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Article pubs.acs.org/Langmuir

Enhanced Separation Performance of PVDF/PVP-g-MMT Nanocomposite Ultrafiltration Membrane Based on the NVP-Grafted Polymerization Modification of Montmorillonite (MMT) Panpan Wang,† Jun Ma,*,†,‡ Zhenghui Wang,† Fengmei Shi,† and Qianliang Liu† †

School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, P. R. China National Engineering Research Center of Urban Water Resources, Harbin 150090, P. R. China

‡

S Supporting Information *

ABSTRACT: A novel hydrophilic nanocomposite additive (PVP-g-MMT), coupling of hydrophilic modifier, selfdispersant, and pore-forming agent (porogen), was synthesized by the surface modification of montmorillonite (MMT) with N-vinylpyrrolidone (NVP) via “grafting from” polymerization in the presence of H2O2−NH3·H2O as the initiator, and then the nanocomposite membrane of poly(vinylidene fluoride) (PVDF) and PVP-g-MMT was fabricated by wet phase inversion onto clean glass plates. The existence and dispersion of PVP-g-MMT had a great role on structures, morphologies, surface composition, and chemistry of the as-prepared nanocomposite membranes confirmed by varieties of spectroscopic and microscopic characterization techniques, all of which were the correlated functions of PVP-g-MMT content in casting solution. By using the dead-end filtration of protein aqueous solution, the performance of the membrane was evaluated. It was seen that all of the nanocomposite membranes showed obvious improvement of water flux and proper BSA rejection ratio, compared to the control PVDF membrane. Meanwhile, dynamic BSA fouling resistance and flux recovery properties were also greatly enhanced due to the changes of surface hydrophilicity and morphologies. All the experimental results indicated that the as-prepared PVDF nanocomposite membranes showed better separation performances than the control PVDF membrane. Hopefully, the demonstrated method of hydrophilic nanocomposite additive synthesis would be applied for commonly hydroxyl group-containing inorganic nanoparticles, which was favorable to fabricate hydrophilic nanoparticle-enhanced polymer membranes for water treatment.

1. INTRODUCTION Membrane filtration has been considered as a highly competitive candidate for the water purification technology in the coming decades.1 Owing to good membrane-forming and excellent physicochemical properties, polymer membranes have gained extremely popularities in water treatment. High permeation flux, proper solute rejection, and lower fouling property constitute the integral requisites of high separation efficiency for membrane filtration. Unfortunately, hydrophobic nature of polymers always accumulates adsorptions or depositions between membrane surfaces and organic foulants (NOMs, pathogens, polysacchrides and proteins, etc.) in impaired water,2−4 which subsequently decreases membrane permeation flux. This organic fouling of polymer membranes influenced by membrane chemistry and morphologies remains a significant obstacle for the further development of membrane application. Among a tremendous amount of efforts, hydrophilicity modification of polymer membranes has become an accepted solution to improve membrane fouling resistance in water treatment.5 Mixed matrix membrane, dispersing small fillers throughout a large polymer matrix, has brought a new concept to fabricate © 2012 American Chemical Society

excellent water-treatment membranes for high water flux, high solute rejection, or lower fouling properties in recent years.6−10 Inorganic nanoparticles have been usually adopted as fillers to form nanocomposite membranes due to their hydrophilicity, large specific surface area, pore channels, and other functional characters. These inorganic nanoparticle fillers commonly adopted refer to varieties of dimensional nanoscale (0D, 1D, 2D) materials. In general, mixed matrix membranes are prepared through adding inorganic nanoparticles into casting solution by wet phase inversion process or into monomer solution by interfacial polymerization process, in which inorganic nanoparticles function as modifiers for improving membrane performance. These recent efforts are briefly reviewed as follows. 0D nanoparticles (TiO2, Al2O3, ZrO2, SiO2, Ag0, zeolite, etc.) and 1D nanotubes (carbon nanotubes, CNTs) are popularly introduced into polymer matrix to form ultrafiltration or reverse osmosis membranes with improved performances. Cao Received: September 5, 2011 Revised: December 14, 2011 Published: February 29, 2012 4776

dx.doi.org/10.1021/la203494z | Langmuir 2012, 28, 4776−4786

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Scheme 1. Schematic Representation of Two-Step Synthesis of the Hydrophilic Nanocomposite Additive, PVP-g-MMT

between clay particles (Cloisite Na, Cloisite 30B, and Cloisite 93A) and casting solvent (NMP) determined the dispersion state of clay particles in polysulfone ultrafiltration membranes, which affected membrane hydophilicity and resultant separation performances.20 Cloisite 30B, modified by a methyl tallow bis(2-hydroxyethyl) quaternary ammonium salt, could be intercalated or exfoliated in polymer matrix, and the membrane had the best separation properties and mechanical strength at 2 wt % loading. Owing to large cation exchange capacity of montmorillonite, Lin et al. prepared PMMA/Na+-MMT cation exchange membranes via emulsion polymerization and phase inversion for cationic dye adsorption.21 Interestingly, Chung et al. took the advantages of hydrophilic and hydrophobic clay to enhance the separation performance and mechanical strength of mixed matrix hollow fiber membranes for distillation.22,23 “Threshold” content, good dispersion, and pore-forming agent (porogen) have become three dominating factors during the preparation process of polymer−nanoparticle mixed matrix membranes by the classical wet phase inversion method. For good dispersion of nanoparticles, other chemical dispersants are necessary.12,13,24 In the present work, a novel hydrophilic nanocomposite additive (PVP-g-MMT), coupling of hydrophilic modifier, selfdispersant, and pore-forming agent (porogen), was synthesized by surface modification of montmorillonite (MMT) with Nvinylpyrrolidone (NVP) via “grafting from” polymerization in the presence of H2O2−NH3·H2O as the initiator, and then the nanocomposite membrane of poly(vinylidene fluoride) (PVDF) and PVP-g-MMT was fabricated by wet phase inversion onto clean glass plates. The structures, morphologies, surface composition, and chemistry of PVDF/PVP-g-MMT nanocomposite membranes were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. Using protein aqueous solution as model foulants, the separation and antifouling properties of the membranes were extensively investigated, and all the properties were correlated to the addition of PVP-g-MMT in PVDF matrix membranes.

et al. compared the effects of TiO2 nanoparticle size on PVDF membrane performance in the presence of poly(ethylene glycol) (PEG) as pore-forming agent (porogen) and demonstrated that smaller TiO2 nanoparticles could improve the antifouling properties of PVDF membranes more remarkably, due to the crystallization changes of PVDF and smaller mean pore sizes on membrane surfaces.10 To avoid agglomeration of nanoparticles, Razmajou et al. fabricated PES/ TiO2 ultrafiltration membranes in the presence of polyvinylidone (PVP K40) as the porogen by phase inversion process, after the mechanical and chemical modification of TiO2 nanoparticles.11 84% of flux recovery ratio and much lower TMP during filtration with bovine serum albumin (BSA) demonstrated the enhanced antifouling properties of PES membrane with 2 wt % TiO2 loading. Yan et al. prepared poly(vinylidene fluoride) (PVDF) ultrafiltration membrane modified by Al2O3 nanoparticles in the presence of hexadsodium phosphate as the dispersant and PVP as the porogen.12,13 The addition of Al2O3 nanoparticles did not affect membrane pore structures but only improved the hydrophilicity of resultant membranes and consequently antifouling performance of membranes. On the basis of the reaction between PVDF and γ-Al2O3 (−OH), Liu et al. demonstrated that pure water flux of PVDF membrane with 2 wt % Al2O3 loading reached 134 L m−2 h−1 with the BSA rejection of 93.4%, due to the increased hydrophilicity of PVDF membrane after modification.14 The hydrophilicity of 0D nanoparticles dispersed in membrane matrix changed the surface hydrophilicity and morphologies of resultant mixed matrix membranes. In addition to the hydrophilicity of nanoparticles, water transport channels of 1D CNT nanotubes also facilitated the permeability and fouling resistance of polymer membranes. On the basis of the strong acid activation of MWCNTs, some researchers demonstrated that the surface morphologies (pore size, porosity, and roughness) and hydrophilicity of MWCNTs/polymer nanocomposite membranes were favorably developed under the “threshold” contents of MWCNTs in casting solution, which improved the performances of water flux, solute rejection, and fouling resistance.15,16 Although the membranes had better separation properties, unselected voids were formed in the membranes due to the poor interfacial compatibility of carbon nanotubes with polymers. Qiu et al.17 functionalized the MWCNTs with isocyanate and isophthaloyl chloride via the reaction between carboxylated carbon nanotubes and 5-isocyanatoisophthaloyl chloride (ICIC) and then blended with polysulfone (PSf). The well-compatible MWCNTs suppressed protein adsorption on membranes and thus alleviated membrane fouling, while the water permeability of membranes was improved. Recently, 2D silicate sheets (clay) were also adopted to improve the hydrophilicity/hydrophobicity and the mechanical strength of water filtration membranes, while mostly were filled in gas separation and proton exchange membranes.18,19 Monticelli et al. demonstrated that the organic compatilizer

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Poly(vinylidene fluoride) (PVDF, FR904, Mn 475 637) was obtained from Shanghai 3F New Materials Co. Ltd. Montmorillonite (Na-MMT) with a cation exchange capacity of 100 mequiv/100 gMMT was purchased from Zhejiang Fenghong New Materials Co. Ltd. γ-Methacryloxypropyltrimethoxysilane (MAPTMS) and N-vinylpyrrolidone (NVP) were supplied by Nanjing Capture Chemical Co. Ltd. and Boai NKY Phamerceuticals Ltd. respectively. Bovine serum albumin (BSA, Mr 68 000) was supplied by Beijing Solarbio Science & Technology Co., Ltd. Phosphate buffered saline (PBS solution, 0.01 M, pH = 7.4) was prepared with KH2PO4 and Na2HPO4·12H2O. N,N-Dimethylacetamide (DMAC) was used as casting solvent, and other reagents were all analytical purification and used directly. Deionized water (18.2 MΩ·cm) was obtained from a Milli-Q ultrapure water purification system. 4777


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2.2. Synthesis of PVP-Grafted Montmorillonite (PVP-gMMT). Montmorillonite (MMT), as a kind of clay mineral, was known to have variable wetting behavior. Before synthesis of PVP-gMMT, the hydrophilicity of Na-MMT was measured by the water contact angle instrument.25 Briefly, 2.5 wt % MMT aqueous suspension was formed, and then 3 mL suspension was spread onto a clean microscope slide (87.76°). After being wetted with the suspension, the MMT-covered slide was dried at room temperature and then 50 °C overnight until a dried thin film was formed. The water contact angle of the MMT film was 43.36°, shown in Figure S1 (Supporting Information). The synthesis of hydrophilic nanocomposite additive, PVP-g-MMT, was carried out via surface “grafting from” polymerization of NVP using H2O2−NH3·H2O as the initiator in aqueous solution; before that, the reactive group (methacryloyl) was anchored on the surface of montmorillonite (MMT) through MAPTMS hydrolysis in acidic solution. This “grafting from” polymerization process is shown in Scheme 1, similar to previous works.26 In brief, 5 g of MMT was dispersed into deionized water by magnetically stirring overnight and then ultrasonic dispersion for 30 min (KQ-300DE, 40 kHz, China), followed by the dropwise of MAPTMS solution (2.5 g of MAPTMS, 30 g of acetic acid, 25 g of ethanol, and 25 g of H2O). The MMT and MAPTMS suspension was then stirred vigorously for 4 h at 90 °C. The obtained MAPTMS-MMT was filtered, washed, and dried in vacuum. Subsequently, 1 g of MAPTMS-MMT was dispersed into 150 g of 30 wt % NVP monomer aqueous solution, followed by the dropwise addition of desired quantities of initiators (1 wt % H2O2, 1 wt % NH3·H2O based on the weight of NVP monomer). After 30 min bubbling of nitrogen, the grafting polymerization was started by raising the temperature of reaction vessel to 60 °C in an oil bath. The polymerization reaction was maintained under a nitrogen atmosphere for 4 h to obtain a gray suspension with some viscidity. After polymerization, the product was centrifuged and washed with ethanol for several times and then dried in vacuum. Finally, the dried PVP-gMMT powders were preserved in a desiccator before use. The viscosity of polymerized supernatant was used to estimate the molecular weight (8115.8) of PVP chain in PVP-g-MMT according to the Mark−Houwink equation.27 2.3. Nanocomposite Membrane Preparation. All the membranes were prepared by classical phase inversion method with PVDF as bulk material, DMAC as the solvent, PVP-g-MMT as the additive, and distilled water at room temperature as the nonsolvent coagulation bath. All the bulk materials and additives were dried at 80 °C for 12 h before use. PVP-g-MMT particles (0, 1, 2, 4, and 6 wt % based on the weight of PVDF) were first imported into DMAC solvent, and then PVP-g-MMT suspension was formed. The suspension was sonicated for 30 min (KQ-300DE, 40 kHz, China) before the addition of PVDF powders. Casting solution consisted of PVDF and PVP-g-MMT was then mechanically stirred at 60 °C for at least 12 h. After fully degassing, the casting solution was spread onto clean glass plates with 200 μm gap and then immersed into coagulation bath (distilled water) for 30 min. After peeling off from the glass plates, the resultant membranes were rinsed in distilled water before ultrafiltration tests. The nanocomposite membranes were denoted as M0, M1, M2, M4, and M6 according to the weight percentage of PVP-g-MMT. The pure PVDF membrane with 0 wt % loading, M0, was used as the control membrane. The composite membranes with the same amount of MMT addition in casting solution were also fabricated through identical conditions, denoted as m1, m2, m4, and m6, respectively. 2.4. Spectroscopic and Microscopic Characterization. The existence and the dispersion of PVP-g-MMT in nanocomposite membranes were characterized by a Fourier transform infrared spectrometer (FTIR, Perkin-Elmer) and a wide-angle X-ray diffraction spectrotometer (WAXRD, XRD 6000, Shimadzu) with a Cu Kα (λ = 0.154 18 nm) generator at 40 kV and 30 mA, respectively. The surface compositions of montmorillonites and membranes were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5700) using Al Kα (1486.6 eV) as the radiation source. Survey spectra were collected over a range of 0−1350 eV. High-resolution spectra were collected for the C 1s, F 1s, N 1s, O 1s, and Si 2p regions. The surface morphologies of

the membranes were analyzed quantitatively by atomic force microscopy (AFM, Nanoscope IIIa; Digital Instruments) with a tapping mode. Root-mean-squared (rms) surface roughness and surface area difference (SAD) were quantified from the recorded roughness parameters. The top surface and cross-section morphologies of membranes were inspected by scanning electron microscope (SEM, Quanta 200F,USA) with an acceleration voltage of 20 kV after the samples were fractured in liquid nitrogen and sputtered with a thin gold layer. Transmission electron microscopy (TEM, JEOL-JEM2010) and differential scanning calorimetry (DSC, SETARAM DSC141) were employed to evaluate the clay dispersion in membranes. Tensile strength tests were conducted by a universal electronic strength measurement (AGS-J, Shimadzu). The water contact angles of the membranes were surveyed by the sessile drop method using contact angle goniometer (QSPJ 360, Jinshengxin Testing Machine Co., China) at room temperature. The reported data of contact angles were averaged from at least five determination at different locations of the membranes. 2.5. Separation and Fouling Properties Evaluation. A deadend stirred cell (model 8200, Millipore Co.) filtration system connected with solution buffer reservoir and nitrogen gas cylinder was conducted to evaluate membrane permeability, retention, and antifouling properties. The filtration process was carried out at 22 ± 1 °C with a near-surface stirring speed of 300 rpm. The model feed solution was BSA-PBS aqueous solution (0.2 g L−1, pH value was 7.4 buffered by phosphate buffer solution), which was a representative of protein-containing wastewater.28 The detailed operation process consisted of three steps: (1) each membrane was initially pressured at 0.15 MPa for 30 min, and then the operation pressure was set at 0.1 MPa during deionized water filtration process; (2) then 0.2 g L−1 BSA−PBS solution was permeated through the membrane and the permeate flux profile with time was recorded; (3) after filtrating the feed solution, the membrane was washed with deionized water for about 20 min, and then the water flux of cleaned membrane was remeasured. The steady water flux JW1, JP, JW2 (L m−2 h−1 bar−1) of three steps was defined as J = V /AΔt ΔP

(1) 2

where V (L) was the volume of permeated water, A (m ) was the membrane area, ΔP (bar) was the transmembrane pressure (TMP), and Δt (h) was the operation time. The rejection ratio of BSA was calculated by determining the corresponding BSA concentration in the feed and permeate solution, using a UV−vis spectrophotometer at a fixed wavelength of 280 nm, respectively. Rejection ratio, r, was calculated by the equation

r = (1 − CP/C f )× 100%

(2)

−1

where Cp and Cf (g L ) are the solute concentration of permeate and feed solution, respectively. To further study the antifouling properties of protein-filtered membranes, the flux recovery ratio (FRR) was calculated by the equation

FRR = JW2 /JW1 × 100%

(3)

To analyze the antifouling properties in details, the degree of membrane fouling based on resistance-in-series model29−31 was calculated quantitively by the equation

Jp = TMP/(ηR t)

(4)

where Jp (L m−2 h−1 bar−1) was the water flux of BSA−PBS feed solution, TMP (0.1 MPa) was the transmembrane pressure, and η (8.9 × 10−4 Pa·s) was the viscosity of BSA−PBS solution measured at experimental temperature.

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R t = R m + R f + Rc

(5)

R m = TMP/(ηJW1)

(6)

R f = TMP/(ηJW2 ) − R m

(7)


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Figure 1. FTIR spectra of (a) MMT, MAPTMS-MMT, PVP-g-MMT particles and (b) PVDF/PVP-g-MMT nanocomposite membrane M0, M1, M2, M4, and M6.

R c = TMP/(ηJp) − R f − R m

were observed for these characteristic bands of montmorrillonite through different modification. However, the shape of Si−O stretching band at 1037 cm−1 was affected by the chemical changes in the Si environment after each modification. The band at 1037 cm−1 decreased in intensity corresponding to broader shape change, which was an indication of amorphous silica present in modified montmorillonite. In the stage of silylation, the MAPTMS first hydrolyzed in acidic solution and then reacted with the hydroxyl groups on montmorillonite surfaces, which were generated by the fully swollen behavior in previous aqueous solution. The new characteristic bands of alkyl groups at 2959 and 2926 cm−1 demonstrated the successful “anchoring” of MAPTMS onto the montmorillonite surfaces. In addition to −CH2 groups, there were some new characteristic bands of 1465 and 1294 cm−1 in PVP-g-MMT, which were assigned to stretching vibrations of amide group and C−N bond, respectively.32 This characteristic bands of poly(N-vinylpyrrolidone) confirmed well-done surface polymerization of NVP by “grafting-from” polymerization. The PVP-g-MMT, as a nanocomposite additive, was used to prepare PVDF/PVP-g-MMT nanocomposite ultrafiltration membranes by the wet phase inversion method. The existence of montmorillonite in PVDF/PVP-g-MMT nanocomposite membrane was confirmed by the FTIR spectra in Figure 1b. In the composite membranes (M1−M6), typical band of silicates at 521 cm−1 was observed, which corresponded to Al− O stretching vibration. Beneficially, MAPTMS modification of montmorillonite endowed more oxygen-containing groups in PVDF/PVP-g-MMT nanocomposite membranes, which were observed at the band of 1638 cm−1. Unfortunately, the characteristic bands of Si−O, −CH2, and amide groups were overlapped by those of the macromolecule chains of poly-

(8)

−1

where Rm (m ) is the intrinsic resistance due to membrane material and structure, Rf (m−1) is the sum of resistances caused by solute adsorption or plug into membrane pores or walls (i.e., irreversible fouling resistances), and Rc (m−1) is the cake resistance formed by cake or gel layer on membrane surface, which could be removed by hydraulic washing easily (i.e., reversible fouling resistances). Obviously, the total resistance, Rt, was the sum of Rm, Rf, and Rc. The lower Rf (i.e., irreversible fouling resistances), the better antifouling performance of the membranes.

3. RESULTS AND DISCUSSION 3.1. Hydrophilic Nanocomposite Additive Synthesis. Water-soluble poly(N-vinyl-2-pyrrolidone) (PVP) has been widely used as a good pore-forming agent (porogen) for the preparation of asymmetric membranes by the wet phase inversion method. However, almost the whole PVP leached out from the rich polymer phase during the phase inversion process, which greatly decreased the hydrophilicity of membranes.32 To alleviate the leaching of PVP, the concept of PVP−MMT nanocomposite additive should be an efficient solution due to the sheet structures of montmorillonite (MMT). The two-step synthesis of PVP-g-MMT is shown in Scheme 1. Fourier transform infrared spectroscopy (FTIR) was conducted to demonstrate the surface modification of montmorillonite by MAPTMS and NVP, shown in Figure 1a. The Na-MMT exhibited bands at 3440 and 1727−1638 cm−1 due to the stretching and bending vibrations for hydroxyl groups of water molecules present in the clay interlayers. Typical bands of silicates at 521 and 467 cm−1 are representatives of Al−O stretching vibration and Si−O bending vibration.33,34 As shown in Figure 1a, no significant changes 4779


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Figure 2. WAXRD spectra of (a) MMT, MAPTMS−MMT, and PVP-g-MMT and (b) PVDF/PVP-g-MMT nanocomposite membrane M0, M1, M2, M4, and M6.

Figure 3. TEM images of MMT or PVP-g-MMT dispersion in the as-prepared nanocomposite membrane M0, m1, M1, M2, M4, and M6.

intercalation of modifier molecules into clay sheets.32 On the basis of the Bragg equation, we can obtain basal spacing of 001 crystal planes of montmorillonite sheets from diffraction angles in Figure 2. In the stage of silylation, MAPTMS molecules were anchored onto outer/inter surfaces of montmorillonite through hydrolysis of silane in acidic solution. According to the surface reaction, basal spacing of MMT was slightly enlarged from 1.46 to 1.53 nm (Figure 2a), due to the small molecules intercalation of clay sheets by MAPTMS surface modification. During “grafted-from” polymerization, the free methacryloyl group of MAPTMS−MMT reacted with vinyl group of NVP in the presence of the NH3·H2O−H2O2 initiator, and then PVP chain grew onto the MAPTMS−MMT surface, shown in Scheme 1.

(vinylidene fluoride) (PVDF). PVDF/PVP-g-MMT nanocomposite membranes could be facilely fabricated through addition of PVP-g-MMT composite additive in casting solution before wet phase inversion process. 3.2.1. Structures and Morphologies of PVDF/PVP-gMMT Nanocomposite Membranes. PVP-g-MMT Dispersion in PVDF Membrane. In this work, two steps were adopted to synthesize PVP-g-MMT hydrophilic nanocomposite additive. Wide-angle X-ray diffraction (WAXRD) and particle size distribution were analyzed to evaluate the shape changes of MMT after surface modification, shown in Figure 2 and Figure S1. Chemical surface modification resulted in d-spacing (d001) changes of montmorillonite crystals, which derived from the 4780


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Figure 4. SEM images of cross-section morphologies for prepared PVDF/PVP-g-MMT nanocomposite membrane M0, M1, M2, M4, and M6.

Table 1. Different Performance Parameters for PVDF/PVP-g-MMT Nanocomposite Membranes membrane M0 M1 M2 M4 M6

thickness (μm) 50.6 59.1 60.7 71.6 103.8

± ± ± ± ±

1.8 2.3 1.3 3.1 5.8

porosityb (%)

dporec (nm)

± ± ± ± ±

14.44 15.06 15.77 15.79 14.71

55.3 79.6 76.3 76.1 81.3

3.1 6.5 1.5 2.4 1.7

contact angle (deg)

Rm/Rtd (%)

Rf/Rt (%)

Rc/Rt (%)

± ± ± ± ±

50.03 55.00 40.81 50.78 60.14

32.73 8.39 7.39 11.85 5.38

17.24 36.61 51.80 37.38 34.48

88.9 82.6 81.3 76.9 77.4

1.8 0.8 1.1 2.2 1.8

a

Wet membrane thickness was measured. bPorosity was measured by weight difference of wet and dry membrane. cPore size, dpore, was calculated by the relationship [r = 1 − 2(1 − λ)2 + (1 − λ)4, λ = dBSA/dpore.37 dMembrane filtration resistance, R, was on the base of resistance-in-series model.

As depicted in Figure 2a, the resultant clay sheets (PVP-gMMT) were partly exfoliated which was evidenced by the mixed basal spacings of 2.29 and 1.47 nm. Compared to the increased d-spacing (d001), the sheet size of montmorrilonite (MMT) was reduced from 923.2 to 581.1 nm after the “twostep” surface modification, which facilitated the dispersion of PVP-g-MMT into PVDF membrane. Through blending in casting solution, PVP-g-MMT was introduced into the PVDF ultrafiltration membrane. Favorably, the characteristic bands of PVP-g-MMT were not observed at lower addition of PVP-g-MMT due to the well exfoliation of montmorillonite sheets (Figure 2b, M0−M4). As PVP-g-MMT content increased, some intercalated montmorillonites appeared in the PVDF/PVP-g-MMT nanocomposite membrane, corresponding to basal spacing of 3.06 nm (Figure 2b, M6). As a direct technical characterization, transmission electron microscopy (TEM) was used to verify the dispersion state of PVP-g-MMT in as-prepared hybrid membranes (Figure 3). As shown in Figure 3, the silicate sheets of MMT/PVP-g-MMT were observed obviously, in the forms of aggregation, intercalation, or exfoliation. Interestingly, most of the silicate sheets existed around membrane pore walls, due to the hydrophilicity of MMT (43.36°, Figure S1) and the surface segregation property of PVP-g-MMT (details in section 3.3). Before surface modification, large aggregates of MMT appeared in the PVDF/MMT membrane (m1), due to the larger size and worse compatibility with polymeric matrix. Contributed to the good compatibility of PVP-g-MMT and PVDF, “grafting from” polymerized surface modification facilitated the dispersion of PVP-g-MMT in the membranes (M1−M6). In addition, smaller sheet size of PVP-g-MMT (581.1 nm, Figure S1) was

also helpful to themselves dispersion in membrane matrix. Obeserved from Figure 3, the intercalated or exfoliated sheets of PVP-g-MMT appeared around the pore walls of membrane M1, M2, and M4, while some aggregation occurred for more addition of PVP-g-MMT (M6). Differential scanning calorimetry (DSC) was conducted to investigate the interaction of PVP-g-MMT and PVDF, as shown in Figure S2 (Supporting Information). The composite membranes displayed single transition (around 164.45 °C) at lower addition of PVP-g-MMT (M1−M4), while the two transition at higher addition (M6) due to some aggregation.31,35 The dispersion states of PVP-g-MMT in membranes were greatly correlated to the interfacial compatibility between PVP-g-MMT and PVDF. 3.2.2. Content-Dependent Structures and Morphologies of PVDF/PVP-g-MMT Nanocomposite Membrane. PVP-gMMT, as a hydrophilic nanocomposite additive, plays a great role on the pore structure development of membranes. SEM micrographs of both cross sections and top surfaces of PVDF membranes with different PVP-g-MMT content had been obtained. Figure 4 shows the cross-section morphologies of PVDF/PVP-g-MMT nanocomposite membranes prepared in the present work. All the PVDF membranes showed the characteristics of asymmetric membranes prepared by wet phase inversion method, consisting of a skin layer as the selective barrier, a much thicker finger-like substructure and a spongy-like bottom support. It could be seen that the thickness of spongy-like layer decreased with the addition of PVP-gMMT and oppositely the finger-like pore structure developed across the whole cross sections of membranes, which may enhance permeability properties of the membranes. Along the 4781


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Figure 5. SEM images of top surface morphologies for prepared PVDF/PVP-g-MMT nanocomposite membranes M0, M1, M2, M4, and M6.

segregated toward membrane surface and then was “dragged” by the 2D nanosheets of MMTs wrapped with PVDF chains, evidenced by the appearance of silicon and nitrogen elements on M4 membrane surface (Figure 6b). This surface segregation behavior was specifically described in section 3.2.2. To provide detailed chemistry changes of membrane surfaces, XPS C 1s core level spectra were resolved into seven peaks representing different chemical environments using a sum of Lorentzian− Gaussian functions, shown in Figure 6c and Figure S3 (Supporting Information). As the grafted PVP segment of nanocomposite additive was the only source of C−N, the surface coverage ratio (14.09%) of hydrophilic PVP chains was directly calculated from the C−N area percentage of XPS C 1s core level spectra resolved results (Figure 6c) for M4. Meanwhile, the area percentages of other oxygen-containing groups were 10.73%, 2.87%, and 6.3%, responding to C−O, CO, and −COO in PVP-g-MMT, respectively, which showed total surface coverage ratio (19.9%) of hydrophilic oxygen-containing groups on the membrane surface. As PVP-gMMT content increased, the surface coverage ratio of hydrophilic PVP chains increased from 0.59%, 2.56% up to 14.09% corresponding to M1, M2, and M4. Because of the aggregation and increased viscosity of casting solution, the surface segregation of PVP-g-MMT was suppressed during membrane M6 formation. Reasonably, C−N coverage ratio on M6 surface reduced to 5.57%, which showed lower C−N peak in Figure S3 (Supporting Information). The surface coverage ratio of hydrophilic oxygen-containing groups was 8.1%, 11.88%, 13.99%, and 13.85% for M1, M2, M4, and M6, respectively. Development of other oxygen-containing groups on the nanocomposite membranes was similar to C−N group of PVP chains, due to the polymerization between MAPTMS and NVP. The appearance of both PVP and oxygen-containing segments on nanocomposite membrane surface identified the PVP-g-MMT nanocomposite additive as a hydrophilic modifier for PVDF membrane. Atomic force microscopy (AFM) results presented in Figure 7 compared the as-prepared membrane morphologies which was dependent on the changes of PVP-g-MMT content. Both rms roughness and SAD of all nanocomposite membranes (M1−M6) were larger than the controlled PVDF membrane (M0), in which the membrane M4 reached the maximal values

development of pore structures, membrane thickness was also increased with the PVP-g-MMT loading and reached the maximal value of 103.8 μm for M6 (Table 1), which was ascribed to the increased viscosity of casting solution with 6 wt % loading. As depicted in Figure 5, the morphologies of membrane surfaces indicated that the pore size and porosity were both increasing with PVP-g-MMT contents up to 4 wt % and then decreased, while the relative pore size characterized by BSA rejection is shown in Table 1. The evolutions found in surfaces as well as in cross sections of membranes confirmed that hydrophilic nanocomposite additive was capable of increasing the exchange of solvent−nonsolvent and then favorably pore forming, as a novel porogen. During the exchange of casting solvent (DMAC) and nonsolvent (water in coagulation bath) in phase inversion process, the hydrophilic PVP-g-MMT segregated toward the top surface of just-formed membrane and simultaneously 2D nanosheets of MMT hindered the leaching of PVP segments from the as-prepared membrane, which was similar to the behavior of amphiphilic “surface segregation materials”.5,28 The surface segregation of PVP-gMMT must change the morphology and composition of PVDF nanocomposite membrane surface, specifically described in section 3.3. 3.3. Interfacial Properties of PVDF/PVP-g-MMT Nanocomposite Membrane. Surface composition and chemistry of membranes dominate the ultrafiltration performances for water purification due to the intensive interaction between foulants and membrane surface. The X-ray photoelectron spectroscopy (XPS) is a surface measurement, probably penetrating a few nanometers below the surface; therefore, the existence of PVP-g-MMT and hydrophilic groups on membrane surface could be analyzed. Figure 6 showed the XPS spectra of MMTs and the resultant PVDF nanocomposite membrane M4. These XPS data quantified the amounts of elemental carbon, fluoride, oxygen, nitrogen, and silicon present on the surfaces of modified-MMTs and membranes. As depicted in Figure 6a, the appearance of carbon and nitrogen elements on modified MMTs confirmed the successful modification of MAPTMS silylation and “grafting-from” polymerization of NVP. During the exchange of DMAC and water in phase inversion process, the hydrophilic PVP-g-MMT 4782


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Figure 7. AFM measured rms roughness and surface area difference (SAD) for PVDF/PVP-g-MMT nanocomposite membrane.

3.4. Separation and Antifouling Performances. Water permeation flux and solute rejection are considered as the two significant property parameters for ultrafiltration membrane in water and wastewater treatment. In this work, protein aqueous solution was used as the model foulant for ultrafiltration experiment. Water permeation flux and BSA rejection results of different nanocomposite membranes are shown in Figure 8a and Figure S5. Water permeation flux increased with the PVPg-MMT loading up to the maximal value of 74.64 L m−2 h−1 bar−1 for membrane M4 in which the content of PVP-g-MMT based on PVDF was 4 wt %, while the BSA rejection was maintained at a reasonable level (at least 81.1%) in water treatment. Subsequently, the water flux of membrane M6 decreased due to the thicker membrane and small pore size of membrane surface. These separation phenomena were in agree with porous structure and surface pore size of nanocomposite membrane, as shown in Figures 4 and 5. To provide more detailed separation parameters, the porosity and surface pore size of the as-prepared nanocomposite membranes were quantified, as shown in Table 1. All the thickness, porosity, and pore size of nanocomposite membranes were larger than the control PVDF membrane, in which the pore size of M6 decreased to 14.71 nm caused by the delayed demixing during the phase inversion process.36 These calculated structure parameters implied the enhanced separation performances of nanocomposite membranes after PVP-g-MMT addition. For PVDF/MMT membranes (m1−m6), the water permeation flux increased with the MMT loading up to the maximal value of 46.49 L m−2 h−1 bar−1 for membrane m1 and then decreased up to 35.63 L m−2 h−1 bar−1 for membrane m6 (Figure S5). Compared to PVP-g-MMT, the hydrophilicity of MMT (43.36°, Figure S1) was not effective enough to form pores during the phase inversion process. On the other hand, increased viscosity of casting solution suppressed the membrane pore forming during phase inversion process.36 PVP-g-MMT, as a porogen, was better to enhance water permeation of composite membrane. Analysis of the time-dependent flux variations during filtration process was allowed for the evaluation of membrane fouling, consisted of irreversible and reversible flux decline. During the pressure-driven membrane filtration process, irreversible fouling was mainly caused by the direct attachment and then adsorption on membrane surface or plug into membrane pores/walls, which was difficult to be cleaned by hydraulic washing. The reversible fouling was usually attributed to cake or gel layer deposited on membrane surface, which

Figure 6. XPS spectra of (a) MMT, MAPTMS, and PVP-g-MMT, (b) PVDF/PVP-g-MMT nanocomposite membrane M0 and M4, and (c) C 1s core level spectra resolved results of membrane M4.

(135.7 nm for rms roughness, 15.7% for SAD). The threedimensional AFM images of membranes and measured results of rms roughness and SAD are shown in Figure S4 and Table S1 (Supporting Information), respectively. Brush-like PVP chains and embedded MMT segments on surfaces improved the membrane roughness dramatically. For membrane M6, the relative smooth surface was caused by probable increased viscosity of casting solution, which decreased the exchange of DMAC and water in phase inversion process and then less surface segregation of PVP-g-MMT. This phenomenon was also observed in other nanocomposite membranes.13,17 Surface roughness greatly correlates with static contact angle measurements, which changes the hydrophilicity of membrane surface consequently. 4783


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Figure 8. Ultrafiltration performances of PVDF/PVP-g-MMT nanocomposite membranes (M0−M6) during BSA−PBS solution (0.2 g L−1, pH 7.4) filtration: (a) pure water permeability and protein rejection, (b) time-dependent water permeation flux, (c) water flux recovery ratio after hydraulic washing, and (d) calculated resistances of membrane fouling based on resistance-in-series model. The operation process included three steps: (1) deionized water filtration, (2) BSA aqueous solution filtration, and (3) 20 min water washing without pressure and the second round deionized water filtration. All the filtration pressure was set at 0.1 MPa, and the model solution was maintained at 22 ± 1 °C.

could be removed by hydraulic washing easily and reversibly. Meanwhile, the flux recovery ratio (FRR) was obtained to identify the fouling extent on membrane surfaces. The higher the water permeation flux and FRR, the better the fouling resistance of membranes. Figure 8b presents the time-dependent water flux variations of nanocomposite membranes with BSA-PBS (0.2 g L−1, pH 7.4) solution as the model foulant, and the resultant flux recovery ratio (FRR) after water washing was plotted as the function of PVP-g-MMT loading, shown in Figure 8c. Obviously, the control PVDF membrane had the worst water permeation flux and FRR during the filtration process of protein aqueous solution. The enhanced antifouling properties of nanocomposite membranes were significantly correlated to the improved hydrophilicity of membrane surface, evidenced by static water contact angles in Table 1. More PVP-g-MMT segregation to membrane surface resulted in the best hydrophilicity (76.9°) for membrane M4, which favored the water permeation flux (Figure 8b). However, membrane M4 did not have the highest flux recovery ratio (FRR) among the nanocomposite membranes, slightly lower than the cases of M1, M2, and M6. As to the definition of flux recovery ratio, FRR may be the combined function of membrane hydrophilicity, pore size distribution, and initial water flux. The control experiments for PVDF/MMT membranes were also conducted under the same conditions, and the results are shown in Figure S5 (Supporting Information). The water

permeation fluxes of PVDF/MMT membranes were less than 25.38 L m−2 h−1 bar−1 at the end of dynamic filtration, while the fluxes of PVDF/PVP-g-MMT membranes were over 30.14 L m−2 h−1 bar−1. In view of FRR, the PVDF/MMT membranes (68.04%−71.56%) were also worse than PVDF/PVP-g-MMT membranes (81.09%−91.78%). Less antifouling performance was contributed to the aggregation and less surface segregation of MMT. The antifouling performance of PVDF membrane was enhanced after the addition of hydrophilic nanocomposite additive PVP-g-MMT. Resistance-in-series model provided a more specifically quantified description for pressure-driven membrane fouling, and thereof varieties of filtration resistance were defined to differentiate irreversible fouling, reversible fouling, and fouling from membrane materials or structures, corresponding to Rf, Rc, and Rm. As depicted in Figure 8d, all the filtration resistances of nanocomposite membranes were dramatically lower than the control PVDF membrane. In detail, the Rm values were remarkably decreased with increase of PVP-g-MMT loading and kept lower than 28.22% of membrane M0, corresponding to the well development of finger-like structures in cross sections (Figure 4). Meanwhile, the Rf and Rc values kept lower than 7.12% and 78.76% of membrane M0, respectively. It was demonstrated that these decreased filtration resistances were in agree with membrane hydrophilicity and structures. Although filtration resistances decreased dramatically, the percentages of Rm in total resistance was not obviously changed and 4784


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ACKNOWLEDGMENTS The authors thank financial support from the National Natural Science Foundation of China (Grants 50978067, 50778049, and 51121062). It is also supported by the Ministry of Science and Technology of China (2009AA06Z310, 2009ZX07424005; 2009ZX07424-006).

maintained between 40.81% and 60.14% (Table 1). After the addition of PVP-g-MMT in nanocomposite membranes, the percentages of Rf decreased and kept lower than 11.85%, while those of Rc increased and kept higher than 34.48%. Resistancein-series was also depicted in Figure S5. Differently, almost equal Rf and Rc (except for m6) demonstrated that irreversible fouling was not greatly relieved after the MMT addition. Compared to the control membranes, irreversible fouling for PVDF/PVP-g-MMT nanocomposite membranes was not dominating and decreased dramatically. It may be explained by the pore-forming and segregation behavior of hydrophilic nanocomposite PVP-g-MMT: well-developed pore structures favored permeability, hydration layer caused by hydrophilic segments segregation endowed the surface with outstanding antifouling resistance potential. Simultaneously, the synergy of hydration layer and “water corona” around protein molecules avoided the conformational transformations of proteins to some extent,28 which also enhanced the fouling resistances of nanocomposite membranes. It was worth noting that well developed pore structures would sacrifice the mechanical strength of fabricated membranes, which was detrimental to long-term operation. Calculated results of mechanical strength are shown in Table S2 (Supporting Information). In the cases of high porosity, proper content and well dispersion of PVP-gMMT could improve the mechanical strength of membranes M2 and M4 compared to membranes M1 and M6.

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In summary, a facile approach for synthesizing hydrophilic nanocomposite additive, PVP-g-MMT, was developed to fabricate PVDF/PVP-g-MMT nanocomposite ultrafiltration membrane for water treatment. As a porogen, PVP-g-MMT induced the well evolution of finger-like pore structures and surface pores in nanocomposite membranes. As a selfdispersant, PVP-g-MMT could be well dispersed in the asprepared nanocomposite membranes attributing to the repulsion each other of surface-grafted PVP chains in casting solution. Consequently, as a hydrophilic modifier, PVP-g-MMT enhanced the hydrophilicity of membrane surface, improved the surface roughness, and changed the surface chemistry composition. These changes resulted from PVP-g-MMT addition were all the functions of PVP-g-MMT content in nanocomposite membranes, in which “threshold” content appeared responding to the best separation and antifouling performances. These novel mixed matrix membranes had great potential for organic pollutants removal in water treatment.

ASSOCIATED CONTENT

S Supporting Information *

Size distribution and hydrophilicity of montmorillonite; DSC spectra, XPS spectra simulation, AFM images, mechanical strength of PVDF/PVP-g-MMT nanocomposite membranes; ultrafiltration performance of PVDF/MMT composite membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 4785


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