Ultrastretchable,Super Tough,and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

It remains a challenge to develop tough hydrogels with recoverable or healable properties after damage. Herein, a new nanocomposite double‐network hydrogel (NC‐DN) consisting of first agar network and a homogeneous vinyl‐functionalized silica nanoparticles (VSNPs) macro‐crosslinked polyacrylamide (PAM) second network is reported. VSNPs are prepared via sol‐gel process using vinyltriethoxysilane as a silicon source. Then, Agar/PAM‐SiO₂ NC‐DN hydrogels are fabricated by dual physically hydrogen bonds and VSNPs macro‐crosslinking. Under deformation, the reversible hydrogen bonds in agar network and PAM nanocomposite network successively break to dissipate energy and then recombine to recover the network, while VSNPs in the second network could effectively transfer stress to the network chains grafted on their surfaces and maintain the gel network. As a result, the optimal NC‐DN hydrogels exhibit ultrastretchable (fracture strain 7822%), super tough (fracture toughness 18.22 MJ m^‐3, tensile strength 431 kPa), rapidly recoverable (≈92% toughness recovery after 5 min resting at room temperature), and self‐healable (can be stretched to 1331% after healing) properties. The newly designed Agar/PAM‐SiO₂ NC‐DN hydrogels with tunable network structure and mechanical properties by multi‐bond crosslinking provide a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels and broaden the current hydrogel research and applications.     

A Facile Strategy for Synergistic Integration of Dynamic Covalent Bonds and Hydrogen Bonds to Surmount the Tradeoff between Mechanical Property and Self-Healing Capacity of Hydrogels

The self-healable hydrogels have attracted increasing attention due to their promising potential for ensuring the durability and reliability of hydrogels. However, they still face a serious challenge to achieve a positive balance between mechanical and healing performance, especially for the room-temperature autonomous self-healable hydrogels. Herein, a simple but efficient strategy to fabricate a kind of dynamic boronate and hydrogen bonds dual-crosslinked double network (DN) hydrogel based on a UV-initiated one-pot in situ polymerization of N-acryloyl glycinamide (NAGA) in polyvinyl alcohol-borax slime is reported. The obtained PN-x/PB hydrogels, especially with high content of PNAGA, are shown to possess high mechanical strength, high toughness, and fatigue-resistance properties as well as excellent self-healability at room temperature (nearly 88% self-healing efficiency based on the strain compression test), due to the dynamic DN structure, and the combination of the adaptable and reconfigurable dynamic boronate bonds and hydrogen bonds. Considering the easily available materials and simple preparation process, this novel strategy should offer not only a kind of dynamic DN hydrogel with robust mechanical performance and high self-healing capability, but also enrich the methodological toolbox for synergistic integration of dynamic covalent bonds and hydrogen bonds to surmount the tradeoff between mechanical properties and self-healing capacity of hydrogels.     

Tough and Tear‐Resistant Double‐Network Hydrogels Based on a Facile Strategy: Micellar Polymerization Followed by Solution Polymerization

How to prepare a hydrogel with high strength and excellent tearing fracture energy is a problem faced by researchers. Here, tough and tear‐resistant double‐network hydrogels (Cx‐SMy gels) are successfully prepared via a facile strategy: micellar polymerization followed by solution polymerization. The strength and fracture energy of these hydrogels are up to 13 MPa and 26500 J m^?2, respectively, which are attributed to the synergy of quatra‐crosslinking interactions inside the double‐network. The quatra‐crosslinking interactions include hydrophobic interaction, crystallization, electrostatic attraction, and hydrogen bonding. Moreover, it is confirmed that the facile strategy is a general way to prepare tough hydrogels by using electrolytic monomers and hydrophobic acrylates.     

A Facile Strategy for Preparing Tough,Self‐Healing Double‐Network Hyaluronic Acid Hydrogels Inspired by Mussel Cuticles

Compared with hydrogel‐like biological tissues such as cartilage, muscles, and blood vessels, current hyaluronic acid hydrogels often suffer from poor toughness and limited self‐healing properties. Herein, a facile and generalizable strategy inspired by mussel cuticles is presented to fabricate tough and self‐healing double‐network hyaluronic acid hydrogels. These hydrogels are composed of ductile, reversible Fe³⁺‐catechol interaction primary networks, and secondarily formed brittle, irreversible covalent networks. Based on this design strategy, the hyaluronic acid hydrogels are demonstrated to exhibit reinforced mechanical strength while maintaining a rapid self‐healing property. In addition, by simply regulating pH or UV irradiation time, the mechanical properties of the hydrogels can be regulated conveniently through variations between the primary and secondary networks.     

Toward Largely Enhanced Toughness and Balanced Strength in PA1012/EPDM Blends via Synergistic Effect of Sacrificial Bonds and Network Structure

Here, zinc-neutralized ethylene propylene diene monomer (EPDM) ionomers with different neutralization levels are prepared through melt blending, and are then incorporated with polyamide 1012 (PA1012) to fabricate PA1012/EPDM ionomer blends. Interestingly, complex crosslinking networks are formed in the blends due to the construction of sacrificial bonds (Zn²⁺-carboxyl, Zn²⁺-amide). The as-formed network structure and sacrificial bond endow the PA/EPDM blends with largely enhanced toughness (16 times higher than that of neat PA), as well as balanced strength and stiffness. Meanwhile, the rheological behaviors of PA1012/EPDM ionomer blends indicate their relative low melting viscosity, which can avoid the processing shortcomings of plastics toughened with rubber. Moreover, PA1012/EPDM ionomer blends show obvious gelation behavior, and a maximum notched Izod impact strength exhibited at the gel point, in which unique double network structure can be observed obviously, indicating that there is a corresponding correlation between the rheological and mechanical parameters. Furthermore, the supper-toughening mechanism of PA1012/EPDM ionomer blends at gel point is explored, which origins from the large deformation and cavitation of rubber particles and the destruction of special double network morphologies. This study provides a novel and effective strategy to fabricate PA materials with outstanding toughness and excellent strength simultaneously.     

Physically Linked PVA/P(AAm‐co‐SMA) Hydrogels: Tough,Fast Recovery,Antifatigue, and Self‐Healing Soft Materials

The application of traditional chemically crosslinked hydrogels is often limited by poor mechanical properties because of their own inhomogeneous network and irreversible crosslinking bonds. Herein, physical interactions are applied to crosslink the interpenetrating network hydrogel, i.e., hydrogen bonding and crystalline domain for polyvinyl alcohol network, and hydrophobic interaction inside micelle for poly (acrylamide‐co‐stearyl methyl acrylate) [P(AAm‐co‐SMA)] network. In this gel network system, reversible energy dissipation mechanism is realized by dissociation and reassociation of weak interactions including hydrogen bonding and hydrophobic interaction inside the micelle. Strong crystalline domains serve as permanent crosslinking interactions to maintain network integrity under large extension. As a result, the synergy of weak and strong interactions leads to tough, antifatigue, fast recovery, and self‐healing properties of the hydrogel. This proposed strategy of achieving versatile hydrogels can broaden the use of hydrogels into load‐bearing applications.     

Self‐Healing,Injectable Gelatin Hydrogels Cross‐Linked by Dynamic Schiff Base Linkages Support Cell Adhesion and Sustained Release of Antibacterial Drugs

A new class of dynamic hydrogels made through Schiff base bonds based on gelatin (type A and B) and polyethylene glycol dibenzaldehyde (diBA‐PEG, 2000 and 4000 g mol^?1) is developed. Hydrogels form in situ by mixing aqueous solutions of gelatin and diBA‐PEG at a carefully adjusted pH. Compression test shows that the samples based on gelatin A are able to withstand at least ten cyclic loading/unloading without crack formation and significant permanent deformation. Self‐healing behavior of the hydrogel is proved by rheological measurements and also visual method. This hydrogel is proven to be injectable and nontoxic. Performance of the hydrogel in loading and delivery of clindamycin hydrochloride, as an antibacterial model drug, is evaluated against Staphylococcus aureus via antibacterial activity test. In vitro release of clindamycin hydrochloride is studied through an innovative method and it becomes clear that the release of clindamycin hydrochloride from this hydrogel follows a zero‐order kinetics.     

Synthesis of poly(acrylic acid)–Fe3+/gelatin/poly(vinyl alcohol) triple‐network supramolecular hydrogels with high toughness,high strength and self‐healing properties

Hydrogels with good mechanical and self‐healing properties are of great importance for various applications. Poly(acrylic acid)–Fe³⁺/gelatin/poly(vinyl alcohol) (PAA‐Fe³⁺/Gelatin/PVA) triple‐network supramolecular hydrogels were synthesized by a simple one‐pot method of copolymerization, cooling and freezing/thawing. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels exhibit superior toughness, strength and recovery capacity compared to single‐ and double‐network hydrogels. The mechanical properties of the synthesized hydrogels could be tailored by adjusting the compositions. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogel with 0.20 mmol Fe³⁺, 3% gelatin and 15% PVA could achieve good mechanical properties, the tensile strength and elongation at break being 239.6 kPa and 12.8 mm mm^?1, respectively, and the compression strength reaching 16.7 MPa under a deformation of about 91.5%. The synthesized PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels have good self‐healing properties owing to metal coordination between Fe³⁺ and carboxylic groups, hydrogen bonding between the gelatin chains and hydrogen bonding between the PVA chains. Healed PAA‐Fe³⁺(0.20)/Gelatin3%/PVA15% triple‐network hydrogels sustain a tensile strength of up to 231.4 kPa, which is around 96.6% of the tensile strength of the original samples. Therefore, the synthesized triple‐network supramolecular hydrogels would provide a new strategy for gel research and expand the potential for their application. © 2019 Society of Chemical Industry     

Iodine‐Catalyzed Intramolecular Oxidative Thiolation of Vinylic Carbon‐Hydrogen Bonds via Tandem Iodocyclization and Dehydroiodination: Construction of 2‐Methylene‐3‐thiophenones

A metal‐free vinylic carbon‐hydrogen bond thiolation has been developed. Under the catalysis of iodine (10 mol%), the cyclization of α‐alkenoyl ketene dithioacetals afforded a broad range of polyfunctionalized 2‐methylene‐3‐thiophenones in good selectivity with moderate to excellent yields via tandem iodocyclization and dehydroiodination. The synthetic strategy can also be extended to the cyclization of ortho‐methylthiophenyl vinyl ketones leading to 2‐methylene‐3‐benzothiophenones.

     

Properties of hybrid materials derived from hydroxy‐containing linear polyester and silica through sol–gel process. I. Effect of thermal treatment

The transparent hybrid material, HLP/SiO₂, was prepared by an in situ sol–gel process of tetraethoxysilane (TEOS) at 30°C in the presence of hydroxy‐containing linear polyester (HLP) obtained by ring‐opening reaction of diglycidyl ether of bisphenol A (DGEBA) with adipic acid under the catalyzation of triphenylphosphine (TPP). The hetero‐associated hydrogen bonds between the HLP and the residual silanol of silica in the hybrids were investigated by FTIR spectroscopy. Upon heating the hybrid, the interfacial force between the HLP matrix and the silica network changed from hydrogen bonds into covalent Si—O—C bonds through dehydration of hydroxy groups in HLP with residual silanol groups in the silica network. The existence of covalent Si—O—C bonds was proved by solid‐state ²⁹Si‐NMR spectra. Other properties such as tensile strength, glass transition temperature (T_g ), solubility, and thermal stability of the hybrids before and after heat treatment were studied in detail. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1179–1190, 2000     

Hybrid Hydrogels Based on Poly(vinyl alcohol) (PVA)/Agar/Poly(ethylene glycol) (PEG) Prepared by High Energy Electron Beam Irradiation: Investigation of Physico‐Mechanical and Rheological Properties

A series of hybrid hydrogels based on poly(vinyl alcohol) (PVA)/agar/poly(ethylene glycol) (PEG) prepared by a solution casting method using e‐beam irradiation are investigated to determine the effect of agar and PEG content (1, 2, and 4 wt%) on their physicomechanical and rheological properties. The gel content of the hydrogels decreases with increasing agar and PEG contents. The equilibrium swelling of PVA hydrogel decreases on blending with agar while adding PEG to PVA/agar increases the swelling by about 400%. No obvious change in the dehydration behavior of the hybrid hydrogels is observed on changing agar and PEG contents. The solid‐like rheological behavior of the hydrogels is not significantly affected by agar content, while it approaches a liquid‐like behavior at high PEG loading. The tensile strength of the hybrid hydrogels is improved by increasing agar content, while its elongation‐at‐break is decreased. On the other hand, the opposite results are found regarding the influence of PEG and its content on the mechanical properties of the hybrid hydrogels.

     

Multiple melting and partial miscibility of ethylene‐vinyl acetate copolymer/low density polyethylene blends

Multiple melting behaviors and partial miscibility of ethylene‐vinyl acetate (EVA) copolymer/low density polyethylene (LDPE) binary blend via isothermal crystallization are investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). Crystallization temperature T (°C) is designed as 30, 50, 70, 80°C with different crystallization times t (min) of 10, 30, 60, 300, 600 min. The increase of crystallization temperature and time can facilitate the growth in lateral crystal size, and also the shift of melting peak, which means the completion of defective secondary crystallization. For blends of various fractions, sequence distribution of ethylene segments results in complex multiple melting behaviors during isothermal crystallization process. Overlapping endothermic peaks and drops of equilibrium melting points of LDPE component extrapolated from Hoffman–Weeks plots clarify the existence of partial miscibility in crystalline region between EVA and LDPE. WAXD results show that variables have no perceptible influence on the predominant existence of orthorhombic crystalline phase structure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009