Desalination 316 (2013) 127–136

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Desalination

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The preparation of antifouling ultrafiltration membrane by surface graftingzwitterionic polymer onto poly(arylene ether sulfone) containing hydroxylgroups membrane

Yang Liu a,b, Shuling Zhang a, Guibin Wang a,⁎a College of Chemistry, Engineering Research Center of High Performance Plastics, Ministry of Education, Jilin University, Changchun 130012, Chinab College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China

H I G H L I G H T S

► Modification of the PES-OH membrane via redox graft polymerization.► Poly(MPC) is immobilized by hydroxyl groups on the PES-OH membrane surface.► The grafting process is facile and effective due to the presence of hydroxyl group.► The zwitterionic membrane exhibits excellent protein antifouling property.

⁎ Corresponding author. Tel./fax: +86 431 8516 8889E-mail address: wgb@jlu.edu.com (G. Wang).

0011-9164/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.desal.2013.02.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 October 2012Received in revised form 9 February 2013Accepted 11 February 2013Available online 15 March 2013

Keywords:AntifoulingSurface modificationRedox graft polymerizationZwitterionic polymerPoly(arylene ether sulfone) containinghydroxyl groups

This work describes the polymerization of a zwitterionic polymer, poly(2-methacryloyloxyethylphosphorylcholine) [poly(MPC)] onto the poly(arylene ether sulfone) containing hydroxyl groups (PES-OH)membrane via redox graft polymerization using ceric (IV) ammoniumnitrate (CAN) as initiator. Due to the pres-ence of the activated hydroxyl groups which were used for the immobilization of MPC monomer, the graftingprocess is facile and effective which avoid the complex and unfavorable pretreatment process and/or hydroxyl-ated treatment. Attenuated total reflectance Fourier transform infrared spectrometer (FTIR-ATR), X-ray photo-electron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to characterize the chemicalcompositions and surfacemorphologies of the unmodified (PES-OH) andmodifiedmembrane, respectively. Stat-icwater contact anglemeasurement indicated that the introduction of poly(MPC) promoted remarkably the sur-face hydrophilicity of the PES-OHmembrane. The cycle ultrafiltration experiments for protein solution revealedthat nonspecific protein adsorption, especially irreversible protein adsorption, for the zwitterionic membranewas significantly reduced, suggesting superior antifouling performance. This work not only introduces a modifi-cation approach to obtain a PES-OHmembrane grafting hydrophilic poly(MPC) chains, but also gives the zwitter-ionic membrane a long time life and excellent ultrafiltration performance.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ultrafiltration (UF) as an effective and powerful technique has beenfound to be an alternative approach for macromolecules separationsuch as proteins. It possesses a number of attractive features mainlylow energy consumption, mild operating conditions, no additive re-quirements, no phase change and environmentally friendly [1], whichhas been integrated into industrial processes as well as economical, eco-logical and safety issues. The state-of-the-art materials used for man-ufacturing of commercial ultrafiltration membrane are generally madefrom polymers [2], such as cellulose [3], poly(vinylidene fluoride) [4],polyetherimide [5], polysulfone [6] and polyethersulfone [7]. Amongall polymeric membrane materials, poly(arylene ether sulfone)s (such

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as polysulfone, polyethersulfone and polyphenylsulfone, etc.) are aclass of especially useful materials which have been used to make ultra-filtration membrane for over 25 years due to their specific characteris-tics. These excellent high-heat plastics offer more toughness, strength,rigidity and hydrolytic stability than other polymer materials, andwithstand prolonged exposure to water, chemicals and temperatures[6,8–10]. However, their applications are often limited by the inherenthydrophobic property of poly(arylene ether sulfone)s. The hydrophobicinteraction between poly(arylene ether sulfone)s membrane and pro-tein molecules in feed solutions often causes nonspecific adsorptionand deposition of proteins on the membrane surface or in pores, andresults in serious membrane fouling [1,11–14], which will reduce per-meate flux, increase production cost, alter membrane selectivity andshorten membrane life. Therefore, increasing need for antifouling ultra-filtration membranes has driven the widespread development of chem-ical or physical modifications of the separation membranes.

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In recent years, many researches have been reported and used suc-cessfully to improve the membrane resistance toward fouling from aca-demic and technological point. According to literatures, four differentmodification strategies, including (I) synthesis of novel polymers withwell-defined structure as membranematerials (synthesis) [1,12], (II) di-rect membrane material modification before membrane preparation(pre-modification) [11,15], (III) blending of membrane material withmodifying additives in casting solutions during membrane preparation(additive) [14,16], and (IV) advanced surface modification or func-tionalization after membrane preparation (post-modification) [17,18]have been proposed. For the synthesis method, the new materials arenot easy to synthesize and handle, and a large number of startingmaterials are wasted. An ideal membrane should retain the excellentmechanical properties of membrane bulk. However, for the pre-modification method, the modified membrane materials could sacrificeintrinsic properties of originalmaterials. For example, a highly sulfonatedmaterial is considerably beneficial to ultrafiltration membrane [19], butit is difficult to obtain amembrane having sufficientmechanical strengthand dimensional stability [15,20]. It is generally accepted that surfaceproperty holds a key factor on antifouling as it determines the interactionbetween proteins and membrane [21]. For the additive method, themodification membrane through a blending technique is simple, andno additional step is needed during membrane preparation. But thesurface coverage of the modifying additives on membrane surface isrelative low, which resulted in poor antifouling property. For thepost-modification method, research efforts have focused on modifyingmembrane surface using hydrophilic materials, which was regard as anefficient way for improving antifouling property. Furthermore, consider-able attention has been directed towards the development of surfacegrafting for modifying membrane surface, which is apparently more re-sistant to variousmembrane-washing processes than surface adsorptionand surface coating [17].

To data, zwitterionic substances, containing a representativestructure of both anionic and cationic units in the same moleculehave been attractive as a class of promising alternative materialswith excellent antifouling ability. Numerous significant works havebeen made in preparing of zwitterionic surfaces were shown tohave the function of reducing protein adsorption, because their hy-drophilic surfaces containing uniform chains with tailorable lengthcould bind a large amount of water, leading to a strong repulsiveforce to protein at specific separation distances or making the proteincontact with the surface without conformation change [22,23]. The2-methacryloyloxyethyl phosphorylcholine (MPC) with a zwitterion-ic phosphorylcholine (PC) group has been synthesized and widelyused in the preparation of antifouling surfaces [24]. For example,MPC has been grafted on polyethylene films by plasma induced atlow temperature [25] and photo induced graft polymerization [26].

Several surface modification techniques have been adopted for im-proving the surface antifouling by grafting polymerization of monomerson the membrane surface. However, the surface grafting modificationsusually require a pretreatment process, including UV-induced [18,21],ozone surface activation [17], plasma induced [25] and γ-radiationinduced [27], which required somewhat expensive equipment and/orcause significant surface aging of the basemembrane [25]. Furthermore,the surface modification techniques mentioned above are incapable ofmodifying the samplewith complex geometries [25]. Consequently, sur-face modification by redox graft polymerization using ceric ammoniumnitrate (CAN) as initiator is a good choice and a well-knownway for thepolymerization of vinylmonomers. The reactionmethodhas a prime ad-vantage of performing atmoderate temperature, whichwouldmake theside reactionminimized [28].Many researches have been focused on thesurface modification of hydroxyl group polymers via redox graft poly-merization by the conventional ceric ion technique [29–31]. To ourknowledge, the membranes were hydroxylated by treatment with theperoxy compounds is unavoidable before redox graft polymerization,especially poly(arylene ether sulfone) membranes.

The present study reports on a simple and efficient method tograft zwitterionic monomer, MPC, onto the membrane surface byredox graft polymerization using CAN as initiator, which only requireone step reaction and avoid the complex and unfavorable pretreat-ment process and hydroxylated treatment. The poly(arylene ethersulfone) containing hydroxyl groups (PES-OH) was synthesized asour previous work [32], which was used to prepare the PES-OH ultra-filtration membrane. Due to the appearance of the pendant activatedhydroxyl group in every unit of PES, an amount of hydroxyl groupswith tailorable distribution density on the surface of the PES-OHmembrane are achievable by immersion precipitation phase inver-sion method, and the PES-OH membrane could be further graftedwith other functional groups. The chemical compositions, hydrophi-licity and surface morphologies of the unmodified and modifiedmembrane were thoroughly investigated, and the antifouling perfor-mance of the modified membrane was examined in detail. It wasexpected that the modified membrane provides a good opportunityto improve antifouling property.

2. Experimental

2.1. Materials

2-Methacryloyloxyethyl phosphorylcholine (MPC) was purchasedfrom Joy-Nature Technology Institute, Nanjing, China. The ceric (IV) am-monium nitrate (CAN, AR grade) was purchased from Aldrich and usedwithout further purification. Polyethersulfone (PES, Ultrason E6020P)was purchased from BASF which was used as polymermaterial in prepa-ration of membrane casting solution. Poly(vinylpyrrolidone) (PVP 30 K),which was used as pore-former, was purchased from Fluka Chemika,Switzerland. Bovine serum albumin (BSA, pI=4.8, Mw=67,000) andphosphate buffer solution (PBS, 0.1 mol/L, pH 7.4) were both purchasedfrom Dingguo Bio-technology Co., Ltd. (Beijing, China). Coomassie bril-liant blue G250 was purchased from Aldrich. (3-Methoxy)aniline wereobtained from Tokyo Chemical Industry, Japan. Hydrochloric acid (36%),sodium bicarbonate, 1,4-benzoquinone and N-methyl-2-pyrrolidinone(NMP) were purchased from Beijing Chemical Reagent, China. Pyridine,zinc powder and sodium nitrite were supplied by Shanghai Chemical Re-agent, China. The polymerization solvents, sulfolane (TMS) and toluenewere purchased from Tiantai Chemical Reagent, China. Commerciallyavailable 4,4′-dichlorodiphenylsulfone (DCDPS) was purchased fromYanji Chemical Plant, China. Anhydrous potassium carbonate was driedunder vacuum at 110 °C for at least 24 h before use. All other chemicalswere obtained from commercial sources and usedwithout further purifi-cation. Deionized water was used throughout this study.

2.2. Synthesis of poly(arylene ether sulfone) containing hydroxyl groups(PES-OH)

Obtained (3-methoxy)phenylhydroquinone was synthesized ac-cording to previously reported procedure [32], and the synthetic route isillustrated in Scheme 1. The synthesis of Poly(arylene ether sulfone)containing hydroxyl groups (PES-OH) was achieved in two steps as ourprevious work [32]. First of all, poly(arylene ether sulfone) containingmethoxyl groups (PES-OCH3) was synthesized by a characteristic nucle-ophilic aromatic substitution (SNAr) reaction by treatment of 4,4′-dichlorodiphenylsulfone with (3-methoxy)phenylhydroquinone. In thesecond step, PES-OH was obtained by demethylation of PES-OCH3 inmolten pyridine hydrochloride. The synthetic route is outlined inScheme2. The advantage of hydroxyl groups being that they can be read-ily transformed into various other funtionalities owing to their goodreactivity. Therefore, the polymer could then be easily modified by reac-tion with monomers or polymers containing proper functionalities togive graft copolymers, allowing the careful tuning of the properties ofthe PES-OH.

Scheme 1. Synthetic procedure for (3-methoxy)phenylhydroquinone.

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2.3. Preparation of PES-OH and PES membranes

The PES-OH membrane was prepared by the conventional immer-sion precipitation phase inversion method. PES-OH was dried at 80 °Cunder vacuum for at least 24 h before use. Homogeneous casting solu-tion was prepared by dissolving PES-OH and PVP in NMP at 50 °Cunder vacuum and filtered by a metal filter. The concentrations ofPES-OH and PVP in the casting solution were 17 wt.% and 8 wt.%, re-spectively. After degassing, the casting solution was cast onto a polyes-ter nonwoven fabric supported by a glass plate using a casting knifewith a nominal thickness of 200 μm, and then the glass plate was im-mersed immediately into a coagulated bath of deionized water atroom temperature (about 18 °C). Finally, themembranewas kept in de-ionized water for at least 48 h until all of solvent and water-solublepolymer were removed. In this work, the PES membrane was used asa benchmark reference for evaluating the membrane performance be-cause it is a commercial ultrafiltration membrane material and itschemical structure similarity to the PES-OH. The reference PES mem-brane was prepared by the same procedure as the PES-OH membrane.

2.4. Ceric (IV) redox surface graft polymerization of MPC on the PES-OHmembrane

The PES-OH membrane was subjected to grafting reaction in anaqueous solution using CAN as initiator [29,30], and the schematic il-lustration is shown in Scheme 3. Firstly, the PES-OH membrane wasimmersed in acetone at room temperature for 10 h, and then treatedby ultrasonication for 30 min to remove surface impurities and driedunder a vacuum oven at 50 °C for 12 h. Then the membrane was im-mersed in an aqueous solution containing 5% (w/v) concentration ofMPC. After displacing oxygen by argon gas about 10 min, the CANinitiator was added in the aqueous solution at a concentration of2×10−3 mol/L. The reaction was conducted at 50 °C and was allowedto proceed for 4 h with continual degassing and stirring. Followingthe modified membrane (PES-g-MPC) was washed with deionizedwater and ethanol to remove ungrafted homopolymer from the sur-face and the pores, and allowed to dry in a vacuum oven at 50 °Cfor 12 h before carrying out ultrafiltration experiments.

Scheme 2. Synthetic procedure for poly(arylene

2.5. Membrane characterization

The characteristics of the membranes were investigated using thefollowing parameters: surface chemical structure, surface chemicalcomposition, contact angle, morphology, porosity and pore size. Themembranes used for characterization were dried under vacuum at50 °C for 24 h.

Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 80 V)with an ATR unit (Attenuated Total Reflection, crystal, 45°) was usedto investigate the surface chemical structures of themembranes. Absor-bance spectra were obtained in the region of 4000–400 cm−1 with res-olution 4 cm−1 for 32 scans. All the spectra were baseline corrected.

The surface chemical compositions of the PES-OH and PES-g-MPCmembranes were performed by X-ray photoelectron spectroscopy(XPS) on an ESCLAB MKII with Al/K (hν=1486.6 eV) anode monoX-ray source. The atomic compositions of the elements were calculatedby their corresponding peak areas.

The static water contact angles of the membranes were estimatedby sessile drop method with a contact angle goniometer from DropShape Analysis (DSA 100 KRUSS GMBH, Hamburg) at room tempera-ture. About 4 μL of deionized water was dropped onto the membranesurface with a microsyringe, and the value of water contact angle wasrecorded after 3 s. At least five measurements in different locations ofthe membrane samples were carried out and averaged to yield thecontact angles. In addition, the surface free energy was also calculatedaccording to the following equation:

cosθ ¼ −1þ 2ffiffiffiffiffiγs

γl

re−β γs−γlð Þ2 ð1Þ

where γs and γl represent the solid and liquid surface free energy, re-spectively. θ is the contact angle of the membrane sample. The valueof water surface free energy is 72.8 mJ/m2. β is the constant coeffi-cient related to a specific solid surface and the value of 0.0001247 isadopted from previously reported literature [25].

The surface morphologies of the unmodified and modified mem-branes were examined by a scanning electron microscope (SSX-550,Shimadzu equippedwith energy dispersive X-ray (EDX) spectroscopy).

ether sulfone) containing hydroxyl groups.

Scheme 3. Schematic illustration of the preparation process of the PES-g-MPC membrane by graft polymerization.

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The porosity of the membrane was measured by the dry-wetweight method which was calculated according to Eq. (2) [33]:

ε ¼ m1−m2

ρwAlð2Þ

where ε is the porosity (%), m1 is the weight of the wet membrane(g),m2 is the weight of the dry membrane (g), ρw is the water density(0.998 g/m2), A is the effective area of the membrane (cm2), and l isthe membrane thickness (μm).

Average pore size rm (m) was determined by the filtration velocitymethod according to the Guerout–Elford–Ferry equation, which canbe calculated as follows [34–36]:

rm ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2:9−1:75εð Þ � 8ηlQt

ε � A� ΔP

rð3Þ

where Qt is the volume of the permeate water per unit time (m3/s),ΔP is the operational pressure (0.1 MPa), and η is the viscosity ofwater (8.9×10−4 Pa.s) at 25 °C. In order to minimize experimentalerror, each membrane was measured for three times and calculatedthe average.

2.6. Protein adsorption experiments

The amount of proteins adsorbed on membrane is one of the mostimportant evidence in evaluating the fouling resistant ability of mem-branes, and BSA was used as model protein to evaluate the anti-protein adsorption performance of three investigated membranes inphosphate buffered saline (PBS, pH 7.4). The membrane sampleswere cut into a round shape with a diameter of about 30 mm, andtreated by ultrasonication for 30 min in 0.1 M PBS solution for clean-liness. Then the pre-treated membranes were immersed into PBS so-lution containing BSA (1.0 mg/mL) at 25 °C for 4 h. After adsorption,each membrane was rinsed three times in the fresh PBS by gentleshaking. Then these membranes were transferred into a well-platefilled with PBS solution, and the protein adsorbed on the membranesurface was completely desorbed by ultrasonic treatment at roomtemperature for 3 min. The obtained PBS solution was dyed withCoomassie Brilliant Blue and measured by a UV–vis spectrophotome-ter (UV3600, Shimadzu) to determine the total amount of adsorbedprotein. The final results were averaged from triplicate specimensfor each membrane.

2.7. Ultrafiltration experiments

The membrane performances were tested using a stirred dead-end filtration cell at room temperature, and the effective membranearea is around 12.6 cm2. At first, eachmembranewas initially subjectedto pure water with pressure of 0.2 MPa for 1 h prior to performing the

ultrafiltration experiments. Then the pressure was lowered to 0.1 MPaand all the ultrafiltration experiments were carried out at this pressure.After compacted, deionized water was passed through the membranefor 1 h to obtain the beginning pure water flux (Jw,0, L/m2h), and thefluxwasmeasured every 5 min. In this present study, three cycles of fil-tration experiment was carried out for each membrane sample. In eachcycle, a BSA solution with a concentration of 1.0 mg/ml in PBS (pH 7.4)was filtrated for 2 h. The flux during protein filtration was recordedwhich called Jp,i (i means the ith cycle). After BSA solution filtration,the membrane was washed thoroughly and passed through deionizedwater for another 30 min (the washing timewas not counted in the fil-tration cycle). Thereafter, the purewaterfluxwasmeasured againwith-in 1 h for themembrane, whichwas recorded as Jw,i. The flux (Jw and Jp)of the membrane was determined by direct measurement of permeatevolume, which was calculated by the following equation:

J ¼ VAt

ð4Þ

where Vwas the volume of permeation, Awas the effective membranearea and t was the permeation time. In order to evaluate the recyclingproperty of these membranes, the flux recovery ratio (FRRi) duringthe ith cycle is calculated using the following expression:

FRRi %ð Þ ¼ Jw;i

Jw;i−1

!� 100 ð5Þ

The higher FRR value, the better the antifouling property of themembrane. The membrane rejection ratio (R) was calculated by usingthe following equation:

R %ð Þ ¼ 1−Cp

Cf

!� 100 ð6Þ

In which Cp (mg/L) is the permeate concentration and Cf (mg/L) isthe feed concentration. The solute concentration of permeation wasmeasured by a UV–vis spectrophotometer (UV3600, Shimadzu).

To study the antifouling property in more detail for the unmodifiedand modified membranes, the degree of total flux loss caused by totalprotein fouling in the ith cycle, Rt,i, was defined as

Rt;i %ð Þ ¼ Jw;i−1−Jp;iJw;i−1

!� 100 ð7Þ

A high value of Rt,i corresponds to a large flux decay and seriousmembrane fouling. The total flux loss was caused by both reversible

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and irreversible protein fouling in the ith cycle. Rr,i was calculated bythe following equation:

Rr;i %ð Þ ¼ Jw;i−Jp;iJw;i−1

!� 100 ð8Þ

which was the reversible fouling ratio caused by reversible fouling,and could be eliminated by hydraulic cleaning. And Rir,i was calculat-ed by the following equation:

Rir;i %ð Þ ¼ Jw;i−1−Jw;i

Jw;i−1

!� 100 ¼ Rt;i−Rr;i ð9Þ

which was the irreversible fouling ratio caused by irreversible fouling,and can only be eliminated by chemical cleaning or enzymatic degra-dation [14,37–39]. Rt,i was the sum of Rr,i and Rir,i.

3. Results and discussion

3.1. Membrane characterization

3.1.1. Scanning electron microscopyThe scanning electron microscopy (SEM) was employed to investi-

gate the surface morphology changes of the unmodified and modified

Fig. 1. SEM images of the surface of the unmodified and modified membranes: (a) the PES-Ounmodified and modified membranes: (c) the PES-OH membrane and (d) the PES-g-MPC m

membranes, and the SEM photographs are shown in Fig. 1. It is apparentthat the top surfaces are much different between the unmodified andmodified membranes. The PES-OH membrane showed a relativelysmooth surface, and the surface of the PES-g-MPC membrane becamemuch rougher after the graft polymerization. Therefore, it could be ten-tatively concluded that the MPC monomer was grafted on the PES-OHsurface by using CAN as an initiator, and resulted in the formation ofmany small dot-like structures on the surface of themodifiedmembrane.The phenomenon of the formation of the “dot-like” structure has oftenbeen reported in the surface-initiated atom transfer radical polymeriza-tion (ATRP) method [40–42]. Furthermore, the energy-dispersive X-ray(EDX) P mapping on the surfaces of the PES-OH and PES-g-MPC mem-branes is also shown in Fig. 1. It could be seen that elemental phosphorus(the bright spots) is evenly distributed on the modified membranesurface, and there is no elemental P on the surface of the unmodifiedmembrane. Since MPC is the only source of elemental P, EDX measure-ment gave valuable information to confirm that the poly(MPC) was suc-cessfully grafted on the PES-OH membrane surface.

3.1.2. FTIR-ATR spectroscopyTo clearly understand, the chemical structures of the top surface of

the unmodified and modified membranes were thoroughly character-ized by FTIR-ATR spectroscopy that plays an important role in surfaceor near-surface chemical composition analysis. Fig. 2 shows the FTIR-

H membrane and (b) the PES-g-MPC membrane. EDX P-mapping on the surface of theembrane.

Fig. 2. FTIR-ATR spectrum of the unmodified and modified membranes: (a) the PES-OHmembrane and (b) the PES-g-MPC membrane.

Fig. 3. XPS spectrum of the unmodified and modified membranes: (a) the PES-OHmembrane and (b) the PES-g-MPC membrane.

Table 1Surface chemical compositions of the PES-OH and PES-g-MPC membranes from XPS.

Samples Compositions (%)

C O S N P

PES-OH 79.73 16.81 3.45 – –

PES-g-MPC 62.14 28.29 0.46 4.62 4.47Pure MPCa (57.89) (31.59) – (5.26) (5.26)

a The values of pure MPC are the theoretical atomic percentages.

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ATR spectrum of the PES-OH and PES-g-MPC membranes. Two absorp-tion peaks were clearly observed at 1149 cm−1 and 1231 cm−1 forboth the unmodified andmodifiedmembraneswhich could be attributedmainly to O_S_O group and Ar\O\Ar group, and these characteristicabsorption peaks were almost unchanged after the graft polymerization.This is because the thickness or the contents of poly(MPC) layer may bebelow the detection limit of FTIR-ATR. Compared to the FTIR-ATRspectrum of the PES-OH membrane, three new absorption peaks at968 cm−1, 1072 cm−1 (−POCH2-) and 1244 cm−1 (P_O) were ob-served only for the PES-g-MPC membrane which could be attributed tothe phosphate (P\O) group in the MPC unit. Additionally, the strong ad-sorption peak at 1722 cm−1 only corresponds to the ketone group in theMPC unit was also observed in the spectrum of the PES-g-MPC mem-brane. The FTIR-ATR spectrum showed that the MPC was successfullygrafted on the surface of the PES-OH membrane.

3.1.3. X-ray photoelectron spectroscopySince the penetration depth of FTIR-ATR spectroscopy is at the level

of a micron, and the layer of graft polymerization of a zwitterionic poly-mer is usually very thin. To further confirm successful graft polymeriza-tion on the PES-OH membrane, XPS, a more surface-sensitive method(XPS only measures a depth of about 10 nm), was used to identify thesurface chemical changes. The comparison between the unmodifiedand modified membranes (Fig. 3) shows clearly that the chemical com-positions are markedly different. In the case of the PES-OH membrane,two strong peaks at 285 (C1s) and 532 (O1s) eV were obviously ob-served, which corresponded to carbon and oxygen atoms. And twosmall peaks at 168 (S2p) and 230 (S2s) eV were also found which wereattributed to sulphur atom in the sulfone group. For the PES-g-MPCmembrane, the content of carbon (C1s) was slightly decreased whilethe content of oxygen (O1s) was slightly increased after the graft poly-merization reaction, and the XPS peaks at 286.9 and 288.8 eV wereattributed to carbon atoms of the ether bond (−C\O\C–) and carbonylgroup (C_O), respectively. Besides, twonewXPS peaks at 134 (P2p) and190 (P2s) eV were attributed to phosphorus atom, and the peak in thenitrogen atom region at 402.5 (N1s) eV attributed to the quaternary am-monium group was detected. These peaks were specific to thephosphorylcholine group in the MPC unit as compared with the surveyscan spectrum of the PES-OH membrane. It could be indicated that thepoly(MPC) was successfully grafted on the PES-OH membrane surface.The detailed analysis results of the surfaces of the unmodified andmodified membranes determined by XPS were shown in Table 1.Based on the only source of elemental P in the MPC unit, the degree of

poly(MPC) surface coverage (CPMPC) on the PES-OH membrane surfacewas calculated as the following equation:

CPMPC %ð Þ ¼ AmP

ApP� 100 ð10Þ

where AmP is the phosphorus atomic percentage on the PES-g-MPCmembrane surface measured by XPS, and ApP is the phosphorus atomicpercentage of poly(MPC) under the condition of membrane surfacecompletely covered with poly(MPC). According to the above equation,the PES-g-MPC membrane presents CPMPC of 84.98%. A large number ofhydroxyl groups arewell distributed on the surface of the PES-OHmem-brane may be responsible for the high degree of poly(MPC) surfacecoverage.

3.1.4. Contact angleSurface hydrophilicity is one of the most important factors in de-

termining antifouling property and performance of ultrafiltrationmembrane. The hydrophilicity and wettability of the unmodifiedand modified membranes in this study were evaluated by contactangle measurement, which was also used to assess the surface (inter-facial) free energies of substrate surfaces. It is commonly acceptedthat the lower contact angle represents the greater tendency forwater to wet the membrane, the higher surface energy and the higherhydrophilicity. Table 2 lists the detailed water contact data from themeasurements on these different membranes.

The PES membrane has the highest contact angle of 82.91°, indicat-ing the lowest hydrophilicity. The PES-OHmembrane has amore hydro-philic surface (67.81°) than the PES membrane, and this tendency wasattributed to the hydrophilic nature of the hydroxyl groups. The de-crease of contact angle indicated that a highly hydrophilic surface wascreated. After the graft polymerization reaction occurred, the contact

Fig. 4. BSA adsorption on the PES, PES-OH and PES-g-MPC membranes.

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angle of the PES-g-MPC membrane was significantly decreased furtherby immobilization of poly(MPC) onto the surface of the PES-OH mem-brane. Comparing with the PES-OH membrane, obvious decrease ofwater contact angle (46.43°) anddramatic increase of the surface energy(55.97 mJ/m2) were observed for the PES-g-MPC membrane, the valueof water contact angle is comparable with those previously reported inthe literatures [24,43]. The zwitterionic groups could form a hydrationlayer via electrostatic interaction in addition to the hydrogen bond[23,44], therefore, the introduction of MPC unit on the membrane sur-face could effectively enhance the hydrophilicity and improve antifoul-ing property of ultrafiltration membrane.

3.2. Membrane performance

3.2.1. Protein adsorptionThe static protein adsorption is one of the dominant factors in

determining the membrane fouling, and the reduction of proteinadsorption will enhance the antifouling property of membrane. Here-in, BSA was used as the model protein to evaluate the static proteinadsorption on the surface of the unmodified and modified mem-branes. The antifouling property of membrane is highly dependenton the membrane surface chemistry, such as surface charge character,surface free energy, surface chemical composition and surface mor-phology. In many cases, the nonspecific protein adsorption on themembrane surface due to the inherent hydrophobic characteristicoften causes serious membrane fouling. Therefore, the increment inthe surface hydrophilicity is a straightforward and effective methodto enhance the antifouling property of membrane.

As is shown in Fig. 4, the adsorption amount for measured proteinexhibited the following order: PES>PES-OH>PES-g-MPC. Interestingly,this tendency is very similar to the tendency found in the staticwater con-tact angle measurement. The PES membrane was found the highest pro-tein adsorption due to its hydrophobic character. BSA adsorption amounton the PES-OH membrane was lower than that on the PES membrane.This may be attributed to the introduction of the hydrophilic hydroxylgroups. The PES-g-MPC membrane exhibited the lowest adsorptionamount among the three investigatedmembranes. In general, the proteinresistant chemical structures are commonly hydrophilic, overall electri-cally neutral, hydrogen-bond acceptors but not hydrogen-bond donors[45], and the zwitterionic groups share all of these common characteris-tics. It is commonly believed that zwitterions form a regular hydrationlayer via electrostatic interaction in addition to hydrogen bond, and theprotein was excluded from the hydration layer to avoid the substantialentropy loss caused by the entrance of large protein molecules into thehighly structural zwitterionic layer [46]. Our study indicated that thegrafting of poly(MPC) onto the surface of the PES-OH membrane de-pressed the protein adsorption effectively. The experimental result wasconsistent with the general antifouling mechanism for zwitterionicmembrane [17,23].

3.2.2. Permeation properties of the unmodified and modified membraneUltrafiltration experiments were carried out to investigate the

separation performance of the unmodified and modified membranes.Fig. 5 presented time-dependent flux during ultrafiltration operation.The pure water flux (Jw,0) of the PES membrane is 156.3 L/m2h, whichis lower than that of the PES-OHmembrane. This result may be attrib-uted to the increased hydrophilicity and surface morphology change

Table 2Water contact angles and surface free energies of the PES, PES-OH and PES-g-MPCmembranes.

Samples

PES PES-OH PES-g-MPC

Contact angle (°) 82.90±2.37 67.81±1.44 46.43±1.93Surface energy (mJ/m2) 33.71±1.45 42.92±0.88 55.97±1.18

of the PES-OH membrane. The PES-g-MPC membrane exhibited thelowest pure water flux among the three investigated membranes.This result could be proved at quantitative degree by determiningthe average pore size. The average pore sizes of the three investigatedmembranes were calculated based on the Eq. (3). In this study, thePES-OH membrane has the highest average pore size (11.76 nm),whereas the PES-g-MPC membrane has the lowest value (7.23 nm),which agree with the water flux measurement. The average poresize of the PES membrane is 10.34 nm, which had inconspicuous dif-ferent from the PES-OH membrane, also clearly had a slight smallpure water flux. Furthermore, the phenomenon could be alsoexplained as follows: the graft polymerization usually occurs in thepores of the ultrafiltration membrane, which will cause these poresbecome smaller or blocked and the surface becomes more dense,meanwhile any defect on the surface is also repaired, and leading tothe pure water flux was seriously decreased as a result of low porosityand reduced pore size.

As shown in Fig. 5, the flux decreased dramatically at the initialoperation of BSA solution ultrafiltration due to membrane foulingcaused by protein adsorption or deposition on the membrane surface.When the adsorption and deposition of protein molecules may reachequilibrium, a relatively steady flux (Jp) was retained in the final op-eration of BSA solution ultrafiltration. The BSA rejection ratio (R) ofthe PES membrane is 98.4%, and that of the PES-OH membrane is95.7%. However, the PES-g-MPC membrane has an excellent BSA

Fig. 5. Time-dependent fluxes of the PES, PES-OH and PES-g-MPC membranes duringthe protein ultrafiltration experiment.

134 Y. Liu et al. / Desalination 316 (2013) 127–136

rejection ratio of 99.9%. The decrease of pore size of the modifiedmembrane had a positive effect on BSA rejection.

After 2 h of BSA solution filtration, the membranes were washedthoroughly and passed through deionized water for another 30 min,and the water fluxes of the cleaned membranes (Jw,1) were measuredagain. FRR is introduced to reflect the resistant fouling ability of themembranes, and higher value of FRR means the higher resistant foul-ing ability. The FRR values were calculated and presented in Fig. 6. TheFRR value is only 60.6% for the PES membrane, meaning the existenceof serious membrane fouling. The PES-OH membrane has a larger FRRvalue (74.5%), suggesting that adsorbed and deposited protein on themembrane surface could be easily washed away. The zwitterionicPES-g-MPC membrane has the highest FRR value (87.1%), which isconsistent with the protein adsorption experiments. The zwitterionicpoly(MPC) chains can prevent direct contact of BSA molecules withthe membrane surface, and the protein fouling in the PES-g-MPCmembrane was suppressed significantly under the ultrafiltrationprocess.

3.2.3. Fouling analysis of the unmodified and modified membranesDuring the ultrafiltration process, the rejected protein molecules

adsorbed or deposited on the surface and inside the membrane porescausing membrane fouling, which could be further divided into revers-ible and irreversible fouling. One part of fouling, recognized as revers-ible fouling, was caused by reversible protein adsorption or depositionand could be eliminated through hydraulic cleaning (hydrodynamicmethod); while the other part of fouling, defined as irreversible fouling,could not be eliminated only through hydraulic cleaning. More detailedresults of the total fouling ratio (Rt), the reversible fouling ratio (Rr) andthe irreversible fouling ratio (Rir) of the three investigated membranesare given in Fig. 6. It can be seen that the Rt value of the PES membraneis larger than that of other two types of membrane. The bigger Rt valueindicates higher total flux loss, corresponding to more protein adsorp-tion and deposition on the membrane surface. Meanwhile, the Rirvalue of the PES membrane is the largest among three types of mem-brane. It can be concluded that the protein fouling on the PES mem-brane is so serious that the fouling cannot be removed by hydrauliccleaning. In addition, the zwitterionic PES-g-MPC membrane has notonly the lowest Rt value but also the lowest Rir value, whichwas alreadyconfirmed in protein adsorption experiments. Generally speaking,when protein molecules contacted with the membrane surface, watermolecules between protein and the membrane surface would be

Fig. 6. Summary of the flux recovery ratio (FRR), the total fouling ratio (Rt), the revers-ible fouling ratio (Rr) and the irreversible fouling ratio (Rir) of the PES, PES-OH andPES-g-MPC membranes during the protein ultrafiltration experiment.

replaced. The high surface coverage of poly(MPC) segments leads to aspontaneous rearrangement of the polymer to form molecular brushand strong hydration layer due to electrostatic interaction by the prox-imity between phosphate group and quaternary ammonium group. Thezwitterionic poly(MPC) can take up large quantities of free water,which possibly prevents protein molecules from close contact withthemembrane surface. These results indicated obviously that the intro-duction of MPC units efficiently reduces total membrane fouling, espe-cially irreversible membrane fouling.

3.2.4. Recycling properties of the unmodified and modified membranesIn the practical application, the ultrafiltrationmembrane should keep

long-term antifouling property which induced a decrease of productioncost by decreasing the energy consumption and the cleaning frequencies.A long-termultrafiltration experimentwith three runswas carried out tofurther investigate the recycling properties of the three investigatedmembranes, and the results are shown in Fig. 7. After three times ofBSA ultrafiltrationwith a total operation time of 10 h and correspondingthree times of hydraulic cleaning, the pure water flux of the PES mem-brane was decreased from 156.3 to 61.2 L/m2h and the flux of BSAsolution was only 40.3 L/m2h. However, the pure water flux of thePES-g-MPCmembrane can retain at 107.8 L/m2h, theflux of BSA solutioncould keep at 64.5 L/m2h after three times of BSA solution ultrafiltration.The performance of the PES-OH membrane was intermediacy betweenthe PES and PES-g-MPCmembrane. The water flux recovery ratio duringeach cycle could be calculated to indicate the extent of cleaning efficiencyor the effect of irreversible fouling resistance of the membranes, and theresults are presented in Fig. 8. For the PES membrane, the water flux re-covery ratio was 60.6% in the first cycle, and the value increased to 76.6%in the second cycle, and reached 84.4% in the third cycle. It reflected anirreversible fouling that was 39.4% in the first cycle, 23.4% in the secondcycle, and 15.6% in the third cycle. The amount of newly formed irrevers-ible adsorption of protein was still significant. For the PES-g-MPC mem-brane, the water flux recovery ratio was 87.0% in the first cycle, 92.3% inthe second cycle, and 95.5% in the third cycle. The degree of irreversiblefouling was significantly reduced after each cycle, which ensured highwater flux recovery in each cycle. The excellent performance of the zwit-terionic PES-g-MPC membrane showed that the modified membranecould be reused for a longer time without compromising the waterflux. On the whole, the incorporation of zwitterionic poly(MPC) seg-ments on the PES-OHmembrane effectively reduced the totalmembranefouling especially irreversible membrane fouling, which is an appropri-ate method for preparation of highly effective antifouling ultrafiltrationmembrane.

Fig. 7. Time-dependent recycling fluxes of the PES, PES-OH and PES-g-MPC membranesduring the protein ultrafiltration experiment of three recycles.

Fig. 8. The flux recovery ratio (FRR) of the PES, PES-OH and PES-g-MPC membranesafter different cycles of the protein ultrafiltration experiment.

135Y. Liu et al. / Desalination 316 (2013) 127–136

4. Conclusions

It is well known that serious fouling of the ultrafiltration membranerestricts its application,which could result from the excessively adsorp-tion of protein through the hydrophobic interaction between themem-brane surface and protein molecules. The main advantage of theintroduction of hydrophilic zwitterionic groups is to modify the surfacechemical property and improve antifouling property of the ultrafiltra-tion membrane. This was achieved in this present article by redoxgraft polymerization using CAN as initiator to successfully graft zwitter-ionic MPC monomer onto the surface of the PES-OH membrane. Thegrafting process is straightforward and effective which avoid the com-plex and unfavorable pretreatment process and/or hydroxylated treat-ment because of the presence of the activated hydroxyl groups on thesurface of the PES-OH base membrane. Thus, the PES-OH membranecould be easily modified by reaction with the zwitterionic monomersto obtain the desired properties. FTIR-ATR and XPS studies confirmedthe graft polymerization performed on the surface of the PES-OHmem-brane. Static water contact angle test showed that some extent of hy-drophilic behavior was obtained for the PES-g-MPC membrane. Themodified membrane has higher flux recovery ratio and lower extentof membrane fouling, as compared to those of the PES and PES-OHmembrane. The excellent antifouling property renders the zwitterionicPES-g-MPC membrane longer operation life, which is quite beneficialfor practical application.

Acknowledgments

This work was supported by a grant from the National High Tech-nology Research and Development Program of China (863 Program)(No. 2012AA03A212).

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