RSC Advances

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Versatile surface

Department of Materials Science and Engi

Gwanak-ro, Gwanak-gu, Seoul 151-744, Kor

† Electronic supplementary informa10.1039/c6ra19020k

Cite this: RSC Adv., 2016, 6, 88959

Received 27th July 2016Accepted 12th September 2016

DOI: 10.1039/c6ra19020k

www.rsc.org/advances

This journal is © The Royal Society of C

charge-mediated anti-fouling UF/MF membrane comprising charged hyperbranchedpolyglycerols (HPGs) and PVDF membranes†

S. Y. Park, Y. J. Kim and S.-Y. Kwak*

We develop a charge-modified PVDF UF/MF membrane that restricts membrane fouling derived from

charged water contaminants. The charge-modified membranes are fabricated through surface assembly

of various charged hyperbranched polyglycerols (HPGs), which act as anti-fouling agents. The native

HPGs are neutrally charged polymers with a lot of hydroxyl end groups that can be modified with

specific charges. We prepare three types of charged HPGs, i.e., neutrally charged HPGs without end

group modifications, positively charged HPG with quaternary ammonium groups, and negatively charged

HPG with sulfonate groups. The combined results for 1H NMR, FT-IR, and XPS results show that the

charged HPGs are successfully synthesized and bound to the surfaces of the PVDF membrane. The

surface hydrophilicity improves upon assembly of the hydrophilic charged HPGs. As expected, zeta

potential results disclose that the differently charged HPGs provide the desired electrical properties to

the membrane surface. In addition, the surface-assembled charged HPGs effectively suppress the

attachment and accumulation of foulants having the same charge due to electrostatic repulsion and

improved surface hydrophilicity.

Introduction

Ultra/microltration (UF/MF) membranes can effectively purifywastewater by removing small pollutants, such as colloidalparticles and bacteria. Membrane ltration processes are rela-tively simple and economical for purifying contaminated watercompared with traditional wastewater treatment methods, suchas physical and chemical coagulation, disinfection, andmicroorganism-mediated decomposition;1,2 however, membraneperformance and lifetime will gradually decrease during a ltra-tion process due tomembrane fouling, which occurs through theaccumulation of contaminants on the membrane surface orinside the pores.3–5 Membrane fouling tends to be mediated byhydrophobic interactions between hydrophobic part in thecontaminants and a hydrophobic membrane surface.6

Approaches to suppressing hydrophobic contaminantmembrane fouling have focused on introducing hydrophilicmaterials, such as hydrophilic polymers or inorganic nano-particles, through coatings,7–9 graing,10–13 or blending5,14–17

methods. The hydrophilic surface layer hinders the adsorption ofhydrophobic foulants on themembrane surface and helps removeaccumulated fouling layers from the membrane surface usinga simple cleaning process. Hydrophilic modications provide

neering, Seoul National University, 599

ea. E-mail: sykwak@snu.ac.kr

tion (ESI) available. See DOI:

hemistry 2016

effective anti-fouling layers against hydrophobic water contami-nants.5,18,19 Nonetheless, some water contaminants include fullyor localized specic charges that promote membrane fouling viaelectrostatic attractions to the membrane surface.20,21 Thus, it isnecessary to introduce hydrophilic charged surface groups toblock the charged matter involved in membrane fouling.

Modifying membranes with charged materials provideshydrophilic surfaces and electrostatic repulsive forces that actagainst foulants with the same charge. Several researchers haveattempted to create charged layers on membrane surfaces byblending, graing polymerization and surface assembly ofcharged materials.22–28 The specically charge-modiedmembranes display a low propensity toward fouling by fou-lants having the same charge due to the introduction of elec-trostatic repulsive forces; however, most of these attempts haveintroduced only one type of charged group onto the membranesurface. Feed solutions that include oppositely chargedcontaminants or mixed charged contaminants may undergorapid membrane fouling as a result of attractive electrostaticforces. The simultaneous introduction of neutral, positive, andnegative charges could potentially yield anti-foulingmembranes with the selective suppression of fouling bydifferent charged contaminants. If various charges i.e., neutral,positive and negative charges, are selectively deposited ontomembrane surface, it can be developed that the anti-foulingmembranes have selective fouling suppression for variouskinds of charged contaminants. Hyperbranched polyglycerol(HPG) as an anti-fouling modier, which is composed of

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a variety branched structures bearing hydrophilic ethyleneglycol chains and numerous hydroxyl end functionalgroups.29–34 HPG is a hydrophilic polymer with a neutral charge.The hyperbranched structure of HPG can hinder the attach-ment of foulants more effectively than single chain hydrophilicpolymers such as polyethylene glycol (PEG). Specic chargesmay be selectively introduced into the HPG by modifying thehydroxyl end groups.

Here, we present an anti-fouling UF/MF membrane fabri-cated through the surface assembly of various charged HPGsonto a polyvinylidene uoride (PVDF) membrane surface. Thecharged HPGs, i.e., anti-fouling surface modifying agents, wereprepared through the anionic ring opening polymerization ofa glycidol monomer with a three-armed core that included anamine functional group. The hydroxyl end groups were thenconverted into cationic quaternary ammonium groups orsulfonic anion groups. The various charged HPGs were cova-lently assembled onto the PVDF membrane surface via a cross-linking agent. The charged surfaces of the membranes effec-tively suppressed membrane fouling against identical chargedwater contaminants via the electrostatic repulsive forces.Selective charge modication of our membranes makes themattractive in a variety of wastewater purication processes forwastewater including specic charged contaminants.

ExperimentalMaterials

The poly(vinylidene uoride) (PVDF) membrane (Durapore®PPHP 04700, pore size 0.1 mm) was purchased from MilliporeCorporation (USA). Benzyl bromide ($99.0%) was suppliedfrom Alfa Aesar (USA). Tri(hydroxymethyl)aminomethane(THAM, $99.8%), glycidol ($96.0%), cesium hydroxide mono-hydrate ($99.5%), palladium on carbon (Pd/C 10 wt%), glycidyltrimethyl ammonium chloride (GTAC, $90.0%), sulfur trioxidepyridine complex (STPC, $98.0%), and 1,4-phenylene diiso-cyanate (PDC, analytic grade) were purchased from Sigma-Aldrich (USA). The membrane fouling resistance was evalu-ated using lysozyme (LYZ) and bovine serum albumin (BSA),supplied from Sigma-Aldrich (USA). All reagents were used asreceived without purication. All aqueous solutions wereprepared using deionized (DI) water with a resistivity exceeding18.0 MU cm.

Synthesis of the amino-functionalized hyperbranchedpolyglycerol (NH2-HPG)

Benzyl-protected tri(hydroxymethyl)aminomethane (Bz2THAM)was synthesized for use as a core material to polymerize amino-functionalized hyperbranched polyglycerol (HPG).35 Thesynthesis and characteristic of the Bz2THAM was described inthe ESI.† A 6.03 g sample of Bz2THAM (20 mmol) was reactedwith 1.01 g cesium hydroxide monohydrate (6 mmol) inbenzene to deprotonate the hydroxyl groups on the Bz2THAM.Next, the benzene solvent was evaporated at 90 �C underreduced pressure. The residual powder used as an initiator wasdissolved in diglyme. Next, glycidol (31.20 g, 420 mmol) was

88960 | RSC Adv., 2016, 6, 88959–88966

slowly injected into the solution using a syringe pump (droprate: 0.02 mL min�1) at 100 �C. The highly viscous product wasdissolved in methanol to terminate the reaction, and 1.2 mL ofa 5 N HCl solution was added to the solution for protonation.The nal product (benzyl-protected HPG, Bz2HPG) was obtainedby precipitation in cold diethyl ether.

The benzyl protecting groups were eliminated from Bz2HPGby treating Bz2HPG with hydrogen gas in the presence ofa palladium catalyst.36 A 5 g sample of Bz2HPG was dissolved in130mL ethanol, and amixture of 1.3mL acetic acid and 0.8 g Pd/C was added to the Bz2HPG/ethanol solution. The reaction vesselwas ushed with hydrogen gas (1 atm) and stirred for 14 h at60 �C. The insoluble solid was removed by ltration using a cel-ite pad, and the ltrate was evaporated under reduced pressure.The nal product (neutral charged amino-functionalized HPG)was dried under vacuum at 80 �C overnight.

End-group modication of the amino-functionalizedhyperbranched polyglycerols by introducing positively andnegatively charged moieties

Charged moieties were introduced into the amine-functionalizedHPG by converting the end groups of the Bz2HPG into positiveand negative charges through a reaction with glycidyl trimethy-lammonium chloride (GTAC)37 and a sulfur trioxide pyridinecomplex (STPC),38 respectively.

Positively charged end-modied HPG was synthesized bydissolving Bz2HPG (11.03 g, 1 equiv.) and NaOH (13.69 g, 2.4equiv.; OH in Bz2HPG) in 60 mL DI water. The aqueous solutionwas cooled to 0 �C and reacted with GTAC (25.94 g, 1.2 equiv.;OH of Bz2HPG) via dropwise addition. The mixture was allowedto react over 16 hours and was subsequently neutralized with5 N HCl (68.42 mL). The solvent was removed by evaporation at60 �C, and the residual product was dissolved in methanol forprecipitation with NaCl. The product was removed by ltration.The nal product (positively charged Bz2HPG) was obtained byevaporating the methanol solvent.

The negatively charged end-modied amine-functionalizedHPG was synthesized by the dropwise addition of a solutionof STPC (11.44 g, 71.2 mM) in 80 mL DMF to a solution con-taining Bz2HPG (5.508 g, 71.2 mM OH-groups)/50 mL DMF over6 h at 60 �C (dropwise rate: 0.11 mL min�1). The solution wasstirred continually at 60 �C for 4 hours and was then cooled toroom temperature.

The pyridine groups were decoupled from the sulfonatedBz2HPG by adding DI water (13 mL) to the above solution. A 1 MNaOH solution was then injected until the solution reached a pH11. The solvent was removed by evaporation, and the residualsolution was precipitated in cold acetone. The nal product(negatively charged Bz2HPG) was dried at 60 �C overnight.

The positively charged Bz2HPG was debenzylated accordingto the protocol for debenzylating Bz2HPG. The negativelycharged Bz2HPG was synthesized by slightly altering thedebenzylation protocol for Bz2HPG due to the poor solubility ofthe negatively charged Bz2HPG in ethanol. A 4.67 g sample ofthe negatively charged Bz2HPG was dissolved in 100mL DI waterand 100 mL ethanol at 60 �C, and the solution was cooled to

This journal is © The Royal Society of Chemistry 2016

Fig. 1 Schematic illustration for the overall preparation procedure ofcharged PVDF membrane by assembly of charged modified HPG ontothe plasma treated PVDF membrane surface.

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room temperature. Next, 4 mL acetic acid and 1.5 g Pd/C wereadded the above solution. The reaction vessel was ushed withhydrogen gas (1 atm), and the reaction was allowed to agitate for4 days at 60 �C. The insoluble solid was removed by ltrationusing a celite pad, and the ltrate was evaporated under reducedpressure. The nal product was dried at 80 �C overnight.

The synthesis of the charge-modied HPG was monitored byFourier transform infrared spectroscopy (FT-IR; Thermo Scien-tic Nicolet 6700 FT-IR spectrometer) at a spectral resolution of4 cm�1 over the range 4000–400 cm�1, and by 1H nuclearmagnetic resonance (1H NMR, Bruker Avance 600) spectroscopyusing DMSO-d6 as a solvent.

Covalent assembly of the charged HPGs on the PVDFmembrane surface

The charge-modied amine-functionalized HPG was assembledonto the PVDFmembrane surface by treating a PVDFmembranewith plasma irradiation to introduce functional groups into thesurface of the membrane. The charged amine-functionalizedHPG was then coupled to the surface-modied PVDFmembrane using phenylene diisocyanate (PDC) as a cross-linker.The PVDF membrane was treated with plasma irradiation at 100W (radio-frequency power) for 4 min under argon gas (8 cm3

min�1) and oxygen gas (20 cm3 min�1) streams to introducereactive functional groups onto the membrane surfaces. Next,the surface-modied PVDF membrane was reacted with PDC tointroduce isocyanate functional groups. Briey, PDC (0.92 g) wasdissolved in 120 mL toluene at room temperature. The plasma-treated PVDF membrane was immersed in a PDC solution with6.5 mL 5 wt% dibutyltin dilaurate (DBTL) as a catalyst. Themodication reaction was incubated at 37 �C for 3 hours. Theresulting membrane was then rinsed with toluene and ethanolseveral times and then dried at room temperature. Finally, thePDC-linked PVDF membranes were immersed in an ethanolsolution containing 10 wt% charged amine-functionalized HPGin 0.1 wt% DBTL. The assembly of the charged HPG onto thePVDF membrane proceeded at room temperature over 24 hourswith gentle agitation. The resulting membranes, denoted PVDF-HPG for the neutral PVDF membrane, PVDF-PHPG for thepositively charged PVDF membrane, and PVDF-NHPG for thenegatively charge PVDF membrane, were rinsed with ethanoland DI water several times and then dried at room temperature.The overall procedure used to introduce charge-modied HPGonto the plasma-treated PVDF membrane surface is illustratedschematically in Fig. 1.

Membrane characterization

The chemical compositions of the membrane surfaces werecharacterized by attenuated total reection Fourier transforminfrared (ATR FT-IR) and X-ray photoelectron spectroscopy(XPS). The IR spectra were collected using a Nicolet 6700 spec-trometer. Spectra were recorded in the ATR mode using a zincselenite (ZnSe) window. The XPS were recorded on an Axis-HSIby Kratos using monochromatic Mg Ka X-ray source. Pure watercontact angles were measured using an Attention® THETALITE, BiolinScientic (Sweden), using DI water at room

This journal is © The Royal Society of Chemistry 2016

temperature. The surface morphologies of the PVDF membraneand the charge-modied PVDF membrane were characterizedusing a JSM-7600 scanning electron microscope (SEM) with anaccelerating voltage of 10 kV. Platinum coating was carried outby sputtering at 10 mA for 100 seconds prior to FE-SEMimaging. The membrane surface charge was analyzed bymeasuring the zeta potential using an electrophoretic lightscattering spectrophotometer (ELS-Z1000, Otsuka Electronics(Japan)). The neat PVDF and the charge-modied PVDFmembranes were prepared in 1 � 3 cm2 sizes and were pre-wetted in ethanol. The membrane sample measurements werecollected in a 10 mM NaCl solution with a pH that wascontrolled by adding 0.1 M NaOH or 0.1 M HCl. The surfacecharge density of the membranes was calculated using eqn (1).23

s ¼ (8kTC03)1/2 sinh(zex/2kT) (1)

the value of k is the Boltzmann constant, T is the absolutetemperature, C0 is the bulk concentration, 3 is the dielectricconstant, z is the ion valence, e is the electron charge, and x isthe zeta potential.

Evaluation of the anti-fouling performance against chargedfoulants

The anti-fouling behavior of the charge-modied PVDFmembranes PVDF-HPG, PVDF-PHPG, and PVDF-NHPG, against

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charged foulants was characterized by conducting a ltrationtest using a charged protein (bovine serum albumin (BSA) asa negatively charged protein, lysozyme (LYZ) as a positivelycharged protein) suspended in an aqueous solution. Thefouling test included three cycles of ltering pure water for 10minutes, followed by a ltration cycle applied to the chargedprotein aqueous solution over 1 hour. Aer ltering the chargedprotein solution, the membrane was backwashed with purewater at 1 bar for 5 minutes to remove loosely bounded foulantsfrom the membrane surface. The aqueous protein solution wasprepared with a 500 ppm concentration by dispersing BSA orLYZ in a phosphate buffer saline (PBS) solution. The PBS solu-tion was prepared by dissolving salts (4.0 g NaCl, 0.1 g KCl, 0.72g Na2HPO4, and 0.12 g KH2PO4) in 800 mL DI water. The pH wasadjusted to 7.4 using 0.1 MHCl, and DI water was added to yielda total volume of 1 L.

The water ux was measured using an Amicon® 8010 dead-end stirred cell (Millipore Corp.) connected to a pressure vessellled with DI water at 0.17 bar. The ux data were collected bymeasuring the permeate weight using an electronic balanceCUW420H (CAS corporation) linked to a computer to automatethe data gathering process at specic time intervals. The ux Jwwas calculated according to eqn (2),

Jw ¼ Q/(At) (2)

where Jw is the water ux (L m�2 h�1), Q is the volume of thepermeate (L), A is the effective area of the membrane (m2), and tis the ltration time (h).

The membrane performance was evaluated using thenormalized ux (eqn (3)) and ux recovery ratio (eqn (4)),

Normalized flux ¼ Jw/Jw0, (3)

FRR ¼ Jw1/Jw0, (4)

where Jw0 is the pure water ux, and Jw1 is the re-measured purewater ux aer themembrane backwashing. As the value of FRRincreased, the antifouling properties of the membranesimproved. The antifouling properties were further tested bycalculating the total fouling (Rt), reversible fouling (Rr), andirreversible fouling (Rir) as follows.

Rt (%) ¼ [1 � Jw/Jw1] � 100, (5)

Rr (%) ¼ [Jw1/Jw0 � Jw/Jw1] � 100, (6)

Rir (%) ¼ [1 � Jw1/Jw0] � 100, (7)

the value of Rt is the sum of Rr and Rir. A small value of Rt

indicates that the ux maintained, indicating an increase in themembrane fouling resistance.

Results and discussionSynthesis of the charge-modied hyperbranched polyglycerol

Hyperbranched polyglycerol (HPG) is composed of neutralcharged hydrophilic ethylene oxide groups and numerous

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hydroxyl end groups, which can provide hydrophilic property tohydrophobic membrane surface. In addition, the neutralcharged HPGwas capable of altering its charge via hydroxyl end-group modication. The charged HPGs were designed aroundan amino group core and numerous pendant charged func-tional groups, including hydroxyl groups that include a neutralcharge, quaternary ammonium groups that introduced a posi-tive charge, and sulfonate groups that introduced a negativecharge into the ends of the HPGs. The amino group at the coreof the HPG was highly reactive compared with the hydroxyl end-groups, which formed amine-based covalent bonds between thecharged HPGs and the membrane surface.

The HPG was polymerized using a tris(hydroxymethyl)ami-nomethane (THAM) derivate as the initiator core, which wasprepared by directly binding two benzyl groups to the aminogroup of the THAM to suppress the reactivity of the aminegroups during anionic polymerization.27 The main chains of theHPGs were synthesized via anionic ring-opening polymerizationof glycidol using the benzyl-protected THAM initiator. Thecharge end-modied HPGs were prepared through a reactionbetween the hydroxyl end groups, glycidyl trimethylammoniumchloride (GTAC), and the sulfur trioxide pyridine complex(STPC), respectively. Finally, the benzyl protective groups of theamino functional group were removed by hydrogenolysis usinga palladium/charcoal catalyst under a H2 atmosphere.

Fig. 2 shows the 1H NMR spectra of the benzyl protectedinitiator and the charged HPGs. As the HPGs were polymerizedfrom the Bz2THAM initiator, new 1H NMR peaks correspondingto the ethylene oxide backbone developed around 3–4 ppm. Thedegree of polymerization (DPn) and the absolute number-average molecular weight (Mn) were calculated to be 21 and1864.5 g mol�1, respectively, in good agreement with the reac-tion ratio obtained from the glycidol monomer and the Bz2-THAM initiator. Aer removing the benzyl protective groupsfrom the Bz2HPG, the 1H NMR peaks corresponding to thebenzene ring around 7.0 ppm disappeared completely. Inaddition, new 1H NMR peak was appeared at 1.9 ppm assignedas NH2 group. These results indicated that the single aminofunctional group in HPG was successfully recovered byremoving the two benzyl protective groups. A positive chargewas introduced onto the HPGs by reacting the hydroxyl endgroups with GTAC under a basic environment. As shown inFig. 2d, new 1H NMR peaks were observed at 4.35 ppm and 3.16ppm, which corresponded to –CH2–N(CH3)3

+ and N(CH3)3+,

respectively.The FT-IR spectra (see Fig. S3†) displayed new IR bands

corresponding to a CH3 bending mode at 1480 cm�1 in thequaternary ammonium group of the GTAC. The degree ofsubstitution (DS) was determined by calculating the integrationratio of the benzyl peaks 4–6 and the ether peak e, and wascalculated to be 99% (see Fig. S3†). The terminal hydroxylgroups were converted to quaternary ammonium groupsthrough an epoxy ring-opening reaction of the GTAC. A negativecharge was introduced into the HPG via sulfonation usingSTPC. The 1H NMR spectra collected aer carrying out thereaction between the end groups of the HPG and the sulfurtrioxide of STPC compound displayed ether bond signals g and f

This journal is © The Royal Society of Chemistry 2016

Fig. 2 (left) 1H NMR spectra and (right) the chemical structures of (a)benzyl protected core material (Bz2THAM), (b) benzyl protectedhyperbranched polyglycerol (Bz2HPG), (c) amino functionalizedhyperbranched polyglycerol (NH2-HPG), (d) positively charged modi-fied hyperbranched polyglycerol (PHPG) and (e) negatively chargedmodified hyperbranched polyglycerol (NHPG).

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(4.24–4.75 ppm). The FT-IR spectra (see Fig. S2†) featuredsulfone-related IR bands, including the S]O stretch 1259 cm�1

and the C–O–S stretch 805 cm�1 due to the end group sulfo-nation process. The degree of substitution (DS) of the endhydroxyl groups was calculated to be 67%, based on a compar-ison of the 1H NMR integral ratio (see Fig. S4†).

The combined results of 1H NMR and FT-IR analysis resultsrevealed that the HPGs were successfully prepared with the desirednumber of branches and molecular weights, and the desiredcharge was introduced at the ends of the HPGs in a high density.

Fig. 3 ATR FT-IR spectra of (right) (a) neat PVDF, (b) PDC-linked PVDF,(c) PVDF-HPG, (d) PVDF-PHPG and (e) PVDF-NHPG membranes, and(left) the enlarge IR spectra, collected over the range 3600–2750cm�1, corresponding to the O–H, N–H and C–H stretches.

Covalent assembly of various charged HPGs onto the PVDFmembrane surfaces

The charged HPGs were bound to the plasma-treated PVDFmembrane surface through a reaction between the amino func-tional group of the charged HPG and the PVDF membranesurface using a 1,4-phenylene diisocyanate (PDC) cross-linker.The amino functional groups in the core of the charged HPGsreacted rapidly with the isocyanate groups in the PDC linker. Thisreaction could be blocked by a reaction between the hydroxyl endgroups in the charged HPGs and the membrane surface.

This journal is © The Royal Society of Chemistry 2016

Fig. 3 shows the ATR FT-IR spectra collected from the neatPVDF membrane, PDC-linked PVDF membrane, and chargedHPG-assembled PVDFmembranes, i.e., PVDF-HPG, PVDF-PHPG,and PVDF-NHPG. New IR bands corresponding to OH and NHstretches at 3600–3200 cm�1, a C–N stretch and an aroma. ]C–H stretch at 1637 cm�1, 1510 cm�1 appeared in the PDC-linkedPVDF membrane. As the charged HPGs assembled onto theplasma-treated PVDFmembrane surface, a OH stretch IR band at3300 cm�1 developed, and a C]O stretch developed at 1748cm�1 corresponding to urethane. These spectral changesprovided evidence for the assembly of charged HPGs onto themembrane via the PDC linker. XPS analysis provided moredetailed surface chemical information about the assembly of thecharged HPGs onto themembrane surfaces. Fig. 4 shows the XPSspectra collected from the surface of the neat PVDF membrane,the neutral HPG-assembled PVDF membrane, the positivelycharged HPG-assembled PVDF membrane, and the negativelycharged HPG-assembled PVDF membrane, respectively.

The C 1s XPS spectrum of the neat PVDF membrane could bet to two main peaks at 284.5 eV and 289.03 eV, corresponding toCH2 and CF2, respectively. As the charged HPGs assembled,peaks corresponding to the C–O binding energy at 285 eV and theC]O binding energy at 288 eV developed, derived from theethylene oxide backbone in the HPG and urethane bonds asa result of the PDC linker-assisted assembly. An N–H speciesappeared at 399 eV in the XPS spectra of the PVDF-HPG andPVDF-PHPG samples. By contrast, no nitrogen species weredetected in the neat PVDF membrane. The N–H species werederived from the amino group in the core of the HPGs and thePDC linker. A new binding energy peak at 402 eV developed in theN 1s XPS spectra of the PVDF-PHPG, corresponding to an N+–Cspecies derived from the quaternary ammonium group of thepositively charged HPG. The XPS spectra of the PVDF-NHPG andsulfone group revealed peaks at 168.8 eV. These new XPS peaks

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Fig. 4 XPS spectra for (1st line) neat PVDFmembrane, (2nd line) PVDF-HPGmembrane and (3rd line) PVDF-PHPGmembrane of (left) C 1s and(right) N 1s binding energies, and (4th line) PVDF-NHPG membrane of(left) C 1s and (right) S 2p binding energies.

Fig. 5 FE-SEM images of the top surface morphologies and (insetimages) pure water contact angles of neat PVDF, PVDF-HPG, PVDF-PHPG and PVDF-NHPG membranes.

Fig. 6 Zeta potential results for neat PVDF, PVDF-HPG, PVDF-PHPGand PVDF-NHPG membranes over the pH range 3–10.

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corresponded to the quaternary ammonium and sulfone groups.As assembly of neutral and charge-modied HPGs onto thesurface of PVDF membranes, XPS atomic concentrations ofcarbon and oxygen were also increased (see Table S1†). Inparticular, the oxygen atomic concentrations which is derivedfrom neutral and charge-modied HPGs were increased by 14.39at% (for HPG-PVDF), 13.22 at% (for PHPG-PVDF) and 12.24 at%(for NHPG-PVDF), respectively. The results of XPS analysis indi-cated the direct assembly of neutral and charged HPGs on themembrane surfaces with similar introduced quantity.

The morphological changes in the membrane surface aerthe assembly of charged HPGs were characterized using FE-SEMmeasurement. Fig. 5 shows the FE-SEM images collected fromthe top surfaces of the neat PVDF, PVDF-HPG, PVDF-PHPG, andPVDF-NHPG membranes. All membranes showed similarsurface morphologies of typical stretched membranes, indi-cating that the plasma-treatment and assembly of the chargedHPGs did not signicantly affect the surface pore structures ofthe PVDF membrane. The pure water contact angles (see theinset images of Fig. 5) decreased with the assembly of thecharged HPGs. The neat PVDF membranes provided a highcontact angle of 136.2 � 5.2� due to the hydrophobicity ofthe surface groups. The charged HPG-assembled membranesprovided smaller contact angles, 80–110�, compared to the neatPVDF membrane. These results indicated that the assembledHPGs endowed the membrane surfaces with hydrophilic

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properties due to the ethylene oxide backbone and the chargedend groups.

The membrane surface charges substantially inuenced thefouling properties during the ltration process.19 The adhesiveforce between the foulants and the partially or fully chargedsurfaces could be controlled by tuning the electrostatic prop-erties of the membrane surface. The charges on the membranesurfaces suppressed adhesion among identical chargedcontaminants due to the electrostatic repulsive forces. Themembrane surface charge strength was determined by the zetapotential measurements. The zeta potential originates from anion stream on a shear plane bordering the electric double layersformed by the immobilized and diffused layers. The zetapotentials of the charge-modiedmembranes weremeasured atpH 3–10. As shown in Fig. 6, the neat PVDF membrane dis-played a negative zeta potential, except at pH 3, under whichconditions the C–F moiety in the neat PVDF membranewas negatively charged. The zeta potential values decreased

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monotonically as the pH increased. The isoelectric point (IEP)was 3.7. The PVDF-HPGmembrane showed a nearly neutral zetapotential across the entire pH range due to the presence ofneutral HPG. By contrast, the zeta potentials of the PVDF-PHPGand PVDF-NHPG membranes were positive and negative,respectively, across the entire pH range. These results indicatedthat the charges of the HPGs (neutral, positive, or negative)provided the desired chemical and electrical properties to themembrane surface and affected the fouling performance duringltration of charged solutes.

The surface analysis data indicated that the charged HPGssuccessfully bound to the membrane surface via the PDC cross-linker. The surface charges on the membrane could becontrolled through the assembly of specic charged HPGs onthe membrane surfaces.

Fig. 7 Filtration tests of neat PVDF, PVDF-HPG, PVDF-PHPG andPVDF-NHPGmembranes using (a) 500 ppm lysozyme (LYZ) (b) bovineserum albumin (BSA) suspended neutral aqueous solutions.

Evaluation of the anti-fouling properties in the presence ofcharged feed solutions

The anti-fouling performances of the charge-modied PVDFmembranes were investigated by conducting ltration testsusing highly concentrated charged protein feed solutions underneutral pH conditions. Lysozyme (LYZ) and bovine serumalbumin (BSA) were used in the fouling resistance tests.

These proteins assumed different charges at neutral pH. Thezeta potential values for LYZ and BSA are +3.5 mV and�22.3 mVat pH 7.4, respectively.20

The neat PVDF and charged PVDFmembranes were fouled byltering a 500 ppm protein solution over 60 minutes. Aerltering the protein solution, the membranes were backwashedto remove loosely attached foulants. Then, the pure water uxthrough the membrane was measured. These ltration processeswere repeated three times. The results obtained from the ltra-tion test are plotted in Fig. 7. Before ltration of protein feedsolution, the all membranes had similar level of pure waterpermeability (neat-PVDF: 2635 � 188 LMH bar�1, PVDF-PHPG:2883 � 240 LMH bar�1, PVDF-HPG 2629 � 206 LMH bar�1,PVDF-NHPG: 2534 � 155 LMH bar�1). The positively chargedprotein, LYZ, was ltered across the membrane (see Fig. 7a andS5 in the ESI†), and the normalized water ux decreased due tothe accumulation of the foulant on themembrane surface. As thepure water passed through the membranes, the normalized uxwas recovered due to the elimination of loosely bound foulantsfrom the membrane surface.

In particular, the PVDF-PHPG membrane showed the highestresistance to fouling among the membranes tested. The totalfouling of the PVDF-PHPG membrane, meaning the summationof the reversible and irreversible fouling measurements, was 16–33% less than the total fouling measured from the neat PVDFmembrane. The PVDF-PHPG membrane provided half of itsinitial water ux, even aer three ltration cycles applied toa highly concentrated contaminant solution. These resultsindicated that the positive surface charges and the hydrophilicityof the PVDF-PHPG membrane suppressed the accumulation ofthe positively charged LYZ. Thus, these groups reduced theadsorption of LYZ and improved detachment of the surface-bound LYZ during the membrane backwashing step. By

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contrast, the neat PVDFmembrane lost about 70% of its originalpermeability aer the ltration test because the neat PVDFmembrane presented a hydrophobic surface with a negativesurface charge. The neutral PVDF-HPG membrane displayed anintermediate level of fouling resistance due to a neutral surfaceand good hydrophilicity. The negatively charged PVDF-NHPGmembrane surface displayed a resistance to fouling against theLYZ foulant comparable to that obtained from the neat PVDFmembrane because the PVDF-NHPG membrane was morehydrophilic than the neat PVDF membrane.

The negative charged BSA ltration test results are presentedin Fig. 7b and reveal that the negatively charged PVDF-NHPGmembrane showed the highest fouling resistance because themembrane presented a negative surface charge and was hydro-philic. About 85% of the permeation performance of the PVDF-NHPGmembrane was recovered aer three BSA ltration cycles.By contrast, the positively charged PVDF-PHPG membrane dis-played a low fouling resistance against the negatively chargedBSA foulant. The ltration tests resulted in an irreversiblefouling that was 16% higher than the irreversible fouling ob-tained in the PVDF-NHPG membranes because the surface waspositively charged. The neutral PVDF-HPG membrane showedan intermediated level of BSA fouling resistance.

Overall, the fouling resistance of the membranes in thepresence of charged contaminants clearly revealed that thesurface charge and hydrophilic properties could be tuned toreduce membrane fouling by suppressing the accumulation ofidentically charged foulants on the membrane surface. These

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results showed similar level anti-fouling performance comparedwith previous reported surface modied UF/MF membranes,which is summarized in Table S2.† Membrane surface graingmodications using a variety of charged HPG groups provide aneco-friendly approach to synthesizing wastewater ltrationmembranes that resist fouling by charged contaminants.

Conclusions

We developed a surface charge-modied UF/MF membrane thatexhibits anti-fouling properties against charged contaminants bygraing a variety of charge-modied HPGs onto the surfaces ofPVDF membranes. The FT-IR and 1H NMR results indicated thatthe charge-modied HPGs were successfully synthesized via theanionic ring-opening polymerization of glycidol and end-groupmodication of the hydroxyl groups. The charge-modiedHPGs were successfully assembled onto the PVDF membranesurfaces using a PDC cross-linker. The surface hydrophilicityimproved upon assembly of the hydrophilic charged HPGs. Asintended, the charged PVDF membranes presented differencesurface charges due to the assembly of charge-modied HPGs, asconrmed by a zeta potential analysis. The fouling resistancetests conducted using feed solutions containing charged proteinsrevealed that the charged HPGs assembled onto the PVDFmembrane surfaces and effectively suppressed the attachmentand accumulation of identically charged foulants through elec-trostatic repulsion and improved surface hydrophilicity. Chargeswere introduced onto membrane surfaces through the assemblyof a variety of charged hyperbranched hydrophilic polymerssynthesized from the corresponding hyperbranched polymers.The selectivemodication of themembrane chargesmakes themattractive for the effective purication of a variety of wastewatercontaining specic charged contaminants.

Acknowledgements

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT & Future Planning (No.NRF-2015R1A2A2A01005651).

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