Highly stretchable,self-adhesive,and self-healable double network hydrogel based on alginate/polyacrylamide with tunable mechanical properties

A number of synthetic hydrogels suffer from low mechanical strength. Despite of the recent advances in the fabrication of tough hydrogels, it is still a great challenge to simultaneously construct high stretchability, and self-adhesive and self-healing capability in a hydrogel. Herein, a new type of double network hydrogel was prepared based on irreversible cross-linking of polyacrylamide chains and Schiff-base reversible cross-linking between glycidyl methacrylate-grafted ethylenediamine and oxidized sodium alginate (OSA). The combination of both cross-linkings and their synergistic effect provided a novel hydrogel with high strength, stretchable, rapid self-healing, and self-adhesiveness to different material. Besides, the hydrogels with diverse OSA content could maintain their original shapes after loading–unloading tensile test. The resulting hydrogel has a great potential in various fields for supporting and load-bearing substance.     

Ultra-Sensitive and Ultra-Stretchable Strain Sensors Based on Emulsion Gels with Broad Operating Temperature

Hydrogels with mechanical elasticity and conductivity are ideal materials in wearable devices. However, traditional hydrogels are fragile upon mechanical loading and lose functions in climate change because the internal water undergoes freeze and dehydration. Herein, we synthesize stable emulsions at high and low temperatures by introducing glycerol into the W/W emulsions. Then the high-stable emulsions are used as templates to produce the freestanding emulsion gels with enhanced mechanical strength and conductivity. The introduction of glycerol endows emulsions and emulsion gels with high and low temperature resistance (−20 to 90 °C). The fabricated strain sensors based on emulsion gels show high sensitivity (gauge factor=6.240), high stretchability (1081 %), fatigue resistance, self-healing and adhesion properties, realizing the repeatable and accurate detection of various human motions. These high-performance and eco-friendly emulsion gels can be promising candidates for next-generation artificial skin and human-machine interface.     

Ultra‐Tough Inverse Artificial Nacre Based on Epoxy‐Graphene by Freeze‐Casting

Epoxy nanocomposites combining high toughness with advantageous functional properties are needed in many fields. However, fabricating high‐performance homogeneous epoxy nanocomposites with traditional methods remains a great challenge. Nacre with outstanding fracture toughness presents an ideal blueprint for the development of future epoxy nanocomposites. Now, high‐performance epoxy‐graphene layered nanocomposites were demonstrated with ultrahigh toughness and temperature‐sensing properties. These nanocomposites are composed of ca. 99 wt % organic epoxy, which is in contrast to the composition of natural nacre (ca. 96 wt % inorganic aragonite). These nanocomposites are named an inverse artificial nacre. The fracture toughness reaches about 4.2 times higher than that of pure epoxy. The electrical resistance is temperature‐sensitive and stable under various humidity conditions. This strategy opens an avenue for fabricating high‐performance epoxy nanocomposites with functional properties.     

Hydrophobic association and ionic coordination dual crossed-linked conductive hydrogels with self-adhesive and self-healing virtues for conformal strain sensors

As a promising functional material, conductive hydrogel has attracted extensive attention, especially in flexible sensor field. Despite the recent developments, current hydrogels still experience several issues, such as limited stretchability, lack of self-recovery and self-healing capability, and insufficient self-adhesion. Herein, dual cross-linked (DC) poly (AA-co-LMA)_SDS/Fe³⁺ hydrogels are fabricated subtly on the basis of ionic coordination interactions and the poly (AA-co-LMA)_SDS hydrophobic association networks, which may provide one plausible routine to compensate the mentioned drawback of hydrogels. The hydrophobic association and ionic coordination networks work synergistically to endow the hydrogels remarkable stretchability (>1200%), high-fracture strength (≈ 820 kPa), and excellent self-healing capability. In addition, the DC hydrogel-based strain sensors displayed a broad sensing range (0 ∼ 900%), conspicuous sensitivity (strain 0% ∼ 200%, gauge factor = 0.53; strain 200% ∼ 500%, gauge factor = 1.23; strain 500% ∼ 900%, gauge factor = 2.09), and pronounced durability. What's more, the self-adhesive feature ensures the strain sensor always forming a good conformal contact with the skin during human movements and displaying remarkable bidirectional detection capability.     

Layered nanocomposites inspired by the structure and mechanical properties of nacre

Nacre (mother-of-pearl), made of inorganic and organic constituents (95 vol% aragonite calcium carbonate (CaCO(3)) platelets and 5 vol% elastic biopolymers), possesses a unique combination of remarkable strength and toughness, which is compatible for conventional high performance materials. The excellent mechanical properties are related to its hierarchical structure and precisely designed organic-inorganic interface. The rational design of aragonite platelet strength, aspect ratio of aragonite platelets, and interface strength ensures that the strength of nacre is maximized under platelet pull-out failure mode. At the same time, the synergy of strain hardening mechanisms acting over multiple scales results in platelets sliding on one another, and thus maximizes the energy dissipation of viscoplastic biopolymers. The excellent integrated mechanical properties with hierarchical structure have inspired chemists and materials scientists to develop biomimetic strategies for artificial nacre materials. This critical review presents a broad overview of the state-of-the-art work on the preparation of layered organic-inorganic nanocomposites inspired by nacre, in particular, the advantages and disadvantages of various biomimetic strategies. Discussion is focused on the effect of the layered structure, interface, and component loading on strength and toughness of nacre-mimic layered nanocomposites (148 references).     

Toughening a Self-Healable Supramolecular Polymer by Ionic Cluster-Enhanced Iron-Carboxylate Complexes

Supramolecular polymers that can heal themselves automatically usually exhibit weakness in mechanical toughness and stretchability. Here we exploit a toughening strategy for a dynamic dry supramolecular network by introducing ionic cluster-enhanced iron-carboxylate complexes. The resulting dry supramolecular network simultaneous exhibits tough mechanical strength, high stretchability, self-healing ability, and processability at room temperature. The excellent performance of these distinct supramolecular polymers is attributed to the hierarchical existence of four types of dynamic combinations in the high-density dry network, including dynamic covalent disulfide bonds, noncovalent H-bonds, iron-carboxylate complexes and ionic clustering interactions. The extremely facile preparation method of this self-healing polymer offers prospects for high-performance low-cost material among others for coatings and wearable devices.     

Mechanically robust,ion-conductive,self-healing glassy hybrid materials via tailored Zn/imidazole interaction

Simultaneously achieving mechanical properties and rapid self-healing under ambient conditions is challenging because of slow diffusion dynamics. Here, we report the design of self-healing hybrids composed of low molecular mass multifunctional silsesquioxane nanoparticles with cross-linked networks formed from non-covalent metal–ligand interactions to address this challenge. Carefully tuning the bond dynamics and strength by changing the counterions and metal–ligand feed ratio enables rapid self-healing and robust mechanical properties (tensile strength = 14.9 MPa and elongation at break = 4.36%) with ion conductivity. Static tensile behavior and rheological response of hybrids revealed dynamic interactions. The hybrids without entanglement can heal from a physical cut at room temperature with a healing efficiency of approximately 90%. This molecular design strategy provides a versatile pathway for the production of self-healing hybrid materials with excellent mechanical properties.     

Development of a novel self-healing dental nanocomposite containing PUF nanocapsules and nanoclay

The aims of this research were to develop the first self-healing dental nanocomposite and to evaluate mechanical properties (compressive and flexural strength), crack-healing, and self-healing longevity after 90 days of water aging. The principal reasons for failure are microcracks formed by polymerization shrinkage, recurrent dynamic mechanical stress, water sorption, and thermal fatigue. N, N-dihydroxyethyl p-toluidine and triethylene glycol dimethacrylate (DEPT-TEGDMA) nanocapsules were synthesized as they have been proven previously to be biocompatible for dental materials. Nanoclay was used as a filler to improve the mechanical properties of self-healing tooth nanocomposites. Nanocapsules were prepared by in situ emulsion polymerization of poly urea-formaldehyde (PUF) shells. The synthesized PUF shells were characterized by FTIR, SEM, and DLS analyses. The results showed that incorporating nanocapsules at a 7.5% mass fraction into the nanocomposite increased the mechanical properties. A good self-healing efficiency ranging from 54.06 to 58% recovery was obtained. The 90 days of water-aging compared to 1 day did not reduce the self-healing efficiency (p > 0.1), showing water-aging did not damage the nanocapsules.     

A high performance solid-state asymmetric supercapacitor based on Anderson-type polyoxometalate-doped graphene aerogel

The manufacture of single-atom transition metal-doping carbon nanocomposites as electrode materials is crucial for electrochemical energy storage with high energy and power density. However, the simple strategy for preparation of such active materials with controlled structure remains a great challenge. Here, cobalt-doped carbon nanocomposites (Co-POM/rGO) were synthesized successfully by deposition of Anderson-type polyoxometalate (POM) on the surface of reduced graphene oxide (rGO) aerogel via one-pot hydrothermal treatment. The resulting Co-POM/rGO possesses three-dimensional graphene-based frameworks with hierarchical porous structure, high surface area and uniform single-atom metal doping. These intriguing features render Co-POM/rGO to be a promising electrode for applications in electrochemical energy storage. As an electrode material of a supercapacitor, Co-POM/rGO shows high-performance electrochemical energy storage (211.3 F g^?1 at 0.5 A g^?1). Furthermore, the solid-state asymmetric supercapacitor (ASC) device, using Co-POM/rGO as a positive electrode, exhibits the outstanding energy density of 37.6 Wh kg^?1 at a power density of 500 W kg^?1, and high capacitance retention of 95.2% after 5000 charge–discharge cycles. These results indicate that the proposed strategy for rational design of single-atom-metal doped carbon nanocomposites for flexible ASC devices with excellent capacitive properties.

     

Ultra-robust,self-healable and recyclable polyurethane elastomer via a combination of hydrogen bonds,dynamic chemistry,and microphase separation

Flexibility, robustness, transparency, and recyclability are critical to the application of self-healing polymer materials in the field of flexible electronics. However, integrating all the above properties remains a huge challenge to date. In this work, we put forward a facile strategy to prepare polyurethane (PU) elastomer with ultra-high strength and self-healing performance based on hydrogen bonds, disulfide dynamic chemistry, and microphase separation at the same time. Three different self-healing PUs were obtained by introducing disulfide bonds and different types of hydrogen bonds. A robust, transparent, and recyclable PU with amino-terminated chain extender (PUA) with fast and efficient self-healing performance was prepared. The mechanical and self-healing properties of the PUA were effectively balanced by the synergistic effect of reversible interaction of disulfide bonds and the formation of microphase separated structure. The results indicated that the PUA exhibited high transparency up to 90% and excellent mechanical property, e.g. the tensile strength and elongation at break can reach 37.10 MPa and 1080%, respectively. Meanwhile, it can achieve a high self-healing efficiency of 96.8% at 80 °C for 4 h and maintain 84% of the initial mechanical strength even after four times of recycling. Moreover, the colloid graphite/PUA flexible strain sensor was prepared by the combination of colloid graphite and PUA, which can accurately detect both large and tiny scale deformations.     

Self-Healable Polyelectrolytes with Mechanical Enhancement for Flexible and Durable Supercapacitors

The practical application of advanced personalized electronics is inseparable from flexible, durable, and even self-healable energy storage devices. However, the mechanical and self-healing performance of supercapacitors is still limited at present. Herein, highly transparent, stretchable, and self-healable poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA)/poly(vinyl alcohol) (PVA)/LiCl polyelectrolytes were facilely prepared by one-step radical polymerization. The cooperation of PAMPSA and PVA significantly increased the mechanical and self-healing capacity of the polyelectrolyte, which exhibited superior stretchability of 938 %, stress of 112.68 kPa, good electrical performance (ionic conductivity up to 20.6 mS cm⁻¹), and high healing efficiency of 92.68 % after 24 h. After assembly with polypyrrole-coated single-walled carbon nanotubes, the resulting as-prepared supercapacitor had excellent electrochemical properties with high areal capacitance of 297 mF cm⁻² at 0.5 mA cm⁻² and good rate capability (218 mF cm⁻² at 5 mA cm⁻²). Besides, after cutting in two the supercapacitor recovered 99.2 % of its original specific capacitance after healing for 24 h at room temperature. The results also showed negligible change in the interior contact resistance of the supercapacitor after ten cutting/healing cycles. The present work provides a possible solution for the development of smart and durable energy storage devices with low cost for next-generation intelligent electronics.     

Quadruple H-Bonding and Polyrotaxanes Dual Cross-linking Supramolecular Elastomer for High Toughness and Self-healing Conductors

Tough and self-healable substrates can enable stretchable electronics long service life. However, for substrates, it still remains a challenge to achieve both high toughness and autonomous self-healing ability at room temperature. Herein, a strategy by using the combined effects between quadruple H-bonding and slidable cross-links is proposed to solve the above issues in the elastomer. The elastomer exhibits high toughness (77.3 MJ m⁻³), fracture energy (≈127.2 kJ m⁻²), and good healing efficiency (91 %) at room temperature. The superior performance is ascribed to the inter and intra crosslinking structures of quadruple H-bonding and polyrotaxanes in the dual crosslinking system. Strain-induced crystallization of PEG in polyrotaxanes also contributes to the high fracture energy of the elastomers. Furthermore, based on the dual cross-linked supramolecular elastomer, a highly stretchable and self-healable electrode containing liquid metal is also fabricated, retaining resistance stability (0.16–0.26 Ω) even at the strain of 1600 %.     

Dynameric Collagen Self-Healing Membranes with High Mechanical Strength for Effective Cell Growth Applications

The fabrication of biocompatible adaptive materials with high stiffness and self-healing properties for medical applications is a challenging endeavor. Collagen is a major extracellular matrix component acting as a substrate for cell adhesion and migration. Dynamers are constitutional polymers whose monomeric components are linked through reversible bonds, able to modify their constitution through reversible exchange of their components. In the current work, we demonstrate that the rational combination of collagen and dynameric networks connected with reversible covalent imine bonds is a very important and previously unreported strategy to provide biocompatible membranes with self-healing ability and excellent mechanical strength. The key challenge in the construction of such membranes is the required adaptive interaction between collagen chains and the dynamic cross-linkers, preventing the formation of defects. For example, by varying structure and molecular lengths of the dynamers, the tensile strength of the dynameric membranes reach over 80 MPa, more than 400 % higher than that observed for the reference collagen membrane, and the highest value for break strain found, was 19 %. The self-healing properties were observed when reconnecting two membrane pieces or even from crushed status of the membranes. Moreover, both MTT assay and confocal laser scanning microscopy method demonstrated the good biocompatibility of the collagen membranes, leaving more than 90 % viability for NIH 3T3 cells after 24 h co-culture.     

Recent Progress in Two-dimensional Nanomaterials Following Graphene for Improving Fire Safety of Polymer (Nano)composites

The high fire safety of polymer nanocomposites is being pursued by research institutions around the world. In addition to intrinsic flame retardancy strategy, the additive-type flame retardants have attracted increasing attention due to low commercial cost and easy fabrication craft. However, traditional additive-type flame retardants usually need high addition amount to achieve a desirable effect, which causes many side-effects on the overall performance of polymer materials, such as deteriorated mechanical property and processability. At present, two-dimensional(2 D) nanomaterials have also been applied to reduce the fire hazards of polymer(nano)composites with the coupling of barrier function and catalysis as well as carbonization effect. Even though most research work mainly focus on graphene-based flame retardants, more emerging two-dimensional nanomaterials are taking away research attention, due to their complementary and unique properties, mainly including hexagonal boron nitride(h-BN), molybdenum disulfide(MoS_2), metal organic frameworks(MOF), carbon nitride(CN),titanium carbide(MXene) and black phosphorene(BP). In this review, except for graphene, the flame retardant mechanism involving different layered nanomaterials are also reviewed. Meanwhile, the functionalization method and flame retardancy effect of different layered nanomaterials are emphatically discussed for offering an effective reference to solve the fire hazards of polymer materials. Moreover, this work objectively evaluates the practical significance of polymer/layered nanomaterials composites for industrial application.     

Effect of surface modification of montmorillonite on the properties of aromatic polyamide/clay nanocomposites

The surface modification of montmorillonite clay was carried out through ion‐ exchange reaction using p‐phenylenediamine as a modifier. This modified clay was employed to prepare aromatic polyamide/organoclay nanocomposite materials. The dispersion behavior of clay was examined in the polyamide matrix. Polyamide chains were synthesized from 4‐aminophenyl sulfone and isophthaloyl chloride (IPC) in dimethylacetamide. These amide chains were suitably end‐capped with carbonyl chloride end groups to interact chemically with modified montmorillonite clay. The resulting nanocomposite films containing 2–20 wt% of organoclay were characterized by TEM, X‐ray diffraction (XRD), thin‐film tensile testing; thermogravimetric analysis (TGA), differential scanning calorimetric (DSC) and water absorption measurements. Mechanical testing revealed that modulus and strength improved up to 6 wt% organoclay loading while elongation and toughness of nanocomposites decreased with the addition of clay content in the matrix. Thermal decomposition temperatures of the nanocomposites were in the range 225–450 °C. These nanocomposites expressed increase in the glass‐transition temperature values relative to pure polyamide describing interfacial interactions among the phases. The percent water uptake of these composites reduced upon the addition of modified layered silicate depicting improved barrier properties. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural characterization and thermal properties of polyamide 6.6/Mg, Al/adipate-LDH nanocomposites obtained by solid state polymerization

A new nanocomposite was obtained by dispersing an adipate-modified layered double hydroxide (Ad-LDH) with adipic acid and hexamethylene diamine. These samples were polymerized in the solid phase under a nitrogen flow for 200 min at 190 °C. The structural and compositional details of the nanocomposite were determined by powder X-ray diffraction (PXRD), fourier transform infrared (FTIR) spectroscopy, focused ion beam (FIB), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The PXRD patterns and FIB images show a partially intercalated and partially exfoliated dispersion of layered crystalline materials in the polyamide 6.6 matrix. The best dispersion level is achieved in polyamide 6.6/LDH nanocomposites with low LDH loading. Some residual tactoids and particle agglomerates are also evident at high concentration. The best thermal stability of the nanocomposites is shown by the sample with 0.1% LDH content, for which it is higher than that of pure polyamide.     

Vibrational energy-harvesting performance of bio-sourced flexible polyamide 11/layered silicate nanocomposite films

The energy-harvesting efficiency of melt processed polyamide 11 (PA11) films and its nanocomposites have been investigated as a function of filler type and content. In the present work, nanoclays have been used as structural modifiers in a PA11 matrix. The nanocomposites were prepared using layered clays, Cloisite 20A, 10A, and Na⁺, by extrusion process through varying the filler content, 1, 2, 4, and 5?wt.%. The crystalline structure of these nanocomposites has been studied by X-ray diffractometer. It has been demonstrated that layered silicates are not significant for the structural quality of the obtained nanocomposites. Regarding the interlayer peak of different clays, it has also been revealed that Cloisite 20A is partially exfoliated, whereas 10A and Na⁺ are totally exfoliated in the PA11 matrix. From mechanical and dynamic mechanical analyses, it was found that the addition of layered silicates results in an increase in mechanical properties. The piezoelectric strain coefficient d₃₃ and dielectric constant ε_R have been measured on polarized films at ambient temperature. Among all the prepared nanocomposites only Cloisite Na⁺-loaded PA11 nanocomposites showed the best piezoelectric constant. This observation showed that piezoelectric constant not only depends on the crystalline phases but also on the nature of the filler. Cloisite Na⁺ is more polar than other modified clays and high polarity leads to a better polarization response. A specific method for the quantification of energy vibration recovery has been developed for these nanocomposites. The capabilities of vibrational energy recovery were studied on PA11 loaded with Cloisite Na⁺.     

Influence of compatibilizer degradation on formation and properties of PA6/organoclay nanocomposites

Nanocomposites based on polyamide 6 (PA6) and commercial layered silicates have been prepared by both in situ polymerization and melt compounding. The main aim of the present work has been centred on compatibilizer degradation, caused by the preparation conditions, in terms of nanocomposite end features. Two montmorillonite (MMT)-type, organically-modified clays (OMLS), namely Cloisite 30B^® and Nanofil 784^®, and a sodium MMT (Cloisite Na^®) have been studied. Thermal properties of the layered silicates have been evaluated by TGA, IR, WAXD and pyrolysis-gas-mass. In order to better assess the influence of high temperature processes on clay modifications, a thermal treatment which mimics the conditions used during the in situ polymerization (4 h at 250 °C) has been applied on layered silicates. The above treatment, besides the elimination of absorbed water from all the clays, turned out to prove noteworthy differences in compatibilizer modification for the two organoclays. Indeed, in the case of Closite 30B^® only a removal of organic molecules outside the silicate galleries and a likely reorganization of those present inside the galleries have been detected, while a relevant chemical modification of Nanofil 784^® compatibilizer has been conversely found.As far as nanocomposite characteristics are concerned, the latter have been found to depend on both the preparation method and clay type. In the case of in situ polymerization, also thermally-treated layered silicates, coded (T), have been used, in order to put more clearly in evidence the role of compatibilizer decomposition on nanocomposite formation and properties. Indeed, nanocomposite samples containing Closite 30B^®(T) have been found to be completely exfoliated, while the same thermal treatment seems to make worse the properties of those based on Nanofil 784^®(T). Furthermore, with respect to nanocomposites based on pristine clays, samples containing thermally-treated silicates turned out to be different in terms of both molecular mass and crystal structure of the polymer matrix. Namely, PA6 γ-form seems to be promoted for all nanocomposites prepared in such a way, probably because of water removal at high temperature, which makes -OH groups of the layered silicate more free to interact with polyamide chains, thus causing a restriction of their mobility.     

Improved self-healing performance of polymeric nanocomposites reinforced with talc nanoparticles (TNPs) and urea-formaldehyde microcapsules (UFMCs)

The present work reports the self-healing performance of the epoxy based polymeric nanocomposite coatings containing different concentrations (1 and 3 wt%) of talc nanoparticles (TNPs) modified with sodium nitrate (NaNO₃), and a fixed amount (5 wt%) of urea-formaldehyde microcapsules (UFMCs) encapsulated with linseed oil (LO). The polymeric nanocomposites were developed, coated on polished steel substrates, and their structural, thermal, and self-healing characteristics were investigated using various techniques. The successful loading (~wt 10%) of NaNO₃ into TNPs, which can be ascribed to the involvement of physio-chemical adsorption mechanism, is validated and proceeds without altering the TNPs parent lamellae structure. The performed tests elucidated that the self-release of the corrosion inhibitor (NaNO₃) from TNPs is sensitive to the pH of the solution and immersion time. In addition, the release of the linseed oil (self-healing agent) from UFMCs in response to the external damage was found to be a time-dependent process. The superior self-healing and corrosion inhibition performance of the protective polymeric nanocomposites coatings containing 3 wt% TNPs and UFMCs/LO are proven using the electrochemical impedance spectroscopy (EIS) studies. A careful selection of smart carriers, inhibitor, and self-healing agent compatible with polymeric matrix has enabled to attain decent self-healing and convincing corrosion inhibition efficiency of 99.9% and 99.5%, respectively, for polymeric nanocomposites coatings containing 3 and 1 wt% TNPs, making them attractive for many industrial applications.