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"id": 1,
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"chunk": "# A Solvent Regulated Hydrogen Bond Crosslinking Strategy to Prepare Robust Hydrogel Paint for Oil/Water Separation \n\nZhongxiang Bai, Kun Jia,\\* Chenchen Liu, Lingling Wang, Guo Lin, Yumin Huang, Shuning Liu, and Xiaobo Liu\\* \n\nHydrogel modified porous matrix with the super-wetting surface (i.e., superhydrophilic/underwater super-oleophobic) is ideal for oil/water separation. However, the deterioration in mechanical strength and separation efficiency during the swelling process and complicated synthesis procedure limits its industrial application. In this study, a strategy of using ethanol to dynamically regulate the hydrogen bond crosslinking between polyvinyl alcohol (PVA) and tannic acid (TA) is proposed to prepare a “hydrogel paint”, which can be simply applied on the porous substrate surface by different one-step operations (dipping, brushing, spraying, etc.) without additional cross-linking. The underline mechanism is attributed to the re-establishment of intermolecular hydrogen bond mediated cross-linking between PVA and TA during ethanol evaporation. Consequently, the resultant hydrogel coating exhibits ultra-high strength $(>10M P a)$ ), swelling volume stability, and excellent oil-water separation efficiency $(>99\\%)$ . This study will provide new insights into the scalable fabrication of hydrogel-coated porous materials for oil/water separation in industrial scenarios.",
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"category": " Abstract"
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"id": 2,
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"chunk": "# 1. Introduction \n\nThe oil/water mixture deriving from industrial wastewater discharge and offshore crude oil leakage have brought great challenges to environmental protection, human health and biological survival.[1] Thus, the technologies or materials that can effectively separate oil from water are of great importance in terms of sustainable development. In recent years, functionalized membrane materials with special wettability are considered to be a more effective water treatment technology due to its high separation efficiency and low energy consumption.[2] Particularly, the super-hydrophilic/super-oleophobic (SHL/SOB) materials exhibit obvious advantages in resisting oil contamination compared to super-hydrophobic (SHB) materials, but their application is still hindered because oil with low surface tension tends to diffuse on most of solid surfaces.[3] Thus, the materials showing super-hydrophilic/underwater super-oleophobic (SHL/UWSOB) properties after water pre-wetting have been considered as superior candidate for oil-water separation. \n\nBiomimetic strategies play important roles in design of advanced functional materials for separation applications. For instance, the underwater oil-repellent surface of marine organisms has brought a new way to develop the research of SHL/ UWSOB materials.[4] In 2009, Liu et al. found that the micro-/nanostructures on the surface of fish scales displayed a unique underwater super-oleophobicity.[5] Inspired by fish scales, various surface functionalization methods, such as chemical corrosion,[6] electrodeposition,[7] heat treatment,[8] etc., have been employed to fabricate SHL/UWSOB materials. However, these methods are usually energyintensive, and the manufacturing process is only suitable for laboratory environments, presenting challenges in real-life industrial applications. Later, Jiang’s group reported a micro-structured hydrogel coating on the surface of stainless-steel mesh (SSM), which opened a new way for the preparation of SHL/UWSOB materials.[9] It is worth noting that the hydrogel coating of porous substrates is more complicated than that of non-porous substrates, because the hydrogel coating on porous substrates also has the risk/defects of blocking the pore structure and reducing flux. In addition, the hydrogel coating applied to oil-water separation should exhibit good swelling stability, as water swelling usually results to deterioration of surface wetting and mechanical properties.[10] \n\nDipping, spraying, brushing or shear coating is regarded as efficient technique to fabricate uniform coatings on large areas.[11] For hydrogel coatings on grid or porous substrates, the complete coating process normally involves the sol-gel transition. The conventional methods to prepare polymer hydrogels often involve monomers polymerization and macromolecular crosslinking,[12] which inevitably leads to the abrupt increase in viscosity of pre-gel solutions caused by rapid reaction kinetics. Therefore, it is difficult to apply a common paint coating method to a hydrogel coating. To avoid this problem, the current commonly used strategy is to form gel coating through a two-step method.[13] The first step is to dissolve the existing polymer and decorate it on the surface of the porous substrate; the second step is to form a 3D crosslinked networks on the surface of the substrate by means of solution immersion assisted cross-linking, repeated freeze-thaw, irradiation crosslinking and other methods.[12,14] Polyvinyl alcohol (PVA), as the largest by production scale existing polymer, has attracted extensive interest for hydrogel coatings due to its low cost, nontoxic, and intrinsic hydrophilicity. For instance, Jiang et al.[13c] used glutaraldehyde as a cross-linking agent to enable crosslinking of PVA on filter paper through a simple aldehyde condensation reaction to prepare a hydrogel-coating for oil/water separation. Liu et al.[15] prepared PVA coated SSM for oil/water separation in complex environment through repeated freezethaw assisted method. Although these PVA hydrogel coatings prepared by physical or chemical methods have achieved success, there are still challenges in practical applications in terms of the cross-linking control, the cost of scalable preparation, as well as the hydrogel coating strength. \n\nHerein, we proposed an ethanol/water solvent system to regulate the hydrogen bond cross-linking between PVA and TA, and prepared a “hydrogel paint” by a one-pot method. Ethanol in the solvent system temporarily hinders the cross-linking of PVA and TA to form a uniform hydrogel paint, which can be coated on the surface of the metal mesh (SSM) by the common paint application method. Interestingly, when ethanol in the hydrogel paint on the surface of the SSM gradually evaporates, the hydrogen bond cross-linking between PVA and TA is reestablished. The obtained hydrogel coating layer possesses stable swelling volume, ultra-high strength and stable repellency to various types of oil, and the gel-coated SSM displays a gravity-driven high separation efficiency and a long-term cyclic stability.",
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"category": " Introduction"
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},
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"id": 3,
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"chunk": "# 2. Results and Discussion",
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"category": " Results and discussion"
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"id": 4,
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"chunk": "# 2.1. Hydrogel Paint Preparation and Formation Mechanism \n\nPoly(vinyl alcohol) (PVA) has been intensively used in the fabrication of hydrogels,[16] and the appropriate crosslinking agents are generally introduced to manipulate the crosslinking kinetics as well as density so as to obtain anti-swelling hydrogels. However, there are contradictions in the conventional hydrogels preparation protocols: weak interactions are preferred to ensure the fluidity of the hydrogel paint during the co-dissolution of PVA and crosslinking agent, while the strong interactions are acclaimed to ensure the high strength and stability of the hydrogel during the crosslinking process. For example, it was recently found that a small amount of plant-derived polyphenol TA could be co-dissolved with PVA without formation of hydrogels;[17] however, when a large amount of TA and PVA were co-dissolved, gelatinous precipitates were generated due to the strong hydrogen bonding (Figure 1a).[18] Inspired by the principle of using non-derived solvents to destroy the intra or intermolecular hydrogen bond to dissolve cellulose,[19] if the PVA/ TA solvent system is added molecules with stronger hydrogen bond with PVA or TA to isolate the intermolecular hydrogen bond between PVA and TA, the resultant mixture solution would exhibit uniform fluidity and good processability for mesh substrate coating. More importantly, when the molecules used to isolate can be removed, the strong hydrogen bond between \n\nPVA and TA will be reformed, enabling the in-situ preparation of hydrogel coatings on the mesh surface. \n\nFor this reason, we chose ethanol to replace part of water to weaken the strong hydrogen bond cross-linking between TA and PVA, and obtained a stable and homogeneous hydrogel paint. The hydrogel paint can be applied to the surface of the porous SSM through one-step method (dipping, spraying, brushing or shearing) similar to that used for common paint (Figure 1b). As shown in Scheme 1, after the ethanol in the coating was removed, the hydrogen bond between PVA and TA is re-established to form a 3D network structure. Ethanol can be removed by volatilization or solvent diffusion. Since the volatilization method is simpler and the resulting coating is more uniform (Figure S1 in the Supporting Information), the subsequent experiment uses the volatilization method to prepare the PVA-TA hydrogel coating. The effect of the volume ratio of ethanol in the solvent on the hydrogel paint (Table S1 and Figures S2 and S3, Supporting Information) was studied, and it was found that the volume ratio of ethanol had an effect on the uniformity of the hydrogel paint, which in turn affected the gel coating rate and water flux of the SSM. For comprehensive performance, a water/ethanol volume ratio of 5:5 is finally selected as the optimized solvent system of PVA/TA. The effect of TA content (Table S2 and Figure S4, Supporting Information) and PVA/TA concentration (Table S3 and Figure S5, Supporting Information) in the hydrogel paint on coating performance is also discussed in detail. In addition, in order to increase the surface roughness of the coating and the stability of the PVA-TA gel, an appropriate amount of nano-sized $\\mathrm{SiO}_{2}$ was added.",
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"category": " Materials and methods"
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"id": 5,
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"chunk": "# 2.2. Characterization of Hydrogel Coatings \n\nFourier transform infrared (FTIR) spectroscopy was used to identify the hydrogen bond cross-linking between TA and PVA molecules (Figure 2a). Compared to PVA, the hydrogel containing TA appeared new absorption peaks at 1714 and $1612\\mathrm{cm}^{-1}$ , which were assigned to the stretching vibration of the $\\scriptstyle{\\mathrm{C=O}}$ and $\\scriptstyle{\\mathrm{C=C}}$ and the peak at $750~\\mathrm{cm}^{-1}$ was attributed to the stretching vibration of C-H on the aromatic ring, the bending deformation of C-OH and benzene ring torsion.[20] The broad and strong absorption bands at 3250 and $3365~\\mathrm{cm}^{-1}$ are attributed to the symmetrical stretching vibration of hydroxyl groups $(\\boldsymbol{\\mathrm{v}}_{\\mathrm{O-H}})$ of PVA and TA,[16a] while the $\\boldsymbol{\\mathbf{v}}_{\\mathrm{O-H}}$ peaks of the PVA−TA and PVATA- $\\mathrm{SiO}_{2}$ shifted to a lower wavenumber of 3232 and $3234~\\mathrm{cm}^{-1}$ , respectively. Simultaneously, the PVA showed typical $\\mathsf{v}_{\\mathrm{C-OH}}$ at $1087\\mathrm{cm}^{-1}$ ,[21] which shifted to a lower wavenumber of $1030~\\mathrm{cm}^{-1}$ after being treated with TA. It is well-known that the formation of intra- or intermolecular hydrogen bond reduces the force constants of the chemical bonds, leading to the red shifting of their vibrational frequencies.[17,22] Therefore, the chemical shift of $\\boldsymbol{\\mathrm{v}}_{\\mathrm{O-H}}$ and $\\mathsf{\\Pi}\\mathsf{v}_{\\mathrm{C.OH}}$ implies that hydrogen bond cross-linking is formed between PVA-TA.[23] In addition, the FTIR spectra of PVA-TA and PVA-TA $\\boldsymbol{\\mathcal{Q}}\\boldsymbol{\\mathrm{SiO}}_{2}$ hydrogels did not show new peaks produced by chemical reactions, indicating that the cross-linking system was exclusively enabled by physical effects. \n\nWe then used X-ray diffraction (XRD) to characterize the structure of the PVA, TA, PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ . As shown in Figure 2b, the XRD pattern of PVA shows three typical peaks at $2\\theta=19.6^{\\circ}$ , $2\\theta=22.9^{\\circ}$ , and $2\\theta=40.8^{\\circ}$ , corresponding to the (101), (200), and (102) planes of PVA crystallites.[17,24] Due to the amorphous nature of TA, there are no sharp crystalline peaks in the XRD pattern of TA, instead of a blunt peak at $2\\theta=25.9^{\\circ}$ . The introduction of TA causes a significant reduction in the typical crystallization peak of PVA, which is due to the fact that the strong hydrogen bond between PVA and TA inhibits the crystallization of PVA chains, and this result is consistent with previous reports.[17–18] \n\n \nFigure 1. a) Photograph of PVA and TA dissolved in $H_{2}O$ and $\\mathsf{H}_{2}\\mathsf{O}/\\mathsf{C}_{2}\\mathsf{H}_{5}\\mathsf{O}\\mathsf{H}$ mixed solvent. b) Schematic fabrication of hydrogel coated SSM by different techniques. \n\n \nScheme 1. Schematic illustration of reconstruction of hydrogen bond of PVA and TA when the ethanol is evaporated. \n\n \nFigure 2. a) ATR-FTIR spectra and b) X-ray diffraction (XRD) patterns of PVA, TA, PVA-TA and PVA-TA $@\\mathsf{S i O}_{2}$ . c) TGA, and d) DTG curves of PVA, PVA-TA, and PVA-TA $\\textcircled{\\sc1}$ hydrogels. \n\nIn addition, the thermal gravimetric analysis (TGA) was conducted for PVA, PVA-TA, and PVA-TA $@\\mathrm{SiO}_{2}$ (Figure 2c,d). The first weight loss platform appearing at $80{-}150^{\\circ}\\mathrm{C}$ was due to the evaporation of bound water in hydrogels. The second weight loss plateau period was detected between 200 and $500~^{\\circ}\\mathrm{C}$ , and the initial weight loss temperatures of PVA-TA and PVA-TA $\\ @\\operatorname{SiO}_{2}$ were slightly lower than PVA. This phenomenon may be due to the fact that TA destroys the crystallization of PVA.[25] Interestingly, in the range of $220{-}350^{\\circ}\\mathrm{C}$ , the thermal degradation rate of PVA and TA alone was higher than that of PVA-TA and PVA-TA $@$ $\\mathrm{SiO}_{2}$ (Figure 2d and Figure S6: Supporting Information), and the weight-loss rates of PVA, PVA-TA, and PVA-TA $\\ @\\operatorname{SiO}_{2}$ were $92\\%$ , $79\\%$ , and $76\\%$ , respectively. \n\nAlthough many previous works have reported high strength hydrogels, maintaining mechanical strength for a long time in oil/water separation environments is still challenging because the heterogeneous swelling of hydrogels likely results in low polymer chain density and small friction between polymer chains.[10,26] Given that hydrogen bond crosslinking is an effective method to restrain swelling,[27] we have characterized the swelling properties and corresponding mechanical properties of PVA-TA and PVA-TA $\\ @\\operatorname{SiO}_{2}$ under different conditions. As shown in Figure 3a, the equilibrium swelling ratio (ESR) of PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ in highly acidic and saline environments was much lower than that of common hydrogels, only $6\\mathrm{-}18\\%$ , and the ESR of PVA-TA $@\\mathrm{SiO}_{2}$ was greater than that of PVA-TA. However, in a highly alkaline environment, the ESR of the hydrogel increased drastically, and PVA-TA $\\ @\\operatorname{SiO}_{2}$ was more stable than PVA-TA. It could also be observed from Figure $3\\mathrm{g}$ that the swelling volumes of PVA-TA and $\\mathrm{PVA\\mathrm{\\cdotTA@SiO_{2}}}$ increased under the highly alkaline environment, while the volume basically did not change under the acidic and saline environment. In addition, we observed that the transparency of the gels decreased after being immersed in pure water, strong acid and high-salt solutions, possibly due to the separation of the hydrophobic phase caused by hydrogen bond.[28] In a highly alkaline environment, the gel cross-linking network was no longer dense, and the hydrophobic phase separation was declined, which leaded to an increase in gel transparency.[29] Twenty-five phenolic hydroxyl groups in the TA molecule, which are protonated under acidic conditions, are excellent hydrogen donors and recipients for binding with hydroxyl groups of PVA.[30] As the pH value increases, the ionization of TA increases, resulting in a decrease in the density of the hydrogen bond cross-linking network of the hydrogel and an increase in water absorption. Since the hydrogen bond is sensitive to $\\mathsf{p H}$ variations, we prepared a series of solutions with a $\\mathrm{\\pH}$ range of 2–12, and then soaked the PVA-TA and $\\mathrm{PVA-TA@SiO}_{2}$ hydrogel samples in these solutions for $24\\mathrm{~h~}$ . As shown in Figure 3b, the ESR of PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ stabilized at around $11\\%$ and $18\\%$ , respectively, in the range of $\\mathrm{pH}=2\\mathrm{-}8$ . At $\\mathrm{pH}=10\\$ , the ESR of the gels showed an apparent upward trend. When $\\mathrm{pH}=12\\$ , the water absorption rate of the hydrogel increased sharply, and the ESR of PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ became 9.7 times and 3.1 times than that of the neutral environment, respectively. The PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ hydrogels exhibit a pH-dependent swelling behavior as a consequence of the ionization of the functional groups in the phenolic structures.[25,31] Correspondingly, we tested the mechanical strength of the hydrogel after reaching swelling equilibrium under different environments. Figure 3c,d showed that the PVA-TA $@\\mathrm{SiO}_{2}$ hydrogel in the pure water swelling equilibrium demonstrated excellent mechanical properties with a tensile strength of $11.8\\mathrm{~MPa}$ , and a breaking strain of $487\\%$ , which is much higher than the currently reported supramolecular polymer hydrogel. [27b,32] As shown in Figure S7 (Supporting Information), the PVA-TA $@\\mathrm{SiO}_{2}$ hydrogel strip with a thickness of ${\\approx}0.3~\\mathrm{mm}$ , width $\\mathrm{\\Delta\\sf{}\\approx10\\ m m}$ was capable of withstanding a weight of $2.5~\\mathrm{kg}$ without any damage. More interestingly, although the PVA-TA $@\\mathrm{SiO}_{2}$ showed a yielding during stretching, no necking phenomenon as that of common high strength hydrogels was observed (Figure S8, Supporting Information), suggesting that hydrogen bond cross-links undergo gradual separation rather than sudden cumulative damage, which implies that the gel could bear greater loads, thereby avoiding catastrophic failure as a coating.[28] Under high acid and salt conditions, the tensile strength of $\\mathrm{PVA–TA@SiO_{2}}$ increased to 15.7 and $12.5\\ \\mathrm{MPa}$ (Figure 3e), respectively, but after swelling and equilibrium in $1\\ \\mathrm{M}\\ \\mathrm{NaOH}$ , the tensile strength decreased sharply to $0.2~\\mathrm{MPa}$ . \n\n \nFigure 3. Swelling and mechanical properties of PVA-TA and PVA- ${\\mathsf{T A@S i O}}_{2}$ hydrogels: The equilibrium swelling rate of hydrogels in a) extreme environments and at b) $\\mathsf{p H}=2-12$ . Photograph of PVA-TA $\\textcircled{a}{\\mathsf{S i O}}_{2}$ in c) tensile test and d) stress–strain curve under extreme environment. The tensile strength of hydrogels in e) extreme environments and f) at ${\\mathsf{p H}}=2{-}12$ g) Photograph of hydrogel swelling under extreme conditions. \n\n \nScheme 2. The preparation of PVA-TA $@\\mathsf{S i O}_{2}$ decorated SSM by dip coating method and its oil/water separation mechanism \n\nAs the $\\mathrm{\\ttpH}$ value of the swelling environment increased from 2 to 12, the tensile strength of $\\mathrm{PVA-TA@SiO}_{2}$ hydrogel decreased from 14.0 to $7.2~\\mathrm{MPa}$ (Figure 3f), which is still an outstanding strength for hydrogel materials. In addition, the breaking energy (Figure S9, Supporting Information) of PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ in different environments was also similar to the change trend of tensile strength. In the whole test conditions, the tensile strength of PVA-TA $\\ @\\operatorname{SiO}_{2}$ was better than that of PVA-TA, especially in the high alkaline environment $\\mathrm{(pH=}$ 12), which was mainly due to the presence of $\\mathrm{SiO}_{2}$ providing more hydrogen bond crosslinking points.",
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"category": " Results and discussion"
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"id": 6,
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"chunk": "# 2.3. Micro-Morphology and Wettability of Hydrogel-Coated Metal Mesh \n\nThe surface chemical composition and microstructure are the two key factors affecting the surface wettability of materials.[33] Herein, we successfully deposited PVA-TA and PVA-TA $@\\mathrm{SiO}_{2}$ hydrogels on SSM substrate through a simple dip-coating process. As shown in Scheme 2, the ultrasonic-washed SSM was placed in the solution of PVA-TA or $\\mathrm{PVA-TA@SiO}_{2}$ at $60~^{\\circ}\\mathrm{C}$ for 2 min, followed by ethanol evaporation in the air to form a hydrogel coating. The surface morphology of the pristine SSM and the hydrogel coated SSM were characterized by scanning electron microscope (SEM). As illustrated in Figure 4, the pristine SSM has a smooth surface structure with an average pore size about $50\\upmu\\mathrm{m}$ . After being decorated with PVA-TA hydrogel, a smooth and dense hydrogel layer was built on the surface of $\\mathrm{{\\ss{M}}}$ , and the coating thickness was about $1.9~{\\upmu\\mathrm{m}}$ . In sharp contrast, the SSM decorated with $\\mathrm{PVA-TA@SiO_{2}}$ hydrogel has a rough surface with nanostructures, and the coating thickness was about $2.1\\upmu\\mathrm{m}$ . EDX analysis also proved that the hydrogel covered the steel wires uniformly and almost no hydrogel existed in the pores of the SSM, which ensures free passage of water through the prepared coated $\\mathrm{{{SSM}}}$ . \n\nThe wettability and oil-adhesion properties of SSM decorated with PVA-TA and PVA-TA $\\ @\\operatorname{SiO}_{2}$ hydrogels were characterized. As shown in Figure 5a, after modification of PVA-TA hydrogel, the water contact angle of SSM in the air decreased from $114.7^{\\circ}\\pm3.2^{\\circ}$ to $55.5^{\\circ}\\pm2.7^{\\circ}$ . Generally, chemical composition and surface roughness are essential factors of super-wettability surface.[34] Even though tannic acid and polyvinyl alcohol molecules are both hydrophilic, since the surface of the gel coating is smooth, the water droplets are not completely spread on the surface. In contrast, M/PVA-TA exhibited super-oleophobic properties underwater, with an oil contact angle (OCA) of $152.3^{\\circ}\\pm2.6^{\\circ}$ and a sliding angle of $7.0^{\\circ}$ (Figure S10a, Supporting Information). When $\\mathrm{SiO}_{2}$ was doped in the PVA-TA hydrogel, $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}@\\mathrm{SiO}_{2}$ exhibited super-hydrophilic properties in the air $(\\mathbb{W}\\mathbb{C}\\mathbb{A}=0^{\\circ}$ ) and super-oleophobic underwater (OCA $=156.3^{\\circ}\\pm1.1^{\\circ}.$ ), and the sliding angle was $3.0^{\\circ}$ (Figure S10b, Supporting Information). According to the surface wettability theory of Cassie-Baxter,[35] the underwater superoleophobicity is achieved in oil/water/solid three-phase system by introducing the repulsive liquid phase into the micro-structured surface. In water, the 3D network hydrogel coatings on the surface of SSM absorb water to its equilibrium state. The rough nanostructure of the PVA-TA $@\\mathrm{SiO}_{2}$ hydrogel surface captures more water, and when oil droplets contact the surface, an oil/water/solid composite interface is formed, which is beneficial to prevent oil in contact with the $\\mathrm{{\\sfSSM}}$ and form an underwater superoleophobic surface. We tested the underwater wettability of various oils and organic solvents (including xylene, cetane, hexane, soybean oil and pump oil) to M/PVA-TA $@\\mathrm{SiO}_{2}$ , and the OCAs were all greater than $150^{\\circ}$ (Figure 5b). Considering that oily wastewater is usually in a variety of complex environments, the underwater oil contact angle of M/PVA-TA (Figure 5c) and $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ (Figure 5d) at $\\mathrm{pH}=2\\mathrm{-}12$ was tested. Both M/PVA-TA and $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ had contact angles greater than $150^{\\circ}$ within the $\\mathrm{\\pH}$ range of 2–12. In comparison, $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ is more oil-repellent, and with the increase of $\\mathrm{\\pH}$ value, the underwater oil contact angle has a tendency to increase, which is due to the increase of water content of gel coating. \n\n \nFigure 4. SEM images of hydrogels decorated SSM and EDX image of $M/P V A-T A@S i O_{2}$ . \n\n \nFigure 5. a) The water contact angles and underwater oil contact angles of original SSM, M/PVA-TA and $M/P V A\\mathrm{-}\\mathsf{T A}@\\mathsf{S i O}_{2}$ . b) Underwater oil contac angles of the M/PVA-TA $\\textcircled{\\sc1}$ for the various oils. c) The oil contact angles of M/PVA-TA in a $\\mathsf{p H}=2-12$ water environment. d) The oil contact angle of $\\mathsf{M}/\\mathsf{P V}\\mathsf{A}\\mathsf{-T A@S i O}_{2}$ in a $\\mathsf{p H}=2–12$ water environment.",
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"category": " Results and discussion"
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"id": 7,
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"chunk": "# 2.4. Oil/Water Separation of Hydrogel-Coated SSM \n\nIn the process of oil-water mixture separation, membrane materials usually suffer from serious oil fouling and pore-blocking, which reduces its separation efficiency and service life. Figure 6a demonstrated the process of oil droplet (chloroform) preloading and adhering to $\\mathrm{{\\sfSSM}}$ underwater. When the oil droplet was loaded on the unmodified SSM, the oil droplet was in full contact with the surface after being pressed in $60~\\mathrm{s}$ After the pressure was removed, the oil droplet was found to adhere to the SSM, indicating poor oil-resistance adhesion performance. However, compared with the pristine SSM, the oil droplets were easily detached from the $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}@\\mathrm{SiO}_{2}$ surface, and there were no oil droplets remaining on the surface. Furthermore, the pre-wetted pristine mesh and the PVA-TA $@\\mathrm{SiO}_{2}$ coated mesh were immersed in hexane to make the surface adhere to oil, and then transferred to deionized water (Figure 6b and Movie S1, Supporting Information). It could be observed that large amount of oil stayed on the pristine SSM surface, while the oil completely slipped off on the PVA-TA $\\ @\\operatorname{SiO}_{2}$ modified SSM, indicating the excellent self-cleaning ability of the gelcoated SSM. Similarly, in Movie S2 (Supporting Information), we found that chloroform adheres to the pristine $\\mathrm{{\\calS}}\\mathrm{{\\calS}}\\mathrm{{\\calM}}$ underwater, but it could easily slip off the surface of the PVA-TA $@$ $\\mathrm{SiO}_{2}$ decorated SSM. These results consistently implied that the PVA-TA $\\ @\\operatorname{SiO}_{2}$ hydrogel coating could effectively prevent the network structure of SSM from being contaminated or blocked by oil during the oil/water separation process, which ensures its potential re-usability. \n\nTo test the oil/water separation capability of the hydrogelcoated mesh, the mixture of oil (xylene, cetane, hexane, soybean oil, and pump oil) and water was continuously injected into the device shown in Figure 7a. During the entire separation process, no external force was applied, and water rapidly permeated through the PVA-TA ${\\mathcal{Q}}\\mathrm{SiO}_{2}$ coated mesh, while the oil was retained in the upper separating funnel (Movie S3, Supporting Information). After the separation process, nearly no visible oil in the permeated water. As a result, the separation efficiency of different oil products by infrared spectrometer was determined to be above $99\\%$ (Figure 7b). Besides, to demonstrate the robust anti-fouling performance and durability of the $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ , 30 cycles of separation experiments of hexane/water mixture were carried out. As shown in Figure 7c, the PVA-TA $@\\mathrm{SiO}_{2}$ coated SSM remained at a high-level after 30 cycles of separation. Specifically, the flux was greater than $6\\times10^{4}\\mathrm{~L~m}^{-2}\\mathrm{~h}^{-1}$ , and the separation efficiency remained above $99\\%$ . The OCAs of $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}@\\mathrm{SiO}_{2}$ were tested after separation in different cycles (Figure 7d). It was found that the OCAs on the surface of the PVA-TA $\\ @\\operatorname{SiO}_{2}$ coated SSM slightly increased and decreased, and it was still in a super-oleophobic state $(153^{\\circ}-157^{\\circ})$ , which further demonstrated the stability of its underwater super-oleophobic performance. Besides, $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ exhibits better flux and separation efficiency than PVA-TA (Figure S11, Supporting Information), which is consistent with the previous results of wettability characterization. We also measured the intrusion pressure of oil flowing through the coated SSM to characterize the maximum height of oil that the PVA-TA $@\\mathrm{SiO}_{2}$ coated SSM can support. As shown in Figure S12 (Supporting Information), the height measured with hexane as the sample oil was about $16.40~\\mathrm{cm}$ , and the penetration pressure of the oil is $1.06~\\mathrm{kPa}$ . In addition, the separation efficiency and intrusion pressure of $\\mathrm{{{SSM}}}$ with different pore sizes modified by hydrogel coating were also characterized in detail (Figure S13, Supporting Information). \n\n \nFigure 6. a) Images show that the adhesion behavior of oil droplets (chloroform) on the surfaces of the pristine SSM and the PVA-TA $\\textcircled{\\sc1}$ coated SSM. b) The underwater self-cleaning ability of the PVA-TA $@\\mathsf{S i O}_{2}$ coated SSM and pristine mesh. \n\nTo further study the separation performance of PVA-TA $@\\mathrm{SiO}_{2}$ coated $\\mathrm{{\\ss{M}}}$ in different environments, we measured its separation flux and efficiency under artificial seawater and different acid-base conditions. As shown in Figure 7e, $\\mathrm{M}/\\mathrm{PVA}{\\cdot}\\mathrm{TA}\\ @\\mathrm{SiO}_{2}$ had higher stability under artificial seawater and $\\mathrm{pH}=2\\mathrm{-}10$ . At $\\mathrm{pH}=12\\$ , the gel network on the surface was destroyed, but the oxidation of TA may form a hydrophilic layer on the surface of the mesh, which still had a separation function when wetted by water. However, it was found in the separation experiment that the TA oxide layer did not possess the water retention function of the gel layer. After the moisture on the grid surface evaporates in a short time, the oil droplets will pass through the filter. Finally, we also demonstrated the potential of PVA-TA $@\\mathrm{SiO}_{2}$ hydrogel coating for separating oilin-water emulsions. Compared with the oil/water mixture, the oil-in-water emulsion contains emulsified oil droplets ranging from hundreds of nanometers to dozens of micrometer, and due to the presence of surfactants on the oil/water interface, the emulsion is extremely stable and difficult to separate by using $\\mathrm{{{SSM}}}$ . Therefore, a polytetrafluoroethylene (PTFE) membrane with a pore size of $1.0\\upmu\\mathrm{m}$ was selected as the porous substrate to apply PVA-TA $@\\mathrm{SiO}_{2}$ paint. As shown in Figures S14 and S15 (Supporting Information), PTFE/PVA-TA $@\\mathrm{SiO}_{2}$ exhibits excellent underwater super-oleophobic properties, with OCAs of $157.8^{\\circ}$ and sliding angle of $2.0^{\\circ}$ . In the emulsion separation experiment, it showed a separation efficiency ${>}99.5\\%$ and stable antifouling performance, demonstrating the effectiveness of the PVA-TA $@\\mathrm{SiO}_{2}$ coating for the separation emulsion \n\n \nFigure 7. a) Typical oil-water separation experiment process. b) Flux and separation efficiency of $M/P V A\\cdot T A@{\\mathsf{S i O}}_{2}$ for the various oil/water mixtures c) Flux and separation efficiency of M/PVA-TA $\\textcircled{\\sc1}$ for 30 cycles test of hexane/water mixture. d) The underwater oil contact angles of M/PVA-TA $@$ $\\mathsf{S i O}_{2}$ during the cyclic separation experiment. e) Flux and separation efficiency of M/PVA-TA $@\\mathsf{S i O}_{2}$ for the various environments. \n\nBesides, the mechanical stability of the PVA-TA $@\\mathrm{SiO}_{2}$ coating in the dry and wet state was studied through sand abrasion and water shock experiments. After rubbed by rolling sand grains $(200{-}600~\\upmu\\mathrm{m})$ from the height of 20 to $100\\ \\mathrm{cm}$ (FigureS16b,c:SupportingInformation),theOCAs(FigureS16d, Supporting Information) of the coating grid decreased slightly as the impact height increased, and the underwater super-oleophobic state was always maintained. This is due to the excellent mechanical properties of the gel in the dry state (Figure S16a, Supporting Information). Similarly, after a $20{-}100~\\mathrm{cm}$ water impact test, the OCAs (Figure S16e, Supporting Information) of PVA-TA $\\ @\\operatorname{SiO}_{2}$ coated SSM remained stable, which was attributed to the ultra-high strength of the hydrogel coating and the strong adhesion strength $(757.9\\mathrm{kPa})$ to the stainless steel substrate (Figure S17, Supporting Information). Meanwhile, this work also exhibited advantages in terms of hydrogel strength and separation efficiency compared with previously reported (Table S4, Supporting Information). In general, the hydrogel coated mesh exhibits stability and durability under complex conditions, and has practical application potential in the field of oil-water separation.",
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"category": " Results and discussion"
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},
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{
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"id": 8,
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"chunk": "# 3. Conclusions \n\nIn conclusion, we reported a simple but effective strategy that exploring ethanol to dynamically modulate the hydrogen bond between PVA and TA in water to prepare a uniform hydrogel coating on SSM surface. When the ethanol evaporates, the initially inhibited hydrogen bond cross-link between PVA and TA is re-formed, which leads to the in situ generation of super-hydrophilic/underwater super-oleophobic PVA-TA $@$ $\\mathrm{SiO}_{2}$ hydrogel coated mesh for robust oil/water separation. The hydrogen bond cross-linked hydrogel composed of PVA, TA and $\\mathrm{SiO}_{2}$ nanoparticles has an ultra-high tensile strength $({>}10\\ \\mathrm{MPa})$ to ensure the stability of the coating under longterm operation. Meanwhile, the gel also shows low expansibility, good adhesion to $\\mathrm{{\\sfSSM}}$ and stable super-wettability in practical application environments. Based on the excellent water retention and stability of the PVA-TA $@\\mathrm{SiO}_{2}$ gel coating, the as-prepared mesh can selectively separate water from the oil/water mixture quickly (flux exceeds $6\\times10^{4}\\mathrm{~L~m^{-2}~h^{-1}})$ , with high separation efficiency $(>99\\%)$ and resistance to oil fouling and they are easy to recycle. We believe that this work has broad application prospects in the fields of underwater petroleum transportation, oil spill accident treatment and domestic sewage purification, and provides important insights into the field of interface wetting modification of hydrogel coatings, and promotes the transformation of hydrogel coating materials from laboratory models to practical applications.",
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"category": " Conclusions"
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},
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{
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"id": 9,
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"chunk": "# 4. Experimental Section \n\nMaterials: All chemicals were obtained from suppliers without additional purification. Polyvinyl Alcohol (PVA-1799, ${\\approx}99\\%$ hydrolyzed) was purchased from Aladdin. Tannic acid (TA, $M\\mathrm{w}=7707\\textrm{g}\\mathsf{m o l}^{-1})$ was obtained from Tianjin Hengxing Chemical Reagent Co. Ltd. Nanoscale $\\mathsf{T i O}_{2}$ $(30\\ n m)$ was purchased from Macklin. The stainless-steel mesh (SSM, 300 mesh) was got from Hebei Anshun Xing Hardware Technology Co., Ltd. The stainless steel sheet (section: $75\\:\\mathrm{mm}\\times0.2\\:\\mathrm{mm};$ was purchased from Hongdu Metal Co., Ltd. Soybean oil and pump oil were purchased from the local market. The polytetrafluoroethylene (PTFE, ${\\mathrm{~1~}}\\upmu\\mathsf{m})$ membrane was purchased from Yibo Filter Equipment Factory. All other chemicals were analytical grade and commercially available from Chron chemicals (Chengdu, China). Deionized water was used throughout the experiment. \n\nPreparation of PVA-TA and PVA- ${\\cdot\\ T A@S i O_{2}}$ Hydrogels Coated Mesh: PVA $(3.0~\\ g)$ and TA $(3.0~\\mathrm{g})$ were dissolved in $94.0~\\mathrm{g}$ mixed solvent (deionized water/ ethanol $=1\\colon1\\ \\mathsf{v}/\\mathsf{v})$ with stirring at ${\\approx}90^{\\circ}\\mathsf C$ for $6\\mathfrak{h}$ to obtain PVA-TA hydrogel paint. The preparation process of PVA- $\\mathsf{T A@S i O}_{2}$ hydrogel paint was similar to that of PVA-TA. Before adding PVA and TA, $0.6\\ g\\ S_{1}\\mathrm{O}_{2}$ was added to water/ethanol solvent under ultra-sonication for $30\\mathrm{\\min}$ . The quality of PVA, TA and water/ethanol solvent was equal to that of PVA-TA paint. Unless otherwise specified, the concentration of $\\mathsf{S i O}_{2}$ , PVA and TA in the pre-gel was as described above. The SSM (the average pore diameter of $\\approx50~{\\upmu\\mathrm{m}}$ ) was immersed in the above pre-gel solution for $2\\min$ . Then the SSM was taken out from the pre-gel and dried at room temperature to get the hydrogel-coated mesh. The SSM decorated with PVA-TA and PVA-TA $\\@{\\sf S i O}_{2}$ were denoted as M/PVA-TA and M/PVA-TA $\\textcircled{0}$ ${\\mathsf{S i O}}_{2},$ , respectively. \n\nInstruments and Characterization: The chemical structures were characterized by Fourier transform infrared (FT-IR) (PerkinElmer, Inc., Waltham, MA) at a resolution of $4c m^{-1}$ in the range of $4000{-}500~{\\mathsf{c m}}^{-1}$ Thermogravimetric analysis (TGA) was conducted on TA $Q50$ under an air atmosphere at a heating rate of $20^{\\circ}\\mathsf{C}\\mathsf{m i n}^{-1}$ from 50 to $700^{\\circ}\\mathsf C$ X-ray diffraction (XRD, XPERT PRO, Netherlands) was applied to characterize the crystallization performance of hydrogels, and the scan angle was $5^{\\circ}$ to $50^{\\circ}$ . An electronic universal testing machine (Instron 5567) was used to test the mechanical properties of hydrogel samples (length $50\\mathsf{m m}$ , width $\\mathsf{10}\\mathsf{m m}$ , thickness $0.3\\mathsf{m m}^{\\mathrm{~.~}}$ ) with a stretching speed of $50\\mathsf{m m}$ $\\mathsf{m i n}^{-1}$ . SEM images of the hydrogel coated mesh were obtained using a field emission scanning electron microscope (FESEM, JMS-6490 LV) at $20\\ \\mathsf{k V}.$ Contact angles were measured on DSA 30 machine at ambient temperature. \n\nThe equilibrium swelling ratio (ESR) of hydrogels in different environments is defined as: \n\n$$\nE S R=\\frac{\\mathbb{W}_{\\mathrm{s}}-\\mathbb{W}_{\\mathrm{d}}}{\\mathbb{W}_{\\mathrm{d}}}\\times100\\%\n$$ \n\nwhere ${\\sf W}_{\\sf d}$ and ${\\sf W}_{\\sf s}$ are the weight of the hydrogels before and after swelling, respectively. \n\nOil/Water Separation Experiments: The hydrogel-coated SSM was used to separate the oil/ water mixture $(30\\mathrm{\\mL},\\mathbb{1};2,\\up v/\\up v)$ . The flux of the SSM \n\nis determined by calculating the filtration time of oil/water mixture and the formula is as follows: \n\n$$\nF I u x=\\frac{V}{A\\times\\Delta T}\n$$ \n\nWhere $V$ (L), A $(\\mathsf{m}^{2})$ , and $\\Delta T$ (h) are the volume of the filtering water, membrane contacting area, and time used, respectively. \n\nThe oil concentration before and after separation was determined by an infrared spectrometer oil content analyzer (OIL460, China). The separation efficiency $(R\\%)$ is calculated by the following formula: \n\n$$\nR^{\\circ}\\rho=\\left(1-\\frac{C_{a}}{C_{b}}\\right)\\times100\\%\n$$ \n\nwhere $C_{a}$ and $C_{\\flat}$ are the oil concentrations after and before separation, respectively. \n\nThe oil intrusion pressure is determined by the maximum height $(h_{\\mathsf{m a x}})$ of oil that the SSM can support and calculated as follows: \n\n$$\nP_{o i l}=\\rho g h_{\\operatorname*{max}}\n$$ \n\nwhere $\\rho$ is the density of oil, $g$ is the gravitational acceleration.",
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"category": " Materials and methods"
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},
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{
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"id": 10,
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"chunk": "# Supporting Information \n\nSupporting Information is available from the Wiley Online Library or from the author.",
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"category": " References"
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},
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{
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"id": 11,
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"chunk": "# Acknowledgements \n\nThis work was supported by National Natural Science Foundation of China (52173068, 51773028, 52073039), the Fundamental Research Funds for the Central Universities (ZYGX2019J026), Sichuan Science and Technology Program (2020YFG0100, 2019YJ0197, 2019YFG0056, 2021YFH0023) and International Science and Technology Cooperation Project from Chengdu municipal government (2019-GH02-00037-HZ).",
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"category": " References"
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},
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{
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"id": 12,
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"chunk": "# Conflict of Interest \n\nThe authors declare no conflict of interest.",
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"category": " References"
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},
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{
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"id": 13,
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"chunk": "# Data Availability Statement \n\nResearch data are not shared.",
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"category": " Results and discussion"
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},
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{
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"id": 14,
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"category": " References"
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] |