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The Dual Functional Epoxy Material with Autonomic Damage Indication and Self-healing
RSC Advances
Abstract
Autonomic indication of mechanical damage and self-healing epoxy materials was conducted using 2′,7′-dichlorofluorescein (DCF) and glycidyl methacrylate (GMA) solution. When mechanical damage occured, the released DCF reacted with the amine group in the crack plane to sense the damage colorimetrically, and the GMA rebound the cracks in the epoxy matrix by chemical or physical interaction.
... In fact, GMA-based healing materials, originating from the reaction of epoxide group with amines, are reported in literature. In such systems, either copolymers of GMA comprise a matrix [68] or GMA is encapsulated as a healing agent in microcapsules or hollow fibers, embedded in a matrix [35,69]. In these cases, healing is apparently not reversible, as the GMA epoxy groups will react once in the scratched area. ...
... In fac GMA-based healing materials, originating from the reaction of epoxide group with amines, are reported in literature. In such systems, either copolymers of GMA comprise matrix [68] or GMA is encapsulated as a healing agent in microcapsules or hollow fibers embedded in a matrix [35,69]. In these cases, healing is apparently not reversible, as th GMA epoxy groups will react once in the scratched area. ...
Self-healing materials and self-healing mechanisms are two topics that have attracted huge scientific interest in recent decades. Macromolecular chemistry can provide appropriately tailored functional polymers with desired healing properties. Herein, we report the incorporation of glycidyl methacrylate-based (GMA) copolymers in waterborne polyurethanes (WPUs) and the study of their potential healing ability. Two types of copolymers were synthesized, namely the hydrophobic P(BA-co-GMAy) copolymers of GMA with n-butyl acrylate (BA) and the amphiphilic copolymers P(PEGMA-co-GMAy) of GMA with a poly(ethylene glycol) methyl ether methacrylate (PEGMA) macromonomer. We demonstrate that the blending of these types of copolymers with two WPUs leads to homogenous composites. While the addition of P(BA-co-GMAy) in the WPUs leads to amorphous materials, the addition of P(PEGMA-co-GMAy) copolymers leads to hybrid composite systems varying from amorphous to semi-crystalline, depending on copolymer or blend composition. The healing efficiency of these copolymers was explored upon application of two external triggers (addition of water or heating). Promising healing results were exhibited by the final composites when water was used as a healing trigger.
... Recently, a technique was introduced for monitoring self-healing coatings based on changes in color or fluorescence at damaged locations on the coating surface. [1][2][3][4][5][6][7][8][9][10] This technique is of great practical utility, as it allows the condition of a coating to be assessed through visual inspection. Nevertheless, most of the recent reported research has been conducted to only detect external impacts or cracks in self-healing coatings. ...
... Nevertheless, most of the recent reported research has been conducted to only detect external impacts or cracks in self-healing coatings. [2][3][4][5][6][7][8][9] However, to fully detect self-healing of a coating surface, a technique for simultaneously detecting cracking and healing is needed [10]. ...
We report the development of an extrinsic, self-healing coating system that shows no fluorescence from intact coating, yellowish fluorescence in cracked regions, and greenish fluorescence in healed regions, thus allowing separate monitoring of cracking and healing of coatings. This fluorescence-monitoring self-healing system consisted of a top coating and an epoxy matrix resin containing mixed dye loaded in a single microcapsule. The dye-loaded microcapsules consisted of a poly(urea-formaldehyde) shell encapsulating a healing agent containing methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS), styrene, a photo-initiator, and a mixture of two dyes: one that fluoresced only in the solid state (DCM) and a second that fluoresced dramatically in the solid than in the solution state (4-TPAE). A mixture of the healing agent, photo-initiator, and the two dyes was yellow due to fluorescence from DCM. On UV curing of this mixture, however, the color changed from yellow to green, and the fluorescence intensity increased due to fluorescence from 4-TPAE in the solid state. When a self-healing coating embedded with microcapsules containing the DCM/4-TPAE dye mixture was scratched, the damaged region exhibited a yellowish color that changed to green after healing. Thus, the self-healing system reported here allows separate monitoring of cracking and healing based on changes in fluorescence color.
... Inspired by this, smart materials were explored to fully or partially simulate these functions via color indication and/or self-healing. [28][29][30][31]33 While color can visualize damage to warn of the vulnerability of the damaged area and prevent further damage to the same site, self-healing can restore the degraded properties to retard or even avoid material failure for enhanced safety and longevity. Feasibility of fabricating polymeric materials with dual functions was explored through possible incorporation of vehicles containing color indicator and healant. ...
... Feasibility of fabricating polymeric materials with dual functions was explored through possible incorporation of vehicles containing color indicator and healant. [28][29][30][31]33 Synthetic materials that are able to visualize damage with the aid of an extra UV source and autonomously repair damage were achieved by using extrinsic carriers with both healants and fluorescein. 28,29,32,34 However, they are passive reporting systems in that the color is always on in both the damaged and the intact sites, leading to the low contrast and therefore less effectiveness for damage indication. ...
Polymers are susceptible to small damages which are difficult to be detected and repaired, and may lead to catastrophic failure if left unattended at their early stage. How to autonomously warn and repair them simultaneously is promising yet challenging, owing to difficulty in integrating different functional elements for packaging and lack of suitable vehicles to carry a multi-role trigger with high reactivity. Herein, inspired by human skin in damage-healing process, we report a genuinely fully autonomous smart material that is capable of warning and healing damages via simply incorporating dual microcapsules containing polyamine as a multi-role trigger and epoxy monomer dyed with a pH indicator, respectively. Both microscopic ‘subcutaneous’ damages and macroscopic surface damages can be warned by conspicuous red color, not only rapidly upon their occurrence, but also permanently after being repaired. Accompanied with the comprehensive warning, the smart material shows high healing performance upon dynamic impact damages with efficiency up to 100% without any external interventions. This facile and ready strategy with fully autonomous warning and healing functions independent of host matrix provides a new avenue to enhance the reliability and longevity of a wide variety of polymeric materials ranging from functional coatings to structural composites.
... GMA, a low viscosity and nontoxic hydrophobic solvent with water solubility of 0.023 g/g at 20°C, was used for dissolving the DCF powder (17,18). As depicted in Figure 1, GMA had a good solubility with DCF due to its high Hansen solubility parameter (19,20). When a droplet of amine (i.e. ...
... The swelling degree was calculated by the weighting method and the mass changes were 0.50 wt%, 1.15 wt%, 3.53 wt% and 5.82 wt%, respectively. The curing degree of epoxy film determined by Fourier transform infrared (FTIR) spectroscopy was 97.38% (19), which indicated some residual amine group was retained in the epoxy matrix. Therefore, it is reasonable to deduce that the color-changing mechanism for epoxy-amine polymer is that DCF molecules react with the residual amine in the epoxy polymer and form red precipitates quickly inside the epoxy matrix. ...
Epoxy polymer with damage indicating ability was very usable for ships and bridges to detect the cracks at an early stage and to prevent corrosion. 2′, 7′-dichlorofluorescein (DCF), as a damage indicator, was used to report the mechanical damage of epoxy-amine polymer by a strong color change from a light yellow to bright red due to the molecular structure transition from the acid molecular form to the base ion form. The effect of water on damage indicator and damaged epoxy-amine polymer film was evaluated by an immersion test and the properties were characterized by ultraviolet-visible spectrophotometry (UV-Vis), scanning electronic microscopy (SEM), energy dispersive X-ray spectrometer (EDS), zeta potential and thermal gravimetric analysis (TGA). The results showed that DCF was an easy, stable and permanent indicator for epoxy-amine polymer and the water only had a slight influence on the indication stability of damaged epoxy polymer.
... Following mechanical damage, repair agents or corrosion inhibitors are released to fill the defects, restoring the coating's protective function. Combining damage detection and self-repair capabilities within the same material can improve reliability [174][175][176][177]. In 2020, Liu [133] designed a coating with a corrosion warning mechanism. ...
Marine biofouling is a well-established and significant challenge for the maritime industry. Self-healing coatings applied to ships have demonstrated superior surface properties, including enhanced corrosion resistance and the ability to mitigate biological contamination. Consequently, numerous studies have been conducted to assess different self-repairing coatings, which incorporate mechanisms such as microcapsules, dynamic covalent bonds, and ion exchange. This review begins with an introduction to the process of biofouling formation. It then provides a comprehensive outline of the self-healing coatings that have been developed to improve wear resistance, summarizing the advancements in this area. Finally, building upon these three coating systems, this paper offers a summary of the fabrication and protection technologies for self-healing coatings, including the preparation of micro/nano containers, corrosion warning mechanisms, and intelligent responsive protection. Furthermore, the review explores the future prospects of self-healing coatings, offering valuable insights for researchers in the field. The potential limitations of their application scenarios are also addressed.
... For example, as shown in Fig. 13(b) in the next section, the emission intensity can be correlated to the impact distance (impact energy). Smart micro-capsule-based composites can be designed such that the color of the capsulated material not only reports the damage but also quantifies the healing state, referred as dualfunction [36] For example, by changing from red, for severe damage, to colourless for complete restoration (Fig. 7) [5]. While researchers have been increasingly investigating smart selfhealing polymers over the last decade [37][38][39], due to the challenges in their manufacturing and implementation, dual function polymers are still limited [30]. ...
Recently emerging mechanochromic systems are becoming highly attractive for structural health monitoring (SHM) purposes in various industries, such as civil, wind, and aerospace, to improve the safety and performance of structures. These are based on self-reporting polymer composites which provide a lightweight sensor with an easy-to-read visual cue for SHM purposes. The present paper reports a critical overview of mechanochromic self-reporting approaches and discusses the outlook for future development in the field. Design principles and cutting-edge applications of the main physical-and chemical-based self-reporting mechanisms, i.e., mechanochromism based on dye-filled materials, modified polymers, structural color materials, and smart hybrid composite sensors, are presented with special attention to SHM. These emerging sensors create a new generation of user-friendly, cheap, and power-free SHM systems, guaranteeing economic and technological advantages that will open up new horizons for innovative, safer, and lighter composite products with significantly lower maintenance costs.
... Recently, a technique was introduced for monitoring self-healing coatings based on changes in color or fluorescence at damaged locations on the coating surface. [1][2][3][4][5][6][7][8][9][10] This technique is of great practical utility as it allows the condition of a coating to be assessed through visual inspection. ...
We report the development of an extrinsic self-healing coating system that shows no fluorescence from the intact coating, yellowish fluorescence in cracked regions, and greenish fluorescence in healed regions, thus allowing the separate monitoring of cracking and healing of coatings. This fluorescence monitoring self-healing system consisted of a top coating, an epoxy matrix resin containing mixed dye-loaded in single microcapsule. The dye-loaded microcapsules consisted of a poly(urea-formaldehyde) shell encapsulating a healing agent containing MAT-PDMS and styrene, a photo-initiator and a mixture of two dyes, one that fluoresces only in the solid state (DCM) and a second that fluoresces dramatically increased in the solid than solution state (4-TPAE). A mixture of the healing agent, photo-initiator and the two dyes was yellow due to fluorescence from DCM. On UV curing of this mixture, however, the color changed from yellow to green and the fluorescence intensity increased due to fluorescence from 4-TPAE in the solid state. When a self-healing coating embedded with microcapsules containing the DCM/4-TPAE dye mixture was scratched, the damaged region exhibited a yellowish color that changed to green after healing. Thus, the self-healing system reported here allows the separate monitoring of cracking and healing based on changes in fluorescence color.
... This is mainly due to the conditions at which the polymer will heal. Healing of the most applicable agent glycidyl methacrylate (GMA) with a modified aliphatic polyamine latent curing agent (EH-4360S) in an epoxy matrix, would require the 20 pole to be heated to 120 o C for 48 hours to return to 90% material integrity [21][22][23]. While this is not feasible today, self-healing polymers could be on the horizon for pole vault. ...
In this paper, a long‐lasting phosphorescent microcapsule based on in‐situ polymerization method is proposed for damage sensing. The successful microencapsulation of core material was evaluated by a Fourier transform infrared spectrometer and an energy dispersive spectrometer. The results demonstrated that the long‐lasting phosphorescent microcapsules prepared with urea‐formaldehyde resin shell can make the microcapsules resistant to the temperature of 170°C. The long‐lasting phosphorescent microcapsules can maintain excellent phosphorescence intensity before being heated to 170°C and after being excited by ultraviolet light. Even after being heated at the limit working temperature for 12 h, they can still maintain phosphorescence emission for more than 10 min after being excited by ultraviolet light. When cracks occur on the surface of the material, the microcapsules are broken and phosphorescence is emitted at the cracks under the irradiation of ultraviolet light. The microcapsules applied to the surface of concrete specimens still had good damage expression effects under different environmental brightness conditions. It provides a new method in the field of concrete crack monitoring based on microcapsule damage sensing materials.
The self-assembling process was conducted to fabricate graphene oxide microcapsules containing light-curing epoxy resin based on the Pickering emulsions in a single step. The chemical stability of microcapsules was improved through chemical stitching of GO nanosheets with polyether amine, while the content of epoxy resin reaching above 85 wt%. The entire highly-efficient process was energy-efficient, which ended up without barely wastes and residuals. In addition, the self-repairing coatings also produced satisfactory self-healing property and protective effect, due to the fracture of microcapsules and the reactions occurring under UV light irradiation on the surface of hot-dip galvanized steel.
Smart protection of metallic materials is currently a hot topic in mitigation of corrosion of metallic structures, and promises reduction in both economic and environmental costs. Degradation of carbon-reinforced composite materials and CFRP-Metal joints is a serious issue in high-performance weight-optimized structures employed in the aeronautical and automobile industries. Since both carbon-reinforced composite materials and CFRP-Metal joints are ubiquitous in these critical industries, there is a need to develop strategies for smart protection of such complex multi-material systems. In this article, the current state of art/practice in protection of carbon-reinforced composite materials and CFRP-Metal joints is reviewed, the theoretical basis, intricacies and limitation of current practice (in protection of carbon-reinforced composite materials and CFRP-Metal joints) highlighted, distinction made between protection of materials and smart protection of materials, and the need for smart protection of carbon-reinforced composite materials and CFRP-Metal joints are emphasized. In addition, the background for smart protection of carbon-reinforced composite materials and CFRP-Metal joints is laid, and drawing from literature and results from our own research efforts perspectives on critical factors to be considered, and plausible strategies for smart protection of carbon-reinforced composite materials and CFRP-Metal joints is provided.
The development of mechanochromic or self‐reporting polymers that can indicate damage or fatigue of materials with an optical signal has become of paramount interest to ensure the reliability of the materials and prevent catastrophic failure. This technology can potentially find usefulness for various applications, including in situ monitoring of mechanical events and structural health monitoring systems. An emerging and versatile approach to achieve mechanochromic properties relies on the encapsulation of dye solutions that can be released and activated (chemically or physically) when the walls of the capsules are mechanically damaged. While the mechanochromic effect can be achieved with different types of dyes and operating principles, this framework can also be designed with encapsulating‐containers of different shapes and shell materials, such as microcapsules, hollow glass fibers, vascular networks, and micelles, making this concept applicable to a broad range of polymer matrices. An overview of the different encapsulation approaches that have been employed to prepare mechanochromic polymers is given, with a focus on the containers used for this purpose. A brief description of the containers’ preparation is provided, and their associated chromic operating principles and progress in their designs are reviewed.
This paper provides an overview on the research progress of self-reporting corrosion protection coatings, which can autonomously indicate coating damages and metal corrosion at early stages. Typical sensing species in self-reporting coatings include fluorescent indicators, color indicators, and mechanically triggered indicators. Upon coating damage and corrosion initiation on the underlying metal substrate, the indicators are released and activated by either pH variation or the presence of metallic cation due to corrosion and other physicochemical reactions within the local defect. In this way, self-reporting coating systems can express strong fluorescence signals or color change to highlight the coating failure. The sensing principles, influencing factors, coloring performances, advantages and drawbacks of different types of self-reporting coatings are discussed. Incorporation of indicators into micro- or nanocapsules is highlighted to enhance their reliability as well as to optimize the fluorescence/coloration performance. In addition, dual-functional coatings combining both the self-reporting and self-healing capacities are introduced, which can not only detect the coating damages but also autonomously recover the barrier property. The development of self-reporting corrosion protection coatings thus holds great potential to improve the safety and lifetime for materials in various industrial applications.
A microcapsule-type visualization sensor for concrete structural damage indication is proposed in this article. Crystal violet lactone, as damage indicator, was microencapsulated within poly(methyl methacrylate) to synthesize the sensor. The successful encapsulation was confirmed by Fourier transform infrared spectrometry. Microcapsules of different diameters and size distributions were obtained by varied stirring speeds. The fabricated microcapsules were embedded into a polymer coating to accomplish the damage indication. When cracks propagated in the coating, the crystal violet lactone in leuco form was released from the ruptured microcapsules. Due to reacting with silicon dioxide in concrete, the released crystal violet lactone turned blue and highlighted the damaged area. It was verified that the visualization performance of the sensor showed good durability in both dry and wet conditions. The proposed microcapsule-type visualization sensor has advantages of easy fabrication, high indication stability, and no special equipment requirements, which will reduce the complexity of concrete structural health monitoring significantly.
Polymeric materials are susceptible to small damage which is undetectable. Without timely and effective repair treatment, the damage may deteriorate the integrity of materials and ultimately result in the material failure and catastrophe. Autonomous warning and repairing the damage simultaneously is of great practical significance yet difficult to realize. Herein, we introduce a smart coating with autonomous warning of and repairing damage by simple incorporating nanosensors embedded with phenanthroline as a corrosion indicator and inhibitor. The electrochemical corrosion resulting from coating damage can be rapidly warned by a prominent orange red color in just five minutes. Accompanied with the warning function, the smart coating exhibits efficient repairing performance on defected region, as reflected from the disappearance of electrochemical admittance peak. This simple and powerful strategy dependent on single active component to achieve autonomous warning and repairing effect is highly expected to provide a new avenue for enhancing the security and longevity of other polymeric materials.
A repeatable self-healing epoxy composite mechanically enhanced by graphene nanosheets (GNS) was prepared from an epoxy monomer with Diels-Alder (DA) bonds, octanediol glycidyl ether (OGE) and polyether amine (D230). The GNS/epoxy composites, with a maximum tensile modulus of 14.52 ± 0.45 MPa and elongation at break more than 100%, could be healed several times under Infrared (IR) light with the healing efficiency as high as 90% through the molecule chain mobility and the rebonding of reversible DA bonds between furan and maleimide. Also, they displayed excellent recyclable ability by transforming into a soluble polymer, which offers a wide range of possibilities to produce epoxy flexible materials with healing and removable abilities.
Inspired by biological systems, self‐healing coatings have been fabricated to protect metals against corrosion. However, in situ monitoring of the corrosion dynamics for various self‐healing strategies generally remains a big challenge due to different working mechanisms. In the present work, a universal intelligent‐sensing coating (SC) system containing pH‐responsive polymer microspheres with a color probe is developed. When corrosion occurs in the self‐healing system, the color around cracks turns pink gradually over time owing to the increased pH value. For the high‐performance self‐healing coatings, the onset and propagation of corrosion is suppressed, thereby leading to a narrow light‐pink‐color area. With this smart SC approach, the corrosion dynamics is established for three self‐healing strategies by the correlation between the width of color lines with time. The anticorrosion ability in 48 h for the three extrinsic self‐healing strategies are evaluated; that is, the SC with benzotriazole‐loaded poly(divinylbenzene)‐graft‐poly(divinylbenzene‐co‐acrylic acid) microspheres (PDVB‐graft‐P(DVB‐co‐AA)‐BTA) is superior to that with BTA‐loaded halloysite (Halloysite‐BTA), which surpasses that with polyurethane/poly(urea‐formaldehyde) microcapsules filled with isophorone diisocyanate (IPDI@PU/PUF). These results are consistent with electrochemical experiments. This smart‐sensing coating system can be a promising alternative for the in situ investigation of the anticorrosion performance of various self‐healing anticorrosion strategies.
Encapsulation of polyamine for practical application of self-healing epoxy is promising yet challenging due to their high reactivity and good solubility in water and most organic solvents. Here, we develop an innovative method to directly synthesize microcapsules containing pure polyamine by integrating microfluidic emulsion and interfacial polymerization. By this integration to make full use of the advantages and avoiding shortcomings of the involved two techniques, properties of the fabricated microcapsule can be delicately tailored according to practical demands of self-healing materials. Superiority of this achieved polyamine microcapsule is demonstrated by a dual-microcapsule high-performance self-healing system with fully autonomous recoverability, high thermal and long-term stability, relatively fast healing kinetics. Highest healing efficiency of 111±12% in term of recovered mode I fracture toughness is achieved at room temperature for 48h without any external interventions. The high performance, environmental stability, as well as low cost and toxicity introduced by the robust microcapsules, promote the potential practical application of this self-healing system.
Sensing of damage, deformation, and mechanical forces is of vital importance in many applications of fiber‐reinforced polymer composites, as it allows the structural health and integrity of composite components to be monitored and microdamage to be detected before it leads to catastrophic material failure. Bioinspired and biomimetic approaches to self‐sensing and self‐reporting materials are reviewed. Examples include bruising coatings and bleeding composites based on dye‐filled microcapsules, hollow fibers, and vascular networks. Force‐induced changes in color, fluorescence, or luminescence are achieved by mechanochromic epoxy resins, or by mechanophores and force‐responsive proteins located at the interface of glass/carbon fibers and polymers. Composites can also feel strain, stress, and damage through embedded optical and electrical sensors, such as fiber Bragg grating sensors, or by resistance measurements of dispersed carbon fibers and carbon nanotubes. Bioinspired composites with the ability to show autonomously if and where they have been damaged lead to a multitude of opportunities for aerospace, automotive, civil engineering, and wind‐turbine applications. They range from safety features for the detection of barely visible impact damage, to the real‐time monitoring of deformation of load‐bearing components.
Autonomous highlighting of damages in protective polymer coatings allows on-demand maintenance and enables lifetime-prolongation of the coated materials. To follow the whole cycle of damage occurrence and successful healing, one has to be able to visualize both processes and display the current health-state of the coating. Herein, we equip coatings with nanocapsules that can self-indicate their damaging via a color development. Hence, whenever the coating is damaged, the capsules break and highlight the damaged spot. As a second feature, the color development is reversed and discoloration occurs in the presence of (self-)healing compounds allowing the user to follow the healing process. Thus, in a first step damages are being highlighted via color “turn-on” and, in a subsequent second step, a propagating healing reaction “turns-off” the damage indication system to trace the healing reaction and allow monitoring of the entire health-cycle.
Cells identify defective mitochondria and eliminate them through mitophagy: this allows cells to rid themselves of unwanted stress to maintain health and avoid the activation of cell death. One approach to experimentally investigate mitophagy is through the use of mitochondrial photosensitizers, which when coupled with light allows one to precisely control mitochondrial damage with spatial and temporal precision. Here we report three far-red fluorophores that can be used as robust mitochondrial photosensitizers to initiate mitophagy. The dyes offer maximal compatibility with multi-color live-cell imaging, as they do not spectrally overlap with commonly used fluorescent proteins. Through the use of these far-red fluorescent photosensitizers we found that mitophagic engulfment and mitophagosome maturation rates are highly correlated with the cellular Parkin-labeled mitochondria levels. This may represent a protective cellular mechanism to avoid membrane and lysosome depletion during mitophagy.
Commercially available polystyrene is coupled with self-healability by exploiting a novel healing chemistry of redox cationic polymerization. In this system, iodonium bis(4-methylphenyl) hexafluorophosphate (IBH)/glycidyl methacrylate (GMA) loaded microcapsules and NaBH4 particles are embedded in the matrix through compression molding. The healant is oxygen insensitive and heat resistant so that it meets the requirement of remendable thermoplastics.
The fluorescence lifetime (τf), emission quantum yield (Φf), absorption and emission spectral data of 20 fluorescein derivatives were measured under the same conditions by using time-correlated single photon counting, steady state fluorescence and absorption methods to get comparable data. Based on the results, the factors and mechanism that control the fluorescence properties of the fluorescein dyes are discussed. Both Φf and τf are remarkably dependent on the substitution on either xanthene or phenyl rings, but their ratio (Φf/τf), i.e. rate constant of radiation process, is a constant value (0.20 × 10(9) s(-1)). The rate constant of nonradiation process, on the other hand, is varied with both the structure and the solvent used.
Carbon-fibre-reinforced polymer composites with an enhanced yellow fluorescent protein (eYFP) at the interface of fibres and resin were prepared. The protein was immobilized on the carbon fibres by physisorption and by covalent conjugation, respectively. The immobilized eYFP fluoresced on the carbon fibres, in contrast to non-protein fluorophores that were fully quenched by the carbon surface. The fibres were embedded into epoxy resin, and the eYFP remained fluorescent within the composite material. Micromechanical tests demonstrated that the interfacial shear strength of the material was not altered by the presence of the protein. Immunostaining of single fibre specimen revealed that eYFP loses its fluorescence in response to pull-out of fibres from resin droplets. The protein was able to detect barely visible impact damage such as fibre–resin debonding and fibre fractures by loss of its fluorescence. Therefore, it acts as a molecular force and stress/strain sensor at the fibre–resin interface and renders the composite self-sensing and self-reporting of microscopic damage. The mechanoresponsive effect of the eYFP did not depend on the type of eYFP immobilization. Fibres with the physisorbed protein gave similar results as fibres to which the protein was conjugated via covalent linkers. The results show that fluorescent proteins are compatible with carbon fibre composites. Such mechanophores could therefore be implemented as a safety feature into composites to assure material integrity and thereby prevent catastrophic material failure.
Covalently crosslinked materials, classically referred to as thermosets, represent a broad class of elastic materials that readily retain their shape and molecular architecture through covalent bonds that are ubiquitous throughout the network structure. These materials, in particular in their swollen gel state, have been widely used as stimuli responsive materials with their ability to change volume in response to changes in temperature, pH, or other solvent conditions and have also been used in shape memory applications. However, the existence of a permanent, unalterable shape and structure dictated by the covalently crosslinked structure has dramatically limited their abilities in this and many other areas. These materials are not generally reconfigurable, recyclable, reprocessable, and have limited ability to alter permanently their stress state, topography, topology, or structure. Recently, a new paradigm has been explored in crosslinked polymers - that of covalent adaptable networks (CANs) in which covalently crosslinked networks are formed such that triggerable, reversible chemical structures persist throughout the network. These reversible covalent bonds can be triggered through molecular triggers, light or other incident radiation, or temperature changes. Upon application of this stimulus, rather than causing a temporary shape change, the CAN structure responds by permanently adjusting its structure through either reversible addition/condensation or through reversible bond exchange mechanisms, either of which allow the material to essentially reequilibrate to its new state and condition. Here, we provide a tutorial review on these materials and their responsiveness to applied stimuli. In particular, we review the broad classification of these materials, the nature of the chemical bonds that enable the adaptable structure, how the properties of these materials depend on the reversible structure, and how the application of a stimulus causes these materials to alter their shape, topography, and properties.
Metals are used extensively in modern society in a range of applications from infrastructure to aircraft to consumer products. The protection of metals, primarily from corrosion, has been an active area of materials science for many years. However, over the last 20 years, changing regulations governing both environmental issues and human health have driven even greater activity in this field. Addressing these regulatory changes presents some of the most exciting challenges in materials science. This review looks at current metal protection schemes, exploring the development of 'green' inhibitors and 'self-healing' paint films that have inbuilt capacity to maintain functionality. Inorganic and organic materials science has undergone rapid development in recent decades and this review looks at how some of those developments, particularly in encapsulation and polymer healing, can be applied to the design of new protective paint systems.
Self-healing polymers and fiber-reinforced polymer composites possess the ability to heal in response to damage wherever and whenever it occurs in the material. This phenomenal material behavior is inspired by biological sys-tems in which self-healing is commonplace. To date, self-healing has been demonstrated by three conceptual approaches: capsule-based healing sys-tems, vascular healing systems, and intrinsic healing polymers. Self-healing can be autonomic—automatic without human intervention—or may require some external energy or pressure. All classes of polymers, from thermosets to thermoplastics to elastomers, have potential for self-healing. The major-ity of research to date has focused on the recovery of mechanical integrity following quasi-static fracture. This article also reviews self-healing during fatigue and in response to impact damage, puncture, and corrosion. The con-cepts embodied by current self-healing polymers offer a new route toward safer, longer-lasting, fault-tolerant products and components across a broad cross section of industries including coatings, electronics, transportation, and energy.
Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).
Cracks in polymer coatings and composites can lead to faster degradation of the underlying substrates or a reduction in the mechanical performance of the composite over time.
High resolution in situ autonomous visual indication of mechanical damage is achieved through a microcapsule-based polymeric material system. Upon mechanical damage, ruptured microcapsules release a liquid indicator molecule. A sharp color change from light yellow to bright red is triggered when the liberated indicator 2′,7′-dichlorofluorescein reacts with the polymeric coating matrix.
Solvents have been used for self-healing materials for swelling mechanism. However, no study has investigated the prediction of solvent healing performance. In this work, we used Hansen solubility parameters (HSPs) to predict crack healing. A 2D solvent map was constructed by polarity and hydrogen bonding parameter. Results showed that the degree of swelling was relative to the HSP distance (D) between solvent and epoxy resin in the map. Moreover, manual healing experiments showed that solvent healing performance was correlated with swelling. Therefore, the healing performance of solvents could be predicted by using HSPs. For epoxy resin, solvents with D<15 had excellent healing performance. This framework simultaneously considered D and water solubility and could be readily extended to fast screen green solvents for microcapsule-loaded self-healing materials.
Self-healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness-enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self-healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.
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The study of fracture behavior of self-healing polymers is the key-tool that issues the efficacy of the advanced healing functionality. Damage mechanisms of autonomously healed polymers resemble the adhesive joints one. Thus, well-established fracture testing configurations of adhesive composites are acquired and adopted in order to monitor the damage initiation (cracking) that calls for healing recuperation. As soon as healing is achieved the loading tests are repeated in order to evaluate the newly built resistance to damage. The innovative analysis of crack arrest (and/or closure) fracture mechanisms due to healing requires simple and sophisticated crack opening (mode I of fracture) settings as the commonly used tapered double cantilever beam (TDCB) test. Experimental research on TDCB shaped polymers has always gone along with self-healing technology (introduced on White's protocol). Furthermore, other classical testing configurations contribute as alternatives of TDCB setting (e.g. single-edge notched beam tensile tests) measuring the fracture toughness or as complementary mechanical features evaluating systems (e.g. tensile, shear, debonding tests).
For the first time, repeatable self-healing was achieved in a cross-linked epoxy polymer by incorporating 2-ethyl-4-methylimidazole (24-EMI) into the matrix as a latent polymerization initiator. Upon material damage and infiltration of liquid EPON 8132 epoxy monomer healing agent into the crack plane, polymerization occurs in the damaged region with a moderate application of heat in the presence of the latent imidazole initiator. Using tapered double cantilever beam (TDCB) fracture testing, greater than 90% recovery of fracture toughness was observed over multiple healing cycles in samples containing 10 wt% 24-EMI, with up to 11 repeat healing cycles possible. The effect of incorporating the imidazole on the host epoxy fracture toughness, complex moduli and glass transition temperature was also investigated. As imidazole concentration increases, a reduction in glass transition temperature and an increase in fracture toughness of the host epoxy is observed.
Force-induced covalent bond changes in mechanophore-linked polymers typically require large, irreversible material deformation, limiting successive activation cycles. Now, repeated force-induced reactions have been achieved by incorporating flex-activated mechanophores into elastomeric networks.
Self-healing materials should take effect immediately following crack generation in principle, but the speed of autonomic recovery of mechanical properties through either extrinsic or intrinsic healing strategy reported so far is not that fast. Mostly, a couple of hours are taken for reaching steady state or maximum healing. To obviously accelerate the healing process, the authors of this work make use of antimony pentafluoride as instant hardener of epoxy, and successfully encapsulate the highly active antimony pentafluoride-ethanol complex in terms of hollow silica spheres. Accordingly, self-healing agent based on microencapsulated antimony pentafluoride-ethanol complex and epoxy monomer is developed. Epoxy material with the embedded healant capsules can thus be healed within a few seconds, as demonstrated by impact and fatigue tests. It is believed that the outcome presented here might help to move the self-healing technique closer to practical application, especially when the engineering significance of epoxy material is concerned.
A bifunctional single-component healant, glycidyl methacrylate (GMA), is encapsulated and employed for fabricating self-healing epoxy materials. The released GMA is able to rebond cracked portions at room temperature through hydrogen and covalent bonds, and hence recover fracture toughness with high efficiency. The main advances of the healing system lie in the following. (i) It simplifies the conventional approach using two-part healing agent and broadens applicability of the therapy since two healing mechanisms, solvent effect and chemical reactions, are involved. (ii) As GMA contains both epoxide groups and CC bonds, ring-opening and nucleophilic addition reactions between GMA and the residual amine in the matrix occur during crack healing and help to reconnect the separated faces. The application of nucleophilic addition, which has not yet been reported as a healing measure, might lead to expansion of the spectrum of self-healing agent because the species of organic molecules enabling nucleophilic addition reaction are far more than those with specific functional groups like epoxide.
Yellow fluorescent protein (YFP) is used as a mechanoresponsive layer at the fiber-resin interface in glass-fiber-reinforced composites. The protein loses its fluorescence when subjected to mechanical stress. Within the material, it reports interfacial shear debonding and barely visible impact damage by a transition from a fluorescent to a non-fluorescent state.
Mechanically responsive polymers harness mechanical energy to facilitate unique chemical transformations and bestow materials with force sensing (e.g., mechanochromism) or self-healing capabilities. A variety of solution- and solid-state techniques, covering a spectrum of forces and strain rates, can be used to activate mechanically responsive polymers. Moreover, many of these methods have been combined with optical spectroscopy or chemical labeling techniques to characterize the products formed via mechanical activation of appropriate precursors in situ. In this tutorial review, we discuss the methods and techniques that have been used to supply mechanical force to macromolecular systems, and highlight the advantages and challenges associated with each.
Strain in polyetherurea thermoplastic elastomers with poly(tetramethylene oxide) soft blocks and well-defined bisurea hard blocks was probed through changes in fluorescence resonance energy transfer between both covalently linked and randomly dispersed donor and acceptor probe molecules. The covalently linked fluorophores are shown to have superior strain sensitivity, while the dispersed probes give information on the affinity of deformation at the nanometer length scale. The method is proposed to be generally applicable to study deformation of elastomers at the molecular scale.
A self-healing system based on conventional epoxy resin was successfully developed in this work. Epoxy and its hardener mercaptan were microencapsulated as two-component healing agent, and then the microcapsules were embedded in epoxy matrix. Attractive healing effect can be acquired at low capsule content (e.g., 43.5% healing efficiency with 1 wt % capsules and 104.5% healing efficiency with 5 wt % capsules at 20 °C for 24 h). Since only a few healant proves to be sufficient for crack repairing, a better balance between strength and toughness restoration can thus be achieved. As a result of high flowability, fast consolidation, and molecular miscibility of the released healing agent consisting of epoxy and mercaptan, self-healing was allowed to proceed rapidly offering satisfactory repair effectiveness.
A two-component healing agent, consisting of epoxy-loaded microcapsules and an extremely active catalyst (boron trifluoride diethyl etherate, (C2H5)2O · BF3)), is incorporated into epoxy composites to provide the latter with rapid self-healing capability. To avoid deactivation of the catalyst during composite manufacturing, (C2H5)2O · BF3 is firstly absorbed by fibrous carriers (i.e., short sisal fibers), and then the fibers are coated with polystyrene and embedded in the epoxy matrix together with the encapsulated epoxy monomer. Because of gradual diffusion of the absorbed (C2H5)2O · BF3 from the sisal into the surrounding matrix, the catalyst is eventually distributed throughout the composites and acts as a latent hardener. Upon cracking of the composites, the epoxy monomer is released from the broken capsules, spreading over the cracked planes. As a result, polymerization, triggered by the dispersed (C2H5)2O · BF3, takes place and the damaged sites are rebonded. Since the epoxy–BF3 cure belongs to a cationic chain polymerization, the exact stoichiometric ratio of the reaction components required by other healing chemistries is no longer necessary. Only a small amount of (C2H5)2O · BF3 is sufficient to initiate very fast healing (e.g., a 76% recovery of impact strength is observed within 30 min at 20 °C).
Nature uses mechanochemical transduction processes to achieve diverse and vital functions, such as hearing, cellular adhesion and gating of ion channels. One fascinating example of biological mechanotransduction is the emission of light on mechanical stimulation. However, molecular-level transduction of force into luminescence in a synthetic system remains a challenge. Here, we show that bis(adamantyl)-1,2-dioxetane emits visible light when force is applied to a polymer chain or network in which this unit is incorporated. Bright-blue luminescence was observed on sonication of solutions of dioxetane-containing linear polymers and on the straining of polymer networks with dioxetane crosslinkers. Light is emitted from the adamantanone-excited state that forms on opening of the four-membered dioxetane ring. Increased sensitivity and colour tuning were achieved by energy transfer to suitable acceptors. High spatial and temporal resolutions highlight the potential to study the failure of polymeric materials in unprecedented detail.
To provide epoxy based composites with self-healing ability, two-component healing system consisting of urea–formaldehyde microcapsules containing epoxy (30–70 μm in diameter) and CuBr2(2-MeIm)4 (the complex of CuBr2 and 2-methylimidazole) latent hardener was synthesized. When cracks were initiated or propagated in the composites, the neighbor microencapsulated epoxy healing agent would be damaged and released. As the latent hardener is soluble in epoxy, it can be well dispersed in epoxy composites during composites manufacturing, and hence activate the released epoxy wherever it is. As a result, repair of the cracked sites is completed through curing of the released epoxy. The present paper studied the preparation of epoxy microcapsules by amino resins, and the influencing factors as well. On the basis of this work, mechanical properties of the epoxy filled with the healing system were evaluated. It was found that incorporation of the two-component healing system nearly did not change the fracture toughness of the neat epoxy, as indicated by the single-edge notched bending test. In the case of 10 wt% microcapsules and 2 wt% latent hardener, the self-healing epoxy exhibited a 111% recovery of its original fracture toughness. Besides, the preliminary result of double-cantilever beam testing showed that the plain weave glass fabric laminates using the above self-healing epoxy as the matrix received a healing efficiency of 68%.
Microcapsules containing curing agent for epoxy were successfully prepared by in situ polymerization with poly(melamine–formaldehyde) (PMF) as the shell material and high-activity polythiol (pentaerythritol tetrakis (3-mercaptopropionate), PETMP) as the core substance. Having been encapsulated, the core material PETMP had the same activity as its raw version. The synthesis approach was so improved that the consumption of polythiol was reduced to a low level. By carefully analyzing the influencing factors including catalyst concentration, reaction time, reaction temperature, feeding weight ratio of core/shell monomers, dispersion rate and emulsifier content, the optimum synthetic conditions were found out. The results indicated that not only core content and size of the microcapsules but also thickness and strength of the shell wall can be readily adjusted by the proposed technical route. The relatively thin shell wall (∼0.2 μm) assured sufficient core content even if the microcapsules were very small (1–10 μm). The polythiol-loaded microcapsules proved to be qualified for acting as the mate of epoxy in making two-part microencapsulated healing agent of self-healing composites.
Self-healing materials are able to partially or completely heal damage inflicted on them, e.g., crack formation; it is anticipated that the original functionality can be restored. This article covers the design and generic principles of self-healing materials through a wide range of different material classes including metals, ceramics, concrete, and polymers. Recent key developments and future challenges in the field of self-healing materials are summarised, and generic, fundamental material-independent principles and mechanism are discussed and evaluated.
Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups-so-called mechanophores-that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.
Unter Spannung: Ändert sich in einem Protein-Polymer-Hybridmaterial die mechanische Spannung der Polymermatrix, so löst dies eine Konformationsänderung des Proteinkomplexes aus: Das Material „meldet“ eine strukturelle Schädigung (siehe Bild). Die Reporterkomponente ist ein Chaperonin, das ein Paar fluoreszierender Proteine kovalent bindet. Wird das Chaperonin deformiert, ändert sich der Abstand zwischen den Fluorophoren und folglich auch das FRET-Signal.
- Jin