Biomimetic Nanowire Structured Hydrogels as Highly Active and Recyclable Catalyst Carriers

Nanowire hydrogels with high specific surface areas have great promise in many practical applications. However, the preparation of nanowire hydrogels using common materials and inexpensive means remains an outstanding challenge. This paper reports a novel method for creating aligned nanowire structured hydrogels by directional freezing and γ-radiation initiated polymerization of 2-hydroxyethyl methacrylate (HEMA) using <i>t</i>-butyl alcohol (TBA) as the solvent. The hydrogels prepared at a monomer concentration lower than 2.0 mol L^–1 and a freezing rate higher than 10 mm min^–1 are structured of nanowires, mimicking the microstructure of jellyfish mesogloea. Silver (Ag) nanoparticles (NPs) are introduced into the hydrogels with a chemical reduction method, and the Ag NPs are formed and deposited on the nanowires. Both size and content of Ag NPs in the hydrogels increase with increasing AgNO₃ concentration. The PHEMA and PHEMA/Ag nanocomposite hydrogels all possess very good compressive properties, and the composite hydrogels show higher compressive strengths and excellent deformation recovery. The PHEMA/Ag NPs composite hydrogels show excellent catalytic activity and reusability for the conversion of <i>o</i>-nitroaniline to 1,2-benzenediamine, with an apparent rate constant (<i>k</i>_app) up to 0.165 min^–1. This facile and efficient method can be applied to fabricate more nanowire hydrogels for many practical applications

Thermosensitive ZrP-PNIPAM Pickering Emulsifier and the Controlled-Release Behavior

Asymmetric Janus and Gemini ZrP-PNIPAM monolayer nanoplates were obtained by exfoliation of two-dimensional layered ZrP disks whose surface was covalently modified with thermosensitive polymer PNIPAM. The nanoplates largely reduced interfacial tension (IFT) of the oil/water interface so that they were able to produce stable oil/water emulsions, and the PNIPAM grafting either on the surface or the edge endowed the nanoplates rapid temperature responsivity. The ZrP-PNIPAM nanoplates proved to be thermosensitive Pickering emulsifiers for controlled-release applications

Poly(vinyl alcohol)–Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors

Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly­(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA–TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the “permanent” cross-link and the weaker H-bonding between PVA chains as the “temporary” cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA–TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively

Interactions Affecting the Mechanical Properties of Macromolecular Microsphere Composite Hydrogels

Macromolecular microsphere composite (MMC) hydrogel is a kind of tough hydrogel fabricated by using peroxidized macromolecular microspheres as polyfunctional initiating and cross-linking centers (PFICC). The contribution of chemical cross-linking (covalent bonding) and physical cross-linking (chain entanglement and hydrogen bonding) to the mechanical properties are understood by testing the hydrogels, which were swollen in water or aqueous urea solutions to different water contents. The as-prepared MMC gels exhibited moderate moduli (60–270 kPa), high fracture tensile stresses (up to 0.54 MPa), high extensibilities (up to 2500%), and high fracture energies (270–770 J m^–2). The moduli of the swollen gels decrease dramatically, but there are no significant changes in fracture tensile strength and fracture strain, even slight increases. More interestingly, the swollen gels show much-enhanced fracture energies, higher than 2000 J m^–2. A gradual decrease in the hysteresis ratio and residual strain is also found in the cyclic tensile testing of the hydrogels that were swollen to different water contents. The covalent bonding determines the tensile strength and fracture energy of the MMC gels, whereas the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels, and they are also the main reason for the presence of hysteresis in the loading–unloading cycles

Threshold and real-time initiation mechanism of urban flood emergency response under combined disaster scenarios

Scientific and reasonable emergency response initiation mechanisms can provide important support for decision making regarding the emergency management of urban floods. However, there is a lack of a unified paradigm on how to calculate the threshold for emergency response initiation and reasonably initiate emergency response. Therefore, this study proposes a loss-driven urban flood emergency response initiation framework from the perspective of combined disasters. A discrimination mechanism of the emergency response initiation level was established based on the optimal threshold and loss function. And the rainfall event that occurred in Zhengzhou, China, on July 20, 2021, was taken as an example to realize real-time emergency response discrimination and initiation driven by forecast data. Results showed that the initiation time of the Level I emergency response using the proposed method was 9.5 h earlier than the time of the government release, thereby significantly increasing the preparation time for flood management personnel. In addition, the results of the optimal threshold selection indicated that the Natural Breakpoint method was the optimal method for loss threshold partitioning, with the comprehensive evaluation index (CEI) being 3.56–9.53 % higher than those of the K-means, Equal Interval, and Quantile method. These results constitute a reference for urban emergency management and related research.</p

Poly(vinyl alcohol)–Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors

Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly­(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA–TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the “permanent” cross-link and the weaker H-bonding between PVA chains as the “temporary” cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA–TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively

Synthesis of Graphene Peroxide and Its Application in Fabricating Super Extensible and Highly Resilient Nanocomposite Hydrogels

Functionalized graphene has been considered as one of the most important materials for preparing polymer nanocomposites due to its unique physical structure and properties. To increase the interfacial interaction between polymer component and graphene oxide (GO) sheets, <i>in situ</i> grafting polymerization initiated by a free radical initiator immobilized on GO sheets is a better choice. We report a facile and effective strategy for preparing graphene peroxide (GPO) <i>via</i> the radiation-induced peroxidation of GO. The formation of peroxides on GO is proven by iodometric measurement and other characterizations. Using GPO as a polyfunctional initiating and cross-linking center, we obtained GO composite hydrogels exhibiting excellent mechanical properties, namely, very high tensile strength (0.2–1.2 MPa), extremely high elongations (2000–5300%), and excellent resilience. This work provides new insight into the fabrication of GO/polymer nanocomposites to fulfill the excellent mechanical properties of graphene

Liquid Crystalline Behavior of Graphene Oxide in the Formation and Deformation of Tough Nanocomposite Hydrogels

In this paper, we report the formation and transformation of graphene oxide (GO) liquid crystalline (LC) structures in the synthesis and deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions form a nematic LC phase, while the addition of poly­(<i>N</i>-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic transition to a lower volume fraction of GO and enhances the formation of nematic droplets. During the gelation process, a phase separation of the polymers and GO nanosheets is accompanied by the directional assembly of GO nanosheets, forming large LC tactoids with a radial GO configuration. The shape of the large tactoids evolves from a sphere to a toroid as the tactoids increase in size. Interestingly, during cyclic uniaxial tensile deformation a reversible LC transition is observed in the very tough hydrogels. The isolated birefringent domains and the LC domains in the tactoids in the gels are highly oriented under a high tensile strain

Thermosensitive ZrP-PNIPAM Pickering Emulsifier and the Controlled-Release Behavior

Asymmetric Janus and Gemini ZrP-PNIPAM monolayer nanoplates were obtained by exfoliation of two-dimensional layered ZrP disks whose surface was covalently modified with thermosensitive polymer PNIPAM. The nanoplates largely reduced interfacial tension (IFT) of the oil/water interface so that they were able to produce stable oil/water emulsions, and the PNIPAM grafting either on the surface or the edge endowed the nanoplates rapid temperature responsivity. The ZrP-PNIPAM nanoplates proved to be thermosensitive Pickering emulsifiers for controlled-release applications

Rheological Behavior of Tough PVP-<i>in Situ</i>-PAAm Hydrogels Physically Cross-Linked by Cooperative Hydrogen Bonding

Rheology studies were performed on tough PVP-<i>in situ</i>-PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and microstructure. The viscoelasticity of the PVP-<i>in situ</i>-PAAm hydrogels was strongly affected by the monomer ratio (<i>C</i>_AAm/<i>C</i>_VP). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus (<i>G</i>′) of the gels was much greater than the loss moduli (<i>G</i>″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and <i>C</i>_AAm/<i>C</i>_VP-dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by decreasing the effective cross-link density or weakening the intermolecular hydrogen bonding