Nanofiltration membrane preparation by photomodification of cardo polyetherketone ultrafiltration membrane

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1 Separation and Purification Technology 51 (2006) Nanofiltration membrane preparation by photomodification of cardo polyetherketone ultrafiltration membrane Changquan Qiu a,b, Q.T. Nguyena Liheng Zhang a,b, Zhenghua Ping a,b, a Department of Macromolecular Science, LMEP, Fudan University, Shanghai , PR China b Rouen University-UFR Sciences-UMR 6522, Mont St. Aignan, France Received 22 January 2005; received in revised form 25 January 2006; accepted 3 February 2006 Abstract Hydrophilic nanofiltration membranes were prepared by UV-assisted graft polymerization of hydrophilic monomers onto the surface of ultrafiltration membranes made of cardo polyetherketone (PEK-C). The effect of irradiation time and monomer nature on the membrane performances was investigated. Analyses by Fourier transform infrared in attenuated total reflection mode, contact angles of the modified-membrane surface showed that the grafting degree and the hydrophilicity increase with the irradiation/grafting time or irradiation intensity. It was found that the UV-assisted grafting can be performed on the PEK-C chains with or without benzophenone (BP). The grafting inside the pores of the UV sensitive PEK-C membrane is demonstrated by the change of the membrane retention characteristic from that of an UF membrane to that of a nanofiltration membrane. The retentions to sodium chloride and sodium sulfate of the membrane with poly(acrylic acid) grafted for 90 min. UV irradiation under a 1.6 mw/cm 2 irradiation were 36 % and 94.1%, respectively, while the water flux through the membrane was ca L m 2 h 1, at 0.4 MPa Elsevier B.V. All rights reserved. Keywords: UV-assisted graft polymerization; Ultrafiltration membrane; Nanofiltration membrane; Cardo polyetherketone (PEK-C); Acrylic acid (AA) 1. Introduction Nanofiltration (NF), a pressure-driven separation process, has various applications in many fields, especially in water treatments. Because of its advantages such as low operating pressures, high fluxes, high retentions of multivalent salts, low investment and operation costs [1], this technology has rapidly developed in the last 10 years. The new applications of nanofiltration in the fields of biotechnology and pharmacy, where the fluid phases to be treated are generally complex and heavily loaded with colloidal matters, require hydrophilic membranes with better fouling resistances. To achieve this goal, there are two kinds of modifications that can be made on the polymers known for their good membrane forming properties, like polysulfones: (i) modification of the polymer then membrane formation with the modified material, or (ii) modification of the membrane surface. Sulfonation is one of the efficient methods for material modifications [2 4]. However, polymers with high sulfonation degrees are hardly suitable for membrane formation in aqueous Corresponding author. Tel.: ; fax: address: zhping@fudan.edu.cn (Z. Ping). coagulation bath, and even if a membrane is formed, it is in general less selective and more fragile because of their high swelling in aqueous media. In fact, membranes with varied hydrophilicities are rather scarce in membrane catalogs. Thus, there has been a considerable interest in the methods capable of altering the chemical nature of the membrane surface. The graft polymerization is an attractive one. Such methods would preserve the main membrane mechanical properties, as the modification takes place at the membrane surface, and does not change the properties of the bulk polymer [5 17]. In the graft polymerization technique, the membrane is simply exposed to an irradiation source in the presence of a monomer in the vapor or solution state. The irradiation source may be any of the sources commonly used in chemistry, like low temperature plasma [5 7,17], electron-beam or UV irradiation [8 16] and X- ray irradiation [12]. Among them, the UV irradiation technique presents the clear advantages of rapid reaction, simple operation and low cost, which make its use very extensive. Sofar, the UV-assisted graft polymerization has been tried for surface modification in almost all types of membranes, such as ultrafiltration (UF) [6 12], nanofiltration (NF) [5], reverse osmosis (RO) [18,19], pervaporation (PV) [20 22] and gas separation [23] membranes. Such surface modifications are generally car /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.seppur

2 326 C. Qiu et al. / Separation and Purification Technology 51 (2006) ried out in dilute solutions, and the pore size is not changed during the reaction unless there is an extended homopolymerization in the pore. However, if the graft polymerization takes place in concentrated monomer solutions, a reduction of the membrane pore size is possible, and the surface hydrophilicity and the pore size can be modified at the same time. This work deals with the preparation of NF membranes from a UF membrane made of cardo polyetherketone (PEK-C) by UV-assisted graft polymerization. PEK-C is a thermally stable polymer with solvent resistance and high mechanical properties. Due to the presence of cardo group in its main chain, PEK-C, unlike polyether ether ketone, the parent commercial product, can be dissolved in dimethyl formamide (DMF) or N-methyl pyrrolidone (NMP) to yield polymer dopes that can be used to prepare ultrafiltration (UF) [24,25] or gas separation membranes [26]. However, our preliminary tests indicated that the nanofiltration (NF) membranes made from sulfonated PEK-C by phase inversion have poor performances. It seems that the reactions on the PEK-C are relatively easy due to the presence of the phenolphthalein cardo group in the polymer chain. For instance, PEK-C and cardo polyethersulfone, both having the cardo group, can be sulfonated under more moderate conditions than the parent polymers without the cardo group. The same facilitation could be expected for the UV-initiated graft polymerization reaction. In this paper, we report a simple method for NF membrane preparation by UV-assisted grafting of hydrophilic polymers on the dense layer of a UF membrane made of PEK-C. We studied the polymerization reaction of two monomers: acrylic acid (AA) and 2-hydroxy-ethyl methacrylate (HEMA), under different UV irradiation times. The chemical and physical changes on the membrane surface were characterized by several surface analysis techniques such as FT-IR ATR and contact angle measurements. The membrane performances, i.e. the water permeability and the NaCl and Na 2 SO 4 retentions were measured. 2. Experimental 2.1. Materials Polyetherketone cardo (PEK-C) was purchased from Xuzhou engineering plastic factory of China. Its chemical formula is shown in Fig. 1. The ultrafiltration membrane of PEK-C (of about 10,000 molecular weight cut-off) was prepared in our laboratory by phase inversion of a dope of PEK-C in NMP. Acrylic Fig. 2. Scheme of UV-assisted grafting polymerization equipment. 1, container; 2, UV lamp; 3, ventilated fan; 4, saturated bluestone solution; 5, quartz glass; 6, nitrogen inlet and outlet; 7, PTFE container; 8, monomer solution; 9, membrane. acid (AA, >99%), 2-hydroxyethyl methacrylate (HEMA), benzophenone (BP) and all other reagents were purchased from Shanghai Chemical Reagent Store (Shanghai, China). AA was distilled in vacuum to remove its inhibitor, and HEMA was recrystallized twice from ethanol before use Photochemical modification technique The apparatus for the UV-assisted graft polymerization is schematically represented in Fig. 2. It is a box of 1000 mm 1000 mm 800 mm equipped with an exhaust fan. A UV lamp of 300 W is placed at the center of its ceiling. The membrane holder of φ150 mm 50 mm made of PTFE was equipped with a nitrogen inlet and outlet for the system purging. On the holder, we put a light filter to screen out the high energy part of the lamp spectrum (i.e. irradiation only with wavelengths λ > 300nm). The distance between the UV lamp and the membrane was 150 mm or 50 mm corresponding to UV irradiation energy of ca. 1.6 mw cm 2 or 8.7 mw cm 2, respectively. After mounting the PEK-C UF membrane in the holder with its dense layer turned towards the UV lamp, 5 ml of methanol solution containing 10 wt.% of AA or HEMA, and eventually a fixed amount of BP were added. Nitrogen was bubbled through the liquid medium in the presence of the membrane for Fig. 1. Formula of PEK-C.

3 C. Qiu et al. / Separation and Purification Technology 51 (2006) min to remove the dissolved oxygen, and then the holder was exposed to UV irradiation (with a) under nitrogen atmosphere. In all experiences, we controlled the reaction time by fixing the irradiation duration. The modified membranes were washed with a water/methanol (50/50) mixture in an ultrasonic bath for 2 h at room temperature to remove any unreacted monomers and non-grafted chains from the membrane surface and pores FT-IR analysis Attenuated total reflection-fourier transform infrared spectra (ATR FT-IR) of the membrane surface were obtained using a NICOLET Nexus 470 spectrometer. For each measurement, a total of 128 scans were performed at a resolution of 4 cm 1 using ZnSe crystal. Membranes were dried overnight in a vacuum oven at room temperature before analysis. As the AA modified membranes exhibit an absorbance peak of carbonyl at 1725 cm 1, we defined the IR grafting ration (GR) as the ratio of the peak height at 1725 cm 1 to that at 1500 cm 1 : GR = H 1725 (1) H 1500 where H 1725 is the peak height of the carbonyl group at 1725 cm 1 of the modified membrane; H 1500 is that of the internal reference peak at 1500 cm 1 assigned to the phenyl carbon carbon stretching vibrations Contact angle measurements Static contact angles of the membrane surface were measured using the sessile drop method by SCA20 (Data Physics Inc. Germany). A total of 0.1 ml of deionized water with a microsyringe was dropped onto a dry membrane in air and the contact angle was measured by using a goniometer as soon as possible. The data were averages of 10 measurements with relative error of ± Nanofiltration experiments A lab scale nanofiltration unit supplied by Shanghai Applied Physics Research Institute was used for all performances measurements. The feed solution was pumped through the flat sheet membrane cell, with an effective membrane area of 15.9 cm 2. The operation temperature was 25 C and the applied pressure across the membrane ranged was operated at 0.4 MPa. The retentate was continuously recycled and the permeate was also recycled to the feed tank of 2000 ml after salt concentration determination by conductimetry. The variations of the feed concentration during the experiments were small in those conditions, and were neglected. The permeation flux is defined as the ratio of the permeate weight to the filtration time brought back to the unit membrane area. The concentration of NaCl or Na 2 SO 4 in the feed was held at 0.1% (in weight), and the ph value at 7.0. The observed retention of the solute by the membranes is calculated with the following equation: ( R obs (%) = 1 c ) P 100% (2) c F where c F and c P are the solute concentrations in the feed and in the permeate, respectively. 3. Results and discussion 3.1. Evidence of polymer grafting by FT-IR ATR study The average penetration depth of the evanescent IR waves in the surface in contact with the crystal is ca. a micrometer; the recorded spectra correspond to the waves with the totality of the chemical bonds within this micrometer layer. As the UV light is screened out efficiently by dense matters, we expect that the photografting is mainly performed at the surface or in the shallow regions of the pores close to their opening. This means that for apparently limited amounts of monomers grafted as shown in the spectra (Fig. 3) the outermost layer would consist of the grafted chains, while the layers inside have gradually less grafts. The PEK-C IR spectrum shows two characteristic peaks at 1765 cm 1 and 1686 cm 1 corresponding to the C O stretching vibrations in the ketone and the phenolphthalein groups, respectively (Fig. 3). After UV photografting on the PEK-C membrane with the AA monomer in the presence of BP as a photoinitiator, it was found a new peak at 1725 cm 1 (Fig. 3a). This peak is assigned to the characteristic stretching vibrations of carboxyl group in AA [5], and its appearance demonstrated that the grafting of AA on the PEK-C membrane surface was successfully achieved under our experiment conditions. Similar results were obtained in the case of HEMA (Fig. 3b). Because the weight change due to the grafting is too small, the relative grafting ratio was measured in this work by FT-IR ATR method. The monotonous increase in the grafting ratio of AA or HEMA on the PEK-C membrane surface with the photografting time (see inserted figures in Fig. 3a and b) shows that the extent of grafting of AA or HEMA chains onto the PEK-C membrane surface can be controlled by this parameter Contact angles of water with the grafted-membrane surface The contact angle of water with a surface is usually used as an index of its hydrophilicity. The smaller the contact angle the more hydrophilic the membrane. PEK-C is not a very hydrophobic polymer because of the presence of the phenolphthalein group in the PEK unit. Its contact angle was 69. The contact angle of water with the modified membrane surface steadily decreases with the photografting time or grafting ratio increasing (Fig. 4), whether the reaction medium contains BP initiator or not. With 90 min of a grafting with AA, i.e. at the highest grafting ratio, the contact angle is quite low ( 30 ), indicating a hydrophilization of the membrane surface. It should be noted that there is no changes in the grafting ratio and in the contact angle, when the membrane is washed several

4 328 C. Qiu et al. / Separation and Purification Technology 51 (2006) Fig. 5. Effect of grafting time on water permeation flux at 25 C and 0.4 MPa. Grafting solution: AA: 10 wt% and (a) BP: 0% (b) BP 0.3 wt.%. Fig. 3. FT-IR ATR spectra of PEK-C membrane modified with (a) AA and (b) HEMA at different grafting time. Grafting solution: methanol containing 10 wt.% of monomer and 0.3 wt.% of BP. Insert figure: the normalized ratio of H(1725 cm 1 )/H(1500 cm 1 ) versus grafting time. times with pure water in an ultrasound bath. We conclude that the poly(acrylic acid) that can be formed by homopolymerization in solution was completely removed from the membrane surface. The photografting of AA on the PEK-C membrane can be performed in absence of the photoinitiator BP, although the grafting ratio of AA on the PEK-C membrane in absence of BP is lower than that in presence of BP at the same irradiation condition (Fig. 4). It means that the PEK-C is a photosensitive polymer, maybe due to benzophenone structure in the PEK-C unit (Fig. 1). To modify the grafting ratio, we increased the irradiation intensity (about 8.7 mw cm 2 ) and prepared an AA grafted membrane with lower contact angle in 5 min irradiation without BP (Fig. 4). Always on the basis of the contact angle, the PEK-C membrane modified with polyhema grafts appears to be less hydrophilic than the one modified with PAA grafts (at similar grafting ratio). As the contact angle was measured with the PAA grafts on the membrane in the anionic form (at ph = 7), this means that the polyacrylate grafts impart to the PEK-C membrane has a better hydrophilicity than the polyhema grafts Performance of grafted membranes Fig. 4. Variation of contact angle vs. irradiation time. Water permeation and salt retention are two important parameters that characterize a nanofiltration membrane. A good nanofiltration membrane must have a higher divalent salt retention and water permeation. Unmodified PEK-C membrane is an UF membrane. Under a pressure of 0.4 MPa, the pure water flux across the starting membrane is two orders of magnitude higher than that of the grafted membranes, without any salt retention. With increasing photografting time, the water flux sharply decreases and reaches a minimum value at about 90 min of irradiation (Fig. 5). The water permeation flux decrease can be explained by pore size reduction due to the formation of polymer grafts in the membrane surface pores. Higher the irradiation ratio, lower the pore

5 C. Qiu et al. / Separation and Purification Technology 51 (2006) size of the modified membrane and water permeation flux, In fact, the sole improvement of the surface hydrophilicity would enhance the pore wettability, thus the pure water flux. The salt retention increases with the grafting time confirms the pore size reduction due to UV-grafting on the dense layer of the starting UF membrane. As show in Fig. 6a, with a 10 min. photografting in a solution of 10% AA, the grafted-membrane tested in a feed containing 0.1% Na 2 SO 4, already presented nanofiltration-like properties: the retention of sodium sulfate. Although the salt retention is not high (24% only), it indicated that the pore of the membrane was reduced to the nanometer scale. With a prolonged grafting, the Na 2 SO 4 retention increases to a value of ca. 61% at 90 min. photografting (Fig. 6a). If the grafting is performed with a stonger UV power of 8.7 mw cm 2 for about 5 min, the salt retention can be very high (>95%, at 0.4 MPa, Fig. 6b). The membrane grafted in the presence of BP has higher salt retention comparing with membrane grafted in the absence of BP at the same irradiation condition. By 20 min irradiation, the Na 2 SO 4 retention was already 52% and it was very high with 90 min irradiation (>93%, at 0.4 MPa), for a water flux of about 31.3 L m 2 h 1 (Fig. 6c). The salt retention of the membranes grafted with polyhema was much lower than those grafted with polyacrylate (Fig. 7). Even with a 90 min photografting, the retentions of sodium chloride and sodium sulfate were only 12% and 40%, respectively. It may seem difficult to explain, as according to IR and contact angle data, the graft ratio of HEMA-modified membrane is higher than that of the AA-modified membrane in the absence of BP in the reaction medium (Fig. 4). However, if we refer to the generally accepted salt retention mechanism, this behavior can be explained by a prevailing electrostatic effect of the anionic sites over the steric effect in the salt rejection. In fact, the mass of grafted chains contribute less to the salt screening than the charge density. It was suggested to improve the salt retention by introducing more strong electronegative chains. We will report it later. UV-induced grafting of hydrophilic polymers was rather used to impart low-protein-adsorption or anti-fouling properties to ultrafiltration membranes [6 7,9,11 13]. The surface-modified membranes are still of the ultrafiltration type. Bequet et al. [27] reported for the first time the successful preparation of nanofiltration hollow fibers by UV-induced grafting of ionic polymers onto UF hollow fibers. To transform an UF membrane to a nanofiltration membrane, the hydrophilic polymers must be fixed on the substrate surface, but also on the pore openings. Substantial changes in the transport of molecular solutes can only be expected when the grafting is large enough to interact significantly with the solute molecules, either sterically or electrostatically, depending on the solute nature. This was achieved in the present work, thanks to the high grafting ratio reached with the photosensitive PEK-C. Even at high grafting, as it is confined to the surface, the membrane mechanical properties would be preserved. In spite of the PEK-C photosensitivity, no sign of changes in the membrane mechanical properties, e.g. those due to a swelling or main chain rupture was observed, at least under the experimental conditions used in this work, Fig. 6. Effect of grafting time on membrane performances at 25 C and 0.4 MPa. Grafting solution: AA 10 wt.% and (a) BP: 0%; (b) BP: 0%, 8.7 mw cm 2 ; (c) BP: 0.3 wt.%. i.e. under a rather low near-uv power and irradiation times not exceeding 90 min. The AA modified nanofiltration membrane with low contact angle has a hydrophilic surface and it can be expected to have

6 330 C. Qiu et al. / Separation and Purification Technology 51 (2006) Fig. 7. Effect of graft time on membrane performances at 25 C and 0.4 MPa. Grafting solution: (a) AA: 10%; BP: 0%; (b) HEMA: 10 wt.%; BP: 0.3 wt.%. FTIR-ATR spectroscopy, contact angles showed a substantial physico-chemical and morphology change of the dense surface layer of the PEK-C starting ultrafiltration membrane upon photografting of acrylic acid or hydroxyethyl methacrylate. The change in the membrane properties by photografting of acrylic acid monomers on the membrane dense surface in the absence of any photoinitiator was sufficient to impart nanofiltration properties to an ultrafiltration membrane, although the presence of the BP photoinitiator improved the grafting ratio. The modified PEK-C UF membrane showed clear characteristics of nanofiltration membrane: retention to sodium chloride and sodium sulfate, up to 94% level. By simply changing the photografting time, it is possible to optimize the membrane performances according to the target retention characteristics. Even at high grafting, as it is confined to the surface, the membrane mechanical properties would be preserved. Our work evidenced the good photosensitivity of PEK-C, which makes it possible to graft-polymerize acrylate-type monomers onto this polymer, even in the absence of any photoinitiator, without noticeable alterations of the membrane mechanical properties. The technique is attractive for the upgrading of ultrafiltration membranes to nanofiltration membranes, due to its simplicity, and its low reagent, equipment, and energy costs (common monomers and low-power lamp). Further works are needed to get a better insight in the mechanism of the reactions of grafting of acrylates on PEK-C, to generalize the method to other membrane materials and other monomers. Acknowledgements The financial support was provided by National Basic Research Program of China, No. 2003CB and Project No supported by National Natural Science Foundation of China. References Fig. 8. Variation of permeation flux with time at 25 C and 0.4 MPa. Grafting condition: AA, 10 wt.%; BP, 0.3%, 90 min; feed: 0.1% Na 2 SO 4. a good anti-fouling property. We tested the membrane in a salt solution of 0.1% Na 2 SO 4 for three days. A sharp decrease of about 33% in water flux followed by a levelling-off was observed (Fig. 8). The sharp decline of flux in the first step is probably a result of the compaction of the membrane under pressure [28]. In the rest of the testing time, the water flux is nearly constant. It seems that the hydrophilic grafts may improve the anti-fouling properties of the membrane. 4. Conclusion [1] I.C. Kim, K.H. Lee, T.M. Tak, J. Membr. Sci. 183 (2001) [2] J.F. Blanco, Q.T. Nguyen, P. Schaetzel, J. Membr. Sci. 186 (2001) [3] P. Staiti, F. Lufrano, A.S. Arico, E. Passalacqua, V. Antonucci, J Membr. Sci. 188 (2001) [4] V. Tricoli, N. Carretta, Electrochem. Commun. 4 (2002) [5] Z.P. Zhao, J. Li, D.X. Zhang, C.N. Chen, J. Membr. Sci. 232 (2004) 1 8. [6] J.E. Kilduff, S. Mattaraj, J.P. Pieracci, G. Belfort, Desalination 132 (2000) [7] D.S. Kim, J.S. Kang, K.Y. Kim, Y.M. Lee, Desalination 146 (2002) [8] H. Yamagishi, J.V. Crivello, G. Belfort, J. Membr. Sci. 105 (1995) [9] M. Ulbricht, H. Matuschewski, A. Oechel, H.G. Hicke, J. Membr. Sci. 115 (1996) [10] M. Ulbricht, K. Richau, H. Kamusewitz, Colloids Surf. A: Physicochem. Eng. Aspects 138 (1998) [11] J. Pieracci, J.V. Crivello, G. Belfort, J. Membr. Sci. 156 (1999) [12] J. Pieracci, D.W. Wood, J.V. Crivello, G. Belfort, Chem. Mater. 12 (2000) [13] J. Pieracci, J.V. Crivello, G. Belfort, Chem. Mater. 14 (2002) [14] R. Mazzei, E. Smolko, D. Tadey, L. Gizzi, Nucl. Instr. Methods Phys. Res. B 170 (2000) [15] M. Ulbricht, G. Belfort, J. Appl. Polym. Sci. 56 (1995) [16] M. Ulbricht, G. Belfort, J. Membr. Sci. 111 (1996) [17] Y.M. Lee, J.K. Shim, Polymer 38 (1997) [18] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. Mccarry, D.R. Gagnon, J. Membr. Sci. 108 (1995) [19] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. Mccarry, D.R. Gagnon, J. Membr. Sci. 135 (1997)

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