Autonomous Damage Detection in Multilayered Coatings via Integrated Aggregation-Induced Emission Luminogens

Photo-modulated activation of organic bases enabling microencapsulation and on-demand reactivity

A method is developed for facile encapsulation of reactive organic bases with potential application for autonomous damage detection and self-healing polymers. Highly reactive chemicals such as bases and acids are challenging to encapsulate by traditional oil-water emulsion techniques due to unfavorable physical and chemical interactions. In this work, reactivity of the bases is temporarily masked with photo-removable protecting groups, and the resulting inactive payloads are encapsulated via an in situ emulsion-templated interfacial polymerization method. The encapsulated payloads are then activated to restore the organic bases via photo irradiation, either before or after being released from the core-shell carriers. The efficacy of the photo-activated capsules is demonstrated by a damage-triggered, pH-induced color change in polymeric coatings and by recovery of adhesive strength of a damaged interface. Given the wide range of potential photo-deprotection chemistries, this encapsulation scheme provides a simple but powerful method for storage and targeted delivery of a broad variety of reactive chemicals, promoting design of diverse autonomous functionalities in polymeric materials.

Optically Sensitive and Magnetically Identifiable Supraparticles as Indicators of Surface Abrasion

Identifying and ensuring the integrity of products plays an important role in today's globalized world. Miniaturized information taggants in the packaging surface are therefore required to monitor the product itself instead of applying external labels. Ideally, multiple types of information are stored in such additives. In this work, micrometer-sized core-shell particles (supraparticles) were developed to provide material surfaces with both an identifier and a surface abrasion indication functionality. The core of the supraparticles contains iron oxide nanoparticles that allow identification of the surface with a spectral magnetic code resolved by magnetic particle spectroscopy. The fluorescent silica nanoparticles in the supraparticle shell can be abraded by mechanical stress and resolved by fluorescence spectroscopy. This provides information about the mechanical integrity of the system. The application as surfaces, that contain several types of information in one supraparticle, was demonstrated here by incorporating such bifunctional supraparticles as additives in a surface coating.

Supraparticles with a Mechanically Triggerable Color-Change-Effect to Equip Coatings with the Ability to Report Damage

Small scratches and abrasion cause damage to packaging coatings. Albeit often invisible to the human eye, such small defects in the coating may ultimately have a strong negative impact on the whole system. For instance, gases may penetrate the coating and consequently the package barrier, thus leading to the degradation of sensitive goods. Herein, the indicators of mechanical damage in the form of particles are reported, which can readily be integrated into coatings. Shear stress-induced damage is indicated by the particles via a color change. The particles are designed as core-shell supraparticles. The supraparticle core is based on rhodamine B dye-doped silica nanoparticles, whereas the shell is made of alumina nanoparticles. The alumina surface is functionalized with a monolayer of a perylene dye. The resulting core-shell supraparticle system thus contains two colors, one in the core and one in the shell part of the architecture. Mechanical damage of this structure exposes the core from the shell, resulting in a color change. With particles integrated into a coating lacquer, mechanical damage of a coating can be monitored via a color change and even be related to the degree of oxygen penetration in a damaged coating.

Aggregation-Induced Emission Boosting the Study of Polymer Science

The past one hundred years witness the great development of polymer science. The advancement of polymer science is closely related with the developing of characterization techniques and methods, from viscometry in molecular weight determination to advanced techniques including differential scanning calorimetry, nuclear magnetic resonance and scanning electron microscopy. However, these techniques are normally constrained to tedious sample preparation, high cost, harsh experimental condition, or ex-situ characterization. Fluorescence technology has the merits of high sensitivity and direct visualization. Contrary to conventional aggregation-causing quenching fluorophores, those dyes with aggregation-induced emission characteristic show high emission efficiency in aggregate states. Based on the restriction of intramolecular motions for AIE properties, the AIE materials are very sensitive to the surrounding microenvironments owing to the twisted propeller-like structures and therefore reveal great potentials in polymer's study. The AIE concept has been successfully used in polymer's study and provides us a deeper understanding on polymer structure and properties. In this review, the applications of AIEgens in polymer science for visualizing polymerization, glass transition, dissolution, crystallization, gelation, self-assembly, phase separation, cracking and self-healing were exemplified and summarized. Lastly, the challenges and perspectives in the study of polymer science using AIEgens are addressed. This article is protected by copyright. All rights reserved.

Stimulus Responsive Zeolitic Imidazolate Framework to Achieve Corrosion Sensing and Active Protecting in Polymeric Coatings

Metal substrates beneath polymeric coatings are susceptible to localized corrosion, which could result in lifetime reduction and catastrophic failure without timely repair treatment. In situ detection of corrosion and repair coating defects are in high demand yet challenging to fulfill so far. Herein, we report a smart polymeric coating by integrating nanosensors into the coating matrix, which is capable of efficient corrosion sensing and active anticorrosion protecting. The nanosensors were constructed by zeolitic imidazolate framework encapsulated with the polyethylene glycol-tannic acid complex. The morphology, chemical constitution, and stimulus responsiveness of nanosensors were systematically analyzed. The generation of local corrosion beneath coating can be promptly sensed and reported by a conspicuous purple color derived from tannic-iron ion coordinates. Meanwhile, local electrochemical impedance spectroscopy results proved that the metal degradation process at the defected interface can be largely inhibited, exhibiting active anticorrosion property. Furthermore, the constructed smart coating possessed superior impermeability and long-term protective performance under simulated seawater and harsh salts spray conditions. This feasible and effective strategy based on simple nanosensors to engineer smart coatings paves a new way to develop high environmental adaptability protective materials with protecting, corrosion sensing, and self-healing functions.

Functional Polymer Systems with Aggregation-Induced Emission and Stimuli Responses

Functional polymer systems with stimuli responses have attracted great attention over the years due to their diverse range of applications. Such polymers are capable of altering their chemical and/or physical properties, such as chemical structures, chain conformation, solubility, shape, morphologies, and optical properties, in response to single or multiple stimuli. Among various stimuli-responsive polymers, those with aggregation-induced emission (AIE) properties possess the advantages of high sensitivity, fast response, large contrast, excellent photostability, and low background noise. The changes in fluorescence signal can be conveniently detected and monitored using portable instruments. The integration of AIE and stimuli responses into one polymer system provides a feasible and effective strategy for the development of smart polymers with high sensitivity to environmental variations. Here, we review the recent advances in the design, preparation, performance, and applications of functional synthetic polymer systems with AIE and stimuli responses. Various AIE-based polymer systems with responsiveness toward single physical or chemical stimuli as well as multiple stimuli are summarized with specific examples. The current challenges and perspectives on the future development of this research area will also be discussed at the end of this review.

Polymer-Based Mechanochromic Composite Material Using Encapsulated Systems

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.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Mechanochromic Fluorescent Polymers Enabled by AIE Processes

Polymeric materials are susceptible to the chain re-conformation, reorientation, slippage, and bond cleavage upon mechanical stimuli, which are likely to further grow into macro-damages and eventually lead to the compromise or loss of materials performance. Therefore, it is of great academic importance and practical significance to sensitively detect the local mechanical states in polymers and monitor the dynamic variations in polymer structures and properties under external forces. Mechanochromic fluorescent polymers (MFP) are a class of smart materials by utilizing sensitive fluorescent motifs to detect polymer chain events upon mechanical stimuli. Taking advantage of the unique aggregation-induced emission (AIE) effect, a variety of MFP systems that can self-report their mechanical states and mechano-induced structural and property changes through fluorescence signals have been developed. In this feature article, an overview of the recent progress on MFP systems enabled by AIE process is presented. The main design principles, including physically doping dispersed or microencapsulated AIE luminogens (AIEgens) into polymer matrix, chemically linking AIEgens in polymer backbones, and utilizing the clusterization-triggered emission of polymers containing nonconventional luminogens, are discussed with representative examples. Perspectives on the existing challenges and problems in this field are also discussed to guide future development.

Synthesis of nanosensors for autonomous warning of damage and self-repairing in polymeric coatings

Polymeric materials are susceptible to minor damage, which is undetectable. Without timely and effective repair treatment, the damage may deteriorate the integrity of the materials and ultimately result in material failure and catastrophe. Autonomous warning and simultaneous damage repair are of great practical significance yet difficult to realize. Herein, we introduce a smart coating with autonomous warning and repairing of damage by the simple incorporation of nanosensors embedded with phenanthroline as a corrosion indicator and inhibitor. The electrochemical corrosion resulting from coating damage can be rapidly indicated by a prominent orange-red color in just five minutes. In addition to the warning function, the smart coating exhibits efficient self-repairing in the defective region, as reflected from the disappearance of the electrochemical admittance peak. This simple and powerful strategy dependent on a single active component to achieve an autonomous warning and repairing effect is highly expected to provide a new avenue for enhancing the security and longevity of other polymeric materials.

A Facile Strategy To Prepare Smart Coatings with Autonomous Self-Healing and Self-Reporting Functions

Herein, we report a smart coating with autonomous self-healing and self-reporting functions by simple integration of one-component microcapsules into the matrix without external intervention. The microcapsules containing hexamethylene diisocyanate (HDI) solution of aggregation-induced emission luminogens (AIEgens) were synthesized, and their properties, such as their composition, thermal stability, morphology, and damage-indicating ability, were investigated systematically. The AIEgen/HDI microcapsule-embedded coatings display adaptive self-repair of scratches and simultaneous high-contrast indication of the healed damage. Two commercialized AIEgens, tetraphenylethylene (TPE) and its derivative with dimethoxyl and benzylidene-methyloxazolone moieties (DM-TPE-BMO), were utilized as examples to demonstrate the feasibility of this concept in diverse polymer matrixes (including blue autofluorescent matrixes). It was found that the content of AIEgens can even be lowered to 0.05 wt %. This facile, economical, and feasible strategy toward the dual functions of self-repairing and self-sensing provides a new route for enhancing the longevity and reliability of polymer coatings, which is appealing and of great importance in practical applications.