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Available online at www.sciencedirect.com

Journal of Membrane Science 310 (2008) 102–109

Novel thin-film composite membranes with improved waterflux from sulfonated cardo poly(arylene ether sulfone)

bearing pendant amino groups

Guang Chen a,b, Shenghai Li a, Xiaosa Zhang a, Suobo Zhang a,∗a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, Chinab Graduate School of Chinese Academy of Sciences, Beijing 100039, China

Received 5 July 2007; received in revised form 24 October 2007; accepted 25 October 2007Available online 1 November 2007

bstract

A new class of polymeric amine, namely, sulfonated cardo poly(arylene ether sulfone) (SPES-NH2) was synthesized and used for the preparationf thin-film composite membrane. The TFC membranes were prepared on a polysulfone supporting film through interfacial polymerization withrimesoyl chloride (TMC) solutions and amine solutions containing SPES-NH2 and m-phenylenediamine (MPDA). The resultant membranes were

haracterized with water permeation performance, chemical structure, hydrophilicity of active layer and membrane morphology including topurface and cross-section. The membrane prepared under the optimum condition showed the salt rejection and water flux reached 97.3% and1.2 L/m2 h, respectively. The high salt rejection and water flux was attributed to the rigid polymer backbone and the presence of strong hydrophiliculfonic groups.

2007 Elsevier B.V. All rights reserved.

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eywords: Reverse osmosis; Thin-film composite; Polyamides; Sulfonated pol

. Introduction

Reverse osmosis separation has attracted significant researchttention as an economic process in the field of water desalina-ion, ultra-pure water production, and wastewater treatment [1].he development of polyamide thin-film composite (TFC) mem-rane is a major breakthrough in the field of membrane sciencend technology [2–5]. The TFC membranes for reverse osmo-is (RO) are the membranes typically consisted of an ultra-thinctive layer formed in situ on the surface of a microporous sub-trate via interfacial polymerization (IP). In TFC membranes,he active layer is the key component, which controls mainly theeparation properties of the membrane, while the microporousupport gives the necessary mechanical strength. Composite

embranes have advantages over single-material asymmetricembranes in that, the top-active layer is formed in situ and

ence the chemistry and performance of the top barrier layer

∗ Corresponding author.E-mail address: sbzhang@ciac.jl.cn (S. Zhang).

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376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.10.039

lene ether sulfone)

nd the bottom porous substrate can be independently modifiedo maximize the overall membrane performance. Moreover, therocess to generate an active barrier via IP allows one to use aariety of cross-linked or linear polymeric materials, whereassymmetric membrane formation process is quite limited toinear, soluble polymers.

To date, a large number of TFC membranes have been suc-essfully developed from different polymers such as polyurea,olyamides, polyurea-amides, polyether-amides, etc. [6–10],hich have shown excellent salt rejection but relatively lowater permeability for RO applications. It is known that lowater permeability of the membranes fabricated from the var-

ous aromatic polyamides (PA) resulted from the excessivelyight cross-linking and excessively low free volume of the PActive layers [1,2,5]. To enhance water permeability, much effortn the area of TFC membranes has been centered on mak-ng membranes with better water flux either through (i) design

nd synthesis of new polymers forming thin films of the ROembranes or (ii) physical/chemical modification of the thinlms. The chemical modification of the active layer materi-ls by the introduction of hydrophilic sulfonic acid [11] or

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arboxylic acid groups [8], or incorporation of various poly-ers with the flexible main chain or with bulky group in the

ide chain such as poly(vinyl alcohol) and poly(vinyl phe-ol) to the aromatic polyamides [12–18] have been employedo improve water permeability. These modifications result inFC membranes of enhancing water flux but simultaneously anccompanying and considerable loss of salt rejection or viceersa. On analyzing both the experimental data reported in theiterature and the mechanism of the transport of the solventhrough the membranes, it has been concluded that the poly-er materials with hydrophilic group and rigid backbone would

e of improved quality to develop RO membranes. In view ofhese considerations, efforts have been made to develop mem-rane materials based on sulfonated cardo poly(arylene etherulfone) (SPES-NH2) bearing pendant amino groups. It wasxpected that the pendant amino groups in SPES-NH2 couldnterfacially react with multifunctional acyl chloride, so thathe hydrophilic sulfonated polymers could be incorporated tohe aromatic polyamides. The preparation of sulfonated cardooly(arylene ether sulfone) and the fabrication of the TFC mem-ranes on a polysulfone supporting film through interfacialolymerization with trimesoyl chloride (TMC) solutions andmine solutions containing SPES-NH2 and MPDA are reportedn this article. The RO performance, chemical structure of theolymer, hydrophilicity of the active layer, and membrane mor-hology, are also discussed in this article.

. Experimental

.1. Synthesis of the monomers

.1.1. MaterialsPhenolphthalein (PPH) was purchased from Beijing

hemical Reagent Company, and purified by recrystal-ization from mixed solvent of ethanol and water. 4,4′-ichlorodiphenylsulfone (DCDPS) and ethylenedianmine

99%) were purchased from Acros Organics and used aseceived. Trimesoyl chloride (TMC) was purified by the vac-um distillation. N-Methylpyrrolidinone (NMP) were firstlyried with anhydrous calcium hydroxide and then distillednder reduced pressure before use. 3,3′-Disulfonate-4,4′-ichlorodiphenylsulfone (SDCDPS) was prepared according tohe literature method [19]. Other reagents and solvents werebtained commercially and used without further purification.

.1.2. Synthesis of N-aminoethyl-3,3′-bis(4-ydroxyphenyl)-1-isobenzopyrrolidonePPH-NH2)

A 1000 mL, three-necked, round-bottom flask equipped withmechanical stirrer, gas inlet and condenser was charged with0.9 g (0.16 mol) of PPH and 200 mL of ethylenediamine. Theixture was stirred at 120 ◦C for 24 h. The excess of ethylene-

iamine was recovered by distillation. The resulting residues

as slowly poured into a mixture of ice-water and concentratedydrochloric acid and filtrated. The solid obtained by filtrationas redissolved in hot water, and then the solution was titratedith aqueous KOH solution (10%) until pH 8. After cooling to

2awc

e Science 310 (2008) 102–109 103

oom temperature, the appeared solid was collected and recrys-allized from a mixture of ethanol and water (1:1). The yield was0%. 1H NMR (DMSO-d6) δ (ppm): 1.87–1.91 (2H, m, CH2).TIR (KBr): 1639 cm−1 (νC O). Anal. Calcd for C22H20N2O3:, 73.32%, H, 5.59%, N, 7.77%. Found: C, 73.45%, H, 5.56%,, 7.60%.

.2. Synthesis of polymer (SPES-NH2)

To a 100 mL three-necked round-bottomed flask equippedith a Dean-Stark trap, N2 inlet, mechanical stirrer and ther-ometer, was added SDCDPS (4.9125 g, 0.01 mol), PPH-NH2

3.6041 g, 0.01 mol), anhydrous K2CO3 (3.04 g, 0.022 mol),0 mL toluene and 12 mL NMP. The reaction mixture was heatedith stirring at 145 ◦C. After toluene and water had been dis-

illed off, the temperature was raised gradually to 170 ◦C andllowed to react at this temperature for 8 h to give a viscousolution. Then, the system was cooled to room temperature andiluted with NMP. The solution was filtrated to remove inor-anic salts. The filtrate was poured into ethanol to give a whiteolymer, which was washed successively with ethanol and driednder vacuum at 80 ◦C for 12 h. The yield was 93%. FTIRKBr): 1672 cm−1 (νC O), 3360–3450 cm−1 (�N H), 1249 cm−1

νO S O).

.3. Synthesis of the cross-linked polyamide

In order to elucidate the chemical structure of the active layer,he cross-linked polyamide was prepared via the IP of a 1% (w/v)queous amine solution containing MPDA and SPES-NH2 (2:1)nd 1% (w/v) TMC dissolved in cyclohexane solution withoutupported layer. The IP was conducted with the unstirred nondis-ersion method for 4 min at room temperature. The solid formedt the interface of the two solutions was retrieved and precipi-ated excess of acetone. The solid was filtered off and washedith 0.1N HCl, methanol, and water to remove the unreactedonomer reactants and occluded salt. The pure solid polymerasses were dried under vacuum at room temperature and then

sed with no additional post-treatment.

.4. Preparation of thin-film composite membrane

The supporting substrate composed of microporous polysul-one (PSF) was prepared according to the method reported in therevious paper [20]. The active skin layer of the composite mem-rane was prepared by interfacial polymerization technology.irst, the aqueous solution (A) containing 1% (w/v) of a mixturef SPES-NH2 and m-phenylenediamine, triethylamine (TEA)1%) and dodecyl sulfonic acid sodium salt (DDS) (0.05%) wasrepared with pH 10 adjusted by camphor sulfonic acid. Therganic phase solution (B) was prepared with a certain amountf TMC in cyclohexane. Then, the aqueous solution (A) wasoured on top of the support membrane and allowed to soak in

h. Excess solution was drained from the coated surface andir-dried at room temperature until no remaining liquids. After-ards, the substrate membrane was covered with solution B for

ertain time. After removing the excess organic solution, the

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embrane was heated in an oven at 70 ◦C for further polymer-zation. Finally, the membrane was rinsed with deionized waternd stored in 1% NaHSO3 solution. All above operations wereerformed in an assembly clean room.

.5. Performance testing

The membrane samples were checked carefully under a flu-rescent lamp to avoid some obvious defects before test. Allests for RO performance were conducted at 2.0 MPa using a000 ppm NaCl solution at room temperature in cross-flow cells.ircular membrane samples were placed in the test cell with thective skin layer facing the incoming feed. The effective mem-rane area (for each cell) was around 19 cm2. The membranesere initially subjected to a pure water pressure of 2.0 MPa forh prior to performing the RO test experiments.

The water flux was determined by direct measurement ofhe permeate flow in terms of liter per square meter per hourL/m2 h). The salt rejection rate was measured by the salt con-entration in the permeate obtained through measurements ofhe permeate and the feed using a conductance meter (DDS-1A, China). The salt rejection rate was calculated by using theollowing equation:

j (%) =(

1 − cp

cf

)× 100

n which cp (mg/L) is the permeate concentration and cf (mg/L)s the feed concentration.

All membrane samples were prepared and tested in at leastuplicated with a total of three membrane tests for RO perfor-ance, results of which have been averaged.

.6. Characterization

.6.1. Characterizations of monomers and polymerThe monomer PPH-NH2 was identified by elemental analy-

is, such as C, H, and N. The 1H NMR spectra of the monomersnd polymer were measured at 300 MHz on a AV300 spectrom-

tF(t

Scheme 1. Synthetic route of monom

e Science 310 (2008) 102–109

ter. The FT-IR spectra were obtained with a Bio-Rad digilabivision FST-80 spectrometer. Inherent viscosity of polymeras determined at 30 ◦C using Ubbelodhe viscometer with.5 g/dL concentration in DMAc. Solubility of polymer wasetermined at a 5% (w/w) concentration. Thermogravimetricnalysis (TGA) was conducted with a SDT 2960 thermal anal-sis station in flowing nitrogen at a heating rate of 10 ◦C/min.

.6.2. Characterization of membranesThe membranes used for the chemical structure and mor-

hology analysis of the skin layer were rinsed with DI water foreveral times. Then, the membranes were dried under vacuumt 40 ◦C for 24 h.

Attenuated total reflectance infrared (ATR-IR) characteriza-ion of the TFC membrane surface was made with a Bio-Radigilab Division FST-80 spectrometer. For ATR-IR analysis ofembrane samples, Irtran crystal at 45◦ angle of incidence was

mployed.The contact angles of the active layer were estimated by Drop

hape Analysis DSA10 (Kruss Gmbh, Germany) at the ambientemperature. Water droplets (about 5 mg) were dropped carefullyn to the layers.

For atomic force microscope (AFM), measurements wereerformed on SPA300HV with an SPI 3800 controller, Seikonstruments Industry, Co. Ltd.

. Results and discussion

.1. Synthesis and characterization of monomerPPH-NH2)

The synthetic route of the monomers is outlined in Scheme 1.onomer 1 (PPH-NH2) was synthesized by condensation of

henolphthalein and ethylenediamine. The chemical composi-

ion and structure of 1 was confirmed by elemental analysis,T-IR and 1H NMR spectroscope. The FT-IR spectroscopyFig. 1) of the monomer 1 showed that the characteristic absorp-ion of ester carbonyl (C O) at 1772 cm−1 was disappeared,

ers SDCDPS and PPH-NH2.

G. Chen et al. / Journal of Membrane Science 310 (2008) 102–109 105

adomap

3

Fig. 1. FT-IR spectrum of monomer PPH-NH2.

nd strong N-phthalimidine carbonyl absorptions (C O) wereetected at 1639 cm−1, which indicated the successful synthesis

f the monomer 1 (PPH-NH2). The 1H NMR spectrum of theonomer 1 is illustrated in Fig. 2. Assignment of each proton is

lso given in this figure and this spectrum agrees well with theroposed molecular structure of the monomer 1.

Fig. 2. 1H NMR spectrum of monomer PPH-NH2.

ca3pvoIcwsi(

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Scheme 2. Synthetic route o

Fig. 3. 1H NMR spectrum of polymer SPES-NH2.

.2. Synthesis and characterization of polymer (SPES-NH2)

Sulfonated cardo poly(arylene ether sulfone) (SPES-NH2)ontaining pendant aminoethyl groups was synthesized bysolution of polycondensation of PPH-NH2 with disodium

,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS) in theresence of anhydrous K2CO3 (Scheme 2). Intrinsic viscosityalue of the polymer was higher than 0.78 dL/g. The structuref SPES-NH2 was confirmed by 1H NMR (Fig. 3) and FT-R spectra (Fig. 4). For the IR spectrum of the polymer, theharacteristic absorption at 1249 cm−1, 1026 cm−1, 690 cm−1

ere assigned to the stretching vibration (O S O) of sodiumulfonate acid groups. The typical absorption bands for phthal-mide are found around 1705 cm−1 (νsymC O), 1659 cm−1

νasymC O) and 1368 cm−1 (νC N imide), respectively.The solubility of the polymer was tested in various solvents.

he polymer showed good solubility in water, DMSO, DMAcnd NMP, but could not soluble in methanol, ethanol and chlo-oform. The good solubility of the polymeric amine in water is arucial for the fabrication of THC membrane via IP technology.

The thermal properties of these polymers were assessed withhermogravimetric analysis (TGA) in nitrogen. As shown inig. 5, the initial weight loss from 100 ◦C to 200 ◦C is ascribed to

he loss of water molecules, absorbed by the highly hydrophilic

f polymer SPES-NH2.

106 G. Chen et al. / Journal of Membrane Science 310 (2008) 102–109

siTd

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icMmtaweaatsrd

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m(pwloitrirtmcTr

Fig. 4. FT-IR spectrum of polymer SPES-NH2.

odium sulfonate groups. The second weight loss around 300 ◦Cs due to the decomposition of the sodium sulfonate groups.he third stage weight loss above 420 ◦C is assigned to theecomposition of polymer main chain.

.3. Permeation properties of TFC RO membranes

To obtain the composite membrane with the best performancen the reverse process, the membranes were prepared by interfa-ial polymerization of TMC with a mixture of SPES-NH2 andPDA on PSF substrate with different conditions such as theolar ratio of MPDA to SPES-NH2 and the time of polymeriza-

ion. The PSF support layer loaded with the aqueous solution ofmines (1%, w/v) containing SPES-NH2 and MPDA was reactedith cyclohexane solution of TMC (1%, w/v). Fig. 6 shows the

ffect of the molar ratio of MPDA to SPES-NH2 on the perme-tion performance of composite membranes prepared with TMCnd mixed amine reactants. It can be seen that the salt rejec-

ion increased very rapidly with the content of MPDA and thenlightly changed when the molar ratio of MPDA to SPES-NH2eached 2, while water flux of those membranes continuouslyecreased with MPDA content.

Fig. 5. TGA curve of the SPES-NH2.

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ig. 6. The effect of the molar ratio of MPDA to SPES-NH2 on the permeationroperties of TMC/MPDA/SPES-NH2 membrane.

Fig. 7 shows the effect of polymerization time on the per-eation properties of the TFC membrane. The salt rejection

Rj) increased and the water flux decreased with the increasingolymerization time. The maximum salt rejection was obtainedithin 4 min, after which both the salt rejection and water flux

eveled off. The relationship between the permeation propertiesf the membrane and the trimesoyl chloride content was alsonvestigated, and the result is illustrated in Fig. 8. Increasinghe amount of trimesoyl chloride in the polymerization solutionesults in a rapid increase in NaCl separation, correspond-ngly, an approximately exponential decrease in water flux. Theesults revealed that the optimum preparation parameters forhe composite membranes are: concentration of acyl chloride

onomer = 1.0%, the ratio of MPDA to SPES-NH2 = 2:1, andontact time with organic solution = 4 min. Under this condition,FC membrane was prepared. The salt rejection and water flux

eached 97.3% and 51.2 L/m2 h, respectively.The skin layer of the most successful commercial product

FT-30) is composed of cross-linked aromatic polyamide, which

s produced by the interfacial polymerization of MPDA andMC. The performance of membranes prepared here was com-ared with that of the membrane produced by the interfacial

ig. 7. The effect of polymerization time on the permeation properties of theMC/MPDA/SPES-NH2 membrane.

G. Chen et al. / Journal of Membrane Science 310 (2008) 102–109 107

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ig. 8. The effect of TMC content on the permeation properties of the TMC/PDA/SPES-NH2 membrane.

olymerization of MPDA and TMC in our laboratory [20]. Thealt rejection of membrane prepared from SPES-NH2 (97.5%)as slightly lower than that of membrane prepared from MPDA

nd TMC (99%). However, water permeability of the former51.2 L/m2 h) was larger than that of the latter (37.4 L/m2 h).igher water permeability of the membrane prepared fromPES-NH2 than that prepared from MPDA and TMC mightome from the hydrophilic sulfonic acid group in the SPES-H2. The contact angle is a measure of the tendency for theater to wet the membrane surface. The lower contact angleeans the greater tendency for water to wet the membrane and

he higher hydrophilicity. The result of contact angle measure-ent is presented in Table 1, which show the presence of the

arge errors during the measurement of different samples. Evenhis, the results may indicate that the (TMC/MPDA/SPES-NH2)embrane exhibits higher hydrophilicity than MPDA/TMCembrane owing to the presence of sulfonic groups in the for-er. This also explains the higher water flux of the former than

he latter.Kim et al. reported the preparation of TFC membranes

y the interfacial polymerization with various acyl chloridend amine solution containing poly(m-aminostyrene-co-vinyllcohol), (PmAS) and MPDA. Due to the incorporation ofydrophilic and flexible poly(vinyl alcohol) to the aromaticolyamides, the water flux of composite membranes continu-usly increased with VA content in copolymer, while declinen the salt rejection was not serious [21]. The similar behav-

or has been observed in this study for SPES-NH2 composite

embranes. However, both the salt rejection and the water fluxf the SPES-NH2 based membranes are higher than these ofmAS-based membranes. We proposed that the good perfor-

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able 1esults of the measurement of contact angle

eactantsa 1 2 3

MC/MPDA/SPES-NH2(2/1) 65.3 64.7 65.6PDA/TMCb 77.6 77.8 78.5

a Numbers in parenthesis present the molar ratio of reactants.b Dates is cited from the reference reported by Li, etc.

ig. 9. (a) ATR-IR spectrum of the cross-linked polyamides prepared by theethod of IP. (b) ATR-IR spectrum of the composite membrane.

ance of the membranes come from the incorporation of theigid and hydrophilic sulfonated cardo poly(arylene ether sul-one). The interfacial polymerization occurs in the organic phase22]. Amine reactants that are dissolved in the aqueous phaseiffuse into the immiscible organic phase and react therein withMC. Water-soluble polymeric amine reactants (SPES-NH2)ould be expected to not diffusing into the organic phase due

o its poor solubility in the organic phase. However, SPES-NH2ould be incorporated into polyamides by the interfacial reac-ion of the amino groups with TMC, thus leading to the increasedater flux.

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.4. Characterization of the active layer

.4.1. ATR-IR spectrumThe chemical structure of active layers prepared by inter-

acial polymerization without supported layer was identifiedy ATR-IR analysis. As shown in Fig. 9(a), peaks around000–3100 cm−1 correspond the aromatic C H stretchingibrations, and those around 2900–3000 cm−1 correspond theliphatic C H stretching vibrations. The band at 1675 cm−1 isharacteristic of amide I (C O stretch), the 1575 cm−1 bands due to amide II (C N stretch), and the 1596 cm−1 bands assigned to polyamide aromatic ring breathing. The disap-earance of the intensive acid chloride band at 1770 cm−1 alsondicates that the interfacial polymerization and cross-linkingeaction has occurred. Additionally, the strong adsorption ofSO3Na at 1241 cm−1, 1085 cm−1 and 691 cm−1 and thebsorption peak at 1640 cm−1 corresponded the amide (C Otretch) generated from aromatic acid chloride and aliphaticnime were also observed in the spectrum. The ATR-IR spec-rum of the TFC membrane was also measured to confirm thehemical structure of active layer (Fig. 9(b)). Peaks at 1487 cm−1

nd 1585 cm−1 correspond the adsorptions of PSF support [23].he typical absorption peaks assigned to amide (1662 cm−1,

O) and -SO3Na (1242 cm−1, 1080 cm−1, 692 cm−1) could be

ound in this figure, which accord with that in Fig. 9(a). However,he adsorption peak at 1640 cm−1 correspond the amide (C Otretch) generated from aromatic acid chloride and aliphaticnime could not be observed in Fig. 9(b). The reason is that the

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ig. 10. (a) The AFM image of the TMC/MPDA membrane surface, RMS: 109.5 nm0.0 nm.

e Science 310 (2008) 102–109

trong adsorption of PSF overlapped with some adsorption peaksf active layer including peak at 1640 cm−1. Both ATR-IR spec-ra confirmed that SPES-NH2 was successfully incorporated tohe aromatic polyamide.

.4.2. Morphology studiesAn atomic force microscope (AFM) was used to investi-

ate surface morphology of active layer overcoated onto theSF support layer. Fig. 10 exhibits surface morphology of

he composite membranes prepared from TMC with MPDAFig. 10(a)), and TMC with a mixture of MPDA and SPES-H2 with 2:1 mole ratio (Fig. 10(b)), which revealed different

xtents and occurrences of surface roughness. The bar at theottom of the each image indicates the vertical deviations inhe sample with the light regions being the highest and theark regions the lowest. Root-mean-square roughness (RMS)efined as the mean of the root for the deviation from the stan-ard surface to the indicated surface, the smaller RMS, themoother membrane surface [24]. The root-mean-square valuef TMC/MPDA/SPES-NH2(2/1) membrane (40 nm) is smallerhan that of TMC/MPDA membrane (109 nm), indicating themoother surface of TMC/MPDA/SPES-NH2(2/1) membranehan TMC/MPDA membrane. During the interfacial conden-ation to form the thin-film active layer, amine reactants that

re dissolved in the aqueous phase diffuse into the immisciblerganic phase and react therein with TMC. Water-soluble poly-eric amine reactants (SPES-NH2) would be expected to not

iffusing into the organic phase due to its poor solubility in the

. (b) The AFM image of TMC/MPDA/SPES-NH2 membrane surface, RMS:

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rganic phase. As a result, the cross-linking between polyamidend SPES-NH2 would decrease the diffusion rate of the amineeactants into the organic phase. Furthermore, the polyamideormed from MPDA and TMC has most of their macromoleculesrafted on the SPES-NH2 on the surface of the membrane, thuseading to a smoother surface morphology of the membrane.

. Conclusions

The novel sulfonated cardo poly(arylene ether sulfone)SPES-NH2) was synthesized and explored as a new mate-ial for preparation of high performance composite membraneor the reverse osmosis process. The structure of SPES-NH2as confirmed by using FT-IR and 1H NMR. The novel TFCembranes were prepared successfully with trimesoyl chloride

TMC) solutions and amine solutions containing SPES-NH2nd m-phenylenediamine (MPDA) through interfacial polymer-zation technique on the polysulfone supporting film. The saltejection and water flux of the composite membrane preparednder the optimum condition reached 97.3% and 51.2 L/m2 h,espectively. The improved water flux is due to the incorpora-ion of hydrophilic sulfonated cardo poly(arylene ether sulfone)SPES-NH2) to the polyamides. The high salt rejection was con-ributed to the chain stiffness of the copolymer and high degreef cross-linking.

cknowledgements

We thank the National Basic Research Program of China (No.003CB615704) and the National Science Foundation of ChinaNo. 5067308) for the financial support.

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