Robust and Durable Superhydrophobic Polyurethane Sponge for Oil

Article pubs.acs.org/IECR

Robust and Durable Superhydrophobic Polyurethane Sponge for Oil/ Water Separation Luhong Zhang,†,‡ Lidong Xu,†,‡ Yongli Sun,†,‡ and Na Yang*,† †

School of Chemical Engineering and Technology and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: With the purpose of purging and recycling oil and organic solvent from a water surface, a superhydrophobic polyurethane (PU) sponge was fabricated through a combined method of interfacial polymerization (IP) and molecular selfassembly. The as-prepared sponge has a superwetting characteristic of superlipophilicity in atmosphere and superhydrophobicity both in atmosphere and under oil, and it can quickly and selectively absorb various kinds of oils up to 29.9 times its own weight. More importantly, because of a covalent combination of the sponge skeleton and the polyamide thin film from IP, the superhydrophobic sponges could be reused for oil/water separation over 500 cycles without losing its superhydrophobicity, showing the highest reusability among the reported absorptive materials. The superhydrophobic sponge also can be used in the continuous absorption and expulsion of oils and organic solvents from water surfaces with the help of a vacuum pump. All of these features make the sponge a promising candidate material for oil-spill cleanups. al.19 fabricated a superhydrophobic sponge reinforced by carbon nanotubes (CNTs); the CNT was first coated with a dopamine film, then anchored on the skeleton of PU sponge by the oxidative polymerization of dopamine, and finally modified with octadecylamine on the polydopamine(PDA) layer. Li et al.20,21 reported modification of PU sponge with ZnO microrods and palmitic acid (PA), and they also reported a strategy for the fabrication of superhydrophobic graphenebased sponges through a facile dip-coating method. However, most of the hydrophobic porous materials face problems, e.g., high cost of reagents/equipment like dopamine or graphene and poor reusability caused by the poor adhesion of the modified layer to the base, thus preventing the use of these materials in practical applications. Interfacial polymerization (IP) is a type of step-growth polymerization in which polymerization occurs at an interface between an aqueous solution containing one monomer and an organic solution containing a second monomer. This technology is currently widely employed in the fabrication of thin-film composite membranes,22−24 capsules,25 fibers,26 etc. The reaction process has many advantages, such as mild reaction conditions, cheap raw materials, and firm adhesion to the base by the covalent bond between the reactant and base.

1. INTRODUCTION With the growth of offshore oil production and transportation, oil spills and chemical leaks have become the most significant threat to the coastal environment and oceanic ecosystem.1−4 As a result, there is a need to develop new materials for the highefficiency collection and separation of large amounts of organic pollutants from water surfaces. Traditional absorbent materials for oil/water separation are almost microporous absorbers, such as wool,5 zeolites,6 activated carbon,7 and exfoliated graphite,8 which have the drawbacks of low absorption capacity, less selective water/oil absorption, and poor durability. Recently, superhydrophobic and superoleophilic materials in the form of meshes, films, and 3D porous materials have attracted broad attention because of their potential capacity for selective separation of oils or organic solvents from water.9−16 Among them, polyurethane (PU) sponge, a kind of porous and hydrophilic polymer, has been extensively employed because of its high porosity, light weight, good elasticity, and low cost for large-scale production. More recently, various superhydrophobic sponges have been prepared for oil/water separation. Jiang and co-workers17 fabricated a superhydrophobic PU composite sponge by the deposition of SiO2/poly(tetrafluoroethylene) composite particles onto the porous structure of PU sponge and then treated it with chemical vapor deposition of silane to obtain a superhydrophobic property. Zhu and co-workers18 fabricated a superhydrophobic sponge by dip-coating and successive hydrolysis of methyltrichlorosilane to form a polysiloxane coating on the surface of the sponge. Wang et © XXXX American Chemical Society

Received: July 29, 2016 Revised: September 4, 2016 Accepted: September 13, 2016

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DOI: 10.1021/acs.iecr.6b02897 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Illustration of the Fabrication Process of a PU−IP−PA Sponge

sponge (15 mm × 10 mm × 10 mm) was first ultrasonically washed with dichloromethane for 30 min to remove stains and pigment and subsequently dried in air. The clean sponge was immersed in the n-hexane solution (contained 0.4 wt % TMC and 0.8 wt % Al2O3 nanoparticles) with magnetic stirring for 3 h; after removal of excess organic solution by squeezing with a tweezers to 75%, the sponge was put in ambient conditions for further evaporation of n-hexane for about 5 min. Then the resulting PU−COCl sponge was dipped in an aqueous solution (containing 2.0 wt % PEI, 0.1 wt % SDS, and 0.1 wt % Na2CO3) for 30 min, during which IP took place on the substrate surface. The resulting PU−IP sponge was taken out, the aqueous solution squeezed off, and then the sponge cured in an oven at 70 °C at ambient pressure for 10 min. After curing, the PU−IP sponge was washed totally with distilled water and dried under vacuum at 70 °C for 2 h, and then the PU−IP sponge was reacted with PA in a dichloromethane solution (the concentrations of PA, DCC, and DMAP were 10, 10, and 1 mM) for 24 h. Finally, the as-prepared PU−IP−PA sponge was washed with dichloromethane and dried in air. In addition, we also prepared a PU−IP−PA sponge without adding Al2O3 nanoparticles as the control. It is worth noting that the organic and aqueous solutions can be reused several times until the concentrations of the reactants decrease to half of what they were are the beginning, which greatly improves the utilization coefficient of raw materials over previous reports and thus reduces the cost.29−31 2.3. Characterization. The water contact angle (WCA) was measured with 3 μL droplets of water using an optical contact angle and an interface tension meter (SL200 KS, KINO, USA) at ambient temperature; average values of the contact angle were obtained from five measurements per sample. Also, in the measurement of the WCA under oil, the asprepared sponge was immersed in a quartz quadrate pot containing octane and the size of the water droplet remained unchanged. The morphology of the surface was observed by scanning electron microscopy (SEM; Nanosem 430, FEI, USA). Elemental analysis was determined by energy-dispersive X-ray spectroscopy (EDS) in combination with SEM. X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000VersaProbe. X-ray diffraction (XRD) patterns were obtained using a X’Pert Pro Panalytical diffractometer. Fourier transform infrared spectroscopy (FTIR) spectral data were

On the basis of these reports, IP was first introduced to fabricate the superhydrophobic sponge, as shown in Scheme 1. Here 1,3,5-benzenetricarbonyl trichloride (TMC) was chosen as the oil-phase monomer and polyethylenimine ethoxylated (PEI) as the aqueous-phase monomer. Relying on the dense film formed by IP, the Al2O3 nanoparticle was anchored on the sponge skeleton and gave it a hierarchical structure like lotus leaf.27 At the same time, excessive amino groups from PEI in the dense polyamide film offer a platform for PA to selfassemble into a polyamide thin film via amidation reaction with the help of N,N′-dicyclohexylcarbodiimide (DCC, dehydrating agent) and 4-dimethylaminopyridine (DMAP, amide catalyst). Because TMC could easily react with the secondary amine of the original PU sponge,28 the resulting film was firmly adhered to the surface of the sponge via a covalent bond, which gave the material excellent stability and durability. The as-prepared sponge showed many excellent properties of high oilabsorption capacity, high mechanical strength, high reusability (>500 cycles) in oil/water separation without losing their superhydrophobicity and elasticity, and better mechanical durability among the reported work,17−20 Therefore, our approach described herein is versatile and facile and can be applied to fabricate a potential superhydrophobic sponge for oil/water separation.

2. EXPERIMENTAL SECTION 2.1. Materials. PU sponge (the average pore size was about 400 μm) was purchased in a local market. Polyethylenimine (PEI; molecular weight 1800 g/mol, 99%) and Al 2 O 3 nanoparticles (0.2 μm, 99.99%) were obtained from Shanghai Macklin Biochemical Co., Ltd. Trimesoyl chloride (TMC; 98%) was supplied by Tianjin Heowns Biochemical Technology Co., Ltd. Palmitc acid (PA; 99%), N,N′-dicyclohexylcarbodiimide (DCC; 99%), and 4-dimethylaminopyridine (DMAP; 99%) was purchased from Aladdin Reagent Inc. Sodium dodecyl sulfate (SDS, analytically pure), Na2CO3 (analytically pure), hexane (analytically pure), and dichloromethane (analytically pure) was obtained from Tianjin Jiangtian Chemical Technology Co., Ltd. Deionized water was used in all experiments. All chemicals were used as received. 2.2. Fabrication of a Superhydrophobic Sponge. The superhydrophobic sponge was obtained via a combined method of IP and molecular self-assembly (Scheme 1). A piece of PU B


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Industrial & Engineering Chemistry Research

of N−H. The bands at 2968 and 2864 cm−1 were attributed to the stretching and bending vibrations of C−H, while the peaks at 1371 and 1449 cm−1 were characteristic of the C−H deformation vibration. The band observed at 1715 cm−1 was the characteristic peak of CO, and the strong peak at 1083 cm−1 corresponded to the C−O stretching vibration. All of the above characteristic peaks reconfirmed that the raw sponge was a kind of polyether−PU.32,33 After immersion in the TMC solution for 3 h, the PU−COCl sponge was covered with Al2O3 nanoparticles and the n-hexane solution contained TMC, and also TMC can easily react with a secondary amine of the original PU sponge, thus grafting itself on the sponge skeleton, which can also be seen in the Supporting Information. ATRFTIR shows that the PU−COCl sponge had shifts in 3281 cm−1 to lower frequencies and appeared as a large band at 3325 cm−1, which was ascribed to the −OH stretching vibration of carboxylic acid (−COOH), resulting from the hydrolysis of acyl chloride (−COCl). Table 1 and Figure S2a show EDS results of the PU−COCl sponge without cleaning, and lots of Al elements could be found on the surface, which indicated that Al2O3 nanoparticles were bare on the skeleton, while the small number of Mg and Ca elements might be due to the impureness of raw Al2O3 nanoparticles. When the PU−COCl sponge was dipped into the aqueous solution, IP took place between the PEI from the aqueous solution and TMC from the hexane solution reserved on the sponge skeleton,34,35 and this polymerization process created a dense film that anchored the Al2O3 nanoparticles tightly on the surface of the sponge, which gave the PU−IP sponge a micronanohierarchical structure. At the same time, the polymerization film was rich in hydrophilic amino groups. These two factors offered the PU−IP sponge great hydrophilicity. The WCA of the PU−IP sponge was 0° (Figure 1b), and a water droplet can be absorbed by the PU−IP sponge within 3 s. At last, while this dense film was rich in amino groups, it precisely supported a platform to graft PA via amidation, forming a monomolecular layer of PA, which introduced lowsurface-energy materials on the sponge. These two key factors (hierarchical structure and low surface energy)27,36,37 gave the PU−IP−PA sponge a superhydrophobic surface. The WCA of the PU−IP−PA sponge sharply reached up to 161° (Figure 1c). More interestingly, the as-prepared sponge is superhydrophobic not only in air but also in oil (Figure 1d). The oil/ water droplet exhibited a quasi-spherical shape on the PU−IP− PA sponge surface with a contact angle of 154°. The superhydrophobicity under oil mainly benefits by the hierarchical surface structure combined with the superlipophilicity of the PU−IP−PA sponge. In oil, the sponge is totally infiltrated by oil because of its superlipophilicity; when the water droplet contacts with the sponge, a high content of oil can be trapped in the rough micro/nanostructures, and this trapped oil will greatly reduce the contact area between water and the sponge surface, resulting in a large WCA in oil. After IP and modification with PA, compared to the other three

acquired on a Biorad FTS6000 spectrometer; before scanning, each sponge was completely cleaned with water or hexane under ultrasound to remove the physical adsorbing substance. UV−vis spectrometry (4802s, Unico, China) was used to detect the concentration of TMC in an n-hexane solution.

3. RESULTS AND DISCUSSION 3.1. Fabrication of a Superhydrophobic and Superoleophilic PU Sponge. The fabrication process of a PU−IP−

Figure 1. WCAs of the (a) pristine PU sponge, (b) PU−IP sponge, (c) PU−IP−PA sponge, and (d) PU−IP−PA sponge under oil. During the WCA measurement under oil, the PU−IP−PA sponge was immersed in octane.

Figure 2. FTIR spectra of the pristine PU sponge, PU−COCl sponge, PU−IP sponge, and PU−IP−PA sponge. The inset is the partial enlarged detail of the spectra around 2864 cm−1.

PA sponge is shown in Scheme 1, and attenuated total reflectance (ATR)-FTIR was used to further confirm the changes. As seen from Figures 1 and 2, the pristine PU sponge had a WCA of about 117° (Figure 1a) and possessed a band at 3281 cm−1, which was associated with the stretching vibration

Table 1. EDS Analysis of the Atom Content of the PU−COCl and PU−IP−PA Sponges atom content (%) sample

C

N

O

Al

Au

Mg

Ca

Cl

PU−COCl sponge PU−IP−PA sponge

19.97 64.95

2.09 4.18

32.67 21.62

35.98 7.16

2.91 2.09

1.82

1.51

3.06

C


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base was covered by a thin film after modification. It is worth mentioning that the sponge’s weight increased by 9.826% on average after modification, which indicates that the coating materials including the polyamide film and Al2O3 nanoparticles accounted for about 9.826% of the total weight of the PU−IP− PA sponge. Also, the detailed data of the content of the coating materials on three sponges with different sizes can be seen in Table S1. EDS analysis of the PU−IP−PA sponge is shown in Table 1 and Figure S2b; compared to the PU−COCl sponge, the content of the Al element on the PU−IP−PA sponge had a sharp decrease, while the contents of the C and N elements increase, which further confirmed that the sponge skeleton and Al2O3 were covered by a thin film after IP. At last, XRD was employed to confirm the successful loading of Al2O3 nanoparticles on the surface of the PU−IP−PA sponge. The XRD patterns of raw Al2O3 and the PU−IP−PA sponge, as presented in Figure S3, clearly demonstrate the PU−IP−PA sponge presents the same characteristic peaks and positions as raw Al2O3 nanoparticles do, suggesting a successful loading. Figure 3 depicts the XPS spectra of the pristine PU sponge and PU−IP−PA sponge. Compared to the pristine PU sponge, the peaks of the C and N elements clearly increased after modification (Figure 3a). Meanwhile, there were only a few Al signals that could be detected (merely 1.2 atom %) on the PU− P−PA sponge. Because XRD had clearly demonstrated the existence of abundant Al2O3 nanoparticles on the PU−IP−PA sponge, we can further confirm that the Al2O3 nanoparticles as well as the PU sponge were well coated by a thin film whose thickness was larger than the detecting depth of XPS analysis (a few nanometers). The C 1s spectrum, as shown in Figure 3b, presents the (C−C, C−H) component at 285.0 eV, (C−O, C− N) at 286.4 eV, and (COO, NCO) at 288.2 eV, which are present in the two samples.38,39 By a comparison of the C 1s spectra, we can find that the intensity of (C−C, C−H) component (285.0 eV) clearly increased after modification, which can be attributed to the long-carbon chain of PA. Because the major component of the polyamide thin film is from PEI, a characteristic signal for the (C−N, H−C) component at 399.3 eV is detected in the N 1s spectrum (Figure 3c) after modification. All of these features indicated that the PU−IP−PA sponge was covered by a polyamide thin film and the outer surface was grafted by abundant long-carbonchain substances. 3.2. Surface Morphology Analysis. The surface morphology was investigated by SEM at different magnifications, and the results are shown in Figure 4. It was observed that the pristine sponge exhibits three-dimensional porous structures with pore sizes in the range of 300−400 μm, and the skeleton surface of the pristine sponge is very smooth (Figure 4a). After IP, the PU−IP−PA sponge was covered with Al2O3 nanoparticles on the surface and eventually formed a hierarchical micro/nanostructure (Figure 4b). Moreover, the pores of the PU−IP−PA sponge were still clear without blockage, which guaranteed the oil absorption ability of the modified sponge. In comparison, we prepared a PU−IP−PA sponge without Al2O3 nanoparticles, and we can see from the SEM image that there was a smooth and dense film on the skeleton of sponge without any node or particle (Figure 4c), although the sponge was ultimately grafting PA after IP. The WCA was only 130° (Figure 4c, right). This indicated the necessity of the addition of Al2O3 nanoparticles. What is more, we also make another interesting experiment in which the order of IP remained

Figure 3. (a) XPS spectra of the pristine PU sponge and PU−IP−PA sponge. (b) C 1s peak fitting of the pristine PU sponge and PU−IP− PA sponge. (c) N 1s peak fitting of the pristine PU sponge and PU− IP−PA sponge.

sponges, the ATR-FTIR spectrum of the PU−IP−PA sponge had a shift for the C−H bending vibration of 2864−2857 cm−1 due to the influence of the long-carbon-chain molecule PA (Figure 2, inset). However, the characteristic peak of the pristine PU sponge in 1083 cm−1 for the C−O stretching vibration was greatly reduced, indicating that the PU sponge D


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Figure 4. SEM images of the (a) pristine PU sponge, (b) PU−IP−PA sponge, and (c) PU-IP sponge without Al2O3 nanoparticles. The right photographs are the WCA images of the PU−IP−PA sponge without Al2O3 nanoparticles. (d) SEM and WCA images of the special PU−IP−PA sponge in which Al2O3 nanoparticles were dispersed in the aqueous solution.

generated a silver mirror-like surface (Figure 5b) and showed Cassie−Baxter nonwetting behavior.40 Water droplets on both the surface and cross section were spherical in shape, indicating uniformity of the superhydrophobic coating on the modified sponge (Figure 5c). Figure 5d shows a high-speed photograph of a 5 μL water droplet rolling down on a slanted PU−IP−PA sponge surface, representing a strong repellance to water. The water droplet even bounced from the surface without any remaining (movie 1). On the contrary, when a 5 μL water droplet was dropped onto the surface of a pristine sponge at the same angle, the water droplet adhered to the surface (Figure 5e). The low sliding angle and superhydrophobicity cause the material to have a self-cleaning performance like a lotus leaf (Figure 5f and movie 2).41 When a water droplet slid down on the surface of the PU−IP−PA sponge, it could carry off the dust particles residing on the foam surfaces from the sponge (we used activated carbon particles as simulated dust particles here). Meanwhile, the PU−IP−PA sponge showed good superlipophilicity, relying on its long-carbon-chain PA on the surface. A droplet of diesel oil could be absorbed within 0.027 s (Figure 5g). 3.4. Oil/Water Separation Research. Owing to its superlipophilicity, superhydrophobicity both in air and under oil, porous structure, and excellent flexibility, the as-prepared

unchanged, but the Al2O3 nanoparticles were added into the aqueous phase instead of the oil phase. After IP and modification with PA, we found that the WSA of this sponge was 134° (Figure 4d, right), and there were plenty of little dents on the surface instead of nanoparticles in the SEM image (Figure 4d). This was possible because the nanoparticles pplaced in the aqueous phase failed to incorporate onto the surface of the polymerization layer by IP after washing with water and dichloromethane. The Al2O3 nanoparticles could be easily washed and left lots of dents, which reconfirmed that the micronanopapilla was necessary for the formation of a superhydrophobic surface. On the basis of the above results, we can further conclude that the Al2O3 nanoparticles of the PU−IP−PA sponge were firmly packed by the PA layer. 3.3. Exhibition of Superhydrophobicity and Superlipophilicity. The PU−IP−PA sponge showed excellent superhydrophobicity and superlipophilicity. A water droplet deposited on the surface formed almost a perfect sphere. The lubricating oil was totally absorbed in the sponge in 3 s (Figure 5a, right). While the water droplets settled on the surface of the pristine sponge, forming a hemisphere, the lubricating oil dispersed into the sponge very slowly (Figure 5a, left). When the modified sponge was immersed in water by an external force, the sponge surface was surrounded by air bubbles, which E


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Figure 5. (a) Image of the lubricating oil droplet (dyed with oil red) and water droplet (dyed with CuSO4) on the pristine sponge (left) and PU−IP−PA sponge (right). (b) Photograph of the PU−IP−PA sponge immersed in water by a force. (c) Photograph of a water droplet standing on the fresh face inside the PU−IP−PA sponge. High-speed photograph of a water droplet dropped on a slanted surface of the PU−IP−PA sponge (d) and pristine PU sponge (e). (f) Demonstration of the self-cleaning ability of the PU−IP−PA sponge through the removal of activated carbon particles from the surface using a dropped water droplet. (g) High-speed photograph of the PU−IP−PA sponge absorbing a diesel oil droplet.

Figure 7. (a) Oil absorption capacities of the superhydrophobic sponges for oils and nonpolar solvents. (b) Absorption recyclabilities of the superhydrophobic sponges for diesel oils.

PU−IP−PA sponge exhibits excellent selective absorption of oil from water. Figure 6a shows that the PU−IP−PA sponge rapidly absorbed the diesel oil floating on the water surface (the diesel oil was dyed with oil red). Once the superhydrophobic sponge touched the oil phase, the diesel oil was absorbed within

a few seconds and left a clear water surface. The mechanization of selective absorption was summarized in previous literature.19 The absorbed oil could be collected just through a simple squeezing process. As expected, no water droplets were visible

Figure 6. Photographs showing the removal process of (a) diesel oil and (b) dichloromethane from water by using the PU−IP−PA sponge. F


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Figure 8. (a) Removal and collection of diesel oil (dyed with oil red) from the surface of water by a piece of the PU−IP−PA sponge. (b) Device and process of continuous absorption and removal of diesel oil (dyed with oil red) from the water surface.

The repeatable absorption capacity of the PU−IP−PA sponge as a function of the absorption/collection cycle for diesel oil was also investigated and is presented in Figure 7b. The adsorption capability was sharply decreased from the second cycles; such a decrease is due to interaction between the diesel oil and as-prepared sponge, resulting in part of the diesel oil being trapped into the sponge. However, the saturated adsorption capacities were not obviously decreased after the second cycle, indicating the PU−IP−PA sponge is still an ideal material to adsorb the oils and organic solvents from water. The as-prepared sponge also can be used to absorb a thin oil layer on water, which simulated an oil spill in the sea. Once the sponge touched the diesel oil (dyed with oil red) on the surface of water, the oil around the sponge was quickly absorbed and could be completely absorbed in about 1 min, leaving a water surface without any contaminations; the absorbed oil could also be collected by squeezing (Figure 8a). Also, Figure 8b shows that the as-prepared sponge could be employed to continuously collect large amounts of oil pollutants from water, as previously report.42 The sponge was fixed at the opening of a tube combined with a vacuum pump and placed between the oil/ water interface; once the vacuum pump was started, the diesel oil was absorbed and pumped into the collector through the tube continuously, while water was repelled by the sponge because of its superhydrophobicity under oil. All of the diesel oil could be removed from the water surface, and no water could be seen in the collected diesel oil, showing the potential for practical applications for continuous oil/water separation. 3.5. Stability and Recyclability of a Superhydrophobic Sponge. The stability with an organic solvent and ultrasound was investigated by dipping the superhydrophobic sponge into ethanol and n-hexane, respectively, for 48 h and ultrasonic cleaning for 1 h subsequently. The WCA and SEM microscopic structure were explored after drying. The result showed that the superhydrophobic sponge had extreme stability against an organic solvent, maintaining WCAs of 154° and 156° after

Figure 9. Variation of the WCAs of the superhydrophobic sponges in repeated absorption/collection processes.

to the naked eye in the collected oils, exhibiting a high separation efficiency. The sponge could also be used to remove organic liquids with higher density than water, as shown in Figure 6b and movie 3. Once the sponge contacted dichloromethane under water, chloroform could be quickly absorbed within a few seconds. The results indicate that the PU−IP−PA sponge is a promising adsorbent for the cleanup and removal of organic pollutants with different densities. To investigate the adsorption capacity, the weight-gain ratio is defined as [(Wafter sorption − Winitial) × 100%]/Winitial. In a typical adsorption measurement, common oils and organic solvents were chosen as adsorbates. Figure 7a shows that the PU−IP−PA sponge had a wide range of absorption capacities for different adsorbates from 16.5 to 29.9, depending on the viscosity and density of the oil or organic solvents. G


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Industrial & Engineering Chemistry Research dipping and ultrasound. Meanwhile, the SEM microscopic structure demonstrated that the Al2O3 nanoparticles were still anchored on the skeleton without obvious loss, as shown in Figure S4. The recyclability was also examined by measuring the WCA change of the PU−IP−PA sponge in repeated absorption/ desorption processes; the sponge was washed with dichloromethane and n-hexane after several absorption/desorption cycles for diesel oil and the WCA was measured after drying. The results shown in Figure 9 demonstrate that the as-prepared sponge has excellent durability, in which it can bear over 500 cycles without losing its superhydrophobicity. With an increase in the cycle times, the contact angle had been gradually falling slowly, but the superhydrophobicity still remained until after 600 cycles. This was attributed to the chemical cross-linking by covalent bonds used in every step during the preparation of a superhydrophobic sponge, giving the PU−IP−PA sponge strong mechanical properties. Further, as shown in Figure S5, the Al2O3 nanoparticles still densely covered the skeleton after 400 cycles, and an observable decrease in the density of the nanoparticles occurred after 600 cycles, with the WCA decreasing to 147°.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.Y.). Tel/Fax: +86 22 27400199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Tianjin Oceanic Administration R&D Program (Grant 19-3BC2014-01)

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REFERENCES

(1) Peterson, C. H.; Irons, D. B. Long-term ecosystem response to the Exxon Valdez oil spill. Science 2003, 302, 2082. (2) Fingas, M. Oil Spill Identification. Anal. Chem. 2008, 48, 179. (3) Schaum, J.; Cohen, M.; Perry, S.; Artz, R.; Draxler, R.; Frithsen, J. B.; Heist, D.; Lorber, M.; Phillips, L. Screening level assessment of risks due to dioxin emissions from burning oil from the BP Deepwater Horizon Gulf of Mexico spill. Environ. Sci. Technol. 2010, 44, 9383. (4) Yang, C.; Kaipa, U.; Mather, Q. Z.; Wang, X.; Nesterov, V.; Venero, A. F.; Omary, M. A. Fluorous metal-organic frameworks with superior adsorption and hydrophobic properties toward oil spill cleanup and hydrocarbon storage. J. Am. Chem. Soc. 2011, 133, 18094. (5) Radetic, M. M.; Jocic, D. M.; Jovancic, P. M.; Petrovic, Z. L.; Thomas, H. F. Recycled Wool-Based Nonwoven Material as an Oil Sorbent. Environ. Sci. Technol. 2003, 37, 1008. (6) Tsai, C. K.; Liao, C. Y.; Wang, H. P.; Chien, Y. C.; Jou, C. J. G. Pyrolysis of spill oils adsorbed on zeolites with product oils recycling. Mar. Pollut. Bull. 2008, 57, 895. (7) Bayat, A.; Aghamiri, S. F.; Moheb, A.; Vakili-Nezhaad, G. R. Vakili-Nezhaad, G. R. Oil Spill Cleanup from Sea Water by Sorbent Materials. Chem. Eng. Technol. 2005, 28, 1525. (8) Inagaki, M.; Konno, H.; Toyoda, M.; Moriya, K.; Kihara, T. Sorption and recovery of heavy oils by using exfoliated graphite Part II: Recovery of heavy oil and recycling of exfoliated graphite. Desalination 2000, 128, 213. (9) Duong, P. H.; Chung, T. S.; Wei, S.; Irish, L. Highly permeable double-skinned forward osmosis membranes for anti-fouling in the emulsified oil-water separation process. Environ. Sci. Technol. 2014, 48, 4537. (10) Cao, Y.; Zhang, X.; Tao, L.; Li, K.; Xue, Z.; Feng, L.; Wei, Y. Mussel-Inspired Chemistry and Michael Addition Reaction for Efficient Oil/Water Separation. ACS Appl. Mater. Interfaces 2013, 5, 4438. (11) Deng, D.; Prendergast, D. P.; Macfarlane, J.; Bagatin, R.; Stellacci, F.; Gschwend, P. M. Hydrophobic Meshes for Oil Spill Recovery Devices. ACS Appl. Mater. Interfaces 2013, 5, 774. (12) Xu, L.; Liu, N.; Cao, Y.; Lu, F.; Chen, Y.; Zhang, X.; Feng, L.; Wei, Y. Mercury Ion Responsive Wettability and Oil/Water Separation. ACS Appl. Mater. Interfaces 2014, 6, 13324. (13) Chen, Y.; et al. Fabrication of a silica gel coated quartz fiber mesh for oil−water separation under strong acidic and concentrated salt conditions. RSC Adv. 2014, 4, 11447. (14) Lee, M. W.; An, S.; Latthe, S. S.; Lee, C.; Hong, S.; Yoon, S. S. Electrospun polystyrene nanofiber membrane with superhydrophobicity and superoleophilicity for selective separation of water and low viscous oil. ACS Appl. Mater. Interfaces 2013, 5, 10597. (15) Wang, S.; Li, M.; Lu, Q. Filter paper with selective absorption and separation of liquids that differ in surface tension. ACS Appl. Mater. Interfaces 2010, 2, 677. (16) Li, B. B.; Liu, X. Y.; Zhang, X. Y.; Chai, W. B. Stainless steel mesh coated with silica for oil-water separation. Eur. Polym. J. 2015, 73, 374.

4. CONCLUSIONS In summary, a robust superhydrophobic and superoleophilic PU sponge can be prepared through a combined method of IP and molecular self-assembly. The micro/nanostructure induced by the addition of Al2O3 nanoparticles, along with the selfassembly of a long carbon chain of PA, synergistically endowed the as-prepared PU sponge with superlipophilicity and superhydrophobicity both in air and under oil. The as-prepared sponge can be used for absorbing floating oils on the water surface and heavy oils under water and also for the continuous separation of large amounts of oil pollutants from the water surface with the help of a vacuum pump. More importantly, the resultant sponges have excellent durability to an organic solvent and could be reused for oil/water separation over 500 cycles without losing superhydrophobicity, showing excellent durability and reusability in an organic solvent. Our investigations offered a novel and facile strategy for fabricating a robust superhydrophobic sponge for the selective removal of oil and organic pollutants from water, which also could be used on other substrates such as fabric, metal mesh, membrane, etc.

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Movie of the absorption of dichloromethane under water with a piece of the PU−IP−PA sponge (movie 3) (AVI)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02897. Research of the TMC enrichment on PU sponge (Figure S1), detailed data of the content of the coating materials on three sponges with different sizes (Table S1), EDS image and data of PU−COCl and PU−IP−PA sponges (Figure S2), XRD patterns of the PU−IP−PA sponge and commercial Al2O3 powder (Figure S3), SEM image of the PU−IP−PA sponge after treatment in ethanol and n-hexane (Figure S4), and SEM images of the superhydrophobic sponges after water/oil separation for 100, 400, and 600 cycles (Figure S5) (PDF) Movie of a 5 μL water droplet rolling down from the surface of the PU−IP−PA sponge (movie 1). (AVI) Movie of the demonstration of the self-cleaning property of the PU−IP−PA sponge (movie 2) (AVI) H


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Robust and Durable Superhydrophobic Polyurethane Sponge for Oil

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