Hyperbranched Zwitterionic Polymers Functionalized Underwater

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Hyperbranched Zwitterionic Polymers Functionalized Underwater Superoleophobic Microfiltration Membranes for Oil-in-Water Emulsion Separation Junqiang Zhao, Dongyang Li, Hongrui Han, Jingjing Lin, Jing Yang, Qiqi Wang, Xia Feng, Ning Yang, Yiping Zhao, and Li Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03231 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Fabrication of Dopamine-Inspired Thiolated Hyperbranched Zwitterionic Polymers Modified PVDF Membranes for Efficient Oil-in-Water Emulsion Separation. 142x110mm (144 x 144 DPI)

ACS Paragon Plus Environment

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Hyperbranched Zwitterionic Polymers Functionalized Underwater Superoleophobic Microfiltration Membranes for Oil-in-Water Emulsion Separation Junqiang Zhao,†, ‡ Dongyang Li,†, ‡ Hongrui Han,‡ Jingjing Lin,‡ Jing Yang,‡ Qiqi Wang,‡ Xia Feng,‡ Ning Yang,‡ Yiping Zhao,‡, * Li Chen‡, #, *

‡

State Key Laboratory of Separation Membranes and Membrane Processes/National Center

for International Joint Research on Separation Membranes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China #

School of Materials Science and Engineering, Tianjin University of Technology, Tianjin

300384, China

1 ACS Paragon Plus Environment

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ABSTRACT: Inspired by mussel adhesion chemistry, a kind of hydrophilic poly(vinylidene fluoride) (PVDF) microfiltration membranes with underwater superoleophobicity was prepared using thiolated hyperbranched zwitterionic poly(sulfobetaine methacrylate) (HPS) as a nanoscale surface modifier. The HPS was first synthesized via RAFT copolymerization and followed by sulfonation reaction, and then coated onto dopamine (PD) adhesive PVDF membranes via thiol-mediated Michael addition reaction. The successful and uniform coating of HPS onto the membrane surface was demonstrated by X-ray photoelectron spectroscopy and energy dispersive X-ray detector. The surface micro-nano morphology and increased roughness of the PD/HPS modified (M-PD/HPS) membrane were also investigated by field emission scanning electron microscope and atomic force microscope. The M-PD/HPS membrane could be wetted completely by water and the underwater oil contact angles were about 160o, indicating the M-PD/HPS membrane with excellent hydrophilicity and underwater superoleophobicity. Compared with the pure PVDF membrane, the M-PD/HPS membrane for hexane-in-water emulsion separation exhibited enhanced water filtration flux of 10707 L m-2 h-1 (0.1 MPa) and oil rejection ratio was above 99.9%. Besides, the excellent anti-fouling ability and recyclable properties of the M-PD/HPS membranes would make them suitable for a long time using. Thus, the approach of mussel adhesion chemistry employed the RAFT mediated nano-sized hyperbranched zwitterionic polymers as postmodification reagents showed a good application prospect in purification of oily waste water and oil recovery.


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KEYWORDS:

Hyperbranched

Zwitterionic

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Polymers,

Dopamine,

Underwater

Superoleophobicity, Microfiltration Membranes, Oil-in-water Emulsion Separation


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1. Introduction As is known to all, industry and frequent oil leakage accidents will continuously produce a large amount of oily waste water, therefore, oil-water separation is a worldwide challenge. The high oil content of oily waste water could cause environmental pollution, food security, and damage public health in a wide range of ways.1-2 The separation materials with special surface wettability, including “oil-removing” type and “water-removing” type, have been extensively prepared for oily waste water separation.3-5 Due to the oil inherent oleophilic property, the “oil-removing” separation materials were easy to be fouled in the separation process. While “water-removing” type separation materials, such as surface modified metallic meshes,6-8 textiles,9-11 and membranes,12-14 were used to separate oily waste water based on their intrinsic superiority of anti-fouling property. However, it is worth noting that the large pore sizes of metallic meshes and textiles were inappropriate to separate oil-in-water emulsions due to their comparatively smaller sizes of 0.1 ~ 20 µm. Membrane separation technology is a common method for sewage treatment and water purification due to its high separation efficiency, relatively low operation cost and space saving. The poly(vinylidene fluoride) (PVDF) microfiltration membranes with good chemical stability, high mechanical strength and suitable pore size have often been used in the treatment of oily waste water.15 But, the hydrophobicity of the pure PVDF membrane was easy to cause membrane fouling, which was an unavoidable issue in application process and could give rise to tremendous reduction of permeation flux and shorten service life. Therefore, hydrophilic modification of hydrophobic PVDF membrane is needed to achieve efficient separation of


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oil-in-water emulsion.16 For instance, by means of interfacial polymerization, a hydrogel layer was constructed on the surface of PVDF membrane to separate oil-in-water emulsion effectively.17 While during the process of fabricating hydrophilic PVDF membranes, alkaline treatment technology towards PVDF membranes was used to produce C=C bonds as surface hydrogel anchoring, which limited their scale production due to the drawback of uncontrolled modification method. Recently, mussel adhesion chemistry has been widely used in the field of material interface modification for its simplicity, universality, and economic practicality.18-22 However, due to the inherent and limited hydrophilicity of the polydopamine (PD) layer, it was difficult for PD coated membranes to obtain underwater superoleophobic surfaces for oil-in-water emulsion separation. Hence, other modifiers based on PD layers have been adapted to decorate PVDF microfiltration membranes. Yang et al. prepared underwater superoleophobic PVDF membrane with dopamine and MWCNTs via mussel-inspired method for oil-in-water emulsion separation.23 The superhydrophilic and underwater superoleophobic PVDF membranes prepared by Shao et al. group via co-depositing dopamine and KH560 for oil-in-water emulsion separation.24 And Shi et al. prepared underwater superoleophobic TiO2 decorated PVDF membranes by dip-coating method to separate water from oil-in-water emulsion.25 Both co-deposition and secondary modification were universal strategies to modify the hydrophobic PVDF membrane. It was of great simplicity and effective approach to enhance hydrophilicity for one-step deposition of dopamine with other polymers in weak alkaline condition. While fewer types of polymer co-deposited with dopamine would limit


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their widespread appilication. To address this problem, some researchers have tried to prepare the polymers with special active groups in the terminal, which could be immobilized on PD layer via Michael addition or Schiff base reaction.18 Recently, 3D hyperbranched polymers with unique structures, such as intramolecular cavities and abundant functional groups, have been gained more attention in the field of membrane surface modification. Hu et al. grafted the hyperbranched polyglycerol (HPG) onto the thin film composite membranes by esterification reaction, which improved the permeability and anti-fouling performance of the membrane.26 Kang et al. prepared HPG coated stainless-steel substrates, which reduced the adhesion of proteins and microorganisms and inhibited the formation of biofilms.27 While except for HPG, the kinds of hyperbranched polymers were relatively homogeneous. Reversible addition fragmentation chain transfer (RAFT) polymerization as a controllable polymerization could be applied to prepare structurally controllable hyperbranched polymers, and the end of RAFT chain transfer agent could also evenly enrich at the terminal of the hyperbranched polymer chains.28 But, to the best of our knowledge, rare studies decorate special 3D hyperbranched polymers prepared by RAFT on PVDF membranes for oil-in-water emulsion separation. Zwitterionic polymers were usually coated onto the surface of hydrophobic materials to improve their hydrophilicity and resistance to protein adhesion.29-31 The negatively and positively charged units were beneficial for generating more stable hydration layer via electrostatic interaction and hydrogen binding interaction. Benefiting from these above attractive attributes, herein, the approach of mussel adhesion chemistry employed the RAFT


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mediated nano-sized hyperbranched zwitterionic polymers as postmodification reagents was used to prepare modified PVDF membranes with hydrophilicity and underwater superoleophobicity

for

oil-in-water

emulsion

separation.

Therefore,

well-defined

hyperbranched zwitterionic poly(sulfobetaine methacrylate) (HPS) polymer was firstly prepared by RAFT polymerization. The thiolated HPS as a kind of surface modifier was coated on PD modified PVDF (M-PD) membrane via Michael addition and thiol-thiol oxide self-crosslinking reaction (Scheme 1). The effect of thiol-terminated HPS on the surface structure and property of PD/HPS modified PVDF (M-PD/HPS) membranes was discussed in detail. The oil-in-water emulsion separation property and the anti-fouling performance of the M-PD/HPS membrane were also investigated. Scheme 1. Fabrication of Dopamine-Inspired Thiolated Hyperbranched Zwitterionic Polymers Modified PVDF Membranes for Efficient Oil-in-Water Emulsion Separation.


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2. EXPERIMENTAL SECTION 2.1. Materials. S-(4-Vinyl) benzyl S'-propyltrithiocarbonate (VBPT) was prepared by our lab according to the previously literature procedure and VBPT structure was confirmed by 1

H NMR (Figure S1).32 2-(Dimethylamino) ethyl methacrylate (DMAEMA, 98%, Tianjin

Heowns Biochem Technologies Co. Ltd) was purified via basic alumina gel column chromatography. The PVDF membranes with the average pore size of 0.45 µm were purchased by Zhejiang Kertice Hi-tech Flour-material Co. Ltd. Sodium dodecyl sulfate (SDS), tris(hydroxymethyl)aminomethane (Tris-HCl), dopamine hydrochloride, ethylenediamine, and 1,3-propanesultone were provided by Tianjin Heowns Biochem Technologies Co. Ltd. Toluene, dichloromethane, hexane, petroluem ether, and triethylamine were obtained from Tianjin Kemiou Chemical Co. Ltd. In addition, dialysis tube (MWCO 3500) was supplied by Shanghai Greenbird Technology Co. Ltd. 2.2. Synthesis of Thiolated HPS. According to the previous research,33,34 the hyperbranched poly(2-(dimethylamino)ethyl

methacrylate)

(HPD)

was

synthesized

via

RAFT

copolymerization of DMAEMA with VBPT (Scheme 2). The typical polymerization procedure was as follows. VBPT (0.29 g, 1.08 mmol), DMAEMA (5.02 g, 32 mmol), AIBN (17.74 mg, 0.11 mmol), and toluene (25 mL) were dissolved in a Schlenk tube (100 mL). After three cycles of freeze-pump-thawing, the mixture under argon atmosphere was reacted at 70 °C for 48 h. The reaction flask was cold down at 25 °C and the reaction solution was precipitated with cold hexane to remove residual monomer and solvent. Finally, yellowish HPD after drying in a vacuum oven at 45 °C for 48 was obtained with the yield of 93.0%.


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Afterwards, 1,3-propanesultone (9.1 g, 74 mmol) and the crude product HPD (3.9 g) were added into methanol (65 mL) to prepare HPS via sulfonation reaction at 60 °C for 48 h. In the process of reaction, HPS would be precipitated out and dried in a vacuum oven at 45 °C for 48 h. In the dark condition, the obtained HPS (1.7 g) was dissolved in 40 mL of deionized water contained with ethylenediamine (15 mL) at room temperature and magnetic stirred for 24 h. Deionized water was used as dialysate and the reaction solution was dialyzed to remove solvents and reaction byproducts. The thiolated HPS as a white products were obtained after lyophilization. Scheme 2. Synthetic Route for Thiolated HPS

2.3. Fabrication of the M-PD/HPS Membranes. The pristine PVDF (M-PVDF)


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membranes were cleaned in ethanol for 10 min and kept in deionized water before used. After pre-wetted by ethanol, the M-PVDF membranes were dipped into dopamine solution (25 mL, 2 mg/ml, pH = 8.5) and shaking at 25 °C for 12 h. The dopamine coated M-PVDF (M-PD) membranes were obtained after washing by ethanol and deionized water for a designed time. Afterwards under nitrogen atmosphere, the thiolated HPS and M-PD membranes were placed into conical flask contained with 25 mL Tris buffer solution (pH = 8.5, 50 mM). The moderate triethylamine was added into conical flask, which was ashaked for 12 h at 25 °C. The HPS modified M-PD (M-PD/HPS) membranes were obtained after washing by deionized water. 2.4. Characterizations. A Bruker AVNCE AV 400 spectrometer was used for 1H NMR measurements using CDCl3 or D2O as solvents. Molecular weight distribution (Mw/Mn) was measured by GPC (Waters 1515, USA) fitted with a refractive index detector and monodispersed polystyrene standards were used. The eluent was tetrahydrofuran and the flow rate was 1.0 mL/min. Dynamic Light Scattering (DLS) measurement was conducted on a BI-200SM (Brookhaven, USA) at 25 oC. The coating yields (μg/cm2) of M-PD membranes and M-PD/HPS membranes were calculated by gravimetric analysis. The chemical compositions of as-prepared membrane surfaces were characterized using X-ray photoelectron spectroscopy (XPS, K-alpha, USA). Field Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800, Japan) were used to characterize the membrane surface morphology. The element mappings of the M-PD/HPS membrane were tested using energy dispersive X-ray spectroscopy (EDX, Hitachi S-4800, Japan). The surface roughnesses of different membranes were carried out on a atomic force microscope (AFM, Veeco,


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NanoScope IIIa Multimode AFM). The contact angles and the dynamic underwater oil adhesion behaviours were recorded at 25 °C by a contact angle instrument (DSA, Kruss GmbH, Hamburg, Germany). The corresponding underwater adhesive force of the M-PD/HPS membrane towards dichloromethane oil droplet (3.0 µL) was evaluated by a micro-electro-mechanical balance system (Data-Physics DCAT11, Germany). The optical microscopy (Nikon, Japan) was used to record the photographs of the feed and filtrate solution. Total organic carbon analyzer (TOC, Shimadzu, Japan) was applied to monitor the oil content of the filtrate solution. 2.5. Preparation of Oil-in-Water Emulsions. The 10 ml of organic solvent and 200 mg of SDS were mixed with 990 mL of deionized water. The mixture was stirred at 1300 rpm for 1 h. And different SDS stabilized oil (hexane, petroleum ether, toluene, and dichloromethane) in water emulsions were also prepared. All the emulsions were stabilized in laboratory. 2.6. Oil-in-Water Emulsion Filtration Experiments. Water permeation flux for oil-in-water emulsion separation was a crucial role to evaluate permeation property. 50 mL of oil-in-water emulsion were permeated through the M-PVDF membrane, M-PD membrane and M-PD/HPS membrane for each separation cycle. The water permeation flux (J) was calculated via flowing equation: J

V A  t

where ∆t (h) was the filtration time, V (L) was the volume of penetrate flow, and A (m2) was the membrane effective area.

3. RESULTS AND DISCUSSIONS


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3.1 Synthesis and Characterization of Thiolated HPS. Herein, we aimed to synthesize water-soluble thiolated HPS with well-defined chemical structure as a nanoscale surface modifier for fabricating underwater superoleophobic PVDF membranes for oil-in-water emulsion separation. As schematically showed in Scheme 2, the hyperbranched HPS was first synthesized via a process of RAFT copolymerization and subsequent sulfonation reaction. During RAFT copolymerization, VBPT was used as a polymerizable RAFT agent to prepare hyperbranced copolymers.34 Finally, the thiolated HPS was obtained by aminolysis reaction.32,35

Figure 1. (A) 1H NMR spectrum of HPD in CDCl3, (B) GPC trace of HPD, and 1H NMR spectra of HPS (C) and thiolated HPS (D) in D2O.


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The structure of the resulted HPD precursor was measured by 1H NMR and GPC, respectively. The typical resonance signal of HPD (Figure 1A) appeared at 4.10 ppm (g), 2.65 ppm (h), and 2.42 ppm (i), respectively.33 Meanwhile, the characteristic peaks of propyl group and H nuclei neighboring carbon in benzene ring of RAFT agent were observed at 3.45 ppm (d) and 7.0 ppm (c), respectively.30 The branch length for HPD was calculated by comparing the integration value of the peak of the hydrogen protons (g) of DMAEMA with the value of the methylene protons (c) of RAFT agent.34 The molar ratio of DMAEMA to VBPT in the HPD was measured as 29.5, which was quite closed to the feed ratio of 30.0, indicating the high yield of HPD and high conversion of DMAEMA and VBPT. As shown in Figure 1B, the GPC curve of HPD was unimodal and the narrow Mw/Mn was 1.33. Therefore, the above 1H NMR spectrum and GPC results confirmed that near-adjustable branch length HPD had been successfully prepared by RAFT copolymerization. The chemical structure of HPS was also measured by 1H NMR (Figure 1C). As a comparison with Figure 1A, a series of new characteristic signals at 2.90 ppm (l), 2.25 ppm (k), and 3.50 ppm (m) attributed for the methylene protons of zwitterionic pendant groups of HPS were readily observed.29 These results confirmed that the HPS were successfully prepared by sulfonation reaction of HPD with 1,3-PS. After aminolysis reaction, the faint yellow HPS aqueous solution was turned into colorless, demonstrating the trithiolcarbonate terminals of HPS were disappeared and changed to thiol groups.35 This experimental result could be further verified by the 1H NMR spectrum of obtained thiolated HPS (Figure 1D) due to the characteristic peak of propyl group of RAFT agent was not appeared. The zwitterionic


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degree of HPS determined by the comparing the characteristic peak area of the hydrogen protons (g') with the value for the methylene protons (g) was about 65.2%, and this lower value was caused by the fact that the resulting HPS were insoluble in methanol solution in the proceeding of zwitterionicalization.30,36 The size distribution of the water-soluble HPS (1.0 mg/mL in water) was determined by DLS (Figure S2). The average diameter of HPS was about 15.2 nm with a size distribution index of 0.18. DLS result showed that single molecular HPS in water exhibited a 3D architecture due to its dendritic architecture and high solubility.28 Therefore, when this nanoscale hydrophilic HPS was used for membrane surface modification, it would not only improve the membrane surface hydrophilicity, also enhance the surface roughness. Both of them were two essential factors to design a hydrophilic PVDF membrane with superhydrophobicity in practical oil-in-water emulsion separation.37 Table 1. Surface elemental composition of the M-PVDF membrane, M-PD membrane and M-PD/HPS membrane determined by XPS. Coating yield

Surface elemental composition (at.%)

Atomic ratio

Membrane

(μg/cm2)

C

F

O

N

S

O/F

S/F

M-PVDF

0

57.0

36.1

4.96

1.95

0

0.13

0

M-PD

113 ±8.00

59.0

27.2

9.81

3.98

0

0.36

0

M-PD/HPS

25.2 ±2.83

62.6

18.7

11.6

5.60

1.51

0.62

0.08

3.2. Membrane Surface Chemical Composition and Morphology. The surface chemical composition and top surface morphology of the M-PVDF membrane, M-PD membrane and


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M-PD/HPS membrane were characterized by XPS, FE-SEM, EDX, and AFM, respectively. The XPS survey scans of the different membranes were shown in Figure S3 and the datas from detailed XPS scans of the different membrane surfaces were summarized in Table 1. Compared with the M-PVDF membrane, it could be observed for M-PD membrane that the composition of oxygen (O 1s) and nitrogen (N1s) increased, while the composition of fluorine (F 1s) decreased, indicating the PD functional layer was deposited on the surface of M-PVDF membrane. The catechol groups contained in PD layer could react with thiolated HPS via Michael addition reaction.38 After M-PD membrane modified by thiolated HPS, a new peak of sulfur (S 2p3 at 167 eV and S 2s at 230 eV) ascribed to -SO3 and disulfides formed by molecular cross-linking between the thiol moieties were detected, which were certainly attributed to the surface self-crosslinked HPS layer.27,29 More significantly, the surface content of oxygen element increased from 9.81% for M-PD membrane to 11.6% for M-PD/HPS membrane and the percentage of fluorine element decreased dramatically from 27.2% for M-PD membrane to 18.7% for M-PD/HPS membrane. As a result, the O/F ratio of 0.36 for M-PD membrane was increased distinctly to 0.62 for M-PD/HPS membrane, revealing the successful formation of HPS coating on the surface of M-PD membrane. The successful coating of PD and HPS on M-PVDF membrane surface confirmed by XPS would affect the surface morphology and surface roughness. As shown in Figure 2, the surface rough structure of micro-nano morphologies of the M-PVDF membrane, M-PD membrane and M-PD/HPS membrane were detected by FE-SEM and their surface roughnesses were also determined by AFM (Figure 3). For the M-PVDF membrane, smoother


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and cleaner surface was observed. As we expected, the obvious PD nano particles with the size about 10 ~ 150 nm were detected on the M-PD membrane surface and the surface average mean roughness (Ra) increased from 296 nm for M-PVDF membrane to 416 nm for M-PD membrane.31 After thiolated HPS coating, the surface nanoparticles of M-PD/HPS membrane were still existed. Interestingly, the Ra of M-PD/HPS membrane was about 609 nm, which was bigger than that of M-PD membrane. Compared with the M-PD membrane, the increase Ra for M-PD/HPS membrane was attributed to the fact that the nano-sized HPS coating would enhance surface roughness (Figure 3).27 Most importantly, PD/HPS coating did not obviously effect the pore structure of the M-PD/HPS membrane and there was no pore blockage, resulting in a minimal negative effect on membrane water permeability.31

Figure 2. FE-SEM surface images of M-PVDF membrane, M-PD membrane and M-PD/HPS membrane. (left, 2000 ×; middle, 10 000 ×; and right, 20 000 ×).


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Figure 3. AFM photographs for M-PVDF membrane, M-PD membrane and M-PD/HPS membrane and the surface roughness (Ra) values were shown under each image. In order to verify whether the thiolated HPS was uniformly distributed on the surface of M-PD/HPS membrane, the EDX mappings of the M-PD/HPS membrane were investigated.39 As shown in Figure S4, various elements, including C, F, O, N, and S, were uniformly distributed on the membrane. Among these elements, S element was a typical element in HPS. Therefore, it may be concluded the HPS was uniformly coated on the M-PD/HPS membrane surface via mussel-inspired surface chemistry and thiol-thiol self-crosslink reaction.27 3.3. Membrane Wettability. The water contact angle (WCA) was tested to demonstrate the surface wetting property of M-PVDF membrane, M-PD membrane and M-PD/HPS membrane. Figure 4A displayed the initial WCA of M-PVDF membrane was around 136.9º and nearly no change after 60 s, which showed the intrinsic hydrophobic property. The initial WCA of M-PD membrane was about 109.2ºand the WCA gradually declined to 96.4ºwithin 60 s. The hydrophilic property of M-PD membrane could be increased after decorating PD because of the membrane surface hydrophilic amino and hydroxyl groups in PD, but the M-PD membrane was not hydrophilic.31 Compared with M-PVDF membrane and M-PD membrane, the M-PD/HPS membrane exhibited improved hydrophilicity with initial WCA of


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52.7ºand the WCA reached 0ºwithin about 40 s. This result indicated M-PD/HPS membrane with strong hydrophilic HPS surface layer and micro-nano morphology exhibited enhanced hydrophilic property.40

Figure 4. (A) Water contact angles and (B) underwater oil contact angles of different oil types towards M-PVDF membrane, M-PD membrane and M-PD/HPS membrane, and (C) dynamic underwater oil resistant adhesion behavior of M-PD/HPS membrane. On the basis of the “Cassie-Baxter” theory, an excellent surface hydrophilic performance could endow membrane with underwater superoleophobicity.41 To demonstrate underwater superoleophobic property of M-PD/HPS membrane, 3.0 µL oil droplet (hexane, toluene, dichloromethane and petroleum ether) was dropped on the top surface underwater. The underwater OCA values of different oil droplets towards M-PVDF membrane and M-PD membrane (Figure 4B) were all below 150º. However, the OCA values of hexane, petroleum


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ether, toluene and dichloromethane on M-PD/HPS membrane were 160º± 1.7º, 156º± 3º, 162º ± 2.5º, and 159º ± 2.2º, respectively, and no obvious oil traces were observed, demonstrating

that

the

M-PD/HPS

membrane

exhibited

outstanding

underwater

superoleophobicity of different oil. Underwater oil droplet adhesion performance of M-PD/HPS membrane was measured using dichloromethane as oil phase. The oil droplet was firstly compelled to entirely contact with the surface of the M-PD/HPS membrane in the water environment (Figure 4C). Once the oil droplet was lifted up, it could gradually separate with membrane and keep the original shape. The corresponding underwater adhesive force of M-PD/HPS membrane towards dichloromethane oil droplet (3 µL) was as low as 50 µN (Figure S5). In addition, there was no oil trace on the membrane surface, proving the hydrophilic and underwater superoleophobic M-PD/HPS membrane exhibited low oil adhesion behavior. This was ascribed to that the water was trapped in the micro/nanostructures formed by HPS coating via electrostatic interaction to form the hydration layer on the surface of the M-PD/HPS membrane, which would restrict oil droplet to contact with the membrane surface. 3.4. Oil-in-Water Emulsion Separation. The enhanced hydrophilicity of M-PD/HPS membrane surface would reduce the resistance of water through the membrane pore and increase water flux.26 The water permeation fluxes were measured by pressure assisted method. As showed in Figure 5A, by contrasted with M-PVDF membrane, the water permeation flux for M-PD/HPS membrane increased from 6333 L m-2 h-1 to 10435 ± 270 L m-2 h-1. According to the result, we could summarize the membrane surface coating of HPS


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could improve water permeation flux.

Figure 5. (A) The pure water flux, (B) water flux and rejection ration, and (C) the oil concentration of permeated water of hexane-in-water emulsion treated by the pure M-PVDF membrane, M-PD membrane and M-PD/HPS membrane, and (D) water flux and rejection ratio values of different oil-in-water emulsions separated by M-PD/HPS membrane. The

M-PD/HPS

membrane

exhibited

outstanding

hydrophilicity,

underwater

superoleophbicity, and low oil adhesion behavior, which could be applied in separating water from oil-in-water separation process.43 To evaluate the permeation and separation performance, different oil-in-water emulsions were filtrated in a vacuum filter (0.1 MPa). The water fluxes of hexane-in-water emulsion for the pure M-PVDF membrane, M-PD membrane


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and M-PD/HPS membrane were measured and showed in Figure 5B. As we expected for M-PVDF membrane, the water flux was relatively low and this result was in accordance with intrinsic hydrophobic property. For M-PD/HPS membrane, the water flux reached 4393 ± 524 L m-2 h-1 and corresponding rejection ratio was over 99.9%, indicating that M-PD/HPS membrane could separate the oil with higher permeation flux and rejection ratio.44,45 Figure 5C showed the oil concentration of hexane-in-water emulsion in filtrate measured by TOC. The theoretical value of the oil concentration in feed was 10 000 ppm. It could be seen that the TOC value in filtrate solution for M-PD/HPS membrane was in the range of 4.5 ± 0.8 ppm and was obviously lower than in feed, demonstrating excellent separation performance of M-PD/HPS membrane for oil-in-water emulsions.

Figure 6. Optical microscopy images of the hexane-in-water emulsion before and after treated by M-PVDF membrane, M-PD membrane and M-PD/HPS membrane. To further evaluate the anti-fouling and separation property of M-PD/HPS membrane, the digital photos of hexane-in-water emulsion and optical microscopies of the feed and permeate solutions were displayed in Figure 6. It could be seen that the feed emulsion was milky white and the permeation solution turned to transparent. The oil droplet size of hexane-in-water


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emulsions determined by optical microscopy was about 0.1 ~ 10 µm in feed emulsion. While, there were almost no oil droplets in the whole image of the permeation solution, proving the oil phase was prevented to permeat in the filtrate solution. Although the filtrate solutions treated by different membranes were transparent, the separation performance for M-PVDF membrane, M-PD membrane and M-PD/HPS membrane was different. Figure 5D showed that the permeation flux for toluene-in-water emulsion, petroleum-in-water emulsion, and dichloromethane-in-water emulsion treated by M-PD/HPS membrane were 3672 ± 352 L m-2 h-1, 3473 ± 300 L m-2 h-1, and 1708 ± 269 L m-2 h-1, respectively. The lower water flux of dichloromethane-in-water emulsion than other three kinds of oil-in-water emulsions was because of that it was difficult for water to get across the oil layer during separation process due to the density of dichloromethane bigger than that of water. The optical microscopy images

of

toluene-in-water

emulsion,

petroleum-in-water

emulsion

and

dichloromethane-in-water emulsion before and after filtration were also showed in Figure S6, indicating the M-PD/HPS membrane was endowed excellent permeation flux and high rejection ratio for different oil-in-water emulsion separation. Meanwhile, the anti-fouling capability and durability of M-PD/HPS membrane were crucial as well as permeation and rejection during a process of oil-in-water emulsion separation. The cyclic experiment could evaluate anti-fouling performance of M-PD/HPS membrane, the hexane-in-water emulsion was separated in three-cyclic experiments and showed in Figure 7. The deionized water was used to wash with the membrane for five minutes after each cycle. The flux dramatically declined due to the compressed oil droplets layer formed on the surface


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of M-PD/HPS membrane under transmembrane pressure.40,42 However, the water permeation flux would be resumed the initial extent and the flux retained a relative high level of 4300 L m-2 h-1 after washing. The high water flux recovery indicated the M-PD/HPS membrane showed excellent anti-fouling capability and reusability for a long time using in practise oil-in-water separation.

Figure 7. Flux recovery of the M-PD/HPS membrane for hexane-in-water emulsion treated under 0.1 MPa.

4. CONCLUSIONS In summary, the nanoscale hyperbranched zwitterionic HPS with well-defined structure was synthesized by RAFT copolymerization and thiol-terminated HPS was employed as a functional surface modifier to prepare hydrophilic PVDF microfiltration membrane with underwater superoleophobicity via mussel-inspired surface chemistry. In comparison to the pure membrane, the obtained HPS modified membrane could selectively separate different oil-in-water emulsions with dramatically high flux and high separation efficiency (higher than 99.9%). Meanwhile, the anti-fouling property and recyclability of the HPS modified


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membranes allowed them to be used for a long-term separation process. This simple and controlled mimetic coating strategy using RAFT mediated nano-sized hyperbranched zwitterionic polymers as modifiers showed a great promise in fabricating multifunctional membranes.

■ ASSOCIATED CONTENT Supporting Information 1

H NMR spectrum of VBPT in CDCl3. Size and size distribution of HPS (1.0 mg/mL in water)

determined by DLS. XPS wide-scan spectra of the M-PVDF membrane, M-PD membrane and M-PD/HPS membrane. EDX scan surface images of the M-PD/HPS membrane. Purple, orange, green, pink and light purple dots denoted C, F, O, N, and S element, respectively. Adhesion force curves of underwater dichloromethane contacted with the surface of M-PD/HPS membrane. Optical microscopy images of the hexane-in-water emulsion before and after treated by M-PD/HPS membrane. Performance comparison of the M-PD/HPS membrane with other reported filtration membranes for oil/water emulsion separation. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Z.). *E-mail: [email protected] (L. C.). Author Contributions †J. Z. and D. L. contributed equally to this work.


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Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was funded by the Natural Science Foundation of Tianjin (18JCQNJC72800, 18JCQNJC83900, and 15JCYBJC17900), Tianjin Education Scientific Research Projects (2017KJ071), National Natural Science Foundation of China (81502624, 51773152 ， 21174103, 31200719, and 51303129), National Undergraduate Training Program for Innovation and Entrepreneurship (201510058001), Program for Innovative Research Team in University of Ministry of Education of China (IRT-17R80), Program for Innovative Research Team in University of Tianjin (TD13-5044), and the Science and Technology Plans of Tianjin (17PTSYJC00040 and 18PTSYJC00180).

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Hyperbranched Zwitterionic Polymers Functionalized Underwater

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