Zwitterionic polyethersulfone ultrafiltration membrane with superior antifouling property

Journal of Membrane Science 319 (2008) 271–278

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

Zwitterionic polyethersulfone ultrafiltration membrane with superior antifouling property Qing Shi a , Yanlei Su a , Wei Zhao a , Chao Li a , Yaohui Hu a , Zhongyi Jiang a,∗ , Shiping Zhu b a b

Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 8 November 2007 Received in revised form 12 February 2008 Accepted 26 March 2008 Available online 1 April 2008 Keywords: Zwitterionic polyethersulfone membrane Ultrafiltration Ion-strength-dependent flux Antifouling property

a b s t r a c t A novel kind of zwitterionic polymer, tertiary amine-modified PES (TA-PES), bearing cationic tertiary amine groups and anionic sulfonic groups, was synthesized in this study. The as-synthesized TA-PES was employed to fabricate ultrafiltration membranes by phase inversion in a wet process using pure water or sodium chloride aqueous solution as coagulation bath. It was found that the as-fabricated ultrafiltration membranes exhibited distinct ion-strength-dependent flux due to the inherent zwitterionic characteristics of TA-PES. The permeation flux of TA-PES membranes can be varied dramatically by simply changing the ionic content in coagulation bath. Furthermore, TA-PES membranes displayed superior anti-protein-fouling property and desirable ultrafiltration performance. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Polymers are most widely used materials in the fabrication of ultrafiltration membranes. However, the application of polymer membranes in the biotechnology field is strongly limited by the undesirable membrane fouling. The adsorption and deposition of biomacromolecules on the membrane surface and/or pore walls result in a sharp decline in permeate flux with operation time [1,2]. The surface property of membranes plays a crucial role in biofouling as it determines the interaction between biomacromolecules and membrane materials. It is well known that an increase of membrane surface hydrophilicity can effectively minimize biomacromolecules (such as protein) adsorption and inhibit membrane fouling. Various methods, including surface coating [3] and surface graft polymerization [4–6] have been reported to introduce hydrophilic moiety onto membrane surfaces. However, both techniques may substantially occlude or block membrane pores, leading to the undesirable reduced permeability [7]. In comparison, covalent linkage of the surface modifying agent (SMA) onto the membrane-forming polymer seems to be an appealing alternative [8–14]. Among all the SMAs, poly(ethylene glycol) (PEG) is most widely employed due to its high hydrophilicity, vigorous chain mobility, low cost and superior biocompatibility [1,13,14]. However, PEG is susceptible to decomposition in the presence of

∗ Corresponding author. Tel.: +86 22 27892143; fax: +86 22 27892143. E-mail address: [email protected] (Z. Jiang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.03.047

oxygen and transition metal ions [15–17]. Inspired by the delicate composition and structure of most outer-cell membranes, the protein-adsorption-resistance property of zwitterionic substances has been gradually recognized and utilized in creating biomimetic surface/interface. Zwitterionic groups, including phosphorycholine (PC), sulfobetaine, and carboxybetaine, have been postulated as new generation of non-biofouling materials [15,18–20]. However, most of these studies focused on nonporous surfaces for biomedical applications. Recently, our group reported the novel use of sulfobetaine copolymer additives to fabricate anti-protein-fouling ultrafiltration membranes via a phase separation method [21,22]. Meanwhile, a few studies reported the incorporation of zwitterionic polymers onto porous polymeric membranes via surface grafting methods. For example, Ulbricht and coworker [23] prepared fouling-resistant ultrafiltration membrane by heterogeneous photograft copolymerization of a kind of betaine onto polyethersulfone (PES) ultrafiltration membrane. The majority of polymer ultrafiltration membranes were prepared by immersion precipitation [24]. In this process, a homogeneous polymer solution was cast as a flat-sheet film or a hollow fiber shape, and then immersed into a nonsolvent coagulant (usually water) bath. However, this conventional procedure is not always assured to generate the desirable membrane structure and properties [25]. The incorporation of some additives (such as poly(vinylpyrrolidone) (PVP), PEG, water, organic acid, LiCl, ZnCl2 , etc.) to the casing solution, and the introduction of some additional steps, like evaporation [26] or annealing [27] are usually required to prepare the desirable membranes. Polyzwitterions bearing both

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anionic and cationic groups along polymer chains are thus deserve more attention. A characteristic of these materials is that their hydrodynamic volume varies with the ionic strength in the solution, due to the screening of Coulombic interaction between charged groups. Therefore, we can conjecture that by using polyzwitterions as membrane materials, the structural morphology and hydraulic permeability of the resulting membrane should be flexibly regulated in response to ionic strength change in coagulation bath. In the present study, a novel kind of zwitterionic polymer based on PES, bearing cationic tertiary amine groups and anionic sulfonic groups, was synthesized. The as-synthesized polymer was cast into flat-sheet ultrafiltration membranes directly by phase inversion method in a wet process. We demonstrated the dual-functional properties of the zwitterionic membranes—tunable transmembrane flux by simply varying the ionic content in coagulation bath, and remarkably improved anti-protein-fouling property. We believe that this novel membrane material is very promising for a wide range of applications in bio-products filtration. 2. Experimental 2.1. Materials and reagents PES 6020P (BASF Co., Germany) was dried at 110 ◦ C for 12 h prior to use. Chlorosulfonic acid (Shanghai Chemical Regent Co., China), dimethylaminoethanol (Shanghai Chemical Regent Co., China), chloroform (Kewei Chemical Co., Tianjin, China), N,N-dimethyl formamide (DMF) (Kewei Chemical Co., Tianjin, China), PEG2000 (Mw = 2000) (Kewei Chemical Co., Tianjin, China), and bovine serum albumin (BSA) (Institute of Hematology, Chinese Academic of Medical Sciences, Tianjin, China) were all of an analytical grade and used without further purification. 2.2. Preparation of tertiary amine-modified PES (TA-PES) The dried PES (10 g) was dissolved in 100 mL of chloroform and the solution was cooled to 0 ◦ C in an ice bath. The mixture of chlorosulfonic acid (7.0 g) and chloroform (50 mL) was added dropwise to the above solution within 1.5 h. The resulting slurry was isolated by decanting the top chloroform solution, the mixture of dimethylaminoethanol (6.0 g) and chloroform (100 mL) was then added gently into the slurry. The reaction mixture was stirred at 30 ◦ C for 4 h. The reaction was terminated by adding fivefold of water into the reaction mixture. The resulting precipitate was collected, washed alternately with water and ethanol for three times. The final product of zwitterionic TA-PES was freeze-dried and placed in oven at 50 ◦ C before utilization. The synthesis and characterization of sulfonated PES (S-PES) were described in detail elsewhere [28]. The synthetic procedure of TA-PES, which contained the chlorosulfonation of PES and the subsequent reaction with dimethylaminoethanol, was outlined in Scheme 1. The chlorosulfonic groups were initially introduced onto the ortho positions of the aromatic rings relative to the ether oxygen atom of the PES chain [29]. The tertiary amine was then grafted onto PES backbone via a facile reaction between chlorosulfonic groups and hydroxyl groups. Chlorosulfonic residues were readily hydrolyzed to anionic sulfonic groups in the presence of water, while tertiary amine groups were readily hydrolyzed to cationic groups. The synthesized zwitterionic polymer, TA-PES, was water insoluble, and could be thus purified and collected by exhaustive washing with deionized water. The chemical compositions of synthesized TA-PES were determined by Element analysis (Vario EL, Element Co., Germany).

Ion-exchange capacity (IEC) of S-PES and TA-PES was determined by back-titration method [30]. Sulfonation degree of these two kinds of PES materials was calculated from the IEC data. 2.3. Membrane preparation Casting solution for PES control membrane contained 18% PES, 10% PEG2000 (use as pore-forming agent) and 72% DMF by weight. Casting solution for modified membranes contained 18% TA-PES, 5% PEG2000 and 77% DMF by weight. The recipe of casting solution for TA-PES membrane was different with that of PES control membrane in order to keep the permeation flux of these two different kinds of membranes in the same magnitude. The mixtures were first stirred at 60 ◦ C for 5 h to ensure a complete dissolution of the polymers. After bubbles were released completely, the solutions were cast on glass plates with a stainless steel knife at a wet thickness of about 250 ␮m, and the glass plates were then immersed in a coagulation bath of deionized water or sodium chloride solution with varied concentrations. Sodium chloride was employed to adjust the ionic content in coagulation bath. The formed membranes were peeled off and subsequently washed with deionized water for 24 h to completely remove the residual solvent and pore-forming agent. The samples were then kept in coagulation bath prior to testing. 2.4. Membrane characterization The surface chemical compositions of the modified PES membranes were analyzed by XPS (PHI-1600, USA) using Mg K␣ as radiation source (the takeoff angle of photoelectron was set to 90◦ ). Surface spectra were collected over a range of 0–1100 eV. The static contact angles of membranes were measured at room temperature using the captive air bubble technique. Membranes were submerged in deionized water and air bubbles were placed in contact with the membrane surface. The contact angles were measured using a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China). At least five sample readings from different locations on the membrane surface were averaged and the angles reported were reliable to ±3◦ . The cross-section morphologies of modified membranes were observed by scanning electron microscopy (SEM) using a Philips XL30E scanning microscope. The membranes frozen in liquid nitrogen were broken and sputtered with gold before SEM analysis. The membrane surface charge was characterized via tangential flow streaming potential measurement [31]. The streaming potentials of membranes were measured using a 0.001 M KCl solution in the range of pH 4–8 at a temperature of 25 ◦ C. The pH was adjusted using 0.1 M HCl and NaOH solutions. The zeta potential  was calculated using the Helmholtz–Smoluchowski equation. 2.5. Ultrafiltration experiments A dead-end stirred cell filtration system was designed to characterize the filtration performance; the details of the filtration were described previously [32]. All ultrafiltration experiments were carried out using a filtration test cell (Model 8200, Millipore Co., USA) with volume capacity of 200 mL. The effective area of the membrane was 28.7 cm2 . The operation pressure in the system was maintained by nitrogen gas. BSA (1.0 g) in 1 L of 0.1 M buffer at varied pH values were used for feed solutions. Both the deionized water and buffer were prefiltered through 0.45 ␮m nylon membrane prior to use. Concentrations of BSA solutions were determined by a UV–vis spectrophotometer (U 2800, Hitachi, Japan) at 280 nm. Each membrane was initially compacted with deionized water for 30 min at 150 kPa. Then the pressure was lowered to the oper-

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273

Scheme 1. Representation of the reactions involved in the synthesis of zwitterionic TA-PES.

ating pressure of 100 kPa and the water flux (Jw1 ) was measured. After 30 min, the filtration was stopped and the cell was emptied. Thereafter, the cell was filled with protein solution immediately and the flux (Jp ) was measured. The rejection ratio (R) of BSA was calculated by the following equation: R=



Cp 1− Cf



× 100%

Jp Jw1

rr =

Jw2 − Jp Jw1

(3)

rir =

Jw1 − Jw2 Jw1

(4)

(1)

where Cp and Cf (g/L) were the protein concentrations of permeate and feed solutions, respectively. Finally, the cell and the solution reservoir were fully emptied and refilled with deionized water. After the stirred cell was thoroughly cleaned with deionized water for 20 min, the water flux (Jw2 ) was measured again. All filtration experiments were carried out at a temperature of 25 ± 2 ◦ C. A high Jp corresponds to low transmembrane flux loss. The flux recovery ratio (FRR), Jw2 /Jw1 , could be employed to represent the antifouling properties of the membranes. To analyze the fouling process in details, we defined several ratios to describe the fouling-resistant ability of TA-PES membrane. The first ratio is rt in the following equation. rt = 1 −

Here, rt was the degree of total flux loss caused by total fouling. A high value of rt corresponds to a large reduction in flux. rr and rir were introduced to distinguish reversible and irreversible fouling. rr and rir were defined by Eqs. (3) and (4), respectively.

(2)

Obviously, rt was the sum of rr and rir . 3. Results and discussion 3.1. Characterization of synthesized TA-PES Elemental analysis showed the weight percentages of C, H, S, O, and N elements in the TA-PES polymer were 54.3%, 3.7%, 16.3%, 24.4%, and 1.4%, respectively. Since tertiary amine was the only possible group containing N species, the appearance of N in TA-PES indicated the covalent linkage of tertiary amine functionality with PES chain. Notably, the sulfonation degree (Ds) and mole fraction of PES units linked with dimethylaminoethanol could be calculated based on elemental analysis. The results were shown in Scheme 1.

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Fig. 1. The SEM images of the cross-section of PES membrane cast in deionized water bath (0#) (a) and TA-PES membranes cast in deionized water bath (1#) (b), 0.01 M NaCl solution (5#) (c) and 0.5 M NaCl solution (d). The temperature of the casting solutions was all maintained at 25 ◦ C.

IEC experiments were also conducted to determine the Ds values of TA-PES. As reported previously, the Ds value was 0.51 for S-PES [28]. Since part of sulfonic groups could be substituted by tertiary amine functionality, the Ds value was decreased to 0.16, which revealed that 16% monomer units of TA-PES were linked with sulfonic groups and 35% linked with tertiary amine. According to these results, we can find that the Ds values determined from the elemental analysis and IEC were quite close. 3.2. Characterization of membranes 3.2.1. Membrane morphology In our previous study, PEG was chemically grafted onto PES backbone to form pegylated PES using similar synthetic procedure [28]. Membranes modified with amphiphilic pegylated PES showed higher pure water flux, though no appreciable change of cross-sectional morphology could be found. In this study, membranes fabricated with zwitterionic TA-PES also showed increased permeation flux. Similar permeation changes were observed with the addition of PVDF-g-PMAA [11], and PVDF-g-POEM [33] to PVDF membranes by Mayes and coworkers. These results indicated that incorporating hydrophilic groups into hydrophobic membrane materials could result in higher water fluxes, which was highly advantageous in filtration applications. Because of the complexity in identifying the antifouling contribution of the membranes with different permeation fluxes, the PEG additive in TA-PES membrane was only of half content compared with that of PES control membrane in order to keep the water flux of the membranes in the same magnitude. The cross-sectional SEM micrographs of the PES control membrane and TA-PES membrane cast into a deionized water bath were shown in Fig. 1(a) and (b), respectively. It can be seen that these two kinds of membranes both displayed the typical asymmetric structures, comprising a skin layer on top, an intermediate layer with finger-like structure, and a bottom layer of fully developed macrovoids. The difference was that the top layer was thicker for

PES control membrane, which fabricated with higher amount of PEG additive. PEG, which is miscible with membrane materials and highly soluble in water and many organic solvents, is one of the most commonly used polymer additives. It had been well known that the morphology of the resulting membrane can be changed by varying the content of PEG additive in the casting solution [34–36]. When the casting solution and coagulation bath came into contact, there was a rapid outflow of the solvent from the casting solution into the coagulation bath, and hence bulk polymer molecules with higher concentration aggregated to form the top dense layer. Higher PEG content would increase the viscosity of the casting solution, slow down the diffusion rate, and thus promote the formation of thicker top layer. In this study, even though the other factors such as different hydrophilicity between PES and TA-PES may affect the phase inversion process and thus the membrane morphology, we still ascribed the formation of thicker top layer mainly to higher PEG concentration. The SEM photographs of Fig. 1(a) and (b) also indicated that after chemically modified with tertiary amine, the excellent membrane-forming property of PES could be well preserved. 3.2.2. Membrane surface Near surface membrane composition was determined by XPS analysis. Fig. 2 presented XPS spectra of PES control membrane and TA-PES membrane. Three characteristic XPS signals for carbon, oxygen, and sulfur were observed at XPS spectra for both membrane samples. However, for TA-PES membrane, one additional emission peak at 401 eV, which was attributed to N 1s, could be observed. As mentioned above, tertiary amine group was the only source of N for TA-PES membrane, the content of tertiary amine functionality on the membrane surface could thus be calculated based on XPS analysis. According to the molecular formula listed in Scheme 1, the theoretical value of N molar ratio was calculated to be 1.6% for TA-PES membrane. As shown in Fig. 2, the experimental results of N molar ratio for TA-PES membrane was 2.3%, which was higher

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Fig. 2. XPS spectra of membranes fabricated with PES (0#) (a) and TA-PES (1#) (b). Both membranes were coagulated in pure deionized water at 25 ◦ C.

than the theoretical value. Surface composition was determined by diffusion of different parts of polymer components prior to gelation in the immersion precipitation process. Because the positively charged tertiary amine groups exhibit lower chemical potential in water than PES, they were more likely localized at the membrane surface during gelation. Surface hydrophilicity is one of the most important factors in determining antifouling property of ultrafiltration membranes. The hydrophilicity of the membranes in our study was evaluated by contact angle measurement, which was commonly used to assess the changes in the hydrophilicity and interfacial energy of substrate surfaces [37]. Fig. 3 presented the contact angle of membranes fabricated with PES, S-PES, and TA-PES. It was found that the contact angle decreased significantly after the chemical modification of hydrophobic PES. This phenomenon was attributed to the hydrophilic nature of the grafted sulfonic groups and tertiary amine groups. The decrease of contact angle indicated that a highly hydrophilic surface was created. The contact angle of S-PES membrane was slightly higher than that of TA-PES membrane, which was probably due to the different water affinity of sulfonic groups and tertiary amine groups.

275

Fig. 4. Zeta potential, , of PES (0#) (a), S-PES (b) and TA-PES (1#) (c) membranes with varying pH calculated from the tangential streaming potential of the outer surface.

When characterizing antifouling property of a membrane surface, it is necessary to consider the additional interaction caused by the surface charge [38]. The surface charge of a membrane can be estimated by measuring the streaming potential and calculating the zeta potential. The profiles of zeta potential of PES control membrane and S-PES membrane (Fig. 4) were in accordance with previous results [39–41]. The negative surface charge of non-ionic PES membrane was caused by specific ionic adsorption. For S-PES membrane, both specific ionic adsorption and the dissociation of the surface sulfonic groups were responsible for the surface charge. The competition between these two processes caused the formation of plateau at pH values >6. By converting parts of sulfonic groups into tertiary amine groups, the absolute values of the zeta potential for TA-PES membrane became much smaller. Two possible explanations could account for the low zeta potential value. First, the charged groups of zwitterionic polymer were distributed over the entire membrane surface, generating an interface with small overall net charge. Second, the extraordinary hydrophilicity of the TA-PES membrane increased the thickness of the swelling layer, and thus decreased the attraction for the adsorption of anions by increasing the distance from the electrolyte to the surface [42]. Another significant difference between TA-PES membrane and PES control membrane could be seen from Fig. 4: the PES control membrane was negatively charged over the entire pH range concerned, while the TA-PES membrane was positively charged at low pH, which was due to the positive overall net charge of the TA-PES membrane. Therefore, although the absolute zeta potential values were reduced by the swelling of surface hydrogel layers, the fixed surface charge on the membrane could still be detected by the tangential flow technique. 3.3. Effect of ionic content in coagulation bath on membrane flux

Fig. 3. Contact angles for PES control membrane (0#) (a), S-PES membrane (b) and TA-PES membrane (1#) (c).

As a kind of polyampholytes, TA-PES carried both acidic and basic groups. Under appropriate conditions, these groups would dissociate, leaving positively and negatively charged groups on the polymer chain [43]. The strongly dipolar structure of the zwitterionic groups endowed polyampholytes with unique solution properties described as the antipolyelectrolyte effect. The synthesized polymer was weakly charged, with considerable hydrophobic character, which rendered it a membrane material with ionicsensitive property. By varying the ionic content in coagulation bath,

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Fig. 5. Near-surface N molar ratio vs. NaCl concentrations in coagulation bath for TAPES membranes. The N molar ratio was determined from XPS analysis. For reference, the theoretical value is indicated by a horizontal solid line.

the salt solution behavior of TA-PES in the process of membrane formation was studied. Fig. 5 showed near surface N molar ratio of TA-PES membrane with different NaCl contents in coagulation bath. The N molar ratio increased at low NaCl content, indicated an increase in surface excess of tertiary amine group. Then, at high NaCl content, the N molar ratio decreased. For charge-imbalanced polyampholytes, salt ions could screen the repulsion between excess charges as well as attraction between opposite charges. At low salt content, the salt in coagulation screened the repulsion by excess charges on large scales. This screening of charge repulsion decreased the chain size. Further addition of salt in coagulation bath could screen the attractive forces between the positive ions on one monomer and negative ions on another, causing the chain to swell to certain extent [43]. We hypothesized that each polymer molecule occupied a certain surface area. Thus the total number of molecules at the near surface was higher for polymer with shrunk chains and lower with swelled chains. In the phase inversion technique, the rapid solvent–nonsolvent exchange led to polymer precipitation and the membrane formation. Thus, coagulation bath properties could strongly influence the morphology and permeability of the membrane. In previous studies, the addition of inorganic salt to the coagulation bath had been tried for preventing macrovoids formation [44,45]. In this study, because of the distinct zwitterionic property of TA-PES, the addition of salt ions in coagulation bath resulted in quite different membrane morphologies. Fig. 1(c) and (d) presented SEM images of the cross-section of TA-PES membranes cast in coagulation bath solutions containing 0.01 M and 0.5 M NaCl, respectively. The membrane cast in 0.01 M NaCl displayed more homogeneous finger-like macrovoids compared with that in salt-free solution (Fig. 1(b)). When the NaCl content in the coagulation bath was raised to 0.5 M, the final membrane possessed significantly enlarged macrovoids. Macrovoids are normally found in membranes prepared by immersion precipitation. In general, macrovoids formation occurs from freshly formed nuclei of the polymer lean phase (solvent-rich phase) in polymer solution. Then, the macrovoids grow around the newly formed nuclei [35,46]. In spite of arguments, it had been known that the diffusional flow of solvent from the casting solution surrounding the initiated nuclei was responsible for the growth of the voids [47]. In general, a rapid outflow of the solvent from the casting solution into the coagulation bath would cause

Fig. 6. Pure water flux through TA-PES membranes vs. NaCl concentrations in coagulation bath at 25 ◦ C.

higher concentration polymer molecules to aggregate. Therefore, higher diffusional flow rate would accelerate the phase separation and lead to the formation of larger macrovoids and vice versa. It can be deduced that with the addition of small amount of NaCl (0.01 M) in coagulation bath, nonsolvent (NaCl solution) inflow and solvent (DMF) outflow decreased, leading to the formation of more homogeneous macrovoids. This flow rate increased with more NaCl (0.5 M) was added, yielding a membrane with much larger macrovoids. This analysis seemed to be reasonable because it agreed well with the subsequent water permeation data in this study (Fig. 6). Enlarged macrovoids formation can adversely affect mechanical properties of the membranes, which caused membranes more susceptible to compaction upon pressure exertion. Therefore, NaCl concentration higher than 0.5 M was not involved in the following studies. The effects of NaCl content in coagulation bath on the pure water flux were studied. As shown in Fig. 6, the TA-PES membranes using 0.001 M NaCl as coagulation bath showed the lowest water flux. This NaCl content was consistent with the critical NaCl content for near surface N molar ratio shown in Fig. 5. When the NaCl content in the coagulation bath was 0.001 M, the repulsion by excess charges was screened in large proportion, the crosslinked zwitterionic polymer chains could contact more closely. Therefore, the membrane cast in 0.001 M NaCl showed the lowest water permeability. With increased salt content in coagulation bath, the attractive forces between the positive and negative monomer were screened, causing the PES chain swelled to certain extent. Thus, the swelled chains would be less curved and the matrix would show more orderly arranged structure, which was responsible for the significantly increased water flux. 3.4. Elucidation of membrane fouling and protein-adsorption-resistant properties Ultrafiltration experiments were carried out to study the protein-adsorption-resistant properties of TA-PES membrane. The results including Jp and flux recovery ratio (R) were plotted in Fig. 7. At pH 5.0, which was near the isoelectric point (IEP) of BSA (IEP is 4.8), the proteins were expected to be electrically neutralized. Therefore, the location of both the minimum Jp and flux recovery ratio at pH 5.0 was probably due to the neutrality of the protein molecules which involves a higher compaction of the polarization layer and a faster adsorption of protein onto the membrane [48]. Apart from pH 5.0, a flux recovery ratio of higher than 90% could

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277

Table 1 Contact angle and ultrafiltration performance of PES membrane cast in deionized water bath (0#) and TA-PES membranes cast in NaCl solutions with different concentrationsa (1–6#) Membrane 0# 1# 2# 3# 4# 5# 6# a b

NaCl concentration (mol/L)b

Contact angle (◦ )

Jw1 (L/m2 h)

Jp (L/m2 h)

FRR (%)

Rejection ratio (%)

rt

rr

rir

0 0 0.0005 0.001 0.002 0.01 0.1

56.0 28.2 29.4 28.6 30.3 28.1 28.5

134.3 197.1 83.0 74.3 91.1 154.4 179.7

57.0 70.2 61.3 52.6 57.2 63.0 68.9

56.1 79.6 93.0 98.5 92.1 89.8 82.9

100 96.6 100 100 100 100 99.4

0.58 0.64 0.26 0.29 0.37 0.59 0.62

0.14 0.44 0.19 0.28 0.29 0.49 0.45

0.44 0.20 0.07 0.01 0.08 0.10 0.17

The ultrafiltration experiments were performed at pH 7.0. NaCl concentration in coagulation bath.

be obtained, which indicated that the zwitterionic TA-PES membrane could resist nonspecific protein adsorption at both acidic and alkaline pH range. More detailed results of ultrafiltration were shown in Table 1. During ultrafiltration, the rejected protein molecules adsorbed or deposited on membrane surface and/or membrane pore wall, result in considerable membrane fouling, which could be further divided into reversible fouling and irreversible fouling [49]. Irreversible fouling, which could not be removed by hydraulic cleaning, constituted the primary challenge for membrane modification. As shown in Table 1, the total membrane fouling, especially the irreversible fouling decreased obviously with a decrease of pure water flux. The reason was that the membrane with higher water flux had more opened pore mouth. During ultrafiltration, protein molecules might be entrapped in the large pores and blocked the channels, which could not be removed by simple hydrodynamic cleaning. While for the membrane cast in 0.001 M NaCl, which had the lowest water flux, there was almost no irreversible fouling. The excellent flux recovery property of TA-PES membrane indicated that the membrane could be reused for several runs. In order to test the recycling potential of TA-PES membrane cast in 0.001 M NaCl solution, long-term ultrafiltration with three repetitive operations was carried out and the experiment results were shown in Fig. 8. After three times of BSA ultrafiltration and hydraulic cleaning, the pure water flux of TA-PES membrane was maintained at 68.9 L/(m2 h), 92.5% of the initial value, which was much higher than that of PES control membrane (43.7%). The outstanding recycling properties of TA-PES membranes render them longer lifespan without obvious decrease of permeation flux. It was interesting to note that the protein solution flux of TA-PES membrane increased slightly with operation time in Fig. 8, which

Fig. 8. Flux change of 0# PES control membrane and 3# TA-PES membrane during three times ultrafiltration of BSA solution. BSA solution with pH 7.0 was used for the ultrafiltration.

was just opposite to the traditional protein ultrafiltration results. During protein ultrafiltration, protein molecules may be adsorbed on the membrane surface and/or entrapped in the membrane pores, resulted in a rapid decline of the flux. Even for the well-modified membranes, flux declines were inevitable because the protein fouling could not be eliminated completely. However, for zwitterionic TA-PES membrane, the substantial swelling due to the presence of ions in buffer solution could affect the mechanical properties of the membrane material as well as the “tortuosity” of the membrane matrix, which gave rise to different permeation properties [50]. 4. Conclusions The water-insoluble zwitterionic polymer, TA-PES, was demonstrated as superior ultrafiltration membrane material. Because of the so-called antipolyelectrolyte effect, TA-PES membranes fabricated showed some unique properties compared with PES membrane and S-PES membrane. The permeability of membrane can be readily adjusted by simply changing the ion content in coagulation bath. Furthermore, fouling-resistance analysis based on the ultrafiltration experiments demonstrated that the TA-PES membranes were of superior protein-fouling-resistant property. Acknowledgements

Fig. 7. BSA solution flux and BSA rejection ratio of 3# TA-PES membrane as the function of pH values. The membranes were cast in 0.001 M NaCl coagulation bath at 25 ◦ C.

This work is supported by Tianjin Natural Science Foundation for International Cooperation, cultivated Item in University for Sci.&Tech. Innovation (No. 706012), and the Programme of Introducing Talents of Discipline to Universities (No. B06006).

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References [1] A. Nayak, H. Liu, G. Belfort, An optically reversible switching membrane surface, Angew. Chem. Int. Ed. 45 (2006) 4094–4098. [2] J.Y. Park, M.H. Acar, A. Akthakul, W. Kuhlman, A.M. Mayes, Polysulfone-graftpoly(ethylene glycol) graft copolymers for surface modification of polysulfone membranes, Biomaterials 27 (2006) 856–865. [3] A.V.R. Reddy, D.J. Mohan, A. Bhattacharya, V.J. Shah, P.K. Ghosh, Surface modification of ultrafiltration membranes by preadsorption of a negatively charged polymer. I. Permeation of water soluble polymers and inorganic salt solutions and fouling resistance properties, J. Membr. Sci. 214 (2003) 211–221. [4] H. Chen, G. Belfort, Surface modification of poly(ether sulfone) ultrafiltration membranes by low-temperature plasma-induced graft polymerization, J. Appl. Polym. Sci. 72 (1999) 1699–1711. [5] J. Pieracci, D.W. Wood, J.V. Crivello, G. Belfort, UV-assisted graft polymerization of N-vinyl-2-pyrrolidinone onto poly(ether sulfone) ultrafiltration membranes: comparison of dip versus immersion modification techniques, Chem. Mater. 12 (2000) 2123–2133. [6] H. Susanto, M. Balakrishnan, M. Ulbricht, Via surface functionalization by photograft copolymerization to low-fouling polyethersulfone-based ultrafiltration membranes, J. Membr. Sci. 288 (2007) 157–167. [7] L. Ying, P. Wang, E.T. Kang, K.G. Neoh, Synthesis and characterization of poly(acrylic acid)-graft–poly(vinylidene fluoride) copolymers and pH-sensitive membranes, Macromolecules 35 (2002) 673–679. ¨ [8] D. Mockel, E. Staude, M.D. Guiver, Static protein adsorption, ultrafiltration behavior and cleanability of hydrophilized polysulfone membranes, J. Membr. Sci. 158 (1999) 63. [9] L.F. Hancock, S.M. Fagan, M.S. Ziolo, Hydrophilic, semipermeable membranes fabricated with poly(ethylene oxide)–polysulfone block copolymer, Biomaterials 21 (2001) 725. [10] L.P. Zhu, L. Xu, B.K. Zhu, Y.X. Feng, Y.Y. Xu, Preparation and characterization of improved fouling-resistant PPESK ultrafiltration membranes with amphiphilic PPESK-graft–PEG copolymers as additives, J. Membr. Sci. 294 (2007) 196–206. [11] J.F. Hester, S.C. Olugebefola, A.M. Mayes, Preparation of pH-responsive polymer membranes by self-organization, J. Membr. Sci. 208 (2002) 375–388. [12] T.F. Schaub, G.J. Kellogg, A.M. Mayes, R. Kulasekere, J.F. Ankner, H. Kaiser, Surface modification via chain end segregation in polymer blends, Macromolecules 29 (1996) 3982–3990. [13] J.F. Hester, P. Banerjee, A.M. Mayes, Preparation of protein-resistant surfaces on poly(vinylidene fluoride) membranes via surface segregation, Macromolecules 32 (1999) 1643–1650. [14] J.F. Hester, P. Banerjee, Y.Y. Won, A. Akthakul, M.H. Acar, A.M. Mayes, ATRP of amphiphilic graft copolymers based on PVDF and their use as membrane additives, Macromolecules 35 (2002) 7652–7661. [15] R.E. Holmlin, X. Chen, R.G. Chapman, S. Takayama, G.M. Whitesides, Zwitterionic SAMs that resist nonspecific adsorption of protein from aqueous buffer, Langmuir 17 (2001) 2841–2850. [16] E. Ostuni, R.G. Chapman, R.E. Holmlin, S. Takayama, G.M. Whitesides, A survey of structure–property relationships of surfaces that resist the adsorption of protein, Langmuir 17 (2001) 5605–5620. [17] Y.-Y. Luk, M. Kato, M. Mrksich, Self-assembled monolayers of alkanethiolates presenting mannitol groups are inert to protein adsorption and cell attachment, Langmuir 16 (2000) 9604–9608. [18] S. Chen, F. Yu, Q. Yu, Y. He, S. Jiang, Strong resistance of a thin crystalline layer of balanced charged groups to protein adsorption, Langmuir 22 (2006) 8186–8191. [19] Y. Chang, S. Chen, Q. Yu, Z. Zhang, M. Bernards, S. Jiang, Development of biocompatible interpenetrating polymer networks containing a sulfobetaine-based polymer and a segmented polyurethane for protein resistance, Biomacromolecules 8 (2007) 122–127. [20] Z. Zhang, S. Chen, Y. Chang, S. Jiang, Surface grafted sulfobetaine polymers via atom transfer radical polymerization as superlow fouling coatings, J. Phys. Chem. B 110 (2006) 10799–10804. [21] T. Wang, Y. Wang, Y. Su, Z. Jiang, Antifouling ultrafiltration membrane composed of polyethersulfone and sulfobetaine copolymer, J. Membr. Sci. 280 (2006) 343–350. [22] Q. Sun, Y. Su, X. Ma, Y. Wang, Z. Jiang, Improved antifouling property of zwitterionic ultrafiltration membrane composed of acrylonitrile and sulfobetaine copolymer, J. Membr. Sci. 285 (2006) 299–305. [23] H. Susanto, M. Ulbricht, Photografted thin polymer hydrogel layers on PES ultrafiltration membranes: characterization, stability, and influence on separation performance, Langmuir 23 (2007) 7818–7830.

[24] A. Akthakul, W.F. McDonald, A.M. Mayes, Noncircular pores on the surface of asymmetric polymer membranes: evidence of pore formation via spinodal demixing, J. Membr. Sci. 208 (2002) 147–155. [25] M. Wang, L. Wu, C. Gao, The influence of phase inversion process modified by chemical reaction on membrane properties and morphology, J. Membr. Sci. 270 (2006) 154–161. [26] D.B. Mosqueda-Jimenez, R.M. Narbaitz, T. Matsuura, G. Chowdhury, G. Pleizier, J.P. Santerre, Influence of processing conditions on the properties of ultrafiltration membranes, J. Membr. Sci. 231 (2004) 209–224. [27] J. Bumsuk, Y.J. Ki, K. Bokyung, R. Hee-Woo, Effect of crystallization and annealing on polyacrylonitrile membranes for ultrafiltration, J. Membr. Sci. 246 (2005) 67–76. [28] Q. Shi, Y. Su, S. Zhu, Chao. Li, Y. Zhao, Z. Jiang, A facile method for synthesis of pegylated polyethersulfone and its application in fabrication of antifouling ultrafiltration membrane, J. Membr. Sci. 303 (2007) 204–212. [29] G.S. Gohil, R.K. Nagarale, V.V. Binsu, V.K. Shahi, Preparation and characterization of monovalent cation selective sulfonated poly(ether ether ketone) and poly(ether sulfone) composite membranes, J. Colloid Interface Sci. 298 (2006) 845–853. [30] J.F. Blanco, Q.T. Nguyen, P. Schaetzel, Sulfonation of polysulfones: suitability of the sulfonated materials for asymmetric membrane preparation, J. Appl. Polym. Sci. 84 (2002) 2461–2473. ¨ [31] D. Mockel, E. Staude, M. Dal-Cin, K. Darcovich, M. Guiver, Tangential flow streaming potential measurements: hydrodynamic cell characterization and zeta potentials of carboxylated polysulfone membranes, J. Membr. Sci. 145 (1998) 211–222. [32] Y. Wang, Y. Su, Q. Sun, X. Ma, Z. Jiang, Generation of anti-biofouling ultrafiltration membrane surface by blending novel branched amphiphilic polymers with polyethersulfone, J. Membr. Sci. 286 (2006) 228–236. [33] J.F. Hester, P. Banerjee, Y.-Y. Won, A. Akthakul, M.H. Acar, A.M. Mayes, ATRP of amphiphilic graft copolymers based on PVDF and their use as membrane additives, Macromolecules 35 (2002) 7652–7661. [34] J.-H. Kim, K.-H. Lee, Effect of PEG additive on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153–163. [35] W.-L. Chou, D.-G. Yu, M.-C. Yang, C.-H. Jou, Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers, Sep. Purif. Technol. 57 (2007) 209–219. [36] A. Idris, L.K. Yet, The effect of different molecular weight PEG additives on cellulose acetate asymmetric dialysis membrane performance, J. Membr. Sci. 280 (2006) 920–927. [37] M. Taniguchi, J.P. Pieracci, G. Belfort, Effect of undulations on surface energy: a quantitative assessment, Langmuir 17 (2001) 4312–4315. [38] D.B. Burns, A.L. Zydney, Buffer effects on the zeta potential of ultrafiltration membranes, J. Membr. Sci. 172 (2000) 39–48. [39] H. Susanto, M. Ulbricht, Influence of ultrafiltration membrane characteristics on adsorptive fouling with dextrans, J. Membr. Sci. 266 (2005) 132–142. ¨ [40] C. Causserand, M. Nystrom, P. Aimar, Study of streaming potentials of clean and fouled ultrafiltration membranes, J. Membr. Sci. 88 (1994) 211–222. [41] C. Werner, H.-J. Jacobasch, G. Reichelt, Surface characterization of hemodialysis membranes based on streaming potential measurements, J. Biomater. Sci. Polym. Ed. 7 (1995) 61–76. [42] H.J. Kreuzer, R.L.C. Wang, M. Grunze, Hydroxide ion adsorption on selfassembled monolayers, J. Am. Chem. Soc. 125 (2003) 8384–8389. [43] A.V. Dobrynin, R.H. Colby, M. Rubinstein, Polyampholytes, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 3513–3538. [44] Y. Termonia, Molecular modeling of phase-inversion membranes: effect of additives in the coagulant, J. Membr. Sci. 104 (1995) 173–179. [45] J.F. Hester, A.M. Mayes, Design and performance of foul-resistant poly(vinylidene fluoride) membranes prepared in a single-step by surface segregation, J. Membr. Sci. 202 (2002) 119–135. [46] C.A. Smolers, A.J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phaseinversion membranes. Part 1. Formation of macrovoids, J. Membr. Sci. 73 (1992) 259–275. [47] L. Broens, F.W. Altena, C.A. Smolders, Asymmetric membrane structures as a result of phase separation phenomena, Desalination 32 (1980) 33–45. ˜ ´ [48] R. Iba´ nez, M.C. Almecija, A. Guadix, E.M. Guadix, Dynamics of the ceramic ultrafiltration of model proteins with different isoelectric point: comparison of ␤-lactoglobulin and lysozyme, Sep. Purif. Technol. 57 (2007) 314–320. [49] J. Pieracci, J.V. Crivello, G. Belfort, Increasing membrane permeability of UVmodified poly(ether sulfone) ultrafiltration membranes, J. Membr. Sci. 202 (2002) 1–16. [50] M.B. Huglin, J.M. Rego, Influence of a salt on some properties of hydrophilic methacrylate hydrogels, Macromolecules 24 (1991) 2556–2563.