The analysis involved thermal characterization, mechanical tests to measure interlaminar fracture toughness, and microscope imaging of the cracked and repaired surfaces. Furthermore, FBG sensors to monitor in-situ damage and healing processes of the composites were employed. It was demonstrated that laminates with a larger proportion of dynamic crosslinker exhibit better repair abilities but lower thermal properties. The average and maximum recorded healing efficiency at the first healing cycle, calculated based on the critical fracture toughness, were at 89% and 95% respectively, while it averaged a steady 50% efficiency after several repairing cycles.

Aero Grade Epoxy Vitrimer towards Commercialization.

Traditional crosslinked aero grade epoxy resins have excellent thermal-mechanical properties and solvent resistance, but they cannot be remolded, recycled, or repaired. Vitrimers can be topologically rearranged via an associative exchange mechanism, endowing them with thermoplasticity. Introducing dynamic bonds into crosslinked networks to obtain more sustainable thermosets is currently an interesting research topic. While recent research into vitrimers has indicated many advantages over traditional thermosets, an important shortcoming has been identified: susceptibility to creep at service temperature due to the dynamic bonds present in the network. In addition, designing aero grade epoxy vitrimers (similar to RTM6 resin) still remains a challenge. Herein, low creep aero grade epoxy vitrimer with thermal and mechanical properties similar to those of aero grade epoxy resins and with the ability to be recyclable, repairable, and reprocessable, has been prepared. In this manuscript, we demonstrate that aero grade epoxy vitrimer with reduced creep can be easily designed by the introduction of a certain fraction of permanent crosslinks, without having a negative effect on the stress relaxation of the material. Subsequently, the mechanical and relaxation properties were investigated and compared with those of classical aero grade epoxy resin. A high Tg (175 °C) epoxy vitrimer was obtained which fulfilled all mechanical and thermal specifications of the aero sector. This work provides a simple network design to obtain aero grade epoxy resins with excellent creep resistance at elevated temperatures while being sustainable.

Aromatic disulfide epoxy vitrimer packaged electronic devices: Nondestructive healing and recycling

Electrical and electronic equipment (EEE) that greatly enhances global living standards has become indispensable in modern societies. In EEE, most electronic devices are protected by epoxy electronic packaging that exhibits outstanding mechanical property, high electrical resistivity and excellent solvent resistance. However, due to thermolability of electronic components and insolubility of epoxy, it is impossible to heal and recycle the packaged electronic devices non-destructively, resulting in a huge amount of e-waste during manufacturing and consumption. Herein, aromatic disulfide epoxy vitrimer packaged electronic devices were proposed, in which nondestructive healing and recycling were achieved. The epoxy vitrimer was prepared from aromatic disulfide-based epoxy and aliphatic amines at moderate temperature (≤100 °C), resulting in highly cross-linked networks, excellent solvent resistance and good thermal stability. The tensile strength was ∼71.9 MPa and the glass transition temperature was ∼115 °C. Moreover, it exhibited comparable electrical resistivity to reference epoxy, while lower dielectric constant was observed. In epoxy vitrimer packaged LED light, cracks could be readily healed at 150 °C through aromatic disulfide metathesis in networks, while the epoxy vitrimer exhibited topology freezing transition at ∼118 °C. Besides, relying on reactions between aromatic disulfide bonds and outer thiol groups, the epoxy vitrimer packaged LED light, printed circuit board and integrated circuit could be non-destructively recycled, while the epoxy vitrimer was degradable on demand. The aromatic disulfide epoxy vitrimer packaged electronic devices with nondestructive healing and recycling properties provide a better way to deal with substandard electric products and retired ones, leading to e-waste reducing, resource recycling and environment-friendly development in EEE.

Tung Oil-Derived Epoxy Vitrimers with High Mechanical Strength, Toughness, and Excellent Recyclability

Tung Oil-Derived Epoxy Vitrimers with High Mechanical Strength, Toughness, and Excellent Recyclability

Insight into the structure-property relationships of intramolecularly-catalyzed epoxy vitrimers

Vitrimers are emerging dynamic polymers with exchangeable associated networks that can potentially be used as substitutes for conventional thermosetting polymers, whose healing and recycling are difficult. However, vitrimers have limited engineering applications due to their process complexity, the addition of extra catalysts, and the unclear factors that affect network rearrangement. Herein, epoxy vitrimers with intramolecularly-catalyzed transesterification were prepared by curing an ester-containing epoxy resin with diethylenetriamine and monoamines. The obtained diethylenetriamine-cured vitrimer had favorable tensile stress and modulus (60.86 MPa and 0.94 GPa). The intramolecularly-catalyzed vitrimers exhibited fast stress relaxation, and the relaxation time of monoamine-containing vitrimers was reduced to 253 s at 180 °C. The effect of structure variation on the vitrimers’ properties was investigated by controlling the influencingparameters. The molality of the tertiary amine and β-hydroxy esters were directly proportional to faster stress relaxation. A higher crosslink density led to better mechanical properties and a higher glass transition temperature but retarded the stress relaxation. Side-chains with low steric hindrance accelerated the stress relaxation. Repair and remolding were more efficient in the vitrimers with lower stiffness. This study contributes to the design and practical applications of various epoxy vitrimers according to the desired final properties.

High-performance epoxy vitrimer with superior self-healing, shape-memory, flame retardancy, and antibacterial properties based on multifunctional curing agent

The lack of functionalities such as degradability, antibacterial, and flame retardancy is the key to limit the further wide application of conventional epoxy thermosets. In this work, a novel bio-based multifunctional amine curing agent (named HVPA), was successfully synthesized via the Schiff base reaction between six-vanillin functionalized cyclophosphazene and N-methylethylenediamine. And an epoxy vitrimer (HVPA/GDE) was fabricated by utilizing HVPA to cure the bio-based epoxy of glycerol diglycidyl ether (GDE). Ascribing to the even distribution of dynamic imine bond in each crosslinking linkage, this synthetic HVPA/GDE vitrimer presented excellent comprehensive properties including fast self-healing (8 min for 100%, without any pressure), excellent shape memory, easy reprocessability (1.5 MPa, 3 min, 100 °C), mild degradation, and superior bacterial resistance to both Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus). Moreover, due to the high content of N/P elements and benzene groups within the network structure, this HVPA/GDE not only exhibited excellent flame retardancy but also possessed good UV resistance and transparency, while maintaining comparable mechanical strength compared to the control group of commercial amine curing agent 4,4-Diaminodiphenyl methane (DDM) cured epoxy resin (DDM/GDE). Therefore, this work provides a facile strategy for fabricating multi-functional and high-performance bio-based epoxy resins.

Facile Surface Depolymerization Promotes the Welding of Hard Epoxy Vitrimer.

Welding via bond exchange reactions has provided advances in obtaining high-quality joining performance. However, the reported welding method requires a relatively high press force, and challenges are still encountered in welding hard vitrimer. In this work, a facile surface depolymerization strategy was introduced to weld high-performance epoxy vitrimer. The vitrimers were firstly dissolved into ethylene glycol for depolymerization based on the solvent-assisted bond exchange reactions. Then, the depolymerized vitrimers were welded under heat and press force. The effect of the depolymerizing time, welding pressure, welding temperature and welding time on the welding strength were further investigated. It was found that there were optimal values for the depolymerizing time, welding pressure, and welding temperature, respectively, for the welding strength, while the welding strength increased with increasing welding time. Through facile surface degradation, the welding pressure was highly reduced, while the welding strength was increased. With surface depolymerization, the welding strength was 1.55-times higher, but the magnitude of press force was 1/1000-times than that with no surface depolymerization. It is elucidative that surface depolymerization can be used to weld hard vitrimer composites alongside reducing the press force effectively.

Thermal-induced self-healing bio-based vitrimers: Shape memory, recyclability, degradation, and intrinsic flame retardancy

The structure of epoxy resin determines its flammable property, non-reprocessing, unrecyclable, and nondegradable, which limits their application. Thus, a novel disulfide bond-containing dynamic covalent thiol-ene crosslinked epoxy vitrimers (TECEVs) as covalently adaptable networks were synthesized. Also, the structure-property relationship of polymeric compounds was investigated. The insoluble vitrimers networks demonstrated faster stress relaxation with 107 s at 200 °C, and obtained lower activation energy between 70.52 and 110.57 kJ·mol−1. In addition, the dynamic nature of the disulfide bond allowed the damaged TECEVs to be thermal-induced self-repairing and self-healing materials with efficiencies 100% at 160 °C. It was discovered that TECEVs can be reprocessed at 160 °C with the recovery of mechanical strength above 80% and be completely decomposed in an ethanol-acid catalyst solution. Moreover, typical TECEVs can function with excellent flame retardancy and illustrated a high limit oxygen index (LOI, 27.45–28.36%), and UL-94 V-0 rating was achieved. This work could provide an efficient strategy for the synthesis of “biobased self-repairing flame retardant” networks and contribute to the development of high-performance sustainable polymers.

Dynamic crosslinking towards well-dispersed cellulose nanofiber reinforced epoxy vitrimer composites

Cellulose nanofiber (CNF) shows great potential in polymer composites but is difficult to disperse in crosslinking polymers because it tends to aggregate during static curing. Herein, we report a new approach to fabricate well-dispersed CNF reinforced epoxy composites through dynamic crosslinking combined with vitrimer chemistry. In this approach, CNF is dispersed in epoxy vtrimer by mixing with epoxy monomer, curing agent, and catalyst and subsequent dynamic crosslinking of the raw materials. The epoxy vitrimer/CNF composite fabricated by dynamic crosslinking showed better CNF dispersion compared to the counterpart prepared by static curing. No obvious aggregation of CNF occurred with the loading ≤0.75% through dynamic crosslinking. The tensile strength and Young's modulus of the epoxy vitrimer were enhanced by 2.26 and 3.61 times respectively by incorporation of 0.75 wt% CNF. We believe this facile approach could be extended to fabricate other vitrimer composites.

Cost-efficient and recyclable epoxy vitrimer composite with low initial viscosity based on exchangeable disulfide crosslinks

Epoxy (EP) vitrimer with excellent processing and recyclability is of great potential for industrial application. In this study, we prepared an EP vitrimer based on the low viscosity homogenous mixture of diglycidyl ether of bisphenol A (DGEBA) and a liquid hardener dimethyl 3,3′-dithiodipropionate (DTDP) catalyzed by 2,4,6-tris (dimethylaminomethyl) phenol (DMP30). The optimal curing condition was determined to be 110 °C × 12 h for the original EP vitrimer. Owing to the exchangeable disulfide crosslinks, rapid stress relaxation was found for the vitrimer with a relaxation time of 1.9 s at 160 °C. Moreover, the prepared EP vitrimer possessed excellent self-healing ability and recyclability, with recovery ratio of at least 94.1% during 3 times recycling. The self-healing ability, recyclability and processing performance remain excellent for the vitrimer even after incorporating 30 wt% boron nitride (BN), which can be recycled using DMF as a solvent.

Bio-based epoxy vitrimer for recyclable and carbon fiber reinforced materials: Synthesis and structure-property relationship

Recycling or degradation of epoxy resins is difficult because of their covalent cross-linking structures, which causes resource crisis, environmental pollution, and resource waste. To address this challenge, we reported a recyclable and degradable epoxy vitrimer with dynamic ester bonds prepared from tung oil and diglycidyl ether of bisphenol-A via epoxy-acid-anhydride crosslinking system. Owing to the compact rigid six-member ring and the anhydride structure, the epoxy vitrimer shows high thermal stability, and excellent dynamic mechanical, and mechanical properties with tensile strength of 59.21 ± 1.02 MPa and glass transition temperature of 76.73 °C. The vitrimer network can achieve topological rearrangement via dynamic transesterification reactions with the addition of a Zn catalyst. Thus, the contrivable vitrimer exhibits excellent self-healing, shape memory, and physical recycling at elevated temperatures. Moreover, the epoxy vitrimer matrix has high joining strength with carbon fibers and can be used to prepare carbon-fiber-reinforced materials (CFRMs). The carbon fibers can be completely recycled from CFRMs through an ethanol-NaOH mixture solvent. This work can assist in preparation of degradable high-strength CFRMs using a recyclable tung-oil-based vitrimer matrix and provides a strategy for efficient recycling of carbon fibers from CFRMs.

Epoxy vitrimer based on borate ester bond for green degradation, closed-loop recycling, and ready reprocessing

Epoxy vitrimer based on borate ester bond for green degradation, closed-loop recycling, and ready reprocessing

Creep Mechanics of Epoxy Vitrimer Materials

Epoxy materials are an indispensable class of polymers due to their excellent mechanical properties and high thermal resistance. However, traditional epoxy materials are typically non-recyclable and face issues with fatigue and creep. To combat the issue of recyclability, research interest is increasingly centered on epoxy vitrimer materials, which exhibit a dynamic covalent bond exchange reaction at high temperatures allowing for self-healing, recycling, and shape reprogramming. Herein, we discuss the creep mechanics of epoxy vitrimers; we postulate that gaining a fundamental understanding of what causes creep in vitrimers will garner advances in vitrimer creep resistance for future applications. Isothermal creep results demonstrate the presence of three distinct creep regimes: primary creep associated with polymer chain mobility, secondary creep associated with network rearrangement due to the dynamic covalent bond exchange reaction, and tertiary creep associated with errors in this reaction. We note at low temperatures (T ≪ Tv) and catalyst concentrations, vitrimers simply behave as a traditional epoxy material. In contrast, at high temperatures and catalyst concentrations, vitrimers exhibit significantly more creep with strong correlations to catalyst concentration, heating rate, and temperature, which are quantified in this work.

Rapid self-healed vitrimers via tailored hydroxyl esters and disulfide bonds

Conventional epoxy vitrimer containing single type of dynamic covalent bonds constitutes a class of polymer materials that integrates the advantages of thermoplastics and thermosets. However, the response of single type of dynamic covalent bond is slow, so it is very meaningful to combine multiple dynamic covalent bonds with adjustable proportions into one network for endowing vitrimers with diversified properties, especially rapid self-healing performances. Based on this consideration, we designed a series of epoxy vitrimers containing β-hydroxyl esters and disulfide bonds with adjusting any proportions. To this end, carboxylic acid-extended epoxy resins containing dynamical β-hydroxyl esters were first prepared through chemical reactions between conventional epoxy resin and suberic acid (SA), and then cured by aromatic disulfides (SS) to form vitrimers containing β-hydroxyl esters and disulfide bonds. Here, the SA part offers soft segments and dynamic β-hydroxyl esters, the curing agent SS part supplies hard segments and dynamic disulfide bonds. The properties of vitrimers with dual dynamic bonds are easily tailored by changing the ratio of SA and SS. Vitrimers with dual dynamic bonds exhibit excellent rapid self-healing properties closing to 80% after healing only 10 min, and the theoretical healing efficiencies are in good agreement with the actual values.

Novel cyclopentane derivative as cross-linker vitrimer: Effect of catalyst contents on the mechanical property and chemical-recyclable ability

A series of epoxy-based vitrimer were synthesized from diglycidyl ether of bisphenol A (DGEBA), sebacic acid (SA) and 3-(carboxymethyl)cyclopentane-1,2,4-tricarboxylic acid (TCAA) with 1,5,7-triazabicylo[4.4.0]dec-5-ene (TBD) as catalyst. Samples that contained only SA showed elastomer-like behavior, which exhibited low mechanical properties and high elongation at break. With the incorporation of SA:TCAA to 2:1, a feature transform from soft to hard was observed, the tensile strength increased from 40 to 70 MPa, the elongation at break decreased from above 200 to 20%, and the values of shore D hardness were also enhanced from 60 to 80. However, the TCAA with rigid structure hindered the transesterification rate, resulting in decreased stress relaxation efficiency. With the TCAA content increased, the stress relaxation time was extended further, while no mechanical properties enhancement was obtained, which might be due to the relatively low extent of reaction in synthesis caused by the steric effect of a higher concentration of TCAA. The effect of concentration for catalyst was also investigated. The stress relaxation efficiency enhanced with the TBD content increased while the thermal stability and mechanical properties decreased. With TBD concentration above 10%, SA:TCAA = 1:0 and 2:1 samples could be dissolved into ethylene glycol (EG) under 180 °C within 300 min, which shows potential for recyclable epoxy vitrimer composite applications.

Weldable and Reprocessable Shape Memory Epoxy Vitrimer Enabled by Controlled Formulation for Extrusion‐Based 4D Printing Applications

Weldable and Reprocessable Shape Memory Epoxy Vitrimer Enabled by Controlled Formulation for Extrusion‐Based 4D Printing Applications

Epoxy Vitrimer Based on Temperature‐Responsive Pure Organic Room Temperature Phosphorescent Materials

AbstractPure organic room temperature phosphorescent (RTP) materials are introduced to polymer matrices in recent researches for their attractive properties like mechanical character, low cost, facile fabrication and so on. Besides, polymers are considered to provide a rigid environment for phosphors to reduce nonradiative pathways and achieve efficient RTP emission. Vitrimers are new thermosetting polymers with dynamic covalent bond networks and temperature‐depended topology structure changes. Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room temperature phosphorescence from purely organic materials in amorphous vitrimer matrices at ambient conditions. Unlike conventional polymer‐based phosphorescent materials, vitrimer matrices show not only rigid environment and optical transparency but thermal dynamic characteristics. A temperature‐responsive phosphorescent system of small phosphor DTPA‐CZ and vitrimer matrices is obtained, with a phosphorescence lifetime of 1.025 s and a photoluminescence efficiency up to 78.7 %.

Multi-functional epoxy vitrimers: Controllable dynamic properties, multiple-stimuli response, crack-healing and fracture-welding

Vitrimers, a new generation of polymeric materials, have been widely investigated to resolve the intersection of thermoplastics and thermosets, relying on the exchange reactions in networks. However, it is still a challenge to develop multi-functional vitrimer materials. Herein, multi-functional epoxy vitrimers that exhibited controllable dynamic properties, multiple-stimuli response, crack-healing and fracture-welding were prepared by introducing Fe3O4 nanoparticles into the networks of dual dynamic epoxy vitrimers. It was revealed that the typical dynamic parameters of vitrimer including the characteristic relaxation time (τ), activation energy (Ea), Arrhenius prefactor (τ0) and topology freezing transition temperature (Tv) could be readily adjusted in the multi-functional epoxy vitrimers. Moreover, the multi-functional epoxy vitrimers were simultaneously responsive to temperature, near infrared and magnetic field, while they could be driven by magnets, heated by near infrared and healed or welded by heating. Upon heating or near infrared, the cracks in samples could be successfully healed. Besides, by the cooperation of heating and magnet or near infrared and magnet, the multi-functional epoxy vitrimers were able to achieve non-touch welding of factures. These multi-functional epoxy vitrimers might have potential application in improving the reliability and extending the lifetime of polymeric materials. Meanwhile, the facile way to functionalize the epoxy vitrimers is estimated to be suitable for other vitrimer materials.

Endowing Thermally Conductive and Electrically Insulating Epoxy Composites with a Well-Structured Nanofiller Network via Dynamic Transesterification-Participated Interfacial Welding

It is urgently required to achieve good thermal conductivity and electrical insulation performance in polymer-based composites with the minimum incorporation of functional fillers. In this work, the selective distribution of multiwalled carbon nanotubes (MWCNTs) in the segregated network of hexagonal boron nitride (h-BN) in the epoxy vitrimer matrix was achieved by compression molding, in which interfacial welding was enabled by small-molecule-participated dynamic transesterification at elevated temperatures. Due to the synergistic contribution of the well-structured nanofiller network, the epoxy vitrimer composites demonstrate enhanced thermal conductivity and electrical insulation at low filler content. The composite containing 1 wt % of MWCNTs and 8 wt % of h-BN shows thermal conductivity and electrical resistivity of 0.83 W/(m·K) and 1.92 × 1011 Ω·cm, respectively. The electrical resistivity can be further improved by increasing the segregated h-BN content. This method provides a novel way to prepare cost-effective polymer composites with excellent thermal conductivity and electrical insulation.