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Modification of polyethersulfone ultrafiltration membranes with phosphorylcholine copolymer can remarkably improve the antifouling and permeation properties
Journal of Membrane Science 322 (2008) 171–177
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Modification of polyethersulfone ultrafiltration membranes with phosphorylcholine copolymer can remarkably improve the antifouling and permeation properties Yanlei Su a , Chao Li a , Wei Zhao a , Qing Shi a , Haijing Wang a , Zhongyi Jiang a,∗ , Shiping Zhu b a b
Key Laboratory for Green Technology of Ministry of Education, 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 9 December 2007 Received in revised form 2 March 2008 Accepted 20 May 2008 Available online 7 July 2008 Keywords: Polyethersulfone Phosphorylcholine copolymer Membrane Ultrafiltration Antifouling
a b s t r a c t The synthesized phosphorylcholine copolymer composed of 2-methacryloyloxyethylphosphorylcholine (MPC) and n-butyl methacrylate (BMA), blended with polyethersulfone (PES), was used to fabricate antifouling ultrafiltration membranes. Water contact angle measurements confirmed that the hydrophilicity of the MPC-modified PES membranes was enhanced to certain extent. X-ray photoelectron spectroscopy (XPS) analysis verified the substantial enrichment of MPC at the surface of the MPC-modified PES membranes. The adsorption experiments indicated that the adsorption amounts of bovine serum albumin (BSA) on the MPC-modified PES membranes were dramatically decreased in comparison with the control PES membrane. Ultrafiltration experiments were carried out to investigate the effect of MPC modification on the antifouling and permeation properties of the PES membranes, it was found that the rejection ratio of BSA was decreased, the flux recovery ratio was remarkably increased, and the degree of irreversible fouling decreased from 0.46 to 0.09. In addition, the MPC-modified PES membranes could run several cycles without substantial flux loss. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ultrafiltration, as a novel and powerful pressure-driven separation technology, is often used to concentrate or fractionate protein solutions. However, the adsorption and deposition of biomacromolecules on membrane surfaces and/or pore walls (the so-called membrane fouling), often cause severe decrease of flux, substantial increase of energy consumption and operation cost. The application of ultrafiltration is seriously limited by membrane fouling. In recent years, many researchers have revealed that increasing the hydrophilicity of the membrane surfaces and pore surfaces can remarkably reduce or suppress membrane fouling [1–7]. Accordingly, hydrophilic molecules, such as poly(ethylene glycol) (PEG) and zwitterionic molecules, have been widely used to modify the ultrafiltration membranes [8–10]. Several methods, including adsorption, coating, and grafting polymerization, constitute the prevalent methods for ultrafiltration membranes modification. However, the adsorbed or coated modification agents may not reside on the membrane surfaces permanently and lead to a decrease of flux. Chemically or radiation induced grafting poly-
∗ 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.2008.05.047
merization suffers from the drawback of requiring additional complicated steps and rigorous conditions. They can obtain only two-dimensional modification instead of three-dimensional modification, namely, the inner surfaces of membranes cannot be modified [7]. In comparison, surface segregation seems to be a promising method because of its facile operation, high efficiency, and low cost [8]. The surface segregation method can be briefly described as follows: amphiphilic additive is blended into membrane casting solution, and during the subsequent phase inversion process, the hydrophilic segments of the additive are segregated spontaneously to the membrane surfaces while the hydrophobic parts of the additive are firmly entrapped in the membrane bulk matrix. The phosphorylcholine (PC), an electrically neutral zwitterionic head group, which represents the dominant property of the phospholipids existing on the external surfaces of cell membranes, can effectively reduce protein adsorption. Jiang et al. have prepared the self-assembly PC monolayer on gold films which displayed the excellent antifouling property [11]. An artificially synthesized monomer of 2-methacryloyloxyethylphosphorylcholine (MPC) has received increasing attention in recent years; many MPC-based materials bearing the PC groups in the side chains are demonstrated to be capable of effectively inhibiting protein adsorption and platelet adhesion [12–17]. Ishihara et al. have reported that
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blending with the MPC copolymer was an effective treatment for both improving hemocompatibility and reducing protein fouling on the cellulose acetate (CA) flat membranes and hollow fiber membranes [18–20], they also investigated that polysulfone (PSf)/MPC polymers blend membranes could improve blood compatibility and reduce protein adsorption and platelet adhesion. Based on these results, the addition of the MPC polymer to the PSf should be a very useful method to improve the functions and blood compatibility [21–24]. Polyethersulfone (PES) possesses excellent membrane-forming property, as well as mechanical strength and physicochemical stability. However, due to its inherent hydrophobic nature, there are serious membrane fouling and thus rapid decline of permeation flux in many application situations. In present study, amphiphilic random copolymer composed of MPC and n-butyl methacrylate (BMA) was synthesized by ordinary radical copolymerization. The synthesized phosphorylcholine copolymer was blended with PES to fabricate antifouling ultrafiltration membranes. Scanning electron microscopy (SEM) was employed to observe the morphologies of the MPC-modified PES membranes. The contact angle measurements and X-ray photoelectron spectroscopy (XPS) analysis were introduced to characterize the hydrophilic properties of the membrane surface. The protein adsorption, pure water flux, rejection ratio, and flux recovery ratio of the MPC-modified PES membranes were extensively investigated. It was found that the modification with phosphorylcholine copolymer could remarkably improve antifouling and permeation properties of the PES ultrafiltration membranes. 2. Experimental 2.1. Materials and reagents PES 6020P (BASF Co., Germany) was dried at 110 ◦ C for 12 h prior to use. MPC was provided by Prof. Shiping Zhu (Mcmaster University, Canada). BMA, N,N-dimethyl formamide (DMF), azobisisobutyronitrile(AIBN), and PEG2000 (Mw = 2000) were purchased from Kewei Chemical Reagent Co. (Tianjin, China). Bovine serum albumin (BSA) was purchased from Institute of Hematology, Chinese Academic of Medical Sciences (Tianjin, China). Other reagents were all of analytical grade and used without further purification. Water used in all experiments was the deionized water.
Table 1 Formulations of casting solutions for preparing the MPC-modified PES membranes Membrane
PES (g)
PEG (g)
MPC–BMA (g)
DMF (g)
MPC–BMA/PES (%)
1# 2# 3# 4# 5#
5.00 5.00 5.12 5.31 5.38
4.17 4.17 4.27 4.42 4.49
0.00 0.08 0.16 0.28 0.47
18.61 18.55 18.91 19.47 19.57
0 1.5 3.0 5.0 8.0
deionized water. The prestine membranes with a wet thickness of about 240 m were peeled off and subsequently rinsed with water to remove the residual solvent and pore-forming agent PEG. The resultant membranes were kept in water prior to ultrafiltration operation. 2.4. Membrane characterization The surface and cross-sectional morphologies of the membranes were observed by scanning electron microscopy (SEM) using a Philips XL30E scanning microscope. The membranes were frozen in liquid nitrogen, broken, and sputtered with gold prior to SEM observation. The water contact angles of the membranes were measured with a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) by the captive bubble method. Wet membranes glued on plastic slides were inverted and floated in water. Air bubbles were then placed in contact with the membrane surface. The water contact angles were calculated from each image with the supporting software. The contact angles were averages of measurements on six different bubbles. A few pieces of the MPC-modified PES membranes were freezedried, the surface chemical compositions were analyzed by XPS instrument (PHI-1600, USA) using Mg K␣ (1254 eV) as radiation source (the takeoff angle of photoelectron was set to 90◦ ). Surface spectra were collected over a range of 0–1100 eV and highresolution spectra of P2p peak were also collected. The mechanical property was tested with a material-testing machine (AXM350–10KN, Testometric Co., UK). Prior to test, membranes were dried and cut into rectangle strips with a dimension of 30 mm × 10 mm (length × width). The thickness of the lyophilized membranes was measured by a micron micrometer. The extension rate was 2 mm/min. Tensile strength, Young’s modulus, and maximum elongation of the membranes were determined.
2.2. Synthesis of MPC–BMA copolymer 2.5. Protein adsorption An amphiphilic random copolymer composed of MPC and BMA was synthesized by ordinary radical copolymerization according to the reported method [18–20,25]. A given amount of monomer was dissolved in ethanol at a concentration of 1.0 mol/L of monomers (MPC/BMA = 3/7 mol/mol). After removing oxygen by bubbling nitrogen, AIBN was added and the flask was sealed and mechanically stirred at temperature of 60 ◦ C for 20 h. The resulting MPC–BMA copolymer was precipitated, washed, collected, and dried in vacuum subsequently.
The membranes were cut into a round shape with external surface of 2 cm × 11.3 cm and were then put into glass vials containing 10 mL of 1.0 mg/mL BSA solution at pH 7.0 (0.1 M phosphate buffer solution (PBS)). After incubation at 25 ◦ C for 24 h to reach equilibrium, the concentrations of protein in the solution were measured with a UV–vis spectrophotometer (Hitach UV-2800, Japan); the apparent amounts of adsorbed BSA were then calculated. 2.6. Ultrafiltration experiments
2.3. Membrane preparation The formulations of casting solutions were given in Table 1. PES (membrane material), MPC–BMA copolymer (modifier), and PEG2000 (pore-forming agent) were added into DMF at given weight percentages to prepare casting solutions. The solutions were stirred for 4 h at temperature of 60 ◦ C to ensure a complete dissolution of the polymers. After the bubbles were released completely, the solutions were cast on glass plates using a stainless-steel knife, and then the glass plates were immersed in a coagulation bath of
A dead-end stirred cell filtration system connected with a N2 gas cylinder and solution reservoir was designed to evaluate the filtration performance of membranes. All ultrafiltration experiments were carried out using a filtration test cell (Model 8200, Millipore Co., USA) whose volume capacity was 200 mL. The effective area of the membrane was 28.7 cm2 . The operation pressure in the system was maintained by nitrogen gas. All the ultrafiltration experiments were carried out at a stirred speed of 400 rpm and a temperature of 25 ± 1 ◦ C.
Y. Su et al. / Journal of Membrane Science 322 (2008) 171–177
173
Fig. 1. SEM images of surface and cross-sectional morphology of the MPC-modified PES membranes.
Each membrane was initially compacted with deionized water for 30 min at 150 kPa. Then the pressure was lowered to the operating pressure of 100 kPa, and the water flux (Jw1 ) was measured. The operation was stopped after 30 min filtration and the cell emptied. The cell was then filled with protein solution (1.0 mg/mL BSA at pH 7.0 in PBS) immediately and the flux of protein solution (Jp ) was measured until the constant value was obtained. The flux (J) is calculated by the following equation: J=
1−
RFR =
J w2 Jw1
(3)
RFR value can be used to reflect the antifouling property of ultrafiltration membrane, the higher the RFR value, the better the antifouling property of the membrane. 3. Results and discussion
V A t
(1) 3.1. Characterization of blend membranes
where V (L) is the volume of permeated water or protein solution, A (m2 ) is the membrane area, and t (h) is ultrafiltration time. The rejection ratio (R) of BSA is calculated by the following equation: R=
the following equation:
Cp Cf
× 100%
(2)
where Cp and Cf (mg/mL) are the protein concentrations of permeate and feed solutions, respectively. Finally, the cell was completely emptied and refilled with deionized water for cleaning for 20 min. After that, the water flux (Jw2 ) was measured again. The flux recovery ratio (RFR ) is calculated by
PES membranes have been widely used in ultrafiltration process since they have excellent temperature, pH stability, and mechanical strength, and are capable to withstand rigorous cleaning. In order to enhance hydrophilicity of the PES membranes, the synthesized amphiphilic MPC–BMA copolymer was blended with PES in the casting solutions. During the precipitation in a water-based coagulation bath, the hydrophilic MPC segments of the additive were segregated spontaneously to the membrane/water interfaces, creating a hydrophilic antifouling surface; whereas the hydrophobic BMA units of the MPC–BMA copolymer were firmly entrapped in
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Table 2 The mechanical property of PES/MPC–BMA blend membranes Membrane
Young’s modulus (MPa)
Maximum elongation (%)
Maximum strength (MPa)
Membrane thickness (m)
1# 2# 3# 4# 5#
98.43 91.48 86.18 84.63 82.08
17.46 18.35 14.50 18.15 19.50
4.85 4.59 4.43 4.40 4.32
145 178 173 182 165
the PES membrane matrix because of the hydrophobic interactions between the PES and BMA units. The cross-sectional morphologies of MPC-modified PES membranes were shown in Fig. 1. All the membranes exhibited a similar asymmetric structure, which was the typical structure of ultrafiltration membranes, with a top dense layer, a porous sublayer, and fully developed macropores at the bottom. It could be deduced that the addition of MPC–BMA copolymer did not appreciably change the morphology of the PES membranes. The mechanical property of blend membranes was of great importance for practical application. Maximum strength, elastic modulus, maximum elongation and thickness of all the membranes were presented in Table 2. The Young’s modulus and maximum strength of the PES/MPC–BMA blend membranes decreased slightly in comparison with the PES control membrane. The mechanical property of the blend membranes could still well meet the requirements for actual ultrafiltration. The hydrophilicity of the MPC-modified PES membranes was evaluated by water contact angle. The advantage of the captive bubble method is complete hydration of the test membranes, the surface energy between water and the membrane does not change with the measurement time [26]. Fig. 2 presents the water contact angles of MPC-modified PES membranes. The control PES membrane (1# membrane without MPC) had the highest contact angle of 71.1◦ , corresponding to the lowest hydrophilicity. The contact angles of the MPC-modified PES membranes showed a gradual decrease with an increase of MPC–BMA content in the membranes (from 2# to 5# membranes). The decrease of water contact angles clearly indicated that the introduction of MPC groups could significantly enhance the hydrophilicity of the PES membranes. The surface compositions of the MPC-modified PES membranes were determined by XPS analysis. Fig. 3 shows a representative XPS spectrum for the MPC-modified PES membrane (4# membrane), the signal for C, O, S, N, and P elements all appeared as expected. The atomic percentages of C, O, S, N, and P elements were 72.6,
Fig. 2. Contact angles of the MPC-modified PES membranes as a function of MPC–BMA copolymer content in casting solution.
Fig. 3. XPS spectrum of the MPC-modified PES membrane (4# membrane).
21.2, 3.6, 2.0, and 0.6% in the membrane surface, respectively. The high-resolution spectra of P2p core level XPS spectra for all the membranes were shown in Fig. 4. No P signal appeared in the control PES membrane. Since MPC groups were the only source of phosphorus, P2p core level XPS peak at 133 eV only appeared in the MPC-modified PES membranes, which demonstrated the actual existence of MPC groups in the membrane surfaces. The P atomic percents measured by XPS for all membranes were listed in Table 3, it was found surprisingly that further increase of MPC–BMA content in casting solutions did not enhance P atomic percentage on the membrane surfaces. 3# membrane had the highest P content of 0.8 at% on the membrane surface. The theoretical P atomic percentages were calculated according to the compositions of the casting solutions and also listed in Table 3. The degree of surface enrichment, E, is calculated by the following equation: E=
Ae At
Fig. 4. High-resolution P2p core level XPS spectra of the PES membranes.
(4)
Y. Su et al. / Journal of Membrane Science 322 (2008) 171–177 Table 3 The degree of surface enrichment (E) for P element on the MPC-modified PES membranes
175
Table 4 A summary of antifouling property, rejection ratio of the MPC-modified PES membranes
Membrane
P (at%)a
P (at%)b
E
Membrane
Rt
Rr
Rir
RFR (%)
Rejection ratio (%)
1# 2# 3# 4# 5#
0 0.07 0.14 0.25 0.36
0 0.5 0.8 0.6 0.5
– 7.14 5.71 2.40 1.39
1# 2# 3# 4# 5#
0.67 0.63 0.60 0.58 0.56
0.21 0.32 0.34 0.42 0.47
0.46 0.31 0.26 0.16 0.09
57 73 67 76 91
99 96 74 70 59
a The theoretical P (at%) was calculated from the original MPC content added in the casting solution. b The P (at%) on the membrane surface was obtained from the XPS measurement.
where Ae and At are the P atomic percentages obtained through experimental measurement and theoretical calculation, respectively. The values of E for the MPC-modified PES membranes listed in Table 3 were all greater than 1, which meant the substantial enhancement of MPC on the membrane surfaces. The highest E value was about 7.14 for 2# membrane, which meant that the surface concentration of MPC was about 7.14 times higher than that in the membrane matrix. However, the value of E was gradually decreased with an increase of MPC–BMA content in the membranes. MPC groups contain an equal number of cationic and anionic species on the same monomer residue in the polymer chain. There is a dipole moment for every PC group in the MPC–BMA copolymer. It has been found that the dipole–dipole assemblies are quite common in zwitterionic polymers, leading to insolubility of these polymers in pure water [27]. In the casting solution of the present study, the existence of dipole–dipole interactions among inter- and intra-polymer chains may lead to the formation of aggregates for the zwitterionic polymers. The higher content of MPC–BMA copolymer in the casting solution should increase the chance of dipole–dipole interactions among the MPC–BMA copolymer chains. The assemblies based on MPC–BMA copolymer should have the lower diffusion velocity in the coagulation process. This may constitute the probable reason that the value of E decreased with the increase of MPC–BMA content in the casting solutions. 3.2. Protein adsorption on PES/MPC–BMA blend membranes Fig. 5 shows that the amounts of adsorbed BSA were decreased with an increase in MPC–BMA copolymer content. When the MPC–BMA content reached to 8 wt.% in the membrane, the amount of BSA adsorption was decreased dramatically to 10.6 g/cm2 . The
Fig. 5. Protein adsorption amount on the MPC-modified PES membranes as a function of the MPC–BMA content in the casting solution.
effective reduction in the amounts of adsorbed BSA was attributed to the residence of MPC groups on the membrane surfaces. The mechanism of protein resistance for PC groups was ascribed to its high hydrophilic property and net neutral charge. In their extensive research, Ishihara et al. found that MPC-based copolymers took up large quantities of free water; which built up the stable defense layer to resist protein invasion [15,28]. 3.3. Permeation and recycling properties of blend membranes Ultrafiltration experiments were carried out to investigate the permeability of the MPC-modified PES membranes. Fig. 6 presents the fluxes for three cycles of ultrafiltration operation. In every cycle, water flux was first measured, then BSA solution was filtrated, the membrane was cleaned with water and water flux was measured again in the final step. The pure water flux of the MPC-modified PES membranes increased from 203.3 to 255.5 L/(m2 h), and the rejection ratio of the membranes was gradually decreased from 99 to 59%, with an increase in the MPC–BMA copolymer content (in Table 4 from 1# to 5# membrane). The pure water flux of an asymmetric ultrafiltration membrane is usually affected by the pore radius and pore density of the skin layer, therefore increasing the pore radius and pore density can increase the pure water flux [29]. Considering the increase of pure water flux and the decrease of BSA rejection ratio of the MPC-modified PES membranes, it could be deduced that the addition of MPC–BMA copolymer increased pore radius in the skin layers. In most cases, flux decline in protein ultrafiltration is attributed to two main sources: concentration polarization and membrane fouling. Concentration polarization can be alleviated by increasing the flow rate over the membrane. Fouling occurs in two common ways: cake formation and adsorption of foulants. Fouling due to
Fig. 6. The time-dependent flux in the three cycles of ultrafiltration operation for the PES membranes. The BSA concentration is 1.0 mg/mL phosphate-buffered saline, pH 7.0.
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cake fouling is generally reversible by water flushing. Fouling due to the adsorption of foulants is essentially irreversible which occurs on both the membrane surfaces and pore walls. In the present study, concentration polarization was ignored because of the rigorous stirring near the membrane surface. Therefore, membrane fouling mostly caused the decline of flux. A simple and direct parameter to characterize the antifouling property of the membranes was the value of RFR , the higher value of RFR meant the more excellent antifouling property. The RFR values in the first cycle for the MPCmodified PES membranes were calculated and listed in Table 4. It can be seen that the value of RFR was increased dramatically from 57% for the control PES membrane (1# membrane) to 91% for the MPC-modified PES membrane (5# membrane). These results demonstrated that the modification of PES membranes with MPC copolymer surely improved the antifouling property. 3.4. Reversible and irreversible fouling To further investigate the antifouling property, several ratios were introduced and defined. The first ratio Rt is calculated by the following equation: Rt = 1 −
Jp Jw1
(5)
which describes the degree of total flux loss caused by total fouling. Rr is calculated by the following equation: Rr =
Jw2 − Jp Jw1
(6)
which describes the degree of flux loss due to reversible fouling, the reversible protein deposition (cake formation) can be eliminated through hydraulic cleaning. Rir is defined by the following equation: Rir =
Jw1 − Jw2 Jw1
(7)
which describes the degree of flux loss caused by irreversible fouling, the irreversible protein adsorption cannot be eliminated through simple hydraulic cleaning. Rt is the sum of Rr and Rir . A summary of Rr , Rir , and Rt of the MPC-modified PES membranes were shown in Table 4. It could be seen that Rt gradually decreased from 0.67 to 0.56 with increasing the MPC–BMA copolymer content, the lower values of Rt indicated lower total flux loss corresponding to less protein adsorption or deposition on membrane surface. Table 4 also shows that the MPC-modified PES membranes had lower Rir than the control PES membrane. Rir decreased remarkably from 0.46 to 0.09 with an increase of MPC content in the membranes. The introduction of MPC copolymer substantially reduced total membrane fouling especially irreversible membrane fouling, which could be attributed to that the highly hydrophilic PC groups held high portion of free water on the membrane surfaces and the resultant water layer associated with the MPC-containing surface preserved the native conformation of the adsorbed proteins [15,28]. As a result, the MPC-modified PES membranes exhibited higher flux recovery compared with the control PES membrane. 3.5. Effect of pH on flux of the MPC-modified PES membranes Fluxes of MPC-modified PES membrane (5# membrane) in ultrafiltration operation for BSA solution under different pH values were shown in Fig. 7. The pH values of protein solution were kept at 4.5, 7.0, and 9.0, the values of RFR of BSA solution were 91, 91, and 90%, correspondingly. These results indicated that the MPC-modified PES membranes possessed antifouling property in a wide pH range.
Fig. 7. Effect of pH values of BSA solution on the flux of the MPC-modified PES membrane (5# membrane).
PC groups rendered the membrane with hydrophilic and electrostatic neutral surface. Although the pH value of protein solution was changed, the interactions between the MPC-modified membranes and BSA were independent of electrostatic interactions [30]. Obviously, the MPC-modified PES membranes would be beneficial for their practical application in wide pH range. 4. Conclusions The MPC-modified PES ultrafiltration membranes were successfully prepared by phase inversion method. Due to the high hydrophilicity and electric neutrality of MPC–BMA copolymer, the antifouling property of the modified membranes was remarkably improved with increasing MPC–BMA copolymer content. The incorporation of MPC groups substantially reduced total membrane fouling, especially irreversible membrane fouling. Therefore, simple hydraulic cleaning could effectively eliminate the reversible fouling. In a wide pH range from 4.5 to 9.0, the MPC-modified PES membranes exhibited superior antifouling property. Acknowledgements This work was funded by Tianjin Natural Science Foundation (No. 07JCYBJC00900); Tianjin Natural Science Foundation for International Cooperation (No. 06YFGHHZ00200), and the Programme of Introducing Talents of Discipline to Universities (No. B06006). References [1] Z.M. Liu, Z.K. Xu, L.S. Wan, J. Wu, M. Ulbricht, Surface modification of polypropylene microfiltration membranes by the immobilization of poly(Nvinyl-2-pyrrolidone): a facile plasma approach, J. Membr. Sci. 249 (2005) 21–31. [2] 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. [3] M. Taniguchi, G. Belfort, Low protein fouling synthetic membranes by UVassisted surface grafting modification: varying monomer type, J. Membr. Sci. 231 (2004) 147–157. ´ Kiss, J. Samu, A. Toth, ´ ´ [4] E. I. Bertoti, Novel ways of covalent attachment of poly(ethylene oxide) onto polyethylene: surface modification and characterization by XPS and contact angle measurements, Langmuir 12 (1996) 1651–1657. [5] J. Pieracci, J.V. Crivello, G. Belfort, Photochemical modification of 10 kDa polyethersulfone ultrafiltration membranes for reduction of biofouling, J. Membr. Sci. 156 (1999) 223–240. [6] 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.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Modification of polyethersulfone ultrafiltration membranes with phosphorylcholine copolymer can remarkably improve the antifouling and permeation properties Yanlei Su a , Chao Li a , Wei Zhao a , Qing Shi a , Haijing Wang a , Zhongyi Jiang a,∗ , Shiping Zhu b a b
Key Laboratory for Green Technology of Ministry of Education, 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 9 December 2007 Received in revised form 2 March 2008 Accepted 20 May 2008 Available online 7 July 2008 Keywords: Polyethersulfone Phosphorylcholine copolymer Membrane Ultrafiltration Antifouling
a b s t r a c t The synthesized phosphorylcholine copolymer composed of 2-methacryloyloxyethylphosphorylcholine (MPC) and n-butyl methacrylate (BMA), blended with polyethersulfone (PES), was used to fabricate antifouling ultrafiltration membranes. Water contact angle measurements confirmed that the hydrophilicity of the MPC-modified PES membranes was enhanced to certain extent. X-ray photoelectron spectroscopy (XPS) analysis verified the substantial enrichment of MPC at the surface of the MPC-modified PES membranes. The adsorption experiments indicated that the adsorption amounts of bovine serum albumin (BSA) on the MPC-modified PES membranes were dramatically decreased in comparison with the control PES membrane. Ultrafiltration experiments were carried out to investigate the effect of MPC modification on the antifouling and permeation properties of the PES membranes, it was found that the rejection ratio of BSA was decreased, the flux recovery ratio was remarkably increased, and the degree of irreversible fouling decreased from 0.46 to 0.09. In addition, the MPC-modified PES membranes could run several cycles without substantial flux loss. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ultrafiltration, as a novel and powerful pressure-driven separation technology, is often used to concentrate or fractionate protein solutions. However, the adsorption and deposition of biomacromolecules on membrane surfaces and/or pore walls (the so-called membrane fouling), often cause severe decrease of flux, substantial increase of energy consumption and operation cost. The application of ultrafiltration is seriously limited by membrane fouling. In recent years, many researchers have revealed that increasing the hydrophilicity of the membrane surfaces and pore surfaces can remarkably reduce or suppress membrane fouling [1–7]. Accordingly, hydrophilic molecules, such as poly(ethylene glycol) (PEG) and zwitterionic molecules, have been widely used to modify the ultrafiltration membranes [8–10]. Several methods, including adsorption, coating, and grafting polymerization, constitute the prevalent methods for ultrafiltration membranes modification. However, the adsorbed or coated modification agents may not reside on the membrane surfaces permanently and lead to a decrease of flux. Chemically or radiation induced grafting poly-
∗ 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.2008.05.047
merization suffers from the drawback of requiring additional complicated steps and rigorous conditions. They can obtain only two-dimensional modification instead of three-dimensional modification, namely, the inner surfaces of membranes cannot be modified [7]. In comparison, surface segregation seems to be a promising method because of its facile operation, high efficiency, and low cost [8]. The surface segregation method can be briefly described as follows: amphiphilic additive is blended into membrane casting solution, and during the subsequent phase inversion process, the hydrophilic segments of the additive are segregated spontaneously to the membrane surfaces while the hydrophobic parts of the additive are firmly entrapped in the membrane bulk matrix. The phosphorylcholine (PC), an electrically neutral zwitterionic head group, which represents the dominant property of the phospholipids existing on the external surfaces of cell membranes, can effectively reduce protein adsorption. Jiang et al. have prepared the self-assembly PC monolayer on gold films which displayed the excellent antifouling property [11]. An artificially synthesized monomer of 2-methacryloyloxyethylphosphorylcholine (MPC) has received increasing attention in recent years; many MPC-based materials bearing the PC groups in the side chains are demonstrated to be capable of effectively inhibiting protein adsorption and platelet adhesion [12–17]. Ishihara et al. have reported that
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blending with the MPC copolymer was an effective treatment for both improving hemocompatibility and reducing protein fouling on the cellulose acetate (CA) flat membranes and hollow fiber membranes [18–20], they also investigated that polysulfone (PSf)/MPC polymers blend membranes could improve blood compatibility and reduce protein adsorption and platelet adhesion. Based on these results, the addition of the MPC polymer to the PSf should be a very useful method to improve the functions and blood compatibility [21–24]. Polyethersulfone (PES) possesses excellent membrane-forming property, as well as mechanical strength and physicochemical stability. However, due to its inherent hydrophobic nature, there are serious membrane fouling and thus rapid decline of permeation flux in many application situations. In present study, amphiphilic random copolymer composed of MPC and n-butyl methacrylate (BMA) was synthesized by ordinary radical copolymerization. The synthesized phosphorylcholine copolymer was blended with PES to fabricate antifouling ultrafiltration membranes. Scanning electron microscopy (SEM) was employed to observe the morphologies of the MPC-modified PES membranes. The contact angle measurements and X-ray photoelectron spectroscopy (XPS) analysis were introduced to characterize the hydrophilic properties of the membrane surface. The protein adsorption, pure water flux, rejection ratio, and flux recovery ratio of the MPC-modified PES membranes were extensively investigated. It was found that the modification with phosphorylcholine copolymer could remarkably improve antifouling and permeation properties of the PES ultrafiltration membranes. 2. Experimental 2.1. Materials and reagents PES 6020P (BASF Co., Germany) was dried at 110 ◦ C for 12 h prior to use. MPC was provided by Prof. Shiping Zhu (Mcmaster University, Canada). BMA, N,N-dimethyl formamide (DMF), azobisisobutyronitrile(AIBN), and PEG2000 (Mw = 2000) were purchased from Kewei Chemical Reagent Co. (Tianjin, China). Bovine serum albumin (BSA) was purchased from Institute of Hematology, Chinese Academic of Medical Sciences (Tianjin, China). Other reagents were all of analytical grade and used without further purification. Water used in all experiments was the deionized water.
Table 1 Formulations of casting solutions for preparing the MPC-modified PES membranes Membrane
PES (g)
PEG (g)
MPC–BMA (g)
DMF (g)
MPC–BMA/PES (%)
1# 2# 3# 4# 5#
5.00 5.00 5.12 5.31 5.38
4.17 4.17 4.27 4.42 4.49
0.00 0.08 0.16 0.28 0.47
18.61 18.55 18.91 19.47 19.57
0 1.5 3.0 5.0 8.0
deionized water. The prestine membranes with a wet thickness of about 240 m were peeled off and subsequently rinsed with water to remove the residual solvent and pore-forming agent PEG. The resultant membranes were kept in water prior to ultrafiltration operation. 2.4. Membrane characterization The surface and cross-sectional morphologies of the membranes were observed by scanning electron microscopy (SEM) using a Philips XL30E scanning microscope. The membranes were frozen in liquid nitrogen, broken, and sputtered with gold prior to SEM observation. The water contact angles of the membranes were measured with a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) by the captive bubble method. Wet membranes glued on plastic slides were inverted and floated in water. Air bubbles were then placed in contact with the membrane surface. The water contact angles were calculated from each image with the supporting software. The contact angles were averages of measurements on six different bubbles. A few pieces of the MPC-modified PES membranes were freezedried, the surface chemical compositions were analyzed by XPS instrument (PHI-1600, USA) using Mg K␣ (1254 eV) as radiation source (the takeoff angle of photoelectron was set to 90◦ ). Surface spectra were collected over a range of 0–1100 eV and highresolution spectra of P2p peak were also collected. The mechanical property was tested with a material-testing machine (AXM350–10KN, Testometric Co., UK). Prior to test, membranes were dried and cut into rectangle strips with a dimension of 30 mm × 10 mm (length × width). The thickness of the lyophilized membranes was measured by a micron micrometer. The extension rate was 2 mm/min. Tensile strength, Young’s modulus, and maximum elongation of the membranes were determined.
2.2. Synthesis of MPC–BMA copolymer 2.5. Protein adsorption An amphiphilic random copolymer composed of MPC and BMA was synthesized by ordinary radical copolymerization according to the reported method [18–20,25]. A given amount of monomer was dissolved in ethanol at a concentration of 1.0 mol/L of monomers (MPC/BMA = 3/7 mol/mol). After removing oxygen by bubbling nitrogen, AIBN was added and the flask was sealed and mechanically stirred at temperature of 60 ◦ C for 20 h. The resulting MPC–BMA copolymer was precipitated, washed, collected, and dried in vacuum subsequently.
The membranes were cut into a round shape with external surface of 2 cm × 11.3 cm and were then put into glass vials containing 10 mL of 1.0 mg/mL BSA solution at pH 7.0 (0.1 M phosphate buffer solution (PBS)). After incubation at 25 ◦ C for 24 h to reach equilibrium, the concentrations of protein in the solution were measured with a UV–vis spectrophotometer (Hitach UV-2800, Japan); the apparent amounts of adsorbed BSA were then calculated. 2.6. Ultrafiltration experiments
2.3. Membrane preparation The formulations of casting solutions were given in Table 1. PES (membrane material), MPC–BMA copolymer (modifier), and PEG2000 (pore-forming agent) were added into DMF at given weight percentages to prepare casting solutions. The solutions were stirred for 4 h at temperature of 60 ◦ C to ensure a complete dissolution of the polymers. After the bubbles were released completely, the solutions were cast on glass plates using a stainless-steel knife, and then the glass plates were immersed in a coagulation bath of
A dead-end stirred cell filtration system connected with a N2 gas cylinder and solution reservoir was designed to evaluate the filtration performance of membranes. All ultrafiltration experiments were carried out using a filtration test cell (Model 8200, Millipore Co., USA) whose volume capacity was 200 mL. The effective area of the membrane was 28.7 cm2 . The operation pressure in the system was maintained by nitrogen gas. All the ultrafiltration experiments were carried out at a stirred speed of 400 rpm and a temperature of 25 ± 1 ◦ C.
Y. Su et al. / Journal of Membrane Science 322 (2008) 171–177
173
Fig. 1. SEM images of surface and cross-sectional morphology of the MPC-modified PES membranes.
Each membrane was initially compacted with deionized water for 30 min at 150 kPa. Then the pressure was lowered to the operating pressure of 100 kPa, and the water flux (Jw1 ) was measured. The operation was stopped after 30 min filtration and the cell emptied. The cell was then filled with protein solution (1.0 mg/mL BSA at pH 7.0 in PBS) immediately and the flux of protein solution (Jp ) was measured until the constant value was obtained. The flux (J) is calculated by the following equation: J=
1−
RFR =
J w2 Jw1
(3)
RFR value can be used to reflect the antifouling property of ultrafiltration membrane, the higher the RFR value, the better the antifouling property of the membrane. 3. Results and discussion
V A t
(1) 3.1. Characterization of blend membranes
where V (L) is the volume of permeated water or protein solution, A (m2 ) is the membrane area, and t (h) is ultrafiltration time. The rejection ratio (R) of BSA is calculated by the following equation: R=
the following equation:
Cp Cf
× 100%
(2)
where Cp and Cf (mg/mL) are the protein concentrations of permeate and feed solutions, respectively. Finally, the cell was completely emptied and refilled with deionized water for cleaning for 20 min. After that, the water flux (Jw2 ) was measured again. The flux recovery ratio (RFR ) is calculated by
PES membranes have been widely used in ultrafiltration process since they have excellent temperature, pH stability, and mechanical strength, and are capable to withstand rigorous cleaning. In order to enhance hydrophilicity of the PES membranes, the synthesized amphiphilic MPC–BMA copolymer was blended with PES in the casting solutions. During the precipitation in a water-based coagulation bath, the hydrophilic MPC segments of the additive were segregated spontaneously to the membrane/water interfaces, creating a hydrophilic antifouling surface; whereas the hydrophobic BMA units of the MPC–BMA copolymer were firmly entrapped in
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Table 2 The mechanical property of PES/MPC–BMA blend membranes Membrane
Young’s modulus (MPa)
Maximum elongation (%)
Maximum strength (MPa)
Membrane thickness (m)
1# 2# 3# 4# 5#
98.43 91.48 86.18 84.63 82.08
17.46 18.35 14.50 18.15 19.50
4.85 4.59 4.43 4.40 4.32
145 178 173 182 165
the PES membrane matrix because of the hydrophobic interactions between the PES and BMA units. The cross-sectional morphologies of MPC-modified PES membranes were shown in Fig. 1. All the membranes exhibited a similar asymmetric structure, which was the typical structure of ultrafiltration membranes, with a top dense layer, a porous sublayer, and fully developed macropores at the bottom. It could be deduced that the addition of MPC–BMA copolymer did not appreciably change the morphology of the PES membranes. The mechanical property of blend membranes was of great importance for practical application. Maximum strength, elastic modulus, maximum elongation and thickness of all the membranes were presented in Table 2. The Young’s modulus and maximum strength of the PES/MPC–BMA blend membranes decreased slightly in comparison with the PES control membrane. The mechanical property of the blend membranes could still well meet the requirements for actual ultrafiltration. The hydrophilicity of the MPC-modified PES membranes was evaluated by water contact angle. The advantage of the captive bubble method is complete hydration of the test membranes, the surface energy between water and the membrane does not change with the measurement time [26]. Fig. 2 presents the water contact angles of MPC-modified PES membranes. The control PES membrane (1# membrane without MPC) had the highest contact angle of 71.1◦ , corresponding to the lowest hydrophilicity. The contact angles of the MPC-modified PES membranes showed a gradual decrease with an increase of MPC–BMA content in the membranes (from 2# to 5# membranes). The decrease of water contact angles clearly indicated that the introduction of MPC groups could significantly enhance the hydrophilicity of the PES membranes. The surface compositions of the MPC-modified PES membranes were determined by XPS analysis. Fig. 3 shows a representative XPS spectrum for the MPC-modified PES membrane (4# membrane), the signal for C, O, S, N, and P elements all appeared as expected. The atomic percentages of C, O, S, N, and P elements were 72.6,
Fig. 2. Contact angles of the MPC-modified PES membranes as a function of MPC–BMA copolymer content in casting solution.
Fig. 3. XPS spectrum of the MPC-modified PES membrane (4# membrane).
21.2, 3.6, 2.0, and 0.6% in the membrane surface, respectively. The high-resolution spectra of P2p core level XPS spectra for all the membranes were shown in Fig. 4. No P signal appeared in the control PES membrane. Since MPC groups were the only source of phosphorus, P2p core level XPS peak at 133 eV only appeared in the MPC-modified PES membranes, which demonstrated the actual existence of MPC groups in the membrane surfaces. The P atomic percents measured by XPS for all membranes were listed in Table 3, it was found surprisingly that further increase of MPC–BMA content in casting solutions did not enhance P atomic percentage on the membrane surfaces. 3# membrane had the highest P content of 0.8 at% on the membrane surface. The theoretical P atomic percentages were calculated according to the compositions of the casting solutions and also listed in Table 3. The degree of surface enrichment, E, is calculated by the following equation: E=
Ae At
Fig. 4. High-resolution P2p core level XPS spectra of the PES membranes.
(4)
Y. Su et al. / Journal of Membrane Science 322 (2008) 171–177 Table 3 The degree of surface enrichment (E) for P element on the MPC-modified PES membranes
175
Table 4 A summary of antifouling property, rejection ratio of the MPC-modified PES membranes
Membrane
P (at%)a
P (at%)b
E
Membrane
Rt
Rr
Rir
RFR (%)
Rejection ratio (%)
1# 2# 3# 4# 5#
0 0.07 0.14 0.25 0.36
0 0.5 0.8 0.6 0.5
– 7.14 5.71 2.40 1.39
1# 2# 3# 4# 5#
0.67 0.63 0.60 0.58 0.56
0.21 0.32 0.34 0.42 0.47
0.46 0.31 0.26 0.16 0.09
57 73 67 76 91
99 96 74 70 59
a The theoretical P (at%) was calculated from the original MPC content added in the casting solution. b The P (at%) on the membrane surface was obtained from the XPS measurement.
where Ae and At are the P atomic percentages obtained through experimental measurement and theoretical calculation, respectively. The values of E for the MPC-modified PES membranes listed in Table 3 were all greater than 1, which meant the substantial enhancement of MPC on the membrane surfaces. The highest E value was about 7.14 for 2# membrane, which meant that the surface concentration of MPC was about 7.14 times higher than that in the membrane matrix. However, the value of E was gradually decreased with an increase of MPC–BMA content in the membranes. MPC groups contain an equal number of cationic and anionic species on the same monomer residue in the polymer chain. There is a dipole moment for every PC group in the MPC–BMA copolymer. It has been found that the dipole–dipole assemblies are quite common in zwitterionic polymers, leading to insolubility of these polymers in pure water [27]. In the casting solution of the present study, the existence of dipole–dipole interactions among inter- and intra-polymer chains may lead to the formation of aggregates for the zwitterionic polymers. The higher content of MPC–BMA copolymer in the casting solution should increase the chance of dipole–dipole interactions among the MPC–BMA copolymer chains. The assemblies based on MPC–BMA copolymer should have the lower diffusion velocity in the coagulation process. This may constitute the probable reason that the value of E decreased with the increase of MPC–BMA content in the casting solutions. 3.2. Protein adsorption on PES/MPC–BMA blend membranes Fig. 5 shows that the amounts of adsorbed BSA were decreased with an increase in MPC–BMA copolymer content. When the MPC–BMA content reached to 8 wt.% in the membrane, the amount of BSA adsorption was decreased dramatically to 10.6 g/cm2 . The
Fig. 5. Protein adsorption amount on the MPC-modified PES membranes as a function of the MPC–BMA content in the casting solution.
effective reduction in the amounts of adsorbed BSA was attributed to the residence of MPC groups on the membrane surfaces. The mechanism of protein resistance for PC groups was ascribed to its high hydrophilic property and net neutral charge. In their extensive research, Ishihara et al. found that MPC-based copolymers took up large quantities of free water; which built up the stable defense layer to resist protein invasion [15,28]. 3.3. Permeation and recycling properties of blend membranes Ultrafiltration experiments were carried out to investigate the permeability of the MPC-modified PES membranes. Fig. 6 presents the fluxes for three cycles of ultrafiltration operation. In every cycle, water flux was first measured, then BSA solution was filtrated, the membrane was cleaned with water and water flux was measured again in the final step. The pure water flux of the MPC-modified PES membranes increased from 203.3 to 255.5 L/(m2 h), and the rejection ratio of the membranes was gradually decreased from 99 to 59%, with an increase in the MPC–BMA copolymer content (in Table 4 from 1# to 5# membrane). The pure water flux of an asymmetric ultrafiltration membrane is usually affected by the pore radius and pore density of the skin layer, therefore increasing the pore radius and pore density can increase the pure water flux [29]. Considering the increase of pure water flux and the decrease of BSA rejection ratio of the MPC-modified PES membranes, it could be deduced that the addition of MPC–BMA copolymer increased pore radius in the skin layers. In most cases, flux decline in protein ultrafiltration is attributed to two main sources: concentration polarization and membrane fouling. Concentration polarization can be alleviated by increasing the flow rate over the membrane. Fouling occurs in two common ways: cake formation and adsorption of foulants. Fouling due to
Fig. 6. The time-dependent flux in the three cycles of ultrafiltration operation for the PES membranes. The BSA concentration is 1.0 mg/mL phosphate-buffered saline, pH 7.0.
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cake fouling is generally reversible by water flushing. Fouling due to the adsorption of foulants is essentially irreversible which occurs on both the membrane surfaces and pore walls. In the present study, concentration polarization was ignored because of the rigorous stirring near the membrane surface. Therefore, membrane fouling mostly caused the decline of flux. A simple and direct parameter to characterize the antifouling property of the membranes was the value of RFR , the higher value of RFR meant the more excellent antifouling property. The RFR values in the first cycle for the MPCmodified PES membranes were calculated and listed in Table 4. It can be seen that the value of RFR was increased dramatically from 57% for the control PES membrane (1# membrane) to 91% for the MPC-modified PES membrane (5# membrane). These results demonstrated that the modification of PES membranes with MPC copolymer surely improved the antifouling property. 3.4. Reversible and irreversible fouling To further investigate the antifouling property, several ratios were introduced and defined. The first ratio Rt is calculated by the following equation: Rt = 1 −
Jp Jw1
(5)
which describes the degree of total flux loss caused by total fouling. Rr is calculated by the following equation: Rr =
Jw2 − Jp Jw1
(6)
which describes the degree of flux loss due to reversible fouling, the reversible protein deposition (cake formation) can be eliminated through hydraulic cleaning. Rir is defined by the following equation: Rir =
Jw1 − Jw2 Jw1
(7)
which describes the degree of flux loss caused by irreversible fouling, the irreversible protein adsorption cannot be eliminated through simple hydraulic cleaning. Rt is the sum of Rr and Rir . A summary of Rr , Rir , and Rt of the MPC-modified PES membranes were shown in Table 4. It could be seen that Rt gradually decreased from 0.67 to 0.56 with increasing the MPC–BMA copolymer content, the lower values of Rt indicated lower total flux loss corresponding to less protein adsorption or deposition on membrane surface. Table 4 also shows that the MPC-modified PES membranes had lower Rir than the control PES membrane. Rir decreased remarkably from 0.46 to 0.09 with an increase of MPC content in the membranes. The introduction of MPC copolymer substantially reduced total membrane fouling especially irreversible membrane fouling, which could be attributed to that the highly hydrophilic PC groups held high portion of free water on the membrane surfaces and the resultant water layer associated with the MPC-containing surface preserved the native conformation of the adsorbed proteins [15,28]. As a result, the MPC-modified PES membranes exhibited higher flux recovery compared with the control PES membrane. 3.5. Effect of pH on flux of the MPC-modified PES membranes Fluxes of MPC-modified PES membrane (5# membrane) in ultrafiltration operation for BSA solution under different pH values were shown in Fig. 7. The pH values of protein solution were kept at 4.5, 7.0, and 9.0, the values of RFR of BSA solution were 91, 91, and 90%, correspondingly. These results indicated that the MPC-modified PES membranes possessed antifouling property in a wide pH range.
Fig. 7. Effect of pH values of BSA solution on the flux of the MPC-modified PES membrane (5# membrane).
PC groups rendered the membrane with hydrophilic and electrostatic neutral surface. Although the pH value of protein solution was changed, the interactions between the MPC-modified membranes and BSA were independent of electrostatic interactions [30]. Obviously, the MPC-modified PES membranes would be beneficial for their practical application in wide pH range. 4. Conclusions The MPC-modified PES ultrafiltration membranes were successfully prepared by phase inversion method. Due to the high hydrophilicity and electric neutrality of MPC–BMA copolymer, the antifouling property of the modified membranes was remarkably improved with increasing MPC–BMA copolymer content. The incorporation of MPC groups substantially reduced total membrane fouling, especially irreversible membrane fouling. Therefore, simple hydraulic cleaning could effectively eliminate the reversible fouling. In a wide pH range from 4.5 to 9.0, the MPC-modified PES membranes exhibited superior antifouling property. Acknowledgements This work was funded by Tianjin Natural Science Foundation (No. 07JCYBJC00900); Tianjin Natural Science Foundation for International Cooperation (No. 06YFGHHZ00200), and the Programme of Introducing Talents of Discipline to Universities (No. B06006). References [1] Z.M. Liu, Z.K. Xu, L.S. Wan, J. Wu, M. Ulbricht, Surface modification of polypropylene microfiltration membranes by the immobilization of poly(Nvinyl-2-pyrrolidone): a facile plasma approach, J. Membr. Sci. 249 (2005) 21–31. [2] 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. [3] M. Taniguchi, G. Belfort, Low protein fouling synthetic membranes by UVassisted surface grafting modification: varying monomer type, J. Membr. Sci. 231 (2004) 147–157. ´ Kiss, J. Samu, A. Toth, ´ ´ [4] E. I. Bertoti, Novel ways of covalent attachment of poly(ethylene oxide) onto polyethylene: surface modification and characterization by XPS and contact angle measurements, Langmuir 12 (1996) 1651–1657. [5] J. Pieracci, J.V. Crivello, G. Belfort, Photochemical modification of 10 kDa polyethersulfone ultrafiltration membranes for reduction of biofouling, J. Membr. Sci. 156 (1999) 223–240. [6] 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.
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