Metal Ion-Induced Acid Hydrolysis Strategy for the One-Step Synthesis of Tough and Highly Transparent Hydrolyzed Polyacrylamide Hydrogels.

Polyacrylamide (PAM) hydrogel is hard to enhance through coordination bonds because amide groups rarely coordinate with metal ions strongly in an aqueous solution. It is known that the aqueous solution of ZrOCl2.8H2O can be strongly acidic depending on its concentration. Consequently, through a facile one-step metal ion-induced acid hydrolysis strategy (MIAHS), tough and highly transparent hydrolyzed PAM physical hydrogels are prepared by using ZrOCl2.8H2O in this work. The formation of the partially hydrolyzed PAM physical hydrogels elucidates that the side reaction of imidization during common acid hydrolysis of PAM can be perfectly overcome because the structure of the Zr(IV) ion and its interaction with amide groups promote selective acidic hydrolysis from amide to carboxyl groups. Compared to most coordination cross-linked hydrogels, which need at least two-step fabrication, the hydrolyzed PAM hydrogel via MIAHS can be obtained by one-step synthesis due to the weak interaction between amide groups and Zr(IV). The obtained PAM hydrogel cross-linked by hydrogen bonds and coordination bond between Zr(IV) and carboxyl is a multibond network (MBN) and can achieve hierarchical energy dissipation, which exhibits excellent mechanical properties (tensile strength of 3.15 MPa, elongation at break of 890%, and toughness of 17.0 MJ m-3), high transparence (transmittance of 95%), and outstanding conductivity (5.6 S m-1) at water content of 80 wt %. The high gauge factor (from 2.24 to 12.8 as the strain increases from 0 to 400%) endows the hydrolyzed PAM hydrogels with promising application as strain sensors. Furthermore, in addition to ZrOCl2.8H2O, the fact that various hydrolyzable compounds of Ti(IV), Zr(IV) Hf(IV), and Sn(IV) can also fabricate tough hydrolyzed PAM hydrogels verifies the universality of MIAHS. Therefore, the simple, efficient, and universal MIAHS will shed new light on preparing functional PAM-based hydrogels.

The Effect of Colloidal Nanoparticles on Phase Separation of Block and Heteroarm Star Copolymers Confined between Polymer Brushes.

The effect of colloidal nanoparticles on the phase changes of the amphiphilic AB linear diblock, A1A2B, and A2B heteroarm star copolymers confined between two polymer brush substrates was investigated by using a real-space self-consistent field theory. By changing the concentrations of nanoparticles and polymer brushes, the phase structure of the amphiphilic AB copolymer transforms from lamellar to core-shell hexagonal phase to cylinder phase. The pattern of A2B heteroarm star copolymer changes from core-shell hexagonal phases to lamellar phases and the layer decreases when increasing the density of the polymer brushes. The results showed that the phase behavior of the system is strongly influenced by the polymer brush architecture and the colloidal nanoparticle numbers. The colloidal nanoparticles and the soft confined surface of polymer brushes make amphiphilic AB copolymers easier to form ordered structures. The dispersion of the nanoparticles was also investigated in detail. The soft surfaces of polymer brushes and the conformation of the block copolymers work together to force the nanoparticles to disperse evenly. It will give helpful guidance for making some new functional materials by nano etching technology, nano photoresist, and nanoprinting.

Highly Conductive Polysiloxane Elastomers with Excellent Transparency, Resilience, and Stretchability.

Flexible transparent conductive materials show great potential in wearable electronics, flexible sensors, and so on. But the most used flexible conductive materials like hydrogels and ionogels suffer from evaporation and solvent leakage. For the application in these fields, integrated performances of preeminent resilience, transparency, stability, and conductivity that do not change with deformation are prerequisites. It is still challenging to handle the trade-off among these performances. Herein, a facile approach is established to balance these properties into one elastomer. Through the thiol-ene click reaction, mercaptopropyl-modified polydimethylsiloxane (mPDMS) is cross-linked and grafted by PEG-based macromonomers to prepare conductive elastomers. By anchoring with mPDMS through carbon-sulfur bonds, PEG can be evenly dispersed, resulting in ultratransparency (97%) and stable conductivity of as high as 1.68 × 10-2 S m-1, comparable to pure PEG/lithium salt conductivity. It also has a wide electrochemical stability window with a high voltage of 4.8 V. Moreover, the multibond network strategy is employed through grafting ligand 1-vinylimidazole to mPDMS to construct dynamic cross-links between Zn(II) and 1-vinylimidazol, bestowing excellent properties to the elastomers. Overall, elastomers with a well-balanced performance of high resilience, good conductivity, and ultratransparency are obtained, providing promising applications for soft electronics, lithium battery electrolytes, and flexible devices.

One-Step in Situ Synthesis of Tough and Highly Conductive Ionohydrogels with Water-Retentive and Antifreezing Properties.

Flexible and conductive gels are promising materials as intelligent and wearable electronics. Herein, through a facile one-step in situ free-radical polymerization, tough VSNPs-PAA-Zr4+ ionohydrogels with integrated multiple functionalities are prepared, which are dually cross-linked by multivalent vinyl-functionalized silica nanoparticles (VSNPs) and metal coordination between Zr4+ and the carboxyl groups in PAA chains. The incorporation of Zr4+ with stable valency during polymerization enables the direct formation of a large number of metal coordination cross-links for adequate energy dissipation, overcoming the inhibition of unstable metal ions on the polymerization process. Meanwhile, VSNPs serve as multivalent cross-linkers and effective stress transfer centers. The obtained VSNPs-PAA-Zr4+ ionohydrogels show high toughness of up to 25 MJ m-3 with a high tensile strength of 3010 kPa and a large elongation at break of 1360%, along with reliable adhesive performance. Attributed to use of an IL/water binary solvent, the ionohydrogels possess excellent water-retentive and antifreezing abilities. Moreover, the existence of large quantities of mobile ions endows the VSNPs-PAA-Zr4+ ionohydrogels with a superior conductivity of 4.77 S m-1 and a high strain sensitivity with a gauge factor (GF) of 9.04, which are promising materials as intelligent and wearable strain sensors.

Biomimetic Gradient Hydrogel Actuators with Ultrafast Thermo-Responsiveness and High Strength.

Most current hydrogel actuators suffer from either poor mechanical properties or limited responsiveness. Also, the widely used thermo-responsive poly-(N-isopropylacrylamide) (PNIPAM) homopolymer hydrogels have a slow response rate. Thus, it remains a challenge to fabricate thermo-responsive hydrogel actuators with both excellent mechanical and responsive properties. Herein, ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogels containing multivalent vinyl functionalized silica nanoparticles (VSNPs) are fabricated. The ultrafast thermo-responsiveness is due to the mobile polymer chains grafted from the surfaces of the VSNPs, which can facilitate hydrophobic aggregation, inducing the phase transition and generating water transport channels for quick water expulsion. In addition, the copolymerization of NIPAM with acrylic acid (AA) decreases the transition temperature of the thermo-responsive PNIPAM-based hydrogels, contributing to ultrafast thermo-responsive shrinking behavior with a large volume change of as high as 72.5%. Moreover, inspired by nature, intelligent hydrogel actuators with gradient structure can be facilely prepared through self-healing between the ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogel layers and high-strength VSNPs-PAA-Fe3+ multibond network (MBN) hydrogel layers. The obtained well-integrated gradient hydrogel actuators show ultrafast thermo-responsive performance within only 9 s in 60 °C water, as well as high strength, and can be used for more practical applications as intelligent soft actuators or artificial robots.

Super Tough and Intelligent Multibond Network Physical Hydrogels Facilitated by Ti3C2Tx MXene Nanosheets.

Stretchable and conductive hydrogels have emerged as promising candidates for intelligent and flexible electronic devices. Herein, based on a multibond network (MBN) design rationale, super tough and highly stretchable nanocomposite physical hydrogels are prepared, where 2D Ti3C2Tx MXene nanosheets serve as multifunctional cross-linkers and effective stress transfer centers. Further MXene-poly(acrylic acid) (PAA)-Fe3+ MBN physical hydrogels fabricated through controlled permeation of Fe3+ exhibit prominent and well-balanced mechanical properties (e.g., the tensile strength can reach 10.4 MPa and elongation at break can be as high as 3080%), attributed to the dual cross-linking network with dense Fe3+-mediated coordination cross-links between MXene nanosheets and PAA chains and sparse carboxy-Fe3+ cross-links between PAA chains. Moreover, both conductive MXene nanosheets and numerous ions endow the hydrogels with superior conductivity (up to 3.8 S m-1), strain sensitivity (high gauge factor of 10.09), and self-healing performance, showing great prospect as intelligent flexible electronics.

Structural reorganization and crack-healing properties of hydrogels based on dynamic diselenide linkages.

We report the dynamic behavior of diselenide-containing hydrophilic polyurethanes and hydrogels based on diselenide exchange reactions in an aqueous media. Diselenide-containing linear and cross-linked polyurethanes were synthesized via polyaddition reactions using diselenide-containing diol in combination with pyridinium diol that enhances the hydrophilicity of the polymer chains. The obtained linear polyurethanes underwent photo-induced diselenide exchange reactions with small diselenide compounds and degraded to smaller fragments, confirming the dynamicity of the obtained hydrophilic polyurethanes. The prepared hydrogels displayed characteristic large swelling behavior based on the structural reorganization through diselenide exchange either under photo-irradiation at 365 nm or even in the dark at room temperature. The diselenide-containing hydrogels also showed crack-healing behavior under the same exchanging conditions, presenting the utility of diselenide linkages as simple and useful units to offer high dynamicity to hydrogels.

How can multi-bond network hydrogels dissipate energy more effectively: an investigation on the relationship between network structure and properties.

Constructing a multi-bond network (MBN), which involves hierarchical dynamic bonds with different bond association energies, is an effective method for achieving super tough hydrogels. In this work, a small amount of poly(vinyl alcohol) (PVA) is introduced into a loosely chemically crosslinked poly(acrylic acid) (PAA) network. The hydrophilic PVA chains can physically interact and form hydrogen bonds with the PAA chains. After a freeze-thaw process, PVA could partially crystallize and the generated microcrystals could become new crosslinking points of the hydrogels. Meanwhile, the hydrogen bonds between PAA and PVA, which connect to the microcrystal "core" through PVA chains, could also become new crosslinking points of the hydrogels. The obtained ternary-crosslinked hydrogels (T-gel 10%) exhibit toughness as high as 8 times that in pure PAA hydrogels. When the PVA content exceeds 15 wt%, PVA chains will run through the whole PAA network. Thus the PVA chains will be crosslinked by microcrystals through freeze-thaw treatment, leading to a double network structure, resulting in a brittle hydrogel. The step-increased modulus of the hydrogels with different PVA contents clearly demonstrates the change in the network structure of the hydrogels. Successively, Fe3+ is introduced into the MBN hydrogels as a third cross-linking point. The obtained quaternary-crosslinked hydrogels (Q-gel 10%-Fe5) (50 wt% water content) exhibit significantly improved mechanical properties: tensile strength as high as 6.83 MPa with a fracture energy of 29.9 MJ m-3. This work provides clear insight into the relationship between network structure and mechanical properties in super tough MBN hydrogels.

Coordination-Driven Hierarchical Assembly of Hybrid Nanostructures Based on 2D Materials.

2D materials have received tremendous scientific and engineering interests due to their remarkable properties and broad-ranging applications such as energy storage and conversion, catalysis, biomedicine, electronics, and so forth. To further enhance their performance and endow them with new functions, 2D materials are proposed to hybridize with other nanostructured building blocks, resulting in hybrid nanostructures with various morphologies and structures. The properties and functions of these hybrid nanostructures depend strongly on the interfacial interactions between 2D materials and other building blocks. Covalent and coordination bonds are two strong interactions that hold high potential in constructing these robust hybrid nanostructures based on 2D materials. However, most 2D materials are chemically inert, posing problems for the covalent assembly with other building blocks. There are usually coordination atoms in most of 2D materials and their derivatives, thus coordination interaction as a strong interfacial interaction has attracted much attention. In this review, recent progress on the coordination-driven hierarchical assembly based on 2D materials is summarized, focusing on the synthesis approaches, various architectures, and structure-property relationship. Furthermore, insights into the present challenges and future research directions are also presented.

Homogeneous and Real Super Tough Multi-Bond Network Hydrogels Created through a Controllable Metal Ion Permeation Strategy.

Poly(acrylic acid) (PAA) hydrogels with a multi-bond network composed of sparse chemical cross-links and carboxyl-Fe3+ coordination are prepared through a controllable permeation strategy utilizing ferric citrate (FeCA). The existing strategies that directly soak PAA hydrogels in Fe3+ solutions usually induce an inhomogeneous network with densely cross-linked shells and uncertain water content of the hydrogels, which brings about ambiguity when investigating strengthening mechanisms because water content significantly affects the mechanical properties of hydrogels. Herein, the controllable permeation of Fe3+ into PAA networks based on the competition between citric acid (CA)-Fe3+ chelation and PAA-Fe3+ coordination guarantees sustained release of Fe3+, facilitating homogeneous distribution of ionic cross-links and a certain water content. The obtained hydrogels exhibit excellent and balanced mechanical properties (high tensile strength of 3.28 to 6.95 MPa with large elongations at break of 1400 to 780% when water content decreases from 80 to 50 wt %). The real robust tensile strength of this hydrogel originates from the effective energy dissipation of the homogeneous PAA-Fe3+ cross-links, and the high water content ensures a large elongation at break. Furthermore, the hydrogel also has pH-responsive and shape-memory properties.

Ultrathin MXene Nanosheets Decorated with TiO2 Quantum Dots as an Efficient Sulfur Host toward Fast and Stable Li-S Batteries.

Being conductive and flexible, 2D transition metal nitrides and carbides (MXenes) can serve in Li-S batteries as sulfur hosts to increase the conductivity and alleviate the volume expansion. However, the surface functional groups, such as OH and F, weaken the ability of bare MXenes in the chemisorption of polysulfides. Besides, they create numerous hydrogen bonds which make MXenes liable to restack, resulting in substantial loss of active area and, thus, inaccessibility of ions and electrolyte. Herein, a facile, one-step strategy is developed for the growth of TiO2 quantum dots (QDs) on ultrathin MXene (Ti3 C2 Tx ) nanosheets by cetyltrimethylammonium bromide-assisted solvothermal synthesis. These QDs act as spacers to isolate the MXene nanosheets from restacking, and preserve their 2D geometry which guarantees larger electrode-electrolyte contact area and higher sulfur loading. The stronger adsorption energy of polysulfides with TiO2 (than with Ti3 C2 Tx ), as proven by density functional theory calculations, is essential for better on-site polysulfide retention. The ultrathin nature and protected conductivity ensure rapid ion and electron diffusion, and the excellent flexibility maintains high mechanical integrity. In result, the TiO2 QDs@MXene/S cathode exhibits significantly improved long-term cyclability and rate capability, disclosing a new opportunity toward fast and stable Li-S batteries.

Elaborate synthesis of black tin oxide-black titanium oxide core-shell nanotubes for ultrastable and fast lithium storage.

Herein, we report on the elaborate synthesis of a novel hybrid architecture, i.e., black tin oxide-black titanium oxide core-shell nanotubes (denoted as TiO2-x@SnO2-x nanotubes) by NaBH4 co-reduction. The synergistic interplay between the SnO2-x core and the TiO2-x shell results in superior cyclability and rate capability for ultrastable and fast lithium storage.

High-performance multi-functional graphene/hexagonal boron nitride/poly(ethylene oxide) nanocomposites through enhanced interfacial interaction by coordination.

In this study, multi-functional nanocomposites with excellent mechanical, electrical and thermal properties were prepared through metal-ion coordination. Reduced graphene oxide (rGO) and hexagonal boron nitride (h-BN) interacted through calcium coordination bonding. Poly(ethylene oxide) (PEO) was added to bridge these two nanomaterials, providing more resistance to tensile deformation. The results of UV-Vis and FTIR spectra proved that coordination bonding was successfully formed among the three compounds. SEM images showed homogenous dispersions of the nanocomposite. After calcium-ion coordination, the mechanical, electrical and thermal properties of Ca2+-coordinated rGO/BN/PEO composite improved significantly, indicating that metal-ion coordination is a potential method for multi-functional nanocomposite fabrication.

Facile and Green Production of Impurity-Free Aqueous Solutions of WS2 Nanosheets by Direct Exfoliation in Water.

To obtain 2D materials with large quantity, low cost, and little pollution, liquid-phase exfoliation of their bulk form in water is a particularly fascinating concept. However, the current strategies for water-borne exfoliation exclusively employ stabilizers, such as surfactants, polymers, or inorganic salts, to minimize the extremely high surface energy of these nanosheets and stabilize them by steric repulsion. It is worth noting, however, that the remaining impurities inevitably bring about adverse effects to the ultimate performances of 2D materials. Here, a facile and green route to large-scale production of impurity-free aqueous solutions of WS2 nanosheets is reported by direct exfoliation in water. Crucial parameters such as initial concentration, sonication time, centrifugation speed, and centrifugation time are systematically evaluated to screen out an optimized condition for scaling up. Statistics based on morphological characterization prove that substantial fraction (66%) of the obtained WS2 nanosheets are one to five layers. X-ray diffraction and Raman characterizations reveal a high quality with few, if any, structural distortions. The water-borne exfoliation route opens up new opportunities for easy, clean processing of WS2 -based film devices that may shine in the fields of, e.g., energy storage and functional nanocomposites owing to their excellent electrochemical, mechanical, and thermal properties.

Aluminothermic reduction enabled synthesis of silicon hollow microspheres from commercialized silica nanoparticles for superior lithium storage.

We report the aluminothermic reduction enabled synthesis of silicon hollow microspheres from commercialized silica nanoparticles by controlled transformation and organization. The synergistically integrated merits of a simple process and delicate structural design lay a basis for developing an industrially viable silicon anode with optimized electrochemical performances.

Dually cross-linked single network poly(acrylic acid) hydrogels with superior mechanical properties and water absorbency.

Poly(acrylic acid) (PAA) hydrogels with superior mechanical properties, based on a single network structure with dual cross-linking, are prepared by one-pot free radical polymerization. The network structure of the PAA hydrogels is composed of dual cross-linking: a dynamic and reversible ionic cross-linking among the PAA chains enabled by Fe(3+) ions, and a sparse covalent cross-linking enabled by a covalent cross-linker (Bis). Under deformation, the covalently cross-linked PAA chains remain intact to maintain their original configuration, while the Fe(3+)-enabled ionic cross-linking among the PAA chains is broken to dissipate energy and then recombined. It is found that the mechanical properties of the PAA hydrogels are significantly influenced by the contents of covalent cross-linkers, Fe(3+) ions and water, which can be adjusted within a substantial range and thus broaden the applications of the hydrogels. Meanwhile, the PAA hydrogels have excellent recoverability based on the dynamic and reversible ionic cross-linking enabled by Fe(3+) ions. Moreover, the swelling capacity of the PAA hydrogels is as high as 1800 times in deionized water due to the synergistic effects of ionic and covalent cross-linkings. The combination of balanced mechanical properties, efficient recoverability, high swelling capacity and facile preparation provides a new method to obtain high-performance hydrogels.

Constructing Novel Si@SnO2 Core-Shell Heterostructures by Facile Self-Assembly of SnO2 Nanowires on Silicon Hollow Nanospheres for Large, Reversible Lithium Storage.

Developing an industrially viable silicon anode, featured by the highest theoretical capacity (4200 mA h g(-1)) among common electrode materials, is still a huge challenge because of its large volume expansion during repeated lithiation-delithiation as well as low intrinsic conductivity. Here, we expect to address these inherent deficiencies simultaneously with an interesting hybridization design. A facile self-assembly approach is proposed to decorate silicon hollow nanospheres with SnO2 nanowires. The two building blocks, hand in hand, play a wonderful duet by bridging their appealing functionalities in a complementary way: (1) The silicon hollow nanospheres, in addition to the major role as a superior capacity contributor, also act as a host material (core) to partially accommodate the volume expansion, thus alleviating the capacity fading by providing abundant hollow interiors, void spaces, and surface areas. (2) The SnO2 nanowires serve as a conductive coating (shell) to enable efficient electron transport due to a relatively high conductivity, thereby improving the cyclability of silicon. Compared to other conductive dopants, the SnO2 nanowires with a high theoretical capacity (790 mA h g(-1)) can contribute outstanding electrochemical reaction kinetics, further adding value to the ultimate electrochemical performances. The resulting novel Si@SnO2 core-shell heterostructures exhibit remarkable synergy in large, reversible lithium storage, delivering a reversible capacity as high as 1869 mA h g(-1)@500 mA g(-1) after 100 charging-discharging cycles.

h-BN Nanosheets as 2D Substrates to Load 0D Fe3O4 Nanoparticles: A Hybrid Anode Material for Lithium-Ion Batteries.

h-BN, as an isoelectronic analogue of graphene, has improved thermal mechanical properties. Moreover, the liquid-phase production of h-BN is greener since harmful oxidants/reductants are unnecessary. Here we report a novel hybrid architecture by employing h-BN nanosheets as 2D substrates to load 0D Fe3O4 nanoparticles, followed by phenol/formol carbonization to form a carbon coating. The resulting carbon-encapsulated h-BN@Fe3O4 hybrid architecture exhibits synergistic interactions: 1) The h-BN nanosheets act as flexible 2D substrates to accommodate the volume change of the Fe3O4 nanoparticles; 2) The Fe3O4 nanoparticles serve as active materials to contribute to a high specific capacity; and 3) The carbon coating not only protects the hybrid architecture from deformation but also keeps the whole electrode highly conductive. The synergistic interactions translate into significantly enhanced electrochemical performances, laying a basis for the development of superior hybrid anode materials.

Robust and self-healable nanocomposite physical hydrogel facilitated by the synergy of ternary crosslinking points in a single network.

Acrylamide (AM) and a small amount of stearyl methacrylate (C18) hydrophobic monomer copolymerize to graft on the surface of vinyl hybrid silica nanoparticles (VSNPs), forming nanobrush gelators, thereby constructing ternarily crosslinked nanocomposite physical hydrogels (TC-NCP gels). The TC-NCP gel is composed of a single network ternarily crosslinked by hydrogen bonds and hydrophobic interactions among the grafting polymer chains as physical cross-linking points and thus the polymer grafted VSNPs as analogous covalent crosslinking points. Under stretching, the physical crosslinking points successively break to gradually dissipate energy and then recombine to homogenize the network. During the stretching process, the polymer chains grafted VSNPs can homogenize the stress distribution as transferring centers. The synergy of the ternary crosslinking points leads the TC-NCP gels to dissipate more energy and redistribute the stress more effectively when compared with hydrogels dually crosslinked by both hydrogen bonds and VSNPs as analogous covalent crosslinking points (without hydrophobic interactions) and by both hydrogen bonds and hydrophobic interactions (without VSNPs). As a result, the TC-NCP gels demonstrate remarkably improved mechanical properties, including tensile strength of 256 kPa, stretch ratio at break of 28.23 and toughness of 1.92 MJ m-3 at a water content of 90%. Pure shear test shows that the TC-NCP gel is able to resist notch propagation by micro-crack development from the notch tip to the whole gel network and has a high tearing energy of 1.21 × 104 J m-2. The dynamic nature of the network endows the TC-NCP gels with excellent self-healing ability. The results evidently indicate that constructing a single gel network with hierarchical crosslinking points is a versatile method to fabricate robust hydrogels.

Self-healable, tough and highly stretchable ionic nanocomposite physical hydrogels.

We present a facile strategy to synthesize self-healable tough and highly stretchable hydrogels. Our design rationale for the creation of ionic cross-linked hydrogels is to graft an acrylic acid monomer on the surface of vinyl hybrid silica nanoparticles (VSNPs) for the growth of poly(acrylic) acid (PAA), and the obtained VSNP-PAA nanobrush can be used as a gelator. Physical cross-linking through hydrogen bonding and ferric ion-mediated ionic interactions between PAA polymer chains of the gelators yielded ionic nanocomposite physical hydrogels with excellent and balanced mechanical properties (tensile strength 860 kPa, elongation at break ∼2300%), and the ability to self-repair (tensile strength ∼560 kPa, elongation at break ∼1800%). The toughness and stretchability arise from the reversible cross-linking interactions between the polymer chains that help dissipate energy through stress (deformation) triggered dynamic processes. These unique properties will enable greater application of these hydrogel materials, especially in tissue engineering.