Highly Transparent, Nanofiller-Reinforced Scratch-Resistant Polymeric Composite Films Capable of Healing Scratches

Healable Supracolloidal Nanocomposite Water-Borne Coatings

Water-borne coatings often contain nanofillers to enhance their mechanical or optical properties. The aggregation of these fillers may, however, lead to undesired effects such as brittle and opaque coatings, reducing their performance and lifetime. By controlling the distribution and structural arrangement of the nanofillers in the coatings and inserting reversible chemical bonds, both the elasticity and strength of the coatings may be effectively improved, while healing properties, via the reversible chemistry, extend the coating's lifetime. Aqueous dispersions of polymer-core/silica-corona supracolloidal particles were used to prepare water-borne coatings. Polymer and silica nanoparticles were prefunctionalized with thiol/disulfide groups during the supracolloid assembly. Disulfide bridges were further established between a cross-linker and the supracolloids during drying and coating formation. The supracolloidal nanocomposite coatings were submitted to intentional (physical) damages, i.e., blunt and sharp surface scratches or cut through into two pieces, and subsequently UV irradiated to induce the recovery of the damage(s). The viscoelasticity and healing properties of the coatings were examined by dynamic, static, and surface mechanical analyses. The nanocomposite coatings showed a great extent of interfacial restoration of cut damage and surface scratches. The healing properties are strongly related to the coating's viscoelasticity and interfacial (re)activation of the disulfide bridges. Nanocomposite coatings with silica concentrations below their critical volume fraction show higher in situ healing efficiency, as compared to coatings with higher silica concentration. This work provides insights into the control of nanofillers distribution in water-borne coatings and strategies to increase the coating lifetime via mechanical damage recovery.

Recent Advances of Self-Healing Polymer Materials via Supramolecular Forces for Biomedical Applications

Noncovalent interactions can maintain the three-dimensional structures of biomacromolecules (e.g., polysaccharides and proteins) and control specific recognition in biological systems. Supramolecular chemistry was gradually developed as a result, and this led to design and application of self-healing materials. Self-healing materials have attracted attention in many fields, such as coatings, bionic materials, elastomers, and flexible electronic devices. Nevertheless, self-healing materials for biomedical applications have not been comprehensively summarized, even though many reports have been focused on specific areas. In this Review, we first introduce the different categories of supramolecular forces used in preparing self-healing materials and then describe biological applications developed in the last 5 years, including antibiofouling, smart drug/protein delivery, wound healing, electronic skin, cartilage lubrication protection, and tissue engineering scaffolds. Finally, the limitations of current biomedical applications are indicated, key design points are offered for new biological self-healing materials, and potential directions for biological applications are highlighted.

Superior Hard but Quickly Reversible Si-O-Si Network Enables Scalable Fabrication of Transparent, Self-Healing, Robust, and Programmable Multifunctional Nanocomposite Coatings

A coating with programmable multifunctionality based on application requirements is desirable. However, it is still a challenge to prepare a hard and flexible coating with a quick self-healing ability. Here, a hard but reversible Si-O-Si network enabled by aminopropyl-functionalized poly(silsesquioxane) and triethylamine (TEA) was developed. On the basis of this Si-O-Si network, basic coatings with excellent transparency, hardness, flexibility, and quick self-healing properties can be prepared by filling soft polymeric micelles into hard poly(silsesquioxane) networks. The highly cross-linked continuous network endows the coating with a hardness (H = 0.83 GPa) higher than those of most polymers (H < 0.3 GPa), while the uniformly dispersed micelles decrease the Young's modulus (E = 5.89 GPa) to a value as low as that of common plastics, resulting in excellent hardness and flexibility, with an H/E of 14.1% and an elastic recovery rate (W_e) of 86.3%. Scratches (∼50 μm) on the coating can be healed within 4 min. The hybrid composition of poly(silsesquioxane) networks also shows great advantages in integration with other functional components to realize programmable multifunctionality without diminishing the basic properties. This nanocomposite design provides a route toward the preparation of materials with excellent comprehensive functions without trade-offs between these properties.

Robust superhydrophobicity: mechanisms and strategies

Superhydrophobic surfaces hold great prospects for extremely diverse applications owing to their water repellence property. The essential feature of superhydrophobicity is micro-/nano-scopic roughness to reserve a large portion of air under a liquid drop. However, the vulnerability of the delicate surface textures significantly impedes the practical applications of superhydrophobic surfaces. Robust superhydrophobicity is a must to meet the rigorous industrial requirements and standards for commercial products. In recent years, major advancements have been made in elucidating the mechanisms of wetting transitions, design strategies and fabrication techniques of superhydrophobicity. This review will first introduce the mechanisms of wetting transitions, including the thermodynamic stability of the Cassie state and its breakdown conditions. Then we highlight the development, current status and future prospects of robust superhydrophobicity, including characterization, design strategies and fabrication techniques. In particular, design strategies, which are classified into passive resistance and active regeneration for the first time, are proposed and discussed extensively.

Polymeric Complex-Based Transparent and Healable Ionogels with High Mechanical Strength and Ionic Conductivity as Reliable Strain Sensors

Transparent and healable ionogels with very high mechanical strength, ionic conductivity, and resilience were fabricated for use as strain sensors with satisfactory reliability. The ionogels were fabricated by casting an aqueous solution of poly(vinyl alcohol) (PVA)-poly(vinylpyrrolidone) (PVP) complexes and 1-ethyl-3-methylimidazolium dicyanamide ([EMIm][DCA]), followed by evaporation of water at room temperature. The use of [EMIm][DCA] endowed the resulting ionogels with ionic conductivity at room temperature as high as 19.7 mS cm^-1. Owing to the synergy between the abundant number of hydrogen bonds between PVA and PVP and the crystallized PVA segments that served as nanofillers, the resulting ionogels had good mechanical properties with a tensile stress of 7.7 MPa, a strain of 821%, and good resilience. In addition, the resulting ionogels showed rapid and repeatable sensing signals over a wide strain range (0.1-400%). This enabled them to detect both vigorous muscle movements, such as walking and jumping, and subtle muscle movements, such as pulse. Moreover, owing to the reversibility of hydrogen bonds, physically damaged mechanical properties, conductivity, and sensing ability of the ionogels could be conveniently healed with the assistance of water.

Transparent and Scratch-Resistant Antifogging Coatings with Rapid Self-Healing Capability

Typical antifogging coatings based on hydrophilic polymers are soft and susceptible to mechanical damage. In this paper, an antifogging coating that is both scratch-resistant and self-healing is fabricated by copolymerizing sulfobetaine methacrylate and 2-hydroxyethyl methacrylate in the presence of sulfobetaine-modified silica nanoparticles in one pot. The coating is highly efficient in preventing fog formation at the surface and reducing ice adhesion, and is resistant to fouling by oil and protein, due to the strong hydration ability of the zwitterionic moieties. The composite coating is resistant to scratching and abrasion under normal use conditions to maintain its transparency due to increased hardness by the filled silica nanoparticles and is able to heal completely within several minutes severe scratches and cuts inflicted in harsh conditions, owing to the water-assisted reversibility of the electrostatic and hydrogen-bonding interactions holding together the polymer components and the silica nanoparticles. The multiple desirable properties demonstrated and the simple fabrication process of the coating offers great potential in many practical applications.

Transparent Polymeric Films Capable of Healing Millimeter-Scale Cuts

Transparent polymeric films have been successfully integrated with self-healing capabilities. However, these films can only heal damages in the scale of several to several tens of micrometers, thereby greatly limiting their practical applications. The present study reports the fabrication of transparent polymeric films capable of healing millimeter-scale cuts by incorporating hydrogen-bonding units into zwitterionic polymer films, which are cross-linked by electrostatic interactions. The intermolecular interactions in the resulting films are greatly reduced when the films absorb water as a result of the reversibility of hydrogen-bonding and electrostatic interactions, thereby promoting the flowability of the film materials. Thus, the transparent films can heal 7.9 mm wide cuts and recover their damaged transparency following exposure to water. Furthermore, owing to their strong binding affinity to water molecules, the healable transparent films can effectively clean up oil fouled on dry films following rinsing with water. The combination of hydrogen bonding and electrostatic interactions provides a new means of design for transparent films with enhanced healing capabilities and an extended service life.

An Epidermis-like Hierarchical Smart Coating with a Hardness of Tooth Enamel

We overcome the fundamental dilemma in achieving hard materials with self-healing capability by integrating an epidermis-like hierarchical stratified structure with attractive mechanical and barrier properties of graphene oxide and show that such biomimetic design enables a smart hierarchical coating system with a synergetic healing effect and a record-high stiffness (31.4 ± 1.8 GPa)/hardness (2.27 ± 0.09 GPa) among all self-healable polymeric films even comparable to that of tooth enamel. A quasi-linear layer-by-layer (LBL) film with constituent graphene oxide is deposited on top of an exponential LBL counterpart as a protective hard layer, forming a hierarchical stratified assembly mimicking the structure of epidermis. The hybrid multilayers can achieve a complete restoration after scratching thanks to the mutual benefit: The soft underneath cushion can provide additional polymers to assist the recovery of the outer hard layer, which in turn can be a sealing barrier promoting the self-healing of the soft layer during stimulated polymer diffusion. The presenting hybridization mode of LBL assembly represents a promising tool for integrating seemingly contradictory properties in artificial materials with potential performances surpassing those in nature.

Nanocontainer-Enhanced Self-Healing for Corrosion-Resistant Ni Coating on Mg Alloy

The ability to manipulate the functionalization of Ni coating is of great importance in improving the corrosion resistance of magnesium (Mg) alloy for many industrial applications. In the present work, MCM-41 type mesoporous silica nanocontainers (MSNs) loaded with corrosion inhibitor (NaF) were synthesized and employed as smart reinforcements to enhance the integrity and corrosion inhibition of the Ni coating. The incorporation of the F-loaded MSNs (F@MSNs) to enhance the corrosion resistant capacity of a metallic coating is reported for the first time. The mesoporous structures of the as-prepared MSNs and F@MSNs were confirmed by transmission electron microscopy (TEM), small angle X-rays scattering (SAXS), and N₂ adsorption-desorption isotherms. The X-ray photoelectron spectroscopy (XPS) data demonstrated the successful immobilization of fluoride ion on the MSNs and formation of a magnesium fluoride (MgF₂) protective film at the corrosion sites of the Mg alloy upon soaking in a F@MSNs-containing NaCl solution. The results from potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) for both bare Mg alloy and Ni coatings with and without F@MSNs have revealed a clear decrease in corrosion rate in a corrosive solution for a long-time immersion due to the introduction of F@MSNs. These findings open new opportunities in the exploration of self-healing metallic coatings for highly enhanced anticorrosion protection of Mg alloy.

Transparent, Healable Elastomers with High Mechanical Strength and Elasticity Derived from Hydrogen-Bonded Polymer Complexes

It is challenging to develop healable elastomers with combined high mechanical strength and good elasticity. Herein, a simple strategy to develop high-performance elastomers that integrate high mechanical strength, enormous stretchability, good resilience, and healability is reported. Through simply complexing poly(acrylic acid) and poly(ethylene oxide) based on hydrogen-bonding interactions, transparent composite materials that perform as elastomers are generated. The as-prepared elastomers exhibit mechanical strength (true strength at break) and toughness (fracture energy) as high as 61 MPa and 22.9 kJ/m², respectively, and they can be stretched to >35 times their initial length and are able to return to their original dimensions following the removal of stress. Further, the elastomers are capable of healing from physical cuts/damages in a humid environment because of reformation of the reversible hydrogen bonds between the polymer components. The high mechanical strength of the elastomers is ascribed to the high degree of polymer chain entanglements and multiple hydrogen-bonding interactions in the composites. The reversible hydrogen bonds, which act as cross-linkages, facilitate the unfolding and sliding of the polymer chains in the composites, thereby endowing the elastomers with good elasticity and healability. Furthermore, flexible conductors with water-enabled healability were developed by drop-casting Ag nanowires on top of the elastomers.

Reduced Graphene Oxide-Reinforced Polymeric Films with Excellent Mechanical Robustness and Rapid and Highly Efficient Healing Properties

The fabrication of nanofiller-reinforced intrinsic healable polymer composite films with both excellent mechanical robustness and highly efficient healability is challenging because the mobility of the polymer chains is suppressed by the incorporated nanofillers. In this study, we exploit the reversible host-guest interactions between nanofillers and the matrix polymer films and report the fabrication of intrinsically healable, reduced graphene oxide (RGO)-reinforced polymer composite films capable of conveniently and repeatedly healing cuts of several tens of micrometers wide. The healable films can be prepared via layer-by-layer assembly of poly(acrylic acid) (PAA) with complexes of branched poly(ethylenimine) grafted with ferrocene (bPEI-Fc) and RGO nanosheets modified with β-cyclodextrin (RGO-CD) (denoted as bPEI-Fc&RGO-CD). The as-prepared PAA/bPEI-Fc&RGO-CD films are mechanically robust with a Young's modulus of 17.2 ± 1.9 GPa and a hardness of 1.00 ± 0.30 GPa. The healing process involves two steps: (i) healing of cuts in an oxidation condition in which the host-guest interactions between bPEI-Fc and RGO-CD nanosheets are broken and the cuts on the films are healed; and (ii) reconstruction of host-guest interactions between bPEI-Fc and RGO-CD nanosheets via reduction to restore the original mechanical robustness of the films.

Self-healing polyelectrolyte multilayered coating for anticorrosion on carbon paper

Ideally, if the corrosion resistance coating on carbon paper (CP) can be endowed with the self-healing property, the service life and the reliability of the carbon paper will be greatly increased as the gas diffusion layer. In this paper, different cycles of s branched poly (ethyleneimine) (bPEI) and poly (acrylic acid) (PAA) were modified on the surface of the carbon paper via layer-by-layer (LbL) self-assembly technology. The prepared polyelectrolyte multilayered coatings can not only protect the carbon fiber from corrosion, but also take advantages of the surrounding water to quickly repair themselves after damaged. The effects of the assembly cycles on morphology, resistance, air permeability and the contact angle of carbon papers were investigated, then the differences of the carbon papers in electrolysis process were explored. The results reveal that all the prepared coatings can protect carbon papers from corrosion, while when the assembly cycles was 10, the coatings are most efficient.

Healable Antifouling Films Composed of Partially Hydrolyzed Poly(2-ethyl-2-oxazoline) and Poly(acrylic acid)

Antifouling polymeric films can prevent undesirable adhesion of bacteria but are prone to accidental scratches, leading to a loss of their antifouling functions. To solve this problem, we report the fabrication of healable antifouling polymeric films by layer-by-layer assembly of partially hydrolyzed poly(2-ethyl-2-oxazoline) (PEtOx-EI-7%) and poly(acrylic acid) (PAA) based on hydrogen-bonding interaction as the driving force. The thermally cross-linked (PAA/PEtOx-EI-7%)*100 films show strong resistance to adhesion of both Gram-negative Escherichia coli and Gram-positive Bacillus subtilis bacteria due to the high surface and bulk concentration of the antifouling polymer PEtOx-EI-7%. Meanwhile, the dynamic nature of the hydrogen-bonding interactions and the high mobility of the polymers in the presence of water enable repeated healing of cuts of several tens of micrometers wide in cross-linked (PAA/PEtOx-EI-7%)*100 films to fully restore their antifouling function.

Stiffer but More Healable Exponential Layered Assemblies with Boron Nitride Nanoplatelets

Self-healing ability and the elastic modulus of polymeric materials may seem conflicting because of their opposite dependence on chain mobility. Here, we show that boron nitride (BN) nanoplatelets can simultaneously enhance these seemingly contradictory properties in exponentially layer-by-layer-assembled nanocomposites as both surface coatings and free-standing films. On one hand, embedding hard BN nanoplatelets into a soft hydrogen bonding network can enhance the elastic modulus and ultimate strength through effective load transfer strengthened by the incorporation of interfacial covalent bonding; on the other hand, during a water-enabled self-healing process, these two-dimensional flakes induce an anisotropic diffusion, maintain the overall diffusion ability of polymers at low loadings, and can be "sealing" agents to retard the out-of-plane diffusion, thereby hampering polymer release into the solution. A detailed mechanism study supported by a theoretical model reveals the critical parameters for achieving a complete self-healing process. The insights gained from this work may be used for the design of high-performance smart materials based on other two-dimensional fillers.

Toward Understanding Whether Interactive Surface Area Could Direct Ordered Macroscopic Supramolecular Self-Assembly

The mismatching phenomena are ubiquitous in complex and advanced self-assembly, such as hierarchical assembly, macroscopic supramolecular assembly, and so on. Recently, for macroscopic supramolecular assembly, the strategy of maximizing the interactive surface area was used and supposed to handle this problem; however, now there is little understanding of whether interactive surface area is the dominant factor to guide the assembly patterns. Herein by taking millimeter cylinder building blocks with different diameter/height (d/h) ratios as model systems, we have investigated the interactive-surface-area-dependent assembling behaviors in macroscopic supramolecular assembly. The results showed that the increasing d/h ratio of cylinders contributed to selectivity of face-to-face assembled pattern over face-to-side or side-to-side geometries, thus having improved the ordering degree of the assembled structures; however, the mismatching phenomena could not be totally avoided due to high colliding chances in kinetics and the thermally favorable stability of these structures. We further confirmed the above hypothesis by in situ measurements of interactive forces of building blocks with different assembled patterns. This work of macroscopic supramolecular assembly provides an in situ visible platform, which is significant to clarify the influences of interactive surface area on the assembly behaviors.

Bioinspired Omniphobic Coatings with a Thermal Self-Repair Function on Industrial Materials

Inspired by the wax regeneration ability of plant leaves and the slippery surfaces of the Nepenthes pitcher plants, we have developed a new form of cross-species bioinspired slippery liquid-infused porous surfaces (X-SLIPS) that can self-repair under thermal stimulation even under large-area physical and chemical damage. The performance and underlying mechanism of the thermal-healing property has been studied and characterized in detail. These thermally self-healing omniphobic coatings can be applied to a broad range of metals, plastics, glass, and ceramics of various shapes and show excellent repellency toward aqueous and organic liquids.

Humidity responsive self-healing based on intermolecular hydrogen bonding and metal–ligand coordination

Self-healing process occurring when a self-healing Co–CS/PAA PEM film is integrated (I), damaged (II), self-healing (III), and self-healed (IV).