Amphiphilic graft copolymers based on ultrahigh molecular weight poly(styrene-alt-maleic anhydride) with poly(ethylene glycol) side chains for surface modification of polyethersulfone membranes

European Polymer Journal 44 (2008) 1907–1914

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Amphiphilic graft copolymers based on ultrahigh molecular weight poly(styrene-alt-maleic anhydride) with poly(ethylene glycol) side chains for surface modification of polyethersulfone membranes Li-Ping Zhu, Zhuan Yi, Fu Liu, Xiu-Zhen Wei, Bao-Ku Zhu, You-Yi Xu * Institute of Polymer Science, Zhejiang University, Hangzhou Yuquan 310027, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 12 November 2006 Received in revised form 7 March 2008 Accepted 18 March 2008 Available online 29 March 2008

Keywords: Polyethersulfone Membranes Amphiphilic graft copolymers Poly(styrene-alt-maleic anhydride) Surface modification

a b s t r a c t Amphiphilic graft copolymers having ultrahigh molecular weight poly(styrene-alt-maleic anhydride) (SMA) backbones and methoxyl poly(ethylene glycol) (MPEG) grafts were synthesized via the esterification between anhydride groups with hydroxyl groups. The synthesized graft copolymers, SMA-g-MPEGs, were used as additives in the preparation of polyethersulfone (PES) membranes via phase inversion process. X-ray photoelectron spectroscopy (XPS) analysis showed the comb-like graft copolymers spontaneously segregated to membrane surface during membrane formation. Water contact angle measurements and water absorbance experiments indicated the PES/SMA-g-MPEG blend membranes were much more hydrophilic than pure PES membrane. The blend membranes had stronger protein adsorption resistance than pure PES membrane did. After washed using de-ionized water for 25 days, the blend membranes exhibited higher hydrophilicity and stronger protein adsorption resistance. This phenomenon was attributed to the further accumulation of SMA-g-MPEG additives on membrane surface in aqueous conditions. SMA-g-MPEGs can be well preserved in membrane near-surface and not lost during membrane washing due to their high molecular weight and comb-like architecture. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Polyethersulfone (PES) is a favorable membrane material due to its excellent chemical stability, good thermal resistance and mechanical strength [1,2]. Despite its advantages as membrane material, the intrinsic hydrophobic character of PES often leads to serious membrane fouling in protein filtration due to protein adsorption on membrane surface, and thus a serious flux loss. Many efforts have been made to enhance the antifouling ability of PES membranes by improving the surface hydrophilicity and reducing undesirable interaction between membrane and protein [3,4]. The main approaches currently used include blending [5], coating [6], surface physical treatment * Corresponding author. Tel./fax: +86 571 87953011. E-mail addresses: [email protected], [email protected] (Y.-Y. Xu). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.03.015

[7], and surface grafting [3] etc. Incorporation with hydrophilic additive or surface modifying macromolecules through solution blending is an effectual and convenient approach to enhance the fouling resistance for phase inversion membranes [8,9]. Amphiphilic graft copolymers consisting of hydrophobic backbones and hydrophilic side chains have attracted increasing attention due to their particular characters and convenient synthesis process [10,11]. In recent years, amphiphilic graft copolymers were employed as additives for the hydrophilic modification of polymer porous membranes [12,13]. Researchers found that the amphiphilic copolymers preferentially segregated to membrane surface during membrane formation. The enrichment and selforganization of hydrophilic chains on membrane surface constructed a barrier against the adsorption of protein and other organisms. Thus, the hydrophilicity and antifouling ability of the resultant blend membranes were

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improved significantly. This phenomenon affords a convenient and practical approach to perform a surface hydrophilic modification for phase inversion membrane by solution blending [14–16]. Styrene–maleic anhydride copolymer (SMA) is a quasiamphiphilic material bearing abundant anhydride groups. The reactive anhydride groups contribute SMA with a convenience for further functionalization. We previously reported a synthesis of SMA copolymers with ultrahigh molecular weights (Mw > 106 g/mol) using supercritical carbon dioxide (SCCO2) as reaction medium in our laboratory. The compositional and structural analysis demonstrated that the SMA copolymers had strictly alternating structure [17]. In our previous work, the synthesized ultrahigh molecular weight SMAs were used as hydrophilic additives in the preparation of PES membranes. The resulted PES/SMA blend membranes exhibited good hydrophilicity and protein adsorption resistance [18]. In the present work, to extend the application of SMAs in membrane modification, amphiphilic graft copolymers based on ultrahigh molecular weight SMA backbones with comb-like MPEG side chains (SMA-g-MPEG) were synthesized. The obtained graft copolymers were added into PES casting solutions to prepare PES/SMA-g-MPEG blend membranes. According to our anticipation, the high molecular weight SMA-g-MPEG with branched hydrophilic chains would be more effectually and more steadily for the hydrophilic modification of PES membranes. The surface compositions, hydrophilicity and protein adsorption resistance for the PES/SMA-g-MPEG blend membranes were compared with those for pure PES and PES/SMA blend membranes. 2. Experimental 2.1. Materials and reagents Polyethersulfone (PES, RADEL A-100, Mw = 53,500 g/ mol) was purchased from Solvay Advanced Polymers and the chemical structure is shown in Fig. 1. Commercially available N,N-dimethylformamide (DMF) (Shanghai SSS reagent company, PR China, chemical purity) served as solvent and was purified by vacuum distillation from calcium hydride before use. Methoxyl poly(ethylene glycol)s (MPEGs) (average molecular weight of 350 and 750 g/mol) were obtained from Aldrich. Bovine serum albumin (BSA, Mw of 67,000 g/mol) was supplied by Bio Life Science and Technology Co. (Shanghai, PR China). pToluenesulfonic acid (PTSA) and diethyl ether were commercially available and used as received. All other chemicals were analytical grade and used without further purification.

2.2. Synthesis and characterization of SMA-g-MPEG graft copolymer Ultrahigh molecular weight SMA as precursor was beforehand synthesized according to our previous reported procedure [17]. In brief, the completely purified styrene (St) and maleic anhydride (Man) monomers (St/Man ratio, 1:1) were copolymerized in SCCO2 under the initiation of 2,20 -azoisobutyronitrile (AIBN). The obtained copolymer was re-precipitated for 3 times from DMF solution into diethyl ether. At last, the product was dried in a vacuum at 120 °C overnight for use. The characteristics of the obtained copolymer are showed in Table 1. The product had a strictly alternating structure with the unit ratio of styrene to maleic anhydride of 1:1. The synthesis route of SMA-g-MPEG is illustrated in Fig. 2. In a typical reaction, DMF (100 mL), SMA (10 g), MPEG (10 mmol) and PTSA (0.3 g) were added into a foul-necked flask equipped with mechanical stirrer, thermometer, and nitrogen inlet tube. The mixture was heated to 100 °C under stirring and the reaction was carried out under a protective nitrogen atmosphere for 12 h. And then the mixture was precipitated into diethyl ether. After filtration, the precipitate was purified by re-precipitation repeatedly with diethyl ether/DMF. The obtained product was dried under vacuum at 50 °C for 72 h. The 1H NMR spectra of the products were determined using a Bruker Avance DMX 500 MHz instrument. 2.3. Preparation of blend membranes Asymmetrical pure PES, PES/SMA and PES/SMA-g-MPEG blend membranes were prepared by immersion precipitation process. PES and SMA-g-MPEG (or SMA) were dissolved in DMF to form the casting solutions. The total fraction of SMA-g-MPEG (or SMA) and PES in the dopes was fixed at 20 wt.%. After vacuum degassed, the solutions were cast on a horizontal glass plate with a glass blade at room temperature. And then the solution films were immediately immersed into a 60 °C de-ionized water bath to gelate. The formed membranes were washed thoroughly with de-ionized water to remove the residual solvent. The resultant membranes were stored in fresh de-ionized water for at least 2 days before characterization. 2.4. Membrane characterization XPS spectra for the top surfaces of these membranes were recorded on a PHI 5000C ESCA System (PHI Co., America) employing Al Ka excitation radiation (1486.6 eV). The measurements were conducted at a

Table 1 Characteristics of ultrahigh molecular weight SMA synthesized in this work

O

S

O

n

O Fig. 1. The chemical structure of polyethersulfone (PES).

Mw (106 g/mol)a

Mn (106 g/mol)

MWD

Tg (°C)

Tm (°C)

Crystallinity (%)

1.99

0.79

2.52

160.9

330.3

34.6

a

Molecular weight were determined by GPC with narrow distribution polystyrene as standard.

L.-P. Zhu et al. / European Polymer Journal 44 (2008) 1907–1914

*

CH2 CH

CH O

CH

HO ( CH2CH2O )n CH3

+

*

1909

O

O

MPEG SMA

PTSA, 100 oC

*

CH2 CH

DMF, N2 protection

CH

CH

C

C

O OH

*

O O ( CH2CH2O )n CH3

SMA-g-MPEG Fig. 2. The synthesis route of SMA-g-MPEGs.

take-off angle of 45°. The X-ray source was run at a power of 250 W (14.0 kV, 93.9 eV). Binding energies were calibrated by using the containment carbon (C 1s = 284.7 eV). Survey spectra were run in the binding energy range 0–1000 eV and high-resolution spectra of C 1s were collected. The hydrophilicity of these membranes was characterized by water contact angle and water absorbance measurements. The contact angle of pure water on membrane surface was determined using a contact angle goniometer (OCA20, Dataphysics Instruments with GmbH, Germany) at 25 °C and 60% relative humidity. Water absorbance is defined as (wwet  wdry)/wwet  100%, where wwet and wdry represent the weight of dried membrane and the membrane soaked in water for 24 h at room temperature, respectively. The reported values were the average of at least 5 measurements. In investigate the durability of the modification effects, the membranes (both PES control membrane and the blend membranes) were immersed into a shaking water bath at 40 °C for 25 days. The washing water was replaced using fresh de-ionized water everyday. Water contact angle and protein adsorption amount for the washed membranes were re-measured. 2.5. Protein adsorption experiments To evaluate the fouling resistance of the control PES membrane and the PES/ SMA-g-MPEG blend membranes, BSA adsorption experiments were performed following a reported procedure [19]. In a typical procedure, a membrane with an area of 8 cm2 was immersed into a 1.0 or 2.0 g/L BSA solution. The pH value of the solution was kept at 7.4 using a phosphate buffer solution (PBS). And then the solution was vibrated for 24 h in a water bath with constant temperature of 30 °C. The concentration of BSA solution was determined using a UV spectrophotometer (UV-1601, Shimadzu). The apparent amount of adsorbed protein by the membrane was calculated from the concentration difference of BSA solution before and after adsorption. The average of at least 3 measurements was reported.

3. Results and discussion 3.1. Synthesis and characterization of SMA-g-MPEG The preparation of comb-like amphiphilic graft polymers based on low molecular weight SMA backbones and MPEG grafts has been reported by some researchers [20– 22]. The cyclic anhydride groups endow SMA with the active sites for the coupling of MPEG into the backbones. In the reactions, the anhydrides are ring opened with the concomitant formation of ester groups, and each ester bond is formed together with one carboxylic acid group, as shown in Fig. 2. In previous reports, the synthesized graft copolymers are often water soluble due to a low molecular weight of the SMA backbone. In the current work, SMA with ultrahigh molecular weight was used as the backbone in order to obtain high molecular weight graft copolymers. MPEGs with molecular weight of 350 and 750 g/mol were used as the grafts. Usually, in the synthesis of graft copolymers based on low molecular SMA backbones and MPEG side chains, gelation of the reaction mixtures was observed when the conversion ratio was too high. The presence of dihydroxylic PEG in MPEG was thought to be responsible for gelation [21,22]. In the present work, to avoid gelation, the mole ratio of MPEG to anhydride group was kept at a low degree of 1:5. A representative 1H NMR spectrum of SMA-g-MPEG (S350-1) and the ascriptions of the peaks are presented in Fig. 3. The broad peak at 7.1 ppm (e) is corresponding to the phenyl protons of the styrene units, and the peaks at 3.5 (g) and 3.2 ppm (h) are attributable to the methylene protons and terminal methoxy protons of MPEG, respectively. In addition, a small peak occurs at about 4.3 ppm (f) corresponding to the methylene protons attached to the ester groups, indicating a substantial covalent linking of MPEG chains with SMA backbones. Based on 1H NMR analysis, the conversion percentage of anhydride groups can be calculated by the area of the proton peak at 4.3 ppm, relative to that of the reference peak at 7.1 ppm. The corresponding equation is: Conversion (%) = 5Af/2Ae  100, where Af and Ae are the areas of the

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*

a b CH2 CH

c CH C O OH

e

d CH

*

C

OO CH2CH2O ( CH2CH2O ) CH3 n-1 g h f

g a cb

d

h e f

10

9

8

7

6

5

4

3

2

1

0

-1 ppm

Fig. 3. Typical 1H NMR spectrum of SMA-g-MPEG (S350-1).

peak f and e, respectively. Moreover, the number of MPEG chains grafted to per SMA molecule, MPEG content in the graft copolymer and the average molecular weight of the graft copolymer can also be calculated. The obtained results are listed in Table 2. From Table 2, it can be seen that the conversion ratio of anhydrides is very low. The highest value is only 11.2%. Despite the low anhydride conversion, the PEG contents in the graft copolymers reach to 22.5 wt.% (S750-1) due to a high content of anhydride group in the polymeric precursor (the unit ratio of styrene to maleic anhydride was 1:1). In addition, it is worth noting that the anhydride conversion of SMA-g-MPEG350 is higher than that of SMA-g-MPEG750 in identical reaction conditions. This phenomenon is reasonably attributed to the higher reactive activity for the ended hydroxyl of MPEG350 than that of MPEG750. At all events, the synthe-

sized SMA-g-MPEGs were highly hydrophilic due to the presence of abundant ether bond, carboxylic group and anhydride group, and they were designed as hydrophilic additives for surface modification of PES membranes. 3.2. Surface enrichment of SMA-g-MPEG The surface aggregation phenomenon has been observed in many polymer blends composed of a hydrophilic component and a hydrophobic component [12–16,23,24]. Generally, if the polymer system is equilibrated in air, the component with the lower surface energy (hydrophobic) will concentrate at the surface to reduce the interfacial energy of the air–polymer interface [23,24]. On the contrary, if equilibrated in aqueous condition, the hydrophilic component with higher surface energy will preferentially

Table 2 Characterization data for SMA-g-MPEG graft copolymersa Sample SMA SMA-g-MPEG350 SMA-g-MPEG350 SMA-g-MPEG750 SMA-g-MPEG750 a

(S350-1) (S350-2) (S750-1) (S750-2)

Conversion of anhydride (%)

PEG incorporation (wt.%)

Average no. of MPEG grafts per molecule

Calculated Mn (106 g/mol)

– 11.2 10.6 7.8 7.6

0 16.2 15.5 22.5 21.9

0 438.0 414.6 305.0 297.2

0.79 0.94 0.93 1.02 1.01

Calculated from NMR analysis.

L.-P. Zhu et al. / European Polymer Journal 44 (2008) 1907–1914

segregate to the water–polymer interface due to the low interfacial energy between the hydrophilic component and water [12–16]. This phenomenon presents a convenient approach to improve membrane surface hydrophilicity by spontaneous enrichment of hydrophilic additives on the membrane surface. Compared to other surface modification methods, surface segregation is carried out simultaneously with membrane preparation and no additional post-treatment step is needed. In the present work, the synthesized SMA-g-MPEG graft copolymers were added to PES casting solutions to perform a hydrophilic modification, and the casting solution compositions are listed in Table 3. The membranes were prepared by phase inversion technique using de-ionized water as coagulant. The coagulation bath temperature was elevated to 60 °C in order to promote the surface migration of hydrophilic additive. The surface chemical composition of pure PES membrane and the blend membranes was characterized by XPS analysis. The Fitted C 1s XPS core level spectra for these membranes are shown in Fig. 4. The peak centers of alkyl carbon (–C–C–), ether carbon (–C–O– and –C–S–) and carboxylic carbon (–COOH) locate at about 284.7, 286.1 and 288.8 eV, respectively [16]. For PES control membrane (M0), the mole fraction of ether carbon detected by XPS is 34.2%, which is in good agreement with the theoretical value, 33.3%. For the blend membranes, the appearance of carboxylic carbon indicates that the presence of hydrolyzed SMA or SMA-g-MPEG in the membrane near-surface. No anhydride group was detected in the PES/SMA and PES/SMA-g-MPEG blend membrane surfaces, suggesting that unreacted anhydride groups had completely hydrolyzed during membrane formation or post-treatment. Due to using hot water as coagulant in membrane formation, the elevated coagulation temperature accelerated the hydrolyzation and surface migration of the additives [12,14]. In order to evaluate the enrichment characteristics of amphiphilic graft copolymers on membrane surface, the near-surface coverage degree of the additives for the blend membranes was calculated from XPS analysis. The obtained results are compared with their theoretical values (namely membrane bulk compositions), as shown in Fig. 5. For all these blend membranes, the near-surface fraction of the additives is significantly higher than the bulk fraction, which is a convincing evidence for the surface enrichment of the amphiphilic additives. For example, the surface concentration of additive for M350-5 is 22.3 wt.%, a 4.5-fold increase relative to the membrane bulk composition. It is worth noting that, for the MS-10, M350-10 and M750-10 membranes, the surface enrichment degree of additives is gradually declined. This phenomenon might be attributed to bigger diffusion retardance for the comb-like molecules, which slowed down the surface migration during the membrane formation. 3.3. Membrane surface hydrophilicity and protein adsorption Static water contact angles and water absorbance ratios for control PES membrane (M0) and the blend membranes are presented in Fig. 6. Since more hydrophilic membrane

1911

is more easily to absorb water, water absorbance ratio generally exhibits a reverse trend compared with contact angle. For the control membrane (M0), static water contact angle is 91.4° and water absorbance is 21.1%, showing a hydrophobic characteristic for pure PES membrane. Compared to the control membrane, the blend membranes have smaller contact angles and higher water absorbance ratios, suggesting that an improved hydrophilicity is obtained after adding amphiphilic SMA or SMA-g-MPEG copolymers into the casting solutions. The surface enrichment of the additives containing abundant MPEG grafts and carboxylic groups is responsible for the achievement of a high hydrophilicity. It is also worth noting that the hydrophilicity of M750-x is higher than that of M350-x (x is 5 and 10) in identical bulk composition, although the additive surface concentration of the latter is bigger. This might be due to the presence of SMA-g-MPEG750 with higher MPEG content and longer MPEG chains relative to SMA-g-MPEG350 in the M750-x (x is 5 and 10) membranes. Significantly, in a same dosage of additives, the membrane M350-10 and M750-10 are more hydrophilic than MS-10 despite a lower surface enrichment of additive. This indicates that the graft copolymers have a better ability for membrane surface hydrophilic modification than the polymeric precursor SMA. The hydrophobic interaction between material surface and protein plays an important role for non-special adsorption of protein on material surface. Usually, hydrophilic material surface shows relatively low non-specific protein adsorption [19]. Fig. 7 indicates the amount of adsorbed BSA on PES control membrane and PES/SMA-gMPEG blend membranes. For all these blend membranes, the amounts of adsorbed BSA are dramatically reduced compared with the control membrane (M0). Combining the results of Fig. 7 with those of Fig. 6, it can be found that the protein adsorption resistance of the membranes is proportional to the membrane surface hydrophilicity. The more hydrophilic membrane possesses a better protein adsorption resistance. The membrane M750-x has a lower BSA adsorbed amount than M350-x (x is 5, 10) due to the longer MPEG graft chains for SMA-g-MPEG750. The longer comb-like MPEG chains could provide bigger repulsion force coming from conformation changes and desolvation, which endows the PES/SMA-g-MPEG blend membranes with higher protein fouling resistance [25,26]. 3.4. Durability of modification effects Some hydrophilic polymers such as polyvinyl pyrrolidone (PVP), PEG are often employed as additives for membrane modification. However, these polymers are watersoluble and easily lost during membrane preparation and application. Thus, they actually act as pore-forming agents, not modification additives. In the present work, the comblike amphiphilic SMA-g-MPEG graft copolymers were used as additives for the hydrophilic modification of PES membranes. The water contact angle and BSA adsorption amount for the PES control membrane and the blend membranes were re-determined after the membranes were washed using de-ionized water for 25 days. Fig. 8 presents the comparison of the results between before washing and

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Table 3 Casting solution compositions for membrane preparation Membrane label

M0 MS-10 M350-5 M350-10 M750-5 M750-10

Bulk composition

Pure PES 10 wt.% SMA 5 wt.% S350-1 10 wt.% S350-1 5 wt.% S750-1 10 wt.% S750-1

Casting solution composition (wt.%) PES

Additive

DMF

20 18 19 18 19 18

0 2 1 2 1 2

80 80 80 80 80 80

after washing. It can be seen that, for the pure PES membrane (M0), no obvious difference in contact angle and BSA adsorption amount is observed between before and after washing. However, for all these blend membranes (including MS-10, M350-5, M350-10, M750-5 and M75010), both water contact angle and BSA adsorption amount decline obviously after water washing. For example, for the M350-5 membranes, the contact angle decreases from 58.1° to 51.2° and the BSA adsorption amount decreases from 19.6 to 15.4 lg/cm2. These results suggest that, after

(a) M0

(b) MS-10

C-C 65.8% C-O/C-S 34.2%

C-C 73.2% C-O/C-S 20.1% COOH 6.7%

-C-C-

-C-C-

-C-O-/-C-S-

-C-O-/-C-S-COOH

292

290

288

286

284

282

280

292

290

288

Binding energy (eV)

286

284

282

280

282

280

282

280

Binding energy (eV) (d) M350-10

(c) M350-5 C-C 66.9% C-O/C-S 29.7% COOH 3.4%

C-C 69.8% C-O/C-S 25.0% COOH 5.2%

-C-C-

-C-O-/-C-S-

-C-C-

-C-O-/-C-S-

-COOH

-COOH

292

290

288

286

284

282

280

292

290

288

Binding energy (eV)

286

284

Binding energy (eV) (f) M750-10

(e) M750-5 C-C 66.9% C-O/C-S 30.8% COOH 2.3%

C-C 67.4% C-O/C-S 28.3% COOH 4.3%

-C-C-

-C-C-

-C-O-/-C-S-

-C-O-/-C-S-

-COOH

-COOH

292

290

288

286

284

Binding energy (eV)

282

280

292

290

288

286

284

Binding energy (eV)

Fig. 4. Fitted C 1s XPS core level spectra for PES control membrane (M-0) and the blend membranes.

1913

50

100

Water contact angle ( )

o

40 30 20 10

80

60 60 40 40

20

M0

140

MS-10 M350-5 M350-10 M750-5 M750-10

0

Membrane label

M350-5 M350-10 M750-5 M750-10

Fig. 5. Comparison of amphiphilic additive concentration in membrane bulk with that in membrane near-surface.

80

2

MS-10

Membrane label

Fig. 8. Comparison in water contact angle and BSA adsorption amount (in 1.0 g/L of BSA solution) between before washing and after washing for 25 days.

140

o

120

100

100

80

80

60

60

40

40

20

20

0

M0

MS-10 M350-5 M350-10 M750-5 M750-10

Water absorbance ratio (%)

Water contact angle Water absorbance ratio

120

Amount of adsorbed BSA (μg/cm )

Fig. 6. Static water contact angle and water absorbance ratio of pure PES membrane and the blend membranes.

100 1.0 g/L BSA solution 2.0 g/L BSA solution

80 60 40 20 0

Fig. 9. Schematic surface enrichment of hydrophilic comb-like (left) and linear (right) polymer additives.

0

Membrane label

2

Before washing After washing for 25 days

20

0 M0

Water contact angle ( )

100

Bulk weight fraction Surface weight fraction from XPS

Amount of adsorbed BSA (μg/cm )

Additive fraction in membrane (wt.%)

L.-P. Zhu et al. / European Polymer Journal 44 (2008) 1907–1914

The hydrophilicity and anti-fouling ability for the membranes with water-soluble additives (e.g. PVP) often deteriorate after water washing due to the loss of additives. However, the additives used in this work (including SMA and SMA-g-MPEGs) have ultrahigh molecular weights and are water insoluble. They can be kept in the membrane more stably than low molecular weight additives. Furthermore, the amphiphilic additives (including comb-like and linear) containing hydrophilic chains can further migrated from membrane bulk to membrane surface in aqueous conditions, as illustrated in Fig. 9. Therefore, the hydrophilicity and protein adsorption resistance for PES/SMA and PES/SMA-g-MPEG blend membranes were improved after water washing. In addition, comb-like SMA-g-MPEG molecules tangle with PES chains in membrane bulk and isn’t easily lost, and thus the modification effects are more persistent. 4. Conclusion

M0

MS-10 M350-5 M350-10 M750-5 M750-10

Membrane label Fig. 7. Amount of adsorbed BSA on pure PES membrane and the blend membranes from 1.0 and 2.0 g/L BSA solution.

water washing, the hydrophilicity and protein adsorption resistance for the blend membranes were not deteriorated, but enhanced.

Amphiphilic graft copolymers based on ultrahigh molecular weight SMA backbones with MPEG grafts were successfully synthesized by the esterification between anhydride groups and MPEG hydroxyls. When used as additives of PES membranes, the synthesized comb-like SMA-g-MPEG copolymers preferentially segregated to membrane surface during membrane formation. The surface hydrophilicity and protein adsorption resistance of

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L.-P. Zhu et al. / European Polymer Journal 44 (2008) 1907–1914

PES membranes were significantly improved after blending with SMA-g-MPEGs. Compared with the polymeric precursor (SMA), the graft copolymers had higher efficiencies in the hydrophilic modification of PES membranes. After water washing for a long period, the blend membranes exhibited higher hydrophilicity and stronger anti-fouling ability due to the further accumulation of SMA-g-MPEGs on membrane surface. SMA-g-MPEG additives can be well kept in the blend membranes and not lost during membrane preparation and application because of their high molecular weights and comb-like architectures. Acknowledgement The authors are grateful to the financial support of the National Nature Science Foundation of China (Grant No. 50673084).

References [1] Qin JJ, Oo MH, Li Y. Development of high flux polyethersulfone hollow fiber ultrafiltration membranes from a low critical solution temperature dope via hypochlorite treatment. J Membr Sci 2005;247:137–42. [2] Kim JH, Kim CK. Ultrafiltration membranes prepared from blends of polyethersulfone and poly(1-vinylpyrrolidone-co-styrene) copolymers. J Membr Sci 2005;262:60–8. [3] Taniguchi M, Belfort G. Low protein fouling synthetic membranes by UV-assisted surface grafting modification: varying monomer type. J Membr Sci 2004;231:147–57. [4] Marchese J, Ponce M, Ochoa NA, Prádanos P, Palacio L, Hernández A. Fouling behaviour of polyethersulfone UF membranes made with different PVP. J Membr Sci 2003;211:1–11. [5] Wang T, Wang YQ, Su YL, Jiang ZY. Improved protein-adsorptionresistant property of PES/SPC blend membrane by adjustment of coagulation bath composition. Colloids Surf B: Biointerf 2005;46:99–105. [6] Reddy AVR, Mohan DJ, Bhattacharya A, Shah VJ, Ghosh PK. 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 2003;214:211–21. [7] Steen ML, Jordan AC, Fisher ER. Hydrophilic modification of polymeric membranes by low temperature H2O plasma treatment. J Membr Sci 2002;204:341–57. [8] Wang YQ, Wang T, Su YL, Peng FB, Wu H, Jiang ZY. Protein-adsorptionresistance and permeation property of polyethersulfone and soybean phosphatidylcholine blend ultrafiltration membranes. J Membr Sci 2006;270:108–14. [9] Zhang L, Chowdhury G, Feng C, Matsuura T, Narbaitz R. Effect of surface-modifying macromolecules and membrane morphology on

fouling of polyethersulfone ultrafiltration membranes. J Appl Polym Sci 2003;88:3132–8. [10] Chiu HC, Chern CS, Lee CK, Chang HF. Synthesis and characterization of amphiphilic poly(ethylene glycol) graft copolymers and their potential application as drug carriers. Polymer 1998;39:1609–16. [11] Guo SR, Shen LJ, Feng LX. Surface characterization of blood compatible amphiphilic graft copolymers having uniform poly(ethylene oxide) side chains. Polymer 2001;42:1017–22. [12] Hester JF, Banerjee P, Won YY, Akthakul A, Acar MH, Mayes AM. ATRP of amphiphilic graft copolymers based on PVDF and their use as membrane additives. Macromolecules 2002;35:7652–61. [13] Zhu L-P, Du C-H, Xu L, Feng Y-X, Zhu B-K, Xu Y-Y. Amphiphilic PPESK-g-PEG graft copolymers for hydrophilic modification of PPESK microporous membranes. Eur Polym J 2007;43:1383–93. [14] Hester JF, Banerjee P, Mayes AM. Preparation of protein-resistant surfaces on poly(vinylidene fluoride) membranes via surface segregation. Macromolecules 1999;32:1643–50. [15] Park JY, Acar MH, Akthakul A, Kuhlman W, Mayes AM. Polysulfonegraft- poly(ethylene glycol) graft copolymers for surface modification of polysulfone membranes. Biomaterials 2006;27: 856–65. [16] Zhu L-P, Xu L, Zhu B-K, Feng Y-X, Xu Y-Y. Preparation and characterization of improved fouling-resistant PPESK ultrafiltration membranes with amphiphilic PPESK- graft -PEG copolymers as additives. J Membr Sci 2007;294:196–206. [17] Qiu GM, Zhu BK, Xu YY, Geckeler KE. Synthesis of ultrahigh molecular weight poly(styrene-alt-maleic anhydride) in supercritical carbon dioxide. Macromolecules 2006;39:3231–7. [18] Zhu L-P, Zhang X-X, Xu L, Du C-H, Zhu B-K, Xu Y-Y. Improved protein-adsorption resistance of polyethersulfone membranes via surface segregation of ultrahigh molecular weight poly(styrene-altmaleic anhydride). Colloids Surf B: Biointerf 2007;57:189–97. [19] Liu Z-M, Xu Z-K, Wang J-Q, Wu J, Fu J-J. Surface modification of polypropylene microfiltration membranes by graft polymerization of N-vinyl-2-pyrrolidone. Eur Polym J 2004;40:2077–87. [20] Hou SS, Kuo PL. Synthesis and characterization of amphiphilic graft copolymers based on poly(styrene-co-maleic anhydride) with oligo(oxyethylene) side chains and their GPC behavior. Polymer 2001;42:2387–94. [21] Rietman EA, Kaplan ML. Single-ion conductivity in comblike polymers. J Polym Sci Polym Lett 1990;28:187–91. [22] De´rand H, Wessle´n B, Wittgren B, Wahlund KG. Poly(ethylene glycol) graft copolymers containing carboxylic acid groups: aggregation and viscometric properties in aqueous solution. Macromolecules 1996;29:8770–5. [23] Kajiyama T, Tanaka K, Takahara A. Surface segregation of the higher surface free energy component in symmetric polymer blend films. Macromolecules 1998;31:3746–9. [24] Suk DE, Chowdhury G, Matsuura T. Study on the kinetics of surface migration of surface modifying macromolecules in membrane preparation. Macromolecules 2002;35:3017–21. [25] Vanderah D, La H, Naff J, Silin V, Rubinson K. Control of protein adsorption: molecular level structural and spatial variables. J Am Chem Soc 2004;126:13639–41. [26] Willem N, Dick G. Interaction of bovine serum albumin and human blood plasma with PEO-tethered surfaces: influence of PEO chain length, grafting density, and temperature. Langmuir 2004;20:4162–7.