On the Thermally Reversible Dynamic Hydration Behavior of Oligo(ethylene glycol) Methacrylate-Based Polymers in Water

Dynamic thermally reversible hydration behavior of a well-defined thermoresponsive copolymer P­(MEO₂MA-<i>co</i>-OEGMA₄₇₅) in D₂O synthesized by ATRP random copolymerization of 2-(2-methoxyethoxy)­ethyl methacrylate (MEO₂MA) and oligo­(ethylene glycol) methacrylate (<i>M</i>_n = 475 g/mol) was studied by means of IR spectroscopy in combination with perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2DCOS). Largely different from poly­(<i>N</i>-isopropylacrylamide) (PNIPAM), P­(MEO₂MA-<i>co</i>-OEGMA₄₇₅) exhibits a sharp change below LCST and a gradual change above LCST due to the absence of strong intermolecular hydrogen bonding interactions between polymer chains, and the apparent phase transition is mainly arising from the multiple chain aggregation without a precontraction process of individual polymer chains. Additionally, the self-aggregation process of P­(MEO₂MA-<i>co</i>-OEGMA₄₇₅) is found to be mainly dominated or driven by the conformation changes of oxyethylene side chains, which collapse first to get close to the hydrophobic backbones and then distort to expose hydrophilic ether oxygen groups to the “outer shell” of polymer chains as much as possible. On the other hand, PCMW easily determined the phase transition temperature to be ca. 32.5 °C during heating and ca. 31 °C during cooling as well as the transition temperature range to be 28.5–37 °C. 2DCOS was finally employed to discern the sequence order of all the group motions during heating and cooling. It is concluded that during the phase transition P­(MEO₂MA-<i>co</i>-OEGMA₄₇₅) chains successively experience “hydrated chains–dehydrated chains–loosely aggregated micelles–densely aggregated micelles” four consecutive conformation changes. The results were further confirmed by temperature-variable ¹H NMR analysis and molecular dynamics simulation

Redox‐Active Iron‐Citrate Complex Regulated Robust Coating‐Free Hydrogel Microfiber Net with High Environmental Tolerance and Sensitivity

Stretchable hydrogel microfibers as a novel type of ionic conductors are promising in gaining skin‐like sensing applications in more diverse scenarios. However, it remains a great challenge to fabricate coating‐free but water‐retaining conductive hydrogel microfibers with a good balance of spinnability and mechanical strength. Here the old yet significant redox chemistry of Fe‐citrate complex is employed to solve this issue in the continuous draw‐spinning process of poly(acrylamide‐co‐sodium acrylate) hydrogel microfibers and microfiber nets from a water/glycerol solution. The resultant microfibers are ionically conductive, highly stretchable, and uniform with tunable diameters. Furthermore, the presence of redox‐reversible Fe‐citrate complex and glycerol endows the fibers with good anti‐freezing, water‐retaining, and environmentally intelligent properties. Humidity and UV light can finely mediate the stiffness of hydrogel microfibers; conversely, the ionic conductance of microfibers is also responsive to light, humidity, and strain, which enables the highly sensitive perception of environmental changes. The present draw‐spinning strategy provides more possibilities for coating‐free conductive hydrogel microfibers with a variety of responsive and sensing applications

Interfacially stable MOF nanosheet membrane with tailored nanochannels for ultrafast and thermo-responsive nanofiltration

Two-dimensional nanosheet membranes with responsive nanochannels are appealing for controlled mass transfer/separation, but limited by everchanging thicknesses arising from unstable interfaces. Herein, an interfacially stable, thermo-responsive nanosheet membrane is assembled from twin-chain stabilized metal-organic framework (MOF) nanosheets, which function via two cyclic amide-bearing polymers, thermo-responsive poly(N-vinyl caprolactam) (PVCL) for adjusting channel size, and non-responsive polyvinylpyrrolidone for supporting constant interlayer distance. Owing to the microporosity of MOF nanosheets and controllable interface wettability, the hybrid membrane demonstrates both superior separation performance and stable thermo-responsiveness. Scattering and correlation spectroscopic analyses further corroborate the respective roles of the two polymers and reveal the microenvironment changes of nanochannels are motivated by the dehydration of PVCL chains

Easy Fabrication of Macroporous Gold Films Using Graphene Sheets as a Template

We demonstrate a facile new and environmentally friendly strategy to fabricate monolithic macroporous gold (MPG) films using graphene sheets as a sacrificial template. Gold nanoparticle (AuNP) decorated graphene sheets were prepared by a one-pot simultaneous reduction of graphene oxide (GO) and gold precursor (HAuCl₄) by sodium citrate. Two thermal annealing methods, direct thermal annealing in air and a two-step thermal treatment (in N₂ first and subsequently in air), were then employed to remove the template (graphene sheets), which can both produce macroporous structures, but with distinctly different morphologies. We additionally investigated the porosity evolution mechanism as well as the effect of graphene/Au weight ratio and annealing temperature on the nanoarchitecture. The two-step treatment has a more significant templating effect than direct thermal annealing to fabricate MPG films because of the existence of a preaggregation process of AuNPs assisted by graphene sheets in N₂. Moreover, the resulting MPG films were found to exhibit excellent surface-enhanced Raman scattering (SERS) activity. Our method can be hopefully extended to the synthesis of other porous materials (such as Ag, Cu, Pt, and ceramic) and much wider applications

Mineral plastic hydrogels from the cross-linking of polyacrylic acid and alkaline earth or transition metal ions

Used polymer materials generate huge environmental problems nowadays. A new solution was recently provided by a physically cross-linked polyacrylic acid-based mineral plastic hydrogel, which is resistant in the anhydrous state and easily recyclable. Here we report that substituting the cross-linker by various cations and controlling the metal content can provide promising materials with analogous properties.publishe

Interface Deformable, Thermally Sensitive Hydrogel–Elastomer Hybrid Fiber for Versatile Underwater Sensing

Underwater sensing plays a vital role in perceiving various hydrodynamic stimuli for underwater operations, while fishes evolve an adaptable, durable, and multifunctional lateral line sensory system to feel mechanical deformations from nearly all sources as well as water temperature changes. Such perfect integration of multiple functions into one biological system poses a great challenge for artificial soft sensors. Here, by constructing a stretchable and water-proof core-cladding hydrogel–elastomer hybrid optical fiber, nearly all the underwater sensations of fish lateral lines can be realized with unprecedented sensing stability. High-refractive-index salt, LiBr, is introduced to the hydrogel core to enable long-range light propagation with a low loss coefficient (≈0.32 dB cm−1), and the dissimilar yet tightly adhered hydrogel–elastomer interface is readily deformable, contributing to the ultrasensitive optical response to subtle environmental stimulations, induced by either motions, hydrostatic pressure variations, ultrasonic/audible sound waves, or water flows. Moreover, the optical loss of the hybrid fiber is linearly responsive to wide temperature changes (5–70 °C), caused by the altered light scattering from hydrogel chain clustering. The present elastomer–hydrogel hybrid optical fiber offers a new designing strategy in developing next-generation underwater stretchable ray-optic sensors

Hybrid Materials from Ultrahigh‐Inorganic‐Content Mineral Plastic Hydrogels: Arbitrarily Shapeable, Strong, and Tough

Natural mineralized structural materials such as nacre and bone possess a unique hierarchical structure comprising both hard and soft phases, which can achieve the perfect balance between mechanical strength and shape controllability. Nevertheless, it remains a great challenge to control the complex and predesigned shapes of artificial organic–inorganic hybrid materials at ambient conditions. Inspired by the plasticity of polymer‐induced liquid precursor phases that can penetrate and solidify in porous organic frameworks for biomineral formation, here a mineral plastic hydrogel is shown with ultrahigh silica content (≈95 wt%) that can be similarly hybridized into a porous delignified wood scaffold, and the resultant composite hydrogels can be manually made into arbitrary shapes. Subsequent air drying well preserves the designed shapes and produces fire‐retardant, ultrastrong, and tough structural organic–inorganic hybrids. The proposed mineral plastic hydrogel strategy opens an easy and eco‐friendly way for fabricating bioinspired structural materials that compromise both precise shape control and high mechanical strength

Colloidally Stable Monolayer Nanosheets with Colorimetric Responses

Despite the discovery of chromogenic‐layered materials for decades of years, fabrication of colloidally stable monolayer organic 2D nanosheets in aqueous media with colorimetric responses is still challenging. Herein reported is the first solution synthesis of chromic monolayer nanosheets via the topochemical polymerization of self‐assembled amphiphilic diacetylenes in aqueous media. The polydiacetylene (PDA) nanosheets are ≈3–4 nm thick in solution and only ≈1.9 nm thick in the dried state, while the lateral size can reach several micrometers. Moreover, the aqueous stability endows PDA nanosheets with excellent processability, which can further assemble into films via vacuum filtration or act as an ink for high‐resolution inkjet printing. The filtrated films and printed patterns exhibit fully reversible blue‐to‐red thermochromism, and the film also displays an interesting reversible colorimetric transition in response to near‐infrared light, which is not reported for other PDA‐only systems. The present colloidal PDA nanosheets should represent a new kind of chromic organic 2D nanomaterials that may be applied as novel building blocks for developing intelligent hybrid materials and may also find diverse sensing, display and/or anticounterfeiting applications

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Abstract Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions