Photoresponsive Self-Healing Polymer Composite with Photoabsorbing Hybrid Microcapsules

Study on Preparation and Performance of Acid pH-Responsive Intelligent Self-Healing Coating

In this paper, microcapsules with acidic pH stimulus responsiveness were prepared through a one-step in situ polymerization method and a layer-by-layer assembly method. The effects of factors such as chitosan (CS) concentration, polymerization time, polymerization process temperature, and the number of polymerization layers on the performance of microcapsules were explored, and microcapsules with optimal performance were prepared and added to the epoxy coating. The morphology and structure of the microcapsules were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and zeta potential testing. The thermal stability and sustained release properties of the microcapsules were studied through thermogravimetric analysis and sustained release curve testing. Through scratch experiments, immersion experiments, salt spray experiments, and electrochemical impedance spectroscopy tests, the impact of the added amount of microcapsules on the self-healing performance and anti-corrosion performance of the coating in complex environments was explored. The results show that the optimal preparation process of acidic pH-responsive microcapsules requires that the concentration of chitosan is 2 mg/mL, the polymerization time of the polyelectrolyte layer is 8 h, the heating temperature during the polymerization process is 75 °C, and the number of polyelectrolyte layers is three. The prepared acidic pH-responsive microcapsules have good morphology, pH sensitivity, and thermal stability. The average particle size is approximately 203 μm, the drug loading rate reaches 59.74%, and the encapsulation rate reaches 63.99%. The optimal added amount of the acidic pH-responsive microcapsule coating is 15 wt%. The coating has a dual-trigger mechanism underlying it stimulus response capability and has an obvious stimulus response to acidic pH. It can inhibit corrosion in non-scratch areas, and its anti-corrosion ability is significantly stronger than that of epoxy coatings and ordinary self-healing coatings. The coating has a stronger repair effect and anti-corrosion ability when the environmental pH becomes acidic.

Nanoparticle-Empowered Core-Shell Microcapsules: From Architecture Design to Fabrication and Functions

Compartmentalization is a powerful concept to integrate multiscale components with diverse functionalities into miniature architectures. Inspired by evolution-optimized cell compartments, synthetic core-shell capsules enable storage of actives and on-demand delivery of programmed functions, driving scientific progress across various fields including adaptive materials, sustainable electronics, soft robotics, and precision medicine. To simultaneously maximize structural stability and environmental sensitivity, which are the two most critical characteristics dictating performance, diverse nanoparticles are incorporated into microcapsules with a dense shell and a liquid core. Recent studies have revealed that these nano-additives not only enhance the intrinsic properties of capsules including mechanical robustness, optical behaviors, and thermal conductivity, but also empower dynamic features such as triggered release, deformable structures, and fueled mobility. In this review, the physicochemical principles that govern nanoparticle assembly during microencapsulation are examined in detail and the architecture-controlled functionalities are outlined. Through the analysis of how each primary method implants nanoparticles into microcapsules, their distinct spatial organizations within the core-shell structures are highlighted. Following a detailed discussion of the specialized functions enabled by specific nanoparticles, the vision of the required fundamental insights and experimental studies for this class of microcarriers to fulfill its potential are sketched.

Magnetically Targeted, Water-Triggered, Self-Healing Microcapsules Based on Microfluidic Techniques Enabling Targeted Healing of Water Tree Damage in Epoxy Resins

Repairing the micro-scale damage of insulating materials under a strong electric field has long been a highly desired but challenging task. Among all kinds of damage, water tree damage in the insulating materials of electrical equipment and electronic devices working in humid environments has long been considered irreparable. The main challenge is that residual water prevents the healing agent from filling the water tree branch channel. To solve this problem, this work reports a magnetically targeted, water-triggered, self-healing microcapsule (MTWTSH-MC) that makes a breakthrough against water tree damage based on microfluidic techniques. Targeted microcapsules driven by a directional magnetic field are concentrated to the vulnerable area of the insulating materials, exerting very limited effects on the dielectric. When damage breaks the microcapsules, the healing agent releases and quickly fills the damage channel and then reacts with water in the air or in the branch channel of the water tree, achieving solidification of the healing agent and self-healing of the damage channels. In this way, we can realize self-perception, self-triggering, and self-healing for both mechanical damage and water tree damage in insulation materials without any external stimulation.

Repeatable Self-Healing of a Protective Coating Based on Vegetable-Oil-Loaded Microcapsules

Generally, microcapsule-based self-healing materials have the limitation of single local self-healing. A few studies have reported repeatable self-healing in these microcapsular materials, but there is a challenge to develop multi-cycle self-healing materials that have the advantages of easier preparation and a more efficient operation. In this work, a mixture of two vegetable oils, soybean and olive oil, was used as a healing agent. The atmospheric oxygen-induced reaction behavior (in the presence of a catalyst) was investigated for various compositions of the vegetable oil mixtures; infrared spectroscopy, recovery testing, and viscoelasticity measurement were performed to find an optimum composition of the healing agent. Microcapsules loaded with soybean oil and catalyst-containing olive oil were separately prepared and used to prepare a dual-capsule self-healing coating. It was demonstrated through optical and scanning electron microscopy that, upon scribing the self-healing coating, the vegetable oils flowed out from microcapsules to self-heal the damaged area. When the healed area of the self-healing coating was re-scribed, self-healing was repeated, which was confirmed by scanning electron microscopy (SEM) and anticorrosion and electrochemical testing. Our new repeatable self-healing coating provides the merits of easy preparation, no need for external intervention such as light irradiation, and an environmentally-friendly nature.

Preparation of dual-chamber microcapsule by Pickering emulsion for self-healing application with ultra-high healing efficiency

This work presented a new concept for designing dual-chamber self-healing microcapsules, which encapsulated both healing and curing species within a single microcapsule via Pickering emulsion photopolymerization. In our strategy, robust SiO₂ spheres encapsulating curing agent were firstly synthesized and used as Pickering emulsifiers to prepare emulsions loaded with self-healing agent and photo-curable monomer. Upon exposure to UV light, the photo-curable monomer underwent photo-crosslinking and converted into microcapsules wall. In the meanwhile, the SiO₂ spheres encapsulating curing agent were trapped in the microcapsule wall. The dual-chamber microcapsule which loaded the healing agent in its core and curing agent within its shell, was thus prepared. The presence of both the encapsulated healing and curing agent within a single capsule was demonstrated by infrared spectrometry and thermogravimetric analysis. Upon fracture, the healing agent and curing agent are released simultaneously from the dual-chamber microcapsule, which facilitates the interaction of the two agents, and enhances the healing efficiency. Up to 85% healing efficiency of the epoxy resin was achieved in 1 h, which was much higher than that of the traditional double microcapsules self-healing system (65%), demonstrating the excellent self-healing performance of the dual-chamber microcapsules. It has been demonstrated that the coating based on dual-chamber microcapsule presented reliable and outstanding self-healing anti-corrosion efficiency. By changing the species of healing agent, curing agent and wall substances (photo-curable monomer), the as-prepared dual-chamber microcapsules can meet different requirements of versatile self-healing system.

Effect of Microcapsules with Waterborne Coating as Core Material on Properties of Coating for Tilia Europaea and Comparison with Other Microcapsules

Urea formaldehyde was used as wall material and waterborne coatings as a core material to prepare microcapsules. So as to explore the influence of mass ratio of core to shell, reaction temperature and standing time on the performance of microcapsules, the orthogonal test of three factors and two levels was put into effect. The orthogonal experimental results showed the mass ratio of core to shell was the most important factor. With the increase of the mass ratio of core to shell, the output and clad ratio of microcapsules increased first and then decreased. The microcapsule with the mass ratio of core to shell of 0.67:1 had better appearance, output, and encapsulation performance. The optical properties of waterborne wood coating with the microcapsules of waterborne coating as core materials did not decrease significantly, while the hardness, impact resistance, and toughness were improved. At the same time, the microcapsules have a certain self-repairing effect on coating micro-cracks. Compared with the properties of waterborne coatings with other microcapsules, the coating with waterborne coating as core material has better comprehensive performance. The results provide a new research idea for the performance enhancement and self-healing of wood waterborne coating.

Challenges and Opportunities of Self-Healing Polymers and Devices for Extreme and Hostile Environments

Engineering materials and devices can be damaged during their service life as a result of mechanical fatigue, punctures, electrical breakdown, and electrochemical corrosion. This damage can lead to unexpected failure during operation, which requires regular inspection, repair, and replacement of the products, resulting in additional energy consumption and cost. During operation in challenging, extreme, or harsh environments, such as those encountered in high or low temperature, nuclear, offshore, space, and deep mining environments, the robustness and stability of materials and devices are extremely important. Over recent decades, significant effort has been invested into improving the robustness and stability of materials through either structural design, the introduction of new chemistry, or improved manufacturing processes. Inspired by natural systems, the creation of self-healing materials has the potential to overcome these challenges and provide a route to achieve dynamic repair during service. Current research on self-healing polymers remains in its infancy, and self-healing behavior under harsh and extreme conditions is a particularly untapped area of research. Here, the self-healing mechanisms and performance of materials under a variety of harsh environments are discussed. An overview of polymer-based devices developed for a range of challenging environments is provided, along with areas for future research.

Novel Smart Insulating Materials Achieving Targeting Self-Healing of Electrical Trees: High Performance, Low Cost, and Eco-Friendliness

It remains challenging to promptly inhibit and autonomically heal electrical trees inside insulating dielectrics, which are caused by sustained strong electrical fields and substantially shorten electronic device lifetimes and even cause premature failure of electrical equipment. Therefore, we demonstrate a magnetically targeted ultraviolet (UV)-induced polymerization functional microcapsule (MTUF-MC) to endow insulating materials with physical and electrical dual-damage self-healing capabilities. Specifically, Fe₃O₄@SiO₂ and TiO₂ nanoparticles, which serve as magnetic targets and UV shields (thereby preventing the healing agent from prematurely triggering), constitute a functional microcapsule shell, ensuring a low dopant concentration and excellent self-healing ability of the epoxy composites without affecting the intrinsic performance of the matrix. By exploiting in situ electroluminescence originating from electrical trees, UV-induced polymerization of healing agent is handily triggered without any applying external stimuli to intelligently, contactlessly, and autonomously self-healing electrical trees inside insulating dielectrics.

Porous Ceramics via Processable Pickering Emulsion Stabilized by Oppositely Charged Colloids

We report a method to produce processable Pickering emulsions in which electrostatic heteroaggregation of oppositely charged fumed silica and alumina particles is exploited. The ability of weakly charged heteroaggregates to adsorb to the oil-water interface favors the formation of highly stable emulsions. A control over the microstructure and rheology of the Pickering emulsion is demonstrated by tuning parameters such as composition, pH, and total concentration of particles. The Pickering emulsions formed under optimal conditions are found to be gel-like and are excellent templates for the fabrication of porous ceramics. Furthermore, the porous mullite ceramic structure which has exceptional thermal and chemical resistance under harsh environments is obtained by drying and sintering of the emulsion template. The phase evolution, microstructure, and porosity of the resulting ceramic are characterized.

Synthesis of UV-Responsive Self-Healing Microcapsules and Their Potential Application in Aerospace Coatings

Advanced polymer composite coatings in the spacecraft are threatened by harsh space environment factors, such as strong UV radiation, atomic oxygen, thermal cycles, space debris, etc. Their service life can be drastically shortened by the unavoidable formation of cracks caused by these factors (especially strong and abundant UV radiation) during long-term flight. Herein, a UV-responsive microcapsule-based coating is developed for in-orbit damage repairing. UV-responsive microcapsules of which the inner polymeric shell can be degraded rapidly by the outer pure TiO₂ shell under UV radiation are produced by UV-initiated polymerization of Pickering emulsions and subsequently embedded into silicon resin matrices. When damaged, some microcapsules will be ruptured under the stimulus of external force, afterward the unbroken ones around the scratched areas will be degraded by UV radiation, as a result, encapsulated healing agents can be released and finally repair cracks. In this system, UV-responsive microcapsules can release more agents more effectively due to the dual release mode, compared with the traditional crack-repairing system. Moreover, the damage of UV radiation in space can be transferred into the favorable ones, which makes it have a potential application in aerospace coatings.

Improvement of Mechanical Properties and Self-Healing Efficiency by Ex-Situ Incorporation of TiO2 Nanoparticles to a Waterborne Poly(Urethane-Urea)

This research work was focused on the incorporation of TiO₂ nanoparticles into synthesized solvent-free waterborne poly(urethane-urea) (WPUU) based on hydrophilic poly(ethylene oxide) (PU0) in order to improve both the mechanical properties and self-healing effectiveness of a polymer matrix. The incorporation of TiO₂ nanoparticles resulted in a successful enhancement of the mechanical properties of nanocomposite films when compared to PU0. Simultaneously, the obtained nanocomposite films did not only maintain the self-healing ability of the PU0 film, measured by means of mechanical properties after successive cutting/recovery cycles, but they also showed a higher self-healing efficiency than the PU0 film. Moreover, the well-dispersed TiO₂ nanoparticles, visualized by atomic force microscopy (AFM), kept their conductive properties when embedded in the PU0 matrix, as was confirmed by electrostatic force microscopy (EFM). This research work described a simple and industrially appealing way to control the dispersion of commercially available TiO₂ nanoparticles in waterborne poly(urethane-urea) for the designing of inorganic/organic hybrid nanocomposites with enhanced mechanical properties and self-healing efficiency, in which TiO₂ nanoparticles preserved their conductive properties within the polymer matrix.

Effect of a Surfactant in Microcapsule Synthesis on Self-Healing Behavior of Capsule Embedded Polymeric Films

Recently, there has been increased interest in self-healing membranes containing functional microcapsules in relation to challenges involving water treatment membranes. In this study, a self-healing membrane has been prepared by incorporating microcapsules with a polyurethane (PU) shell and a diisocyanate core in a poly(ether sulfone) (PES) membrane. Depending on the characteristics of the microcapsule, to precisely quantify the self-healing behavior and performance of the produced microcapsule embedded membranes, it is important to understand the effect of a used surfactant on microcapsule synthesis. It is noteworthy that mixed surfactants have been employed to control and tailor the size and morphology of microcapsules during the synthetic process, and the surfactant system employed was one of the most dominant parameters for affecting the healing capability of microcapsule embedded membranes. Various techniques including microscopy (optical and electron), thermal analyses (DSC and TGA), and water flux measurements have been employed. This article provides essential and important information for future research into the subtle relation between microcapsule properties with varied synthetic parameters and the self-healing behavior of membrane.

Novel Dual-Component Microencapsulated Hydrophobic Amine and Microencapsulated Isocyanate Used for Self-Healing Anti-Corrosion Coating

Dual component microencapsulated hydrophobic amine and microencapsulated isocyanate were designed and fabricated for self-healing anti-corrosion coating. In this system, novel hydrophobic polyaspartic acid ester (PAE) and isophorone diisocyanate (IPDI) were microencapsulated respectively with melamine-formaldehyde (MF) as shell via in situ polymerization. To reduce the reaction activity between shell-forming MF prepolymer and PAE, another self-healing agent tung oil (TO) was dissolved in PAE and subsequently employed as core material. With field-emission scanning electron microscopy (FE-SEM) and optical microscopy (OM), the resultant microencapsulated IPDI with diameter of 2⁻5 μm showed a spherical shape and smooth surface. More importantly, both the morphology and microstructure of microencapsulated PAE enhanced significantly after addition of TO. Fourier transform infrared spectra (FTIR) analysis confirmed the molecular structure of chemical structure of the microcapsules. Thermal gravimetric analysis (TGA) indicated that both kinds of microcapsules exhibit excellent thermal resistance with the protection of MF shell. Furthermore, the self-healing epoxy coating system containing microencapsulated IPDI and microencapsulated PAE/TO was prepared and investigated. From the micrographs of true color confocal microscope (TCCM), the self-healing coating containing dual-component microcapsules showed excellent self-repairing performance compared to single microencapsulated IPDI system, and the optimal content of dual-component microcapsules in epoxy coating was 20 wt % approximately.

Photothermal Conversion Triggered Precisely Targeted Healing of Epoxy Resin Based on Thermoreversible Diels-Alder Network and Amino-Functionalized Carbon Nanotubes

In the present work, we demonstrated the recyclability and precisely targeted reparability of amino functionalized multiwall carbon nanotubes-epoxy resin based on dynamic covalent Diels-Alder (DA) network (NH₂-MWCNTs/DA-epoxy) by exploring the photothermal conversion of CNTs to trigger the reactions of dynamic chemical bonds. The covalent cross-linked networks of NH₂-MWCNTs/DA-epoxy resin change their topology to linear polymer by thermally activated reverse Diels-Alder (r-DA) reactions at high temperatures, which endues the resin with almost 100% recyclability. The self-healing property of the epoxy resin was confirmed by the complete elimination of cracks after the reconstruction of DA network induced by heating or near-infrared (NIR) irradiation. For heat-triggered self-healing process, heat energy may also act on those uninjured parts of the resin and cause the dissociation of the whole DA network. Therefore, redundant r-DA and DA reactions, which have no contribution to self-healing, are also triggered during thermal treatment, resulting in not only a waste of energy but also the deformation of the sample under external force. Meanwhile, for the NIR-triggered self-healing process, the samples can maintain well their original shape without observable deformation after irradiation. The NIR-triggered healing process, which uses MWCNTs as the photothermal convertor, have very good regional controllability by simply tuning the MWCNTs content, the distance from NIR laser source to sample, and the laser power. The injured samples can be locally repaired with high precision and efficiency without an obvious influence on those uninjured parts.

UV-Triggered Self-Healing of a Single Robust SiO2 Microcapsule Based on Cationic Polymerization for Potential Application in Aerospace Coatings

UV-triggered self-healing of single microcapsules has been a good candidate to enhance the life of polymer-based aerospace coatings because of its rapid healing process and healing chemistry based on an accurate stoichiometric ratio. However, free radical photoinitiators used in single microcapsules commonly suffer from possible deactivation due to the presence of oxygen in the space environment. Moreover, entrapment of polymeric microcapsules into coatings often involves elevated temperature or a strong solvent, probably leading to swelling or degradation of polymer shell, and ultimately the loss of active healing species into the host matrix. We herein describe the first single robust SiO2 microcapsule self-healing system based on UV-triggered cationic polymerization for potential application in aerospace coatings. On the basis of the similarity of solubility parameters of the active healing species and the SiO2 precursor, the epoxy resin and cationic photoinitiator are successfully encapsulated into a single SiO2 microcapsule via a combined interfacial/in situ polymerization. The single SiO2 microcapsule shows solvent resistance and thermal stability, especially a strong resistance for thermal cycling in a simulated space environment. In addition, the up to 89% curing efficiency of the epoxy resin in 30 min, and the obvious filling of scratches in the epoxy matrix demonstrate the excellent UV-induced healing performance of SiO2 microcapsules, attributed to a high load of healing species within the capsule (up to 87 wt %) and healing chemistry based on an accurate stoichiometric ratio of the photoinitiator and epoxy resin at 9/100. More importantly, healing chemistry based on a UV-triggered cationic polymerization mechanism is not sensitive to oxygen, extremely facilitating future embedment of this single SiO2 microcapsule in spacecraft coatings to achieve self-healing in a space environment with abundant UV radiation and oxygen.

Reversible dissolution/formation of polymer nanoparticles controlled by visible light

Noncovalent crosslinking between polyvinyl pyridine and a copolymer of acrylic acid led to the formation of a polymer nanoparticle. In the presence of a metastable-state photoacid, reversible dissolution and formation of the nanoparticle can be controlled by visible light. Photo-induced proton transfer from the photoacid to the polymers broke the hydrogen bonding and ionic bonding and led to the dissolution of the nanoparticle. Cycles of dissolution and formation were controlled by turning on and off irradiation, and were demonstrated by the transmittance change.

Preparation and property of epoxy resins-penetrated aligned carbon nanotube bundle hybrid microcapsules for self-healing polymers

The epoxy resins-penetrated aligned carbon nanotube bundle (ACNTsB) hybrid system (epoxy@ACNTsB) was prepared by soaking ACNTsB in the acetone solution of epoxy resins. Epoxy@ACNTsB was encapsulated in aqueous solution containing an amine curing agent to prepare hybrid microcapsules (MCs). The structure and morphology of MCs were characterized by Fourier transform infrared spectroscopy, scanning electronic microscopy, transmission electron microscopy, and laser scanning confocal microscopy. The thermal properties of MCs were carried out by differential scanning calorimetry and thermogravimetric analysis. The solvent resistance of MCs in acetone was also investigated. The results show that MCs exhibit high initial decomposition of temperatures (266°C). The content of epoxy resin in MCs is about 74% and the polymer film thickness of MCs is from 1 to 2 μm. MCs show high thermal and chemical stability below 200°C and excellent solvent resistance. MCs embedded in cyanate ester (CE) resins can significantly toughen the matrix. The fracture MCs can release the epoxy resins under heating condition, and the released epoxy resins can react with the reactive –OCN group/triazine rings in the CE matrix to rebond the crack surfaces. When the temperature schedule of 100°C/1 h + 200°C/2 h is applied, 10–15 wt% MCs can recover from 82.7% to 99.8% of the original fracture toughness of CE matrix.