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Journal of Membrane Science 349 (2010) 421–428

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

endrimer-based coatings for surface modification of polyamide reversesmosis membranes

bhijit Sarkar ∗, Peter I. Carver, Tracy Zhang, Adrian Merrington, Kenneth J. Bruza,oseph L. Rousseau, Steven E. Keinath, Petar R. Dvornicichigan Molecular Institute, 1910 West St. Andrews Road, Midland, MI 48640-2696, USA

r t i c l e i n f o

rticle history:eceived 12 May 2009eceived in revised form 1 December 2009ccepted 3 December 2009vailable online 11 December 2009

a b s t r a c t

The first use of dendrimer nanotechnology in the modification of reverse osmosis (RO) membranes isreported. The effects of dendrimer surface coatings on the advancing water contact angle (i.e., coatings’hydrophilicity), permeate flux and % salt rejection of commercial polyamide membranes were stud-ied. The membranes were coated by in situ crosslinking of amine-functional polyamidoamine (PAMAM)dendrimers and PAMAM–polyethylene glycol (PAMAM–PEG) multi-arm stars with difunctional PEG

eywords:embranes

iofoulingendrimersAMAMolyethylene glycol

crosslinkers. The resulting coatings significantly reduced contact angles of membrane surfaces with-out affecting their % salt rejection and only moderately reducing their permeate fluxes. Lower contactangles indicated more hydrophilic membranes with the potential for increased resistance to fouling byhydrophobic foulants, such as biofoulants and organic pollutants.

Published by Elsevier B.V.

ydrophilic coatingeverse osmosis

. Introduction

During the past two decades, reverse osmosis (RO) has becomehe main technology for the production of drinking and ultra-ure water for The electronics and pharmaceutical industry [1–6].owever, although over 3 billion gallons of water are currently pro-uced this way every day using commercially available membraneshat possess excellent separation properties (i.e., % salt rejectionnd permeate flux), frequent membrane fouling by inorganic andrganic/biological contaminants in the feed water still representsmajor problem from both technological and economical perspec-

ive [7–13]. Some of the most successful commercially availableO membranes are composite structures with aromatic polyamideeparation layers. Unfortunately, such surfaces attract organic andiological species which on contact easily form biofilm as the firsttep in the biofouling process that creates an additional filtrationarrier and reduces the flux of permeate water. Because of this,ctions must be taken in order to compensate for undesired flux

ecline, including increasing the operating pressure by as muchs 50% in some cases, interrupting production for cleaning theembranes by frequent chlorination, and replacing the membrane

lements approximately every 3 years [10–13]. In fact, the costs of

∗ Corresponding author. Tel.: +1 989 832 5555; fax: +1 989 832 5560.E-mail address: sarkar@mmi.org (A. Sarkar).

376-7388/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.memsci.2009.12.005

energy to run the high-pressure pumps and to remedy membranefouling (where the latter often amounts to ca. 30% of the total oper-ating expense) are two main factors that control the economics ofpresent day water production by RO processes [14]. Table 1 summa-rizes several important features of some of the best commerciallyavailable RO membranes, indicating that most of them are quiteprone to biofouling.

Because of this situation, there is a fast growing need for effec-tive prevention of membrane fouling in RO processes, and amongthe methods under consideration, surface modification of exist-ing, commercially available membranes by antifouling coatingshas attracted considerable attention [4–6,10–12]. Clearly, the keyrequirement of such a modification is to eliminate the fouling whileretaining rejection characteristics and permeate fluxes comparableto those of unmodified, non-fouled membranes. However, addi-tional benefits would also include: avoidance of expensive anddifficult development of completely new types of membranes, anda cost-effective solution to the problem, considering that only avery thin coating (of the order of hundreds of nanometers) wouldlikely satisfy the purpose and that it could be realized by a rela-tively simple procedure added at the end of the existing membrane

fabrication process. Following this line of thought, in this workwe prepared and evaluated two types of dendrimer-based coat-ings for polyamide-type RO membranes aimed at elimination offouling by organic contaminants (such as humic acid, petroleumproducts, weed killers and other frequently encountered organic

422 A. Sarkar et al. / Journal of Membrane Science 349 (2010) 421–428

Table 1Characteristic properties of selected RO membranes.

Membrane HR98PP CA995PE SEPA-MS05 SEPA-SS1C DESAL-3B Elix® 3 ROCartridge

Manufacturing company Dow/FilmTec Dow/FilmTec GE-Osmonics GE-Osmonics Desalination Systems, Inc. MilliporeComposition Polyamide thin

film compositeDiacetate/polyester Polyamide Cellulose acetate Polyether-sulfone Polyamide thin

film compositeTypical NaCl rejection (%) >97.5 >95 >98 >98 98.5 94–99Chlorine tolerance Low Low Low Low Low Low

3–1High10180

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pH range 2–11 2–8.5Tendency for biofouling High LowMaximum operating pressure (psi) 870 1015Maximum operating temp (◦C) 60 35

ontaminants) and biological species (such as microorganisms andacteria). To the best of our knowledge this is the first time thatendrimers have been utilized for this purpose.

Dendrimers are highly branched, globular, nanoscopic macro-

olecules composed of two or more tree-like dendrons emanating

rom a central core which can be either a single atom or an atomicroup [15]. They are built up of three-dimensional “branch cells”hat are organized in concentric layers (usually referred to as “gen-

ig. 1. Some different ways to present planar projections of ideal dendrimer structuresimensionality added for artistic effect. (B) Chemical structure of a G2 PAMAM dendrim

ts constitutive building blocks. The ellipse with an “I” in the center denotes the core. Tf the structure. Medium size circles denote dendritic branch cells with Arabic numberonds between the adjacent cells. Small circles (both filled and unfilled) in the center ofunctionality. Symbols X represent end-groups, which may be chemically reactive or unr

1 2–8 4–11 4–11Low High High

5 1015 650 6550 50 35

erations”) around the core, and these branch cells can be considereddendritic analogues of traditional linear polymer repeat units. Theycan be divided into two main types: interior branch cells, whichform the intramolecular dendrimer armature, and exterior (or sur-

face) branch cells, which carry either reactive or inert end-groups,as shown in Fig. 1 [16]. The structure of the branch cells is deter-mined by the nature and arrangement of their contributing atoms,bond lengths and angles, directionality and conformational bond

. (A) An idealized presentation of a tridendron dendrimer with a sense of three-er. (C) A generalized representation of a tetradendron dendrimer architecture andhe big circle in the center with four small zeros denotes the generation zero parts indicating their generation level and small letters in squares denote connectingeach cell represent branch junctures which in this case are all of 1 → 2 branching

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A. Sarkar et al. / Journal of Mem

exibility. They can be either the same (homodendrimers) or dif-erent (copolymeric dendrimers), but in either case each of them

ust contain at least one branch juncture, the functionality ofhich (i.e., the number of branches emanating away from the

uncture) usually equals to 2 or 3 and defines the intramolecularendrimer branching density. Reactive end-groups may be used forontinuation of dendritic growth during the synthesis of the nextenerations, for modification of the existing “dendrimer surface”,r for synthesis of more complex structures involving dendrimerss nanoscopic building blocks. A prime example of the last men-ioned use is the crosslinking of dendrimers into honeycomb-like,hree-dimensional nano-domained networks using di- or multi-unctional crosslinking agents that can be either small moleculareight reagents or macromolecular compounds [17]. Similarly, a

ery good example of the modification of dendrimer surfaces ishe attachment of linear chains to create multi-arm star moleculeshat can have several tens to several hundreds of arms attachedo the dendrimer core. Utilization of these two synthetic strategiesor the preparation of highly hydrophilic membrane coatings withynamic brush-like surfaces was the focus of the study described

n this report.The methods of dendrimer synthesis permit an exceptional

egree of synthetic control over the resulting structures, whichrovides a very high (i.e., almost mathematically precise) degreef structural regularity. As a consequence, dendrimers show prop-rties that are not typical for any other type of synthetic polymers,ncluding: very high isomolecularity (i.e., their polydispersity coef-cients, Mw/Mn, are routinely lower than 1.2, even at very higholecular weights of several hundreds of thousands or generations

s high as 10), very well-defined molecular sizes (i.e., their hydro-ynamic diameters regularly increase by about 1 nm per generationithin the 1–10 nm range), almost spherical molecular shapes andnusually high density of functionality, which ranges into hundredsr even thousands of reactive, exo-presented functional groups perolecule at higher generations (see Fig. 1) [15]. Surprisingly, how-

ver, in spite of these unique and extremely versatile properties,he use of dendrimers in membrane separation processes has notttracted significant research attention. The reasons for this maynclude the fact that commercial quantities of dendrimers haveecome readily available only relatively recently, but also that theromise of these exceptional building blocks for solving some ofhe existing RO problems has simply been overlooked.

The coatings developed in this study consist of nano-tructured, honeycomb-like networks prepared from amine (NH2)nd/or polyethylene glycol (PEG) terminated polyamidoamine,[(CH2)2–C(O)–N(H)–(CH2)2–N]n<, dendrimers (referred to asAMAM and PAMAM–PEG, respectively), and �,�-telechelic linearEG crosslinking agents. Their preparation involved a bottom-upynthetic strategy capable of providing a synergistic combinationf two main structural features that are well known to lead to effec-ive biofouling protection: (a) an increased hydrophilicity relativeo uncoated membranes [18–23] (both PAMAM dendrimers andEG are well known for their high hydrophilicity), and (b) dynamic,rush-like topology of coated surfaces [24–26]. While the formeras expected to repel organic and biological contaminants, the

atter was to provide cilia-like dynamic membrane surfaces, therownian motions of which would create a few nanometers thinwall” of ultrapure water at the membrane-feed interface and pre-ent the contaminants from settling [26].

. Experimental

.1. Procedures

1H NMR spectra were obtained on a Varian 400 MHz NMR spec-rometer. Samples were dissolved in CDCl3 with tetramethylsilane

e Science 349 (2010) 421–428 423

as the internal reference standard. Infrared spectra were obtainedon a Nicolet 20 DXB FTIR spectrometer either from neat liquids onpolished KBr plates or, if the sample was a solid, as KBr pellets. SEMphotomicrographs were obtained on an Amray scanning electronmicroscope.

2.2. Materials

All starting chemicals were used as received from the providerand the solvents were ACS reagent grade. Generation 2 PAMAMdendrimers (G2 PAMAM) were obtained from Dendritech, Inc.,Midland, MI, and also used as received. The polyamide RO mem-branes (XLE) were obtained from FilmTec, Inc., a division of theDow Chemical Company, Minneapolis, MN.

2.3. Synthesis of N-hydroxysuccinimide mono-functionalizedpolyethylene glycol (NHS-PEG)

In a 250 mL round-bottom flask equipped with a nitrogeninlet and outlet, a dropping funnel and a magnetic stirringbar, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochlo-ride (EDC) (1.31 g, 6.83 mmol) and N-hydroxysuccinimide (0.780 g,6.78 mmol) were added and the flask was placed on a magneticstirrer. Methylene chloride (120 mL) was added to dissolve thereagents and PEG mono carboxylic acid (4.09 g, 6.739 mmol) wasadded to the stirred solution drop-wise over 6 min. After stirring atroom temperature for 42 h, the reaction mixture was transferredto a 1-neck flask and the solvent was removed on a rotavap. Theresidue was dissolved in water and extracted 4 times with chlo-roform (4× 200 mL). The resultant chloroform extract was washedwith 500 mL of saturated sodium bicarbonate solution, followed bydeionized water (2× 250 mL). The organic extract was dried overanhydrous sodium sulfate, filtered, and the solvent was strippedunder reduced pressure on a rotavap to obtain 1.316 g (49%) of ayellowish solid product. 1H NMR (CDCl3): ı 3.50 (s, PEG backbone);3.21 (s, –OCH3), 2.81 (s, succinimide, 4H), 2.63 (m, CH2CH2, 4H).

2.4. Synthesis of 50% PEGylated PAMAM dendrimers(PAMAM–PEG)

1.316 g (1.87 mmol) of NHS-PEG was placed in a 100 mL round-bottom flask and dissolved in 25 mL of methylene chloride. Amethanolic solution of G2 PAMAM (0.7608 g in 3.55 g methanol,0.2337 mmol, 3.74 mmol equivalent NH2) was slowly pipetted intothis solution with stirring and the resulting mixture was stirredat room temperature for 48 h. The solvent was removed underreduced pressure and the product was redisolved in methanoland ultrafiltered using a 1000 Da nominal molecular weight cut-off (MWCO) regenerated cellulose membrane and methanol as thesolvent. The solution obtained from ultrafiltration was strippedof solvent under reduced pressure on a rotavap to yield 1.2793 g(yield 64% with respect to the reactants) of a caramel coloredsolid product. The product was characterized by FTIR and NMR.1H NMR (CDCl3): ı 7.95 (s, CONHCH2CH2N ), 6.41 (d, NHCONH–),5.91 (d, NHCONH–), 4.30 (d, CHCH–), 4.15 (d, CHCH–), 3.50 (s, PEGbackbone), 3.21 (s, OCH3), 2.63 (m, CH2CH2, 4H), 2.51–2.46 (m,CONHCH2CH2N ), 1.61 (b), 1.43 (b), 1.23 (s) (CH3). FTIR (KBr):3263 cm−1 (�, NH), 1650 cm−1 (�, C O), 1547 cm−1 (�, CNH ofamide).

2.5. General procedure for preparation of PAMAM–PEG networks

A rectangular section of approximately 19 cm × 23 cm of aFilmTec XLE RO membrane was cut from continuous roll stockand affixed to a glass plate of similar dimensions via masking tapeat the top and the bottom only. Coating solutions were prepared

424 A. Sarkar et al. / Journal of Membrane Science 349 (2010) 421–428

Table 2Properties of XLE membranes with G2 PAMAM–PEG coatings with 50% free amino groups.

Coating Sample Flux (mL/cm2 min) Salt rejection (%) Contact angle (◦)

Uncoated membrane – 0.11 99.0 60

G2 PAMAM–PEG with 50% free amines

1 0.09 99.0 342 0.10 98.0 343 0.08 99.0 334 0.08 99.0 365 0.09 99.0 36Average 0.09 98.8 35

Table 3Properties of XLE membranes with G2 PAMAM–PEG brush-PEG coatings.

Coating Sample Flux (mL/cm2 min) Salt rejection (%) Contact angle (◦)

Uncoated membrane – 0.11 99.0 60

1 0.10 99.0 152 0.08 99.0 193 0.09 97.0 15

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3. Results and discussion

The goal of this study was to prepare and evaluate PAMAMdendrimer-based networks with PEG crosslinkers and tetheredchains as unique novel coatings for RO membranes (see Fig. 2),

Fig. 2. Schematic representation of antifouling dendrimer-based coating on theactive (polyamide) layer of a standard asymmetric RO membrane. The coating is

G2 PAMAM–PEG brush-PEG 456Average

ith varying weight percents of G2 PAMAM dendrimer (eitherAMAM or PAMAM–PEG) and stoichiometric equivalents of digly-idyl ether–PEG, DGE-PEG, in water. The solution was poured acrosshe top of the membrane and spread using a size 25 Mayer rod. Theoating was allowed to crosslink for 30 s in a stationary horizontalosition, and then an aluminum roller was rolled over the mem-rane with little to no vertical pressure to remove excess solution.

.6. Membrane evaluation procedures

.6.1. Permeate flux and % salt rejection measurementsSince the membranes evaluated in this work were aimed at

heir use with brackish water eighty gallons of 1000 ppm aque-us NaCl (pH ∼6.3) in deionized water were placed in a reservoironnected to a SEPA CF II (GE-Osmonics) dual cell test system. Theells were connected in parallel, allowing for comparative evalu-tions of pairs of membranes. The system was equipped with anutomated data collection unit for obtaining permeate flux and %alt rejection values as a function of time. The set-up was calibratedsing standard FilmTec XLE RO membranes of known performancearameters and all coated membranes were evaluated using theollowing procedure.

First, the membranes were compacted with deionized water inhe test cell for 4 h, and then exposed to the salt solution for 30 mint ambient pressure. Following this, a back pressure of 100 psi waspplied and the first measurement of separation properties wasaken 1 h later. The cross flow velocity was chosen to be 13.2 cm/secause it was convenient for our experimental setup and it is wellnown that fouling strength is not strongly affected by the physi-al operating parameters, such as cross flow velocity and pressure.epeated measurements were taken every 30 min over a periodf 6–9 h. Permeate flux was measured with a flux meter as theumber of milliliters of permeate per minute, while % salt rejec-ion was measured as total dissolved solids (TDS) in ppm from theonductance in microsiemens for the permeate and the reservoirater, respectively. Coated membranes were kept at room temper-

ture in an environment containing moisture where they remained

nchanged for at least 6 months.

.6.2. Advancing water contact angle (AWCA) measurementsAWCA values of coated membranes were measured using a KSV

nstruments CAM 200. In each measurement dry membrane was

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placed on the test stand and affixed via two-sided adhesive tape. A0.01 mL drop of deionized water was metered out of the dispensertip and slowly lowered to touch the membrane, whereupon it waspulled away from the dispenser tip via the hydrophilic character ofits surface. AWCA values were measured in one of two ways. In thedynamic method, the images of the water droplet were taken usinga camera, first rapidly, 33 ms apart, and then slowed down to 1 sintervals when the drop shape stabilized. In the second method, asingle image was captured 5 s after the water droplet was placed incontact with the membrane. AWCA values were calculated from theYoung–Laplace fitting equation under conditions which allowed fortilt and an automatic baseline and the means of obtained values arereported in Tables 2 and 3.

a crosslinked honeycomb-like network of dendritic cells (represented by large cir-cles with pearly rims) created from highly hydrophilic G2 PAMAM dendrimers withattached hydrophilic linear PEG tethered chains/brushes (top). Note that dendriticcells are crosslinked by linear PEG segments (represented by straight lines connect-ing the circles). This schematic is not to scale: in reality, the thickness of the coatingwill be less than 5% of the thickness of the membrane substrate.

A. Sarkar et al. / Journal of Membrane Science 349 (2010) 421–428 425

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nd to correlate their hydrophilicity and topology with the result-ng % salt rejection and permeate flux of the coated membranes.t was expected that the well-defined, nano-domained 3D archi-ecture of these coatings would provide two main advantages tohe resulting membranes. First, the choice of the coatings’ com-osition (i.e., PAMAM and PEG building blocks) and the ability ofendrimer technology to control organization of matter at the nanocale [15] would enable tailoring of these membranes’ hydrophilic-ty and surface topology more precisely and more effectively than ishe case with traditional methods [27], and second, the incorpora-ion of dendrimer nano cells into these structures would allow safencapsulation of potent antimicrobial agents for further improve-ent of antifouling properties in the next stage of this research

rogram [28].Because of a favorable combination of properties (including den-

rimer size of about 2 nm in hydrodynamic diameter and relativelyarge number (16) of functional end-groups per molecule), techni-al grade generation 2 (G2) NH2-terminated PAMAM dendrimersG2 PAMAM) and their 50% PEGylated derivatives (PAMAM–PEG)ere used on FilmTec’s Extra Low Energy (XLE) membranes.

he dendrimers were crosslinked on the active (polyamide)ides of these membranes with glycidyl ether-functionalized �,�-elechelic PEG (DGE-PEG, Mw = 526). Since the primary aminend-groups as well as the interior tertiary amine and amide unitsf these dendrimers were all capable of engaging in attractiventeractions with polyamide membrane surfaces, they providedor very good adhesion of these coatings to polyamide membraneubstrates and no delamination was observed.

.1. PEGylated PAMAM dendrimers

The 50% PEGylated PAMAM dendrimers (PAMAM–PEG) wererepared by reacting amine-terminated G2 PAMAM precursorith a half-equivalent of monfunctional N-hydroxysuccineimide-

EG (NHS-PEG), as shown in Reaction Scheme 2. NHS-PEG waselected for PEGylation because it was more reactive with amine-

erminated PAMAMs than the commercially available carboxy-PEGerivative, and it enabled better control of the syntheses andigher yields of the resulting PAMAM–PEG product. It was pre-ared from a commercially available PEG acetic acid (PEG-AcA;w = 550, DP = 19) in the presence of EDC catalyst, as shown in

Scheme 2

.

Reaction Scheme 1, and the reaction was monitored by Fouriertransform infrared spectroscopy (FTIR) by following the disappear-ance of the COOH peak to give the NHS-PEG product in quantitativeyield.

PEGylation of NH2-terminated G2 PAMAM was performed ina methylene chloride/methanol mixture at room temperature, asshown in Reaction Scheme 2. The disappearance of ester groupswas monitored by FTIR, aiming at 50% PEGylation (i.e., substitutionof 8 of the 16 available NH2 end-groups). The PAMAM–PEG productwas purified by ultrafiltration and its structure was verified by NMRspectroscopy.

The tethered PEG chains were estimated to be approximately8 nm long if extended away from the coating surface (a situationthat is expected to occur in an aqueous environment), and theremaining eight NH2 groups per dendrimer molecule were leftunreacted for use in a subsequent crosslinking reaction to form thenetwork coatings on the polyamide RO membranes.

3.2. Preparation of dendrimer network coatings on ROmembranes

Coating solutions were prepared by dissolving the desiredquantities of difunctional DGE-PEG crosslinker and G2 PAMAMdendrimers (either PAMAM or PAMAM–PEG) in deionized water(a good solvent for both reagents). Since the reaction of glycidylether and amino groups at room temperature is very fast, solutionswere poured on the membranes immediately after mixing and thecoatings were formed within minutes. This procedure was simpleand versatile, and it enabled variation of the network crosslinkingdensity by selecting the PAMAM/DGE-PEG ratio used. Three typesof networks were prepared. In one type, all available dendrimeramino groups were used in crosslinking with DGE-PEG to create avery dense network, while in the second type 50% of these groupswere left unreacted in order to create a looser network and to deter-mine their effect on the hydrophilicity of the coated surfaces and onthe separation properties of the coated membranes. These reactions

can be represented, for example, for the case of crosslinking 50% ofthe amine groups of unmodified PAMAM dendrimers, as shown inReaction Scheme 3.

In the third type of coatings, PEGylated PAMAM dendrimers ofReaction Scheme 2 were crosslinked with DGE-PEG, as shown in

.

426 A. Sarkar et al. / Journal of Membrane Science 349 (2010) 421–428

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eaction Scheme 4, into dendrimer networks with tethered PEGhains to form dynamic, brush-like surfaces that mimic the behav-or of biological cilia. This topology was aimed at creating a dynamicarrier to organic and/or biological contaminants in the feed stream

n RO experiments resulting in nanometers thin layers of ultrapureater on the polyamide side of the membranes, leading to repelling

f such contaminants from membrane surfaces and providing anffective mechanism for extended antifouling protection.

.3. Properties of coated membranes

The coverage of membrane surfaces by crosslinked dendrimeroatings was evaluated by exposure of coated samples to 254 nmV light after a 4 h drying period. Under these conditions, the coat-

ngs exhibited a faint fluorescence which clearly distinguished itrom the uncoated polyamide surfaces (see Fig. 3). It was also found

Scheme

3.

that: (1) there was a minimum concentration of the coating solutionnecessary for achieving continuous coverage (i.e., no islands anduncoated areas), and this concentration was between 0.5 and 1 wt.%solids; (2) the coating solution concentration and the permeate fluxthrough the coated membrane were inversely proportional, indi-cating that the thicker coatings obstructed the permeate flow; and(3) the advancing water contact angles (AWCA) of the coated mem-branes were independent of the coating solution concentration (i.e.,of the thickness of the coatings applied). The retentates showed nopresence of dendrimers confirming that they were quantitativelyincorporated into the network structure and incapable of leach-

ing out of it. These observations clearly showed that the benefitsfrom these surface coatings were maximized and the costs wereminimized when the lowest concentrations of the coating solutions(i.e., the concentrations sufficient to just achieve uniform coverage)were used in the process.

4.

A. Sarkar et al. / Journal of Membrane Science 349 (2010) 421–428 427

Fig. 3. Visualization of the uniformity of membrane coverage with fluorecein-containing G2 PAMAM–PEG network coatings painted in a cross shape on the membranesurface. (A) Wet membrane under laboratory light; (B) membrane from A after 4 h drying and also under laboratory light; (C) membrane from B viewed under the UV lightsource.

urface

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Fig. 4. SEM photomicrographs of uncoated (A) and coated (B) XLE membrane s

The surfaces of selected samples of uncoated and coated mem-ranes were evaluated by scanning electron microscopy (SEM)see Fig. 4). It was found that surface roughness and physicalrregularities were more pronounced on the uncoated membranesnd that the coating generally smoothed their surfaces. It seemsikely that this smoothing may positively contribute to the mem-rane’s antifouling characteristics (since it is generally believedhat smoother surfaces offer less potential for contaminants to set-le), although this contribution may not be as significant as thatrom increased hydrophilicity (see below).

As expected, a significant increase in hydrophilicity wasbserved by coating the polyamide membrane surfaces withAMAM–PEG crosslinked networks, as indicated by an almostwofold drop of the AWCA value (see Table 2). Furthermore, intro-uction of tethered PEG chains further increased the coating’sydrophilicity, by another twofold decrease of the AWCA valueselative to those of the non-tether-containing coatings (compareables 2 and 3). It was interesting to note, however, that AWCAalues determined after 2 h of membrane drying were significantly

igher (by about 10◦) than those obtained after 10 h or more. Thisould be attributed to absorbed water, the relative amount of whichas always higher in the more hydrophilic coated membranes.

Tables 2 and 3 summarize the results obtained for separationroperties (i.e., % salt rejection and permeate flux) of uncoated and

s. The coating was a G2 PAMAM–PEG network with no free amino end-groups.

of both types of coated membranes. It can be seen from these datathat a significant increase in surface hydrophilicity expressed bya fourfold decrease in AWCA values from uncoated membranes tothose coated with tethered PEG chains was accompanied by no sig-nificant change in the % rejection and by about a 20% decrease inpermeate flux values (compare columns 3 and 4 of Tables 2 and 3).These results appear quite promising considering that these coat-ings had high crosslinking density (of about 50–100% dendrimerNH2 groups) and that it should be expected that flux reductionwould either dramatically decrease or completely disappear ifcrosslinking densities were held to no more than 10–20%. Thisprovides further confidence that with optimal coatings’ composi-tions no deterioration of membrane separation properties is to beexpected.

4. Conclusions and future prospects

We have demonstrated that dendrimer nanotechnology canvery effectively regulate the hydrophilicity of polyamide-based RO

membranes without substantially affecting their separation prop-erties. For this purpose, the crosslinked dendrimer-based coatingsfrom highly hydrophilic polyamidoamine, PAMAM, dendrimersand polyethylene glycol, PEG, linear crosslinkers were developedand evaluated. The coatings were prepared by a simple solution

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[[27] J.F. Youngblood, L. Andruzzi, C.K. Ober, A. Hexemer, E.J. Kramer, J.A. Callow, J.A.

Finlay, M.E. Callow, Biofouling 19 (2003) 91.

28 A. Sarkar et al. / Journal of Mem

asting technique and conditions were optimized to obtain thin,ontinuous and uniform coverage without pinholes or other detri-ental defects. It was found by advancing water contact angleeasurements that membrane hydrophilicity could be quadrupled

epending on the coating type applied, without deterioration of thesalt rejection and acceptable (about 20%) reduction of the per-eate flux. However, since the latter depends on the crosslinking

ensity of the dendrimer coating, this suggests that the observedux reduction can be eliminated by reducing the crosslinking den-ity to lower levels. In fact, the main novelty that dendritic polymerechnology brings to RO membranes is the ability to control theoatings’ nanostructure and to use the extremely high molecularunctionality of dendritic molecules to enable easy, versatile andighly controlled preparation of brush-like surfaces and preciseontrol of crosslinking density. Such a combination of high coatingydrophilicity, dynamic surface topology and very low crosslink-

ng density should provide effective antifouling protection whileetaining all of the desired separation properties of the RO mem-rane substrate.

To the best of our knowledge, this study represents the firstpplication of dendrimer-based networks for coatings of RO mem-ranes and its results show the significant promise of this approachor the control of membrane surface properties at the nano scale.ince both hydrophilicity and dynamic brush-like topology arenown to enhance antifouling properties, the coatings describedn this work may provide a novel approach to the solution of thisery important problem in membrane filtration, particularly withespect to organic and biological contaminants.

eferences

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