Self-healable interfaces based on thermo-reversible Diels–Alder reactions in carbon fiber reinforced composites

'Donor-acceptor', 'interpenetrating polymer network' and 'electrostatic self-assembly' work in tandem to achieve extraordinary specific shielding effectiveness

The exploration of 'electrostatic self-assembly' on solid surfaces has garnered significant interest across various fields. With the sophistication of gadgets, managing electromagnetic interference (EMI) from stray signals, especially in stealth applications, necessitates materials that can absorb microwaves. A promising approach involves integrating lightweight self-healing polymeric materials. This study employs electrostatic self-assembly to design a carbon nanotube structure on an interpenetrating polymer network (IPN) made of PVDF and bismaleimide (BMI)-grafted dopamine hydrochloride, enhancing mechanical integrity through well-formed IPNs. Graphene oxide (GO) is introduced before IPN formation to facilitate an 'acceptor-donor' interaction via the Diels-Alder adduct between BMI and GO, which binds with multi-walled carbon nanotubes (MWCNTs). MWCNTs, modified with PQ7 or PDDA for a positive charge, self-assemble onto the IPN-GO construct, creating a lightweight and chemically stable structure capable of absorbing electromagnetic radiation. The 21 μm thick construct exhibits enhanced microwave absorption within the X-band (8.2-12.4 GHz), with a specific shielding effectiveness of 8637 dB cm2 g-1 and a green index (gs ≈ 1.41). The construct is coated with self-healable polyetherimide (PEI) containing exchangeable disulfide bonds to address maintenance challenges, providing heat-triggered self-healing properties. These innovative structures offer solutions for 5G and IoT applications where lightweight, durable, and multifunctional properties are essential for effectively shielding electronic devices from stray signals.

Self-Healable Interfaces with Improved Mechanical Properties Induced by Dynamic Network Reconfiguration in Carbon Fiber-Reinforced Epoxy Laminates

Interfacial failure in carbon fiber-reinforced epoxy (CFRE) laminates is a prominent mode of failure, attracting significant research attention. The large surface-energy mismatch between carbon fiber (CF) and epoxy results in a weaker interface. This study presents a facile yet effective method for enhancing the interfacial adhesion between CF and epoxy with self-healable interfaces. Two variants of a designer sizing agent, poly(ether imide) (PEI), were synthesized, one without a self-healing property termed BO, and the second one by incorporating disulfide metathesis in one of its monomers that renders self-healing properties at the interface-mediated by network reconfiguration, termed BA. 0.25 wt % of CF was found to be the optimum amount of BO and BA sizing agents. The surface free energy of CF drastically increased and became quite close to the surface energy of epoxy after the deposition of both sizing agents and the higher surface roughness. The improved surface wettability, presence of functional groups, and mechanical interlocking worked in tandem to strengthen the interface. The interlaminar shear strength (ILSS) and flexural strength (FS) of CFRE laminate sized with BO consequently increased by 35% and 22% and of CFRE laminate sized with BA increased by 26% and 19%, respectively. Fractography analysis revealed outstanding bonding between epoxy and PEI-CF, indicating that matrix fracture is the predominant mode of failure. The self-healable interfaces due to the preinstalled disulfide metathesis in the sizing agent resulted in 51% self-healing efficiency in ILSS for BA-sized CFRE laminate. Interestingly, the functional properties, deicing, and EMI shielding effectiveness were not compromised by modification of the interface with this designer sizing agent. This study opens new avenues for interfacial modification to improve the mechanical properties while retaining the key functional properties of the laminates.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Self-Healing Polymeric Soft Actuators

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

An Overview of Self-Healable Polymers and Recent Advances in the Field

The search for materials with better performance, longer service life, lower environmental impact, and lower overall cost is at the forefront of polymer science and material engineering. This has led to the development of self-healing polymers with a range of healing mechanisms including capsular-based, vascular, and intrinsic self-healing polymers. The development of self-healable systems has been inspired by the healing of biological systems such as skin wound healing and broken bone reconstruction. The goal of using self-healing polymers in various applications is to extend the service life of polymers without the need for replacement or human intervention especially in restricted access areas such as underwater/underground piping where inspection, intervention, and maintenance are very difficult. Through an industrial and scholarly lens, this paper provides (a) an overview of self-healing polymers, (b) classification of different self-healing polymers and polymer-based composites, (c) mechanical, thermal, and electrical analysis characterization, (d) applications in coating, composites, and electronics, (e) modeling and simulation, and (f) recent development in the past 20 years . This review highlights the importance of healable polymers for an economically and environmentally sustainable future, the most recent advances in the field, and current limitations in fabrication, manufacturing, and performance. This article is protected by copyright. All rights reserved.

Tuneable chemistry at the interface and self-healing towards improving structural properties of carbon fiber laminates: a critical review

Carbon fiber reinforced epoxy (CFRE) laminates have become a significant component in aircraft industries over the years due to their superior mechanical and highly tunable properties. However, the interfacial area between the fibers and the matrix continues to pose a significant challenge in debonding and delamination, leading to significant failures in such components. Therefore, since the advent of such laminated structures, researchers have worked on several interfacial modifications to better the mechanical properties and enhance such laminated systems' service life. These methods have primarily consisted of fiber sizing or matrix modifications, while effective fiber surface treatment has utilized the concept of surface energy to form an effective matrix locking mechanism. In recent times, with the advent of self-healing technology, research is being directed towards novel methods of self-healing interfacial modifications, which is a promising arena. In this review, we have provided comprehensive insight into the significance, historical advances, and latest developments of the interface of CFRE laminates. We have analysed the significant research work undertaken in recent years, which has shown a considerable shift in engineering the interface for mechanical property enhancement. Keeping in view the latest developments in self-healing technology, we have discussed reversible interfacial modifications and their impact on future improvements to service life.

Intrinsic Self-Healing Epoxies in Polymer Matrix Composites (PMCs) for Aerospace Applications

This article reviews some of the intrinsic self-healing epoxy materials that have been investigated throughout the course of the last twenty years. Emphasis is placed on those formulations suitable for the design of high-performance composites to be employed in the aerospace field. A brief introduction is given on the advantages of intrinsic self-healing polymers over extrinsic counterparts and of epoxies over other thermosetting systems. After a general description of the testing procedures adopted for the evaluation of the healing efficiency and the required features for a smooth implementation of such materials in the industry, different self-healing mechanisms, arising from either physical or chemical interactions, are detailed. The presented formulations are critically reviewed, comparing major strengths and weaknesses of their healing mechanisms, underlining the inherent structural polymer properties that may affect the healing phenomena. As many self-healing chemistries already provide the fundamental aspects for recyclability and reprocessability of thermosets, which have been historically thought as a critical issue, perspective trends of a circular economy for self-healing polymers are discussed along with their possible advances and challenges. This may open up the opportunity for a totally reconfigured landscape in composite manufacturing, with the net benefits of overall cost reduction and less waste. Some general drawbacks are also laid out along with some potential countermeasures to overcome or limit their impact. Finally, present and future applications in the aviation and space fields are portrayed.

Facile immobilization of graphene nanosheets onto PBO fibers via MOF-mediated coagulation strategy: Multifunctional interface with self-healing and ultraviolet-resistance performance

The surface of poly (p-phenylene benzobisoxazole) (PBO) fibers with self-healing and ultraviolet (UV)-resistance performance play the key role in prolonging their service lifespan. Although great advances have been made in the single aspect of above two properties, integration of self-healing and anti-UV performance into the surface of PBO fiber is still a challenge. In this study, the coagulation strategy mediated by metal-organic framework (MOF) is proposed to construct the multifunctional surface of PBO fibers. The spindle-like iron (III)-based MOF (MIL-88B-NH₂) nanocrystals are firstly immobilized onto the surface of PBO-COOH through hydrothermal reaction, then serving as the medium layer to further immobilize sufficient graphene oxide (GO) nanosheets. Benefitting from the favorable near-infrared (NIR, 808 nm) photothermal conversion performance of GO nanolayers, the monofilament composite-PBO@Fe-MIL-88B-NH₂-GO-TPU (thermoplastic polyurethane) exhibited a stable and high self-healing efficiency (approximately 80%) within five cycle times. Meanwhile, the cooperative adsorption and shielding weaken effects of MOF-GO nanolayers enabled PBO fibers with excellent anti-UV properties that are superior to much reported literatures after 96 h aging time and eventually increased by 75% compared with untreated PBO fiber. In view of the varieties and multifunctionalities of MOFs and carbon nanomaterials, MOF-mediated coagulation strategy would provide guidance for preparing multifunctional composite materials.

Self-healing and self-cleaning clear coating

Coatings exhibiting both self-cleaning and self-healing properties are envisioned for a wide range of applications. Herein we report a simple fabrication approach toward poly(urea-urethane) (PU) coatings having self-healing and self-cleaning properties. The self-cleaning component is a poly(dimethylsiloxane) (PDMS), which is affordable in cost and also has a lower environmental footprint relative to its fluorinated counterpart. The self-healing properties are imparted by dynamic urea bonds of the matrix. The obtained surfaces are evaluated for their anti-smudge properties such as water-, oil- and ink-repellency, as well as optical properties. The self-healing properties of these coatings are evaluated by making scores with a doctor blade and monitoring the healing under different conditions using optical microscopy. The resultant coatings are also investigated for their good mechanical properties. The surface chemical compositions are determined x-ray photoelectron spectroscopy, while atomic force microscopy is used for microstructural analysis of these coatings.

Ionic Liquids-Containing Silica Microcapsules: A Potential Tunable Platform for Shaping-Up Epoxy-Based Composite Materials?

In this work, silica microcapsules containing phosphonium ionic liquid (IL), denoted SiO₂@IL, were successfully synthesized for the first time using the one step sol-gel method in IL/H₂0 emulsion. The morphologies of the obtained micron-size microcapsules, including their diameter distribution, were characterized using dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The thermal behavior of these microcapsules and the mass fraction of the encapsulated IL in the silica microcapsules were determined using thermogravimetric analysis, showing an excellent thermal stability (up to 220 °C) and highlighting that an amount of 20 wt.% of IL is contained in the silica microcapsules. In a second step, SiO₂@IL microcapsules (1 wt.%) were dispersed into epoxy-amine networks to provide proof of concept of the ability of such microcapsules to act as healing agents as microcracks propagate into the epoxy networks.

Simultaneous Recovery of Matrix and Fiber in Carbon Reinforced Composites through a Diels-Alder Solvolysis Process

Efficient and comprehensive recycling of fiber-reinforced thermosets is particularly challenging, since the irreversible degradation of the matrix component is necessary in order to separate the fiber component in high purity. In this work, a new approach to fully recyclable thermoset composites is presented, based on the thermal reversibility of an epoxy-based polymer network, crosslinked through Diels-Alder (DA) chemistry. Carbon fiber composites, fabricated by compression molding, were efficiently recycled through a simple solvolysis procedure in common solvents, under mild conditions, with no catalysts. Specifically, the purity of reclaimed fibers, assessed by thermogravimetric analysis and scanning electron microscopy, was very high (>95%) and allowed successful reprocessing into second generation composites. Moreover, the dissolved matrix residues were directly employed to prepare smart, thermally healable coatings. Overall, DA chemistry has been shown to provide a convenient strategy towards circular economy of thermoset composites.

Control of Interfacial Diels-Alder Reactivity by Tuning the Plasma Polymer Properties

Functionalizing the surface of a material with a smart plasma polymer coating is an interesting alternative strategy to obtain a thermoresponsive material without changing its formulation. On the basis of a low-pressure plasma polymerization process, the present work first aims to fabricate polymer thin films that react via the well-known thermoreversible Diels-Alder (DA) reaction (diene/dienophile cycloaddition). A two-step surface modification process based on (pulsed) plasma polymerization enables the design of functional coatings that contain furan (diene) groups. The reactivity of these surfaces with maleic anhydride (dienophile) in solution is thoroughly investigated, mainly by studying the kinetics of the DA reaction by advancing contact angle measurements. The determination of rate constants of reactions at various temperatures leads to the quantification of thermodynamic parameters such as the activation energy of the reaction as well as the enthalpy and entropy of activation related to the formation of the transition-state complex involved in the DA reaction. More interestingly, the design of furan-functionalized coatings with various physicochemical properties enables the understanding of the role played by the density of functional groups and the cross-linking rate of the polymer on the interfacial reactivity. Thus, we show in this work how to control the interfacial DA reaction on plasma coatings by tailoring the operating conditions of plasma polymerization.

A comprehensive review on self-healing of asphalt materials: Mechanism, model, characterization and enhancement

Self-healing has great potential to extend the service life of asphalt pavement, and this capability has been regarded as an important strategy when designing a sustainable infrastructure. This review presents a comprehensive summary of the state-of-the-art investigations concerning the self-healing mechanism, model, characterization and enhancement, ranging from asphalt to asphalt pavement. Firstly, the self-healing phenomenon as a general concept in asphalt materials is analyzed including its definition and the differences among self-healing and some viscoelastic responses. Additionally, the development of self-healing in asphalt pavement design is introduced. Next, four kinds of possible self-healing mechanism and corresponding models are presented. It is pointed out that the continuum thermodynamic model, considering the whole process from damage initiation to healing recovery, can be a promising study field. Further, a set of self-healing multiscale characterization methods from microscale to macroscale as well as computational simulation scale, are summed up. Thereinto, the computational simulation shows great potential in simulating the self-healing behavior of asphalt materials from mechanical and molecular level. Moreover, the factors influencing self-healing capability are discussed, but the action mechanisms of some factors remain unclear and need to be investigated. Finally, two extrinsic self-healing technologies, induction heating and capsule healing, are recommended as preventive maintenance applications in asphalt pavement. In future, more effective energy-based healing systems or novel material-based healing systems are expected to be developed towards designing sustainable long-life asphalt pavement.

Light triggered interfacial damage self-healing of poly(p-phenylene benzobisoxazole) fiber composites

The interfacial microcracks in the resin matrix composites are difficult to be detected and repaired. However, the self-healing concept provides opportunities to fabricate composites with unusual properties. In the present study, photothermal conversion Ag-Cu₂S nanoparticles were immobilized onto poly(p-phenylene benzobisoxazole) (PBO) fibers via a polydopamine chemistry. Benefitting from the photothermal effects of Ag-Cu₂S, the obtained PBO fibers (Ag-Cu₂S-PBO) efficiently converted the light energy into heat under Xenon lamp irradiation. Then, single PBO fiber composites were prepared using thermoplastic polyurethane as the matrix. It was found that the interfacial damage caused by single fiber pull-out was simply self-healed by Xe light irradiation. This wonderful interfacial damage self-healing property was mainly attributed to the in situ heating generation via photothermal effects of Ag-Cu₂S in the composite interface. This paper reports a novel strategy to construct advanced composites with light-triggered self-healing properties, which will provide inspiration for preparing high performance composite materials.

Effect of epoxy matrix architecture on the self-healing ability of thermo-reversible interfaces based on Diels–Alder reactions: demonstration on a carbon fiber/epoxy microcomposite

Thermally reversible Diels–Alder adducts formed between furan and maleimide groups have been introduced into the interphase of a carbon fiber-reinforced composite material to design a self-healable composite material.

Effect of hydrothermal modified carbon fiber through Diels–Alder reaction and its reinforced phenolic composites

The intensity of peaks 1 and 2 changed little indicating no significant change of the carbon fiber skin region. Peak 3 obviously increased after modification indicating the increase in carboxyl groups, and revealing that the maleic anhydride was grafted on the carbon fibers.