DSpace at VNU: Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes

Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes Thu Hong Anh Ngo,1 D T Tran,1 Cuong Hung Dinh2,3 Department of Chemical Technology, Faculty of Chemistry, Hanoi University of Science (HUS), Vietnam National University (VNU), 334 Nguyen Trai, Thanh Xuan District, Hanoi 10000, Vietnam Laboratory for Materials and Engineering of Fibre Optics, Institute of Material Science (IMS), Vietnamese Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay District, Hanoi 10000, Vietnam International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 105-0044, Japan Correspondence to: D T Tran (E-mail: tranthidung@hus.edu.vn) ABSTRACT: Surface modification is an effective approach to enhance the properties of polymeric membranes In this work, the UVphoto-induced graft polymerization of acrylic acid (AA) onto the surfaces of polyamide thin film composite (TFC-PA) membranes was carried out using an immersion method performed under ambient conditions The experimental results indicate that the membrane surface becomes more hydrophilic because of the appearance of new carboxylic groups on the surface after the modification This reduces the water contact angle and increases the water permeability compared with the unmodified membrane The membrane surface is relatively compact and smooth due to the formation of the polymeric AA-grafted layer The separation performance of the modified membrane is improved with enhancements of the permeate flux and the retention of humic acid from aqueous feed solutions compared with those of the unmodified membrane The fouling resistance of the membrane is also improved because of the higher maintained flux ratios and the lower irreversible fouling factors for the removal of various organic compounds from feed soluC 2016 Wiley Periodicals, Inc J Appl Polym Sci 2016, 133, 44418 tions V KEYWORDS: grafting; membranes; morphology; polyamide; radical polymerization Received 28 March 2016; accepted 26 August 2016 DOI: 10.1002/app.44418 INTRODUCTION Polymeric membranes have been widely used for many different applications such as the production of pure water, treatment of polluted water, filtration of beer and beverages, and separation of proteins However, one of the major problems with membrane processes is fouling, which is considered to be a severe limitation for membrane applications Fouling may result in a significant decrease in the membrane separation capacity, shorten the membrane lifetime and increase the operational costs of membrane processes Therefore, improving the membrane antifouling property is one of the most important goals in membrane separation technologies Surface modification is a very useful method for developing fouling resistance in polymeric membranes Different techniques can be used for the surface modification of thin film polymeric membranes such as UVinduced graft polymerization,1,2 plasma-induced polymerization,3,4 plasma treatment,5,6 chemical functionalization,7 physical coating of the polymer,8,9 and nanoparticle treatment.10 Using these methods, hydrophilic or charged functional groups can be introduced onto polymeric membrane surfaces, changing the surface chemistry, and topology, thus potentially reducing the fouling factors and enhancing the membrane separation performance.11,12 For enhanced membrane resistance towards fouling, UV-photo-induced graft polymerization is a convenient technique Using this approach, various vinyl monomers have been used for graft polymerization, and the reactions occur only on the surfaces of the membranes The first step involves the absorption of UV light to generate free radicals that serve as nucleation sites on the surface, which prepare the substrate for the grafting of vinyl monomers and their subsequent polymerization The attractive features of UV-induced grafting are the easy and controllable introduction of graft chains with a high density and an exact localization on the polymeric membrane surface Furthermore, the covalent attachment of the graft chains onto a polymer surface is stable, which is in contrast to physically coated polymeric layers.13 In this method, the UV irradiation time and the monomer concentration are the most important parameters that alter the degree of grafting The C 2016 Wiley Periodicals, Inc V WWW.MATERIALSVIEWS.COM 44418 (1 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP modified membranes show more resistance to fouling and a higher rejection than the unmodified ones, but the reduction of permeability due to pore constriction or even blocking by grafting has been observed.14 The antifouling property of a membrane is highly influenced by the surface characteristics such as hydrophilicity and surface electrical charge Membranes with hydrophilic surfaces are less susceptible to fouling than hydrophobic ones, while the ability to recover performance upon washing is higher for the membranes with chemically neutral surfaces than it is for charged membranes.15 The enhanced antifouling property of polymeric membranes has also been obtained via the surface grafting of a zwitterionic copolymer via UV-initiated polymerization.16 Among the polymeric membranes, thin film composite polyamide (TFC-PA) membranes have been widely used for water treatments because of their superior water flux, good resistance to pressure compaction, wide operating pH range and good stability to biological attack.17,18 However, TFC-PA membranes are also sensitive to fouling because of the surface roughness and electrical charge.19–21 In general, the fouling phenomenon can be reduced when the membrane surface becomes more hydrophilic, smoother and/or has the same electrical charge as the foulants.22–24 Recently, many research efforts have been devoted to modifying the membrane surface to improve the TFC-PA membrane filtration performance Belfer et al.25,26 modified a TFC-PA reverse osmosis (RO) membrane by a redox-initiated radical grafting of hydrophilic polymers and the modified membranes exhibited improved fouling resistance Van Wagner et al.27 modified a TFC-PA RO membrane by grafting poly(ethylene glycol) diglycidyl ether onto the membrane surface and found that the modified membranes showed an improved fouling resistance but a lower water flux In another work, the modification of a TFC-PA membrane through in situ polymerization for the purpose of coating with a layer of sorbitol polyglycidyl ether was studied by Kwon et al.28 The modification resulted in a more neutral, hydrophilic and smooth membrane surface, and the formed membrane also showed an improved chlorine stability Yu et al.29 modified the surface of TFC-PA RO membranes by coating with N-isopropylamide-co-acrylic acid copolymers to improve the membrane properties Mondal and Wickramashinghe30 reported that polyamide TFC membrane surfaces were successfully modified by the UV-photo-induced grafting of N-isopropyl acrylamide The grafted membranes had better characteristics in terms of their separation properties, because of their high salt rejections compared with that of the unmodified one The modified membranes also showed good fouling resistance for brackish water desalination Mansourpanah and Habili31 modified thin film polyamide nanofiltration membranes with acrylic acid (AA) and UV irradiation using two methods (1) the finalization of the modification during the formation of the polyamide thin layer and (2) the finalization of the modification after the formation of the polyamide thin layer The first method was very effective due to the simultaneous increases in the flux and rejection, as well as an improved antifouling property for desalting In another work, Cheng et al.32 modified a commercial TFC-PA RO membrane via the redoxinitiated surface graft polymerization of N-isopropylacrylamide WWW.MATERIALSVIEWS.COM (NIPAm) followed by AA The graft polymerization of NIPAm reduced the membrane salt rejection but increased the water permeability, while the following graft polymerization of AA resulted in a decreased water flux and an increased salt rejection The modified membrane possessed an improved fouling resistance to bovine serum albumin (BSA) In general, surface modifications via coating and grafting are effective methods for improving the antifouling and/or chlorine resistance of TFC-PA membranes However, the modified membrane usually shows a lower water flux and/or a decreased salt rejection compared with the unmodified one Therefore, the modification of TFC-PA membranes for the simultaneous improvement of fouling resistance, flux and retention is still very challenging for the development of membrane applications In this work, the UV-induced graft polymerization of AA onto the surfaces of commercial TFC-PA membranes was studied The UV-induced grafting method was chosen due to the advantages of the technique such as mild reaction conditions at a low temperature with high selectivity and easy incorporation into the end stages of membrane manufacturing processes.33 The influences of the surface modification conditions such as UV irradiation time and AA monomer concentration on the membrane characteristics were investigated in terms of the membrane surface properties and the filtration performance EXPERIMENTAL Materials A commercial TFC-PA membrane (Filmtec BW30) was used as the substrate material for the surface modification experiments The TFC-PA membrane consists of a topmost ultrathin polyamide active layer, which was synthesized in situ via interfacial polymerization of m-phenylenediamine and trimesoyl chloride on a reinforced polysulfone porous substrate The commercial TFC-PA membrane developed by Filmtec Corporation demonstrates up to 99.1% NaCl rejection with flux as high as 42.5 L/ m2 h at a pressure of 5.5 MPa.34 The membrane samples used for the surface modification were cut (U 47 mm) and soaked in a 25 v/v % aqueous solution of isopropanol (purity 99.9%, Sigma-Aldrich) for 60 min, carefully rinsed with pure water and then kept wet until they were used for surface grafting AA (liquid, purity 99.0%, Xilong Chemicals, China) was used as the monomer for the graft polymerization without further purification Deionized water with a conductivity of less than lS/cm was used to prepare the aqueous solutions and to rinse the membrane samples Other reagents, such as humic acid (HA) (Wako, Japan), reactive red dye RR261 (China), and pure-grade BSA (Wako, Japan), were used for the preparation of aqueous feed solutions for membrane filtration experiments Modification of TFC Membrane The membrane surface was modified using the UV-photoinduced graft polymerization method, under a UV light (300 nm, 60 W) Aqueous solutions of the monomer (AA) were prepared with different concentrations ranging from to 50 g/ L The membranes were immersed in the AA solutions in Petri dishes under UV irradiation for different times ranging from to 10 After grafting, the membranes were washed carefully 44418 (2 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP spectroscopy (FTIR-ATR, Spectro 100 PerkinElmer) The measurements were performed at a nominal incident angle of 458 with 100 scans at a resolution of cm21 The membrane samples used for the FTIR-ATR analysis were dried at 258C under vacuum before characterization Contact Angle Measurements The wettability of the membrane was examined through the water contact angle (WCA) measurements using a goniometer (DMS012) equipped with a camera, which captured images of deionized water drops on the dried surfaces of the membranes at 258C, and the contact angles were calculated For each sample, three drops (3 lL) were placed at different positions on the membrane surface, and the average value of the contact angles was obtained Scheme Potential mechanism of the surface graft polymerization of AA onto the TFC-PA membrane with deionized water and then kept wet until they were used for the filtration experiments The graft degree was determined based on the difference in the membrane weight before and after grafting The grafting yield in percentage (G, %) was used for the calculation of the graft degree over the surface area of the membrane sample (U 47 mm)    ðm1 –m0 Þ :100 ð%Þ G m0 where m0 and m1 are the weights of the dried membrane samples before and after grafting, respectively A potential mechanism of the UV-photo-induced graft polymerization of AA onto the TFC-PA membrane surface is given in Scheme The first step is the absorption of UV light to abstract hydrogen atoms from amide groups on the base of the polymeric TFC-PA membrane This produces the radical sites required for grafting with AA monomer free radicals, which then form the polymeric AA-grafted chains on the membrane surface Characterization of the Membranes FTIR-ATR Analysis The surface chemical functionalities of the unmodified and modified membranes were characterized using attenuated total reflectance Fourier transform infrared WWW.MATERIALSVIEWS.COM SEM and AFM Images The membrane surface morphology was observed via scanning electron microscopy (SEM) using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) The membrane samples were sputter coated with a nm thick Pt layer before imaging Quantitative surface roughness analysis of the membranes was measured by atomic force microscopy (AFM), using a MultiMode scanning probe microscope The samples were dried under vacuum before analysis Several different positions over a lm lm area were analyzed for each membrane sample to obtain an average value of the root mean square (RMS) roughness The AFM images were obtained from different places on each membrane surface under the same conditions, i.e., temperature, air medium, and scale The surface roughness was calculated using the data analysis software provided by the equipment manufacturer (NanoScope Analysis, Brucker) Evaluation of the Membrane Filtration Properties The membrane filtration experiments were performed in a dead-end membrane filtration system consisting of a stainless steel cylindrical cell of 300 cm3 supplied by Osmonics (USA) and a stirrer connected to a nitrogen gas cylinder, which provided a working pressure in the cell of 15 bar The experiments were carried out at room temperature using a membrane area of 13.2 cm2 The membrane was compacted by pure water at 15 bar for 15 before carrying out the filtration measurements In all of the experiments, the membrane cell was carefully rinsed with pure water before and after using The membrane pure water permeability was determined by   Vw ðL=m2 h barÞ Jw ðA:t:PÞ where Vw is the water volume obtained through a membrane area of A within a time of t at a determined pressure of P The normalized pure water permeability ratio (Jw/Jwo), where Jw and Jwo are the average pure water fluxes of the modified and unmodified membranes, respectively, was used to evaluate the changes in the pure water permeability of the membranes resulting from the surface graft polymerization The retention (R) for the removal of a certain object in the feed solution was determined by 44418 (3 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP RESULTS AND DISCUSSION Membrane Surface Characteristics FTIR-ATR Spectra The functionality of the membrane surface was studied by FTIR spectroscopy using the FTIR-ATR technique to verify the successful graft polymerization of AA onto the surfaces of the membranes The results (Figure 1) show a comparison of the FTIR-ATR spectra of the unmodified membrane and the modified membranes grafted with AA under different grafting polymerization conditions Figure FTIR-ATR spectra of unmodified and modified membrane surfaces: (a) 10 g/L AA - UV (10AA-1), (b) 10AA-5, (c) 50AA-5    C0 2C :100 ð%Þ R C0 where C0 and C are the concentrations of the object in the feed and filtrate, respectively The permeate flux (J) was evaluated by   V ðL=m2 hÞ J ðA:tÞ where V is a filtrate volume obtained through a membrane area of A within a separation time of t at the determined pressure driving force The normalized permeate flux ratio (J/Jo) was used to evaluate the changes in the membrane flux caused by the surface modification, where J and Jo are the average permeate fluxes of the modified and unmodified membranes, respectively For the raw TFC-PA membrane surface, the absorptions of the polyamide active layer are characterized by NAH (1550– 1640 cm21), C@O (1640–1690 cm21), C@C (1400–1600 cm21), and CAN (1080–1360 cm21) signals For the modified membranes, the spectra show new peaks at approximately 1730 cm21, which are ascribed to the carboxylic group of the grafted AA In addition, the absorption intensity of the carboxylic group increases with prolonged graft polymerization times and/or a higher AA monomer concentration This could be due to the differences in the grafting degree on the membrane surfaces modified under the different conditions As shown in Figure 2, the surface modification conditions such as UV irradiation time and AA monomer concentration could greatly influence the graft degree For the same UV irradiation time, the grafting yield clearly increases when the concentration of AA was varied from 10 to 50 g/L Meanwhile, for the same AA concentration, the grafting yield increases more slowly as the UV irradiation time was varied from to 10 The experimental results regarding the grafting yield under the different graft polymerization conditions are also in good agreement with the FTIRATR spectral data Wettability Changes in the membrane wettability that resulted from the graft polymerization of AA were investigated through WCA measurements, which revealed the changes in hydrophilicity that occurred in the outermost layer of the membrane surface after modification Figure shows a comparison of WCAs of the unmodified membrane and modified membrane surfaces formed under the different graft polymerization conditions The obtained results Evaluation of Membrane Antifouling Property The antifouling property of the membranes was estimated through the maintained flux ratio (%) during the filtration of different feed solutions containing high fouling tendency compounds such as HA, dyes or proteins An irreversible fouling factor (FRw) of the membranes was calculated and compared through    Jw1 2Jw2 :100 ð%Þ FRw Jw1 where Jw1 and Jw2 are the pure water fluxes of the membranes before and after using them for the filtration of feed solutions, respectively The higher the maintained flux ratio and the lower the irreversible fouling factors, the better the antifouling property of the membranes WWW.MATERIALSVIEWS.COM Figure Graft degrees on the surfaces of the modified membranes 44418 (4 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP Figure Water contact angles of the unmodified and modified membrane surfaces indicate that the modified membrane surface becomes more hydrophilic due to a significant decrease of WCA, from 518 for an unmodified membrane to 23–258 for the modified ones This is due to the formation of the hydrophilic polymeric AA-grafted layer on the membrane surface after modification The enhancement of the surface hydrophilicity is desirable because it could reduce foulants adsorbed on membrane surface during filtration In addition, as shown in figure, the WCA is clearly decreased and nearly stable for a prolonged UV irradiation time of up to 10 This is also observed for the AA concentrations ranging from 10 to 40 g/L The almost stable WCA values could be due to the stability of the poly(acrylic acid) (PAA) layer formed on the modified membrane surfaces Although the graft degree has been gradually increased for the prolonger grafting time and/or with the higher AA concentration in graft solution, the chemical functionality of the grafted PAA layer is maintained for the modified membranes prepared at the different graft polymerization conditions, as shown in the FTIR-ATR spectra (Figure 1) The formation of PAA-grafted layer leads to the increased hydrophilicity of the membranes, and it is the reason for the reduced WCA; thus, the wettability of the modified membranes improved The improvement in the hydrophilicity of the modified membranes can result not only in enhanced water flux but also in reduced fouling because hydrophilic surfaces are preferentially adsorbed by water and lead to the lower tendency for adsorption of foulants.35,36 In addition, it is also known from the literature that the grafting of acrylic monomers with carboxylic groups can enhance the negative surface charge of the TFC-PA membrane, thereby changing the surface energy of the membrane,25,32 which also affects the membrane antifouling property Figure SEM images of the cross-section and surface of the unmodified membrane (a) and the modified membranes: (b) 10AA-7 and (c) 50AA-7 WWW.MATERIALSVIEWS.COM 44418 (5 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP Figure AFM images of the unmodified membrane (a) and the modified membranes: (b) 10AA-7 and (c) 50AA-7 [Color figure can be viewed at wileyonlinelibrary.com] Membrane Surface Morphology The surface morphological structure of the membrane was characterized by SEM and AFM images Figure shows the SEM images of the unmodified membrane and the modified ones, which were grafted with AA concentrations of 10 g/L and 50 g/L under a UV irradiation time of (10AA-7 and 50AA-7) The cross-section images demonstrate the formation of the polymeric AA-grafted layer on the top, and the grafting polymerization mainly occurred on the surface of the membrane As shown in the figure, the surface of the modified membrane becomes more compact than the unmodified one The AFM images (Figure 5) show the changes in the TFC-PA membrane surface morphology after the grafting polymerization of AA The values of the average and RMS roughnesses (Ra and Rms) are given in Table I and clearly demonstrate that the modification highly influences the surface roughness of the composite polyamide membranes The membrane surface becomes smoother with reduced roughness values of Ra and Rms compared with the unmodified one The lower surface roughness is also desirable because it improves the antifouling properties for membranes Kang and Cao18 and Sagle et al.37 suggested that a smoother surface is commonly expected to experience less fouling, presumably because foulant particles are more likely to be entrained by rougher topologies than by smoother membrane surfaces Consequently, the decrease of surface roughness can improve antifouling property of RO membranes Kochkodan et al.38 suggested also that there is a strong correlation between the fouling and the surface roughness for RO and NF membrane Van der Bruggen et al.19 suggested that surface roughness may also increase membrane fouling by increasing the rate of attachment onto the membrane surface and it is accepted that membranes with a rough surface are more prone to fouling than membranes with a smoother surface Vrijenhoek et al.22 indicated that particles preferentially accumulate in the “valleys” of rough membranes, resulting in “valley clogging” which causes more severe flux decline than in smooth membranes Ishigami et al.39 investigated the antifouling property of polyelectrolyte multilayered RO membranes The results illustrated that the antifouling capacity increased with increasing layer number due to enhanced hydrophilicity and smoothed surface morphology The changes in the membrane surface characteristics after modification confirm the successful grafting polymerization of AA onto the surface of the TFC-PA membrane The changes in the membrane surface functionality and morphological structure could lead to changes in the membrane filtration performance, especially on the antifouling property, because the hydrophilicity and brush-like form of the grafted layer, as well as the lower surface roughness, may reduce the adsorption of foulants on the surface of the modified membrane during filtration Membrane Filtration Performance Pure Water Permeability The difference in the pure water permeability between the unmodified and modified membranes was obtained using the normalized pure water permeability (Jw/ Table I Membrane Surface Roughness Membranes Ra (nm) Rms (nm) Unmodified 93.0 121.0 Modified 10AA-7 24.6 33.3 Modified 50AA-7 39.1 51.1 WWW.MATERIALSVIEWS.COM Figure Normalized pure water permeability of the membranes 44418 (6 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP 25–30% compared with the unmodified one The membrane retention is expressed through the values of R1 for UV254 and R2 for the TOC measurements The results indicate that R1 is higher than R2, and for all of the modified membranes, the values of R1 are nearly maintained (95%); meanwhile, the values of R2 increased from 63% for the unmodified membrane to higher than 80% for the modified ones The improvement in the membrane retention is due to the formation of a polymeric layer grafted on the membrane surface after modification, leading to the relative compactness of the membrane surfaces Meanwhile, the increase in the membrane surface hydrophilicity is the reason for the enhancement of the membrane flux The results concerning membrane retention and flux are in good agreement with the changes in the membrane surface wettability and morphology Figure Retention and flux of the membranes Jwo) The results given in Figure indicate the increased water permeability of the modified membranes, which is 20 to 24 % higher than that of the unmodified one The changes in water permeability caused by the graft polymerization of AA lead to an enhancement in the membrane wettability The obtained experimental results are also in good agreement with the WCA results Retention and Flux The membrane separation property was determined through the possibility for the removal of HA in a neutral feed solution with 50 ppm HA The concentration of HA was evaluated using ultraviolet (UV254) absorption spectroscopy and a total organic carbon (TOC) analyzer The TOC analyzer has a higher oxidation efficiency for smaller and more aliphatic compounds, and UV254 is better for large aromatic compounds The smaller aliphatic compounds would be able to pass more easily through the membrane than the larger aromatic compounds.40 The experimental results (Figure 7) demonstrate that all of the modified membranes have improved separation performance with flux enhancements of more than Antifouling Property The membrane fouling resistance was evaluated through filtration experiments using different aqueous feed solutions containing 50 ppm HA, 50 ppm dye (RR261), or 50 ppm BSA The modified 10AA-7 membrane was selected to perform the fouling experiments and to compare with the base TFC-PA membrane Figure shows a comparison of the decrease in flux based on the maintained flux ratio between the unmodified and the modified membranes The surface characteristics highly impact the membrane antifouling property, which could be improved if the surfaces have a higher hydrophilicity, lower roughness, and/or the same charge as the foulants.11,12 When the filtration time increases, the fluxes of both the unmodified and modified membranes gradually decrease as a result of membrane fouling However, the degree of fouling differs between the two membranes The experimental results illustrate that the flux decrease for all of the modified membranes is less than that of the unmodified one, resulting in a higher flux maintenance during filtration For example, after 60 of filtration, the maintained flux ratios of the unmodified membrane for RR261, HA, and BSA feed solutions are 70, 80, and 70%, while they are 88, 94, and 85% for the modified one, respectively After 300 of filtration, these values for the unmodified membrane are reduced to 65, 70, and 60%, while the values for the modified one are 82, 83, and 72% After 600 Figure Filtration performance of the unmodified and modified membranes WWW.MATERIALSVIEWS.COM 44418 (7 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ARTICLE WILEYONLINELIBRARY.COM/APP Table II Irreversible Fouling Factors of the Membranes FRW (%) Feed solutions Unmodified Modified 30 ppm HA 5.1 4.5 100 ppm HA 8.3 5.1 30 ppm RR261 7.5 5.5 200 ppm RR261 17.9 10.9 1000 ppm BSA 18.1 15.1 5000 ppm BSA 19.5 17.4 of filtration, the maintained flux ratios of the membranes are reduced further; however, the values for the modified one are still higher, indicating the improved fouling resistance of the membrane after the surface grafting of AA The separation performance of the modified membrane was evaluated through the normalized flux (J/Jo) and retention (R) for RR261, HA, and BSA The experimental results reveal that the separation performance of the AA-grafted membrane remains well kept after prolonged usage After 10 h of filtration, the retentions for RR261 and BSA are still maintained at 99.8 and 99.9%, respectively; the retention for HA remains at 99.8% (RUV) and 88.0% (RTOC) Furthermore, the fluxes of the modified membranes are greatly enhanced compared with the base, with normalized flux values of 1.31, 1.27, and 1.28 for the filtration of RR261, HA, and BSA feed solutions, respectively A comparison of the irreversible fouling factors between the unmodified and modified membranes is given in Table II, which indicates that all of the modified membranes have irreversible fouling factors that are lower than the unmodified one The experimental results also point out that the irreversible fouling factor of the membrane increased with the concentration of fouling objects in the feed solution The experimental results illustrate that the antifouling property of the TFC-PA membranes is clearly improved by the graft polymerization of AA onto the membrane surface The improvement in the membrane fouling resistance is mainly due to the improved surface hydrophilicity and the reduced surface roughness of the modified TFC-PA membrane, leading to less fouling and thereby higher maintained flux ratios, as well as lower irreversible fouling factors CONCLUSIONS A commercial TFC-PA membrane surface was successfully modified through the UV-photo-induced graft polymerization of AA using an immersion technique carried out under ambient conditions The experimental results demonstrate that the grafting polymerization led to changes in the membrane surface characteristics and membrane filtration performance The FTIR-ATR spectra illustrate the appearance of new carbonyl groups on the surface after the modification, and the membrane’s surface becomes more hydrophilic with a highly reduced WCA The morphological characterization of the modified membranes observed through SEM and AFM analysis demonstrate the formation of a polymeric AA-grafted layer on the membrane WWW.MATERIALSVIEWS.COM surface, which is relatively compact with a low surface roughness compared with the unmodified one The AA-grafted membranes possess an improved separation performance with an enhancement of both the membrane flux and the retention for the removal of HA in an aqueous feed solution The antifouling of the membrane is also clearly improved after the surface modification, which is due to the higher maintained flux ratios and the lower irreversible fouling factors during the filtration of feed solutions containing strongly fouling objects such as HA, dye and BSA Thus, the surface modification of the TFC-PA membrane via the UV-photo-induced graft polymerization of AA results in an enhancement of both the membrane antifouling property and filtration performance, in terms of simultaneously increasing the membrane retention and flux ACKNOWLEDGMENTS The authors would like to thank the National Foundation for Science and Technology Development (NAFOSTED) for financial support under grant number 104.02-2013.42 They are grateful to the Vietnamese Ministry of Education and Training for support through the Program No 911 Substantial contributions to research design by Dung Thi Tran Acquisition of the data by Thu Hong Anh Ngo, Dung Thi Tran, and Cuong Hung Dinh Interpretation of the data and drafting the paper by Dung Thi Tran and Thu Hong Anh Ngo REFERENCES Wei, X.; Wang, Z.; Wang, J.; Wang, S J Membr Sci 2010, 351, 222 Zhang, Z.; Wang, Z.; Wang, J.; Wang, S Desalination 2013, 309, 187 Yamagishi, H.; Crivello, J V.; Belfort, G J Membr Sci 1995, 105, 237 Tran, D T.; Mori, S.; Tsuboi, D.; Suzuki, M Plasma Process Polym 2009, 6, 110 Kochkodan, V M.; Sharma, V K J Environ Sci Health Part A 2012, 47, 1713 Michelle, L.; Steen, M L.; Hymasa, L.; Havey, E D.; Capps, N E.; Castner, D G.; Fisher, E R J Membr 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Elimelech, M J Membr Sci 2001, 188, 115 36 Tang, C Y.; Kwon, Y N.; Leckie, J O Desalination 2009, 242, 168 23 Ahman, A L.; Ooi, B S J Membr Sci 2006, 255, 67 37 Sagle, A C.; Van Wagner, E M.; Ju, H.; Mc Closkey, B D.; Freeman, B D.; Sharma, M M J Membr Sci 2009, 340, 92 24 Li, Q Z.; Xu, Z.; Pinnau, I J Membr Sci 2007, 290, 173 25 Belfer, S.; Purinson, Y.; Fainshtein, R.; Radchenko, Y.; Kedem, O J Membr Sci 1998, 139, 175 26 Belfer, S.; Gilron, J.; Daltrophe, N.; Priel, M.; Tenzer, B.; Toma, A Desalination 2001, 139, 169 27 Van Wagner, E M.; Sagle, A C.; Sharma, M M.; Na, Y H.; Freeman, B D J Membr Sci 2011, 367, 273 WWW.MATERIALSVIEWS.COM 38 Kochkodan, V.; Johnson, D J.; Hilal, N Adv Colloid Interface Sci 2014, 206, 116 39 Ishigami, T.; Amano, K.; Fuji, A.; Ohmukai, Y.; Kamio, E.; Maruyama, T.; Matsuyama, H Sep Purif Technol 2012, 99, 40 Lowe, J.; Hossain, M M Desalination 2008, 218, 343 44418 (9 of 9) J APPL POLYM SCI 2016, DOI: 10.1002/APP.44418 ... modified thin film polyamide nanofiltration membranes with acrylic acid (AA) and UV irradiation using two methods (1) the finalization of the modification during the formation of the polyamide thin. .. via UV-initiated polymerization. 16 Among the polymeric membranes, thin film composite polyamide (TFC-PA) membranes have been widely used for water treatments because of their superior water flux,... RO membrane via the redoxinitiated surface graft polymerization of N-isopropylacrylamide WWW.MATERIALSVIEWS.COM (NIPAm) followed by AA The graft polymerization of NIPAm reduced the membrane salt