Thermoresponsive Melamine Sponges with Switchable Wettability by

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Thermo-Responsive Melamine Sponges with Switchable Wettability by Interface-Initiated Atom Transfer Radical Polymerization for Oil/Water Separation Zhiwen Lei, Guangzhao Zhang, Yonghong Deng, and Chaoyang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14565 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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ACS Applied Materials & Interfaces

Thermo-Responsive Melamine Sponges with Switchable Wettability by Interface-Initiated Atom Transfer Radical Polymerization for Oil/Water Separation Zhiwen Lei,† Guangzhao Zhang,† Yonghong Deng,*,‡ and Chaoyang Wang*,†

†

Research Institute of Materials Science, South China University of Technology, Guangzhou

510640, China ‡

Department of Materials Science and Engineering, South University of Science and Technology

of China, Shenzhen 518055, China

*Corresponding Authors: E-mail: [email protected] (CY Wang); [email protected] (YH Deng)

ACS Paragon Plus Environment

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ABSTRACT Here we have obtained a temperature responsive melamine sponge with a controllable wettability between super-hydrophilicity and super-hydrophobicity by grafting the octadecyltrichlorosilane and thermo-responsive poly (N-isopropyl acrylamide) (PNIPAAm) onto the surface of melamine sponge skeletons. The whole process included the silanization in which step the rough surface with low surface energy and the -NH2 were provided, and the atom transfer radical polymerization which ensured the successful grafting of PNIPAAm onto the skeleton’s surface. The product exhibits a good reversible switch between super-hydrophilicity and superhydrophobicity by changing the temperature below or above the lower critical solution temperature (LCST, about 32 oC) of PNIPAAm, and the modified sponge still remains a good responsiveness after undergoing two temperature switching for 20 cycles. Simultaneously, the functionalized sponges could be used to absorb the oil underwater at 37 oC, and released the absorbed oil in various ways underwater at 20 oC, showing the wide potential applications including oil/water separation.

KEYWORDS: melamine sponge， PNIPAAm， thermo-responsive， controlled wettability， oil/water separation


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1. Introduction The control of surface wettability plays a very important role in many applications including controllable drug release,1 microfluidic devices,2,3 catalytic reaction,4 energy storage,5 oil/water separation,6-9 and so on. It is well known that the surface wettability depends on the surface chemical composition and surface roughness. The first factor contributes to the hydrophilicity or hydrophobicity, and the other amplifies the wettability, showing an access to the super-hydrophilicity or super-hydrophobicity.10-12 Based on the theory, an interesting modification had been taken into consideration that many responsive molecules with a reversible switch between hydrophilicity and hydrophobicity in response to different environments could be introduced to the substrate surface by chemical or physical methods to form the smart interfacial materials. Owing to the addition of responsive molecules or polymers, the wettability of this modified product could be changed from hydrophilicity to hydrophobicity under external stimuli. To be more excitedly, the combination of responsive molecules or polymers with rough structure provided a potential for the preparation of responsive super-wettability materials with a switching behavior between super-hydrophobicity and super-hydrophilicity. So far, the light,13 pH,14-16 and temperature-responsive surface materials have been reported by many researchers.1719

Poly(N-isopropyl acrylamide) (PNIPAAm) was widely used for the synthesis of smart materials due to its temperature responsiveness. This polymer shows hydrophilic or hydrophobic property when the environmental temperature is below or above its lower critical solution temperature (LCST).20 With such a favorable property, PNIPAAm was employed to modify the rough substrate by Jiang to achieve a reversible switching material between super-hydrophilicity and super-hydrophobicity at different temperature.21 Except for acting as an surface modifier, the


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PNIPAAm could also be a part of the block copolymer with poly(methyl methacrylate) (PMMA) to form different chemically bearing polymers, which showed a reversible switching behavior between hydrophilicity/oleophobicity and hydrophobicity/oleophilicity in a narrow temperature range.22 Most two dimensional temperature responsive wettability materials have been investigated,23 but few of three dimensional temperature responsive materials have been reported. Melamine sponge consisting of melamine formaldehyde (MF) is a candidate of oil/water separation materials due to its low cost, low density, high porosity, good flexibility and easy modification.24-26 In most modification, the melamine sponges acted as the support which was coated by hydrophobic polymer to form hydrophobic or super-hydrophobic oil absorbed materials. Besides this physical modification, the melamine sponges could be pyrolysed in nitrogen atmosphere at high temperature to obtain the hydrophobic carbon foams.27 Recently, an interesting chemical modification was found that the siloxane was hydrolyzed by the absorbed water on the skeleton surface of melamine sponge to obtain the silanol compounds, then the hydrogen bond between the silanol and the secondary amine on the surface of melamine sponge skeleton was formed during the condensation of Si-OH. Finally the Si-N bond was achieved through the dehydration reaction between Si-OH and N-H at high temperature.28 Although the melamine sponge was used as a substrate for different hydrophobic modifications including dip coating, phase separation, high temperature treatment and so on, 29-31 the responsive melamine sponges were barely obtained.32 Inspired by those excellent works about temperature responsiveness and hydrophobic modifications of melamine sponges, we have fabricated a temperature responsive melamine sponge with a reversible switching behavior between

super-hydrophilicity

and

super-hydrophobicity

by

grafting

hydrophobic

octadecyltrichlorosilane (OTS) and temperature responsive PNIPAAm onto the surface of


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melamine sponge skeletons. The wettability of the obtained product could be controlled via changing the environmental temperature rather than changing the media pH values or introducing the light irradiation due to the different conformations of PNIPAAm macromolecule structures. More importantly, this responsive material might be applied in oil absorption from oily water and oil desorption underwater in a collector without destroying the structure and polluting the air, resulting in a low-cost, controllable and environment-friendly oil/water separation.

2. Experimental Methods 2.1. Materials. Melamine sponges (MF sponges) were obtained from GreenCARE International

(Guangzhou)

Ltd.

Octadecyltrichlorosilane

(OTS),

(3-ami-nopropyl)

trimethoxysilane (ATMS), N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (PMDETA) and 2bromoisobutyryl bromide were purchased from Shanghai Macklin Biochemical Co., Ltd. NIsopropylacrylamide (NIPAAm, 98%) was obtained from Beijing J&K Scientific Ltd. The common organic solvents including toluene, dichloromethane, acetone and methanol were provided by Guangzhou Chemical Reagent Factory. Pyridine and Copper (I) bromide were bought from Shanghai Rich Jiont Chemical Reagents Co., Ltd. 2.2. Preparation of OTS/ATMS Modified Sponge (MF-OTS/NH2). Melamine sponges (112 cm3, 8 mg/cm3) with about 30 wt% deionized water were immersed in the 10 mL toluene solution containing 30 L ATMS and 30 L OTS. After immersing for 1.5 h, the sponge was taken out for 5 times repeated washings by clean toluene to remove the unreactive siloxane, finally the silanized MF-OTS/NH2 sponge was obtained after drying at 115 oC for 2 h.


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2.3. Synthesis of Thermo-Responsive Sponge (MF-OTS/PNIPAAm). The MF-OTS/NH2 sponge was introduced into the dry dichloromethane with 2 % (V/V) pure pyridine and stirred at 0 oC, then 30 L 2-bromoisobutyryl bromides was added dropwise; after 1h, the reaction solution was placed at ambient temperature for 12 h. Subsequently, the initiator grafted sponge (MF-OTS/Br) was taken out from the dichloromethane solution and washed with acetone and toluene, followed by drying under nitrogen atmosphere. Finally, the MF-OTS/Br sponge was immersed into the degassed solution of deionized water (2.5 mL) and methanol (2.5 mL) containing copper (I) bromide (0.016 g) and PMDETA (80 L) and N-isopropylacrylamide (0.2 g) for 2 h at 60 oC. The target product (MFOTS/PNIPAAm sponge) (the conversion of NIPAAm was about 10 wt %) was obtained after washing with deionized water three times and drying with a nitrogen stream.33 2.4. Characterization. The surface topography of melamine sponge was observed by scanning electron microscopy (SEM Zeiss EVO18) at 10 KV after gold coating. The x-ray photoelectron spectroscopy (XPS, ESCA Axis Ultra DLD) was used to confirm the presence of modifying agent on the surface of melamine sponge. The contact angle of a 4 L water/oil droplet on the surface was measured by the contact angle analysis system (OCA20LHTTEC700-HTFC1500, Dataphysics, Germany). The temperature was controlled by a temperature controller (THX-05, Ningbo Tianheng Instrument Factory, China). Before every contact angle measurement, the sample was dried with a flow of nitrogen and kept at the required temperature for 30 min.34,35 The extent of modification reaction can be also expressed with the assistance of attenuated total reflection-Fourier transform infrared spectroscopy and the purity of the oily water after separation can be characterized via an infrared spectrometer (ATR-FTIR Vector 33 & Nicolet 6700, Germany).


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3. Results and Discussion As we know that the rough structure plays an important role to determine the wettability as well as the chemical composition, so the rough melamine sponge with high porosity (about 99 %) might be an ideal candidate of substrates for wettability modifications. The addition of OTS provided the low surface energy chains to improve the hydrophobicity when the responsive product was in a higher temperature environment. In addition, ATMS with NH2 groups was used as the active site to introduce PNIPAAm. The whole preparation mechanism of MFOTS/PNIPAAm sponge is shown in Figure 1. Firstly, the OTS was hydrolyzed by the absorbed water on the melamine sponge surface skeleton to produce silanol and hydrogen chloride which was used as a catalyst to accelerate the hydrolysis of ATMS containing -NH2 group.36 Then the condensation of silanol compounds occurred; simultaneously, the hydrogen bonds between the N-H of melamine sponges and unreacted Si-OH of silanol compounds were formed; after drying at high temperature, the N-Si bond was formed due to the dehydration reactions between N-H and Si-OH, resulting in covalently bonding between siloxane oligomer and melamine sponges. Subsequently, the initiator 2-bromoisobutyryl bromide was introduced onto the skeleton’s surface via the acylation reaction between NH2 of ATMS and Br-C=O groups of initiators. Followed by the atom transfer radical polymerization, the MF-OTS/PNIPAAm sponge was synthesized through atom-transfer radical polymerization (ATRP).37 The immobilization of ATMS containing NH2 groups onto the surface of melamine sponge skeletons played a very important role in the whole process, so the XPS was used to verify this modification (Figure S1). Compared with the OTS modified melamine sponge which had been reported by Dickerson, 28 the Si content of the OTS/ATMS modified melamine sponge increased from 5.48 wt % (OTS) to 10.8 wt % (OTS/ATMS). The same tendency was found for N content


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which increased from 3.15 wt % to 4.84 wt % owing to the extra addition of ATMS. This phenomenon implied that hydrolysis, condensation and dehydration reactions could happened to ATMS as OTS. Through the silanization, the N-Si bonds were formed (Figure S2). Meanwhile, the silanization time of 90 minutes was determined by the ATR-FITR spectra through the increasing intensity of those peaks at 2850 cm-1 and 2920 cm-1 attributed to the -CH2 and -CH3 groups of the OTS and ATMS (Figure S3).38 The successful grafting of ATMS onto the surface of melamine sponge skeletons set the stage for the immobilization of PNIPAAm.

Figure 1. Schematic illustration of preparation of MF-OTS/PNIPAAm sponges.


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After the silanization, the melamine sponge was modified by the initiator (2bromoisobutyryl bromide). To confirm the successful grafting of the initiator, the surface chemical compositions of MF and MF-OTS/Br sponges were analyzed by XPS in Figure 2a. Compared with the MF, the intensity of O 1s and Si 2p of MF-OTS/Br showed an apparent increase due to the successful silanization. The appearance of Br 3d peak (67.9 eV) of MFOTS/Br sponge confirmed the perfect graft modification of 2-bromoisobutyryl bromide onto the skeleton’s surface of melamine sponges, which suggested that the acyl bromide groups of 2Bromoisobutyryl bromide reacted with the NH2 of ATMS, and another Br of the 2Bromoisobutyryl bromide was well preserved. Simultaneously, the O 1s peak of the MF-OTS/Br sponge was also measured by XPS (Figure 2b). There were three signals at 529.6 eV (O=C), 530.3 eV (O-C), and 531.6 eV (O-Si) in the spectrum. The signal at 530.3 eV came from the C-O groups of melamine sponges while the signal at 531.6 eV was attributed to the O-Si groups of OTS/ATMS and the last signal at 529.6 eV was ascribed to the C=O groups of 2Bromoisobutyryl bromide. So the XPS spectra powerfully verified the successful introduction of OTS/ATMS and the initiator 2-Bromoisobutyryl bromide respectively.39


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b

a

c

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e

d

Figure 2. (a) XPS spectra of MF and MF-OTS/Br sponges, (b) O 1s peaks of MF-OTS/Br sponges. SEM images of (c) MF, (d) MF-OTS/NH2, and (e) MF-OTS/PNIPAAm sponges.

Followed by the interface-initiated atom transfer Radical polymerization, all the elements of MF-OTS/PNIPAAm sponges were the same as the MF-OTS/Br sponges, and intensity of C 1s and O 1s of MF-OTS/ PNIPAAm sponges was stronger than that of MF-OTS/Br sponges due to the good implementation of atom-transfer radical polymerization (Figure S4). The surface microcosmic topography of samples was obtained by SEM as shown in Figure 2c-e. Different from the unmodified melamine sponge skeleton with smooth surface (Figure 2c), the silanization sponge (MF-OTS/NH2 sponge) showed some uneven coating covering the whole skeleton (Figure 2d). It was obviously that the uneven coating consist of siloxane oligomers with low surface energy, and the siloxane oligomers contributed to the super-hydrophobicity of MF-


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OTS/NH2 (the water contact angle was 152o shown in Figure S5a) as the wettability was determined by the surface chemical composition and roughness. It was well known that the single chemical modification without improving the surface roughness just made the flat substrate hydrophobic and the water contact angle was lower than 120o.40, 41 In other words, every skeleton’s surface coated by OTS/ATMS with low surface energy was hydrophobic and the sponges with high porosity about 99 % provided a large area of pores to trap air, leading to a large area of air/water interface, forming super-hydrophobic sponges. This super-hydrophobic modification might be well illustrated by the Cassie equation,42, 43 and the water contact angle for air was assumed as 180o.

cos *  f1 cos   f 2 cos180  f1 cos   f1 1 Here * and  are the water contact angles on the rough surface and corresponding smooth surface, respectively. f1 and f2 are the fractions of liquid/solid and liquid/air interfacial areas, respectively (f1 + f2 =1). The equation indicates that the smaller of f1 lead to the greater of the apparent contact angle (*). As for melamine sponges, f2 is very greater than f1 due to the highly porosity. So it is easy to obtain super-hydrophobic melamine sponges through simple hydrophobic modifications such as the introduction of siloxane with low surface energy. After the atom transfer Radical polymerization, many aggregates made up of nano-sized chain-like compounds covering on the partial siloxane oligomers coating of skeleton’s surface (Figure 2e), which could be described as that only the ATMS with NH2 groups, combining with OTS without amine groups to form the super-hydrophobic coating, could be successful grafted by the 2-bromoisobutyryl bromide which initiated the polymerization of NIPAAm, resulting in temperature responsive chain-like PNIPAAm compounds covering on the partial surface.


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It was well known that the wettability of PNIPAAm could be switched between hydrophilicity and hydrophobicity through making the environment temperature below or above the lower critical solution temperature (LCST) of about 32 oC,44 and the rough surface had been provided by original melamine sponge with high porosity which acted an important role in changing the super-amphiphilic melamine sponges into a super-hydrophobic sponge at the step of silanizaiton. Therefore, when comparing with the smooth substrate modified by PNIPAAm, the wettability of MF-OTS/PNIPAAm sponge would be amplified. In air condition, the responsive product showed super-hydrophobic at 45 oC (the water contact angle was 150o) (Figure 3a) owing to the original porous structure and the low energy surface containing the collapsed PNIPAAm chains and outstretched OTS chains, while exhibited super-hydrophilic at 25 oC (the water contact was 0o) (Figure 3c) due to the outstretched PNIPAAm chains and highly porous sponges. When being placed on the top of the water at 25 oC, the responsive sponge firstly floated on the water, then immersed in the water, finally sunk in the bottom. The oil droplet (dichloroethane died with Sudan I) which could wetted the responsive sponge both below and above the LCST in air (Figure S5b, c) could stand on the surface of the submerging sponge underwater (Figure 3b), showing a highly oleophobic and super-hydrophilic performance underwater at 25 oC.


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a

b

c

d

e

f

Figure 3. (a) Water contact angle at 45 oC in air and (b) oil contact angel at 25 oC underwater of MFOTS/PNIPAAm sponges. (c) Water contact angles of MF-OTS/PNIPAAm sponges in air as a function of temperatures. (d) Reversible water wettability of MF-OTS/PNIPAAm sponges in air at 45 oC and 25 oC. (e) A spherical water droplet was standing on the surface over 1260 s at 37 oC in air. (f) The water droplet permeated into the sponge within 150 s at 25 oC in air.

Meanwhile, the surface wettability of the as-prepared product at different temperature from 15 to 45 oC in air condition was also investigated (Figure 3c). The water contact angle increased from 0o to 150o in a narrow temperature range from 25 to 40 oC, showing a reversible switch behavior between super-hydrophilicity and super-hydrophobicity. Compared with the PNIPAAm directly modified melamine sponges which justly showed high hydrophobic property in air at 45 oC (the water contact angle was 130o) and hydrophilic property at 25 oC (the water contact angle was 33o) as shown in Figure S6, the wettability change of MF-OTS/PNIPAAm sponge to the environmental temperature was amplified. This thermo-responsive transition of as prepared sponge was conducted on two temperature stages at 25 and 45 oC in air condition. This


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product showed a good switching behavior between super-hydrophilicity at low temperature and super-hydrophobicity at high temperature. The reversible water/oil wettability of the modified sponge in air/underwater at two temperature stages could be repeated 20 cycles without changing the temperature responsiveness of the sample (Figure 3d, S7), showing a good stability and reproducibility of the PNIPAAm modified sponge. The performance of water droplets on the surface of responsive sponges at different temperature in air were investigated. At 37 oC (above the LCST of PNIPAAm), the spherical water droplet stood on the surface over 1260 s and its spherical shape was remained despite of a smaller size due to the evaporation of water (Figure 3e), showing a stable super-hydrophobicity of MF-OTS/PNIPAAm sponges. However, at 25 oC, a part of the water droplet was swallowed instantaneously when touching with the surface of the responsive sponge, and the rest permeated into the sponges within 150 s (Figure 3f), indicating a good super-hydrophilicity of MFOTS/PNIPAAm sponges. The reversible switching between super-hydrophilicity and super-hydrophobicity of the thermo-responsive sponges might be illustrated perfectly in Figure 4. From a molecular perspective, the intermolecular and intramolecular hydrogen bonding of PNIPAAm played a very important role in the wettability of MF-OTS/PNIPAAm sponges. When the responsive sponges were touched with water under the LCST of PNIPAAm, the intermolecular hydrogen bonding between PNIPAAm and water molecules was formed. The hydrated, swollen and outstretched PNIPAAm was fabricated by the influence of hydrogen bonds and van der waals forces, showing a super-hydrophilic and water filled MF-OTS/PNIPAAm sponge. While the temperature was above the LCST, the C=O groups combined with N-H groups to form the intramolecular hydrogen bonds, leading to a compact and collapsed PNIPAAm, which made the


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hydrophilic C=O and N-H groups surrounded by the hydrophobic isopropyl groups. Then the hydrophobic PNIPAAm and uncovered OTS on the skeleton’s surface played the leading roles, resulting in a super-hydrophobic and oil filled MF-OTS/PNIPAAm sponge. When applied to the oil/water separation, the as prepared sponges can absorb the oil from the oily water at higher temperature, and release the absorbed oil underwater at lower temperature to recover the oil without polluting the air.

Figure 4. Schematic diagram of the absorption performance of MF-OTS/PNIPAAm sponges at different temperatures.

Based on the above work, the functionalized sponge was employed to absorb the oil under water as shown in Figure 5a (Movie S1), when immersed into the water by external force at 37 o

C, the MF-OTS/PNIPAAm sponge acted like a sliver mirror, and quickly absorbed the oil


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underwater on the bottom of the beaker when touching the oil (dichloromethane, density of 1.325 g/cm-3), showing a good potential in oil /water separation performance. At the same temperature, many organic liquids could be absorbed by the modified sponges from oily water with the maximum absorption capacity up to 70 times of its own weight (Figure 5c). It is possible that the oil absorbed sponge could quickly release the absorbed oil underwater at low temperature due to its temperature responsiveness. unexpectedly, when the heavy oil (dichloromethane) absorbed sponges was immersed into the low temperature water at 20 oC, the absorbed oil was desorbed as air in oil bubbles, the whole desorption was very slowly and finished in several hours (Figure 5b). This phenomenon might be explained like that: Firstly, owning to the low thermal conductivity of sponges, only the surface contacted with water was switched from super-hydrophobicity into super-hydrophilicity, leading to a weak interaction between oil and the skeleton’s surface. Then the air bubbles captured by the porous responsive sponge played an important role during the oil desorption. Air bubbles were encapsulated when touching with the oil in oily sponges as a result of positive spreading coefficient, and spontaneously grew to get away from the absorbent mainly due to the increasing bubble buoyancy. However, the mixed bubbles were sank underwater due to the heavy oil contributing to the greater gravity than buoyancy.45 And the original space of sponges was occupied by low temperature water after the escape of air bubbles with heavy oil, leading to a further wettability switching and oil release. Almost all the absorbed oil had been desorbed from the oily sponge 6 hours later, and the air bubbles encapsulated by heavy oil were located in the bottom. Inspired by this performance, one assumption was made that the organic liquid drugs could be absorbed by this thermo-responsive sponge, which could release the drugs slowly below the LCST of about 32 oC in a fishpond, keeping a lasting drug effect.


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a

b

c

Figure 5. (a) Fast oil (dichloromethane died with Sudan I) absorption at 37 oC and (b) slow oil desorption at 20 oC of MF-OTS/PNIPAAm sponges. (c) The absorption capacities of MF-OTS/PNIPAAm sponges for different oils or organic liquids at 37 oC.

Interestingly, when the responsive sponge was cut into smaller pieces (about 0.50.51 cm3), the absorbed oil could be quickly desorbed from the oily sponges underwater at 20 oC (Figure 6a). The oil desorption performed perfectly due to relatively effective contact area


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between oily sponges and water. The PNIPAAm grafted surface was changed from superhydrophobicity to super-hydrophilicity immediately when contacting with the low-temperature water molecules. And the air in heavy oil bubbles in the sponge grew to escape the sorbent, then came up due to the greater buoyancy than its gravity (Figure S8 and Movie S2). The whole desorption took about 135 s in despite of little residues containing dyes in the sponges, and the water occupied the space of the sponge instead of the oil. The temperature responsiveness of the functionalized sponge was remained after the drying. Though with so perfect performance, this product with such small size might be merely used for microfluidic application.

a

b

c

Figure 6. (a) The quick oil desorption of oily MF-OTS/PNIPAAm sponges at 20 oC. (b) The absorbed oil was recovered by gently squeezing the thermo-responsive sponge underwater at 20 oC, and (c) the oil absorption at 37 oC/desorption by gentle compression at 20 oC of MF-OTS/PNIPAAm sponges at different temperatures underwater.


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Obviously, the oil could be absorbed from oily water by the responsive sponge at higher temperature than the LCST of PNIAAm and recovered by squeezing the oil absorbed sponges in air. After the oil absorption of the n-hexane/water mixture, the treated water was characterized via an infrared spectrometer (Figure S10). The absence of the characteristic peaks of n-hexane showed that the treated water was very pure after oil absorption.46 Just like other traditional hydrophobic sponges, some oil residues were still remained in the modified sponges after squeezing method (Figure S9).47 If the oil filled thermo-responsive sponge was immersed in the water below the LCST of PNIPAAm, almost all oil could be recovered by gentle compression in a short time. During this recovery, the responsive sponge was changed from superhydrophobicity into super-hydrophilicity due to the existence of PNIPAAm and rough structure, resulting in a weak interaction force between sponges and oil (Figure 6b). Therefore, the responsive sponge could keep clean after the quick oil recovery and be used for other oil recovery after drying. The recyclability of the modified sponge for the oil adsorption and desorption has been tested. As shown in Figure 6c, the reversible oil (dichoromethane) absorption at 37 oC/desorption at 25 oC of the as-prepared sponge was repeated five times, the absorption capacity nearly remained unchanged. And few of oil residues were kept in the absorbent after oil desorption. The smart oil absorption/desorption methodology can be applied for oil/water separation without incurring secondary air or water pollution.

4. Conclusions In summary, the thermo-responsive melamine sponges have been fabricated through the introduction of OTS and PNIPAAm onto the skeleton’s surface, showing a good reversible switching between super-hydrophilicity and super-hydrophobicity below and above the LCST of


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PNIPAAm. With such special wettability, the MF-OTS/PNIPAAm sponge could not only be used to absorbed the oil underwater at 37 oC, but also released the absorbed oil when dropped into the low-temperature water at 20 oC to keep clean for once more oil/water separation. We believe that this study will extend the application of PNIPAAm and melamine sponges in the oil/water separation and provide an interesting design for more responsive three dimensional materials. ASSOCIATED CONTENT Supporting Information This supporting information is available free of charge on the ACS Publications website at DOI: XPS, ATR-FTIR spectra, wettability and oil/water separation of sponges. (PDF) Movie S1: Absorption of dichloromethane at 37 oC. Movie S2: Desorption of dichloromethane in water at 20 oC. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (CY Wang). [email protected] (YH Deng). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21474032) and the Natural Science Foundation of Guangdong Province (2016A030311031).


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ABBREVIATIONS PNIPAAm, poly (N-isopropyl acrylamide); LCST, lower critical solution temperature; MF, melamine

formaldehyde;

OTS,

octadecyltrichlorosilane;

ATMS,

(3-ami-nopropyl)

trimethoxysilane; PMDETA, N,N,N’,N’’,N’’-Pentamethyldiethylenetriamine; SEM, scanning electron microscopy; XPS, x-ray photoelectron spectroscopy; ATR-FTIR, attenuated total reflection-Fourier transform infrared spectroscopy.

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Thermoresponsive Melamine Sponges with Switchable Wettability by

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