Improved Antifouling Property of Polyethersulfone Hollow Fiber

ARTICLE pubs.acs.org/IECR

Improved Antifouling Property of Polyethersulfone Hollow Fiber Membranes Using Additive of Poly(ethylene glycol) Methyl Ether-b-Poly(styrene) Copolymers Weifeng Zhao, Chao He, Huiyuan Wang, Baihai Su, Shudong Sun, and Changsheng Zhao* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: Well-defined block copolymers, poly(ethylene glycol) methyl ether-b-poly(styrene) (mPEG-b-PS), in which the PS blocks had different molecular weights, were synthesized by atom-transfer radical polymerization (ATRP) and characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). The block copolymers were then used as amphiphilic additives to modify polyethersulfone (PES) hollow fiber membranes to improve the antifouling property. Static contact angle measurement indicated the increase of the membrane surface hydrophilicity, and scanning electron micrograph (SEM) suggested that the modified membranes preserved asymmetric structure. Protein ultrafiltration experiments showed that the antifouling ability of the modified membranes enhanced. After three cycles of BSA solution (1.0 mg/mL) ultrafiltration and three times of hydraulic cleaning, the water flux recovery ratios (FRR) of the mPEG-b-PS modified membranes were still as high as 85.6%. The hydrophilic modification with mPEG-b-PS copolymers is a good method to improve the antifouling property of PES hollow fiber membranes.

1. INTRODUCTION Block copolymers are composed of long sequences of the same monomer unit, covalently linked chains of chemically distinct type. The blocks can be connected in a variety of ways, and the chemical distinction among different chains leads to ordered equilibrium mesophases, known as microphase separation. Amphiphilic block copolymers are of considerable interest for various applications, as steric barriers at solid surfaces to restrain undesired protein adhesion,1-4 as stabilizers of emulsions5 to avoid flocculation, as carrier for controlled drug release6 and vector for gene therapy.7 The combination of the two (or more) different chemical character and solution properties, such as hydrophilic and hydrophobic properties, of these materials may be utilized. The synthesis of amphiphilic block copolymers with controlled block lengths requires efficient controlled/”living” polymerization. Controlled polymerization processes such as atom-transfer radical polymerization (ATRP)8-12 provides access to synthesize various well-defined copolymers with narrow molecular weight distribution and controlled topology. Although there are some challenges for ATPR polymers, such as cost-performance for commercial products,13 ATRP shows a tolerance to a large variety of functional groups and monomers such as styrene,14 (meth)acrylates,15 (meth)acrylamides,16 and acrylonitrile.17 Poly(ethylene glycol) methyl ether (mPEG) derivatives show good solubility in most organic and aqueous media and are of great interest in the chemical modification of synthetic polymers due to the unique properties and general compatibility with biologic systems.9 In previous studies, several PEG derivatives were used as ether macroinitiators14,15 or macromonomers18 in ATRP, and amphiphilic block copolymers containing PEG and polystyrene (PS) were synthesized.14,19,20 The PS-b-PEG-b-PS triblock copolymers with controlled structures had been r 2011 American Chemical Society

synthesized and characterized, which had not been obtained previously by direct polymerization from a macroinitiator.19 Bromine-terminated diblock copolymer mPEG-b-PS-Br was precursor to synthesize a series of well-defined amphiphilic mPEG-b-PS-b-PCL block copolymers.20 The MPEG-b-PS, which had different molecular weights of PS blocks, was synthesized to investigate the effects of the self-assembled structures on the crystallization behavior of mPEG blocks.14 However, the other properties of mPEG-b-PS copolymers were not mentioned. Polyethersulfone (PES) is a favorable polymer applying for manufacturing membranes due to its excellent thermal tolerance, chemical stability, oxidation resistance, and mechanical characteristics.21 However, the application of the PES membranes sometimes is limited by their hydrophobic nature, which led to membrane fouling due to the adsorption of nonpolar solutes, hydrophobic particles, or bacteria.22 In our recent research, functional terpolymers of poly(acrylonitrile-acrylic acid-vinyl pyrrolidone) (P(AN-AA-VP)) and poly(methyl methacrylate-acrylic acid-vinyl pyrrolidone) with different monomer proportion were synthesized to modify PES membranes for decreasing protein adsorption and increasing protein fouling resistant.23,24 In addition, PES membranes were modified by blending amphiphilic block copolymers Pluronic F127 to investigate the effects of coagulation bath temperature (CBT) on the separation performance and antifouling property.25 Comb copolymer PS-b-PEG was synthesized with methoxy-poly(ethylene glycol) 2000 (PEGME2000) and styrene as monomers Received: November 9, 2010 Accepted: January 14, 2011 Revised: January 11, 2011 Published: February 07, 2011 3295

dx.doi.org/10.1021/ie102251v | Ind. Eng. Chem. Res. 2011, 50, 3295–3303

Industrial & Engineering Chemistry Research through anionic living polymerization to study the protein adsorption-resistant property and antifouling property of the flat-sheet membranes, and the water flux recovery ratios (FRR) were as high as 80.6%.3 In the present study, amphiphilic block copolymers mPEG-bPS were synthesized by ATRP and characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and gel permeation chromatography (GPC). The copolymers were blended with PES to prepare hollow fiber membranes by using a dry-wet spinning technique. The water contact angle and protein antifouling property of the modified membranes were investigated, and the effect of PS chains on the membrane properties were also studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ethylene glycol) methyl ether (mPEG5000, Mn = 5000, Aldrich), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich), 2-bromoisobutyryl bromide (97%, Alfa Aesar), bovine serum albumin (BSA, fraction V, 95%, Sigma), and polyethersulfone (PES, Ultrason E6020P, BASF) were used as received. 4-(Dimethylamino)pyridine (DMAP) was recrystallized in toluene at 80 °C. Methylene dichloride (CH2Cl2) was shaken with concentrated H2SO4 until the acid layer remained colorless, then washed with water, with aqueous 5% Na2HCO3, and with water again until the water phase remained neutral, and finally distilled from CaH2.19 CuBr (Shanghai Sinpeuo Fine Chemical Co., A.R. grade) was purified by stirring in glacial acetic acid overnight, filtered off, washed with ethanol, and then dried in a vacuum oven at 60 °C for 24 h. Triethylamine (TEA) and styrene (St) (Chemical Reagent Factory of Kelong, A.R. grade) was distilled from CaH2 before use. Spectra/Por Dialysis membrane (MWCO 3500) was purchased from Chengdu Kebite Biotechnologies Co. Ltd. Phosphate-buffered saline (PBS, pH = 7.2-7.4) solution is a buffer solution commonly used to dissolve BSA. All of the other chemicals (analytical grade) were obtained from the Chemical Reagent Factory of Kelong and were used without further purification. 2.2. Synthesis and Characterization of mPEG-b-PS20. The synthesis route of mPEG-b-PS is shown in Scheme 1. 2.2.1. Preparation of mPEG Macroinitiators. The macroinitiators were prepared as follows: a certain amount of mPEG (10.0 g, 2 mmol) and DMAP (1.0 g, 8.2 mmol) was mixed with TEA (2 mL, 14 mmol) in 80 mL of CH2Cl2. The solution was transferred into a 250 mL three-neck round-bottom flask equipped with a condenser, a dropping funnel, gas inlet/outlet, and a magnetic stirrer. After cooling to 0 °C, 2-bromoisobutyryl bromide (3 mL, 24.3 mmol) in 37 mL of CH2Cl2 was added dropwise into the three-neck round-bottom flask in 1 h under nitrogen, and then the temperature was allowed to rise to room temperature. After stirring for 24 h, the solution was moved in a rotary evaporator. The crude mPEG macroinitiator was precipitated in cold absolute ethyl ether, recrystallized in absolute ethanol for 3-4 times, and dried in vacuo at room temperature for 48 h. 2.2.2. Synthesis of mPEG-b-PS by ATRP. mPEG-b-PS block copolymers were prepared by bulk polymerization. The procedure was as follows: mPEG-Br (1.0 mmol), PMDETA (173.0 mg, 1.0 mmol), and CuBr (143.0 mg, 1.0 mmol) were added to a Schlenk flask. Styrene (50/100/500 mmol) was added under N2 atmosphere, and the Schlenk flask was closed by a three-way stopcock. After three freeze-pump-thaw cycles, the tube was

ARTICLE

Scheme 1. Synthesis Route of mPEG-b-PS Amphiphilic Block Copolymers

sealed off in vacuo. The mixture was heated at 90 °C in an oil bath. The reaction was stopped after 12 h, and the mixture was then quickly cooled to room temperature with cool water. The Schlenk flask was opened and exposed to air, and the sample was further diluted with N-methylpyrrolidone (NMP). Afterward, the crude product solution was dialyzed against deionized water for 24 h, then against water/ethanol mixture (50/50 (v/v)) for 72 h, and finally against deionized water for 48 h, and then the resulting suspension was freeze-dried. The final mPEG-b-PS copolymers with different molecular weights of PS blocks were named as mPEG-PS50, mPEG-PS100, and mPEG-PS500, respectively. The actual extents of the polymerization were characterized by using a weighing method, the values of which were 50, 58, and 40% for mPEG-PS50, mPEG-PS100, and mPEG-PS500, respectively. 2.3. Characterization of mPEG-b-PS Copolymers. Fourier transform infrared spectra were obtained on a Nicolet-560 spectrophotometer (Nicol American) between 4000 and 600 cm-1 with the resolution of 2 cm-1. 1 H NMR data were obtained with a BRUKER spectrometer (400 MHz). The mass spectra of these compounds were obtained on an HP1100-LC/MSD with atmospheric pressure chemical ionization (positive mode). The number-average molecular weight of the copolymer was determined by gel permeation chromatography on an HP1100 using two PLgel columns (10 μm, 104 Å; 10 μm, 500 Å) using monodisperse polystyrene as the standards. The mobile phase was tetrahydrofuran (THF). The sample concentration was 1.0 g/L. The detector was RID, and the flow rate was 1.0 mL/min. 2.4. Preparation and Characterization of the Modified PES Membranes. Different flat-sheet membranes were prepared using the copolymers (mPEG-PS50, mPEG-PS100, and mPEGPS500) with different molecular weights of PS blocks, and named FSM-50, FSM-100, and FSM-500, respectively. PES (16 wt %) and the block copolymer (2 wt %) were dissolved in NMP by vigorous stirring until clear homogeneous solution was obtained. After being vacuum degassed, the casting solutions were prepared into membranes by spin coating coupled with a liquidliquid phase separation technique at room temperature. The substrate (glass plate) was placed on the spin coater tray, and then the polymer solution was cast on the glass plate and spun at 1000 rpm for the definite time; double distilled water was used as the coagulant. The membranes were rinsed thoroughly with distilled water to remove the residual solvent. The pristine PES membrane was prepared in the same manner, and named FSM-0. All the prepared membranes were in a uniform thickness of about 60-70 μm, which was determined by micrometer. To study the antifouling property of the copolymer modified PES membranes, PES/copolymer hollow fiber membranes were 3296

dx.doi.org/10.1021/ie102251v |Ind. Eng. Chem. Res. 2011, 50, 3295–3303

Industrial & Engineering Chemistry Research prepared using NMP as solvent. The PES/copolymer solution (16/4, wt %) was degassed to remove the air bubble. And then a dry-wet spinning technique was used to fabricate the PES hollow fiber membranes. The PES/copolymer solution was filtered and then was extruded through a spinneret. After passing an air gap, the nascent hollow fiber was immerged into water (coagulation bath). The membranes were incubated in water for over 24 h to elute the residual NMP from the membranes, and the extracted water was changed every 3 h. Then the membranes were placed in 50 wt % glycerol aqueous solution for 24 h. The resultant fibers were then placed in air at room temperature. The pristine and modified PES hollow fiber membranes were named HFM-0, HFM-50, HFM-100, and HFM-500, respectively. The hollow fiber membrane filters were prepared by employing epoxy resin as the potting material, with an effective area of about 120 cm2. The morphologies of the flat-sheet and hollow fiber membranes were observed by scanning electron microscopy (SEM) using an XL 30ESME scanning microscope. The membranes frozen in liquid nitrogen were broken and sputtered with gold before SEM analysis. The hydrophilicity of the flat-sheet membrane surface was characterized on the basis of contact angle measurement using a contact angle goniometer (OCA20, DataPhysics, Filderstadt, Germany) equipped with video capture. For the static contact angle measurements, a total of 3 μL of double distilled water was dropped on the air-side surface of the membrane at room temperature, and the contact angle was measured after 10 s. At least eight different positions for one sample were chosen randomly to ensure that the measurement was practical. The results were averaged to get a reliable value, and the measurement error was (3°. 2.5. Ultrafiltration of Protein Solution Experiment. For the experiments, PES and the modified PES hollow fiber filters were used. The BSA solution was prepared by dissolving the BSA in PBS (adjusted to pH 7.2-7.4, with hydrochloric acid) solution with a concentration of 1.0 mg/mL. The test hollow fibers were precompacted at an inside pressure of 135 mmHg and outside pressure of 100 mmHg for 30 min by PBS (pH 7.2-7.4) solutions flow to get to steady state, then the inside pressure was reduced to 90 mmHg and the outside pressure was reduced to 60 mmHg, and the PBS solution flux was measured. After 30 min filtration, the feed solution was switched to 1.0 mg/mL BSA solution (pH 7.2-7.4), and the flux was measured. Finally, the filter and the solution reservoir were fully emptied and refilled with deionized water. The hollow fiber membranes were cleaned with deionized water for 30 min, and the water was changed every 30 min. Then the PBS solution flux was measured again. The PBS solution or BSA solution flux of the hollow fibers was calculated by the following equation: V ð1Þ permeability ðmL=ðm2 3 h 3 mmHgÞÞ ¼ SPt where V (mL) is the volume of the permeated solution, S (m2) is the effective membrane area, t (h) is the time of the solution collecting, and P (mmHg) is the pressure applied to the hollow fiber membrane. The permeability of BSA was also investigated. The BSA solution was applied to the membrane using the filter described above at the same pressure. The permeated solution was collected, then the concentration of the BSA solution was determined by an UV-vis spectrophotometer (Model 756, Shanghai Spectrophotometer Instrument Co., Ltd., China) at

ARTICLE

the wavelength of 278 nm. The BSA rejection ratio (R) was calculated by the following equation: Cp 100 ð2Þ R ¼ 1Cf where Cp is the permeate concentration of the solution and Cf is the bulk concentration. To evaluate the fouling-resistant ability of membranes, the flux recovery ratio (FRR) was calculated using the following expression: F2 FRR ð%Þ ¼ 100 ð3Þ F1 where F1 and F2 (mL/(m2 3 h 3 mmHg)) are the PBS solution flux before and after protein ultrafiltration experiments.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of mPEG-b-PS Amphiphilic Block Copolymer. Table 1 shows the synthesis condi-

tions for the mPEG-b-PS copolymers by ATRP. Styrene and its derivatives are usually polymerized at relatively high temperatures (80-130 °C) by ATRP.15 In this study, the reaction temperature of styrene was controlled at 90 °C. As described in Scheme 1, an ATRP macroinitiator from mPEG was prepared first, then it was allowed to react with a styrene monomer at the temperature mentioned above to get the diblock copolymer with the dormant halogen in the junction point, and other monomers could be further polymerized to obtain triblock copolymer for the broad applications. 3.1.1. FTIR Spectra Analysis of the mPEG-b-PS Copolymers. The FT-IR spectra of mPEG, mPEG-Br, and mPEG-b-PS copolymers are given in Figure 1. The FTIR spectrum of mPEG showed a broad peak at 3200-3700 cm-1 corresponding to the -OH group of the mPEG. Strong absorption peaks at 1109 and 1148 cm-1 were corresponding to the ether bonds. After introducing a -Br group onto the mPEG terminal, the FTIR spectrum of mPEG-Br showed a new sharp peak around 1734 cm-1 assigned to the -OCO groups in the macroinitiator. By the polymerization of styrene, the peaks at 700 and 759 cm-1 were newly formed for mPEG-PS50, mPEG-PS100, and mPEGPS500, which were due to the presence of phenyl proton in the copolymers. The peaks at 1450-1610 cm-1 were the characteristic stretch vibrations peaks of CdC in the phenyl group. The three peaks at 3000-3100 cm-1 were attributed to the stretch vibrations of =CH in the phenyl group. Meanwhile, all of the absorption peaks in mPEG-Br mentioned above also appeared in mPEG-PS50, mPEG-PS100, and mPEG-PS500 amphiphilic copolymers. Although the -OH group disappeared when the mPEG reacted with 2-bromoisobutyryl bromide to form mPEG-Br, the peak at 3200-3700 cm-1 still appeared in the spectrum of mPEG-Br. It was attributed to the OH bond of the residual water because of the hydrophilicity of mPEG-Br, and the residual water in the polymer was difficult to remove completely. This peak in the spectra of mPEG-PS50, mPEGPS100, and mPEG-PS500 became weak until it disappeared as the hydrophobic styrene chain increased. All of these peaks mentioned above were marked by arrows in Figure 1. These results demonstrated that both the PEG chain and the styrene chain were in the mPEG-PS50, mPEG-PS100, and mPEG-PS500 copolymers. 3297


Industrial & Engineering Chemistry Research

ARTICLE

Table 1. Synthesis Conditions for the mPEG-b-PS Copolymers by ATRP entry

macroinitiator/St/CuBr/PMDETA

Mn(NMR) a (g/mol)

mPEG mPEG-Br

Mn(GPC) b (g/mol)

Mw/Mn b

mPEG b (wt %)

PS b (wt %) 0

6 113

1.13

100

5 080

6 912

1.09

100

0

mPEG-PS50

1/50/1/1

7 605

9 929

1.26

69

31

mPEG-PS100

1/100/1/1

10 958

12 847

1.26

53

47

mPEG-PS500

1/500/1/1

27 080

30 644

1.29

22

78

a

As determined by 1H NMR spectroscopy from the relative intensities of the phenyl protons of St derived repeating units at d = 6.3-7.3 ppm and the methylene protons of mPEG sequence at d = 3.4-3.8 ppm (CDC13). b As determined by GPC in tetrahydrofuran (THF) with reference to PS standards.

Figure 2. 1H NMR spectra of mPEG, mPEG-Br, and mPEG-b-PS copolymers.

Figure 1. FT-IR spectra of mPEG, mPEG-Br, and mPEG-b-PS copolymers.

3.1.2. 1H NMR Spectra of the mPEG-b-PS Copolymers. 1H NMR spectra of the mPEG, mPEG-Br, and mPEG-b-PS copolymers are shown in Figure 2. The 1H NMR [CDCl3, ppm from TMS] for mPEG indicated the resonance signals of -O-CH3 (3.38 ppm), -CH2- (3.40-3.83 ppm), and -OH (2.21 ppm). After the substitution reaction, the signal of the proton in -C-CH3 was at 1.92 ppm while the signal of -OH disappeared, and the new signal observed in the spectra of mPEG-Br was at 4.30 ppm corresponding to the -CH2-O-CdO. As the styrene was copolymerized with mPEG, the signal of the phenyl protons from styrene repeating units appeared at 6.30-7.30 ppm, and the methylene protons were at 1.43-1.72 ppm. As shown in Table 1, the Mn determined by 1H NMR spectroscopy was calculated from the relative intensities of the phenyl protons of St derived repeating units at d = 6.3-7.3 ppm and the methylene protons of the mPEG sequence at d = 3.4-3.8 ppm (CDCl3). All of these signals indicated that the mPEG-b-PS block copolymer was synthesized successfully.15,26 3.1.3. GPC Characterization. Figure 3 shows the GPC curves of the diblock copolymers and the precursors of mPEG-b-PS. Symmetrical unimodal peaks are observed as shown in Figure 3, and the peaks shifted to a higher molecular weight region with an increase in the PS block content in the diblock copolymers from

Figure 3. GPC curves of the diblock copolymers and the precursors of mPEG-b-PS.

31 to 78%. With the decrease of the elution time from 12.74 to 11.90 min, Mn increased from 9929 to 30 644 g/mol. The resulting diblock copolymer mPEG-b-PS was purified by being dialyzed against deionized water and a water/ethanol mixture. After purification and drying, no peaks of their corresponding precursors in the GPC curves were observed. FTIR spectra, 1H NMR spectra, and GPC analysis indicated that the well-defined block copolymers mPEG-b-PS, in which the 3298



ARTICLE

Table 2. Compositions of the Flat-Sheet and Hollow Fiber Membranes concentration in casting solution (wt %) sample

type

copolymer

PES

copolymer

NMP

FSM-0

flat-sheet membrane

18

0

82

FSM-50


mPEG-PS50

16

2

82

FSM-100


mPEG-PS100

16

2

82

FSM-500


mPEG-PS500

16

2

82

HFM-0

hollow fiber membrane

20

0

80

HFM-50


mPEG-PS50

16

4

80

HFM-100


mPEG-PS100

16

4

80

HFM-500


mPEG-PS500

16

4

80

Figure 5. Static contact angles for the modified membranes. Each contact angle represents the average of eight independent measurements.

Figure 4. SEM images of the cross-sections of the PES flat-sheet membranes. FSM-0-(a and b) are for PES membranes without mPEG-b-PS copolymers; FSM-50-(a and b), FSM-100-(a and b), and FSM-500-(a and b) are for mPEG-PS50, mPEG-PS100, and mPEGPS500 modified membranes, respectively. Voltage, 20 kV; magnification, (a) 500 and (b) 2000.

PS blocks had different molecular weights, were successfully synthesized by ATRP. 3.2. Membrane Preparation and Characterization. The morphology and surface properties of the PES and the amphiphilic block copolymer modified membranes were investigated.

The compositions of the flat-sheet and hollow fiber membranes are shown in Table 2. 3.2.1. Flat-Sheet Membranes. The flat-sheet membranes were prepared to investigate the surface properties of the blending membranes. Figure 4 shows the SEM pictures of the cross-sections for the PES and the modified PES membranes. As shown in the total cross-section of Figure 4 (FSM-0-(a), FSM50-(a), FSM-100-(a), and FSM-500-(a)), which was magnified 500, a skin layer was found. Under which was a fingerlike structure, as shown in Figure 4 (FSM-0-(b), FSM-50-(b), FSM100-(b), and FSM-500-(b)), which was magnified 2000. The PES transformation from polymer solution to the solid state occurred quickly in poor water solvent and was followed by instantaneous demixing of PES precipitation, which thus led to the asymmetric structures.27,28 And there was no significant difference in the morphology when the amphiphilic block copolymers of mPEG-b-PS were blended into the PES membranes with a concentration of 2 wt %, although PS blocks had different molecular weights in the block copolymers. Contact angle measurements have been usually used to characterize the relative hydrophilicity or hydrophobicity of a polymer surface29,30 and provide information on the interaction energy between the surface and liquid. The static water contact angles for all of the samples had been evaluated as shown in Figure 5. It was found that the addition of the mPEG-b-PS block copolymers enhanced the hydrophilicity of the membranes, and the contact angles decreased with the decrease of the molecular weights of the PS blocks in the copolymers. The contact angle of 3299



ARTICLE

Figure 6. SEM images of the cross-sections of the PES hollow fiber membranes. HFM-0-(a-d) are for PES membranes without mPEG-b-PS copolymers; HFM-50-(a-d), HFM-100-(a-d), and HFM-500-(a-d) are for mPEG-PS50, mPEG-PS100, and mPEG-PS500 modified hollow fiber membranes, respectively. Voltage, 20 kV; magnification, (a) 100, (b) 1000, and (c, d) 3000.

the pristine PES membrane was as high as 87.7°, which was consistent with the results reported by others.31-33 The water contact angles of FSM-500, FSM-100. and FSM-50 were 85.6, 79.6, and 68.4°, respectively. The results indicated that the hydrophilicity of PES membranes was improved by blending with mPEG-b-PS. In addition, the water contact angles distinctly depended on the contact time between the membrane and water, and the angles decreased with the time. The increased hydrophilicity of the membranes could decrease the water contact angles and increase the water flux,34,35 which is discussed in the next section. 3.2.2. Hollow Fiber Membranes. The prepared hollow fiber membranes were opaque in appearance and satisfactorily strong for a high-pressure application; SEM pictures of the cross-section views of the hollow fiber samples are shown in Figure 6. The wall thicknesses of the hollow fiber membranes were 50-60 μm both for pristine PES hollow fiber and the modified PES hollow fiber. For the pristine PES hollow fiber, the inner diameter was 738 μm (as shown in Figure 6 HFM-0-(a), which was magnified 100), and that was 623 μm for the mPEG-b-PS modified hollow fiber membrane (as shown in Figure 6 HFM-50-(a), HFM-100-(a), and HFM-500-(a), which were magnified 100). The difference of the inner diameter between the pristine and copolymer modified PES hollow fiber membranes might be caused by the different viscosity in the casting solutions. A skin layer was found on both sides of the membrane wall, under which were a fingerlike structure and then the porous structure. Furthermore, it was clearly observed that the fingerlike structure was interdicted in the middle of the membrane. This was caused by the exchange between NMP and water during the membrane formation.36 The hollow fiber membrane was spun by the dry-wet spinning method. Exchange between NMP and water occurred rapidly from the internal side of the nascent hollow fiber membrane when the polymer solution was extruded through the spinneret. After the nascent fiber was immersed in the coagulation bath, the exchange began from the outside of the membrane.

Thus, a porous wall formed in the middle of the hollow fiber. By manipulating the membrane wall thickness and the air gap, the porous wall could be controlled.37 Comparing Figure 6 HFM0-(b) with HFM-50-(b), HFM-100-(b), and HFM-500-(b), which were magnified 1000, it was noticed that there was no obvious difference between the anterior three membranes but for HFM-500-(b), whose structure was changed. This interesting pheomenon was distinctly observed in Figure 6 HFM-0-(c and d), HFM-50-(c and d), HFM-100-(c and d), and HFM-500-(c and d), which were magnified 3000. Some small particles and their aggregation, which were marked by arrows, were observed. These results might be caused by the self-assembly of the amphphilic block copolymers and microphase separation induced by blending the copolymers. Because of the amphiphilic property of the block copolymer, the PEG blocks might move to the membrane surface and the pore surface while the PS blocks favor the reverse; then the block copolymer assembled as spheres when the phase separation began during the membrane preparation. In addition, with the increase of the molecular weights of the PS blocks, the copolymers would be difficult to migrate and to be self-assembled as spheres. Moreover, with the increase of the PS blocks in the copolymers, the hydrophilicity of the membranes decreased, the fingerlike structure slightly changed, and the selfassembly behavior of the amphphilic copolymers evidently arose and the microphase separation became serious as the concentration of mPEG-PS copolymer increased to 4 wt %. 3.3. Ultrafiltration Property of Modified Polyethersulfone Hollow Fiber Membranes. The effect of the molecular weight of PS blocks in the copolymers on the permeation properties was investigated by ultrafiltration experiments. Figure 7 shows the time-dependent flux of the pristine and the modified PES hollow fiber membranes. At the first 80 min, it was pure water ultrafiltration; then the feed solution was switched to BSA solution. After 80 min ultrafiltration of BSA solution, the membrane was refreshed under hydraulic cleaning for 60 min, and the water flux was measured again for another 80 min. As seen in Figure 7, the 3300



ARTICLE

pure water flux after cleaning could not completely reach the initial value before BSA ultrafiltration. Table 3 summarized the pure water fluxes and BSA rejection ratios of the pristine and modified hollow fiber membranes. The pure water fluxes of HFM-50, HFM-100, and HFM-500 were 170, 167, and 158 mL/(m2 3 h 3 mmHg), respectively. All the modified membranes exhibited higher pure water fluxes than the pristine membrane (110 mL/(m2 3 h 3 mmHg)), which was probably due to the increased hydrophilicity of the membrane surface and the pore-forming effect of the mPEG-b-PS amphiphilic copolymer. Generally, the membrane pore size and distribution, and the membrane hydrophilicity had great effect on the pure water flux of the membrane. With the increase of the membrane pore size, the flux increased. On the basis of our recent study,23 the amount of the amphiphilic copolymers had an effect on the membrane pore size, though the hydrophilicity of the HFM-500 membrane was identical with that of HFM-0 membrane. And with the increase of the molecular weight of the PS blocks in the block copolymer, the water fluxes slightly decreased, which was also due to the decrease of the hydrophilicity of the modified hollow fiber. However, all of the membranes had the BSA rejection ratios of about 93%. It was concluded that although the addition of the copolymer had some influence on the membrane-forming process, the surface segregation did not remarkably affect the permeation properties of the PES hollow fiber membranes. Conducting fouling experiments on the flat sheet membranes showed results similar to those for the hollow fiber membranes. And it was found that the flux decline for the flat sheet membrane was larger than that for the hollow fiber membrane when the solution switched to BSA solution, since it was dead-end filtration for the flat sheet membrane and was

Figure 7. Time-dependent flux of the pristine PES and modified hollow fiber membranes with mPEG-b-PS copolymers with different molecular weights of the PS blocks in casting solutions. Duplicate experiments showed similar results.

cross-flow filtration for the hollow fiber membrane. And the antifouling property of the modified hollow fiber membranes was also investigated by the ultrafiltration experiment, which will be discussed in the follow section. Generally, membrane fouling is caused by the following mechanisms: pore constriction within the membrane pores, pore blocking at the membrane surface, and cake formation on the membrane surface.3,38,39 On the basis of Jiang's analysis,3 pore constriction could be neglected and the irreversible fouling was mainly due to the pore blocking. When higher molecular weight of PS blocks was present in the copolymers, the mPEG chain density at the membrane surface was relatively low. Although the hydrophilicity and flexibility of mPEG chains could inhibit some protein adsorption, few BSA could still easily penetrate the mPEG layer and adsorb onto the hydrophobic membrane matrix and the surface of the block copolymer mPEG-b-PS modified PES membranes was characterized by contact angle goniometer and X-ray photoelectron spectroscopy, etc., in Jiang's research.3 With lower molecular weight of PS blocks in the copolymers, the adsorption sites at the membrane surface were reduced, and a concentrated PEG layer was formed. Thus the adsorption amount of protein was thus remarkably decreased. 3.4. Flux Recovery Property of Modified Polyethersulfone Hollow Fiber Membranes. Most of the protein in the filter cake and some of the protein adsorbed or deposited at the membrane surface could be removed by simple hydraulic cleaning several times; thereby, the declined flux could be recovered to a certain value. The flux recovery ratio was defined as FRR, which described the flux recovery property of membranes. A higher FRR, which indicated a greater part of fouling was reversible, was desired for practical application. A long-term ultrafiltration was carried out to further investigate the antifouling property of the modified membranes, as shown in Figure 8. The FRR values for HFM-0, HFM-50, HFM100, and HFM-500 were 83.6, 85.3, 80.8, and 75.3%, respectively (as also shown in Table 3) after the first run of ultrafiltration. After three times of BSA ultrafiltration with a total operation time of 14 h and the corresponding three times of hydraulic cleaning, the pure water flux of the modified membrane was maintained at 147.56 mL/(m2 3 h 3 mmHg), 86.5% of the initial value for HFM50, which was higher than that (80.8%) of the flat sheet membranes modified by PS-b-PEG,3 while the pure water flux of the pristine membrane was only 62.90 mL/(m2 3 h 3 mmHg), 56.3% of the initial value. For HFM-100 and HFM-500, the FRR after three runs of the BSA ultrafiltration were 78.4 and 73.4%, respectively. And all of the modified hollow fiber membranes had a higher water flux and flux recovery property than the pristine PES membrane, indicating that the modified hollow fiber membranes could be reused for a longer time without an obvious degeneration of permeation property.

Table 3. Permeation Properties of Pristine PES and Modified Hollow Fiber Membranes with mPEG-b-PS in Casting Solution flux recover ratios of BSA ultrafiltration (%) sample

wall thickness ( μm)

pure water flux (mL/(m h mmHg)) 2

rejection ratio (%)

first time

second time

third time

HFM-0

50 ( 5

110

97

83.6

60.9

56.3

HFM-50 HFM-100

55 ( 5 60 ( 5

170 167

95 93

85.3 80.8

85.3 80.8

86.5 78.4

HFM-500

60 ( 5

158

95

73.4

73.4

73.4

3301



Figure 8. Time-dependent fluxes of the hollow fiber membranes during a process of three recycles of BSA ultrafiltration at room temperature. PES membrane (HFM-0, 9), the modified membrane (HFM-50, b), the modified membrane (HFM-100, 2), and the modified membrane (HFM-500,1). Distilled water: 0-80, 270-340, 530-600, and 790860 min; BSA solution: 140-210, 400-470, and 660-730 min. Duplicate experiments showed similar results.

4. CONCLUSIONS The copolymers mPEG-b-PS, which had different molecular weights of PS blocks, were successfully synthesized by atomtransfer radical polymerization using mPEG-Br as the macroinitiator. These amphiphilic block copolymers could be directly blended with PES to prepare PES modified membranes. The morphology of the PES membrane was slightly changed due to the self-assembly of the amphiphilic copolymers and the microphase separation between the copolymer and the PES matrix, which was characterized by SEM observation. The hydrophilicity and the water flux of the modified membranes increased with the decrease of the molecular weight of PS blocks in the copolymers. The flux recovery property of the PES membranes was significantly enhanced, indicating that the modified hollow fiber membranes could be potentially used for favorable long-term utilization. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-28-85400453. Fax: þ86-28-85405402. E-mail: [email protected] or [email protected].

’ ACKNOWLEDGMENT This work was financially sponsored by the National Natural Science Foundation of China (Grant Nos. 50973070, 51073105, and 30900691) and Sichuan Youth Science and Technology Foundation (Grant 08ZQ026-038). We should also gratefully acknowledge the help of Ms. H. Wang, of the Analytical and Testing Center at Sichuan University, for the SEM micrographs. Moreover, we thank our laboratory members for their generous help. ’ REFERENCES (1) Weinman, C. J.; Gunari, N.; Krishnan, S.; Dong, R.; Paik, M. Y.; Sohn, K. E.; Walker, G. C.; Kramer, E. J.; Fischer, D. A.; Ober, C. K. Protein adsorption resistance of anti-biofouling block copolymers containing amphiphilic side chains. Soft Matter 2010, 6, 3237–3243.

ARTICLE

(2) Tan, J.; McClung, W. G.; Brash, J. L. Nonfouling biomaterials based on polyethylene oxide-containing amphiphilic triblock copolymers as surface modifying additives: Protein adsorption on PEOcopolymer/polyurethane blends. J. Biomed. Mater. Res., Part A 2008, 85A, 873–880. (3) Ma, X. L.; Su, Y. L.; Sun, Q.; Wang, Y. Q.; Jiang, Z. Y. Preparation of protein-adsorption-resistant polyethersulfone ultrafiltration membranes through surface segregation of amphiphilic comb copolymer. J. Membr. Sci. 2007, 292, 116–124. (4) Wang, Y. Q.; Su, Y. L.; Sun, Q.; Ma, X. L.; Jiang, Z. Y. Generation of anti-biofouling ultrafiltration membrane surface by blending novel branched amphiphilic polymers with polyethersulfone. J. Membr. Sci. 2006, 286, 228–236. (5) Chausson, M.; Fluchere, A. S.; Landreau, E.; Aguni, Y.; Chevalier, Y.; Hamaide, T.; Abdul-Malak, N.; Bonnet, I. Block copolymers of the type poly(caprolactone)-b-poly(ethylene oxide) for the preparation and stabilization of nanoemulsions. Int. J. Pharm. 2008, 362, 153–162. (6) Huang, L. H.; Hu, J.; Lang, L.; Chen, X. S.; Wei, Y.; Jing, X. B. Synthesis of a novel electroactive ABA triblock copolymer and its spontaneous self-assembly in water. Macromol. Rapid Commun. 2007, 28, 1559–1566. (7) Morille, M.; Passirani, C.; Letrou-Bonneval, E.; Benoit, J. P.; Pitard, B. Galactosylated DNA lipid nanocapsules for efficient hepatocyte targeting. Int. J. Pharm. 2009, 379, 293–300. (8) Wang, J. S.; Matyjaszewski, K. Controlled “living” radical polymerization: Atom transfer radical polymerization in the presence of transition metalcomplexes. J. Am. Chem. Soc. 1995, 117, 5614– 5615. (9) Lee, S. B.; Russell, A. J.; Matyjaszewski, K. ATRP Synthesis of amphiphilic random, gradient, and block copolymers of 2-(dimethylamino)ethyl methacrylate and n-butyl methacrylate in aqueous media. Biomacromolecules 2003, 4, 1386–1393. (10) Bencherif, S. A.; Sheehan, J. A.; Hollinger, J. O.; Walker, L. M.; Matyjaszewski, K.; Washburn, N. R. Influence of cross-linker chemistry on release kinetics of PEG-co-PGA hydrogels. J. Biomed. Mater. Res., Part A 2009, 90A, 142–153. (11) Bencherif, S. A.; Srinivasan, A.; Sheehan, J. A.; Walker, L. M.; Gayathri, C.; Gil, R.; Hollinger, J. O.; Matyjaszewski, K.; Washburn, N. R. End-group effects on the properties of PEG-co-PGA hydrogels. Acta Biomater. 2009, 5, 1872–1883. (12) Cho, H. Y.; Gao, H. F.; Srinivasan, A.; Hong, J.; Bencherif, S. A.; Siegwart, D. J.; Paik, H. J.; Hollinger, J. O.; Matyjaszewski, K. Rapid cellular internalization of multifunctional star polymers prepared by atom transfer radical polymerization. Biomacromolecules 2010, 11, 2199– 2203. (13) Matyjaszewski, K. Controlled/Living Radical Polymerization: Progress in ATRP; American Chemical Society: Washington, DC, 2009. (14) Chen, Y. F.; Zhang, F. B.; Xie, X. M.; Yuan, J. Y. Effects of micelle structures formed in selective solvents on crystallization behaviors of poly(ethylene glycol)-b-poly(styrene) copolymers. Polymer 2007, 48, 2755–2761. (15) Dong, Y. Q.; Dong, B. T.; Du, F. S.; Meng, J. Q.; Li, Z. C. An all ATRP route to PMMA-PEO-PS and PMAA-PEO-PS miktoarm ABC star terpolymer. Polymer 2009, 50, 125–132. (16) Teodorescu, M.; Matyjaszewski, K. Controlled polymerization of (meth)acrylamides by atom transfer radical polymerization. Macromol. Rapid Commun. 2000, 21, 190–194. (17) Kowalewski, T.; Tsarevsky, N. V.; Matyjaszewski, K. Nanostructured carbon arrays from block copolymers of polyacrylonitrile. J. Am. Chem. Soc. 2002, 124, 10632–10633. (18) Dong, Y. Q.; Tong, Y. Y.; Dong, B. T.; Du, F. S.; Li, Z. C. Preparation of tadpole-shaped amphiphilic cyclic PS-b-linear PEO via ATRP and click chemistry. Macromolecules 2009, 42, 2940–2948. (19) Jankova, K.; Chen, X. Y.; Kops, J.; Batsberg, W. Synthesis of amphiphilic PS-b-PEG-b-PS by atom transfer radical polymerization. Macromolecules 1998, 31, 538–541. (20) He, X. H.; Liang, L. Y.; Xie, M. R.; Zhang, Y. Q.; Lin, S. L.; Yan, D. Y. Synthesis of novel linear PEO-b-PS-b-PCL triblock copolymers by 3302


Industrial & Engineering Chemistry Research the combination of ATRP, ROP, and a click reaction. Macromol. Chem. Phys. 2007, 208, 1797–1802. (21) Shi, Q.; Su, Y. L.; Zhao, W.; Li, C.; Hu, Y. H.; Jiang, Z. Y.; Zhu, S. Q. Zwitterionic polyethersulfone ultrafiltration membrane with superior antifouling property. J. Membr. Sci. 2008, 319, 271–278. (22) Van der Bruggen, B.; Braeken, L.; Vandecasteele, C. Flux decline in nanofiltration due to adsorption of organic compounds. Sep. Purif. Technol. 2002, 29, 23–31. (23) Li, L. L.; Yin, Z. H.; Li, F. L.; Xiang, T.; Chen, Y.; Zhao, C. S. Preparation and characterization of poly(acrylonitrile-acrylic acid-Nvinyl pyrrolidinone) terpolymer blended polyethersulfone membranes. J. Membr. Sci. 2010, 349, 56–64. (24) Zou, W.; Huang, Y.; Luo, J.; Liu, J.; Zhao, C. S. Poly (methyl methacrylate-acrylic acid-vinyl pyrrolidone) terpolymer modified polyethersulfone hollow fiber membrane with pH sensitivity and protein antifouling property. J. Membr. Sci. 2010, 358, 76–84. (25) Peng, J. M.; Su, Y. L.; Chen, W. J.; Shi, Q.; Jiang, Z. Y. Effects of coagulation bath temperature on the separation performance and antifouling property of poly(ether sulfone) ultrafiltration membranes. Ind. Eng. Chem. Res. 2010, 49, 4858–4864. (26) He, X. H.; Zhang, H. L.; Yan, D. Y.; Wang, X. Y. Synthesis of side-chain liquid-crystalline homopolymers and triblock copolymers with p-methoxyazobenzene moieties and poly(ethylene glycol) as coil segments by atom transfer radical polymerization and their thermotropic phase behavior. J. Polym. Sci., Polym. Chem. 2003, 41, 2854–2864. (27) Zhao, C. S.; Yu, B. Y.; Qian, B. S.; Wei, Q.; Yang, K. G.; Zhang, A. M. BPA transfer rate increase using molecular imprinted polyethersulfone hollow fiber membrane. J. Membr. Sci. 2008, 310, 38–43. (28) Zhao, W. F.; Fang, B. H.; Li, N.; Nie, S. Q.; Wei, Q.; Zhao, C. S. Fabrication of pH-responsive molecularly imprinted polyethersulfone particles for Bisphenol-A uptake. J. Appl. Polym. Sci. 2009, 113, 916–921. (29) Wavhal, D. S.; Fisher, E. R. Hydrophilic modification of polyethersulfone membranes by low temperature plasma-induced graft polymerization. J. Membr. Sci. 2002, 209, 255–269. (30) Wavhal, D. S.; Fisher, E. R. Membrane surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling. Langmuir 2003, 19, 79–85. (31) Fang, B. H.; Ling, Q. Y.; Zhao, W. F.; Ma, Y. L.; Bai, P. L.; Wei, Q.; Li, H. F.; Zhao, C. S. Modification of polyethersulfone membrane by grafting bovine serum albumin on the surface of polyethersulfone/ poly(acrylonitrile-co-acrylic acid) blended membrane. J. Membr. Sci. 2009, 329, 46–55. (32) Bolong, N.; Ismail, A. F.; Salim, M. R.; Rana, D.; Matsuura, T. Development and characterization of novel charged surface modification macromolecule to polyethersulfone hollow fiber membrane with polyvinylpyrrolidone and water. J. Membr. Sci. 2009, 331, 40–49. (33) Yoon, K.; Hsiao, B. S.; Chu, B. Formation of functional polyethersulfone electrospun membrane for water purification by mixed solvent and oxidation processes. Polymer 2009, 50, 2893–2899. (34) Yang, Q.; Xu, Z. K.; Dai, Z. W.; Wang, J. L.; Ulbricht, M. Surface modification of polypropylene microporous membranes with a novel glycopolymer. Chem. Mater. 2005, 17, 3050–3058. (35) Liu, F.; Du, C. H.; Zhu, B. K.; Xu, Y. Y. Surface immobilization of polymer brushes onto porous poly(vinylidene fluoride) membrane by electron beam to improve the hydrophilicity and fouling resistance. Polymer 2007, 48, 2910–2918. (36) Qin, J. J.; Chung, T. S. Effect of dope flow rate on the morphology, separation performance, thermal and mechanical properties of ultrafiltration hollow fibre membranes. J. Membr. Sci. 1999, 157, 35–51. (37) Wang, D. L.; Li, K.; Teo, W. K. Polyehtersulfone hollow fiber gas separation membranes prepared from NMP/alcohol solvent systems. J. Membr. Sci. 1996, 115, 85–108. (38) Katsoufidou, K.; Yiantsios, S. G.; Karabelas, A. J. A study of ultrafiltration membrane fouling by humic acids and flux recovery by backwashing: experiments and modeling. J. Membr. Sci. 2005, 266, 40–50.

ARTICLE

(39) M€ockel, D.; Staude, E.; Guiver, M. D. Static protein adsorption, ultrafiltration behavior and cleanability of hydrophilized polysulfone membranes. J. Membr. Sci. 1999, 158, 63–75.

’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the web on February 7, 2011, with errors in Tables 1-3. The corrected version was reposted on February 23, 2011.

3303


Improved Antifouling Property of Polyethersulfone Hollow Fiber

Recommend Documents