Good Self-healing Properties Research Articles

Design and development of self-healing coating based on thiol–epoxy reactions

Cracks caused by the effects of mechanical or chemicals forces on coatings may cause their performance to deteriorate and may shorten their lifetime. The microcapsules used for self-healing coatings provide automatic recovery and extend the life of the coatings. In this study, polyurea-formaldehyde epoxy resin (PUF-epoxy) microcapsules were synthesized by in-situ polymerization; because, they allow the size control of the microcapsules, the thickness of the shell can be controlled, its production is easy, it is cheap and combinable with industry. Synthesized microcapsules were characterized by Fourier Transform Infrared Spectroscopy (ATR-FTIR), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), optical microscope, UV spectrum, zeta analyzer and contact angle measurements. As a result of the thermal analysis, the core (epoxy) content of the microcapsules was 57% by weight and 60% by the extraction method based on the principle of selective dissolution, and these two values were compatible with each other. The average microcapsule diameter was found as 185 nm. For the first time in the literature, an epoxy-containing microcapsule was added to the thiol-containing coating material and a self-healing coating was prepared. The epoxy-containing microcapsules were artificially cracked and the epoxy resin released from it reacted with the thiol received in the coating. For this reason, it was observed that the artificial scratches formed on the coatings showed good self-healing properties.

Biodegradable Poly(acrylic acid-co-acrylamide)/Poly(vinyl alcohol) Double Network Hydrogels with Tunable Mechanics and High Self-healing Performance.

We proposed a novel strategy in the fabrication of biodegradable poly(acrylic acid-co-acrylamide)/poly(vinyl alcohol) (P(AAc-co-Am)/PVA) double network (DN) hydrogels with good mechanical and self-healing properties. In the DN hydrogel system, P(AAc-co-Am) polymers form a network through the ionic coordinates between –COO– and Fe3+ and hydrogen bonding between –COOH and –CONH2, while another network is fabricated by the complexation between PVA and borax. The influences of the composition on the rheological behaviors and mechanical properties of the synthesized DN hydrogels were investigated. The rheological measurements revealed that the viscoelasticity and stiffness of the P(AAc-co-Am)/PVA DN hydrogels increase as the acrylamide and Fe3+ concentrations increase. At 0.05 mmol of Fe3+ and 50% of acrylamide, tensile strength and elongation at break of P(AAc-co-Am)/PVA DN hydrogels could reach 329.5 KPa and 12.9 mm/mm, respectively. These properties arise from the dynamic reversible bonds existed in the P(AAc-co-Am)/PVA DN hydrogels. These reversible bonds also give good self-healing properties, and the maximum self-healing efficiency of P(AAc-co-Am)/PVA DN hydrogels is up to 96.4%. The degradation test of synthesized DN hydrogels was also conducted under simulated physiological conditions and the weight loss could reach 74% in the simulated intestinal fluid. According to the results presented here, the synthesized P(AAc-co-Am)/PVA DN hydrogels have a potential application prospect in various biomedical fields.

Tough hydrophobic association hydrogels with self-healing and reforming capabilities achieved by polymeric core-shell nanoparticles

The application range of hydrogels can be greatly widened by improving their mechanical properties. It is still a great challenge to develop hydrogels with good mechanical properties, reliable self-healing properties and remolding ability at the same time. Inspired by biological soft tissue with excellent mechanical properties and self-healing properties, here, a facile method to fabricate poly (styrene-acrylic acid) (P(S-AA)) core-shell nanoparticles with plenty of carboxyl groups on their surface, and their enhancement to hydrophobic association hydrogels was reported. Under stress, the dynamic physical bonds including hydrogen bonding between polymer chains and P(S-AA) core-shell nanoparticles (NPs), and entanglement of hydrophobic chains were destroyed to effectively dissipate energy, and uniform hydrogel network leads to smooth stress-transfer, which makes the core-shell nanoparticles composite hydrophobic association hydrogels (MHA gels) excellent mechanical properties, such as excellent mechanical properties, toughness and ductility, and good self-healing properties as well. These features make the MHA gels have great potential in biomedical applications such as tissue engineering, articular cartilage and artificial skin.

Preparation of konjac glucomannan–borax hydrogels with good self-healing property and pH-responsive behavior

In this study, konjac glucomannan (KGM)–sodium tetraborate (borax) hydrogels with good self-healing property and pH-sensitivity were successfully prepared via a simple one-step method. The effects of KGM and borax contents on the morphology, rheological viscoelasticity, mechanical properties and self-healing performance of the hydrogels were systematically investigated. The results of infrared spectroscopy and scanning electron microscope confirmed the generation of boron-oxygen bonds in the hydrogels. The rheological behavior of the hydrogels further revealed that the cross-linking density of the hydrogel system is positively correlated with borax concentration. Due to this fact, the mechanical properties are enhanced with increasing the borax content. For their self-healing performance, we observed that the recovery rate of the tensile strength is significantly faster than that of the elongation rate at break. Particularly, we found that hydrogel with 6 wt% of KGM exhibits the best self-healing performance, i.e., after 5 min of self-healing, the mechanical properties can recover by 95%. Besides, the KGM–borax hydrogels also show a good and recyclable pH-responsive behavior.

Tough and conductive nanocomposite hydrogels for human motion monitoring

Conductive hydrogels have received great attention owing to their promising applications in electrodes, supercapacitors, actuators, fuel cells, biosensors and biomedical devices and so on. However, it is still a challenge to achieve conductive hydrogels with integrated high performances, including high strength, large stretchability, high toughness, rapid self-recovery and good self-healing. In the present work, multiple physically cross-linked conductive hydrogels with integrated high performances were fabricated by in situ doping conductive PEDOT:PSS during the formation of dual physically cross-linked poly(N-hydroxyethyl acrylamide)/Laponite hydrogels. At the optimal condition, the conductive gels exhibit high tensile strength, high extensibility, high tearing energy, rapid self-recovery property without any external stimuli, high conductivity, good self-healing property, as well as sensitive strain sensing for monitoring human activities (e.g. the movements of the finger and wrist, and tiny breathing movements). Considering easy preparation and integrated high performances, our conductive gels have potential applications in wearable electronic sensors and bioelectronics.

Corrosion Studies of Low-Alloyed FeCrAl Steels in Liquid Lead at 750\xa0\xb0C

New ductile experimental FeCrAl alloys, based on the composition of Fe–10Cr–4Al, were exposed to stagnant liquid lead at 750 °C for up to 1970 h. Two exposures with different test conditions were performed: one with addition of oxygen (as H2O) to the liquid lead and one without. The experimental alloys showed generally good oxidation and self-healing properties. The exposures showed that this specific category of steels has the potential to operate in liquid lead at very high temperatures with only minor oxidation. With this new material development, new energy technologies such as the CSP plants may be able to utilize liquid lead at very high temperatures as heat transfer fluid, thus achieving increased thermal efficiency.

Cover Feature: Self‐Healing Polymer Electrolytes Formed via Dual‐Networks: A New Strategy for Flexible Lithium Metal Batteries (Chem. Eur. J. 72/2018)

A self-healing polymer electrolyte fabricated via dual-network structures, which is crosslinked by quadruple hydrogen bonding and chemical bonds, shows enhanced ionic conductivity, high thermal and electrochemical stabilities, and good self-healing properties. This dual-network self-healing polymer electrolyte also exhibits excellent interfacial stability with lithium metal, and displays stable cycling performance. More information can be found in the Full Paper by D. He, Z. Xue et al. on page 19200.

Mechanically strong and tough hydrogels with excellent anti-fatigue, self-healing and reprocessing performance enabled by dynamic metal-coordination chemistry

Strong and tough hydrogels with good self-healing property and processability are of great interest in the recent decades, however, how to simultaneously realize high mechanical properties and self-healing performance still remains a challenge. Here, we report a fully physical dual-network (DN) hydrogel consisting of poly(acrylic acid) (PAA) network crosslinked by ferric ions and chitosan (CS) network crosslinked with calcium ions and chain entanglement. The resulting hydrogel displays a tensile strength of 12.2 MPa and a tensile strain of 1290%. Under the strain of 1000%, 35.7 MJ m−3 of energy can be dissipated by the hydrogel network. Due to the reversibility of the physical crosslinks, particularly, the hydrogel presents high self-recovery and self-healing performances, that is, its tensile strength is recovered to 2.3 MPa after healing. Additionally, even if fragmented, the hydrogel can still acquire a tensile stress of 4.3 MPa after re-processing. This finding indicates the effective strategy to construct high-performance hydrogels can bring benefits to more soft materials.

Developing a Self-Healing Supramolecular Nucleoside Hydrogel Based on Guanosine and Isoguanosine.

Recently, supramolecular hydrogels have attracted increasing interest owing to their tunable stability and inherent biocompatibility. However, only few studies have been reported in the literature on self-healing supramolecular nucleoside hydrogels, compared to self-healing polymer hydrogels. In this work, we successfully developed a self-healing supramolecular nucleoside hydrogel obtained by simply mixing equimolar amounts of guanosine (G) and isoguanosine (isoG) in the presence of K+ . The gelation properties have been studied systematically by comparing different alkali metal ions as well as mixtures with different ratios of G and isoG. To this end, rheological and phase diagram experiments demonstrated that the co-gel not only possessed good self-healing properties and short recovery time (only 20 seconds) but also could be formed at very low concentrations of K+ . Furthermore, nuclear magnetic resonance (NMR), powder X-ray diffraction (PXRD), and circular dichroism (CD) spectroscopy suggested that possible G2 isoG2 -quartet structures occurred in this self-healing supramolecular nucleoside hydrogel. This co-gel, to some extent, addressed the problem of isoguanosine gels for the applications in vivo, which showed the potential to be a new type of drug delivery system for biomedical applications in the future.

Effect of a functional polymer on the rheology and microstructure of sodium alginate

Here, we report on the effect of functional copolymer poly(N-isopropylacrylamide-co-4-vinyl-phenylboronic acid) (NIBA) on the rheology and network structure formed by sodium alginate (SA) through linear and nonlinear viscoelasticity measurements. The hydrogel moduli at pH 3 increased with increasing NIBA addition, while the yield point decreased. Furthermore, these hydrogels showed strain-softening behavior, weak G″-overshoot marking the onset of nonlinearity, and good self-healing properties after large deformation. The zero-strain nonlinearity parameter (Q0) was found to be more sensitive to NIBA-addition than the linear viscoelastic properties. The blends showed a clear peak in the startup test except for SA alone and the peak intensity increased with increasing NIBA-concentration. Finally, based on all data, gelation mechanism and interaction of SA and NIBA will be clarified.

Fluorescent Metallacage-Core Supramolecular Polymer Gel Formed by Orthogonal Metal Coordination and Host-Guest Interactions.

Herein, we report the preparation of a multifunctional metallacage-core supramolecular gel by orthogonal metal coordination and host-guest interactions. A tetragonal prismatic cage with four appended 21-crown-7 (21C7) moieties in its pillar parts was first prepared via the metal-coordination-driven self-assembly of cis-Pt(PEt3)2(OTf)2, tetraphenylethene (TPE)-based sodium benzoate ligands and linear dipyridyl ligands. Further addition of a bisammonium linker to the cage delivered a supramolecular polymer network via the host-guest interactions between the 21C7 moieties and ammonium salts, which formed a supramolecular gel at relatively higher concentrations. Due to the incorporation of a TPE derivative as the fluorophore, the gel shows emission properties. Multiple stimuli responsiveness and good self-healing properties were also observed because of the dynamic metal coordination and host-guest interactions used to stabilize the whole network structure. Moreover, the storage and loss moduli of the gel are 10-fold those of the gel without the metallacage cores, indicating that the rigid metallacage plays a significant role in enhancing the stiffness of the gel. The studies described herein not only enrich the functionalization of fluorescent metallacages via elegant ligand design but also provide a way to prepare stimuli-responsive and self-healing supramolecular gels as robust and smart materials.

Strong and tough fully physically crosslinked double network hydrogels with tunable mechanics and high self-healing performance

Suffering weak mechanical properties, hydrogels are generally limited for high load-bearing applications. Herein, chitosan and poly(acrylic acid) (PAA) are employed as double networks (DN), and metal-coordination and chain entanglement are exploited as dual crosslinks to develop fully physically crosslinked DN hydrogels. In spite of the non-covalent crosslinking, the hydrogel exhibits impressive tensile properties (3.7 ± 0.12 MPa) and toughness (1200 ± 50% of elongation at break and 2.8 × 103 J m−2 of fracture energy). In addition, good fatigue resistance and self-recovery performance are achieved for the hydrogel because of their supramolecular nature. Notably, the DN hydrogel display good self-healing property with recovered fracture stress and strain of 1.4 MPa and ∼700%, respectively. The results indicate that the supramolecular design proposed in this work can bring more benefits for wide applications of hydrogel.

Development of a novel chitosan based biocompatible and self-healing hydrogel for controlled release of hydrophilic drug

Smart polymeric hydrogels of chitosan and acryloyl-phenylalanine having potential of fast intrinsic shape memory properties (self-healing), non-toxic, biocompatible with moderate mechanical strength have been developed. The hydrogel has been formed by linking its network with flexible pendant side chains of chitosan and acryloyl-phenylalanine (exhibiting optimal balance of hydrophilic and hydrophobic moieties). The non-toxic and biocompatible behavior of the synthesized chitosan based hydrogel reveals its potential use towards the biomedical field. The side chain of hydrogel consists of amine and carboxylic acid groups and these moieties allow non-covalent interactions (H-bonding) across its interface. Thus, synthesized hydrogel shows very good self-healing property. Further, it has shown remarkable swelling (at different pH viz.- 2, 7, 9), cell viability (HEK-293 cells up to 200 μg/mL), cell proliferation, and controlled drug release and thus found multi-responsive.

Mechanically strong ionogels formed by immobilizing ionic liquid in polyzwitterion networks

Recently, ionogels have obtained widespread attention as excellent functional materials due to their advantageous properties, such as nonvolatility, nonflammability, and good electrochemical properties. Herein, a novel two-component ionogel system exhibiting superior mechanical strength and flexibility, satisfactory self-healing property, as well as comparable electrochemical property is constructed through in situ photopolymerization. This kind of ionogels is free of any crosslinker and comprised of a polymerized imidazolium-type zwitterionic salt as polymer backbone and an ionic liquid 1-butyl-3-methylimidazolium chloride (bromide) as solvent. The zwitterionic groups interact strongly with each other and the ions of ionic liquid solvent, resulting in the formation of the physical crosslinking within the polymer chains and the ion-dipole interaction between the zwitterionic group in the polymer backbone and ionic liquid solvent which endow the ionogel with superior mechanical strength without any crosslinker. The compression strength and tensile strength can reach 58MPa and 2.3MPa, respectively. The mechanical performance of these polyzwitterion ionogels is even comparable to chemically crosslinked high strength ionogels reported. These tough ionogels also show good self-healing properties at their glass transition temperature and have 80% recovery of their original strength. Moreover, these ionogels exhibit comparable ionic conductivities and can be well-maintained even after successive bending or folding.

Synthesis of cellulose-based double-network hydrogels demonstrating high strength, self-healing, and antibacterial properties

Novel antibacterial double-network (DN) hydrogels with superior mechanical and self-healing properties are developed via the UV-initiated copolymerization of polyacrylic acid (PAA)-grafted quaternized cellulose (QCE) and polyvinyl alcohol (PVA). The QCE functioned as an antibacterial agent, resulting in excellent antibacterial capability (antibacterial rate >93%). The hydrogels are thus protected from microbial attack in natural environments, prolonging their lifetime. The PVA functioned as a physical cross-linker, resulting in superior mechanical properties. At PVA and QCE contents of 8% and 1.5%, respectively, the strain and stress at break of hydrogel were 465.37% and 1.13MPa, respectively. The hydrogel maintained good self-healing properties owing to ionic bonding between the ferric ions and carboxylic groups, and hydrogen bonding between the PVA molecules. The hydrogel was responsive to pH; its water-holding ability could be controlled by changing the pH. The material is simply prepared and used. Hydrogels with such excellent properties could be applied in various biomedical fields.

Mechanical Properties of Hybrid Double Crosslinked Hydrogel Regulated by Sodium Chloride Solution

Polymer hydrogels are a type of soft and wet materials with three-dimensional network structure, large water content and hydrophilicity. It plays a very important effect in the human tissue engineering because it has some excellent characteristics as human active tissue. In this study, physical/chemical hybrid cross-linked hydrogels were prepared by "one-pot" method, and the mechanical properties of hydrogels were effectively controlled by immersing in different concentration of sodium chloride solution. The tensile strength of the hydrogels can be increased from 0.1 MPa to 1.8 MPa by the controlling the concentration of the sodium chloride solution, and the elongation at break increased from 770.3% to 1320.9%. It was found that the hybrid cross-crosslinked hydrogels has good self-healing properties. Put the cut gels section together, it can be self-healing, and the elongation at break of healing the gels was restored to the original gels of the 30%. Therefore, this high ionic content of the hydrogels have excellent performance in the human cartilage materials, medical materials and electrical properties. The hybrid dual crosslinked hydrogels has excellent overall properties, which are characterized by high-strength hydrogels obtained from polyelectrolytes and "high ionic solutions". So it is of potential value in the application of high performance ionic conductive hydrogels.

Atomic force microscopy–based study of self-healing coatings based on reversible polymer network systems

A self-healing polymer system is created by incorporating reversible covalent bonds into an epoxy–amine-based network structure. The self-healing concept is based on the reversible Diels–Alder reaction between furan and maleimide functional groups. The thermal and mechanical properties of the reversible network structure are tailored in order to achieve good self-healing properties for the corrosion protection of metal surfaces. Atomic force microscopy is proposed as a technique to study the self-healing behavior of coatings. Local thermal analysis techniques are used to study the local thermomechanical behavior of the reversible network. Nanosized defects in the coatings are made by means of nanolithography. The actual self-healing behavior is studied by atomic force microscopy imaging before and after the heating steps. The healing capability of elastomeric and glassy model systems is compared.