Unusual Phase Transition Behavior of Poly(<i>N</i>‑isopropylacrylamide)-<i>co</i>-Poly(tetrabutylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in the Poly(<i>N</i>‑isopropylacrylamide) Part

In this paper, one LCST-type thermoresponsive poly­(ionic liquid) (PIL), poly­(tetrabutylphosphonium styrenesulfonate) (P­[P_4,4,4,4]­[SS]), was introduced to poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) by two different ways, mixing and copolymerization. Interestingly, they show distinct thermoresponsive phase transition behaviors, evidenced by temperature-variable ¹H nuclear magnetic resonance and Fourier transform infrared in combination with the perturbation correlation moving window (PCMW) technique. The PNIPAM/P­[P_4,4,4,4]­[SS] mixture exhibits a sharp and drastic phase transition, similar to that of pure PNIPAM. In the statistical copolymer, PNIPAM-<i>co</i>-P­[P_4,4,4,4]­[SS], the thermosensitivity of P­[P_4,4,4,4]­[SS] is largely suppressed, resulting in a linear, mild, and incomplete phase transition, which has never been reported before. This abnormal phenomenon is shown to arise from the outstanding hydration ability of P­[P_4,4,4,4]­[SS]. Our findings should be conducive to improving our understanding of the interaction between LCST-type polymers with distinct structures and provide a new perspective for preparing thermoresponsive materials with linear phase transition behavior

Aqueous Phase Exfoliation of Two-Dimensional Materials Assisted by Thermoresponsive Polymeric Ionic Liquid and Their Applications in Stimuli-Responsive Hydrogels and Highly Thermally Conductive Films

With the increasing attention for various two-dimensional (2D) materials in recent years, developing a universal, facile, and eco-friendly method to exfoliate them into single- and few-layered nanosheets is becoming more and more urgent. Herein, we use a thermoresponsive polymeric ionic liquid (TRPIL) as a universal polymer surfactant to assist the high-efficiency exfoliation of molybdenum disulfide (MoS₂), graphite, and hexagonal boron nitride in an aqueous medium through consecutive sonication. In this case, the reliable interaction between 2D materials and the TRPIL would facilitate the exfoliation and simultaneously achieve a noncovalent functionalization of the exfoliated nanosheets. Interestingly, the dispersion stability of exfoliated nanosheet suspensions can be reversibly tuned by temperature because of the thermoresponsive phase transition behavior of the TRPIL. As a proof of potential applications, a temperature and photo-dual-responsive TRPIL/MoS₂ coloring hydrogel with robust mechanical property and an artificial nacre-like BN nanosheet film with high thermal conductivity were fabricated

Graphene Quantum Dot Hybrids as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction

The doping of heteroatoms into graphene quantum dot nanostructures provides an efficient way to tune the electronic structures and make more active sites for electro-catalysis, photovoltaic, or light emitting applications. Other than the modification of chemical composition, novel architecture is very desirable to enrich the research area and provides a wide range of choices for the diverse applications. Herein, we show a novel lotus seedpod surface-like pattern of zero-dimension (0D) seed-like N-GODs of ca.3 nm embedded on the surface of a two-dimension (2D) N-GQD sheet of ca.35 nm. It is demonstrated that different photoluminescence (PL) could be tuned easily, and the novel multidimensional structure displays excellent performance toward oxygen reduction reaction in alkaline solutions. Thus, the fabricated N-GQD hybrids show bright perspective in biomedical imaging, biosensors, and conversion and storage of energy

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

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

Composite Proton-Exchange Membrane with Highly Improved Proton Conductivity Prepared by in Situ Crystallization of Porous Organic Cage

Porous organic cage, a kind of newly emerging soluble crystalline porous material, is introduced to proton-exchange membrane by in situ crystallization. The crystallized Cage 3 with intrinsic water-meditated three-dimensional interconnected proton pathways working together with Nafion matrix generates a composite membrane with highly improved proton conductivity. Different from inorganic crystalline porous materials, like metal–organic frameworks, the organic porous material shows better compatibility with Nafion matrix due to the absence of inorganic elements. In addition, Cage 3 can absorb water up to 20.1 wt %, which effectively facilitates proton conduction under both high- and low-humidity conditions. Meanwhile, the selectivity of Nafion–Cage 3 composite membrane is also elevated upon the loading of Cage 3. The proton conductivity is evidently enhanced without obvious increased methanol permeability. At 90 °C and 95% RH, the proton conductivity of NC3-5 reaches 0.27 S·cm^–1, highly improved compared to 0.08 S·cm^–1 of recast Nafion under the same condition. This study offers a new strategy for modifying proton-exchange membrane with crystalline porous materials

Novel Slightly Reduced Graphene Oxide Based Proton Exchange Membrane with Constructed Long-Range Ionic Nanochannels via Self-Assembling of Nafion

A facile method to prepare high-performance Nafion slightly reduced graphene oxide membranes (N-srGOMs) via vacuum filtration is proposed. The long-range connected ionic nanochannels in the membrane are constructed via the concentration-dependent self-assembling of the amphiphilic Nafion and the hydrophilic–hydrophobic interaction between graphene oxide (GO) and Nafion in water. The obtained N-srGOM possesses high proton conductivity, and low methanol permeability benefitted from the constructed unique interior structures. The proton conductivity of N-srGOM reaches as high as 0.58 S cm^–1 at 80 °C and 95%RH, which is near 4-fold of the commercialized Nafion 117 membrane under the same condition. The methanol permeability of N-srGOM is 2.0 × 10^–9 cm² s^–1, two-magnitude lower than that of Nafion 117. This novel membrane fabrication strategy has proved to be highly efficient in overcoming the “trade-off” effect between proton conductivity and methanol resistance and displays great potential in DMFC application

Novel Composite Proton Exchange Membrane with Connected Long-Range Ionic Nanochannels Constructed via Exfoliated Nafion–Boron Nitride Nanocomposite

Nafion–boron nitride (NBN) nanocomposites with a Nafion-functionalized periphery are prepared via a convenient and ecofriendly Nafion-assisted water-phase exfoliation method. Nafion and the boron nitride nanosheet present strong interactions in the NBN nanocomposite. Then the NBN nanocomposites were blended with Nafion to prepare NBN Nafion composite proton exchange membranes (PEMs). NBN nanocomposites show good dispersibility and have a noticeable impact on the aggregation structure of the Nafion matrix. Connected long-range ionic nanochannels containing exaggerated (−SO₃^–)_<i>n</i> ionic clusters are constructed during the membrane-forming process via the hydrophilic and H-bonding interactions between NBN nanocomposites and Nafion matrix. The addition of NBN nanocomposites with sulfonic groups also provides additional proton transportation spots and enhances the water uptake of the composite PEMs. The proton conductivity of the NBN Nafion composite PEMs is significantly increased under various conditions relative to that of recast Nafion. At 80 °C–95% relative humidity, the proton conductivity of 0.5 NBN Nafion is 0.33 S·cm^–1, 6 times that of recast Nafion under the same conditions

“Evaporating” Graphene Oxide Sheets (GOSs) for Rolled up GOSs and Its Applications in Proton Exchange Membrane Fuel Cell

In the present work, we prepare rolled up graphene oxide sheets (GOSs) by “evaporating” GOSs from their dispersion to a remote aluminum foil surface. The topological structure of the rolled up GOSs on the aluminum foil surface, which is determined by the quantity of the formed Al³⁺ ions from the reaction between the alumina on the aluminum foil surface and the weak acidic condensed vapor of the GOS dispersion, can be easily controlled via simply changing the H₂O content in the original GOS dispersion. Meanwhile, a GO/Nafion composite membrane for proton exchange membrane fuel cell is successfully prepared utilizing the as-obtained hole-like self-assembled structure of the rolled-up GOSs as a supporting material. The resultant composite membrane exhibits excellent proton conductivity compared to that of the recast Nafion membrane, especially under low-humidity conditions. An increase in proton conductivity by several times could be easily observed here, which is mainly attributed to the rearrangement of the microstructures of Nafion matrix to significantly facilitate the proton transport with rolled up GOSs being independently incorporated. The method reported here offers new degrees of freedom to achieve such transformations among the allotropic forms of carbon and/or develop new carbon material/polymer composite materials with excellent properties

Development of Hybrid Ultrafiltration Membranes with Improved Water Separation Properties Using Modified Superhydrophilic Metal–Organic Framework Nanoparticles

Metal–organic frameworks (MOFs) are being intensively explored as filler materials for polymeric membranes primarily due to their high polymer affinity, large pore volumes, and alterable pore functionalities, but the development of MOF-based ultrafiltration (UF) membranes for water treatment lags behind. Herein, poly­(sulfobetaine methacrylate) (PSBMA)-functionalized MOF UiO-66-PSBMA was developed, and incorporated into polysulfone (PSf) casting solution to fabricate novel hybrid UF membranes via phase-inversion method. The resultant UiO-66-PSBMA/PSf membrane exhibited significantly improved water flux (up to 602 L m^–2 h^–1), which was 2.5 times that of the pristine PSf membrane (240 L m^–2 h^–1) and 2 times that of UiO-66-NH₂/PSf membrane (294 L m^–2 h^–1), whereas the rejection of UiO-66-PSBMA/PSf membrane was still maintained at a high level. Moreover, UiO-66-PSBMA/PSf membrane exhibited improved antifouling performance. The improvement of membrane performances could be attributed to the well-tailored properties of UiO-66-PSBMA. On one hand, the excellent dispersion and compatibility of UiO-66-PSBMA ensured the formation of a uniform structure with few defects. On the other hand, the superhydrophilicity of UiO-66-PSBMA could accelerate the exchange rate between solvent and nonsolvent, resulting in a more hydrophilic surface and a more porous structure. Besides, UiO-66-PSBMA nanoparticles in the thin layer provided additional flow paths for water permeation through their hydrophilic porous structure as well as the tiny interspace between PSf matrix. This study indicates the great application potential of UiO-66-PSBMA in fabricating hybrid UF membranes and provides a useful guideline to integrate other modified hydrophilic MOFs to design UF membranes for water treatment