Iranian Polymer Journal20 (9), 2011, 757-767

surface properties;polyamide thin films;grafting copolymerization;physically adsorbed free radial;hydrophilicity.

(*) To whom correspondence to be addressed.E-mail: yongyiyao@scu.edu.cn

A B S T R A C T

Key Words:

Surface Properties of a Reverse Osmosis MembraneUndergoing Grafting Process

Yongyi Yao1*, Yurong Wu2, and Ruili Xiang3

(1) Textile College, Sichuan University, Chengdu 610065, Sichuan, P.R. China(2) College of Chemistry; (3) Analytical and Testing Center, Sichuan University,

Chengdu 610064, Sichuan, P.R. China

Received 7 March 2011; accepted 17 August 2011

Radical grafting of acrylic acid (AA) monomer onto the aromatic polyamidereverse osmosis (APRO) membrane was performed. The physical adsorption byfree radical initiator technique was applied to modify the surface of the

asymmetric membrane. The hydrophobic initiator adsorbed on the APRO membranewas used to initiate the water-soluble monomer in aqueous phase. The APRO membrane was immerged into ether solution of benzoyl peroxide (BPO). The BPO wasphysically adsorbed onto the membrane surface. The hydrophobic initiator was inhibited to diffuse through membrane surface into the aqueous solution of the acrylicacid monomer because of a hydrophilic/hydrophobic barrier. The grafting process wastriggered by the residing BPO on the membrane surface in an aqueous medium at70°C for a certain time interval. The surface changes of grafted membrane were characterized with ATR-FTIR, SEM, AFM and contact angle measurements. The resultsshowed the PAA is successfully grafted on the APRO membrane surface. When thegrafting rate of PAA was increased from 0% to 8.02%, the edge of leaf-like surface ofAPRO membrane was gradually curled and the roughness of APRO membrane wasdecreased about 36% (Ra). These changes of membrane surface increased thehydrophilicity of the APRO membrane surface and the contact angle decreased byabout 50%.

INTRODUCTION

In membrane desalination, waterdiffuses through a membrane,while salts are almost completelyretained [1]. The membranes usedfor removing the salts are always ofreverse osmosis type membrane ofpolymeric materials including cellulose acetate and aromaticpolyamide. Aromatic polyamidereverse osmosis (APRO) mem-brane has excellent performance inseawater desalination and ultra-pure water preparation due to its

high selective performance andthermal-chemical stability. APROmembranes are usually synthesizedby the interfacial polymerizationfrom hydrophobic monomer, suchas aromatic polyamides and theiranalogues, to form the functionalthin-film-composite (TFC) layer,an asymmetric membrane [2-4].This hydrophobic functional layerof the aromatic polyamide usuallyleads to unwanted adsorption ofnon-polar solutes, hydrophobic

Available online at: http://journal.ippi.ac.ir

particles and protein in the treatment process [5-7].This unwanted adsorption restricts its application. Tosolve this problem, much research has been devoted to the surface modification [8-10]. Surface modification is not only to introduce the functionalmaterials onto a membrane surface but to essentiallypreserve the bulk structure of the membrane. Surfacemodification methods usually involve both chemicaland physical processes. These methods include surface grafting [11-13], surface coating [14,15], surface cross-linking [16,17] and plasma treatment[18,19]. Although these methods can improve the surface hydrophilic APRO membrane, the surfacegrafting with hydrophilic chains has several advantages that include easy and controllable intro-duction of high density grafted chains accompaniedby minor changes in the membrane bulk properties.

Grafting methods are generally divided into two classifications known as grafting-from and grafting-to [7,20,21]. The grafting-to process occurs whenpre-formed polymer chains with reactive functionalgroups are covalently coupled onto the surface.Several researchers [22-24] used the grafting-tomethod successfully to induce the polymeric chainsonto the surface of a thin-film composite polyamidereverse osmosis membrane. However, the densities ofthe polymeric chains by grafting-to method often arelower due to the steric hindrance. In grafting-frommethod, the copolymerization is carried out by reaction of a vinyl monomer with the primary radicalof the membrane surface [25] and the monomer poly-merizes to form a dense grafted chain from the surface. The primary radical can be formed by ionbeam [26,27], plasma [12,19,28], and irradiation[7,29,30] or by electron transfer from a free radicalinitiator. The free radical is generated by hydrophobiccomponents such as benzoyl peroxide or azodiiso-butyronitrile as initiators using heat or by hydrophiliccomponents using a redox reaction between a suitableoxidizer and reductant. For a water-soluble monomer,the grafting process usually starts in aqueous media atroom temperature without an external activation.Belfer et al. [31,32] have modified the polyamidereverse osmosis membranes by grafting hydrophilicchains using the oxidant-reductant initiator system.The grafting copolymerization is performed in the aqueous medium at room temperature to obtain

higher hydrophilic membranes by the grafting-frommethod. However, the water-soluble monomer inaqueous phase forms the monomer radical by a redoxinitiator. This process is poorly controlled because themonomer radical hardly meets the membrane surfaceand it is easily homopolymerized in aqueous phase.Recently, a novel physisorbed free radical graftingtechnique has been developed [21,33]. In this technique, a hydrophobic/hydrophilic diffusional barrier is created between the membrane surface andthe aqueous solution of the monomer. This barrierobstructs the hydrophobic initiator diffusing throughmembrane surface to enter into the aqueous solution.Thus, the initiator remains on the membrane surfacebefore the generation of a primary radical. Finally, the primary radical is generated by immersion of the membrane having the absorbed initiator in a hot monomer solution. Hu et al. [21] reported a grafting-from technique in which hydrophilic poly-mers were created on the surface of poly(dimethylsiloxane) using a physisorbed free radical initiator. Itwas speculated that this method exploited the polari-ty disparity between the hydrophobic initiatingspecies and the polar polymerization medium.McGinty et al. [33] applied this physisorbed free rad-ical grafting technique to modify the surfacehydrophilic property of the poly(vinyl chloride) filmby creating a hydrophobic/hydrophilic diffusionalbarrier that promotes radical generation upon heatingat the polymer surface. The surface polymerizationwas initiated by a physically adsorbed free radicalinitiator system. However, the aim of their research

was to modify the surface hydrophilicity of a homogeneous membrane or complex geometries. Forasymmetric membranes, their works are rarelyapplied.

In the present paper, we report the results achievedby the physically adsorbed free radical initiator system to modify the surface of the asymmetric membrane. In the modification process, thehydrophobic initiator was used to initiate the water-soluble monomer such as an acrylic acid selectively grafting-from the top surface of an asymmetric membrane. The effect of grafting processon the reverse osmosis membrane surface propertieswas investigated. The properties of the grafted membranes were characterized by ATR-IR, SEM,

Surface Properties of a Reverse Osmosis Membrane ... Yao Y et al.

Iranian Polymer Journal / Volume 20 Number 9 (2011)758

AFM and contact angle technique.

EXPERIMENTAL

MaterialsA commercially available aromatic polyamide reverseosmosis membrane such as ULP21 from VontronEnviro-Tech Co. (China) was used in the presentstudy. The active layer of ULP21 was aromaticpolyamide prepared by interfacial polymerizationwhich employed m-phenylene diamine and trimesoylchloride as monomers. Acrylic acid (AA) (analyticalreagent, Chengdu Kelong Agent Co., China) was distilled under reduced pressure at 78 ~ 80°C forremoving the inhibitor before use. Benzoyl peroxide(BPO), ether, acetone and ethyl acetate (analyticalreagent, Chengdu Kelong Agent Co., China) wereused without further purification.

Graft CopolymerizationThe process of the graft polymerization on the top surface of APRO membrane by pre-adsorbed initiators is shown in Figure 1.

The solvent aimed to dissolve the BPO should beselected carefully. The solvents such as acetone, ethylacetate and ether were used to prepare the BPO solution. In our experiments, the APRO membranewas first immersed in the BPO/acetone. The acetonemight make the polyamide layer of reverse osmosismembrane separated from the polysulphone supportlayer. Another trial was to employ BPO/ethyl acetatesolution which led to dissolution of polysulphone support layer. We finally selected ether as the solventfor dissolving the BPO. The support layers of APROmembranes were protected using stainless steel sheet(3.0 cm × 3.0 cm). The APRO membrane with theweight of W1 was dipped into an ether solution of 5%BPO (w/w) for 15 min. The membranes wereremoved from solution and placed into a vacuumoven drying at 25°C for 48 h. The monomer solution(15% AA monomer) and the BPO-pre-adsorbedreverse osmosis membranes were transferred into asealed glass vessel, purged with nitrogen for 1 h andheated at 70°C for 10 min to 90 min. In order toremove the residual monomers and copolymers, thesamples were then exhaustively washed with hot

distilled water (80°C) until neutral pH. The sampleswere dried in vacuum oven at room temperature for48 h. The weight of the samples was designated asW2. The graft ratio was determined by the percentageincrease in weight as the following equation:

where W1 and W2 represent the weights of initial andgrafted APRO membranes, respectively.

Characterization of APRO MembranesFourier Transform Infrared SpectraThe functional group composition of the membranesurfaces was measured by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy.FTIR Spectra were obtained on a Fourier transforminfrared spectrometer (FTS3000, Digilab LLC, USA).The samples were fixed on ATR accessory (PIKETechnologies Inc., USA). The instrument was purgedwith dry nitrogen to prevent interference of atmospheric moisture with the spectra. All spectra (10 scans at a resolution of at least 4 cm-1) wererecorded at 25°C.

Electron Microscopy AnalysisTo characterize the morphology of the membrane surface, a scanning electron microscope (SEM) andan atomic force microscope (AFM) were employed.

For SEM, the surfaces of the membrane sampleswere sputtered with gold in 2-3 nm thickness usingthe EIKO IB-3 ion coater (EIKO Engineering Co.,Japan). The SEM microphotographs were taken byJSM-5900LV (Jeol Co., Japan).

The AFM images of dry modified membranes withdifferent degrees of the polymer grafting and non-modified APRO membrane were taken on theAFM/SPM instrument (SPA400, Seiko Instruments,Japan). The dynamic force mode was used in the surface analysis. The surface area of the imagesscanned was 5×5 μm2. The surfaces of various membranes were compared in terms of roughnessparameter [34]. The roughness parameters obtainedfrom the AFM images should be considered as the relative roughness value. The same tip was used in all experiments and all the captured surfaces were

759Iranian Polymer Journal / Volume 20 Number 9 (2011)

Surface Properties of a Reverse Osmosis Membrane ...Yao Y et al.

1

12(%) ratioGraft W

WW −=

treated in the same way. All AFM images wereobtained at 25°C.

Contact Angle Static contact angle measurements were conductedusing a DataPhysics contact angle system (OCA200,Germany). The samples of 25×50 mm2 and 0.2 mmthickness were used for the determination of surfacehydrophilicity of membranes. A 7.5 μL droplet of filtered, de-ionized water (carbon dioxide in thewater was removed by heating to its boiling point for10 min before use) was placed on the surface of thesamples at room temperature. Each reported value isthe average of five independent measurements.

RESULTS AND DISCUSSION

Grafting Acrylic Acid onto the Top Layer ofAPRO Membranes by Physisorbed Free RadicalInitiation To modify the hydrophilic property of APRO membrane surface, the hydrophilic monomer such as

acrylic acid was grafted by initiation of a pre-adsorbed initiator. The process of graft copoly-merization is shown in Figure 1. In order to selective-ly graft the acrylic acid chains onto the top layer ofthe APRO membrane, the support layer of APROmembrane was protected by stain steel sheet. TheAPRO membrane was immersed into the ether solution of BPO. The BPO as an initiator wasadsorbed on the top layer of APRO membrane. Thepenetration of BPO into the polymer matrix was relatively restrained, with a depth of less than 1 μmfrom the top layer [21]. When the APRO membranewith absorbed BPO, as a hydrophobic organic compound, was immersed into the aqueous solutionof acrylic acid, a barrier was formed between themembrane surface and the aqueous solution. BPOcould not diffuse into the aqueous solution andremained on the top layer of APRO membrane. Whenthe reactive system was heated up to 70°C, thedecomposition of BPO yielded a pair of primary freeradicals (Scheme I).

The free radical on the top surface might undergothe reactions such as combination type reaction with

Iranian Polymer Journal / Volume 20 Number 9 (2011)760

Surface Properties of a Reverse Osmosis Membrane ... Yao Y et al.

Figure 1. Process of graft polymerization.

C O

O

O C

OΔΔ

C

O

O2

Scheme I. Schematic decomposition of benzoyl peroxide.

another benzoyl radical, the addition of an acrylic acidmonomer in the aqueous solution to a primary radicalto yield a chain radical, or abstraction of hydrogenfrom the aromatic polyamide backbone (Scheme II).Because the free radical was also hydrophobic, it wasobstructed to diffuse from the top surface into theaqueous solution. The free radicals still remained onthe top surface of APRO membrane and extracted thehydrogen from the aromatic polyamide backbone toform the surface radicals. Then the top surface radicals initiated the graft copolymerization of AAmonomer in the aqueous solution. The schematicillustration of grafting process is shown in Figure 2.After an appropriate time interval, the membrane wasremoved from the vessel and washed with water until Figure 3. Graft ratio versus reaction time.

Surface Properties of a Reverse Osmosis Membrane ...Yao Y et al.

Iranian Polymer Journal / Volume 20 Number 9 (2011) 761

C

O

O

C

O

O

Δ

H

Top surface

Surface layer

BPO

H C

O

C O O C

C

O

H

O

OH

O

OH

PAA graft on APRO

O O

HN NHCOOC

CONHn

Figure 2. Possible mechanism of membrane surface grafted free radical polymerization.

Scheme II. Structure of the top layer of APRO membrane.

the pH of washing water was neutral in order toremove the monomers and homopolymer.

The change in grafting ratio with reaction time inthe grafting process of AA onto APRO at a monomerconcentration of 15% (by weight) is shown in Figure3. It is clear that the grafting ratio increases with theincreased reaction time from 10 to 90 min. The grafting ratios are from 0.98% to 8.02%.

Fourier Transform Infrared SpectraOriginal membrane and modified membranes werecharacterized by ATR-FTIR spectroscopy. The ATR-FTIR spectra of original membrane and modifiedAPRO membranes are shown in Figure 4. The newgrafted polymers on the APRO membranes were identified in two regions as follows. The spectra in theregion of 3600-2600 cm-1 of membranes modifiedwith AA are given in Figure 4a. A remarkable feature of the spectra in Figure 4a is the significantchange occurred in the shape of the central broad bandand its spreading and flattening after a certain stage ofgrafting. In the spectrum of original membrane, thisband has a complex profile due to the overlapping ofthe hydroxyl stretching mode, 3400 cm-1 and the NHstretching mode, 3301 cm-1. Because of grafting andintrusion of the polyacrylic acid the carboxyl groupsassociation may occur through hydroxyl groups. Thisis reflected in the IR spectra as a band shift attributed

to hydroxyl stretching to a lower frequency [20]. Thestriking similarity between our spectra and the spectrafor reverse acid-amide transformation described in literature [35], provided additional support for theoccurrence of grafting. In the region of aliphaticvibration 2690-2870 cm-1, the growth of the peak at2850 cm-1 and 2942 cm-1 was identified as an evidence of AA grafting (Figure 4a). In the region ofcarbonyl absorbance, the appearance of the two newpeaks attributed to the C=O vibration in the acidgroup (1699 cm-1 and 1716 cm-1) was observed in thespectra of membranes modified with AA (Figure 4b). The intensity of these peaks was greater with increas-es in grafting ratio. We see here the direct relationshipbetween the macroscopic behaviour of the polymerand the microscopic picture of the grafted layer

Morphological Analysis by SEM The morphology of the surface of original membraneand modified membranes was characterized by SEM.The SEM images of the membranes are shown inFigure 5. Figures 5a to 5e illustrate the morphologychanges during the surface grafting. We observe that the surface of original APRO membrane is porous and has many leaf-like sheets (arrowed in the photographs) above the surface of the membrane (Figure 5a). However, for the AA-graftedmembranes (Figures 5b to 5e), we observed that the

Surface Properties of a Reverse Osmosis Membrane ... Yao Y et al.

762 Iranian Polymer Journal / Volume 20 Number 9 (2011)

(a) (b)

Figure 4. ATR-FTIR Spectra of modified reverse osmosis membranes: line 0 is original membrane, grafted rate lines 1-4 are0.98%, 1.31%, 4.45% and 8.02%, respectively.

pores were filled by grafted PAA and the edges of theleaf-like sheets were curled due to the PAA graftingfrom the APRO membrane. The curling degree ofthese edges has increased with the increase of the

grafting ratio. This change was the result of new poly-mer chains onto the membrane surface. Because thenew PAA chains were introduced onto the skeleton ofthe aromatic polyamide, changes occurred in tension

Surface Properties of a Reverse Osmosis Membrane ...Yao Y et al.

Iranian Polymer Journal / Volume 20 Number 9 (2011) 763

(c) (d)

(e)

Figure 5. Morphology of original and modified reverse osmosis membranes: (a) original membrane,(b) to (e) grafted rates are 0.98%, 1.31%, 4.45% and 8.02%, respectively.

(a) (b)

764 Iranian Polymer Journal / Volume 20 Number 9 (2011)

Surface Properties of a Reverse Osmosis Membrane ... Yao Y et al.

(a) (c)

(b) (d)

(e)

Figure 6. Three-dimensional AFM images of original and grafted APRO membranes: (a) originalmembrane, (b) to (e) grafted rates are 0.98%, 1.31%, 4.45% and 8.02%, respectively.

μm

μm

μm

μm

μm

nmnm

nm nm

nm

of the skeleton on the leaf-like sheets surface. Thehigher the graft ratio, the larger the deformation of theleaf-like sheets occurred. These results are consistentwith the supposition that the AA monomer was successfully grafted onto the APRO membrane surface.

Morphological Analysis by AFMThe AFM was used to further study the morphology and roughness of the original membraneand grafted membranes [15,22,35]. The results areshown in Figure 6 and Table 1. According to the AFMimages in Figure 6a, we observed that the surface ofthe original membrane shows the typical nodular morphology. There appeared a peak-and-valley structure consistent with that in literature [23]. In the membrane surface, we could also observe the porousstructure the same as the SEM result. However, thegrafted membranes (Figures 6b to 6e) were differentnot only in the surface morphology but in the porousstructure. With increasing the duration of grafting reaction, the AA copolymers gradually grafted fromthe top surface of the APRO membrane to form a thincoating and filling the valleys. This thin coatingclosed some pores in the membrane. For open pores,we speculated that the inner surface of these poresshould be simultaneously modified by hydrophilicpolymers. This is because the hydrophobic initiatorshould be adsorbed on the inner surface of APROmembrane [21]. Therefore, the graft copolymerizationwas permitted to occur on the inner surface at appropriate condition.

In contrast, the graft copolymerization led to lowersurface roughness. The surface roughness is described

by roughness parameters such as the mean roughness(Ra), the square mean roughness (RMS) and the maximum difference of peak and valley (P-V) [34].The values of the surface roughness of each sampleare shown in Table 1. The surface roughness values ofRa and RMS of original membrane are 119.3 nm and155.5 nm, respectively. When the grafted rate of theAPRO membranes increased from 0.00% to 8.02%,the mean roughness, Ra, decreased from 119.3 nm to75.5 nm and the square mean roughness, RMS,decreased from 155.5 nm to 94.0 nm. These resultsdemonstrate that not only the grafting process of theAA monomers took place on the top surface of theAPRO membrane but the PAA chains were propagat-ed along the normal direction of the top surface. The data of P-V further supported the results(Table 1).

AFM analysis results again confirmed that thePAA chains were grafted from the top surface of theAPRO membrane.

Contact AngleThe objective of surface modification is to improvethe hydrophilic properties of the surface of APROmembrane. The contact angle is usually used to char-acterize the hydrophilic and hydrophobic propertiesof the surface of the membrane. Table 2 shows theeffect of the grafting rate of PAA on the water contactangle of the surface of the modified membranes. Thecontact angle of water decreased with an increase ofthe grafting ratio from 0.00% to 8.02%. As the graftratio was 8.02%, the contact angle of water decreasedto 33.9°. This means the hydrophilicity increased withhigher PAA grafting ratio.

Surface Properties of a Reverse Osmosis Membrane ...Yao Y et al.

Iranian Polymer Journal / Volume 20 Number 9 (2011) 765

Grafting ratio(%)

Roughness

Ra* (nm) RMS# (nm) P-V § (nm)

0.000.981.314.458.02

119.3116.7113.083.675.7

155.5150.7149.0105.1

94.0

132613201211764595

Table 1. Surface roughness of original and grafted APRO membranes.

(*) Mean roughness; (#) square mean roughness; (§) maximum difference of peak and valley.

Table 2. Water contact angles for original and grafted APROmembrane.

CONCLUSION

The physically adsorbed free radical technique wasdemonstrated to successfully apply and modify anasymmetric membrane. In this modification processof the asymmetric membrane, the solvent selection fordissolving the hydrophobic initiator was the key step.The copolymerization of AA monomers initiated bythe physically adsorbed BPO on the APRO membranewas performed. The results of characterization byATR-IR, SEM, AFM and contact angle demonstratedthat the PAA chains are covalently grafted-from theAPRO membrane surface. In the grafting process, thegraft polymerization had a significant impact on thesurface morphology of reverse osmosis membrane.The surface roughness decreased and the hydrophilic-ity increased about 50%.

ACKNOWLEDGEMENT

This work is financially supported by the NationalNatural Science Foundation of P.R. China (Grant No.50473050) and by the Application and Basic Researchof Sichuan Province (Grant No.2010JY0015).

REFERENCES

1. Fritzmann C, Lowenberg J, Wintgens T, Melin T,State-of-the-art of reverse osmosis desalination,Desalination, 216, 1-76, 2007.

2. Chen SH, Chang DJ, Liou MR, Hsu CS, Lin SS,Preparation and separation properties ofpolyamide nanofiltration membrane, J Appl Polym

Sci, 83, 1112-1118, 2002.3. Roh IJ, Greenberg AR, Khare VP, Synthesis and

characterization of interfacially polymerizedpolyamide thin films, Desalination, 191, 279-290,2006.

4. Zou H, Jin Y, Yang J, Dai HJ, Yu XL, Xu J,Synthesis and characterization of thin film com-posite reverse osmosis membranes via novel inter-facial polymerization approach, Sep Purif Technol,72, 256-262, 2010.

5. Koh M, Clark MA, Howe KJ, Filtration of lakenatural organic matter: adsorption capacity of apolypropylene microfilter, J Membr Sci, 256, 169-175, 2005.

6. Li QL, Xu ZH, Pinnau I, Fouling of reverse osmosis membranes by biopolymers in wastewatersecondary effluent: role of membrane surfaceproperties and initial permeate flux, J Membr Sci,290, 173-181, 2007.

7. Bhattacharya A, Misra BN, Grafting: a versatilemeans to modify polymers techniques, factors andapplications, Prog Polym Sci, 29, 767-814, 2004.

8. Ulbricht M, Advanced functional polymer mem-branes, Polymer, 47, 2217-2262, 2006.

9. Mansouri J, Harrisson S, Chen V, Strategies forcontrolling biofouling in membrane filtration systems: challenges and opportunities, J MaterChem, 20, 4567-4586, 2010.

10. Li D, Wang HT, Recent developments in reverseosmosis desalination membranes, J Mater Chem,20, 4551-4566, 2010.

11. Rana D, Matsuura T, Surface modifications forantifouling membranes, Chem Rev, 110, 2448-2471, 2010.

12. Van der Bruggen B, Chemical modification ofpolyethersulfone nanofiltration membranes: areview, J Appl Polym Sci, 114, 630-642, 2009.

13. Kato K, Uchida E, Kang ET, Uyama Y, Ikada Y,Polymer surface with graft chains, Prog PolymSci, 28, 209-259, 2003.

14. Coronell O, Marinas BJ, Zhang XJ, Cahill DG,Quantification of functional groups and modelingof their ionization behavior in the active layer ofFT30 reverse osmosis membrane, Environ SciTechnol, 42, 5260-5266, 2008.

15. Wu DH, Liu XS, Yu SC, Liu MH, Gao CJ,Modification of aromatic polyamide thin-film

Surface Properties of a Reverse Osmosis Membrane ... Yao Y et al.

766 Iranian Polymer Journal / Volume 20 Number 9 (2011)

Grafting ratio (%) Contact angle of water (°)

0.000.981.314.458.02

67.5±0.556.3±0.552.4±0.543.1±0.533.9±0.5

composite reverse osmosis membranes by sur-face coating of thermo-responsive copolymersP(NIPAM-co-Am). I. Preparation and characteri-zation, J Membr Sci, 352, 76-85, 2010.

16. Liu LF, Yu SC, Zhou Y, Gao CJ, Study on a novelpolyamide-urea reverse osmosis composite mem-brane (ICIC-MPD) I. Preparation and characteri-zation of ICIC-MPD membrane, J Membr Sci,281, 88-94, 2006.

17. Ghosh AK, Jeong BH, Huang XF, Hoek EMV,Impacts of reaction and curing conditions onpolyamide composite reverse osmosis membraneproperties, J Membr Sci, 311, 34-45, 2008.

18. Wu SY, Xing J, Zheng C, Xu GF, Zheng GD, XuJP, Plasma modification of aromatic polyamidereverse osmosis composite membrane surface, JAppl Polym Sci, 64, 1923-1926, 1997.

19. Zou L, Vidalis I, Steele D, Michelmore A, LowSP, Verberk JQJC, Surface hydrophilic modifica-tion of RO membranes by plasma polymerizationfor low organic fouling, J Membr Sci, 369, 420-428, 2011.

20. Buonomenna MG, Gordano A, Drioli E,Characteristics and performance of newnanoporous PEEKWC films, Eur Polym J, 44,2051-2059, 2008.

21. Hu SW, Brittain WJ, Surface grafting on polymersurface using physisorbed free radical initiators,Macromolecules, 38, 6592-6597, 2005.

22. Freger V, Gilron J, Belfer S, TFC polyamidemembranes modified by grafting of hydrophilicpolymers: an FTIR/AFM/TEM study, J MembrSci, 209, 283-292, 2002.

23. Kang GD, Liu M, Lin B, Cao YM, Yuan Q, Anovel method of surface modification on thin-film composite reverse osmosis membrane bygrafting poly(ethylene glycol), Polymer, 48,1165-1170, 2007.

24. Van Wagner EM, Sagle AC, Sharma MM, La YH,Freeman BD, Surface modification of commer-cial polyamide desalination membranes usingpoly(ethylene glycol) diglycidyl ether to enhancemembrane fouling resistance, J Membr Sci, 367,273-287, 2011.

25. Bernstein R, Beller S, Freger V, Surface modifi-cation of dense membranes using radical graftpolymerization enhanced by monomer filtration,

Langmuir, 26, 12358-12365, 2010.26. Chennamsetty R, Escobar I, Effect of ion beam

irradiation on two nanofiltration water treatmentmembranes, Sep Sci Technol, 43, 4009-4029,2008.

27. Mi BX, Cahill DG, Marinas BJ, Physico-chemi-cal integrity of nanofiltration/reverse osmosismembranes during characterization byRutherford backscattering spectrometry, JMembr Sci, 291, 77-85, 2007.

28. Boricha AG, Murthy ZVP, Preparation and per-formance of N,O-carboxymethyl chitosan-poly-ether sulfone composite nanofiltration membranein the separation of nickel ions from aqueoussolutions, J Appl Polym Sci, 110, 3596-3605,2008.

29. Hegde C, Isloor AM, Padaki M, WanichapichartP, Yu LD, Synthesis and desalination performanceof Ar+-N+ irradiated polysulfone-based new NFmembrane, Desalination, 265, 153-158, 2011.

30. Chen H, Peng CR, Yao YY, Wang JX, Chen ZP,Yang ZR, Xia LJ, Liu SY, Preparation and characterization of poly(L-lactide) membranesprepared by gamma-radiation-induced grafting ofN-vinyl pyrrolidone, J Appl Polym Sci, 114,3152-3157, 2009.

31. Belfer S, Fainshtain R, Purinson Y, Gilron J,Nystrom M, Manttari M, Surface modification ofcommercial composite polyamide reverse osmo-sis membranes, J Membr Sci, 239, 55-64, 2004.

32. Belfer S, Purinson Y, Fainshtein R, Radchenko Y,Kedem O, Modification of NF membrane proper-ties by in situ redox initiated graft polymerizationwith hydrophilic monomers, J Membr Sci, 139,175-181, 1998.

33. McGinty KM, Brittain WJ, Hydrophilic surfacemodification of poly(vinyl chloride) film andtubing using physisorbed free radical graftingtechnique, Polymer, 49, 4350-4357, 2008.

34. Yao YY, Guo SW, Zhang YX, Surface propertiesof reverse osmosis membrane, J Appl Polym Sci,105, 1261-1266, 2007.

35. Li CL, Huang SH, Liaw DJ, Lee KR, Lai JY,Interfacial polymerized thin film compositemembranes for pervaporation separation of aqueous isopropanol solution, Sep Purif Technol,62, 694-701, 2008.

Surface Properties of a Reverse Osmosis Membrane ...Yao Y et al.

Iranian Polymer Journal / Volume 20 Number 9 (2011) 767