Protein fouling resistant membrane prepared by amphiphilic pegylated polyethersulfone

Bioresource Technology 102 (2011) 2289–2295

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Protein fouling resistant membrane prepared by amphiphilic pegylated polyethersulfone Jinming Peng, Yanlei Su, Qing Shi, Wenjuan Chen, Zhongyi Jiang ⇑ Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

a r t i c l e

i n f o

Article history: Received 28 July 2010 Received in revised form 10 October 2010 Accepted 11 October 2010 Available online 15 October 2010 Keywords: Protein fouling resistance Ultrafiltration Membrane Polyethersulfone Poly(ether glycol) methyl ether methacrylate

a b s t r a c t A mild and facile grafting of poly(ether glycol) methyl ether methacrylate (PEGMA) monomers onto polyethersulfone (PES) was carried out. Then, the PES-g-PEGMA membranes with integrally anisotropic morphology were fabricated through the coupling of non-solvent induced phase inversion and surface segregation. Compared with PES control membrane, the surface hydrophilicity of PES-g-PEGMA membranes was remarkably enhanced due to the drastic enrichment of poly(ethylene glycol) (PEG) segments on the membrane surface; protein adsorption was significantly inhibited due to the hydrogen bonding interactions between hydrophilic groups and water molecules. Ultrafiltration experiments were used to assess the permeability and protein fouling resistance of the PES-g-PEGMA membranes. It was found that the PES-g-PEGMA membranes with higher surface coverage of PEG segments displayed stronger antibiofouling property. Moreover, the stable antibiofouling property for PES-g-PEGMA membranes was acquired due to covalent bonding interactions between hydrophilic PEGMA side chains and PES main chains. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Proteins are essential parts of organisms and take part in virtually every process within cells. Protein separation and purification become the indispensable operations for biomedical and biotechnological applications (Atra et al., 2005; Yang et al., 2009a). As a green, efficient and versatile technology, ultrafiltration is more and more widely utilized for concentrating or fractionating protein solutions. However, fouling caused by nonspecific protein adsorption on surface seriously hinders the working efficiency and application range of ultrafiltration membrane (Yang et al., 2010; Zhao et al., 2010). In recent years, numerous efforts have been devoted to suppress or eradicate membrane fouling by implementing the different process intensification strategies or by creating the novel membrane fabrication or modification methods. It was reported that adding diatomite (Yang et al., 2010) or green bioflocculant (Ngo and Guo, 2009), utilizing ozonation (Wu and Huang, 2010) or utilizing moving bed (Yang et al., 2009b), the membrane fouling was dramatically alleviated. It was also demonstrated that membranes fabricated or modified by poly(ethylene glycol) (PEG)-based polymers (Guo and Ulbricht, 2010; Su et al., 2009) or zwitterionic polymers (Jiang and Cao, 2010; Li et al., 2008) displayed prominent and long-term fouling resistance property. Particularly, PEG based modifiers are much more extensively utilized owing to their ⇑ Corresponding author. Tel./fax: +86 22 23500086. E-mail address: [email protected] (Z. Jiang). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.045

excellent protein resistance, superior biocompatibility, easy availability and low cost. Till now, several approaches including surface coating, surface grafting and surface segregation have been attempted to construct fouling resistant membrane surface with desirable protein fouling resistance property (Du et al., 2009; Hester and Mayes, 2002; Wang et al., 2005; Zhu et al., 2007). As post-treatment approaches, surface coating and surface grafting both suffer from the drawback of significant reduction of intrinsic permeability owing to substantial blocking of surface pores. The internal pores are unmodified and remain prone to fouling. Furthermore, the extra manufacturing steps add to membrane cost (Carroll et al., 2002; Shannon et al., 2008). Surface segregation, an in situ approach, has evolved as a facile and effective approach to modify the membrane surface. In this approach, amphiphilic copolymers comprising hydrophobic backbones and hydrophilic side chains are added into the membrane casting solution. Upon immersion into the coagulant bath, hydrophilic segments can spontaneously segregate upon membrane surface and construct the hydrophilic fouling resistant layer. Simultaneously, hydrophobic segments could firmly anchor with membrane matrix. As an in situ modification, surface segregation hardly blocks the pores, preventing the decrease of permeability. More importantly, surface segregation achieves a three-dimensional modification both on membrane surface and on the internal pores surface. In addition, the resultant surface is rendered with the unique self-healing and self-repairing properties.

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In the present work, amphiphilic comb polymer with hydrophobic polyethersulfone (PES) backbone and hydrophilic poly(ether glycol) methyl ether mathacrylate (PEGMA) side chains was synthesized using the graft polymerization method described in our previous work (Shi et al., 2010). Subsequently, ultrafiltration membranes were prepared using the resultant polymer with different graft yield (GY) via the non-solvent induced phase separation and surface segregation. The morphologies of PES-g-PEGMA membranes were characterized by scanning electron microscopy (SEM). Water contact angle and X-ray photoelectron spectroscopy (XPS) measurement were employed to characterize the surface hydrophilicity and surface composition of resultant membranes. Bovine serum albumin (BSA) was used as the model protein to investigate the protein adsorption resistance. Ultrafiltration experiments were conducted to evaluate the antibiofouling and separation properties of PES-g-PEGMA membranes.

2. Methods 2.1. Materials Polyethersulfone (PES) in flake form (6020P) was purchased from BASF Co. (Germany) and dried at 110 °C for 12 h before use. Poly(ether glycol) methyl ether methacrylate (PEGMA) monomers ((PEG)4.5MA with Mn of 300, (PEG)8.5MA with Mn of 475, (PEG)22.7MA with Mn of 1100) were purchased from Sigma–Aldrich Co. (US). Benzoyl peroxide (BPO), poly(ethylene glycol) (PEG) (Mw = 2000) and N,N-dimethyl formamide (DMF) were purchased from Kewei Chemical Reagent Co. (Tianjin, China) and used as received. Bovine serum albumin (BSA) was purchased from Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China).

2.2. Synthesis and characterization of the PES-g-PEGMA graft copolymers The copolymer PES-g-PEGMA was synthesized using our previously reported method (Shi et al., 2010). The graft polymerization was carried out in a heterogeneous polymer–monomer reaction system using BPO as initiator. BPO was proved to be efficient for grafting polymerization of vinyl and acrylic monomers on numerous polymeric backbones, such as polyamide, polypropylene, and polystyrene (Guan et al., 2009; Makhlouf et al., 2007; Wang et al., 2009b). First 4.0 g PES flakes were added to three-necked flask with 24 mL deionized water. Before adding the initiator BPO, a steady stream of nitrogen was injected into the mixture for 20 min in order to remove oxygen. After initiated for 30 min, 16 mL aqueous solution containing specific amount of PEGMA was added dropwise into the reaction vessel. The polymerization was carried out under nitrogen atmosphere and temperature was kept by water bath. The polymer product was washed sufficiently with hot water. The grafted PES flakes left were collected and dried in an oven at 80 °C until reaching a constant weight. The graft yield (GY) was calculated gravimetrically according to the Eq. (1):

GY ¼

Wf  Wi  100% Wi

ð1Þ

where Wf is the weight of grafted PES flakes while Wi is the initial PES weight. To confirm the successful polymerization of PEGMA onto PES, the compositions of PES-g-PEGMA were then determined by solid-state 13C cross-polarization/magic angle spinning (CP/MAS) NMR experiments using a Varian Unity plus 300WB spectrometer at resonance frequencies of 75 MHz (Liu et al., 2007).

2.3. Preparation of the PES-g-PEGMA membranes Ultrafiltration membranes were fabricated via non-solvent induced phase separation and surface segregation described in our previous work (Ma et al., 2007; Peng et al., 2010; Zhao et al., 2008). To prepare membrane casting solution, the unmodified PES or synthesized PES-g-PEGMA (3.6 g, 18 wt.%) as membrane bulk material and 2.0 g PEG (10 wt.%) as pore forming agent, were dissolved in 14.4 g DMF. The solution was stirred at 60 °C about 4 h to ensure the homogeneous mixing. Subsequently, casting solution was left for 6 h to allow complete release of bubbles. After cooled to room temperature, the solution was cast on glass substrate with a steel knife, and immediately immersed into the coagulation bath of deionized water at 20 °C. The pristine membranes were peeled off and washed thoroughly with deionized water to remove residual solvent. The as-prepared membranes had an average wet thickness of 250 lm and were kept in deionized water before use. The resultant membranes were designated as PES-g-(PEG)nMA-GY membranes, where n indicated the average polymerization degree of PEG segment within each PEGMA monomer molecules (4.5, 8.5, and 22.7 for PEGMA with molecular weight of 300, 475, and 1100, respectively) and GY indicated the graft yield of synthesized PES-g-PEGMA. In addition, the PES membrane represented the PES control membrane which was prepared using unmodified PES following the protocol as above.

2.4. Characterizations of the PES-g-PEGMA membranes Environment scanning electron microscope (ESEM, Philips XL30E) and field emission scanning electron microscope (FESEM, Nanosem 430) were utilized to investigate the cross-section and surface morphologies of the membranes. The membrane samples frozen in liquid nitrogen were broken and sputtered with gold for producing electric conductivity prior to SEM observation. The pore size of skin layer was measured based on the surface morphologies visualized by FSEM. To minimize the experimental error, the average of five different locations of each membrane was reported. Water contact angle was employed to evaluate the hydrophilicity of the PES-g-PEGMA membranes. The measurement was carried out at room temperature by a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China). Membranes were lyophilized for 3 h and then pressured with a rolling machine. Similarly, the contact angle was measured at five different locations for each membrane and the average was reported. The surface chemical compositions of PES membranes were investigated by X-ray photoelectron spectroscopy (XPS) instruments (PHI-1600, USA) using Mg Ka (1254.0 eV) as radiation source. Since the effective detect depth of XPS measurement was proportional to the cosh, the takeoff angle h of the photoelectron was set at 90°, which indicated only the near surface composition of membranes could be measured (Zhu et al., 2009). Survey spectra were collected over a range of 0–1100 eV. BSA was used as the model protein to evaluate the protein adsorption resistance of PES-g-PEGMA membranes. The membranes (30 mm in diameter) were immersed in 1.0 mg/mL BSA solutions at 25 °C for 24 h to reach adsorption–desorption equilibrium. The pH of protein solutions was maintained at 7.0 using 0.1 M phosphate buffered solution. The concentrations of BSA in the solution before and after contact with the membranes were measured with a UV–Vis spectrophotometer (Hitach UV-2800) at 280 nm (Khaparde and Singhal, 2001; Stoschek, 1990; Zhang et al., 2008). Subsequently, the apparent adsorbed BSA amount was calculated. The reported data were the mean values of triplicate samples for each polymer membrane.

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2.5. Permeation and antibiofouling properties of the membranes A dead-end stirred cell filtration system connected with a nitrogen gas cylinder and solution reservoir was designed to characterize the separation performance of membranes. The system consisted of a filtration cell (model 8200, Millipore Co.) with a volume capacity of 200 mL and an inner diameter of 62 mm. The effective area of the membrane was 28.7 cm2. The feed side of the system was pressed by nitrogen gas. All the ultrafiltration experiments were carried out at a stirring speed of 400 rpm and a temperature of 25 ± 1 °C. Each membrane was initially pressurized at 0.15 MPa for 30 min, then the pressure was lowed to the operating pressure of 0.1 MPa. The water flux Jw1 (L/(m2h)) was calculated by the following equation:

J w1 ¼

V A Dt

ð2Þ

where V (L) was the volume of permeated water, A (m2) was the membrane area and Dt (h) was the permeation time. In the following step, the stirred cell and solution reservoir were emptied and refilled rapidly with protein solution (1.0 mg/mL BSA solution, pH was kept at 7.0 with 0.1 M phosphate buffer solution). The flux for protein solution Jp (L/(m2h)) was measured based on the water quantity permeating the membranes at the same pressure (0.1 MPa). The rejections (R) of BSA were calculated by the following equation:

R ¼ ð1 

Cp Þ  100% Cf

ð3Þ

where Cp and Cf were the protein concentration of permeate and feed solutions, respectively. After ultrafiltration of BSA solution, the membranes were washed with deionized water for 20 min, then the water flux of cleaned membranes Jw2 (L/(m2h)) was measured again. The flux recovery ratio (FRR) was calculated using the following expression:

FRR ¼

J w2  100% J w1

ð4Þ

The higher value of FRR, the better antifouling property of the ultrafiltration membrane. To analyze the fouling process in details, several ratios were defined to describe the fouling resistance of the PES-g-PEGMA membranes (Peng et al., 2010; Zhao et al., 2008). The total fouling ratio Rt was defined and calculated as following:

Rt ¼ ð1 

Jp Þ  100% J w1

ð5Þ

Here, Rt was the degree of total flux loss caused by total fouling. Reversible fouling ratio Rr and irreversible fouling ratio Rir were also defined and calculated as following, respectively.



 Jw2  J p  100% J  w1  J  J w2  100% Rir ¼ w1 J w1

Rr ¼

system, in which diffusion controlled the chain growth and chain termination of the grafting procedure. After BPO thermal dissociation, free radicals were generated which could attract aromatic protons from the backbone of PES to generate macroradicals to enable the grafting of monomer molecules onto PES chains. Another possible reaction was that radicals initiated PEGMA monomers to form homopolymer. To reduce homopolymerization, PES flakes were first initiated by BPO and then PEGMA monomer aqueous solution was added dropwise into reaction mixture. The influences of reaction temperature, monomer concentration, and molecular weight of monomer on graft yield (GY) were investigated subsequently. The influence of reaction temperature on the GY with (PEG)8.5MA at monomer concentration of 0.1 g/mL was shown in Fig. S1. The GY was first increased and then reduced when reaction temperature was increased from 50 to 80 °C. The GY reached the highest value of 7.4% when reaction temperature was kept at 70 °C. An increase of reaction temperature could accelerate reaction rate of both graft polymerization and homopolymerization. However, the acceleration effect of higher temperature on homopolymerization was more significant than on graft polymerization. Hence, there was an optimal reaction temperature. The influence of (PEG)8.5MA concentration on the GY was represented in Fig. S2. Initially, the GY was improved from 5.3% to 7.4% when monomer concentration was increased from 0.06 to 0.10 g/ mL. Subsequently, with a further increase of monomer concentration to 0.14 g/mL, GY maintained at about 7.4%. Three PEGMA monomers with different Mn (300, 475, 1100 g/mol) were grafted onto PES flakes at reaction temperature of 70 °C and monomer concentration of 0.10 g/mL. Fig. S3 represented the GY as a function of monomer molecular weight. The GY of (PEG)4.5MA was 17.4% and dropped sharply to 7.4% for (PEG)8.5MA and 5.9% for (PEG)22.7MA. The steric hindrance between macroradicals and long chain monomers hindered the graft polymerization of (PEG)8.5MA and (PEG)22.7MA. Therefore, monomer with short chain (PEG)4.5MA should be preferentially utilized if a higher GY was desired. The chemical graft of PEGMA monomer onto PES flakes was validated with 13C NMR. Fig. S4 depicted the 13C CP/MAS NMR spectrum of pure PES (a) and synthesized PES-g-PEGMA (b). Compared with the curve of pure PES, there were three new peaks presented at 18.8, 59.2, and 72.1 ppm which belonged to PEGMA according to literatures (Ahmad et al., 2009; Jon et al., 2003; Liu et al., 2007; Wang et al., 2009a). These 13C NMR results proved the achievement of graft polymerization of PEGMA onto PES chains.

3.2. Morphologies of the membranes The above PES-g-PEGMA copolymers were utilized to fabricate ultrafiltration membranes via phase inversion process. The cross

ð6Þ ð7Þ

Table 1 XPS analysis of PES-g-PEGMA membrane surfaces. Membrane

Obviously, Rt was the sum of Rr and Rir. PES PES-g-(PEG)8.5MA-5.3% PES-g-(PEG)8.5MA-6.4% PES-g-(PEG)8.5MA-7.4% PES-g-(PEG)4.5MA-17.4% PES-g-(PEG)22.7MA-5.9%

3. Results and discussion 3.1. Synthesis of the PES-g-PEGMA copolymers poly(ether glycol) methyl ether methacrylate (PEGMA) monomer was grafted onto polyethersulfone (PES) chains using benzoyl peroxide (BPO) as an initiator in aqueous solution. The polymerization was carried out in a heterogeneous polymer–monomer

a

Composition of surface elemental (mol%a) C

O

S

72.8 73.6 73.3 72.9 72.3 72.7

19.1 21.2 21.7 22.4 23.6 22.7

6.1 5.2 5.0 4.7 4.1 4.6

Surface coverage of PEG segmentsb

0 16.8 20.0 24.8 34.4 26.4

The atomic molar ratios were determined from XPS spectra. The surface coverage of PEG segments was calculated based on atomic mol% from XPS measurements. b

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Table 2 Pore size, water contact angles, rejection of BSA, and antibiofouling property of PES-g-PEGMA membranes.

a b c

Membrane

Pore size (nm)

Water contact angle (°)

Ra (%)

FRRb (%)

Rt (%)

Rr (%)

Rirc(%)

PES PES-g-(PEG)8.5MA-5.3% PES-g-(PEG)8.5MA-6.4% PES-g-(PEG)8.5MA-7.4% PES-g-(PEG)4.5MA-17.4% PES-g-(PEG)22.7MA-5.9%

12.5 ± 2 12.7 ± 1 13.9 ± 2 13.4 ± 1 14.2 ± 1 13.7 ± 2

64.5 ± 2 61.2 ± 1 59.6 ± 1 57.4 ± 2 46.7 ± 1 59.7 ± 2

97.8 96.7 97.1 97.2 98.1 96.4

56.6 66.2 69.6 76.2 88.1 74.0

57.3 55.2 54.5 62.3 52.8 52.2

13.9 21.4 24.1 38.4 40.8 26.2

43.4 33.8 30.4 23.8 12.0 26.0

The rejection (R) of BSA of each membrane. The flux recovery ratio (FRR) of each membrane, Higher FRR value indicated better antifouling property. The total fouling ratio (Rt), reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) of each membrane.

section morphologies of the membranes were observed with environment scanning electron microscope (ESEM). All of the membranes displayed the typical asymmetric morphology with a thin skin layer on top and a porous bulk at the bottom. The skin layer was responsible for the permeation and rejection of solutes while the bulk played a role as a mechanical support. There was no apparent variation of cross section morphology between PES control membrane and PES-g-PEGMA membranes, which indicated that the excellent film-forming ability of PES was preserved after grafting PEGMA. Field emission scanning electron microscope (FESEM) was employed to investigate the structural morphology of skin layer. There were observable pores in the surfaces of PES-g-PEGMA membranes and the pore size was estimated based on ESEM photographs and listed in Table 2. The pore size of skin layer for PES control membrane was 12.5 ± 2 nm. The pore sizes of skin layer for PES-g-PEGMA membranes were slightly increased compared with PES control membrane. No apparent change in porosity was observed among all the membranes.

O 1s

100000 S 2p

C/S

150000

C 1s

During non-solvent induced phase inversion process, the hydrophilic modifier molecules containing in casting solution were entropically driven to the interface and immobilized at the surface when solidification occurred. The surface compositions of the asprepared membranes were characterized by X-ray photoelectron spectroscopy (XPS). Fig. 1 showed the typical XPS spectrum of PES-g-PEGMA membrane. The characteristic XPS signals for carbon, oxygen and sulfur were observed in the XPS spectra. Table 1 listed the XPS analyses of PES control membrane and PES-g-PEGMA membranes. Apparently, the oxygen molar ratios on the surface of all the PEG-g-PEGMA membranes were higher than PES control membrane, which indicating that PEG segments were

Atomic % C 1s 72.3 O 1s 23.6 S 2p 4.1

50000

0 1000

800 600 400 Bonding Energy (eV)

200

Fig. 1. XPS analysis result of PES-g-(PEG)4.5MA-17.4% membrane.

us ¼ ð1  16  rs Þ  100%

ð8Þ

where rs was the sulfur molar ratio based on XPS analysis. The factor 16 accounted for the fact that there was one sulfur atom in each PES chain unit containing 16 atoms. The surface coverage of PEG segments of all the as-prepared membranes were listed in Table 1. With an increase of GY from 5.3% to 7.4%, the surface coverage of PES-g-(PEG)8.5MA membrane was increased from 16.8% to 26.8%. PES-g-(PEG)4.5MA-17.4% membrane possessed the highest surface coverage of PEG segments of 34.4%, which might be ascribed to the highest GY for (PEG)4.5MA. The highest surface coverage of hydrophilic PEG segments implied the strongest surface hydrophilicity, the largest protein resistance, and best antibiofouling property for the PES-g-(PEG)4.5MA-17.4% membrane. 3.4. Hydrophilicity and protein adsorption resistance of the membranes

3.3. Surface segregation of PEG segments during membrane preparation

200000

enriched on the membrane surface. Since only PES contained sulfur element, the surface coverage of PEG segments, us, was calculated as follows:

0

The hydrophilicity of PES-g-PEGMA membrane surfaces was characterized by water contact angle. Lower water contact angle means stronger hydrophilicity. The water contact angles of all the prepared membranes were listed in Table 2. The PES control membrane possessed the highest water contact angle of 64.5 ± 1.9°, while the PES-g-PEGMA membranes had lower water contact angles. For PES-g-(PEG)8.5MA membranes, with GY was increased from 5.3% to 7.4%, water contact angles were reduced from 61.2 ± 1.5° to 57.4 ± 1.7°, which indicated the hydrophilicity of membrane surface was improved with an increase of GY. The increased surface coverage of hydrophilic PEG segments was the dominant reason. PES-g-(PEG)4.5MA-17.4% membrane possessed the lowest water contact angle of 46.7 ± 1.2° among all these membranes, suggesting the strongest surface hydrophilicity. The water contact angles for PES-g-(PEG)8.5MA-7.4% membrane and PES-g(PEG)22.7MA-5.9% membrane were 57.4 ± 1.7° and 59.7 ± 1.6°. The protein adsorption was carried out to evaluate the protein resistance of the prepared membranes. The protein adsorption of the membranes using bovine serum albumin (BSA) as the model was showed in Fig. 2. The BSA adsorption of PES control membrane was 57.8 lg/cm2. After graft modification, the BSA adsorption for PES-g-PEGMA membranes remarkably declined. The BSA adsorption of PES-g-(PEG)8.5MA-5.3% membrane was 36.2 lg/cm2. When GY was increased to 7.4%, the BSA adsorption of PES-g-(PEG)8.5MA7.4% membrane was reduced to 15.7 lg/cm2. It could be concluded that the higher surface coverage of PEG segments, the less amount of BSA molecules adsorbed on PES-g-PEGMA membrane surface. PES-g-(PEG)4.5MA-17.4% membrane adsorbed the least amount of BSA molecules (only 7.8 lg/cm2), while PES-g-(PEG)22.7MA-5.9% membrane possessed BSA adsorption of 15.2 lg/cm2, almost the same as PES-g-(PEG)8.5MA-7.4% membrane. These results could

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70

50

250

PES-g-(PEG)8.5MA-7.4%

200

PES-g-(PEG)8.5MA

2

GY=6.4%

30 PES-g-(PEG)8.5MA

20

PES-g-(PEG)8.5MA-6.4%

PES-g-(PEG)8.5MA GY=5.3%

40

PES PES-g-(PEG)8.5MA-5.3%

PES

Flux(L/m h)

BSA adsorption µg/cm

2

60

300

GY=7.4%

PES-g-(PEG)22.7MA

GY=5.9% PES-g-(PEG)4.5MA GY=17.4%

10

(a)

150 100 50

0

0 0.0

Membrane Fig. 2. Protein adsorption of the PES-g-PEGMA membranes. The concentration of model BSA solution was 1.0 mg/mL (pH 7.0).

0.5

1.0 Time(h)

1.5

2.0

300 PES-g-(PEG)8.5MA-7.4%

250

PES-g-(PEG)4.5MA-17.4% PES-g-(PEG)22.7MA-5.9%

200

(b)

2

Flux(L/m h)

be interpreted as follows. In aqueous solution, membranes with hydrophilic surface lead to a spontaneous rearrangement of hydrophilic groups (Shi et al., 2008). The hydrogen bonds formed between hydrophilic PEG groups and water molecules induced the formation of hydration layer and the effect of steric exclusion which could consequently inhibit protein adsorption (Zheng et al., 2004, 2005). Accordingly, membrane with higher hydrophilicity possessed lower protein adsorption and stronger antibiofouling property.

150 100 50

3.5. Separation properties of the membranes Ultrafiltration experiments were conducted to assess the permeability of the PES-g-PEGMA membranes. Water fluxes as a function of filtration time for all the prepared membranes were first measured and showed in Fig. 3. Compared with PES control membrane, PES-g-PEGMA membranes possessed higher water fluxes owing to the improved hydrophilicity. With an increase of GY, the water flux of PES-g-(PEG)8.5MA membrane was increased from 163.5 to 226.2 L/(m2h), while water flux of PES control membrane was only 135.2 L/(m2h). The water fluxes of PES-g-(PEG)4.5MA17.4% membrane and PES-g-(PEG)22.7MA-5.9% membrane were almost the same (approximate 170 L/(m2h)). PES-g-(PEG)8.5MA-7.4% membrane possessed the maximum water flux of all the PES-gPEGMA membranes. The rejection, which was mostly depended on pore size of the skin layer, was a key parameter to investigate the selectivity of membranes. During ultrafiltration, the molecules, smaller than the pores on skin layer, were driven to penetrate through the membrane, while the bigger molecules were mostly rejected. BSA was used as a model protein to evaluate the rejection of the membranes and the results were listed in Table 2. For PES control membrane, the BSA rejection was 97.8%. The BSA rejections of all the PES-g-PEGMA membranes maintained at approximate 97% with a little fluctuation. 3.6. Antibiofouling and recycling properties of the membranes Membrane fouling was primarily caused by the adsorption and deposition of proteins on the membrane surface and entrapment of proteins in the pores. Basically, membrane fouling consisted of reversible fouling and irreversible fouling. Reversible protein adsorption led to reversible fouling which could be removed by simple hydraulic cleaning. On the contrary, irreversible fouling was caused by firm adsorption of protein molecules on the surface or entrapment of protein molecules in pores (Pieracci et al., 2002).

0 0.0

0.5

1.0 Time(h)

1.5

2.0

Fig. 3. Time-dependent fluxes of PES-g-PEGMA membranes during the ultrafiltration: the sequence constituted three-steps: water flux in first 0.5 h, BSA solution (1 mg/ml, pH 7.0) permeation for 1 h, and water flux for 0.5 h after 20 min hydraulic cleaning.

In the current work, water fluxes of the fouled PES-g-PEGMA membranes were measured again after hydraulic cleaning. Then, flux recovery ratio (FRR), total fouling ratio (Rt), reversible fouling ratio (Rr), and irreversible fouling ratio (Rir) values were calculated to evaluate the antibiofouling properties (Table 2). The FRR value of PES control membrane was only 56.6% which meant a poor antibiofouling property. Rir value of PES control membrane was 43.4% (more than 75% in total fouling), implying the irreversible fouling dominated the total fouling. After graft modification, the FRR values of all the PES-g-PEGMA membranes were increased. With GY was increased from 5.3% to 7.4%, the FRR values of PESg-(PEG)8.5MA membranes were increased from 66.2% to 76.2%. Simultaneously, the Rir values were reduced from 33.8% to 23.8%. The irreversible fouling percentage in the total fouling sharply dropped to about 38%. It could be concluded that the antibiofouling capability of PES-g-PEGMA membrane was considerably improved with an increase of GY. For the three PEGMA monomers with different PEG chain length, PES-g-(PEG)4.5MA-17.4% membrane had the highest FRR value of 88.1% and the lowest Rir value of 12.0%. The irreversible fouling percentage in total fouling for PES-g-(PEG)4.5MA-17.4% membrane was dramatically decreased to 23%. The FRR value and Rir value of PES-g-(PEG)22.7MA-5.9% membrane was 74.0% and 26.0%, the poorest antibiofouling property among these three types of membranes. These phenomena could be interpreted that membrane with more PEG segments could induce denser and more stable hydration layer, which could endow the better

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References

250

150

2

Flux(L/m h)

200

100

50 1st

0

0

2nd

1

2

3rd

3

4

5

Time(h) Fig. 4. Recycling property of PES-g-(PEG)4.5MA-17.4% membrane.

antibiofouling property. The largest GY resulted in the highest surface coverage of PEG segments and the strongest fouling resistance for PES-g-(PEG)4.5MA-17.4% membrane. Therefore, short chain (PEG)4.5MA was preferential for antibiofouling PES membrane fabrication. The stable antibiofouling property for PES-g-PEGMA membranes was acquired due to covalent bonding interactions between hydrophilic PEGMA side chains and PES main chains. The three cycle of experiment of PES-g-PEGMA membranes was employed to evaluate the durability of antibiofouling property. Fig. 4 exhibited the three-cycle of ultrafiltration experiment of PES-g-(PEG)4.5MA17.4% membrane. The FRR values in the three cycles were 84.1%, 93.4%, and 93.8%, respectively, indicating that the reversible fouling was dominant in total fouling and hydraulic cleaning still maintained high efficiency after multi-run. 4. Conclusions The graft polymerization of PEGMA onto PES in aqueous solution was successfully conducted. The PES-g-PEGMA membranes were fabricated through the coupling of non-solvent induced phase inversion and surface segregation. The incorporation of PEGMA could inappreciably affect the morphology of PES membranes. With an increase of grafting yield, surface hydrophilicity and protein fouling resistance of the PES-g-PEGMA membranes were enhanced due to the considerable surface enrichment of PEG segments. It was found that the PES-g-PEGMA membranes with higher surface coverage of PEG segments displayed stronger antibiofouling property. Moreover, the stable antibiofouling property for PES-g-PEGMA membranes was acquired. Acknowledgements This research was supported by the Research Fund for the Doctoral Program of Higher Education of China (20060056032), Drug Separation and Purification Project in Programme for Development of Novel Drug (No. 2009ZX09301-008), the Program of Introducing Talents of Discipline to Universities (No. B06006), and State key laboratory of precision measuring technology and instruments (Tianjin University). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.10.045.

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