A silane-based interfacial crosslinking strategy to design PVDF membranes with versatile surface functions

Journal of Membrane Science 520 (2016) 769–778

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

A silane-based interfacial crosslinking strategy to design PVDF membranes with versatile surface functions Yunze Wang a,b,1, Haibo Lin a,1, Zhu Xiong a, Ziyang Wu a, Yi Wang a, Lingchao Xiang a, Aiguo Wu a, Fu Liu a,n a b

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 May 2016 Received in revised form 17 August 2016 Accepted 18 August 2016 Available online 25 August 2016

An up-scaling strategy to produce multi-functional PVDF membrane is highly desirable for both academia and industries. It is now reported that a novel silane-based interfacial crosslinking technique was developed to endow PVDF membrane with versatile superwettability, excellent protein desorption and antibacterial properties respectively. Prepolymers were first synthesized through the free radical copolymerization between crosslinker triethoxyvinylsilane (VTES) and N-Vinyl-2-pyrrolidone (NVP), oligo (ethylene)glycol methacrylate (OEGMA), dimethyl aminopropyl methacrylamide (DMAPMA). And then the superhydrophobic PVDF membrane was facilely modified by silane-based interfacial crosslinking of the corresponding prepolymer. PVP-VTES crosslinked PVDF membrane showed reversible separation performance for both oil-in-water and water-in-oil emulsions. POEGMA-VTES crosslinked PVDF membrane exhibited excellent BSA desorption. The halo zone test verified the good elimination to Escherichiacoli (E. coli) for the PDMAPMA-VTES crosslinked PVDF membrane. FTIR, XPS, TGA, SEM and laser scanning confocal microscope were utilized to investigate the chemistry and morphology of functional layer. All results demonstrate that versatile PVDF membranes with multiple functions can be facilely designed by the efficient interfacial crosslinking strategy. & 2016 Elsevier B.V. All rights reserved.

Keywords: PVDF membrane Interfacial crosslinking Superwettability Protein desorption Antibacterial

1. Introduction Polyvinylidene fluoride (PVDF) membranes have been extensively investigated and applied in versatile separation processes e.g. sewage treatment [1,2], drinking water [3,4] and oil/ water separation [5,6] due to the excellent chemical and physical stabilities [7]. PVDF membranes can be feasibly endowed with specific surface virtues to satisfy the corresponding requirements. Superwettability e.g. superhydrophilicity and superoleophobicity under water, or superoleophilicity and superhydrophobicity under oil is essential to achieve the efficient separation of oil-in-water or water-in-oil emulsions via the superwetting PVDF membrane [8,9]. Besides, for water or wastewater treatment applications, PVDF membranes with anti-fouling surface are commonly recognized to overcome the serious fouling and reduce the energy consumption and cleaning cost caused by frequent washing, backwashing or chemical washing. Usually, the hydrophilic surface modification is very effective to improve the fouling resistance to n

Corresponding author. E-mail address: [email protected] (F. Liu). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.memsci.2016.08.029 0376-7388/& 2016 Elsevier B.V. All rights reserved.

organic matters and proteins [10–15]. Antibacterial property is of great significance for application of medical membranes and drinking water treatment membranes [16,17]. The bio-bacterial formation on the inactive membrane surface induces the bacterial infection and increases the drinking water security risk. The versatile functions of the PVDF membranes are mainly determined by the surface or interface properties. Therefore the various interface design is widely focused to gain necessary properties on polymeric membranes. Several principles are needed to be paid attention: 1) high efficiency of realizing the purposed function, 2) long term stability during service cycle, 3) feasibility and easy scaling up, 4) less negative effects on the land environment, aquatic ecosystems and human health. So far, various approaches have been employed to introduce different chemical groups and physical morphologies into PVDF membranes, such as blending [18,19], surface coating [20–22] and surface grafting [23,24]. However, none of all techniques can satisfy all above criteria. The additional components involved in blending usually alter the phase inversion mechanism and the morphology. In addition, the compatibility difference between bulk polymer and additives will cause the membrane performance deterioration due to the inevitable elution of additives [18,24]. Surface coating can only work well in a short term due to the weakly physical

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adsorption [20,23]. Surface chemical grafting via UV, electron beam etc. is restricted to be scaled up due to the process complexity. To simplify the surface grafting process and overcome the instability of surface coating, quaternization crosslinking was adapted to endow polypropylene membrane with good hydrophilicity or positive charges. Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) can be introduced into polypropylene or polysulfone microporous membranes by selecting ρ-xylylene dichloride for quaternization crosslinking to form a positively charged coating layer, which significantly enhanced the hydrophilicity or CO2 permeability [25,26]. Besides, polyethylenimine (PEI) can also be formed on polypropylene microporous membranes via the crosslinking of ρ-xylylene dichloride and subsequent quaternization with iodomethane [27]. However, the membrane needs to be pretreated and activated by the plasma to produce the free radicals, which may compromise the nascent membrane structure or mechanical stability. PVA coating can also be generated on membrane surface via the glutaraldehyde crosslinking to improve the hydrohilicity and fouling resistance to some extents [28,29]. Different from the previously reported strategy e.g. blending, surface grafting, surface coating and quaternization crosslinking, we aim to develop a novel silane-based interfacial crosslinking to produce functional PVDF membrane with superwetting behavior for oil/water emulsion separation, excellent protein desorption and bacterial inhibition respectively. The precopolymers based on silane coupling agent VTES were first synthesized through free radical copolymerization and then applied to modify pristine PVDF membrane via simple interfacial crosslinking strategy. The modification is simple and environmentally benign and no particular pretreatment is needed on the membrane. In our previous work, we have realized the direct in-situ crosslinking of PVP-VTES in PVDF casting solution [30]. To our best knowledge, this is the first time to unveil the feasibility, mechanism and versatility of interfacial crosslinking of PVP-VTES, POEGMA-VTES, PDMAPMA-VTES on superhydrophobic PVDF membrane.

2. Experimental section 2.1. Materials Triethyl phosphate (TEP), Vinyltriethoxysilane (VTES) and citric acid monohydrate were provided by Sinopharm Chemical Reagent Co. Ltd., China, 1-Vinyl-2-pyrrolidone (NVP, 99%), oligo(ethylene glycol) methacrylate (OEGMA) (Mn ¼475 g/mol), 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 99%), 2,2′-Azobis(2-methyl propionitrile) (AIBN, 99%), fluorescein isothiocyanate (FITC), triethoxyvinylsilane (VTES), Congo red and bovine serum albumin (BSA, Mw¼67 kDa, 96 wt%) were purchased from Shanghai Aladdin Chemistry Co. Ltd., China. Benzyl chloride (BC, 99%) was supplied by J&K Scientific Ltd. Escherichia coli (E. coli) and other bacterial culture materials were supplied by nano biomaterials group, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences. Superhydrophobic PVDF membrane with the contact angle 150° was produced and selected as the control pristine membrane M0. Casting solution containing 15 wt% PVDF (Kynar 761-A, Arkema) in 85 wt% TEP was casted onto non-woven fabric (NWF, 90 g/m2 ) by a casting knife with the thickness of 300 mm to form the nascent membrane through phase separation, and the nascent membrane was immediately immersed into 25 °C coagulation bath composed of TEP/water mixture (v/v: 5/5) for 5 s, and then moved to pure water bath for complete solidification. Afterwards, the membrane was entirely stripped off from the support nonwoven fabric to obtain the superhydrophobic PVDF membrane

Table1 The synthetic recipe of the precopolymers. Precopolymer

M1 M2 M3

Recipe of synthesis solution (g) NVP

OEGMA

DMAEMA

VTES

AIBN

3.75 0 0

0 4.5 0

0 0 4.0

2.75 1.0 1.6

0.08 0.06 0.07

with the hierarchical surface. The detailed preparation of membrane M0 can refer to our previous work [8,31]. 2.2. Synthesis of functional precopolymer The functional precopolymers were first synthesized through the free radical polymerization. We first dissolved the certain amount of functional monomer (NVP, OEGMA, DMAEMA) and crosslinker (VTES) into TEP at 25 °C, and added AIBN to initiate the polymerization at 60 °C for 24 h. The resulted precopolymer solutions (PVP-VTES, POEGMA-VTES, PMAEMA-VTES) were applied to prepare the superhydrophilic, anti-fouling, antibacterial PVDF membrane marked as M1, M2 and M3 respectively. The synthetic recipe of the precopolymers was seen in Table 1. In order to obtain the synthesized copolymer, the precopolymer solutions were mixed with equal amount of water at 60 °C for 48 h to complete the VTES segment thermal hydrolysis and coupling, and then dried in vacuum at 80 °C to obtain the crosslinked polymer for further characterization. 2.3. Interfacial crosslinking on PVDF membrane The synthesized precopolymer solution was diluted by adding equivalent amount of deionized water, and then the superhydrophobic membranes were immersed in the diluted solutions for 3 h respectively. Subsequently, the membranes were immersed in 1 wt% citric acid solution for 24 h at 60 °C, and then moved into pure water for 48 h at room temperature to eliminate the residual monomers and solvent, finally the modified PVDF membranes were dried in air. In the immersion process, the precopolymer was adsorbed onto the membrane surface via the swelling of diluted solvent. The addition of citric acid boost the hydrolysis, condensation and crosslinking between silane segments (-Si-O-Si-). Finally, the functional precopolymer was crosslinked and immobilized firmly on the textured PVDF membrane surface. The interfacial crosslinking mechanism was illustrated in Fig. 1. It is worth noting that the pH of immersion solution of M3 was adjusted to 10 for adequate surface crosslinking of PDMAPMA-VTES.

Fig. 1. The interfacial crosslinking strategy to accomplish the superwetting, antifouling and antibacterial properties.

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Fig. 2. Schematic illustration of quaternization of PDMAEMA by benzyl chloride (BC).

The crosslinking can be boosted and prompted under pH 10 through the hydrolysis and condensation. M3 membrane was further quaternized by benzyl chloride (BC). 0.1 g (5.0 cm  5.0 cm) of membrane was immersed into in a mixture of 10 mL acetone and 1.5 g BC, and vibrated at 25 °C for 16 h for complete reaction. Subsequently the membrane was fully cleaned with acetone and deionized water and dried in air. The quaternized membranes were named as M3-Q. The quaternization process of PDMAEMA was described in Fig. 2. 2.4. Membrane chemical composition characterization Attenuated total reflectance fourier transform infrared spectra (ATR-FTIR) of control membrane (M0) and the modified PVDF membranes (M1, M2 and M3) were measured by Thermo-Nicolet 6700 FTIR spectrometer (US) over a range of 4000–400 cm  1. X-ray Photoelectron Spectroscopy (XPS) was conducted to analyze the membrane composition by using XPS (AXIS UTLTRADLD, Japan), which Al-Kα was as radiation resource. The take-off angle of the photoelectron was set at 90°. Thermo-gravimetric Analysis (TGA, Mettler Toledo, Switzerland) was carried out to calculate the copolymer content in the membrane at a heating rate of 10 °C  min  1 over a range of 80 °C∼700 °C under nitrogen atmosphere. 2.5. Membrane morphology characterization The membrane was sputtered with gold before morphology observation by Scanning Electron Microscopy (SEM, Hitachi S-4800, Japan). The 3D roughness images of the membranes were observed through laser scanning confocal microscope (Zeiss LSM 700). 2.6. Water Flux of Membrane The membrane (effective area was 6.15 cm2) was operated for 1 h under a pressure of 0.15 MPa for compromising the compacting effect. Then the pure water flux was measured 5 times every 5 min at 0.1 MPa. The pure water flux was defined according to the following equation:

J=

V At

(1)

Where V was the permeate volume (L), A was the membrane area (m2) and t was the operation time (h).

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2.7.2. Emulsion separation test For water-in-toluene (W/T) emulsions, 114 mL toluene, 1.0 mL of water were mixed with 0.5 g Span80 (HLB ¼4.3) as emulsifier. For toluene-in-water (T/W) emulsions, 0.6 g Tween80 (HLB ¼15) as emulsifier and 4 mL toluene were mixed 120 mL water. The mixtures were stirred violently for 3 h to produce emulsions that were stable for at least 3 h, and no demulsification or precipitation was observed. The two kinds of sizes of W/T and T/W emulsions were 2–20 mm and 1–30 mm respectively [31]. Optical microscopy images were obtained on a BX 51 TF Instec H601 (Olympus, Japan). Meanwhile, the size of T/W emulsion of before and after separation was measured by Laser particle size analyzer (S3500). M1 membrane was sealed between a vertical glass tube with effective area 3.14 cm2 and a conical flask. A certain volume of emulsion was poured onto the membrane under the vacuum ( 0.09 MPa) by the laboratory filtration system. At least 5 samples were assessed to obtain an average value for each emulsion. The W/T filtrates finally obtained were collected and evaluated its purity by UV–VIS–NIR spectrometer (Lambda 950, PerkinElmer, USA). The purity of the purified T/W filtrates was analyzed by Karl Fischer titrator (Mettler Toledo DL, Switzerland). The rejection of emulsions was defined according to the following equation:

⎛ C ⎞ R(%) = ⎜ 1 − 1 ⎟ × 100% C0 ⎠ ⎝

(2)

Where C1 represented the purity of the filtrate, and C0 represented the purity of the feed. 2.7.3. Acid and alkali treatment stability The membranes were placed in aqueous solution with pH 2 and pH 12 for 24 days, respectively. The contact angle of the sample was tested and recorded every 6 days. 2.8. Characterization of anti-fouling M2 2.8.1. FITC-labeled BSA adsorption test BSA (10.0 mg) and FITC (1.0 mg) were dissolved in NaHCO3 buffer solution (0.1 M, pH 9) at 25 °C. After 3 h, the solution was dialyzed against PBS buffer solution (pH 7.4) for 3 days at 4 °C to remove the unreacted FITC. Then the purified BSA-FITC solution was diluted with PBS buffer solution to a concentration of 20 mg mL  1 and stored at 4 °C prior to use [32]. The M0 and M2 (1.5  1.5 cm2) were separately immersed into 10 mL of the BSAFITC solution in a beaker and shaken in darkness at 25 °C for 12 h. Subsequently, the samples were thoroughly washed at least 5 times with PBS buffer solution to remove the loosely absorbed BSA-FITC. The rough surfaces of membrane samples were observed by Laser scanning confocal microscope (Leica TCS SP5). 2.8.2. Static BSA adsorption experiment The experiments were measured in physiological saline. The 3 cm  3 cm M0 and M2 were thrown into 50 mL the BSA solution of 1 g/L in physiological saline (0.9 wt%) at 25 °C. The membranes were maintained for 36 h for adsorption equilibrium. Moreover, the BSA concentration of the solution was measured every 12 h by UV–VIS–NIR spectrometer (Lambda 950, Perkin Elmer, US) at 280 nm. The reduction of BSA in the solution was approximately equal to the adsorption of BSA on the membrane.

2.7. Characterization of M1 for oil/water separation 2.7.1. Contact angle measurements The contact angle was recorded by a water contact angle system (OCA20, Dataphysics, Germany), 5 locations were measured for the average value.

2.8.3. Dynamic BSA adsorption measurement The operating conditions and BSA solution are described above, in 2.6 and 2.8.2. To determine the stability of the membrane for anti protein contamination, repeated protein retention experiments were conducted. After each experiment, the membrane was

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washed with ethanol to remove possible deposited BSA and water flux was measured before and after membrane cleaning. 2.9. Characterization of antibacterial M3 2.9.1. Stability experiment The quaternized membrane M3-Q was thoroughly washed with ethanol and water for 25 times. The washed M3-Q was named as M3-Q-W. 2.9.2. Halo zone test E. coli served as the testing organism to qualitatively

investigate the broad spectrum antibacterial behavior. Before testing, all materials were autoclaved at 120 °C for 20 min in order to ensure sterility. E. coli was cultivated in sterilized Luria-Bertani (LB) broth and then incubated overnight at 37 °C in a shakingincubator. For qualitative evaluation, an LB agar plate was covered with E. coli containing 106 colony forming units (CFU). Then, M0, M3-Q and M3-Q-W with a diameter of 1.5 cm were placed in the LB agar plate and the whole agar plate was incubated overnight at 37 °C. The antibacterial activity was identified and estimated by a clear zone of inhibition in the indicator lawn around the membrane [33,34].

Fig. 3. (a) FTIR-ATR spectra and (b) XPS wide-scan spectra of control PVDF membrane (M0) and modified PVDF membranes (M1, M2, M3), (c) TGA curves for control PVDF membrane (M0), modified PVDF membranes (M1, M2, M3) and crosslinked precopolymers.

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3. Results and discussion

bulk of the membrane through the interfacial crosslinking.

3.1. Chemical composition of membranes

3.2. Surface Physical Structure of Membranes

As illustrated in Fig. 1, the functional precopolymer was first deposited on the PVDF membrane through the simple immersion. Subsequently, the acid catalyzed hydrolysis and coupling reaction among –Si–O– groups, which induced the self-crosslinking assembly of precopolymer on PVDF membrane. The chemical composition of surface functionalized membrane was investigated by FTIR in Fig. 3(a). The peak of 1636 cm  1 of all modified membranes (M1–M3) disappeared, implying the absence of monomer with CQC bonds. Compared to M0, all modified PVDF membranes exhibited a new absorption at 841 cm  1 attributed to the linking group –Si–O–. In contrast to M2 and M3, M1 showed the new absorption at 1660 cm  1 (CQO stretching vibration), implied the presence of PVP segments. As for M2 and M3, two new bands appeared at 1729 cm  1 and 1151 cm  1 assigned to symmetrical CQO stretching and C–O stretching, indicating the immobilization of POEGMA and PDMAEMA segments on the membrane surface. From XPS spectra in Fig. 3(b), we can see that the control PVDF membrane (M0) only showed peaks at 284 and 686 eV assigned to carbon (C) and fluorine (F) respectively. In contrast, the modified M1, M2 and M3 membranes showed new peaks attributed to oxygen (O), nitrogen (N) and silicon (Si) respectively, as distinctively marked in the corresponding spectra. Table 2 summarized the element mass concentration of the membranes. It is clearly shown that M1, M2 and M3 contains 7.95 wt% O and 5.25 wt% Si, 9.07 wt% O and 5.99 wt% Si, as well as 8.58 wt% O and 6.76 wt% Si on the membrane surface respectively. Besides, M1 and M3 displayed 3.57 wt% and 3.22 wt% N assigned to PVP and PDMAEMA segments. TGA was further carried out to calculate the content of crosslinking copolymers on the modified membranes. The crosslinked copolymer was synthesized and extracted for comparison. As shown in Fig. 3(c), it is calculated that all three modified membranes contained 9–10 wt% of the crosslinked copolymer based on the curve differences (the decomposition temperature of the pure PVDF membrane was about 458 °C in nitrogen). In summary, it was inferred that different functional copolymers were successfully incorporated onto PVDF membrane surface through the interfacial crosslinking. The superhydrophobic PVDF membrane M0 was micro-swelled by the diluted solvent TEP containing functional precopolymer, and then the precopolymer was able to be adsorbed into the membrane surface and infiltrated into the bulk, the subsequent crosslinking binds the copolymer firmly with the hierarchical surface and the porous bulk. The chemical crosslinked copolymer has a good entanglement with PVDF chains, which overcomes the traditional weak adsorption in surface coating. To identify the nature of the interaction, the EDX mapping of the membrane M0, M1, M2 and M3 was taken as shown in Fig. 4, The clear and uniform distribution of Si, O, N for M1 and M3, Si and O for M2 accompanying with the XPS results demonstrated that the functional copolymer was well immobilized both on the surface and in the

Besides the surface chemical evolution, the interfacial crosslinking will alter the surface morphology accordingly. As shown in Fig. 5, the pristine M0 membrane showed an extraordinary roughness (RSa 13.116 mm) with massive pores and crystalline micro-structures, which facilitated and enhanced the adsorption of precopolymer. After interfacial crosslinking, the modified PVDF membrane showed decreased roughness (RSa 4.213 mm, 4.230 mm, 4.485 mm for M1, M2, M3), and the irregular macrovois around 100 mm completely vanished. The mean pore size of the modified membranes decreased accordingly compared to the pristine M0 membrane, as shown in Fig. 6(a). In the immersion and swelling process, the diluted TEP swollen and plasticized the rigid crystalline surface [35], eliminated the stress concentration, narrowed the irregular macrovoids, reassembled the surface micro-/nanostructure and decreased the pore size. Therefore, the interfacial crosslinking smoothed the hierarchical structure and altered the morphologies substantially. Accordingly, both the elongation and the tensile strength was improved to some extents due to the interfacial crosslinking, as shown in Fig. 6(c).

Table 2 Element mass concentration calculated by XPS. Sample

M0 M1 M2 M3

XPS mass concentration (%) C

F

O

N

Si

46.86 42.48 42.83 43.32

53.14 40.75 42.11 38.12

0 7.95 9.07 8.58

0 3.57 0 3.22

0 5.25 5.99 6.76

3.3. Membrane permeability The pure water flux was measured to evaluate the permeability of membranes. As shown in Fig. 6(b), the modified PVDF microfiltration membranes (M1, M2 and M3) with versatile surface functionization exhibited significantly improved pure water flux (6064 7136.5, 57727 216.5 and 5830.5 7195 L/m2 h), compared with that of pristine superhydrophobic PVDF microfiltration membrane (1755 797.5 L/m2h). Due to the decrease of pore size of modified membranes, we believe that the improved hydrophilicity dominated the enhanced permeability. The superhydrophobicity of M0 with the contact angle  150° enhanced the hindrance of water flow and caused the low permeability. While the functional networks were thought to endow the membrane with certain hydrophilicity and enhance the permeability consequently. 3.4. Superwetting membrane for oil/water separation For more obvious contrast, the superhydrophobic PVDF membrane M0 was chosen as the control sample. PVP-VTES was crosslinked on M0 to achieve the superwetting PVDF membrane M1. As shown in Fig. 7(a) and (b), the contact angle of the superhydrophobic membrane M0 was maintained stable at about 150°, while the M1 showed the instantaneous wetting behavior in 2 s. Both the physical micro-/nano- texture surface and the chemical affinity of crosslinked PVP contributed to the superhydrophilicity. PVP is a common additive widely used to improve the hydrophlicity of the membrane [36,37]. Herein, PVP-VTES was firmly anchored on the hierarchical and rough surface of PVDF membrane through the silane coupling interaction, which endowed the membrane with excellent superhydrophilic stability [30]. M1 were immersed in aqueous solution of pH 2 and 12 for 24 days for chemical corrosion test, respectively. As depicted in Fig. 7(c), the initial contact angle of M1 was maintained almost unchanged below 25° despite the acidic or alkali corrosion. Therefore, the interfacial crosslinking strategy successfully realized the transition from superhydrophobicity to superhydrophilicity through the synergistic effect between PVP-VTES affinity and hierarchical texture. The modified PVDF membrane exhibited long-term superhydrophilic stability even under the harsh chemical corrosion. The superhydrophobic membrane M0 is incapable of separating oil/water emulsion. However, the superhydrophilic PVDF

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Fig. 4. The element mapping of M0, M1, M2 and M3 via EDX.

Fig. 5. (a) 3D confocal microscope images, (b) SEM images of M0, M1, M2, M3.

membrane M1 gained the new capability to separate both the oil/ water and the water/oil emulsion. Water-in-toluene (W/T) and toluene-in-water (T/W) were filtered by M1 separately under a vacuum of 0.09 MPa as shown in Fig. 8(a). There is a notable

difference between the feed and the corresponding filtrate observed by the naked eye or optical microscope. Compared to milky white feeds, the collected filtrates were totally colorless, indicating that the toluene in T/W and water in W/T was effectively removed.

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Fig. 6. The mean pore size (a), pure water flux (b), tensile strength and elongation at break (c) of M0, M1, M2, M3.

Fig. 7. (a) Superhydrophilic modification (M1) was performed on the left side of M0. Top is a vertical view and Bottom is a front view. (b) Contact angle of M0 and M1. (c) Contact angle of M1 under acid and alkali corrosion over 24 days.

UV–VIS spectrometer and Karl Fischer titrator were used to further analyze the purity of the filtrates. The characteristic peak of toluene for the filtrate of T/W was absent in the UV–VIS spectrum (Fig. 8(b)), indicating the high purity of 99.73%. The toluene in

water emulsion feed had a larger size range droplet (1 mm to 105 mm), while the maximum size of the corresponding filtrate is less than 7 mm (Fig. 8(c)), displaying the high purity of 99.93%. The high purity of the filtrate is up to 99.93%. High permeability of M1

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Fig. 8. Separation performances for water/oil and oil/water emulsions. (a) Toluene in water emulsion (left), separating device (middle) and water in toluene emulsion (right). (b) UV–VIS spectra for toluene in water emulsion. (c) Droplet size distribution of toluene in water emulsion. Table 3 The permeate flux, purity and rejection of filtrates for both W/T and T/W. Emulsion

Permeate flux (L/ m2 h)

toluene-in-water (T/ 9554 7758 W) water-in-toluene 2729 7341 (W/T)

Purity of filtrate Rejection (%) (%) 99.73

91.63

99.93

91.95

was obtained for both W/T (2729 7341 L/m2 h) and T/W (9554 7 758 L/m2 h) emulsions as shown in Table 3. The modified PVDF membrane by PVP-VTES crosslinking showed superhydrophilicity due to the micro-/nano- texture surface and the hydrophilicity. The hydration film was therefore generated and binding closely on PVDF membrane surface. The new water barrier allows the continuous permeation of water, while repelling the oil droplet. Therefore, the oil/water emulsion was successfully separated by the superhydrophilic PVDF membrane with the high permeability and selectivity. Water is heavier than toluene, which allows the rapid flow of water through the superhydrophilic membrane for toluene-in-water emulsion separation. In case of water-in-toluene emulsion separation, the heavier water can form the barrier layer to hamper the flow of toluene, which reduced the toluene flux comparing to water. However, the oleophilic PVDF membrane still can acquire high permeability for water-in-toluene emulsion. 3.5. Anti-fouling modification of membrane As we all know, OEGMA based polymer has an excellent biorepellency [38]. In order to investigate the protein adsorption

behavior on the membranes surface modified by POEGMA, FITClabeled BSA (BSA-FITC) was used as a model. Fig. 9(a) represented typical protein adsorption of these membranes. Compared with white modified membrane M2, the control membrane M0 was stained light yellow due to the massive fluorescent protein adsorption. Obvious fluorescence was also observed on M0 compared to M2, indicating the enriched presence of adsorbed BSAFITC on the surperhydrophobic membrane surface. In Fig. 9(b), the control PVDF membrane M0 displayed a serious BSA adsorption around 428.337 13 μg/cm2. In comparison, M2 showed a substantially reduced BSA adsorption with the maximum amount of 35.75 710 μg/cm2 over 36 h. In dynamic BSA adsorption test, the pure water flux of M2 was decreased to about 4800 L/m2 h after filtering 1 g/L BSA, and then recovered to the initial value 5700 L/ m2 h after washing with ethanol and water multiple in the multicycle filtration (Fig. 9(c)). All static and dynamic protein fouling results indicated that the POEGMA-VTES modified PVDF membrane via interfacial crosslinking showed excellent and long term fouling resistance to protein. 3.6. Antibacterial modification of membrane M3 with surface crosslinked copolymer P(DMAEMA-coVTMOS) was further quaternized by BC to obtain antibacterial M3Q. Meanwhile, in order to verify the long term antibacterial property, M3-Q was repeatedly washed with ethanol and water, which was named M3-Q-W. As shown in the XPS wide scan data in Fig. 10(a), N 1s core-level spectra were curve-fitted into two peak components. The quaternary ammonium cations (C-N þ ) at 402.1 eV could be observed [39] and accounted for roughly 82.49% of nitrogen element total weight.

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Fig. 9. (a) Top: comparison of M0 and M2; Bottom: Fluorescence microscopy images of the membrane surfaces after exposure to a BSA-FITC solution (b) BSA static adsorption on M0 and M2 (c) Dynamic filtration fouling behavior of M2, the orange ball represented BSA polluted water flux, and the pink ball represented recovered water flux after cleaning.

Fig. 10. (a) N 1 s core-level XPS spectra of M3 and M3-Q (b) Antibacterial properties of the control membrane M0, the quaternized membrane M3-Q, and M3-Q-W were washed by ethanol and water for 25 times.

It is well known that the quaternized PDMAEMA has outstanding antibacterial effect due to the pednant positive amino groups [40,41]. The cytoplasma membrane will be disintegrated by the positively charged PVDF membrane, which caused the death of the microorganism. Compared with M0, as expected, M3-Q exhibited a distinct inhibition zone with the width of 2.5 cm on LB agar plate from Fig. 10(b). Meanwhile, M3-Q-W also exhibited the clear area without E. coli growth even after 25 times washing. Therefore, in spite of washed with ethanol and water for more than 20 times, it was testified that the crosslinked networks PDMAEMA-VTES were firmly immobilized on the membrane surface and effectively inhibited the growth of E. coli.

4. Conclusion We have developed a feasible and efficient interfacial crosslinking strategy to design PVDF membrane with versatile surface functions. PVP-VTES, POEGMA-VTES, and PDMAEMA-VTES precopolymer was separately synthesized and anchored firmly onto the superhydrophobic PVDF membrane through the silane coupling interaction. FTIR-ATR, XPS and TGA confirmed the immobilization of crosslinked networks on the membrane surface. SEM also showed the morphology e by the micro-swelling and plasticization of diluted TEP containing hydrophilic copolymer. All modified membranes exhibited significantly enhanced permeability. PVP-VTES

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modified membrane M1 showed excellent and stable hydrophilicity over a wide range of pH, which was allowed to separate both waterin-toluene (W/T) and toluene-in-water (T/W) with high permeability and purity. Effective and stable fouling resistance to BSA for POEGMA-VTES modified membrane M2 was verified by both static and dynamic adsorption. The quaternized membrane M3-Q showed outstanding and stable antibacterial properties. The interfacial crosslinking strategy provided a versatile platform for functionalizing porous polymeric membranes.

[18]

[19]

[20]

[21]

Author contributions Notes. The authors declare no competing financial interest.

[22]

[23]

Acknowledgments [24]

This work is supported by National Natural Science Foundation of China (51673209, 51473177) and Youth Innovation Promotion Association of Chinese Academy of Sciences (2014258).

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