Polybenzoxazine-Functionalized Melamine Sponges with Enhanced

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Surfaces, Interfaces, and Applications

Polybenzoxazine Functionalized Melamine Sponges with Enhanced Selective Capillarity for Efficient Oil Spill Cleanup Jianlong Ge, Fei Wang, Xia Yin, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14052 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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ACS Applied Materials & Interfaces

Polybenzoxazine

Functionalized

Melamine

Sponges

with

Enhanced Selective Capillarity for Efficient Oil Spill Cleanup Jianlong Ge†, Fei Wang†, Xia Yin†,‡, Jianyong Yu‡, Bin Ding*,†,‡ † Key

Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua

University, Shanghai 201620, China. ‡Innovation

Center for Textile Science and Technology, Donghua University, Shanghai 20051, China

*Corresponding author e-mail address: [email protected] KEYWORDS: polybenzoxazine, in-situ polymerization, superhydrophobic-superoleophilic sponges, selective capillarity, oil spill cleanup

ACS Paragon Plus Environment

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ABSTRACT Severe environmental and ecological issues arising from the frequent oil spill accidents have been great worldwide concerns. Considering the abruptness, complex condition, and long-term perniciousness of the spilled oil, the development of economic and versatile materials to quickly remove oil contaminants, especially for oil with high viscosity from a large water body is of significant importance, but remain a big challenge. Herein, we demonstrated a facile strategy to fabricate a versatile hierarchical structured sponge with superhydrophobicity and powerful oil capillarity via the in situ polymerization of a novel phenolic resin (polybenzoxazine) composite open-cell sponges. The tuneable hierarchical structures of the as-prepared sponge significantly improved its water repellence and oil capillarity; meanwhile, a plausible mechanism is also proposed. With the merits of high porosity, excellent water repellence, enhanced oil capillarity, and robust mechanical stability, the obtained sponge exhibited an intriguing oil spill clean-up performance with fast oil absorption speed, good recyclability, and high absorption capacity. Besides that, the modified sponge could also be utilized for the separation of oil/water mixture with individual phase and the surfactant-stabilized emulsion solely under the driven of gravity. The robust oil/water separation performance, low cost, and facile synthesis strategy make the resultant sponges a competitive material for the large scale oil spill emergency remediation.


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1. INTRODUCTION Over the past few decades, frequently occurred oil spillages, industrial chemical leakages, and the discharge of oily domestic sewage have become the most serious threat to the watery environment and ecosystem.1,2 The large scale oil spill accident, such as the Gulf of Mexico oil spill, released about 4.9 million barrels of crude oil to the marine, have caused significant damages to the marine organisms and the public health in the nearshore water. What’s more, the perniciousness of the spilled oils may last for several decades, and more similar accidents may happen with the development of industry.3,4 To solve these severe issues, various techniques such as skimming,5 air flotation,6 centrifuge,7 membrane separation,8 chemical coagulation,9 and absorbtion,10 were developed. Among these strategies, absorption method based on various porous materials is considered as an ideal choice for the urgent removal of spilled oils from the water surface owing to its low operational cost, easy accessibility, and relatively high efficiency.11-13 Up to now, a few commercial oil absorbents include natural materials,14-16 synthetic polymers,17,18 and inorganic absorbents19,20 were utilized in the treatment of oil spills. However, these traditional absorbents still suffer from several critical limitations of poor selectivity, low absorption capacity, unsatisfactory recyclability, and low absorption speed. To address the above-mentioned problems, concerted efforts have been made to develop an ideal absorbent with fine buoyancy, excellent selectivity, high adsorption capacity, good recyclability, and satisfying absorption speed to effectively remove various oil contaminants from water.21,22 Till now, a variety of advanced functional porous oil/water separation materials have been successfully obtained via the synergistic regulation of the surface energy and surface roughness to realize the simultaneous superhydrophobicity and superoleophilicity. Among those materials, three dimensional (3D) bulky porous materials are believed to be beneficial for the improvement of absorption capacity owing to their intrinsic high porosity and larger size along the z-direction compare with the two dimensional (2D) membranes.23 Examples include the carbon aerogels or carbonized sponges derived from graphene,24 carbon nanotube-graphene hybrid,25-28 biomass,29-31



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and polymerized sponges,32-34 have been intensively fabricated for the oil absorption, and quite high absorption capacities were obtained ascribing to their ultralow density, high porosity, and high surface area, confirming the effect of 3D materials on enhancing the absorption performance.35,36 Most recently, comparing with the carbon based absorbents, commercial available polymer sponges such as polyurethanes and melamine sponges have attracted great researching attentions for their comparable open-cell structures with low density (< 10 mg cm-3) and high porosity (> 99%), robust mechanical property, low cost, and easy to be functionalized, which are utilized as a promising template for the synthesis of high-performance absorbents.36,37 However, challenges still remained on improving the absorption speed, which is one of the most important criteria for the oil spill clean-up, especially for the oil contaminants with high viscosity and large scale.38 More importantly, the relationship of the sponge’s porous structures with its absorption rate was not clarified, which is crucial for the improvement of the oil/water separation performance. Recently, inspired by the natural organisms such as the lotus leaf, mussel, diatoms, and natural sponges, immobilization of nanostructured materials onto 3D porous materials to construct hierarchical architectures has been proven to be an effective way to further improve their application performances.39 Similar idea could also be adapted to the design of hierarchical structured porous materials with 2D or 3D structures to further enhance their oil/water separation capability.40-42 Additionally, we reported a few works about the hierarchical structured membranes based on a facile technology of in situ polymerize the polybenzoxazine (PBZ), which is a novel phenolic resin with different types according to its monomer segments. Benefited from the advantages of hydrophobic and oleophilic property, fine chemical stability, excellent thermostability, and the nearly zero shrinkage rate during polymerization, the PBZ could act as both bonding agent as well as functionalized layers to construct the hierarchal porous membranes with high permeation flux and good separation efficiency for oil/waster mixtures.43-47 However, these 2D porous membranes were impractical for the separation of the spilled oil floating on a large area water surface due to their limited porosity, thin thickness with less storage


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space for oil. Herein, we demonstrated a facile approach to synthesis a hierarchical structured sponge with superhydrophobicity and enhanced oil capillarity for the fast separation of both oil spill and oil/water mixture. For the first time, we carried out the in situ polymerization of a novel monomer of PBZ (BAF-oda) on the surface of skeletons of melamine sponges (MS), meanwhile, hydrophobic SiO2 nanoparticles were introduced to construct the hierarchical roughness. As a result, the as-prepared hierarchical structured sponges exhibited a significant transition of wettability from superhydrophilic to superhydrophobic with ultralow water adhesion force. More importantly, benefited from its hierarchical structures, the capillarity of sponges for oil were significantly enhanced, exhibiting a fast oil absorption speed, and ultrahigh oil/water separation flux solely under the driven of gravity. Furthermore, a plausible mechanism is proposed to explain the effect of hierarchical structures on enhancing the selective superwettability and capillarity of the sponges, which is crucial for the design and development of next generation of oil absorption materials. 2. EXPERIMENTAL SECTION 2.1 Materials. Melamine sponges were commercially obtained from the Chengdu Jiasideng Technology Corporation. Organic solvents (toluene, ethanol, n-hexane, acetone, petroleum ether, dimethylformamide (DMF), and dichloromethane) were purchased from the Shanghai Chemical Corporation. Diesel and vacuum pump oil were provided by the China National Petroleum Corporation. Silicone oil was commercially obtained from the Shanghai Longxu Chemical Corporation. Sunflower oil was purchased from the local market. Hydrophobic SiO2 nanoparticles (diameters range in the range of 7-40 nm), anhydrous magnesium sulfate, 4,4΄-(hexafluoroisopropylidene) diphenol (BAF, 98%), paraformaldehyde, octadecylamine (oda), Span 80, and sodium hydroxide were obtained from the Aladdin Chemistry Corporation. Deionized water was obtained via using a Heal-Force system. All the chemicals were of analytical grade and were used as received without further purification.



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2.2 Fabrication of the hierarchical structured sponges. As shown in Scheme 1, the hierarchical structures were constructed via combining the dip coating method and in-situ polymerization. Firstly, a precursor solution was prepared by dissolving the BAF-oda (Figure S1 in the Supporting Information) in the acetone at room temperature, and then a certain amount of SiO2 NPs was dispersed in the BAF-oda solutions with the treatment of ultrasonic for 1 h. The pristine MS was cut in to pieces with the size of 2.0 × 2.0 × 2.0 cm3, and subsequently washed in alcohol for three times to remove the pollutants. The cleaned MS was sufficiently dried and then dipped into the as-prepared precursor solutions with various content of BAF-oda and SiO2 NPs for 10 min with continuous shaking. After that, the MS was removed from the solutions and was extruded out the residual solution between skeletons, then dried in a hot air circulating oven at 80℃ for 15 min, and the weight gain of MS before and after dip coating was calculated (Figure S2). During the in situ polymerization process, the MS was directly put into a hot air circulating oven at 200℃ for 1 h. In order to identify the different samples, the MS treated with solutions containing BAF-oda was denoted as FPBZ@MS. Accordingly, the MS modified from a precursor solution with the BAF-oda and SiO2 NPs was denoted as FPBZ/SiO2@MS. 2.3 Characterization The morphology of the pristine MS, FPBZ@MS, and FPBZ/SiO2@MS were characterized by using a scanning electron microscopy (SEM, Hitachi S4800). FT-IR spectra was obtained via using a total reflection-Fourier transform infrared spectroscopy (Thermo Scientific, Nicolet 8700). N2 adsorption-desorption isotherms were tested via using a physisorption analyzer (Micromeritics, ASAP 2020). Water contact angle (WCA) and oil contact angle (OCA) were checked by using a contact angle goniometer Kino SL200B. The water contact angle hysteresis (WCAH) was measured through the increment and decrement method. The compression performance was evaluated using an Instron 3365 testing system equipped with two flat-surface compression stages. 2.4 Oil Spill Absorption


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The absorption capacity and the saturated absorption time for various oils were evaluated via dipping the FPBZ/SiO2@MS into oils on the water surface till it was saturated with the target liquid, and then was taken out for the weight measurement quickly to avoid the influence of the evaporation of the absorbed liquid. According to the previous studies the gravimetric absorption capacity (Qm/m) and volumetric absorption capacity (Qv/v) were calculated according to the following equations:36 Qm/m = Qv/v =

(𝒎𝟏 ― 𝒎𝟎)

(1)

𝒎𝟎

(𝒎𝟏 ― 𝒎𝟎) 𝒅𝒔 𝒎𝟎

(2)

𝒅𝒍

where 𝒎𝟎 and 𝒎𝟏 are the masses of absorbent before and after absorption, 𝒅𝒔 and 𝒅𝒍 are the bulk density of absorbents and the density of absorbed liquid, respectively. The cycling performance test was carried out via repeating the absorption-squeezing process. In each squeezing stage, the absorbent was compressed to extract the absorbed liquid. 2.5 Oil/water Mixture Separation The surfactant free layered oil/water mixture was prepared by directly adding relevant oils or organic solvents into water with a volume ratio of 1:1 v/v. To be easy for identification, the oil was dyed with oil red. For the preparation of model water-in-oil emulsions, n-hexane was selected as the model oil and Span 80 was chosen as the surfactant; typically, the volume ration of the water and oil was 1: 10, the concentration of the Span 80 was 0.1 mg mL-1, the mixture was stirred vigorously for 30 min at room temperature and then ultrasonic for 1 min, the up side semi-transparent solution was chosen as the model emulsion, the prepared emulsions could keep without obvious stratification for at least 4 h. During the separation, FPBZ/SiO2@MS with a thickness of 10 mm was fixed between two glass vessels with a diameter of 15 mm. The oil/water mixture were poured into the upside vessel directly and the separation was conducted solely under the driven of gravity. The flux was



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estimated by calculating the time needed for the permeation of all oil. The residual water content in the collected oil was checked using a Karl Fischer 831KF coulometer.

Scheme 1. Schematic illustration for the construction of hierarchical structured FPBZ/SiO2@MS via the in situ polymerization. 3. RESULTS AND DISCUSSION Generally, MS exhibits open-cell foam structures with numerous skeletons consist of formaldehyde-melaminesodium bisulfite copolymer. With the merit of these uniform inter-connected 3 D networks, the MS possesses extremely high porosity, and good elasticity. 36 Inspired by the 3D porous structures of MS and the hierarchical nano/micro-structured surface of lotus leaf, we designed a superhydrophobic and superoleophilic sponge based on the followed principles: (i) tuning the surface energy of skeletons in MS to make it hydrophobic and oleophilic, (ii) constructing hierarchical roughness on the surface of skeletons to enhance the Cassie’s state wettability. Herein, the first criteria was realized by introducing FPBZ layer on the surface of skeletons via in situ polymerization of the BAF-oda monomer. The incorporated fluorine components could bring down the


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surface energy effectively, which endowing MS hydrophobicity, meanwhile the oda groups enhanced its affinity with oil. Figure 1a-d show the pristine MS, and FPBZ@MS derived from solutions with different concentration of BAF-oda. It can be found that the pristine MS (Figure 1a) possessed a typical open-cell structure with pores sizes of 100-150 μm, and the skeletons with smooth surfaces were well conjugated. After being coated with BAF-oda and in situ polymerized, a thermal setting FPBZ layer was generated on the surface of skeletons. As shown in Figure 1b-d, the FPBZ layer loaded on the skeleton surface could be well recognized towards the increment of BAF-oda concentration with more beads appeared at the junction points. Moreover, it can be found that some pores may be blocked by beads formed from the cross-linked FPBZ owing to the excessive BAF-oda at higher concentration (Figure S2a). These phenomena may be attributed to the aggregation of the BAF-oda during the thermal treatment. In addition, the presence of FPBZ was further verified by the FT-IR spectra analysis, as shown in Figure 1e, the pristine MS displayed prominent peaks around 812, 1544, and 3391 cm-1, which were assigned to the triazine ring bending, C=N stretching, and N-H stretching, respectively.48 Besides that, the FPBZ@MS exhibited additional characteristic peaks around 966, 1250, 2854, and 2927 cm-1, which were corresponded to the C-O stretching, and wagging of -CH2, and the intramolecular hydrogen bonding (O…H+N and OH…N), respectively, confirming the polymerization of BAF-oda monomer.44,49



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Figure 1. SEM images of (a) Pristine MS, and FPBZ@MS modified with different content of BAF-oda: (b) 0.5 wt%, (c) 1 wt%, and (d) 2 wt%. (e) FT-IR spectra of pristine MS, and the representative FPBZ@MS. (f) WCAs of pristine MS, and FPBZ@MS. As aforementioned, the pristine MS consisting formaldehyde-melamine-sodium bisulfite copolymer is amphiphilic without the selectivity between oil and water (Figure S3) due to its relatively high surface energy (> 0.045 J·m-2).50 Fortunately, with the existence of fluorine groups on the main chain and the oda segments on the side, the FPBZ possessed an extremely low surface energy of 0.015 J·m-2.43 As a result, the wetting ability of MS transformed significantly from superhydrophilic to hydrophobic after being coated with FPBZ. As shown in Figure 1f the WCA of pristine MS was nearly 0o, upon being coated with FPBZ, the WCA of the obtained FPBZ@MS greatly increased to more than 130o, and a highest WCA of 140o could be obtained when the BAF-oda monomer concentration was 1 wt%. However, no obviously increment of the WCA was presented


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when further increase the concentration of BAF-oda monomer. This phenomenon may be attributed to the presence of FPBZ beads filling in the open-cell structures reduced the roughness of the sponge, indicating a critical concentration of BAF-oda monomer which endow the MS an appropriate hydrophobicity without overloading the FPBZ.

Figure 2. SEM images of FPBZ/SiO2@MS derived from precursor solutions with different concentration of SiO2 NPs: (a, a’) 0.5wt%, (b, b’) 1 wt%, (c, c’) 3 wt%, and (d, d’) 5 wt%.



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Recently, researchers have found that constructing two-tier micro-/nanostructures on a surface will further enhanced its intrinsic wettability.51,52 Based on this principle, a variety of superwettable interfacial materials have been successfully deigned via creating sufficient micro/nanoscale hierarchical roughness on the surface of appropriate substrates. In this work, we combined SiO2 NPs with FPBZ, in which the FPBZ acted as both the binder and the hydrophobic functional layer. As shown in Figure 2 and Figure S4, the SiO2 NPs were successfully anchored on the surface of skeletons forming the nanoscale roughness with the cement of FPBZ. It can be found that these nanoparticles could not fully cover the surface of the skeletons until its concentration reach up to 3 wt%. Figure 2c and c’ demonstrates that a uniform layer with hierarchical roughness in both micro/nano scale had been successfully constructed on the surface of skeletons. However, further increment of the SiO2 NPs content (5 wt%) resulted in a significant aggregation (Figure 2d and d’). Moreover, the overloaded SiO2 NPs may fall off from the skeletons because they were beyond the cementation capacity of FPBZ layer, which could also be verified by the result of Figure S2b. Herein, it should be mentioned that the MS after functionalized could maintain its uniform open-cell 3D porous structures without shrinkage or broken of the inter-connected networks. This phenomenon was benefited from the excellent thermal stability of melamine skeletons and the nearly zero shrinkage of BAF-oda monomer during the in situ polymerization.46,49 To further quantitatively investigate the hierarchical roughness, the N2 adsorption-desorption testing and an associated fractal analysis were conducted. Figure 3a demonstrates the N2 adsorption-desorption curves of the sponges at 77 K. It can be found that all these isotherms were belong to the type IV according to the IUPAC classification, in which the typical adsorption behaviours including monolayer adsorption, multilayer adsorption, and capillary condensation were recognized, resulting a pore size distribution of 10-100 nm (Figure S5). Considering that the pore sizes of the pristine sponge were several micrometres, these mesopores were mainly resulted from the stacking of SiO2 NPs. Meanwhile, the sharply increment of N2 adsorption occurring at P/P0 > 0.9 and the tiny H1 type hysteresis loops locating in this region indicate that


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these mesopores were open and exhibited nonparallel slit-shapes, which was beneficial to trap more air in the porous structures as well as to enhance the capillarity.53 Besides that, the surface area were significantly increased with the increment of SiO2 NPs concentration, indicating that the SiO2 NPs played a crucial role in the construction of the roughness. Furthermore, the fractal dimension (Df), which could quantitatively demonstrate the roughness, was calculated by using a modified Frenkel-Halsey-Hill (FHH) equation based on the isotherm of N2 adsorption.54 The FHH plots depicted in Figure 3b demonstrate distinct differential slopes in the high coverage region. Accordingly, the related Df values of FPBZ/SiO2@MS derived from precursor solutions with various content of SiO2 NPs were 1.96, 2.02, 2.04, and 2.03, respectively, revealing the typical surface fractal features, which means that the nano/micro scaled roughness has been constructed on the surface of the skeletons of the sponges with the presence of SiO2 NPs. The increased Df values towards the increment of SiO2 NPs content confirmed the enhancement of fractal roughness structures,

55,56

which is crucial for a

superwettable surface.51,52 The tiny decrease of Df value at the high SiO2 NPs concentration (5 wt%) was ascribed to the aggregation of SiO2 NPs, which was in agreement with the aforementioned SEM results. Here it should be mentioned that the fractal roughness structures is crucial for a superwettable surface, which could enhance the Cassie wetting effect of the materials, and it directly influence the selective capillary effect of the sponges.51,53 As a supporting evidence, the variation tendency of the data shown in Figure 4 and Figure 5 were consistent with this Df results.



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Figure 3. (a) N2 adsorption-desorption isotherms of FPBZ/SiO2@MS modified with precursor solutions with various concentration of SiO2 NPs. Inset is the corresponding BET surface area. (b) Plots of ln(V/Vmono) against ln(ln(p0/p)) reconstructed from the adsorption isotherms based on the FHH equation. The linear area labelled as high coverage region were employed to calculate the fractal dimensions. Benefited from the low surface energy and hierarchical roughness structures, the FPBZ/SiO2@MS exhibited enhanced selective wettability. As shown in Figure 4a, the WCAs of the relevant FPBZ@MS were up to 140o, 142o, 145o, 152o, and 153o, respectively, indicating a remarkable enhancement of hydrophobicity with the introduction of SiO2 NPs. This phenomenon was well corresponded to the transformation of Wenzel’s wetting state to Cassie’s wetting state with the presence of fractal hierarchical roughness.51 As can be seen from the


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aforementioned SEM results, the pores between skeletons of all FPBZ/SiO2@MS were in the range of microscale, thus the water tend to penetrate into the pores according to the Wenzel’s wetting theory; whereas, with the increment of hierarchical roughness on the surface of skeletons, the trapping air in the mesopores acted as cushions to reduce the water-solid contact area, resulting a higher apparent contact angle. Figure 4b demonstrates the water contact angle hysteresis (WCAH), and a small WCAH of 4.7o was obtained, indicating that the water could not intrude the sponge to a large extent but sat on the asperities of the surface with the minimum water-solid adhesion, which matched well with the Cassie model.52,57 In addition, a corresponding theoretical water retention force was also estimated based on the WCAH and droplet volume (10 μL) using the followed equations:58 𝑭𝒓 =

(𝝅𝟐)𝜸𝑫𝒄(𝐜𝐨𝐬 𝜽𝒓 ― 𝐜𝐨𝐬 𝜽𝒂)

𝑫𝒄 = 𝟐

𝟏 𝟑

(3)

( ) {𝐭𝐚𝐧 ( 𝟐 )[𝟑 + 𝐭𝐚𝐧 ( 𝟐 )]} 𝟔𝑽 𝝅

𝜽𝒂

𝟐

𝜽𝒂

𝟏

―𝟑

(4)

where 𝑭𝒓 is the retention force, 𝜸 is the surface tension of liquid, 𝑫𝒄 is the contact diameter, 𝜽𝒂 is the advancing contact angle, 𝜽𝒓 is the receding contact angle, V is the volume of the droplet. Here, the calculated water retention force of the FPBZ@MS was ~12.2 μN, and for FPBZ/SiO2@MS, it decreased from ~7.9 to ~2.4 μN, which was comparable to those of the pioneer lotus-leaf inspired hydrophobic surfaces.47,57,58 Besides, the dynamic water repellence of the FPBZ/SiO2@MS (3 wt% SiO2 NPs) was investigated using a high-speed camera. As shown in Figure 4c, a droplet of water (5 μL) was forced to attach the surface of FPBZ/SiO2@MS till the presence of obviously deformation. After that, the droplet was lifted up gradually, and it can be found that, upon leaving the solid surface no obvious deformation was exhibited, revealing an extreme low water adhesion with the surface, which was well agreement with the aforementioned WCAH and retention force analysis.



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Figure 4. (a) WCAs of the FPBZ/SiO2@MS as a function of SiO2 NPs content in the modification solutions. (b) WCAHs and the calculated retention force as a function of SiO2 NPs concentrations. (c) Photographs of droplets during the dynamic measurements of water adhesion. Additionally, the oil wettability of the FPBZ/SiO2@MS was further studied, Figure 5a demonstrates the snapshots of dynamic contact process of oil droplet (sunflower oil, 5 μL) on the surface of FPBZ@MS and various FPBZ/SiO2@MS. Interestingly, it was found that the spreading and permeating speed of FPBZ/SiO2@MS with sufficient hierarchical roughness was almost 3 times higher than that of the FPBZ@MS, indicating a significantly enhanced oil absorption capability. This phenomenon could be explained by that the hierarchical architectures and the extremely oleophilic and hydrophobic constituent endowed the pores tremendous selective wettability between oil and water based on the capillarity, which could be well explained based on the Young-Laplace theory:53 ∆𝒑 = ―

𝟐𝛄𝐜𝐨𝐬 𝜽 𝒓

(5)


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where the ∆p is the intrusion pressure difference between ambient pressure and liquid interior, γ is the surface tension of liquid, θ is the contact angle, r is the diameter of pore. As mentioned above, the WCAs on the surface of FPBZ/SiO2@MS was > 90o, thus the resulted ∆p was > 0, which means that an intrusion pressure of Δp has to be overcome before the water can penetrate the pore, as a result water was repelled outside the pores without an additional pressure. In contrast, the oil contact angle (OCA) was < 90o, and the corresponding ∆p for oil was < 0, which means that the oil can spontaneously pass through the pores, exhibiting a wicking effect for oil.53 According to the Wenzel’s wetting theory, the established roughness further enhanced the surface affinity with oil, resulting in a smaller OCA, so that the wicking effect for oil increased with the introduction of hierarchical structures. Moreover, the FPBZ/SiO2@MS showed superhydrophobicity even after being fully wetted by oil with the under oil WCA of > 150o (Figure S6). This phenomenon could be attributed to the trapped air in the pores was equally replaced by oil under the driven of capillary force, and the absorbed oil in the pores could act as a slippery shield to reduce the adhesion of water on the solid surface, which is vital for the selectivity of oil and water.47,52 In order to further verify the effect of the hierarchical roughness on enhancing the selective capillarity, FPBZ@MS and FPBZ/SiO2@MS with the same size were simultaneously dipped onto the surface of sunflower oil (46 mPa·s, 20℃) to check their absorption speed. As shown in Figure 5b, and Movie S1, the saturated absorption time decreased sharply from 40 s to 22 s with the introduction of SiO2 NPs, indicating a significant enhancement of the oil capillarity, which is well consist with the Lucas-Washburn law:59 𝒉𝟐 =

𝜸𝒓𝐜𝐨𝐬 𝜽 𝟐𝜼

t

(6)

in which 𝒉 is the permeating height, 𝜸 is the surface tension of liquid, 𝜽 is the contact angle, 𝒓 is the diameter of pore, 𝜼 is the liquid viscosity, t is the time needed for the permeation. In this experiment, the 𝒉 is related to the thickness of sponges, 𝜸 and 𝜼 are constant (sunflower oil), 𝒓 could considered as the



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average diameter of sponges, and there was no obvious change according to the SEM images. Thus the t is in inverse proportion of 𝐜𝐨𝐬 𝜽. As aforementioned, owing to the lipophilicity, the value of 𝐜𝐨𝐬 𝜽 will increase with the increment of roughness, therefore the time needed for the permeation of oil decreased accordingly.

Figure 5. (a) Snapshots of dynamic contact process of oil droplets on the surface of FPBZ/SiO2@MS derived from modification solutions with various content of SiO2 NPs. (b) Saturated absorption time of the relevant FPBZ/SiO2@MS for sunflower oil. Inset illustrates a plausible mechanism for the enhanced capillary effect.


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Besides the intriguing selective superwettability, the FPBZ/SiO2@MS (taking 3 wt% as an example) also possessed robust mechanical stability with outstanding compressive performance. Figure 6a depicts the representative compressive stress-strain curves at maximum strain ε of 20, 40, 60, and 80 %. It can be found that all the curves exhibited closed hysteresis as well as a tiny permanent strain loss, indicating a low energy loss rate. The whole loading process could be distinguished as three characteristic regions, the elastic region at ε < 10% was resulted from the elastic bending of the skeletons, the plateau region at 10% < ε < 60% was corresponded to the creep of polymer and the friction between skeletons, from which a part of the absorbed energy was dissipated, the rapid stress increasing region at 80% was contributed by the densification of skeletons. Furthermore, a cyclic compression test was further carried out to evaluate the durability at a large compressive strain (ε = 60%). The curves shown in Figure 6b depict the representative stress-strain curve at cycle 1, 10th, 100th, and 1,000th with a slight plastic deformation (3.4% at 100th, and 10.8% at 1,000th), respectively, which was smaller than that of the traditional fibrous polymer foams.46 In addition, the maximum compressive stress and Young’s modulus could achieve the equilibrium state after 50 cycles, and a compressive stress of 21.3 kPa, and a Young’s modulus of 13.2 kPa were obtained even after 1,000 times cyclic compression (Figure S7), and the inset SEM image in Figure 6b demonstrates that the FPBZ/SiO2@MS could maintain its 3D inter-connected networks without obvious collapse and a WCA of 150o even after suffering 1,000 times cyclic compression, highlighting an intriguing structural robustness to maintain the valid absorption space.



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Figure 6. (a) Compressive stress-strain curves of FPBZ/SiO2@MS (3 wt% SiO2 NPs) loaded with various compressive strain. Inset are digital photos illustrating the compression and recovery. (b) The selected stressstrain curves of the relevant FPBZ/SiO2@MS during long cycling compression at the first, 10th, 100th, and 1,000th cycles. Inset: the SEM image and the corresponding WCA of the FPBZ/SiO2@MS after 1,000 cycle compression. The open-cell 3D structures of FPBZ/SiO2@MS provided sufficient space to store the absorbed liquid. As a result, the maximum sorption capacity of the FPBZ/SiO2@MS (3 wt% SiO2 NPs) was systematically


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investigated via simply dipping it in various oils, which are common organic pollutants from our daily life or industry. It was revealed that the gravimetric absorption capacities for different oils range from 46 to 96 times of its own weight (Figure S8), mainly depending on the property (e.g. density, viscosity) of the absorbed liquids (see detail in Figure S9 and the supplementary discussion). Obviously, the gravimetric absorption capacity of the FPBZ/SiO2@MS was much superior to that of the commercial PP nonwovens (< 20 times),37 and was comparable to the previous reported polymerized absorbents such as pomelo peel (< 30 times),60 cross-linked poly(tetrahydrofuran) (< 20 times),61 conjugated microporous polymers (< 20 times),62 PU sponges (< 35 times),63 nanocellulose sponges (< 40 times),64 and polydimethylsiloxane-functionalized melamine sponge (< 75 times),35 indicating a competitive absorption capacity. Actually, considering the difference of liquids density, and the valid volume of absorbents, the volumetric absorption capacity should be a more appropriate parameter to evaluate the practical absorption capacity. Figure 7a demonstrates the volumetric absorption capacities of the FPBZ/SiO2@MS for various oils, revealing high volumetric absorption capacities of > 90%, especially for the oil with higher viscosity such as silicone oil, the volumetric absorption capacities could reach to 98%, indicates that almost all the pore volume of FPBZ/SiO2@MS was valid for storing the absorbed oil, which was comparable to state-of-the-art studies.32,36,37 For an appropriate absorbent, the recyclability, and the recoverability of the absorbed liquids are key factors for the practical applications. To evaluate the cyclic absorption performance of FPBZ/SiO2@MS, the sunflower oil was employed as the model adsorbate for its moderate viscosity and the popularization in our daily life. Fortunately, the FPBZ/SiO2@MS exhibited a satisfying stability during the cyclic absorption-squeezing test. As shown in Figure 7b, the FPBZ/SiO2@MS could maintain more than 90% of its 1st absorption capacity and a WCA of 150o even after 50 repeated absorption-squeezing process, indicating a relatively good reusability. And at every cycle nearly 80% of the absorbed oil could be extruded during the squeezing stage. This excellent recyclability was mainly resulted from the robust mechanical stability of the interconnected skeletons and the SiO2 NPs hierarchical roughness



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with the cement of FPBZ. To further verify the facile practical application performance for water remediation from oil spill, two kind of oils with different density, sunflower oil (ρ = 0.96 g cm-3) and dichloromethane (ρ = 1.33 g cm-3) were used as the model adsorbates. As shown in Figure 7c (up) and Movie S2, upon dropping the FPBZ/SiO2@MS in the light oil layer on the water surface, the oil (dyed with oil red) was absorbed immediately, and the whole absorption process was finished within a few seconds, which was attributed to the powerful capillary force as mentioned above.

Figure 7. (a) Volumetric absorption capacity for different oils. (b) Absorbed and remnant capacity retention of sunflower oil over 50 absorption and squeezing cycles. Inset is the WCA of 150o after the cycling test. (c)


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Digital photos illustrate the absorption and recovering process of floating n-hexane spill and the underwater dichloromethane. Owing to its superhydrophobicity and low density, the FPBZ/SiO2@MS absorbed with oil could stably float on the surface of water, which was convenient for the collection. As the floated sponge absorbed with oil was taken off, none oil spill could be observed on the water surface, demonstrating an efficient oil spill clean-up performance. Meanwhile, FPBZ/SiO2@MS could also quickly absorb the heavy oil, which would sink under the water owe to its higher density than water. As shown in the photograph in the bottom of Figure 7c and Movie S3, it can be found that a silver mirror-like surface appeared after immerging the FPBZ/SiO2@MS under water, which was contributed by the trapped air on the surface. Upon contacting the oil, the trapped air layer was broken and oil was quickly absorbed under the driving of vigorous capillary force, and the trapped air was extruded gradually forming air bubbles. In addition, after the absorption, most of the absorbed oils could be recovered, and none water droplets could be observed in the collected oil, which was contributed by the stable superhydrophobicity of FPBZ/SiO2@MS even be filled with oil. Besides the absorption capacity and recyclability, the absorption speed is another crucial factor to evaluate the practical application performance of the oil absorbents, the absorption speed of FPBZ/SiO2@MS for various oils were systematically investigated. During the test process, a few pieces of FPBZ/SiO2@MS (3 wt% SiO2 NPs) with the same size were dipped into the target oils without additional force, the time consumed for the saturated absorption were recorded to estimate the capillary effect of the sponge for different oils and organic solvents. From Table S1 we can find that for oils and organic solvents with relatively low viscosity ( n-hexane, 0.29 mPa·s, 20℃), the absorption procedure could be finished within 1 s, which was corresponded to a absorption rate of 5.95 L min-1, whereas, for the oils with high viscosity (silicone oil, 254 mPa·s, 20℃), the saturation absorption time increase to 219 s with a corresponding absorption rate of 0.019 L min-1. The significantly increment of saturated absorption time for high viscosity oils could be well explained by the



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aforementioned Lucas-Washburn theory, in which the liquid viscosity is proportional to the absorption time. Furthermore, a conceptual oil water separation system was designed for the continuous collection of oil spill. As shown in Figure 8a and Movie S4, through an embedded pipe in the sponge the absorbed oil could be quickly exported under the driving of a pump, and none water was absorbed by the sponge. This continuous separation system could act as a novel oil skimmer for the collection of oil spill on the large area water body (e.g., sea).35,42

Figure 8. (a) A designed system for the continuous separation of light oil/water mixture (take n-hexane as an example stained with oil red). (b and c) Gravity-driven separation for oil/water mixture with indidual phase and the surfactant-stabilized emulsion.


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To further evaluate the practical application performance of the in oil/water mixture separation, a piece of FPBZ/SiO2@MS with thickness of 10 mm was compressed and fixed in a filtering apparatus to act as the separation membrane. Before the oil/water mixture separation, the intrusion pressure of water was measured. According to the previous studies,23 the intrusion pressure was calculated from the maximum height of water that the FPBZ/SiO2@MS could supported. As a result, the intrusion pressure was about 1.44 kPa, which was comparable to the previous reported works.23 In this work, two kind of oil/water mixtures were used as the models, which were the mixture with two individual phases and the emulsified water-in-oil mixture. Figure 8b and Movie S5 depict a typical oil/water mixture separation process solely under the driven of gravity, firstly when 20 mL oil/water mixture (individual phase) subjected to the FPBZ/SiO2@MS, the oil phase (dyed red) could pass through quickly, whereas, water was selectively blocked, and during the whole separation process the water was stably retained in the up glass funnel, which was benefited from the aforementioned under oil superhydrophobicity of FPBZ/SiO2@MS. It is noteworthy that although no external force was applied, the whole separation process was finished with 3 s, indicating an extremely high oil penetration flux of 57140 L m-2 h-1, which was several times higher than that of the thin membranes.45 More importantly, an excellent separation efficiency up to 99.8 % could be obtained. Which was comparable to the pioneer works.11,23 Similar to that of the absorption, the mechanism for the ultrafast and high efficient oil/water mixture separation could be explained by the superhydrophobicity and superoleophilicity of FPBZ/SiO2@MS. Besides these, the high porosity and open-cell pores with hierarchical roughness provided abundant capillary tunnels allowing the oil pass through quickly, while water was well repelled. Additionally, considering that there may be some special case that the oil and water mixture may be emulsified due to the existence of surfactant and mechanical forces, so we also checked the separation performance of the sponges for a water-in-oil emulsion. As can be seen from Figure 8c, and Movie S6, the surfactant stabilized oil emulsion with numerous micron size water droplets inside could be effectively separated solely under the drive of gravity with a flux of ~ 1500 L m-2 h-1, which was also



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comparable to the commercial membranes45; the relatively low separation flux compare with the separation flux of mixture with individual phase may be due to the block of water droplets stabilized by the surfactant. After separation none water droplets could be found in the oil, indicating a relative high separation efficiency, and the water separation efficiency estimated by the Karl Fischer coulometer could reach 97.6% even with the existence of surfactant. Moreover, the as-prepared FPBZ/SiO2@MS possessed intriguing chemical (Figure S10) and thermal stability (see detail in Figure S11 and the supplementary discussion), revealing a robust durability for different regeneration method, such as by cleaning with an appropriate solvent or distillation.

4. CONCLUSION In summary, a novel hierarchical structured polybenzoxazine/melamine sponge with simultaneous superhydrophobic and enhanced oil capillarity has been developed via a versatile strategy of constructing nano/microscale roughness on melamine sponge based on the in situ polymerization of BAF-oda, which acted as both the binder and the oleophilic functional component. The established fine hierarchical roughness on the surface of skeletons in sponges played critical role in improving the water repellence and selective oil capillary performance. In addition, a conjecture of the mechanism behind the enhanced hydrophobic and oil capillarity was carried out. Benefiting from the high porosity, open-cell pores, excellent water repellence, strong oil capillarity, and superelasticity, the as-prepared sponges possessed an excellent oil spill clean-up performance with fast oil absorption speed, high oil absorption capacities (up to 96 times its own weight or almost 98% of its own volume), and robust recyclability. Furthermore, the functional sponge could also be utilized as a separation membrane effectively separate the oil/water mixtures with individual phase and surfactant-stabilized emulsion solely under the driven of gravity. With the merit of intriguing oil/water separation performance, low cost and facile synthesis strategy, the FPBZ/SiO2@MS exhibited a promising prospect in the industrial applications of oil spill clean-up.


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ASSOCIATED CONTENT Supporting Information Available: Detailed descriptions of the synthesis procedure and structural confirmation of BAF-oda; weight gain of the MS after dip coating by pure BAF-oda, and BAF-oda/SiO2 NPs with different concentration; photographs show the wettability of the pristine sponge, element mapping analysis of the corresponding FPBZ/SiO2@MS; pore size distribution of the sponges; under oil WCA; maximum stress and the Young’s modulus; gravimetric absorption capacity; correlation of the gravimetric absorption capacity of FPBZ/SiO2@MS for various oils; WCAs shown the chemical and thermal stability of the functionalized sponges; absorption speed for different oils. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (B. Ding) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 51503030, 51873031, 51673037, and 51773033), the Fundamental Research Funds for the



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Polybenzoxazine-Functionalized Melamine Sponges with Enhanced

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