Improved Antifouling Properties of Poly(Ether Sulfone) Membrane by

Aug 24, 2015 - ... properties after three cycles of BSA filtration and chemical washing. It is promising that the present study can offer an effective...
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Improved Antifouling Properties of Poly(Ether Sulfone) Membrane by Incorporating the Amphiphilic Comb Copolymer with Mixed Poly(Ethylene Glycol) and Poly(Dimethylsiloxane) Brushes Fan Gao, Guangfa Zhang, Qinghua Zhang,* Xiaoli Zhan, and Fengqiu Chen College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China ABSTRACT: In this study, a novel amphiphilic comb copolymer P(poly(ethylene glycol) (PEG)-r-poly(dimethylsiloxane) (PDMS)) with mixed hydrophilic and low surface energy side chains was synthesized by the free radical polymerization method. The microstructure of the synthesized amphiphilic copolymers was characterized by Fourier transform infrared (FTIR) and the nuclear magnetic resonance proton spectra (1H NMR). The copolymers were blended with poly(ether sulfone) (PES) to fabricate low-fouling ultrafiltration membranes through nonsolvent induced phase separation (NIPS). The surface distribution of the hydrophilic PEG segments and hydrophobic PDMS segments on the membranes via surface segregation during the NIPS process was confirmed by the X-ray photoelectron spectroscopy (XPS), the wetting property, and the surface energy measurements. The multidefense mechanisms from fouling resistant to fouling release for the membranes modified by amphiphilic copolymers P(PEG-r-PDMS) were investigated with bovine serum albumin (BSA) aqueous solution as a model foulant. The antifouling properties of the modified membrane, especially for membrane PES/P(PEG-r-PDMS)∼31.3% (M4), was effectively improved in comparation with pure PES membrane (M0). The flux decline rate of the membrane M4 was as low as 15.6%, and the flux recover ratio was up to 96.6%. The modified membrane also possessed stable and durable antifouling properties after three cycles of BSA filtration and chemical washing. It is promising that the present study can offer an effective approach to construct a low fouling membrane in the application of wastewater treatment and water purification.

1. INTRODUCTION Organic membranes are frequently used in membrane separation processes for marine, medical, industrial, and environmental applications because of their good processability, stability, and low cost.1−4 However, the intrinsic hydrophobic membranes, such as poly(ether sulfone) (PES), poly(vinylidene fluoride) (PVDF), and polysulfone (PSF), are easily contaminated by the attachment and accumulation of foulants on the membrane surface due to the nonspecific interaction.5,6 Serious fouling would reduce the service life of the membrane and increase the cost of operation, which limits the wide application of membrane technology.7 Therefore, it is necessary to find an effective solution to improve the antifouling properties of the membranes. The basic approach of fabricating antifouling membranes is to weaken the interactions between foulants and membrane surface.8 One of the effective methods is to improve the surface hydrophilicity.9 Poly(ether sulfone) (PEG)-based systems are commonly introduced to the membrane surface in order to improve the hydrophilic and low-biofouling properties.10 These hydrophilic surfaces can prevent the foulants from attaching the surface by constructing a compact hydration layer barrier and robust steric repulsion effect on the surface (fouling resistant mechanism).11−13 However, the hydrophilic PEG would be easily washed out of the matrix because of the low compatibility with the membrane matrix. The amphiphilic additive containing hydrophobic anchoring groups and hydrophilic moieties would be a good candidate for membrane modification.14−18 The hydrophobic anchoring groups in the amphiphilic additive are well compatible with the hydrophobic matrix, and hydrophilic moieties can improve the surface wettability. When blending © 2015 American Chemical Society

the amphiphilic additive with the matrix during the membrane forming process, the hydrophilic groups in amphiphilic copolymers will migrate to the surface owing to the completely opposite compatibilities with water and the matrix.19 In order to minimize the interfacial free energy, the driving force of thermodynamics causes the surface segregation of the amphiphilic copolymers.20,21 The fluoropolymers and polysiloxanes with low surface energy and a low Young’s modulus are usually used as the fouling release coating materials.22−24 The principle of the fouling release mechanism is to minimize the intermolecular forces of interactions between the foulants and the synthetic surface, promoting the release of accumulated foulants with the aid of the hydrodynamic forces generated by movement through the water.25,26 A lot of effort has been invested in the fabrication of heterogeneous amphiphilic surfaces, combining synergistically hydrophilic moieties and low surface energy moieties.27,28 The coexistence of multiple defense mechanisms should be more efficient in inhibiting the nonspecific adsorption and deposition from a much broader variety of foulants.29,30 The fluorinated segments are the most used fouling release moieties, while the chemical and physical effects of longer chain perfluorinated compounds on the environments and human health have become a major concern.31,32 The silicones, such as poly(dimethylsiloxane) (PDMS), are considered to have low toxic potential and are evaluated as a Received: Revised: Accepted: Published: 8789

August August August August

4, 2015 21, 2015 24, 2015 24, 2015 DOI: 10.1021/acs.iecr.5b02864 Ind. Eng. Chem. Res. 2015, 54, 8789−8800

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Industrial & Engineering Chemistry Research Table 1. Recipes for Amphiphilic Comb Copolymers of P(PEG-r-PDMS)∼x%a at 80 °C reactive flask expt.

PEGMA (g)

PDMS (g)

AIBN (g)

butylaetate (g)

mole percentage of PDMSb (mol %)

Mnc (g/mol)

P(PEG-r-PDMS)∼0% P(PEG-r-PDMS)∼12.7% P(PEG-r-PDMS)∼21.6% P(PEG-r-PDMS)∼31.3%

10.0 9.0 8.0 7.0

0.0 1.0 2.0 3.0

0.1 0.1 0.1 0.1

22.1 22.1 22.1 22.1

0 12.7 21.6 31.3

10 000 12 300 10 700 11 400

a P(PEG-r-PDMS)∼x%, x denoted the PDMS molar content in the random copolymers, which were calculated by 1H NMR. bThe molar contents were calculated by 1H NMR. cMn determined by GPC analysis.

promising candidate in constructing antifouling surfaces.27,33−35 The fundamental properties of PDMS are related to the specific characteristics of the siloxane bonds and the combination of a flexible backbone and low surface energy side groups.36 The “ambiguous” amphiphilic polymer brush coating incorporating both hydrophilic and hydrophobic components is expected to be capable of resisting biofouling and releasing fouling organisms.37 As far as we know, few studies have been reported about the amphiphilic comb copolymer additive with PDMS and PEG brush-like side chains in membrane antifouling modification. In this study, the novel amphiphilic comb copolymer P(PEGr-PDMS) was synthesized by free radical polymerization and then incorporated into PES to fabricate an amphiphilic ultrafiltration membrane by the wet phase inversion method. During the phase separation process, the hydrophobic methacrylic backbone worked as an anchor to prevent the copolymer removal, and the hydrophilic PEG segments mixed with PDMS chains spontaneously migrated to the membrane surface to form amphiphilic brushes by surface segregation.38,39 The surface composition and wetting property of the modified membranes were determined by X-ray photoelectron spectroscopy (XPS) and contact angle measurement. In addition, the antifouling properties and the stability of the membranes were assessed by a series of ultrafiltration experiments. By adjusting the mole ratio of the PEG and PDMS, the fouling resistant and fouling release multidefense mechanisms were investigated in detail.

The general procedure was as follows. The monomer weight ratio of PEGMA/PDMS was 9:1, 8:2, and 7:3, respectively. Using P(PEG-r-PDMS)∼12.7% as an example, 9 g of PEGMA and 1 g of PDMS were dissolved in 25 mL of butyl acetate and then put into a three-necked round-bottom flask. The reaction flask was placed in an oil bath at a temperature of 80 °C. Then, the flask was purified with nitrogen gas for 15 min to remove the oxygen, adding 0.1 g of AIBN into the reaction device after that. Another 0.1 g of AIBN was put into the flask after a 5 h reaction in order to improve the conversion of monomers. The whole polymerization lasted about 10 h under the nitrogen atmosphere. The reaction solution was purified by rotary evaporation to remove most of the solvent. Then, the copolymer was precipitated in the n-hexane for three times. The obtained copolymer was dried in a vacuum oven at 70 °C for 2 h. The as-synthesized copolymers were designated as P(PEG-rPDMS)∼x%, where x represented the molar percentage of PDMS segments in the copolymer. The recipes of polymerization were summarized in Table 1. Besides, we also prepared the homopolymer of P(PEGMA) following a similar procedure, which is shown as P(PEG-r-PDMS)∼0%. The successful polymerization of the comb copolymer was verified by Fourier transform infrared (FTIR; Nicolet 5700), 1H NMR (Advance DMX500), and gel permeation chromatography (GPC, Waters 1525/2414) characterization after thorough purification. 2.3. Preparation of Membranes. All the membranes were prepared by the nonsolvent induced phase separation method (NIPS), using DMF as the solvent. The casting solution was prepared by dissolving PES and P(PEG-r-PDMS)∼x% amphiphilic copolymer into the organic solvent DMF, fixed at a weight percentage of 15 and 1.0 wt %, respectively. PEG400 was used as pore-forming agent, and its concentration in the solution was 8.0 wt %. The solutions were stirred for 6 h at 60 °C and then left for 6 h to ensure complete release of bubbles. Then, the casting solutions were cast on glass plates with a steel knife and immersed in a coagulation bath. The newly formed membranes were kept in water for 24 h, and water was changed once during that period of time, ensuring a total removal of residual solvent and pore-forming agent. All the membranes were stored in deionized water prior to utilization. The resultant membranes were labeled with Mx membranes, where M0, M1, M2, M3, and M4 represented for the membrane pure PES, PES/P(PEG-r-PDMS)∼0%, PES/P(PEG-rPDMS)∼12.7%, PES/P(PEG-r-PDMS)∼21.6%, and PES/P(PEG-r-PDMS)∼31.3%, respectively. As the purely hydrophilic modifier, the concentration of homopolymer PES/P(PEG-rPDMS)∼0% in the casting solution of was fixed at 1.0 wt %. 2.4. Membrane Structure and Surface Characterization. The cross-section (broken in liquid nitrogen) and top surface morphologies of the membrane were observed by

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ether sulfone) (PES, A100, Mw = 52 000 g/mol, PDI ≈ 2) was purchased from Solvay Advanced Polymers and dried at 110 °C for 12 h before use. Polydimethylsiloxane methyl ether methacrylate (X-22-2475, 420 g/mol) was purchased from Shin-Etsu Chemical Co. Ltd. Poly(ethylene glycol) methyl ether methacrylate (PEGMA, 500 g/mol, Aldrich) was filtered through a basic alumina column to remove the radical inhibitor. Dimethylformamide (DMF), nhexane, critic acid, sodium hydroxide (NaOH), ethanol, and butyl acetate were obtained from Sinopharm Chemical Reagent and used as received. Polyethylene glycol (PEG, Mw = 400 g/ mol, Aladdin) was used as pore-forming agent. Bovine serum albumin (BSA) was purchased from the local reagent corporation and was of commercial analytical grade. Phosphate buffered saline (PBS, 0.01 M) was prepared by dissolving preweighed quantities of potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (Na2HPO4· 12H2O) in deionized water. Deionized water was used throughout the experiments. 2.2. Synthesis of Amphiphilic Comb Copolymer P(PEG-r-PDMS). The amphiphilic comb copolymer P(PEG-rPDMS) was synthesized by free radical solution polymerization. 8790

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Industrial & Engineering Chemistry Research scanning electron microscopy (SEM, Nanosem 430, FEI Co., Ltd.). The membrane samples were sputtered gold prior to SEM analysis. To estimate the surface pore size of the membranes, their molecular weight cutoff (MWCO) was characterized by measuring the rejection of dextran (70, 250, and 500 kDa from Aladdin).40 The UV−vis spectrophotometer was used to measure the concentration of feed and permeate dextran solution for the purpose of calculating the retentions.41 The surface composition of the membranes were analyzed by X-ray photo electron spectroscopy (XPS, PerkinElmer Phi1600 ESCA system) using Mg Kα (1254.0 eV) as the radiation source. Survey scans were taken in the range of 0−1100 eV at takeoff angle of 90°. The surface wettability characteristics of membranes were investigated by measuring the water on the membrane surfaces with a contact angle instrument (KSV Co., Ltd.) at room temperature. The advancing and receding contact angles on the same drop were measured by inserting the needle of the syringe into the drop throughout the experiment while adding and removing small amounts of water to/from the drop. The total surface energy, as well as the dispersive (γsd) and polar (γsp) parameters of all membrane surfaces were determined by using the Owens and Wendt’s42 two-liquid geometric method. The polar component could be further subdivided into electron-accept (or acidic, γs+) and electrondonor (or base, γs−) components. The electron-acceptor (γs+) and electron-donor (γs−) parameters of the surface energies were calculated using the three-liquid Lifshitz-van der Waals acid−base model proposed by van Oss et al.43 The test liquids used in both methods included two polar liquids (water and glycerol) and one nonpolar liquid (hexadecane). Contact angle measurements were taken at five different spots per liquid on each membrane. It should be mentioned here that, considering the permeability of porous membrane, the computed surface energy of the porous membrane surfaces was used for semiquantitative comparisons of surface properties in this study. 2.5. Ultrafiltration Experiments and Antifouling Properties Evaluation. Currently, pressure-driven membrane processes are the dominant membrane processes owing to their energy-saving and eco-friendly features. Because the adsorption and deposition of foulants on membrane surfaces could often result in sharp flux decline during filtration, the antifouling properties (including flux decline resistant and flux recovery properties) were evaluated through room temperature. The separation properties and antifouling performances of membranes were evaluated by the cross-flow ultrafiltration model (Figure 1). The effective area of the membrane was 22.05 cm2. At the beginning of the ultrafiltration experiments, each membrane was compacted for about 30 min at 0.35 MPa to obtain a stable flux; then, the pressure decreased to the operation pressure of 0.3 MPa. The pure water flux Jw1 (L/(m2 h)) was determined by measuring the pure water volume and calculated by the following equations:

Jw1 =

V AΔt

Figure 1. Schematic diagram of the filtration test equipment.

based on the water quantity permeating the membranes. The rejection of model foulant was calculated from the feed and the permeate concentrations via UV-spectrophotometer (UV9200) according to the following equation: ⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(2)

where Cp and Cf (g/L) referred to the concentration of foulants in permeate and feed solutions, respectively. The concentrations were quantified using UV absorbance at 278 nm for BSA. After filtration of the feed solution for 60 min, the membranes were washed with deionized water for about 30 min; then, the water flux of cleaned membranes Jw2 (L/(m2 h)) was measured again. The deleterious effect of biofouling on membrane durability and separation performance is a serious impediment to the development of efficient membrane processes. To examine the antifouling properties of these membranes during foulants solution filtration in detail, several ratios, including the total flux decline ratio (DRt), reversible flux decline ratio (DRr), irreversible flux decline ratio (DRir), and flux recovery ratio (FRR) were defined in the following equations: ⎛ Jp ⎞ ⎟⎟ × 100% DR t = ⎜⎜1 − Jw1 ⎠ ⎝

DR r = DR ir =

FRR =

Jw2 − Jp Jw1

× 100%

Jw1 − Jw2 Jw1

Jw2 Jw1

(3)

(4)

× 100% (5)

× 100% (6)

The higher value of FRR and lower DRt indicated the better antifouling properties of the membranes. 2.6. Antifouling Stability and Durability of Membrane. The antifouling stability after chemical cleaning was measured through the static adsorption of the BSA test in the following steps. Each sample membrane (square shape, 2.0 cm × 2.0 cm) was rinsed by phosphate buffer (0.1 M pH = 7.0) and then wiped with a piece of filter paper to remove the water on the surface. After that, the clean membrane samples (2.0 cm × 2.0 cm) were separately placed in individual wells of a 6-well tissue culture plate at 25 °C, and 8.0 mL of 1 g/L BSA in PBS (0.1 M pH = 7.0) was dropped on the sample surfaces and

(1)

where V (L) is the volume of permeated water, A (m2) is the membrane effective area, and Δt (h) is the permeation time. Subsequently, the stirred cell and solution reservoir were emptied and refilled rapidly with model foulant feed solution. The flux for the feed solution was recorded as Jp (L/(m2 h)) 8791

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Figure 2. (a) FTIR spectra of P(PEG-r-PDMS)∼x%; (b) 1H NMR spectra of P(PEG-r-PDMS)∼x% in deuterated chloroform.

copolymers was analyzed by 1H NMR. Figure 2b shows the H NMR spectra of copolymer P(PEG-r-PDMS) with different compositions. The chemical shift at 3.94 ppm can be assigned to COOCH2− from both PEG and PDMS. Signals at 3.53 and 3.26 ppm are assigned to −OCH2 and −CH3 from PEG, respectively. The peaks observed at 0 ppm are assigned to the −CH3 group in PDMS. The weight percentage of PDMS in the copolymers could be calculated from the intensity ratio of the proton peaks for PEG and PDMS in the 1H NMR spectra. As described in Table 1, the composition of the copolymer calculated by 1H NMR was in good agreement with the feed mole fraction. Both of the FTIR and 1H NMR results showed that the amphiphilic comb copolymers P(PEG-r-PDMS)∼x% were successfully synthesized by the free radical polymerization method. 3.2. Morphology of the Membranes. The surface and cross section morphologies of the prepared membranes are shown in Figure 3. In this study, all the PES membranes were prepared via the NIPS method with P(PEG-r-PDMS)∼x% as modifier. In general, nonsolvent induced phase separation is the predominant method in fabricating the porous asymmetrical membranes composed of a skin layer with micropores on the top and a supported layer with fully developed macrovoids on the bottom. Instantaneous liquid−liquid demixing is thought to provide conditions for macrovoids formation.44,45 As depicted in the Figure 3, all membranes exhibit typically anisotropic cross-sectional morphologies, consisting of a skin layer as selective barrier and a much thicker fingerlike substructure. The formation of this cross-sectional structure was mainly influenced by the high mutual diffusivity of water and DMF. Only a slight change of the fingerlike pore size is observed from the SEM images, indicating that PES still retains good membrane-forming ability after incorporating P(PEG-rPDMS)∼x% copolymer additives. Some pores on the top surface of the membrane are also observed, and the pore size is less than 20 nm by SEM, which demonstrates that the membranes are in the range of ultrafiltration. In order to further characterize the surface pore size of the ultrafiltration membranes, the molecular weight cutoff (MWCO) experiment was carried out. Molecular weight cutoff curves for the membranes M0, M1, and M3 were shown in the

incubated for 12 h to reach adsorption−desorption equilibrium. Next, the samples were rinsed attentively with PBS; the concentrations of BSA in the solution before and after incubation were measured with a UV−vis spectrophotometer, and then, the amount of adsorbed BSA on the membrane was calculated. Triplicate samples for each blend membrane were determined, and the mean values were obtained. The cyclic filtration of BSA aqueous solution was tested to verify the antifouling durability of the modified membrane. Each filtration cycle could be divided into three steps, which were the same as the above ultrafiltration experiments. The ith cycle permeation flux (Jwi or Jpi) was checked from time to time until steady, where the parameters Jwi and Jpi meant the pure water and BSA aqueous solution flux in the ith cycle permeation flux, respectively.

1

3. RESULTS AND DISCUSSION 3.1. Synthesis and Analysis of Amphiphilic Comb Copolymer P(PEG-r-PDMS). Amphiphilic comb copolymers P(PEG-r-PDMS)∼x% were synthesized via free radical polymerization using AIBN as initiator in butyl acetate solution. The detailed synthesis formulations of copolymers are shown in Table 1. The hydrophilic PEG and low surface energy PDMS side chains were distributed randomly in the hydrophobic methacrylic main chain to form amphiphilic comb copolymer P(PEG-r-PDMS). The GPC test results show that the molecular weight of the copolymers is from 10 000 to 12 300 g/mol. The chemical composition of the amphiphilic comb copolymers was identified by FTIR and 1H NMR spectra. The FTIR spectra was recorded before and after polymerization to verify the functional group of the copolymer P(PEGr-PDMS). As shown in the Figure 2a, the peak at 1727 cm−1 is assigned to the stretching vibration of the ester CO in acrylate. The existence of the C−O−C stretching band is confirmed by the band at 1114 cm−1 in the spectrum of P(PEGMA). The peak corresponding to the asymmetrical stretching vibrations of Si−O−Si occur at 1057 cm−1 in PDMS. The adsorption band at 1256 cm−1 should be assigned to the Si−CH3 bending deformation, and the adsorption bands at 843 and 758 cm−1 are assigned to the C−Si−C stretching vibrations. The chemical composition of the amphiphilic 8792

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and the decrease of sulfur molar ratio were observed to be obvious for the PES/P(PEG-r-PDMS)∼x% membranes. This phenomenon would be attributed to the migration of the hydrophilic PEG segments and PDMS segments to the membrane surfaces.33 Since PDMS segments are the only source of silicone, the XPS peak at 101.4 eV (Si 2p) appearing on the membrane demonstrates the actual existence of PDMS segments on the membrane surfaces caused by forced surface segregation. For further determining the quantitative information, the C 1s core level spectra was resolved into peaks representing different chemical environments using a sum of Lorentzian−Gaussian functions: binding energy location around at 284.7 eV for C−C, 286.0 eV for C−O/S, 288.4 eV for CO, and 284.2 eV for C−Si.33 The PDMS segments are the only sources of C−Si species; the near-surface coverage of PDMS (ΦPDMS) for PES/ P(PEG-r-PDMS)∼x% can be calculated from the following expression:33 ΦPDMS =

Figure 3. SEM on top surface and cross section morphologies: (a, b, c) pure PES (M0), (d, e, f) PES/P(PEG-r-PDMS)∼0% (M1), and (g, h, i) PES/P(PEG-r-PDMS)∼31.3% (M4). The magnifications were 100 000 for top surface images and 1000 and 5000 for cross sections, respectively.

A C−Si /(A C−Si + A C−C + A C−O/S + ACO) 10/15 (7)

where AC−Si, AC−C, AC−O/S, and ACO are the areas of fitted C− Si, C−C, C−O/S, and CO peaks, respectively. The factor 10/15 accounts for the fact that, in 15 carbon atoms per repeat unit of PDMS segments, there is 10 (C−Si) atom. Meanwhile, the near surface coverage of PEG for PES/P(PEG-r-PDMS)∼x % membranes was estimated by a similar method. The carbonyl carbon molar ratio, PCO, is calculated by eq 8:38

Figure 4. Results shows that the MWCO at the rejection of 90% shifts from approximately 100 kDa (for membrane M0) to

PCO =

ACO AC−C + AC−Si + AC−O / S + ACO

(8)

The near surface coverage of PEG for PES/P(PEG-rPDMS)∼x% membranes is calculated by Eq9:46 PCO = ΦPEG ×

1 1 + ΦPDMS × 23 15

(9)

MCO is obtained from XPS analysis results; 1/23 represents the theoretical mole ratio of CO in PEG. The factor 1/15 accounts for the fact that, in 8 carbon atoms per repeat unit of PDMS segments, there is one CO carbon atom. The corresponding information is listed in Table 2. As listed in Table 2, the values of ΦPEG and ΦPDMS for PES/P(PEG-r-PDMS)∼x% membranes are much higher than the theoretical values determined from the actual copolymers composition and the formulation of casting solutions. During the phase inversion process, a thin film of casting solution was immersed in the coagulation bath and the subsequent diffusive exchange of solvent and nonsolvent triggered the precipitation of polymer. The pronounced enrichment of hydrophilic PEG segments and low surface energy PDMS segments on the membrane surface should be ascribed to the free and forced surface segregation during the phase inversion process.38 It is generally believed that the hydrophobic PDMS is inclined to be entrapped in the matrix membrane because of the unfavorable solution thermodynamics.47 Due to the intrinsic hydrophilic and the low interfacial free energy with water, the PEG segments tended to freely migrate to the polymer−water interfaces during the phase inversion process. Meanwhile, the covalently binding nonpolar PDMS segments were dragged onto membrane surfaces by hydrophilic PEG segments via forced surface segregation.26 Thus, the amphiphilic membrane

Figure 4. Molecular weight cutoff for membranes M0, M1, and M3.

between 270 and 320 kDa (for membrane M3 and M1). It implied that the incorporation of comb polymer P(PEG-rPDMS) had led to membranes with a larger surface pore size.40 3.3. Surface Chemical Features. The near-surface chemical compositions of the control and modified membranes were determined by XPS analysis. Figure 5 shows the wide scans, C 1s core level resolving results of the XPS spectra of the modified membranes. In the wide-scan XPS spectra, the surface elemental mole percentages of the prepared membranes were calculated, and XPS characteristic signals 284.8 eV (C 1s), 545.2 eV (O 1s), 101.4 eV (Si 2p), and 175.2 eV (S 2p) were observed in the modified membranes. The surface elemental mole percentages of the modified membranes are listed in the Table 2. The values of oxygen and sulfur signals of the pristine PES membrane are 23.4% and 4.5%, which are determined by XPS analysis. Meanwhile, the increase of oxygen molar ratio 8793

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Figure 5. XPS wide scans, C 1s core level resolving results of (a, b) M1, (c, d) M2, and (e, f) M4 membranes.

Table 2. Surface Elemental Mole Percentages and Surface Segment Coverage of PES and Modified Membranes surface elemental (mol %)

surface segment coverage (mol %)

membrane ID

C

O

S

Si

a ⌀XPS PEG

b ⌀theo. PEG

a ⌀XPS PDMS

b ⌀theo. PDMS

theo. c ⌀XPS PDMS/⌀PDMS

M0 M1 M2 M3 M4

72.1 70.8 69.9 70.0 69.1

23.4 24.8 24.3 23.9 24.2

4.5 4.4 4.2 4.1 4.0

1.6 2.0 2.7

36.4 44.0 38.3 30.0

27.1 20.6 19.8 17.4

11.3 12.5 15.1

3.0 5.5 7.9

3.8 2.3 1.9

a The actual value of surface segment coverage was calculated from the XPS results. bThe theoretical value of surface segment coverage was based on the actual content of segments in the casting solutions. cThe ratio of actual surface coverage based on theoretical surface coverage of PDMS segments.

8794

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Figure 6. Schematic diagram of mixed amphiphilic brushes P(PEG-r-PDMS)∼x% and its migration behavior in the PES blend solution during the NIPS process.

hydrophilicity slightly decreased due to more PDMS segments distributed on the membrane surface. However, the water contact angles were still lower than 90°. In order to further study the influence of amphiphilic copolymers on the wetting property of the membranes, the dynamic change of the contact angles in 600 s was recorded after dropping water on the membrane surface.48 The dynamic wetting property can be used to estimate the amphiphilic heterogeneity surface, and the results are shown in the Figure 8a. After 600 s, the contact angle of pure PES membrane (M0) decreases modestly to 18.2° with the prolongation of the contact time. Owing to the lack of hydrophilic PEG segments, the membrane M0 showed a minimum change of contact angle. In contrast, the modified membranes exhibit more obvious changes due to the amphiphilic segments migrating to the surface. Considering that the values of surface roughness of the modified membranes were similar, the different decaying tendencies of the contact angle were mainly dependent on the hydrophilicity of the membrane surface.49 In addition, the advancing and receding contact angles were also performed to investigate the heterogeneous amphiphilic membrane surface modified with amphiphilic copolymer brushes. It is well know that the advancing water contact angle θa is sensitive to the hydrophobic surface component, and the receding contact angle θr is sensitive to the hydrophilic surface component.50 As shown in the Figure 8b, because of the presence of hydrophilic component (PEG), the receding contact angle of membrane M1 is 48.1°, which is lower than that of M0 (70.8°). The content of hydrophobic PDMS segments on the surface plays a key role in the increase of the advancing contact angle. As the prepared membranes were expected to have a similar surface roughness, the hysteresis contact angle demonstrated chemical heterogeneity of the membrane surface combining the mixed polar and nonpolar brushes.33,50 The surface free energy was also calculated according to the contact angles of probe liquids. The total surface energies (γs) along with the polar (γsp), dispersion (γsd), Lifshitz-van der Waals (γsLW), electron-donor (γs−), and electron-acceptor (γs+) components were evaluated using a two-liquid geometric model and three-liquid Lifshitz-van der Waals acid−base model (Table 3). The surface energy of the (M1) membrane is as high as 44.9 mJ/m2, which is due to the high surface energy PEG segments on the near-surface region. However, the surface free energy of the PES/P(PEG-r-PDMS)∼x% membranes decreased gradually with the increase of surface coverage of PDMS segments.

surface with a heterogeneous structure of hydrophilic PEG and low surface energy PDMS brushes were obtained in one step. Figure 6 shows the schematic diagram of mixed amphiphilic brushes P(PEG-r-PDMS)∼x% and its migration behavior in the PES blend system during the phase inverse separation process. Moreover, the ΦPDMS value for M2 is nearly 4 times higher than the theoretical value, while the ΦPDMS value for M4 decreases to 1.9 times. This indicated that, with the increase of PDMS content in the amphiphilic copolymer, the dragging effect of hydrophilic PEG became weaker, limiting the efficiency of forced segregation behavior of PDMS. 3.4. Surface Wetting Property. The surface wettability property is one of the most important factors affecting the separation performance and antifouling properties of ultrafiltration membranes. The wetting property of the membrane can be easily obtained from the static water contact angle measurements. Figure 7 shows the static water contact angle of

Figure 7. Static water contact angle of membranes: pure PES membrane (M0), PES/P(PEG-r-PDMS)∼0% (M1), PES/P(PEG-rPDMS)∼12.7% (M2), PES/P(PEG-r-PDMS)∼21.6% (M3), and PES/ P(PEG-r-PDMS)∼31.3% (M4).

membranes blended with different amphiphilic copolymers. The static contact angle of pure PES membrane M0 is about 70.8°. The surface hydrophilicity of the membrane M1 is significantly improved with a water contact angle of about 60.5°. The obvious improvement in the hydrophilicity was believed to originate from the migration of PEG segments to the membrane surface via free surface segregation. For the PES/P(PEG-r-PDMS)∼x% membranes, with the increase of the PDMS contents in copolymer P(PEG-r-PDMS), the surface 8795

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Figure 8. Dynamic water contact angle measurements: (a) water contact angle change in 600 s; (b) advancing, receding, and hysteresis contact angles.

trend when the distribution of PDMS increased on the surface. It was found that the similar results were obtained using the three-liquid Lifshitz-van der Waals acid−base model as shown in Table 3. 3.5. Membrane Permeation and Antifouling Properties. Pressure-driven filtrations by porous membranes are considered as the most promising candidate for water treatment. Compared with the nonporous surfaces, porous membrane surfaces confront a great challenge in preventing the fouling,51 because convective flow through the membranes give the foulants a direct chance to attach the membrane surface. In this study, the modified membranes were assessed in a crossflow filtration model employing BSA aqueous solution as model foulant. To evaluate the antifouling properties of the membranes, flux decline ratio (DRt), flux recovery rate (FRR), reversible flux-decline ratio (DRr), and irreversible flux-decline ratio (DRir) are needed. Membrane fouling includes the reversible fouling and the irreversible fouling. Reversible fouling is generated from the formation of a cake layer on the membrane surface, due to the deposition of foulants and weak interaction between the foulants and the membrane surface, which could be removed by simple

Table 3. Surface Energy of Membranes Calculated by the Two-Liquid Geometric Method and Three-Liquid Lifshitzvan der Waals Acid−Base Model two-liquid model (mJ/m2)a membrane

γs

γs

M0 M1 M2 M3 M4

39.8 44.9 37.4 32.7 28.2

27.1 27.3 26.9 26.6 24.9

d

γs

three-liquid model (mJ/m2)b p

12.8 17.5 10.5 6.6 3.5

γs

γsLW

γs+

γs−

37.91 39.99 29.48 28.28 25.65

27.06 27.34 26.99 26.16 24.71

2.40 1.95 0.00 0.11 0.02

12.27 20.48 20.45 10.81 7.69

Two-liquid model: γs = γsd + γsp, γL(1 + cos θ) = 2(γLdγsd)1/2 + 2(γLpγsp)1/2. bThree-liquid model: γs = γsLW + 2(γs+γs−)1/2, γL(1 + cos θ) = 2((γsLWγLLW)1/2 + (γL+γs−)1/2 + (γs+γL−)1/2). a

The M4 membrane has a minimum value of 28.2 mJ/m2 when the PDMS segment surface coverage is 15.1 mol % (Table 2). The γsp value of PES/P(PEG-r-PDMS)∼x% membranes decreased significantly, which was attributed to the high contents of PDMS segments. First, the dispersion force (γsLW) increased slightly, owing to the presence of polar segments PEG. Then, the dispersion force displayed a declining

Figure 9. (a) Time-dependent water permeation during BSA solution filtration of the pure PES membrane (M0), PES/P(PEG-r-PDMS)∼0% (M1), PES/P(PEG-r-PDMS)∼12.7% (M2), PES/P(PEG-r-PDMS)∼21.6% (M3), and PES/P(PEG-r-PDMS)∼31.3% (M4). The filtration operation included three steps: pure water permeation, BSA solution filtration, and pure water permeation after membrane cleaning. (b) A summary of the corresponding DRt, DRr, DRir, and FRR. 8796

DOI: 10.1021/acs.iecr.5b02864 Ind. Eng. Chem. Res. 2015, 54, 8789−8800

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Figure 10. Tentative illustration of multidefense mechanisms for heterogeneous membrane.

PDMS brushes on the membrane surface or pore wall effectively migrated the membrane fouling. The antifouling model of constructing multidefense mechanisms of amphiphilic membrane is showed in Figure 10. Due to the hydrogen-bonding interaction, the hydrophilic PEG brushes could strongly bind the water molecules to generate a hydration layer, preventing the foulants from attaching directly to the membrane surface and achieving fouling resistant ability.10 However, some irreversible foulants firmly attached to the membrane surface, which could not be removed by simple hydraulic washing. It would be impossible for PEG segments to prevent foulants adsorption completely. In contrast, the low surface energy PDMS brushes could reduce the adsorption strength of the foulants and tended to promote the release of the foulants by the shearing stress near the membrane surface region.33 Therefore, the collaborative complementary function of optimized hydrophilic segments (PEG) and low surface energy segments (PDMS) on the developed membrane surfaces was found to be responsible for the desired comprehensive antifouling performances. In short, the amphiphilic heterogeneous PES/P(PEG-r-PDMS)∼x% membrane surface, combining the fouling resistance and fouling release properties, endowed the membrane with satisfactory excellent antifouling performances. 3.6. Antifouling Stability and Durability of Membrane. Due to the membrane fouling, a regular chemical cleaning is generally used in the membrane operation process in order to sustain the filtration efficiency in the long term operation. Hence, the membrane’s antifouling stability after cleaning should be concerned.54 According to the chemical cleaning technology applied in the industry, acid and base cleaning are often used. The membrane PES/P(PEG-rPDMS)∼21.6% (M3) was immersed in 1.0 wt % critic acid and 1.0 wt % NaOH solutions with ultrasonic treatment during 0.5, 1, 3, and 6 h. Subsequently, the static water contact angle and BSA adsorption on membranes were measured, and the results were shown in the Figure 11. Obviously, the water contact angles and the BSA adsorption remained constant after 1 h of the chemical cleaning in both acidic and alkaline solutions. It was demonstrated that the modified PES membrane with mixed PEG and PDMS brushes had stable antifouling properties after the chemical cleaning.

hydraulic cleaning and near-surface sheer force, whereas irreversible fouling is caused by the entrapment of foulants in pore channels and firm adsorption, which could not be removed by simple hydrophilic cleaning.39 It is regarded as the main reason for the deterioration of membrane performances. The time-dependent fluxes of the prepared membranes are showed in the Figure 9. For the membrane M0, it exhibited a severe flux decline and lower flux recovery after washing, indicating a bad antifouling property. The reversible fluxdecline ratio (DRr) and irreversible flux-decline ratio (DRir) are as high as 54.8% and 45.7%, respectively, and the flux recovery ratio (FRR) is as low as 54.3%. It had the most serious irreversible fouling because of the adsorption of protein on the membrane pore wall or surface. For the membrane M1, the DRir value decreases to 28.3% due to the surface hydrophilization, and DRr still remains at a high level (29.9%). On account of the strong interaction between the proteins and the membrane, the fouling behavior had not effectively changed and the flux recovery was inhibited to recover to 100%. With the increase of PDMS surface coverage on the membrane surface, the DRr and DRir successively decrease, while the FRR accordingly exhibits an opposite tendency, indicating the improvement of the antifouling performances (both fouling resistance and fouling release) of the membranes. Moreover, the DRr/DRt (reversible fouling percentage) gradually became larger, which revealed that the reversible fouling developed more dominantly than the irreversible fouling. With the highest ΦPDMS values (16.7%), the membrane M4 has the minimum level of total flux decline (15.6%). The membrane also exhibits only 3.4% irreversible flux-decline and 12.1% reversible fluxdecline, corresponding to 96.6% flux recovery. It was indicated that the M4 membrane possessed the best antifouling property. Previous works have proved that membrane fouling was more severe for the larger pore size as compared to the smaller pore size membrane, and the main type of membrane fouling for the larger pore size membranes is pore blocking followed by cake layer formation.52,53 In the present work, the pore size of the comb-polymer blend membranes (higher molecular weight cutoff) are larger than that of pure PES membrane M0. However, the modified membranes exhibited a better antifouling property in comparison with the M0. It was demonstrated that the incorporation of mixed PEG and 8797

DOI: 10.1021/acs.iecr.5b02864 Ind. Eng. Chem. Res. 2015, 54, 8789−8800

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Industrial & Engineering Chemistry Research

of the membrane. The modified membranes covered with mixed hydrophilic and low surface energy brushes combine multidefense mechanisms of fouling resistance and fouling release. The membrane fouling was effectively suppressed by adjusting the surface coverage ratio of hydrophilic PEG and low surface energy PDMS brushes. For the membrane M4, PES/ P(PEG-r-PDMS)∼31.3%, the flux decline rate was as low as 15.6%, and the flux recovery ratio was up to 96.6%. The modified membrane also possessed a stable and durable antifouling property after three cycles of BSA filtration and chemical washing. It is promising that the present study can offer an effective approach to construct a low fouling membrane in the application of wastewater treatment and water purification.

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Figure 11. Antifouling stability of membranes M3 in critic acid and NaOH solutions.

AUTHOR INFORMATION

Corresponding Author

The long-term antifouling durability of the prepared membrane was investigated in detail. The three cycles of ultrafiltration tests were employed to evaluate the durability of antifouling properties. To compare the flux recovery/ antifouling performance of virgin (M0) and modified PES membrane (M4), all of permeation fluxes were normalized by the first-cycle pure water flux and defined as a normalized flux (Figure 12). It is observed that the normalized flux decline rate

*E-mail: [email protected]. Tel: +86-571-8795-3382. Fax: +86-571-8795-1227. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by National Natural Science Foundation of China (Nos. 21176212, 21276224, 21476195) and Zhejiang Provincial Natural Science Foundation of China (LY14B060008).

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Figure 12. Time dependence on recycling flux for the M0 and M4 membrane. The entire process was operated with three cycles of 1 g/L BSA solution filtration at room temperature.

of the M0 continuously increases from 54.7% to 75.4% to 87.7%, and the flux recovery rate after simply pure water washing decreases from 54.3% to 46.4% to 22.1%. The fouling situation on the M0 was gradually severe after several times of operation, indicating the low durability. However, the normalized flux decline rate of the M4 increases slowly from 4.9% to 10.5% to 13.4%, and the flux recovery ratio still remains at a high level, which is more than 86%. It implied that the modified PES membrane’s antifouling properties remained stable after three cycles of filtration and had durability in the long-time operation.

4. CONCLUSIONS In this study, a novel amphiphilic comb copolymer P(PEG-rPDMS) was synthesized and blended into the PES matrix to fabricate a low fouling membrane. The backbones of the comb copolymer have good compatibility with the membrane matrix, and the amphiphilic side chains tend to migrate to the surface 8798

LIST OF SYMBOLS Mn = molecular weight of P(PEG-r-PDMS)∼x% polymers γs = total surface energy (mJ/m2) γsd = dispersive parameter of the surface energy (mJ/m2) γsp = polar parameter of the surface energy (mJ/m2) γsLW = Lifshitz-van der Waals parameter of the surface energy (mJ/m2) γs− = electron-donor parameter of the surface energy (mJ/ m2) γs+ = electron-acceptor parameter of the surface energy (mJ/ m2) Jw1 = initial pure water flux (L/(m2h)) Jp = flux for model foulant feed solution (L/(m2h)) Jw2 = pure water flux of cleaned membrane (L/(m2h)) V = volume of permeated water (L) A = membrane effective area (m2) Δt = permeation time (h) Cp = the concentration of foulant in permeate solutions (g/ L) Cf = the concentration of foulant in feed solutions (g/L) DRt = total flux decline ratio during foulants filtration DRr = reversible flux decline ratio during foulants filtration DRir = irreversible flux decline ratio during foulants filtration FRR = flux recovery ratio during foulants filtration ⌀XPS PEG = surface coverage of PEGMA segments measured by XPS ⌀theo. PEG = theoretical surface coverage of PEGMA segments ⌀XPS PDMS = surface coverage of PDMS segments measured by XPS ⌀theo. PDMS = theoretical surface coverage of PDMS segments theo. ⌀XPS PDMS/⌀PDMS = ratio of actual surface coverage based on theoretical surface coverage of PDMS segments M0 = pure PES membrane M1 = membrane PES/P(PEG-r-PDMS)∼0% DOI: 10.1021/acs.iecr.5b02864 Ind. Eng. Chem. Res. 2015, 54, 8789−8800

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M2 = membrane PES/P(PEG-r-PDMS)∼12.7% M3 = membrane PES/P(PEG-r-PDMS)∼21.6% M4 = membrane PES/P(PEG-r-PDMS)∼31.3%

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