Desalination 328 (2013) 58–66

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Desalination

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Modulating pore size and surface properties of cellulose microporousmembrane via thio-ene chemistry

Tao Yuan, Jianqiang Meng ⁎, Xiangzhuang Gong, Yufeng Zhang, Mingli XuState Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, China

H I G H L I G H T S

• A regenerated cellulose membrane (RCM) was grafted using thio-ene chemistry.• The graft density decreases obviously with the graft PEG chain length.• The membrane pore size decreases with the increased graft density.• The RCM-g-PEG2000 membranes have the best antifouling performance.• The graft density is more influencing than chain length on membrane properties.

⁎ Corresponding author. Tel.: +86 22 83955078; fax: +E-mail address: jianqiang.meng@hotmail.com (J. Men

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2013.08.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 June 2013Received in revised form 22 August 2013Accepted 24 August 2013Available online xxxx

Keywords:Regenerated cellulose membraneGraft chain lengthGraft densityAntifoulingPore size

Thio-ene chemistry was used to modulate pore size and surface properties of regenerated cellulose membranes(RCM). A series of thiol PEGs (PEG-SH) (Mn, ca. 2000, 6000 and 10,000 g/mol)were tethered onto vinylated RCMsurface. The membrane surface was analyzed by ATR–FTIR, XPS and SEM. The thio-ene reaction conditions, in-cluding the UV irritation time, the 2,2-dimethoxy-2-phenylacetophenone (DMPA) concentration and the PEG-SH concentration, were optimized to achieve high graft yields. The highest graft densities for PEG2000,PEG6000 and PEG10,000 are 1.37 nm−2, 0.32 nm−2 and 0.10 nm−2, respectively. Membrane pore size wasdetermined using the Guerout–Elford–Ferry method. The antifouling performance was evaluated by the proteinfiltration experiment. Effects of graft chain length and graft density onmembranemorphology, pore size, perme-ation and antifouling properties were systematically studied. The results indicate that the change of membranemorphology, pore size and permeation properties with PEG grafting is more closely related to graft densitythan the chain length. The attainment of high graft density is more important than long chain length to achieveexcellent antifouling performance. A series of membranes with pore size ranging from 70 to 140 nm wereobtained.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Surface modification of microporous membranes is a major interestformembrane chemists. Significant amount of work has been dedicatedto membrane surface modification to introduce novel functionality[1–3], to improve antifouling performance [4–8] as well as to adjustthe pore size and separation scales [9–11]. Because more permanentsurface modification can be achieved, surface grafting has attractedmore attention than surface coating [12,13]. Grafting-to and grafting-from are two approaches for surface grafting. For the grafting-to process,end-functionalized polymer chains are grafted to the solid substrate;while for the grafting-from process, the grafting reaction proceeds bypolymerization initiated from the surface. Although the grafting-tometh-od enables tailoring of the structure and individual characterization of the

86 22 83955055.g).

ghts reserved.

tethered polymer chains beforehand, it often has limited graftingdensities. The click chemistry has opened up a new path towardshigh density grafting of polymer chains onto the membrane surfacevia the grafting-to method [14–17]. We have successfully synthesizeda series of nanoporousmembranes via surface grafting on a polysulfonemembrane [17]. The membrane was demonstrated to have molecularsieving capacity when filtrating dye solution and antifouling propertieswhen filtrating protein solution and oily microemulsion. The pore sizecan be tuned by themolecularweights of the grafted poly(ethylene gly-col) (PEG) and the antifouling properties can be ensured by denselygrafted PEG forming a hydrophilic layer. The tethering of PEGs withpredetermined molecular weight (MW) and high graft density wasachieved via click chemistry—the propargyl PEGs were grafted ontoazide functionalized polysulfone via copper(Ι) catalyzed azide-alkynecycloaddition (CuAAC) reaction. However, copper catalysis can be anissue for biomaterial application [18,19]. In recent years, metal-freeclick chemistries have attracted increasing attention. The thio-ene

59T. Yuan et al. / Desalination 328 (2013) 58–66

click chemistry is one of them [20]. The thio-ene reaction can becompleted efficiently in a few minutes under UV light or heating. Inaddition, the starting membrane we used was synthesized in the lab.The strategy of modulating membrane pore size and surface propertiesvia click chemistry need to be evaluated on commercial membranes.In this contribution, a commercial microfiltration (MF) membrane,regenerated cellulose membrane (RCM), which is considered to be aversatile platform for surface modification due to the presence of largeamount of hydroxyl group, was used as the startingmembrane. A seriesof thio PEGs (PEG-SH) having different MWs (Mn = 2000, 6000 and10,000 g/mol) were tethered onto RCM surface via thio-ene couplingreaction. The RCM surface was anchored with the vinyl group before-hand with the alkoxysilane chemistry via the reaction of the hydroxygroup on the membrane surface with trimethoxyvinylsilane (seeScheme 1). The effects of the graft chain length and graft density (GD)on membrane pore size and antifouling performance were systemati-cally studied.

2. Materials and methods

2.1. Materials

Regenerated cellulose membrane (RCM) discs (average pore diame-ter 0.2 μm, 47 mm diameter, 200 μm thickness) were purchased fromSartorious Stedium Biotech Inc. (Göttingen, Germany). Thio-lated PEGs(PEG-SH) (Mn = 2000, 6000, 10,000 g/mol) were synthesized followingthe procedure in literature [21].

Mercaptoacetic acid (≥98%), trimethoxyvinylsilane (95%), 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%), p-toluenesulfonic acid(98%) and bovine serum albumin (BSA, Fraction V) were purchasedfrom Aldrich. Deionized (DI) water was supplied by a water treatmentsystem composed of reverse osmosis and ion exchange, Tianjin Motian

O CH2 CH2 O Cn

O

CH2 SH= C

OH2CHS

RCM-g-PEG

UV or heat

RCM

OHOH OHOH

OHOHOH

OH

OH

OH

OH

OH

OH

OH

OHOH

OH OH

OH

OH OH OHOH

OH

RCM-Vi

= Thiol

= Alkene

= RCM-g-PEG

O CH2 CH2 O Cn

O

CH2 SH= C

OH2CHS

ethanolpH=3.5

2 h

trimethoxyvinylsilane3 h

Si

O

OO

Scheme 1. Scheme of the RCM grafting via the thio-ene reaction.

Membrane Engineering Technology Co., Ltd. All the chemicals wereused as received unless otherwise noted.

The bovine serum albumin solution in phosphate buffer (BSA/PBS)was used as the model protein solution for fouling test. To preparethe BSA/PBS solution, 1.00 g of BSA was dissolved in 1 L of PBS hav-ing 8.00 g (137 mmoL) of sodium chloride, 0.20 g (2.7 mmoL) ofpotassium chloride, 1.44 g (10.1 mmoL) of disodium hydrogenphosphate and 0.24 g (1.76 mmoL) of potassium dihydrogen phos-phate (pH = 7.4).

2.2. Synthesis of the vinylated cellulose membrane (RCM-Vi)

Before the synthesis of RCM-Vi, cellulose membranes were rinsedby 95% ethanol for 1 h at 30 °C to remove impurities. 2.0 mL oftrimethoxyvinylsilane was dissolved in 33.7 mL of ethanol and the pHwas adjusted to 3.5 with glacial acetic acid. The solution was stirred for2 h at room temperature in order to hydrolyze trimethoxyvinylsilanecompletely. Then the rinsed cellulose membrane was added to the solu-tion and the mixture was stirred at room temperature for 2 h. After that,the cellulosemembranewas dried at 120 °C for 2 h. Then themembranewas rinsed with ethanol for 24 h and with DI water for another 24 h.Finally the vinylated cellulose membrane was dried to constant weightunder vacuum at 40 °C and stored in desiccator.

2.3. Synthesis of the RCM-g-PEG membrane via thio-ene reaction

The RCM-g-PEGmembranes were synthesized via thio-ene reactionbetween the vinyl group on RCM-Vi membrane surface and PEG-SHmolecules. The reaction was conducted in a quartz flask. For a typicaloperation, 2.00 g of PEG-SH was dissolved in 15 mL of DI water andwas stirred for 30 min under nitrogen environment. 0.05 g of DMPAwas dissolved in 20 mL of acetone and added in the flask. The reactionwas kept under UV light (λ = 365 nm) for 10 min. Then the mem-brane was washed with ethanol for 24 h and with DI water for another24 h. Finally, themodifiedmembrane was dried under vacuum at 50 °Cfor 3 h and stored in desiccator. The grafting yield was calculated byEq. (1)

GY ¼ m1−m0

Að1Þ

where m0 is the mass of the RCM-Vi membrane (g), m1 is the mass ofRCM-g-PEG membrane (g), and A represents the area of the membrane(cm2). All the results were the average value of three membranes. Themembrane was weighed with an analytical balance to a precision of0.1 mg.

2.4. Polymer and membrane characterization

The chemical structure of PEG-SH was analyzed by 1H-NMR using aBruker 400-MHz spectrometer with CDCl3 as the solvent. FTIR spectraof RCM, RCM-Vi and RCM-g-PEG membranes were recorded using aVector-22 spectrometer (Bruker Daltonic Inc., Germany) with zincselenide (ZnSe) as an internal reflection element at an incident angle of45°. Each spectrum was collected by the accumulation of 16 scans at aresolution of 4 cm−1. The spectra were recorded in wave number of600–4000 cm−1 at 25 °C. The XPS measurements were conducted on aQuanta200 spectrometer (FEI Co., Ltd. USA) using monochromated AlK radiation generated from an electron beam operating at a passenergy of 93.9 eV. Survey spectra were run in the binding energyrange 0–1000 eV, followed by high-resolution scan of the C1s region.Binding energies (BEs) were calibrated using the containment carbon(C1s = 285.0 eV). The surface morphology of the membranes was ob-served by a field emission electronmicroscopy (FESEM, Hitachi S-4800,Japan). Representative samples of the membranes were cut into0.5 cm2, attached with double-sided tape to steel stabs, and shadowed

SH CH2 C

O

O CH2 CH2 OnC

O

CH2 SH

F1

Fig. 1. 1H-NMR spectrum of PEG10,000-SH.

500 1000 1500 2000 2500 3000 3500 4000 4500

δ (=CH2,=CH)

vs(CH2)

vas (Si-O-Si),

Si-OH,Si-O-C

vs(Si-O-Si)

vas (C-Si)

cb

Wave number (cm-1)

a

v(C=O)v(C=C)

Fig. 2.ATR–FTIR spectra of RCMmembrane (a), RCM-Vimembrane (b) and RCM-g-PEG6000membrane (c).

60 T. Yuan et al. / Desalination 328 (2013) 58–66

with gold prior to SEM measurements. Specific surface area (SSA) ofmembranes was measured by the BET method using a MicromeriticsASAP-2020 (Micromeritics, USA). All samples were degassed at 105 °Cfor 4 h prior to each adsorption experiment to remove adsorbedcontaminant. The membrane porosity can be obtained and calculatedby Eq. (2) following the procedure in the literature [22].

ε ¼ fluid uptake volumetotal volume

¼Wwet−Wdry

� �ρdry

Wwet−Wdry

� �ρdry þWdry ρsol

ð2Þ

where ρdry is the dry membrane density (g/cm3), ρsol is the ethanolaqueous solution density (g/cm3), and Wwet and Wdry are the wetmembrane weight and the dry membrane weight (g), respectively.

2.5. Filtration experiment

The filtration experiments were conducted using a combination oflab-scale units containing a dead end testing cell and a crossflow testingcell. The dead end testing cell has an effective area of 5.7 cm2 and thecross flow testing cell has an effective area of 12.5 cm2. The feed solu-tion temperature of the cross flow cell was maintained at 25 °C by acooling coil connected to a cycle refrigerator inside the feed tank. Thewater flux of membranes was calculated by Eq. (3)

J ¼ VS � Δt ð3Þ

where J is thewater flux ofmembranes (L/m2·h·bar),V is the volume ofthe filtrate water (L), S is the effective area of membrane (m2) and Δtrepresents the testing time (h).

For the antifouling experiments, at the beginning, the membranewas pressurized with pure water under 1.0 bar. The initial pure waterflux Jwi was recorded. Then the feed solution was changed into theBSA/PBS solution. When the membrane flux became steady, the purewater flux of the resulted membrane was recorded as Jm. After that,the membrane was rinsed with DI water for 1 h. The pure water fluxof the cleaned membrane was recorded as Jww. The water flux recovery(FR) was calculated from Jwi and Jww by Eq. (4) [23]

FR ¼ Jww

Jwi� 100%: ð4Þ

The above equation can be written with irreversible fouling byEq. (5)

R ¼ Jww

Jwi¼ 1− Rir

Rm þ Rir

� �� 100%: ð5Þ

where Rir is irreversible filtration resistance (m−1) and Rm is the intrinsicmembrane resistance (m−1). Rm, Rir and Rc can be calculated by Eqs. (6),(7) and (8).

Rm ¼ Δpη Jwi

ð6Þ

Rir ¼Δpη Jww

−Rm ¼ Δpη Jww

− Δpη Jwi

ð7Þ

Rc ¼Δpη Jm

sm−Rm−Rir ð8Þ

whereΔp is trans-membrane pressure (Pa), η is the feed viscosity (Pa·s),and Rc is the cake layer formation resistance (m−1).

3. Results and discussion

3.1. Synthesis of PEG-SH

High functionality degree of the thiol group in PEG-SH is a prerequisitefor fair yield of the thio-ene graft reaction. The nominal pore size of theRCM membrane is about 0.2 μm, which is much bigger than that usedin our previous work [17]. Therefore PEG-SHs having high MW wereused tomodulatemembrane size. Herein the acid catalyzed esterificationwas used to prepare PEG-SH from PEG-OH. Fig. 1 shows the 1H-NMRspectra of PEG-SH (Mn, 10,000 g/mol) in CDCl3. The peaks at 3.25 ppm,4.29 ppm and 3.65–3.83 ppm can be assigned to the methylene of the\CH2SH,\CH2\O\C(_O)\ and\OCH2\CH2\O\ group, respective-ly. By comparing the intensity of these groups, the functionality degrees ofthe thiol group for PEG2000, PEG6000 and PEG10,000 were calculated tobe 97%, 90% and 79%, respectively.

3.2. Chemical composition of the membrane surface

The surface chemical composition of RCM membrane, RCM-Vimembrane and RCM-g-PEG membrane was characterized by ATR–FTIRspectroscopy. As shown in Fig. 2, the adsorptions at 1400 cm−1 (δ_CH2

and δ_CH), 1600 cm−1 (νC_C) and 2960 cm−1 (νs CH2) in the spectrum

of RCM-Vi membrane are characteristic vibrations of the vinyl group.

0 200 400 600 800 1000 1200 1400

cb

Binding energy (eV)

a

Fig. 3. XPS spectra of RCM membrane (a), RCM-Vi membrane (b) and RCM-g-PEG mem-brane (c).

0.15

0.20

0.25

0.30

0.35

0.40 c

0 2 4 6 8 10

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

DMPA Concentration (mmol/L)

b

4 6 8 10 12 14 16 18 20 220.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Gra

ftin

g yi

eld

(mg/

cm2 )

G

raft

ing

yiel

d (m

g/cm

2 )

raft

ing

yiel

d (m

g/cm

2 )

UV irritation time (min)

a

61T. Yuan et al. / Desalination 328 (2013) 58–66

The adsorptions at 1276 cm−1 (νSi–C) and those between 1180 and690 cm−1 (νSi–C, νas Si–C, νas Si–O–Si and νs Si–O–Si) are characteristic ofsilicon based linkages. In the spectrum of RCM-g-PEGmembrane, the ad-sorptions at 1750 cm−1 (νC_O) and 2960 cm−1 (νs CH2

) can be attributedto the ester carbonyl group and themethylene group, respectively. Theseobservations indicate successful grafting of the vinyl group and the PEGchains.

The membrane surface was analyzed by XPS in order to obtain amore quantitative characterization of the surface chemical composi-tions. The XPS spectra for RCM, RCM-Vi and RCM-g-PEG membranesare depicted in Fig. 3. The spectrum of RCM displays two main signalsat 533 eV and 285 eV, which are associated with oxygen (O1s) and car-bon (C1s), respectively. For the spectra of RCM-Vi and RCM-g-PEGmembranes, the presence of the peak at 103 eV assigned to Si 2p indi-cates successful vinylation reaction and the depression of this peak inRCM-g-PEG spectrum is due to the formation of a grafted PEG layer onmembrane surface. The corresponding composition of these membranesurfaces is listed in Table 1. The C/O ratio increases from 0.79 to 0.92after the vinylation reaction, indicating that RCM surface is richer in ox-ygen atom than RCM-Vi membrane. This happens because the attachedvinylsilane group has lower oxygen content than the cellulose. The de-crease of C/O content ratio after PEG grafting indicates successful tether-ing of PEG chains on the membrane surface. Similar to our previouswork, control experiments without UV irradiation also indicate thatthe presence of PEG on membrane surfaces was due to the chemicalreaction rather than physical adsorption [17].

3.3. Optimization of the thio-ene reaction conditions

Tethering of polymer chains ontomembrane surface via the “graft-to”method remains a challenging task to polymer chemists. The develop-ment of click chemistry opens up a new route and enables individual syn-thesis and characterization of the pre-formed polymers before grafting.However, the “click” coupling reaction conditions still need to be tailoredto pursue high graft yields (GY) [24]. In this study, the reaction conditionsintrinsic for UV-induced polymerization were studied systematically to

Table 1Surface element analysis of RCM membrane, RCM-Vi membrane and RCM-g-PEG mem-brane.

Sample O1s [mol%] C1s [mol%] Si [mol%] n(C)/n(O)[mol/mol]

RCM 55.94 44.06 0 0.79RCM-Vi 47.86 44.14 8.00 0.92RCM-g-PEG 54.13 41.69 4.18 0.77

investigate their effects on grafting yields. The conditions include theUV irritation time, the DMPA concentration, and the PEG-SH concentra-tion. The results are shown in Fig. 4. It can be seen from Fig. 4(a) thatthe GY increases progressively with UV irritation time up to 10 min.This happens because the number of active sites on the membranesurface increasedwith the irritation time. However, no additional benefitwas observed for longer UV irritation time because the number of

0 5 10 15 20 25 30 35 400.00

0.05

0.10

PEG-SH concentration (g/L)

G

Fig. 4. Effects of UV irritation time (a), DMPA concentration (b) and PEG-SH concentration(c) on the GY of thio-ene reaction.

62 T. Yuan et al. / Desalination 328 (2013) 58–66

available sites for photo-grafting decreased due to themembrane surfacebeing covered by grafted PEG.

The effect of DMPA concentration on the GY is shown in Fig. 4(b). Itcan be seen that 2 mmol/L of DMPA is sufficient to achieve a fair GY. Theincrease of DMPA concentration only brought about trivial benefits onthe GY. We believe that the increase of DMPA concentration shouldlead to high concentration of PEG macro-radicals. But this benefitcould be discounted by the diffusion hindrance of the macro-radical tothe membrane surface. For the DMPA concentration over 7 mmol/L,higher DMPA concentration even led to lower GY. The reason may bethat the concentration of PEG-SH radicals increases with DMPA concen-tration, leading to intensive termination reactions, which is a competi-tive reaction of the thio-ene reaction [25,26].

The effect of PEG-SH concentration on the GY is shown in Fig. 4(c). Itcan be seen that PEG-SH has an obvious effect on the GY. At lower con-centrations, GY increasedquicklywith the PEG-SH concentration. This iseasily understood because PEG-SH is one of the reactants for thio-enereaction. However, the GY decreased quickly when the PEG-SH concen-tration exceeded 17 g/L. This can be explained by the increased viscosityof the PEG-SH solution and the cumbered diffusion of PEG-SHmoleculesfrom the solution to the membrane surface.

3.4. The graft density

There are quite a few reports making detailed studies on effects ofthe graft density (GD) on membrane adsorption and filtrationproperties [27–29]. The “graft-from” strategy was used in these reportsand the reported GDs are actually based on the assumption of 100% ini-tiator efficiency. This probably is far from the actual situation of asurface-initiated polymerization process because the premature termi-nation is inevitable [30,31]. The “graft-to” strategy enables characteriza-tion of the polymer chain for grafting and more accurate calculation ofthe GD as the grafted molecular weight is predetermined [14,17]. TheGD can be calculated as following

GD ¼ GYSSA �MW

ð9Þ

where GY is the grafting yield of PEG (μg/cm2), MW is the molecularweight of grafted PEG (g/moL) and SSA is the specific surface area ofthe membrane (m2/g). By tuning the thio-ene reaction condition, wewere able to synthesize a series of membranes with varying GDs foreach predetermined PEGMW so that the GD effects can be decomposedand studied independently. The GD of the obtained RCM-g-PEG mem-branes was shown in Table 2. It can be seen that the highest GDachieved for each PEG decreased dramatically with the grafted MW.The highest GD for PEG2000, PEG6000 and PEG10,000 are 1.37 nm−2,0.32 nm−2 and 0.10 nm−2, respectively. The highest GD for PEG10,000 is less than one tenth of that for PEG2000. We believe thatthis is due to the increased diffusion hindrance and steric hindrancewith increased PEGMW and embedding of the\SH group in the PEGcoil structure. It should be mentioned that the GDs achieved in thisstudy are all far below those through the copper(Ι) catalyzed azide-alkyne cycloaddition (CuAAC) reaction in our previous work [17]. Webelieve it was also most related to the higher grafted MW used in thiswork.

Table 2The GY and GD of RCM-g-PEG membranes.

ID RCM RCM-g-PEG2000-1

RCM-g-PEG2000-2

RCM-g-PEG2000-3

RCM-g-PE6000-1

UV time [min]a – 6 8 10 6GY [μg/cm2] – 185 225 275 69GD [nm−2] – 0.92 1.12 1.37 0.12

a The grafting reaction was conducted at DMPA concentration of 2 mmol/L and PEG-SH con

3.5. Morphology of the membrane surface

Themorphology of the pristine RCMmembrane and the RCM-g-PEGmembranes was observed by FESEM. The FESEM images for the surfaceof RCM membrane, RCM-g-PEG2000 membrane, RCM-g-PEG6000membrane, and RCM-g-PEG10,000 membrane are shown side by sidein Fig. 5 to investigate the effects of grafting chain length. It should bementioned that it is hard to obtain the membranes having exactly thesame GD for different PEGs. So the membranes having the highest GDfor each grafted PEG were used as the representatives. It is obviousthat a novel layer of hydrogel was developed on the membrane surfaceand the pore size was altered. This phenomenon is most obvious for theRCM-g-PEG2000 membrane. It seems that the grafted hydrogel layerof RCM-g-PEG2000 membrane is thicker than those of both RCM-g-PEG6000 and RCM-g-PEG10,000 membrane, indicating that the mor-phology change with the PEG grafting is not closely related to thegraft chain length. In Fig. 6, the images of RCM-g-PEG2000 membraneshaving different GDs are shown together to investigate the GD effect. Itcan be seen that the grafted hydrogel layer appears thicker for themem-brane with the higher GD. But even for the membrane having thehighest GD, the pore shrinkage is limited and the pore filling is notobvious.

3.6. Permeation and antifouling properties of the membrane

The effects of the grafted PEG chain length and density on purewater permeance of the membrane were studied and the results areshown in Table 3. It can be seen that PEG grafting brings about dramaticflux decline. The unmodified RCM membrane has a pure waterpermeance of 1447 ± 11.6 L/m2·h·bar, while the lowest pure waterpermeance of RCM-g-PEG membranes is only 200 L/m2·h·bar or so. Inaddition, the effect from the GD is also obvious. The RCM-g-PEG2000membrane with the lowest GD has a pure water permeance five timesthat of the RCM-g-PEG2000membranewith the highest GD. In contrast,as for the SEM results discussed in Section 3.4, pore filling was notobserved and the RCM-g-PEG2000 membranes having different GDsshowed quite similar morphologies. This happens because the graftedPEG chains are in a collapsed conformation in a completely dry stateunder SEMmeasurements while they are stretched in a highly swollenstate under the permeation tests.

Non-specific adsorption plays a key role in membrane fouling for itsapplication in bioseparation. As one of the most adhesive proteins, BSAwas used as a model foulant and the antifouling properties ofmembranes were evaluated via cross flow filtration of the BSA solution.Although significant permeability loss was observed from PEG grafting,the RCM-g-PEG membranes prepared by grafting PEG with differentMWs and densities are supposed to target at separation of moleculesat different scales. So it makes more sense to compare antifouling per-formance using the membrane flux normalized by the initial flux thanusing the apparent flux value. The flux profile of membranes underBSA solution filtration was shown in Fig. 7. Fig. 7(a) shows the fluxplots of RCM-g-PEG2000 membranes having different GDs. It can beseen that all the membranes show flux decline due to protein adsorp-tion and convective deposition. The RCM membrane shows quickerflux decline and lost nearly 90% of the initial flux after 60 min of the fil-tration, indicating severe cake layer formation and pore blockage. On

G RCM-g-PEG6000-2

RCM-g-PEG6000-3

RCM-g-PEG10,000-1

RCM-g-PEG10,000-2

RCM-g-PEG10,000-3

8 10 6 8 10115 190 31 82 101

0.19 0.32 0.03 0.08 0.10

centration of 17 g/L.

c

a b

d

Fig. 5. Membrane surface morphologies of RCM membrane (a), RCM-g-PEG2000 membrane (GD = 1.37 nm−2) (b), RCM-g-PEG6000 membrane (GD = 0.19 nm−2)(c) and RCM-g-PEG10,000 membrane (GD = 0.10 nm−2) (d).

63T. Yuan et al. / Desalination 328 (2013) 58–66

the other hand, RCM-g-PEG2000 membranes reached to a steady flux,which is 70% of the initial flux, within 30 min of filtration, indicatingmuch better anti-fouling performance. In addition, the flux recovery ofthree RCM-g-PEG2000 membranes after pure water rinsing was morethan 85%, indicating effective depression of the irreversible membranefouling and high rinsing efficiency. Yang and coworkers [28] have stud-ied adsorption behavior of three proteins on grafted glycopolymerbrushes and found that increasing grafting density caused loweradsorbed amounts for all proteins and larger protein caused more

c

a

Fig. 6. Membrane surface morphologies of RCM membrane (a), RCM-g-PEG2000 membr(c) and RCM-g-PEG2000 membrane with GD of 1.12 nm−2 (d).

pronounced adsorption reduction. In our study, the three membraneswith different GDs for PEG2000 show almost the same flux profile. Webelieve that this happens due to the lowest GD, which is 0.92 nm−2, isalready sufficient for PEG2000 to form a decent hydrophilic PEG layeron membrane surface precluding the adsorption of BSA molecules.

It can also be seen from Fig. 7(b) and (c) that RCM-g-PEG6000 andRCM-g-PEG10,000 membranes have better antifouling performancesthan the RCM. However, more obvious effects from the GD can beobserved for RCM-g-PEG6000 and RCM-g-PEG10,000 than for RCM-

b

d

ane with GD of 0.63 nm−2 (b), RCM-g-PEG2000 membrane with GD of 0.92 nm−2

Table 3The permeation property, filtration resistance and pore size of RCM and RCM-g-PEG membranes.

ID rp [nm] re [nm] HT [nm] Jwi [L/m2·h·bar] Jm [L/m2·h·bar] Jww [L/m2·h·bar] Rm [×1011 m−1] Rir [×1011 m−1] Rc [×1011 m−1] Rf [×1011 m−1]

RCM 198 76 – 1447 ± 12 212 ± 8 534 ± 7 2.9 5.1 12.3 20.3RCM-g-PEG2000-1 189 122 38 1321 ± 10 549 ± 8 686 ± 5 3.2 3.0 1.6 7.8RCM-g-PEG2000-2 138 107 45 699 ± 8 427 ± 4 481 ± 3 6.1 2.8 1.1 10.0RCM-g-PEG2000-3 95 87 51 334 ± 6 265 ± 6 283 ± 5 12.7 2.4 0.9 16.0RCM-g-PEG2000-4 85 80 56 265 ± 11 227 ± 8 239 ± 7 15.9 2.0 0.8 18.6RCM-g-PEG2000-5 77 73 61 219 ± 9 190 ± 7 196 ± 6 19.9 1.6 0.7 22.2RCM-g-PEG6000-1 143 122 28 761 ± 12 450 ± 8 556 ± 8 5.7 2.0 1.9 9.5RCM-g-PEG6000-2 134 120 31 695 ± 12 450 ± 7 535 ± 6 6.2 1.8 1.6 9.6RCM-g-PEG6000-3 112 103 43 463 ± 9 354 ± 7 392 ± 9 9.3 1.7 1.1 12.1RCM-g-PEG10,000-1 192 141 3 1355 ± 29 547 ± 6 735 ± 8 3.1 2.7 2.0 7.8RCM-g-PEG10,000-2 157 134 20 919 ± 15 485 ± 7 664 ± 7 4.7 1.8 2.3 8.8RCM-g-PEG10,000-3 127 105 36 595 ± 12 406 ± 5 484 ± 6 7.2 1.6 1.7 10.5

64 T. Yuan et al. / Desalination 328 (2013) 58–66

g-PEG2000. There is a trend that higher GD leads to lower flux de-cline and higher recovery ratio, which is especially true for RCM-g-PEG10,000 membrane. This corresponds well with other reports thatthe protein adsorption decreases with the graft density [32–34]. It isbelieved that a brush conformation is more effective in preventingproteins from adsorption [35,36]. Wu et al. [37] have measured andfound that the crossover between the mushroom and brush regimeswas at the GD of 0.065 nm−2 for the grafted polyacrylamide with Mn of10,000 g/moL. In this study, all the RCM-g-PEG2000 membranes have

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Fig. 7. Normalized water flux of RCM-g-PEG2000 membranes with different GDs (a), RCM-g-PGDs (c) and RCM-g-PEGmembranes with different chain lengths (d), ( , RCM; , RCM-g-PEG20.92 nm−2; , RCM-g-PEG6000, GD = 0.32 nm−2; , RCM-g-PEG6000, GD = 0.19 nm−2

RCM-g-PEG10,000, GD = 0.08 nm−2; , RCM-g-PEG10,000, GD = 0.03 nm−2).

GDs well above this crossover. Therefore, all the membranes show fairantifouling performance and no obvious GD effects were observed.On the other hand, for the RCM-g-PEG6000 and RCM-g-PEG10,000membranes, the low GD RCM-g-PEG membranes only have the GDfalling at themushroom regime. The PEG grafting did not show enoughadvantage on antifouling performance and the GD effects are more ob-vious. It should be noted that the residue vinyl groups on membranesurface could also interfere and provoke nonspecific protein adsorptionat low GDs.

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EG6000 membranes with different GDs (b), RCM-g-PEG10,000 membranes with different000, GD = 1.37 nm−2; , RCM-g-PEG2000, GD = 1.12 nm−2; , RCM-g-PEG2000, GD =; , RCM-g-PEG6000, GD = 0.12 nm−2; , RCM-g-PEG10,000, GD = 0.10 nm−2; ,

65T. Yuan et al. / Desalination 328 (2013) 58–66

Fig. 7(d) compares the flux profile of RCM grafted with three PEGshaving different MWs. Noted that the GD varies with the graft chainlength for the “graft-to” method. Herein we compared PEG graft chainlength effects at their highest GDs achieved. It is different from the stud-ies using the “graft-from” method that high graft chain length was notable to show its benefit on fouling resistance [7]. This happens due tohighly discounted GD for high graft chain length. In fact, both the exper-imental and theoretical results show that the attainment of high graftdensity is more important than long chain length to achieve optimalprotein resistance [27–29].

Filtration resistancewas calculated to quantitatively describe the anti-fouling properties of membranes. The results were listed in Table 3. Forthe pristine RCM, Rc is much bigger than Rm, indicating that the cakelayer formation is the main reason for the fouling behavior. For theRCM-g-PEG membranes, the Rc values are all lower than Rm values andare all greatly depressed, indicating the depress of the cake layer forma-tion. It can also be seen that the RCM-g-PEG membranes have muchhigher Rm values than RCMmembrane. The pore shrinkage was believedto be the main reason. For the effects of the GD, it can be easily observedthat higher GD leads to lower Rc and Rir values for each grafted PEG. Butfor the effects of graft chain length, there is not an obvious trend by com-paring the average Rc and Rir values for different PEGs.

3.7. Pore size of the membrane

Since the objective of this study is to synthesize a series of antifoul-ing membranes having a wide range of screening capacities, the effectsof PEG grafting on pore size were addressed. The method based on fil-tration velocity is an importantmethod to determine average pore radi-us (rp). The average pore radius of membranes is calculated by thefollowing Guerout–Elford–Ferry (Eq. (10)) [22]

rp ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2:9−1:75εð Þ � 8η � l � Q

ε � A � Δp

sð10Þ

where η is the water viscosity (8.4 × 10−4 Pa·s), l is the membranethickness (m), Q is the volume of the permeate water per unit time(m3·s−1), A is the effective area (m2), ε is the membrane porosity andΔp is the trans-membrane pressure (Pa).

The calculated average pore radius (rp) is shown in Table 3. It can beseen that all of the rp values are decreased by PEG grafting for RCM-g-PEG2000 membranes with higher GDs. The pore radius of RCM-g-PEG2000 membrane having the highest GD was narrowed more than ahalf of the pristine RCM. Similar to the observations on permeation andantifouling properties, the effects of GD are obvious but the effects ofthe graft chain length are obscure. Again, tuning the GD is more effectivethan tuning the graft chain length to modulate the pore size. Herein wealso put forward an equilibrium pore size re, which is calculated fromthe steady flux under BSA filtration and is believed to bemore accuratelyreflecting the membrane filtration properties in real applications. It canbe seen that a wide range of pore radius (re) was obtained ranging from73 nm to 122 nm, even this range was narrowed by the BSA fouling.

Based on the difference between the hydrodynamic pore radii of thepristine and the RCM-g-PEG membranes, the average hydrodynamicthickness (HT) of the grafted PEG hydrogel can be obtained. The resultsare also shown in Table 3. Apparently higher graft density leads tohigher HT. In addition, the RCM-g-PEG2000 membranes have thehighestHT values, all of which are above 38 nm. This can be used to ex-plain the very low Rc values in the fouling experiment and excellent an-tifouling performance of the RCM-g-PEG2000 membranes. It shouldalso be mentioned that the HT values of the grafted PEG2000 layer ishigher than the completely stretched chain length of PEG2000. Thiscan be due to the high affinity of water molecules to the grafted PEGchains, which bind a large amount of water molecules [38–40].

4. Conclusion

A series of thiol PEGs (PEG-SH) having differentMWswere tetheredonto the RCM surface via thio-ene coupling reaction. The results showthat the graft density (GD) plays a more important role in adjustingmembrane pore size and surface properties. The RCM-g-PEG mem-branes exhibit better antifouling performance than theRCMmembrane.RCM-g-PEG2000membraneswith fair GDs have normalized purewaterflux recovery ratios over 90%. The resulted membranes have apparentpore size ranging from 70 to 140 nm or so in pure water conditionand equilibrium pore size ranging from 70 to 120 nm in protein filtra-tion condition, as shows potential in a wide range of filtration applica-tions. The click chemistry is a powerful and well-defined methodtuning the pore size and surface properties of microporousmembranes.

NomenclatureList of symbolsm0 the mass of the RCM-Vi membrane (g)m1 the mass of RCM-g-PEG membrane (g)A the area of the membrane (m2)S the effective area of membrane in filtration experiment (m2)GY the grafting yield (mg/cm2)GD the grafting density of RCM-g-PEG membrane (nm−2)SSA membrane specific surface area (m2/g)MW molecular weight of grafted PEG (g/moL)ε the membrane porosity (%)ρdry the dry membrane density (g/cm3)ρsol the ethanol aqueous solution density (g/cm3)Wwet the wet membrane weight (g)Wdry the dry membrane weight (g)J permeation flux of membrane (L/m2·h·bar)V the permeate volume (L)Δt the testing time (h)Jwi the initial pure water flux (L/m2·h·bar)Jm pure water flux of the polluted membrane without cleaning

(L/m2·h·bar)Jww the pure water flux of the cleaned membrane (L/m2·h·bar)FR the pure water flux recovery (%)Δp the trans-membrane pressure (bar)η the feed solution viscosity (Pa·s)Rm the intrinsic membrane resistance (m−1)Rir the irreversible filtration resistance (m−1)Rc the cake layer formation resistance (m−1)rp membrane average pore radius in aqueous solution (m)re equilibrium pore size of membrane in aqueous solution (m)HT the average hydrodynamic thickness of grafted hydrogel layer

(nm)L the average hydrodynamic thickness of grafted hydrogel layer

(nm)Q the volume of the permeate water per unit time (m3·s−1)

Acknowledgments

We gratefully thank the support from the National Nature ScienceFoundation of China (Grant no. 21274108) and the National High Tech-nology Research and Development Program of China (863 Program ofChina, 2012AA03A602). Prof. MJQ also thanks Prof. Zi-Chen Li (PekingUniversity) for helpful discussion and BET measurements.

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