Enhancing the Antifouling and Antimicrobial Properties of Poly(ether

Aug 28, 2014 - As a result, PES ultrafiltration (UF) membranes may undergo serious ..... the National High Technology Research and Development Program...
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Enhancing the Antifouling and Antimicrobial Properties of Poly(ether sulfone) Membranes by Surface Quaternization from a Reactive Poly(ether sulfone) Based Copolymer Additive Yi-Fan Zhao, Li-Ping Zhu,* Jin-Hong Jiang, Zhuan Yi, Bao-Ku Zhu, and You-Yi Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR. China S Supporting Information *

ABSTRACT: A facile and practical method for fabricating antifouling and antimicrobial poly(ether sulfone) (PES) membranes is developed in this work. A PES blend membrane was first fabricated by a traditional nonsolvent induced phase separation (NIPS) process with a reactive amphiphilic copolymer poly(ether sulfone)-block-poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA-b-PES-b-PDMAEMA) as the blending additive. The hydrophilic PDMAEMA chains in the copolymer additive spontaneously migrated toward the interfaces between membrane and coagulant bath (usually water) during membrane formation. The enriched PDMAEMA chains on the as-made blend membranes were then quaternized by reacting with 1,3propane sultone (1,3-PS) and 1-bromodecane, respectively. The zwitterionic PES membranes obtained from 1,3-PS treatment showed significantly improved antifouling ability and hemocompatibility. In another case, the resulting cationic PES membranes treated with 1-bromodecane exhibited excellent antibacterial activities against Escherichia coli (E. coli) and Staphyloccocus aureus (S. aureus). The surface quaternization from the reactive polymeric additive provides a facile and versatile strategy for further functionalization of phase inversion membranes. This method is also suitable for other membrane materials. Moreover, it is easy to scale up in industrial applications.

1. INTRODUCTION Poly(ether sulfone) (PES) is one of the most favorable membrane materials thanks to its good chemical stability, mechanical strength, and thermal resistance.1−5 However, since the inherent hydrophobic characteristics of PES, some nonpolar solutes, hydrophobic particles, bacteria, etc., are apt to be adsorbed onto the membrane surfaces or pore walls. As a result, PES ultrafiltration (UF) membranes may undergo serious membrane fouling in aqueous filtration and bring rapid decline in permeation flux and increase in energy consumption.6,7 When used as a blood-contact material, PES membranes with poor hemocompatibility may lead to undesirable platelet adhesion, activation, and blood coagulation.8 In addition, during storage and conveyance, the growth and colonization of bacteria on membrane surface usually causes the irreversible deterioration in physiochemical and separation/permeation properties of PES membrane.9 Therefore, to improve the antifouling and antimicrobial properties of PES membranes, the modification of membrane prior to use is often necessary and helpful. Numerous literature works have been published about the modification of polymeric membranes prepared by phase inversion process. The commonly used modification methods for PES membranes include coating, blending, surface grafting, etc.10−16 These methods have achieved many successes in improving the fouling resistance of PES membranes. However, they are often companied by various shortcomings. For example, the leaching of hydrophilic coating or water-soluble additive is nearly unavoidable, which leads to the lack of longterm durability in modification validity. Chemically or radiation induced grafting techniques often require complicated post© 2014 American Chemical Society

treatment steps and extreme conditions. Moreover, chemical grafting may cause the damages in membrane bulk structures and thus the deterioration of mechanical properties.8,17 In recent years, amphiphilic copolymers have been developed as the promising macromolecular additives to improve membrane properties via blending modification.18−20 Actually, compared to the traditional water-soluble polymers or inorganic particles, amphiphilic copolymers are more durable in membranes due to the chain entanglements between their hydrophobic chains and membrane-preparation host polymers. More importantly, due to the surface segregation phenomenon, a small quantity of amphiphilic copolymer addition can achieve a high coverage of hydrophilic chains on membrane surface. Nonionic water-soluble poly(ethylene glycol) (PEG)-based materials are often used as the hydrophilic chains of amphiphilic copolymers. However, in long-term applications, PEG chains are susceptible to oxidative degradation and even lose their capability in biological media.21−23 So it is desirable to find novel and more durable polymers as the hydrophilic chains of amphiphilic copolymers. In recent years, some ionic polymers are becoming more popular than PEG in the hydrophilic and antifouling modification of polymer membranes due to their higher chemical stability. Polyzwitterions including phosphorylcholine, sulfobetaine, carboxybetaine polymers, etc., have received growing attention as the next-generation antifouling materials Received: Revised: Accepted: Published: 13952

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Figure 1. Scheme for the fabrication and surface quaternization of the PES/PDMAEMA-b-PES-b-PDMAEMA blend membranes.

Figure 2. Scheme for the synthesis of (A) PES−OH, (B) PES-CTA, and (C) PDMAEMA-b-PES-b-PDMAEMA.

due to their highly hydration, excellent blood compatibility, and antifouling properties.23−26 After surface modification with polyzwitterions, the hydration layers formed via electrostatic interactions are able to greatly inhibit the attachments of proteins or bacteria onto material surfaces. Up to now, many literature works have reported the modification of some polymer membranes through grafting of polyzwitterions.27−32 These investigations indicated that the zwitterionic membranes

exhibit improved protein adsorption resistance, suppressed platelet adhesion, and prolonged plasma-clotting time. In addition, previous studies have also demonstrated that the polymers having quaternary ammonium salt groups are capable of resisting the propagation and growth of bacteria and, thus, can be used as antimicrobial materials.32−35 As discussed above, ionic polymers are favorable as the hydrophilic and antibiofouling modifier of polymer membranes. 13953

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2.3. Fabrication of PES/PDMAEMA-b-PES-b-PDMAEMA Blend Membranes. The blend membranes were fabricated via traditional NIPS process. To prepare the homogeneous casting solutions, PES and the block copolymer PDMAEMA-b-PES-b-PDMAEMA together with the poreforming agent PEG-400 were fully dissolved in DMAc with mechanical stirring overnight at 40 °C. The total concentration of PES and the block copolymer was fixed at 20.0 wt % and that of PEG-400 was constant at 4.0 wt %. The weight percentage of the block copolymer relative to PES varied at 0, 10, 15, 20 wt %. After releasing bubbles, a casting solution was cast on a glass plate using a blade of 150 μm. Subsequently, the glass substrate together with the solution film was immersed into a coagulation bath of DMAc and deionized water mixture (the volume ratio of DMAc/H2O was 40/60). After the membrane detached from the glass plate, it was thoroughly washed with deionized water. The as-made membranes with different copolymer concentrations (0, 10, 15, and 20 wt %) were named as M0, M1, M2, and M3, respectively. 2.4. Quaternization of PES/PDMAEMA-b-PES-bPDMAEMA Blend Membranes. The process for fabricating zwitterionic PES membranes was depicted as follows. A membrane sample M2 (∼200 mg) was immersed into a flask containing methanol (50 mL) and 1,3-PS (5 g) and shaken at 60 °C for a given time. Afterward, the obtained membrane (referred to as ZMt, where t means the modification time) was rinsed with ethanol and deionized water alternately. In a typical process of preparing cationic membrane, an asmade blend membrane sample (M1, M2, or M3) was immersed into a flask containing ethanol (50 mL) and 1-bromodecane (5 g) and shaken at 60 °C for 12 h. After reaction, the resulted cationic membranes (named as CM1, CM2, and CM3) were taken out and thoroughly washed with ethanol and deionized water repeatedly. 2.5. Membrane Characterizations. The surface chemistry of the investigated membranes was analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR) and X-ray photoelectron spectroscopy (XPS). XPS analysis was performed on a spectrometer (XPS, PHI 5000C ESCA system) using Mg Kα as radiation source. A scanning electron microscope (SEM, SIRION-100, FEI Co., Ltd.) was used to observe the membrane surface and cross-sectional morphologies. Water contact angle measurement was employed to evaluate the hydrophilicity of membranes and done using a Dataphysics Instrument (Germany, OCA20) with sessile drop model at 25 °C. For each sample, the reported result was the average of at least five measurements. An electrokinetic analyzer (SurPASS Anton Paar, GmbH, Austria) was utilized to test the zeta potential of membrane surface. The pH of solution varied from 3 to 11 by adding NaOH or HCl, and the measurements were performed at room temperature.37 The zeta potential was calculated according to Helmholtz−Smoluchowski eq 1

However, the ionic polymers as well as the amphiphilic copolymers containing ionic hydrophilic chains are often not well soluble in the commonly used aprotonic polar solvents such as N,N-dimethylformide (DMF) and N,N-dimethylacetamide (DMAc) for membrane casting. Therefore, amphiphilic copolymers containing high concentration of ionic hydrophilic chains cannot be directly used as the blending additive of casting solution for membrane preparation. Fortunately, the postquaternization of amphiphilic copolymers containing reactive poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) chains provides a feasible approach for the surface ionization of phase inversion membranes. More importantly, surface chemistry of the modified membranes can be designed and tuned by the selection of specific quaternization agents. In this work, the synthesized amphiphilic copolymer PDMAEMA-b-PES-b-PDMAEMA was designed as a macromolecular additive for preparing PES blend UF membranes via nonsolvent induced phase separation (NIPS) process. The surface-enriched PDMAEMA chains were then transformed into polyzwitterions and polycations via quaternization, respectively (as shown in Figure 1). The hydrophilicity, permeability, antifouling properties, and blood compatibility as well as antibacterial activity of the prepared membranes were discussed in detail. The objective of this work is to prepare antifouling and antimicrobial PES membrane via postquaternization from the reactive PDMAEMA-b-PES-b-PDMAEMA additive.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ether sulfone) (PES, A100, Mw = 52 000 g/mol, PDI ≈ 2) was purchased from Solvay Advanced Polymers. Bis(4-fluorophenyl) sulfone (FPS, 99%) and bis(4hydroxyphenyl) sulfone (HPS, 99%) were obtained from Sigma-Aldrich. 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 99%) was obtained from Aladdin and passed through a silica gel column prior to use. 1,3-Propanesulfonate (1,3-PS, ≥99%) and 1-bromodecane (≥99%) were purchased from Aladdin and used as received. Escherichia coli (E. coli) and Staphyloccocus aureus Rosenbach (S. aureus) were supplied by the college of Life Sciences, Zhejiang University. The plateletrich plasma (PRP) was supplied by the Blood Center of Zhejiang Province, China. DMAc, toluene, methylene dichloride (CH2Cl2), and other chemicals were commercially analytical grade. 2.2. Synthesis of PDMAEMA-b-PES-b-PDMAEMA via RAFT Polymerization. The chemical structure and synthesis route of ABA type copolymer PDMAEMA-b-PES-b-PDMAEMA are shown in Figure 2. The detailed synthesis process and characterization are described in the Supporting Information. In brief, the synthesis process included the following steps: (A) The hydroxyl-terminated poly(ether sulfone) (PES−OH) was synthesized via a condensation polymerization between FPS and HPS.36 (B) The macro chain transfer agent (PES-CTA) was synthesized via esterification between 4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanyl pentanoic acid (CDP) and PES− OH. (C) The triblock copolymer PDMAEMA-b-PES-bPDMAEMA was prepared through reversible addition− fragmentation chain transfer (RAFT) polymerization. The weight-average molecular weight (Mw) and polydispersity index (PDI) of the used triblock copolymer in the present work are 29 900 g/mol and 1.52 (determined by GPC), respectively. The Mw of PES block in the copolymer was 20 000 g/mol.

ζ=

ΔE ηκ ΔP εε0

(1)

where ζ is the zeta potential, ΔE is the streaming potential, ΔP is the hydrodynamic pressure in the tunnel, η is the solution viscosity, κ is the solution conductivity, ε and ε0 are the permittivity of the test solution and free space, respectively.38 2.6. Filtration Experiments. The filtration performances of membrane were determined by a self-designed dead-end 13954

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Figure 3. (A) XPS wide scans of pure PES membrane M0 and blend membrane M2 and the elemental atomic percentages of the membranes (M0− M3, takeoff angle was 45°). (B) Surface elemental mole percentage of the blend membrane M2 detected by XPS with different probe depth. (C) Water contact angles for the membranes (M0−M3). (D) Permeability (Jw1, Jp, and Jw2) and BSA rejection of the membranes (M0−M3). Data were means ± SD (n = 3).

filtration apparatus (Supporting Information Figure S4). The filtration test mainly included three steps.39,40 First, a membrane sample was prepressured under 0.25 MPa with deionized water for 30 min, then the pressure was reduced to 0.2 MPa and the pure water flux (Jw1) was measured after stable filtration for 60 min and calculated by eq 2. Afterward, the feed liquid was displaced with 1 g/L BSA solution (PBS, pH 7.4) and the real-time flux was recorded as JP. The BSA rejection (R) was calculated by eq 3. Finally, the membrane after protein solution permeation was rinsed thoroughly with water. The pure water flux for the cleaned membrane was remeasured as Jw2. The flux recovery ratio (FRR) was calculated by eq 4.

J=

V ST

reported method.10 A membrane sample (2 cm × 2 cm) was incubated with 20 μg mL−1 BSA-FITC solution in dark for 8 h at 25 °C followed by several rinses with PBS buffer solution. A Zeiss Axiovert 200 inverted microscope (Axiovert 200 M, Zeiss, Germany) was used to observe the adsorption of BSA-FITC on membrane sufaces.40,41 Platelet adhesion tests were conducted as described in the literature.39−41 Some clean membrane samples (1.0 cm × 1.0 cm) were separately placed in individual wells of a 24-well tissue culture plate at 25 °C, and platelet-rich plasma (PRP, 80 μL) was dropped on the sample surfaces and incubated for 1 h. Next, the samples were rinsed attentively with PBS and then fixed with glutaraldehyde solution (2.5 wt %) for 30 min. After being cleaned with deionized water at least three times, the membranes were gradually dehydrated with a serials of ethanol aqueous solutions (20, 40, 60, 80, 100 vol %). Finally, the air-dried samples were observed with SEM after sputter-coated with gold. 2.8. Antimicrobial Assay. Two typical bacteria E. coli and S. aureus were used to evaluate the antibacterial activities of the investigated membranes. In a typical test, a membrane sample (2 cm × 2 cm) was incubated with 10 mL of bacterial suspension at 37 °C for 24 h. After adding physiological saline, the bacterial dilutions were placed into agar culture medium at 37 °C for 24 h, then the surviving colony forming unit (CFU) were counted and presented. Three paralleled measurements were carried out and the antimicrobial activity of each membrane was characterized as follows:

(2)

⎛ C permeate ⎞ R = ⎜⎜1 − BSAfeed ⎟⎟ × 100% C BSA ⎠ ⎝

(3)

⎛J ⎞ FRR = ⎜⎜ w2 ⎟⎟ × 100% ⎝ Jw1 ⎠

(4)

Where V (L) was the volume of the permeated solution, S (m2) was the effective filtration area, and T (h) was the operation permeate time. Cfeed represented the BSA concentrations in BSA and CBSA the feed and the permeate solutions, respectively, measured by UV spectrophotometer. 2.7. Protein Adsorption and Platelet Adhesion. FITClabeled BSA (BSA-FITC) was prepared according to the 13955

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Figure 4. (A) FTIR/ATR spectra for the blend membrane M2 and the zwitterionic membranes with different modification time (ZM2h, ZM4h, ZM8h, and ZM12h). (B−F) N 1s core-level spectra for the blend membrane M2 and the zwitterionic membranes ZM2h, ZM4h, ZM8h, and ZM12h, respectively.

death rate (%) =

CFU0 − CFUM × 100% CFU0

surface modification for phase inversion membranes can be achieved with minimal impact on membrane bulk properties by blending with amphiphilic macromolecule.18 In this work, PES itself was designed as the hydrophobic block of the amphiphilic additive PDMAEMA-b-PES-b-PDMAEMA used for the blending modification of PES membranes. The PES backbones in the additive were amalgamated with PES membrane bulk and acted as an anchor of PDMAEMA chains. The enrichment of PDMAEMA chains on membrane surface was confirmed by XPS analysis. As shown in Figure 3A, pure PES membrane (M0) showed no nitrogen while the signal was observed for the blend membranes (M1−M3), and PDMAEMA was the only source of nitrogen. With the increase of copolymer dosage, the percentage of nitrogen (N) increased while sulfur (S) decreased evidently and the N/S mole ratio increased from 0 (M0) to 1.6 (M3). As we know, the XPS probe depth usually increases along with the increase of the takeoff angle. Angle-resolved XPS

(5)

where CFU0 and CFUM were the original and surviving CFU in the bacterial suspension, respectively.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the PES/PDMAEMA-b-PES-bPDMAEMA Blend Membranes. In the blending modification of polymer membranes using an amphiphilic macromolecule, the hydrophilic chains in the amphiphilic macromolecule are often immiscible with host polymer and thus are segregated onto membrane surface spontaneously during membrane preparation. The hydrophobic part of the amphiphilic additive is usually designed to be well miscible with the host polymer and able to anchor the hydrophilic chains onto membrane surface. Therefore, a highly efficient and long-term stable 13956

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Figure 6. (A) Permeability (Jw1) and BSA rejection of the zwitterionic membranes with the modification times. (B) Summary of Rt, Rir, and FRR of the zwitterionic membranes with the modification times. Data were means ± SD (n = 3). Figure 5. (A) Water contact angles for the control samples (pure PES membrane M0 and blend membrane M2) and the zwitterionic membranes (ZM2h, ZM4h, ZM8h, and ZM12h). (B) Zeta potentials of the control samples (M0 and M2), the zwitterionic membrane ZM8h, and the cationic membrane CM2 at various pH values.

The permeation experimental results are shown in Figure 3D. The initial pure water flux (Jw1) increased steadily while the BSA rejection decreased slightly with the increase of the block copolymer concentration. This phenomenon can be explained by two aspects. First, the porosity and pore size of the membranes increased with the increase of the block copolymer concentration and thus improved the water permeability of membrane. On the other hand, the improved hydrophiliciy and wettability decreased the resistance of water permeating through membrane and, thus, greater water fluxes were achieved. After BSA solution filtration, the membranes were rinsed and the pure water flux (Jw2) was measured. Based on the obtained flux data, FRR was calculated to quantify the fouling resistance of membranes. It was obvious that FRR data of the blend membranes were higher than the original PES membrane. The FRR of the pure PES membrane was only 50%, while this value increased to 70% for M3. This phenomenon indicated that the antifouling property of PES membrane was improved to some extent by the addition of PDMAEMA-b-PES-b-PDMAEMA copolymer, but the antifouling ability was unsatisfactory. As we know, the isoelectric point of BSA is at about pH 4.8 and the BSA carries negative charges at pH 7.4. Whereas PDMAEMA is a weak polyelectrolyte and positively charged at pH 7.4.42 The electrostatic attractions between the positively charged DMAEMA groups and negatively charged BSA molecules may lead to the adsorption of BSA on membrane surface. Therefore, in order to minimize the electrostatic interaction between the foulants (e.g proteins) and membrane surface, the transformation of PDMAEMA into polyzwitterions by quaternization may be a feasible way to improve further the antifouling ability of the blend membranes.

data (Figure 3B) showed that the mole percentage of N in PDMAEMA decreased, while S element in the PES matrix increased with the increase of XPS probe depth. The XPS results suggested that the PDMAEMA-b-PES-b-PDMAEMA concentration in the top layer of the blending membranes was higher than those in the sublayer, indicating the substantial surface segregation of PDMAEMA chains during NIPS process. Figure 3C shows the results of water contact angle measurements. The original PES membrane had the highest contact angle (89°), indicating the poor wettability. Furthermore, with the increase of copolymer additive, the contact angle of the blend membranes decreased constantly, showing the better water affinity. The obviously improved hydrophilicity showed that the copolymer addition was effective for hydrophilic modification of PES membrane. The surface morphologies of the pure PES and the blend membranes were observed by SEM (Figure S4 in the Supporting Information). All the membranes were featured with a typical asymmetric structure with finger-like macrovoids beneath a skin layer. No obvious pore was observed on the surface of pristine PES membrane at the magnification of 20 000 times. But the blend membranes had visible pores on the surface. With the increase of the additive concentration, the nanoscale porous structure on membrane surface became more and more obvious. The formation of finger-like macrovoids and surface nanopores in the blend membranes was partially attributed to the pore-forming capacity of the block copolymer additive.39,41 13957

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Figure 7. (A) Representative fluorescence microscopy images for the control samples (M0 and M2) and the zwitterionic membranes (ZM2h, ZM4h, ZM8h, and ZM12h). (B) SEM images of platelet adhesion for the control samples (M0 and M2) and the zwitterionic membranes (ZM2h, ZM4h, ZM8h, and ZM12h).

3.2. Surface Chemistry and Antifouling Properties of Zwitterionic PES Membranes. 3.2.1. Surface Chemistry of Zwitterionic Membranes. Owing to the highly hydration capability and the unique conformational variability, zwitterionic polymer is an effective and stable biomaterial with excellent protein-adsorption resistance and biocompatibility, and has been extensively studies and used as an antifouling material. In the present work, the PDMAEMA chains having pendent tertiary amine groups were designed as the hydrophilic chains of amphiphilic additive for PES membrane modification. The pendent tertiary amine groups can be conveniently transformed into zwitterionic or cationic species by quaternization.43,44 The introduction of reactive PDMAEMA chains on membrane surface provide high flexibility in the design of membrane surface chemistry. As a typical sample, M2 was employed to react with 1,3-PS for further surface zwitterionicalization of PES membranes. The surface chemistry of the investigated membranes was analyzed by ATR-FTIR and XPS. The ATR-FTIR spectra of the modified membranes are presented in Figure 4A. In the spectra of the quaternized samples, a new peak appears at 1036 cm−1, which is attributed to the vibration of the sulfonate (−SO3−) groups. This result shows the successful introduction of −SO3− onto membrane surface by the ring opening reaction of 1,3-PS.43 The existence of zwitterionic species on membrane surface was confirmed by the XPS nitrogen (1s) high resolution scan (Figure 4B−F). The results of N 1s peak fitting comprising C−N component peak at 399.5 eV and C−N+ component peak at 401.5 eV shows that, with the elongation of the modification time, the conversion ratio of PDMAEMA increased.43 When the modification time is longer than 8 h, the conversion ratio gets up to ∼90%. These results shows the PDMAEMA chains on the surfaces of the blend membranes were successfully transformed into zwitterionic species by quaternization with 1,3-PS. 3.2.2. Hydrophilicity and Surface Charge characteristics of Zwitterionic Membranes. The results of water contact angle measurements are shown in Figure 5A. The contact angle of water droplet on the zwitterionic membranes decreased obviously with the elongation of the treatment time. The contact angle of the membrane quaternized for 12 h (ZM12h) was only 29°, exhibiting a high surface hydrophilicity. These

results show that the resulted PDMMSA chains from quaternization brought superhydrophilic character to PES membranes due to the high affinity between polyzwitterions and water molecules. Moreover, the zwitterionic PES membranes show excellent durability, which was verified by the consecutive contact angle monitoring during membrane long-term rinse (Figure S5 in the Supporting Information). Zeta potentials were determined as a function of pH in order to characterize the surface charge characteristics of the modified PES membranes. As presented in Figure 5B, the pure PES membrane shows an isoelectric point at pH 4.9. Since the hydrophilic PDMAEMA chains in the copolymer additives were segregated onto PES membrane surface during the NIPS procedure, the addition of macromolecular copolymer modified the effective surface charge of the blend membranes. It was observed that the blend membrane M2 had an isoelectric point at pH 8.7. At pH < 8.7, the membrane M2 was positively charged due to the protonation of tertiary amine groups in PDMAEMA chains. After reacting with 1,3-PS, the surface zeta potential of the membrane M2 decreased due to the transformation of PDMAEMA into zwitterionic species. The surface charge of the resultant zwitterionic membrane ZM8h was close to neutral at pH 7.4. This result was in agreement with the balanced oppositely charged groups on the zwitterionic membrane surface, and it also confirmed the effectiveness of surface modification. 3.2.3. Permeation and Antifouling Properties of Zwitterionic Membranes. It is well-known that adsorption of organic contaminants especially proteins on membrane surface and in membrane pores is the predominant cause for membrane fouling in aqueous separation and purification. In this work, BSA was employed as a model protein to investigate the fouling resistance of the investigated membranes. As presented in Figure 6A, the water flux (Jw1) decreased while the BSA rejection increased as the modification time prolonged. As the modification time reached 8 h, the BSA rejection exceeded 86% but the flux decreased to 69.2 L m−2 h−1. This was ascribed to the smaller membrane pore size and greater steric hindrance of PDMMSA hydration layer. FRR is a parameter that reflects the reversibility of membrane fouling. A higher FRR indicates that the membrane fouling is often more recoverable. As shown in Figure 6B, with the 13958

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Figure 8. (A−D) N 1s core-level spectra for the control sample M2 and the cationic PES membranes (CM1, CM2, and CM3). (E) Water contact angle of M0 and the blend membranes (M1, M2, and M3) before and after quaternization. (F) Water flux of M0 and the blend membranes (M1, M2, and M3) before and after quaternization.

zwitterionic membranes was attributed to the powerful hydration capabilities of polyzwitterions with strongly ionic solvation. It is generally accepted that zwitterionic polymers bind water molecules via electrostatic interaction which is more strongly and stably than hydrogen bonding, and thus reduce the irreversible fouling greatly.45 The experimental results in the present work confirmed that membrane fouling, especially the irreversible fouling could be substantially controlled by the introduction of polyzwitterions onto PES membrane surface. Therefore, the zwitterionic PES membrane is fairly valuable in practical applications. 3.2.4. Blood Compatibility of the Zwitterionic Membranes. In blood-contact applications, nonspecific adsorption of proteins on material surface is recognized as the first step to induce a full-scale platelet adhesion and activation.46 In this work, FITC-labeled protein (BSA-FITC) was used to investigate the nonspecific protein adsorption of the zwitterionic membranes. Figure 7A shows the fluorescence images of BSA-FITC on the investigated membranes. The uniform and strong fluorescence was observed on the control samples

elongation of quaternization time, the FRR value of the resultant zwitterionic membranes increases to some extent. For example, the FRR of ZM8h is up to 87%. To quantify the fouling resistance of the investigated membranes, the antifouling ability of the membranes was further described by the total fouling ratio (Rt), the reversible fouling ratio (Rr), and the irreversible fouling ratio (Rir), which were calculated according the following equations, respectively. (Rt = 1 − Jp/ Jw1, Rir = 1 − Jw2/Jw1, Rr = (Jw2 − Jp)/Jw1). Usually, reversible fouling ratio (Rr) reflects the flux decline caused by the cake layer formation which can be removed by hydraulic cleaning. Irreversible fouling (Rir) describes the flux decline induced by deposition of contaminants on membrane surface or wall pore which is difficult to be eliminated by physical cleaning. As shown in Figure 6B, the Rt value decreases from 0.57 to 0.39 with the elongation of quaternization time from 0 to 8 h. The lower Rt value indicates lower total flux loss and less protein fouling for the investigated membranes. In addition, the Rir value of the zwitterionic membranes also decreased with the quaternization time. The excellent fouling resistance of the 13959

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3.3. Surface Characterizations and Antibacterial Properties of Cationic PES Membranes. The introduction of reactive PDMAEMA chains into PES membrane by blending with the block copolymer PDMAEMA-b-PES-b-PDMAEMA also provided a possibility of fabricating cationic PES membranes by quaternization of PDMAEMA chains. As we know, positively charged material surface often demonstrates good antibacterial properties. Therefore, in another case of this work, the surface-enriched PDMAEMA chains on the blend membranes were transformed into polycations by quaternization with 1-bromodecane. The surface chemistry of the resultant modified membranes was analyzed using XPS. The N 1s core-level spectra of the blend membranes before and after quaternization are shown in Figure 8A−D. The shift in binding energy of the N 1s peak component from 399.5 to 402.1 eV indicates the production of quaternary amine groups by reacting with 1-bromodecane. After the N-alkylation, the N 1s line shape of the membranes is nearly dominated by the C− N+ peak. The C−N+ component area ratio reaches about 80%, 88% and nearly 100% for the membrane CM1, CM2, and CM3, respectively. As presented in Figure 5B, compared to the pure PES membrane and the blend membrane, the cationic membrane CM2 has a higher zeta potential. After quaternization, the positive electricity of the cationic membranes increased due to the higher pKa of quaternary amine than tertiary amine. The water contact angle and the water permeability of the cationic membranes were also tested. As presented in Figure 8E, the contact angle of the cationic membranes decreased obviously compared to the unmodified blend membrane. This result shows the quaternized PES membranes have higher hydrophilicity than the PES/PDMAEMA-b-PES-b-PDMAEMA blend membrane. The data in Figure 8F show that the water flux of the blend membrane was reduced after quaternization. This phenomenon can be attributed to the coverage and steric hindrance of alkyl groups on the cationic membrane surface. The changes in membrane surface hydrophilicity and water permeability also proved the success of postquaternization on membrane surface using 1-bromodecane. In this work, two typical microorganisms E. coli (Gramnegative) and S. aureus (Gram-positive) were used to evaluate the antibacterial activity of the investigated membranes. The initial concentrations of E. coli and S. aureus were 1.0 × 107 and 6.3 × 107 CFU mL−1, respectively. As shown in Figure 9, the survival colonies of bacteria in the agar culture medium for the quaternized membranes (CM1, CM2, and CM3) are much less than those for the control samples M0 and M2. The death rates of bacteria were further calculated to quantify the antibacterial activities of the membranes, as shown in Figure 9C. It can be seen that the death rates of the quaternized cationic membranes (CM1, CM2, and CM3) increase substantially compared with the pure PES membrane (M0) and the typical blend membrane (M2). Moreover, the higher block copolymer concentration in the membranes with same quaternization time (8 h) brought the higher death rate of bacteria. For example, the death rate for CM3 reaches 99%, showing a strong antibacterial ability. Generally, the quaternary ammoniums are prone to permeate and destroy the cell walls of bacteria and thus lead to the destruction of bacterial body. As a result, a cationic surface usually exhibits good antibacterial activity.35 In summary, the cationic PES membranes with excellent antibacterial activity were successfully fabricated via the combination of the blending membrane-preparation process and surface postquaternization.

Figure 9. Antibacterial activities of the control sample (M0 and M2) and the cationic PES membranes (CM1, CM2, and CM3) against (A) E. coli and (B) S. aureus. (C) Death rate of both bacteria after 24 h of contact time of the control samples (M0 and M2) and the cationic PES membranes (CM1, CM2, and CM3).

including the pure PES membrane and the blend membrane M2, indicating that a great quantity of BSA was adsorbed onto the membrane surfaces. In comparison, the fluorescence intensities of the zwitterionic membranes were weakened obviously. Moreover, the fluorescence intensity became gradually unobvious with the elongation of the quaternization time. These results show the zwitterionic membranes had better antifouling ability than the pure PES membrane and the PES/PDMAEMA-b-PES-b-PDMAEMA blend membrane. Actually, this conclusion has been testified by the results of contact angle measurements and dynamic antifouling evaluations, as discussed in section 3.2.3. The elongation of quaternization time in same reaction conditions is advantageous to increase the coverage ratio of polyzwitterions on membrane surface, and thus improve further the antifouling ability of the blend membranes. Figure 7B shows some typical surface SEM images after the membranes were contacted with PRP solution for 60 min at 37 °C in vitro. It can be seen that a lot of platelets aggregate on the surfaces of M0 and M2. Moreover, a great of pseudopodia of the platelets are observed. Compared to M0 and M2, a much less quantity of attached platelets are seen on the surfaces of ZM2h and ZM4h and the platelets are dispersive with nearly no pseudopodia. Furthermore, the membranes ZM8h and ZM24h exhibited higher resistance to platelet adhesion than ZM2h and ZM4h. The results allow us to conclude that the platelet adhesion and activation was significantly controlled by the block copolymer incorporation and further surface zwitterionicalization. The zwitterionic PES membranes developed in this work are a preferable choice as blood-contact separation membrane. 13960

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4. CONCLUSIONS The incorporation of triblock copolymer PDMAEMA-b-PES-bPDMAEMA containing reactive PDMAEMA chains into PES membranes in membrane preparation offered a possibility to improve the antifouling and antibacterial properties of PES membranes by postquaternization. The hydrophilic PDMAEMA chains in the copolymer additive were concentrated onto PES membrane surface due to the surface segregation phenomenon in NIPS process. The fabricated PES/PDMAEMA-b-PES-b-PDMAEMA blend membranes were further functionalized by the quaternization of PDMAEMA chains with 1,3-PS and 1-bromodecane, respectively. The resultant zwitterionic PES membranes quaternized with 1,3-PS exhibited significantly enhanced antifouling ability and blood compatibility. And the obtained cationic PES membranes treated by 1bromodecane demonstrated excellent antibacterial activities. The membrane modification strategy presented in this work can be scaled up conveniently and is applicable to more membrane materials. This work develops a facile technology for designing and fabricating antifouling and antibacterial polymer UF membranes.

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ASSOCIATED CONTENT

S Supporting Information *

Details for the synthesis and characterization of PES-bPDMAEMA, the scheme of the homemade dead-end ultrafiltration apparatus, surface morphologies of membranes, the details for stability test, and the permeability of cationic membranes. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. & Fax: +86-571-87953011. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 51273176), the National High Technology Research and Development Program of China (863 Program of China, Grant No. 2012AA03A602), and the Fundamental Research Funds for the Central Universities (Grant No. 2014QNA4038).

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