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Hydrophilic modification of polyethersulfone porous membranes via a thermal-induced surface crosslinking approach
Applied Surface Science 255 (2009) 7273–7278
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Hydrophilic modification of polyethersulfone porous membranes via a thermal-induced surface crosslinking approach Li-Jun Mu *, Wen-Zhen Zhao School of Material Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 November 2008 Received in revised form 22 March 2009 Accepted 24 March 2009 Available online 1 April 2009
A thermal-induced surface crosslinking process was employed to perform a hydrophilic surface modification of PES porous membranes. Difunctional poly(ethylene glycol) diacrylate (PEGDA) was used as the main crosslinking modifier. The addition of trifunctional trimethylolpropane trimethylacrylate (TMPTMA) into the reaction solutions accelerated the crosslinking progress of PEGDA on PES membranes. The membrane surface morphology and chemical composition were characterized by scanning electron microscopy (SEM) and FTIR-ATR spectroscopy. The mass gains (MG) of the modified membranes could be conveniently modulated by varying the PEGDA concentration and crosslinking time. The measurements of water contact angle showed that the hydrophilicity of PES membranes was remarkably enhanced by the coating of crosslinked PEGDA layer. When a moderate mass gain of about 150 mg/cm2 was reached, both the permeability and anti-fouling ability of PES membranes could be significantly improved. Excessive mass gain not only contributed little to the anti-fouling ability, but also brought a deteriorated permeability to PES membranes. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Polyethersulfone membranes Hydrophilic modification Surface crosslinking Anti-fouling ability
1. Introduction As the classical pressure-driven membrane separation processes, ultrafiltration (UF) and microfiltration (MF) have been widely applied in the fields of waste water treatment, water purification, hemodialysis, protein separation, etc. [1,2]. In UF or MF process, the small molecules in feed solution are driven to pass through membrane tunnel, while the components with large sizes are mostly rejected. Usually, some of the rejected organic particles such as colloids, proteins, bacteria deposit or adsorb on membrane surface and pore walls, causing membrane fouling. As a result, the permeation flux gradually declines over time, and the operation pressure has to be elevated in order to keep high separation rate [3,4]. It is well-known that the surface chemistry and physical characteristics of polymer membrane is one of the main factors influencing membrane fouling. Generally, a hydrophilic membrane surface has a strong anti-fouling ability due to the hydration of surface which suppresses the adsorption of organic substances [5,6]. The hydrophilic polymers are usually unable to serve as membrane bulk material as they are susceptible to the swelling in aqueous system. Therefore, in order to obtain anti-fouling UF or MF membranes, most reports focused on the surface hydrophilization of hydrophobic membranes.
* Corresponding author. Tel.: +86 029 86978697; fax: +86 029 86978697. E-mail address: [email protected] (L.-J. Mu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.03.081
Polyethersulfone (PES) is a favorable membrane material due to its excellent chemical resistance, good thermal endurance and mechanical strength [7]. Despite its popularity as a membrane material, the hydrophobic character of PES is disadvantageous in aqueous filtration systems. Many efforts have been made to improve the hydrophilicity and anti-fouling ability of PES membranes. Methods currently used include blending, coating, surface grafting polymerization, as well as chemical modification of membrane bulk materials [8–11]. Most of these processes achieved well the enhancements of fouling resistance. However, each of these techniques is difficult to reach a perfect criterion. Chemical modification of bulk materials or bending with hydrophilic polymers often bring a deterioration in the mechanical properties of membrane. Hydrophilic coatings or additives may be lost during membrane preparation and application in aqueous environments, weakening the effectiveness of hydrophilic modification. Covalent grafting of hydrophilic moieties onto membrane surface requires to introduce reactive groups by exposure to plasma, UV, g-ray, or electron beam etc., which may be complex and difficult to popularize in industry application [12– 14]. Here we report a simple and effectual method for hydrophilic modification of PES porous membranes via a thermal-induced surface crosslinking process. PES membranes were first soaked in the blend solutions of poly(ethylene glycol) diacrylate (PEGDA) and trimethylolpropane trimethylacrylate (TMPTMA). And then the dip-coated membranes were introduced into a box with hot
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2.2. Surface modification of PES membranes by thermal-induced crosslinking
Fig. 1. The chemical structures of PES, TMPTMA, and PEGDA.
air to perform a surface crosslinking step. The changes in membrane morphologies and properties after the surface modification were investigated in detail. It is expected that a convenient and practical technique for membrane hydrophilization is developed to enhance the anti-fouling ability of polymeric UF and MF membranes.
The schematic representation for the surface modification of PES membranes by a thermal-induced crosslinking process is shown in Fig. 2. In a typical procedure, a blend solution of PEGDA and TMPTMA was prepared in advance using ethanol as the solvent. A small amount of BPO was added into the blend solution as the initiator, and BPO concentration was 2.0 g/L. A clean and dried PES membrane was immersed into the blend solution for 30 min. Then the membrane was transferred into a box filled hot air (70 8C) to carry out the crosslinking reaction for a predesigned time. Subsequently, the membrane was taken out and washed thoroughly in an ultrasonic bath filled with deionized water or ethanol. Finally, the modified membrane was fully dried for characterization. The mass gain (MG, mg/cm2) was calculated according to the following equation: W2 W1 MG ¼ Am where W1 and W2 represent the masses of the pristine membrane and the corresponding modified membrane, respectively. Am is the area of the membrane. For each sample, the average value of at least three parallel measurements was reported.
2. Experimental 2.3. Membranes characterization 2.1. Membrane preparation and reagents PES (Ultrason E 6020P, Mw = 58,000) resin was purchased from BASF Co. (Germany) and dried fully before use. PES porous membrane was prepared via a dual-bath coagulation method [15]. N,N-Dimethyl acetamide (DMAc) obtained from SSS Reagent Co. (China) was used as the solvent and poly(vinyl pyrrolidone) (PVP, K30, Sinapharm Group Co.) as a pore-forming agent in membrane preparation. In a typical procedure, casting solution was first prepared by dissolving PES and PVP into DMAc according to a predetermined proportion. PES dope film on a glass plate was immersed into a mixture of DMAc and water (v/ v = 70/30) to pre-gelate. Then the nascent membrane was transferred into a water bath to solidify further. Finally, the as-made membrane was thoroughly rinsed by deionized water and dried for characterization and further surface modification. Poly(ethylene glycol) diacrylate (PEGDA, molecular weight of PEG is 400) was purchased from Shanghai Xingtu Chemical Co. (China). Industrial grade TMPTMA was produced by Jiangsu Tianpeng Chemical Co. (China). The chemical structures of PES, TMPTMA and PEGDA are illustrated in Fig. 1. Benzoyl peroxide (BPO) obtained from Shanghai Ruiteng Chemical Co. (China) was used as the initiator of the crosslinking reaction. All other chemicals in this work were of analytical grade and used without further purification.
The surface chemical changes between the unmodified membrane and the modified membranes were qualitatively investigated using Fourier transform infrared spectroscope (NEXUS 670, Nicolet) with an attenuated total reflectance unit (FTIR/ATR). A contact angle goniometer (CTS-200, Mighty Technology Pvt. Ltd., China) was used to evaluate the hydrophilicity of the unmodified and modified PES membranes. The dynamic permeation process of water drop on porous membranes was recorded using the movie mode. A scanning electron microscope (SEM, Philips XL 30 E, USA) was used to observe the surface and cross-sectional morphologies of these membranes. The membranes were fractured in liquid nitrogen and then sputtered with gold before examination. All these measurements are performed at ambient environments. A dead-end filtration system (Model 8200, Millipore Co., USA) was used to investigate the permeation and anti-fouling properties of the unmodified and modified PES membranes. The effective area of the membrane was 5.3 cm2. The filtration experiments were carried out at a stirring speed of 400 rpm at room temperature. The filtration protocol is similar to that of Shim et al. [16]. In a typical procedure, the membrane was initially pressurized at 0.10 MPa for 30 min. Then the pressure was reduced to 0.05 MPa and the stable pure water flux (Jw1) was measured. Next, the feed solution was displaced by 1 g/L of BSA solution (in 0.01 M PBS, pH 7.4) and the
Fig. 2. Schematic diagram of thermal-induced surface crosslinking on PES membranes.
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steady flux under 0.05 MPa (Jp) was recorded. To investigate the flux recovery properties of the protein-filtered membrane, the membrane was washed for 30 min by pure water flushing, and then the steady-state pure water flux was remeasured (Jw2). The relative flux reduction (RFR) and the flux recovery ratio (FRR) were calculated according to the following equations: Jp J 100%; FRR ¼ w2 100% RFR ¼ 1 J w1 Jw1
3. Results and discussion 3.1. Effects of TMPTMA and PEGDA concentrations on mass gain PEG is a hydrophilic and biocompatible polymer, and often used as a surface modifier of porous polymer membranes. In previous reports, to tether firmly PEG chains onto membrane, reactive groups had to be introduced to membrane surface, and thus destructive surface treatments (such as UV, plasma, electron beam, g-ray irradiation, etc.) were commonly required in advance [17,18]. In the present work, simple surface crosslinking process was employed for surface modification of PES membranes, and PEGDA containing terminating double bonds was used as the main crosslinking monomer. TMPTMA is a trifunctional compound and often used as a crosslinker of printing ink and rubber etc. Both PEGDA and TMPTMA are multifunctional monomers and they are able to crosslink itself or each other at a high temperature. Here, in order to accelerate the crosslinking of PEGDA during surface modification, TMPTMA was used as an assistant crosslinker. The dependence of the mass gain (MG) of PES membranes on crosslinking time in different TMPTMA-added concentration is illustrated in Fig. 3. It is found that, in various TMPTMA concentrations, the MG increases obviously with the increase of crosslinking time. Moreover, the higher the TMPTMA concentration, the more quickly the MG value increases. It is worth noting that in a low TMPTMA concentration (e.g. 0 or 5 g/L), the MG increases almost linearly with the crosslinking time. But in a relatively higher TMPTMA concentration (e.g. 15 g/L), the value of MG ascends with the crosslinking time at first, and then tends to level off when the reaction time exceeds 25 min. When the crosslinking time reaches 30 min, the MG value in the reaction system with 10 g/L of TMPTMA is approximately 151 mg/cm2, which is very close to that in the system with 15 g/L of TMPTMA. These results indicate that the addition of TMPTMA promotes obviously the progress of crosslinking reaction due to the high content of double bonds in TMPTMA. However, the accelerating
Fig. 3. Effect of crosslinking time on the mass gain of PES membranes in various TMPTMA concentrations.
Fig. 4. Effect of PEGDA concentration on the mass gain of PES membranes.
effect is not obvious at the end stage when the crosslinking time is longer than 30 min. Based on the results above, 10 g/L was selected as the TMPTMA concentration, and the investigating crosslinking time was locked at 30 min in the following discussion. PEGDA concentration is another important factor to affect the MG value in the surface modification of PES membranes. Fig. 4 presents the effect of PEGDA concentration on the MG of PES membranes. As can be seen, with the increase of PEGDA concentration from 50 to 150 g/L, the MG value increases almost linearly from 72 to 253 mg/cm2. The increase of the MG value with PEGDA concentration is reasonable because more PEGDA is absorbed into the porous PES membranes during dipping process in a relatively higher PEGDA concentration. The absorbed PEGDA crosslinks to form a hydrophilic layer under the initiation of BPO, and thus the membranes become weightier. When the PEGDA concentration increases further, the rise of MG value tends to slow down. The MG value in the reaction system with 300 g/L of PEGDA is only a little bigger than that in the system with 250 g/L of PEGDA. This phenomenon may be due to a saturated content of PEGDA being reached at the concentration of approximate 250 g/L. Higher concentration of PEGDA solution contributes little to the uptake amount of PEGDA. From the above-mentioned results, it is concluded that the MG value can be conveniently modulated at the range of 0–350 mg/cm2 by varying the crosslinking time and the PEGDA concentration.
Fig. 5. FT-IR/ATR spectra of the unmodified and modified PES membranes: (A) pristine PES membrane; (B)–(D) the modified PES membranes with the mass gain of 115.4, 151.0, 345.6 mg/cm2, respectively.
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Fig. 6. SEM images of the unmodified and modified PES membranes: (A) pristine PES membrane; (B)–(D) the modified PES membranes with the mass gain of 115.4, 253.1, 345.6 mg/cm2, respectively. 1 denotes the separation surface, 2 is the cross-section.
3.2. FT-IR/ATR and SEM characterization of PES membranes Fig. 5 shows the FT-IR/ATR spectra of the pristine and modified PES membranes. Compared to the spectrum of the pristine membrane, a new absorption at about 1728 cm1 is observed in those of the modified membranes. Obviously, this peak is ascribed to the C O stretching vibration in the crosslinked PEGDA and TMPTMA. Moreover, the absorption becomes stronger with the increase of MG value, which indicates that a thicker PEGDA layer is formed. In addition, the peak at about 1150 cm1 seems to become stronger with the MG value, which may be attributed to the C–O–C stretching vibration in PEG chains. These results prove the
presence of the crosslinking layers from PEGDA and TMPTMA on the membrane surfaces. The surface and cross-sectional SEM images of the membranes before and after surface modification are presented in Fig. 6. It can be seen that the pristine PES membrane (A) exhibits a typical microfiltration membrane structure with a porous surface and a network cross-section. The surface pore size is in the range of 0.2– 0.4 mm. After a surface modification, the coverage of crosslinking layer reduces the pore size on membrane surface. With the increase of mass gain (B, C and D), both pore size and number gradually decrease. When the MG value reaches 345.6 mg/cm2, nearly no pore is observed on membrane surface. No obvious
L.-J. Mu, W.-Z. Zhao / Applied Surface Science 255 (2009) 7273–7278
Fig. 7. Dynamic declines of water contact angle with drop time for the pristine and modified PES membranes with various mass gains.
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Fig. 8. Initial water flux (Jw1), BSA solution flux (Jp) and recovery flux (Jw2) for the unmodified and modified PES membranes with different mass gains.
difference is found in the cross-section morphologies between the pristine PES membrane (A2) and the modified ones with lower MGs (B2, C2). However, the cross-section of the modified membrane with a higher MG (D2) seems to become tighter, suggesting that the modification might be able to reach membrane bulk. 3.3. Membrane hydrophilicity and permeability Water contact angle measurements were performed to evaluate the hydrophilicity of the membrane surfaces. Fig. 7 shows the dynamic changes of water contact angle with drop time for the pristine and modified PES membranes with various mass gains. The initial water contact angle for the pristine membrane is about 898, and the contact angle attenuates with time due to the permeation of water drop into membrane pore. The contact angle declines to 668 at the drop time of 185 s. Compared to the unmodified membranes, the initial contact angles of the modified membranes decrease gradually with the increase of mass gain. When the mass gain reach 345.6 mg/cm2, the initial contact angle declines to about 378, which indicates that a highly hydrophilic surface is obtained via this surface crosslinking approach. In addition, it is worth noting that the attenuation of contact angle with time becomes more rapidly when the mass gain of membrane increases. For the membrane with mass gain of 345.6 mg/cm2, the water contact angle decreases to about 68 within the time of 30 s. This result shows that the hydrophilic surface modification is able to touch the pores inside the membranes, which is in agreement with the phenomenon observed by SEM. The results of the initial water flux (Jw1), BSA solution flux (Jp) and recovery flux (Jw2) for the unmodified and modified PES membranes are illustrated in Fig. 8. It can be seen that, with the increase of mass gain, all of Jw1, Jp and Jw2 increase significantly at first, and then decrease obviously when the mass gain exceeds 151.0 mg/cm2. In a relatively lower range, the increase of mass gain raises the membrane hydrophilicity and decreases the resistance of water permeation through membrane, and thus the fluxes go up. However, too high mass gain is disadvantageous to the improvement of membrane permeability due to the excessive coverage of crosslinking layer on membrane pores. That is to say, rather than being dominated by a single aspect, the flux of the modified membranes is affected by the trade-off between membrane structure and surface hydrophilicity. Therefore, a moderate mass gain should be controlled to maximize the membrane permeability.
Fig. 9. Relative flux reduction (RFR) and flux recovery ratio (FRR) for the unmodified and modified PES membranes with different mass gains.
In order to evaluate the anti-fouling abilities of the pristine and modified PES membranes, the anti-fouling factors, including the relative flux reduction (RFR) and the flux recovery ratio (FRR) after washing, were calculated according to the flux data Jw1, Jp and Jw2. The calculated results are shown in Fig. 9. Obviously, it can be found that the RFR value decreases and the FRR value increases with the increase of mass gain. For the pristine PES membrane (mass gain is 0), the RFR and FRR values are 75.5% and 50.7%, respectively. After a surface modification, the RFR and FRR values for the membrane with a mass gain of 151.0 mg/cm2 become 50.8% and 77.8%, respectively. These results allow us to conclude that the modified membranes are less susceptible to fouling than the pristine one. Moreover, the modified membranes have greater flux recoveries after cleaning, which shows that the fouling is more reversible due to the hydrophilic character of the modified membranes. In addition, it is noted that the RFR and FRR values vary only slightly when the mass gain is greater than 151.0 mg/ cm2. This result indicates that the excessive crosslinked PEGDA coating contributes less to the anti-fouling abilities of the modified PES membranes. 4. Conclusion A hydrophilic surface modification of PES porous membranes was successfully performed by a convenient thermal-induced crosslinking process. The addition of trifunctional monomer,
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TMPTMA, into the reaction liquid (PEGDA solution) promoted the crosslinking rate on membrane surface. The mass gain of PES membranes could be conveniently modulated by the crosslinking time and PEGDA concentration. The pore size and number on the surfaces of modified membranes decreased with the increase of mass gain due to the coverage of crosslinking layer. The hydrophilicity of PES membranes was improved significantly by the surface crosslinking process. When a moderate mass gain was obtained (about 150 mg/cm2), both the permeability and antifouling ability of PES membrane could be optimized nicely.
Acknowledgement The financial support from the Tackling Key Problem of Shanxi Science & Technology Bureau (grant no. 2004K07-G12) is gratefully acknowledged.
References [1] M. Cheryan, Ultrafiltration and Microfiltration Handbook, Technomic Publishing Company, Lancaster, Pennsylvania, 1998. [2] R. van Reis, A. Zydney, Curr. Opin. Biotechnol. 12 (2001) 208. [3] I. Huisman, P. Pradanos, A. Hernandez, J. Membr. Sci. 179 (2000) 79. [4] C. Herrero, P. Pradanos, J.I. Calvo, F. Tejerina, A. Hernande, J. Colloid Interface Sci. 187 (1997) 344. [5] J.A. Koehler, M. Ulbricht, G. Belfort, Langmuir 13 (1997) 4162. [6] D. Mockel, E. Staude, M.D. Guiver, J. Membr. Sci. 158 (1999) 63. [7] J.J. Qin, M.H. Oo, Y. Li, J. Membr. Sci. 247 (2005) 137. [8] M.-L. Luo, J.-Q. Zhao, W. Tang, C.-S. Pu, Appl. Surf. Sci. 249 (2005) 76. [9] L.-P. Zhu, B.-K. Zhu, L. Xu, Y.-X. Feng, F. Liu, Y.-Y. Xu, Appl. Surf. Sci. 253 (2007) 6052. [10] Y.-Q. Wang, Y.-L. Su, Q. Sun, X.-L. Ma, Z.-Y. Jiang, J. Membr. Sci. 286 (2006) 228. [11] H. Susanto, M. Balakrishnan, M. Ulbricht, J. Membr. Sci. 288 (2007) 157. [12] D.S. Wavhal, E.R. Fisher, Langmuir 19 (2003) 79. [13] M. Taniguchi, J. Pieracci, W.A. Samsonoff, et al. Chem. Mater. 15 (2003) 3805. [14] L.-P. Zhu, B.-K. Zhu, Y.-X. Feng, et al. Appl. Surf. Sci. 253 (2007) 6052. [15] J.A. van’t Hoff, A.J. Reuvers, R.M. Boom, et al. J. Membr. Sci. 70 (1992) 17. [16] J.K. Shim, H.S. Na, Y.M. Lee, H. Huh, Y.C. Nho, J. Membr. Sci. 190 (2001) 215. [17] H. Ma, L.F. Hakim, C.N. Bowmana, R.H. Davis, J. Membr. Sci. 189 (2001) 255. [18] M. Ulbricht, H. Matuschewski, A. Oechel, H.-G. Hicke, J. Membr. Sci. 115 (1996) 31.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Hydrophilic modification of polyethersulfone porous membranes via a thermal-induced surface crosslinking approach Li-Jun Mu *, Wen-Zhen Zhao School of Material Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 November 2008 Received in revised form 22 March 2009 Accepted 24 March 2009 Available online 1 April 2009
A thermal-induced surface crosslinking process was employed to perform a hydrophilic surface modification of PES porous membranes. Difunctional poly(ethylene glycol) diacrylate (PEGDA) was used as the main crosslinking modifier. The addition of trifunctional trimethylolpropane trimethylacrylate (TMPTMA) into the reaction solutions accelerated the crosslinking progress of PEGDA on PES membranes. The membrane surface morphology and chemical composition were characterized by scanning electron microscopy (SEM) and FTIR-ATR spectroscopy. The mass gains (MG) of the modified membranes could be conveniently modulated by varying the PEGDA concentration and crosslinking time. The measurements of water contact angle showed that the hydrophilicity of PES membranes was remarkably enhanced by the coating of crosslinked PEGDA layer. When a moderate mass gain of about 150 mg/cm2 was reached, both the permeability and anti-fouling ability of PES membranes could be significantly improved. Excessive mass gain not only contributed little to the anti-fouling ability, but also brought a deteriorated permeability to PES membranes. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Polyethersulfone membranes Hydrophilic modification Surface crosslinking Anti-fouling ability
1. Introduction As the classical pressure-driven membrane separation processes, ultrafiltration (UF) and microfiltration (MF) have been widely applied in the fields of waste water treatment, water purification, hemodialysis, protein separation, etc. [1,2]. In UF or MF process, the small molecules in feed solution are driven to pass through membrane tunnel, while the components with large sizes are mostly rejected. Usually, some of the rejected organic particles such as colloids, proteins, bacteria deposit or adsorb on membrane surface and pore walls, causing membrane fouling. As a result, the permeation flux gradually declines over time, and the operation pressure has to be elevated in order to keep high separation rate [3,4]. It is well-known that the surface chemistry and physical characteristics of polymer membrane is one of the main factors influencing membrane fouling. Generally, a hydrophilic membrane surface has a strong anti-fouling ability due to the hydration of surface which suppresses the adsorption of organic substances [5,6]. The hydrophilic polymers are usually unable to serve as membrane bulk material as they are susceptible to the swelling in aqueous system. Therefore, in order to obtain anti-fouling UF or MF membranes, most reports focused on the surface hydrophilization of hydrophobic membranes.
* Corresponding author. Tel.: +86 029 86978697; fax: +86 029 86978697. E-mail address: [email protected] (L.-J. Mu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.03.081
Polyethersulfone (PES) is a favorable membrane material due to its excellent chemical resistance, good thermal endurance and mechanical strength [7]. Despite its popularity as a membrane material, the hydrophobic character of PES is disadvantageous in aqueous filtration systems. Many efforts have been made to improve the hydrophilicity and anti-fouling ability of PES membranes. Methods currently used include blending, coating, surface grafting polymerization, as well as chemical modification of membrane bulk materials [8–11]. Most of these processes achieved well the enhancements of fouling resistance. However, each of these techniques is difficult to reach a perfect criterion. Chemical modification of bulk materials or bending with hydrophilic polymers often bring a deterioration in the mechanical properties of membrane. Hydrophilic coatings or additives may be lost during membrane preparation and application in aqueous environments, weakening the effectiveness of hydrophilic modification. Covalent grafting of hydrophilic moieties onto membrane surface requires to introduce reactive groups by exposure to plasma, UV, g-ray, or electron beam etc., which may be complex and difficult to popularize in industry application [12– 14]. Here we report a simple and effectual method for hydrophilic modification of PES porous membranes via a thermal-induced surface crosslinking process. PES membranes were first soaked in the blend solutions of poly(ethylene glycol) diacrylate (PEGDA) and trimethylolpropane trimethylacrylate (TMPTMA). And then the dip-coated membranes were introduced into a box with hot
L.-J. Mu, W.-Z. Zhao / Applied Surface Science 255 (2009) 7273–7278
7274
2.2. Surface modification of PES membranes by thermal-induced crosslinking
Fig. 1. The chemical structures of PES, TMPTMA, and PEGDA.
air to perform a surface crosslinking step. The changes in membrane morphologies and properties after the surface modification were investigated in detail. It is expected that a convenient and practical technique for membrane hydrophilization is developed to enhance the anti-fouling ability of polymeric UF and MF membranes.
The schematic representation for the surface modification of PES membranes by a thermal-induced crosslinking process is shown in Fig. 2. In a typical procedure, a blend solution of PEGDA and TMPTMA was prepared in advance using ethanol as the solvent. A small amount of BPO was added into the blend solution as the initiator, and BPO concentration was 2.0 g/L. A clean and dried PES membrane was immersed into the blend solution for 30 min. Then the membrane was transferred into a box filled hot air (70 8C) to carry out the crosslinking reaction for a predesigned time. Subsequently, the membrane was taken out and washed thoroughly in an ultrasonic bath filled with deionized water or ethanol. Finally, the modified membrane was fully dried for characterization. The mass gain (MG, mg/cm2) was calculated according to the following equation: W2 W1 MG ¼ Am where W1 and W2 represent the masses of the pristine membrane and the corresponding modified membrane, respectively. Am is the area of the membrane. For each sample, the average value of at least three parallel measurements was reported.
2. Experimental 2.3. Membranes characterization 2.1. Membrane preparation and reagents PES (Ultrason E 6020P, Mw = 58,000) resin was purchased from BASF Co. (Germany) and dried fully before use. PES porous membrane was prepared via a dual-bath coagulation method [15]. N,N-Dimethyl acetamide (DMAc) obtained from SSS Reagent Co. (China) was used as the solvent and poly(vinyl pyrrolidone) (PVP, K30, Sinapharm Group Co.) as a pore-forming agent in membrane preparation. In a typical procedure, casting solution was first prepared by dissolving PES and PVP into DMAc according to a predetermined proportion. PES dope film on a glass plate was immersed into a mixture of DMAc and water (v/ v = 70/30) to pre-gelate. Then the nascent membrane was transferred into a water bath to solidify further. Finally, the as-made membrane was thoroughly rinsed by deionized water and dried for characterization and further surface modification. Poly(ethylene glycol) diacrylate (PEGDA, molecular weight of PEG is 400) was purchased from Shanghai Xingtu Chemical Co. (China). Industrial grade TMPTMA was produced by Jiangsu Tianpeng Chemical Co. (China). The chemical structures of PES, TMPTMA and PEGDA are illustrated in Fig. 1. Benzoyl peroxide (BPO) obtained from Shanghai Ruiteng Chemical Co. (China) was used as the initiator of the crosslinking reaction. All other chemicals in this work were of analytical grade and used without further purification.
The surface chemical changes between the unmodified membrane and the modified membranes were qualitatively investigated using Fourier transform infrared spectroscope (NEXUS 670, Nicolet) with an attenuated total reflectance unit (FTIR/ATR). A contact angle goniometer (CTS-200, Mighty Technology Pvt. Ltd., China) was used to evaluate the hydrophilicity of the unmodified and modified PES membranes. The dynamic permeation process of water drop on porous membranes was recorded using the movie mode. A scanning electron microscope (SEM, Philips XL 30 E, USA) was used to observe the surface and cross-sectional morphologies of these membranes. The membranes were fractured in liquid nitrogen and then sputtered with gold before examination. All these measurements are performed at ambient environments. A dead-end filtration system (Model 8200, Millipore Co., USA) was used to investigate the permeation and anti-fouling properties of the unmodified and modified PES membranes. The effective area of the membrane was 5.3 cm2. The filtration experiments were carried out at a stirring speed of 400 rpm at room temperature. The filtration protocol is similar to that of Shim et al. [16]. In a typical procedure, the membrane was initially pressurized at 0.10 MPa for 30 min. Then the pressure was reduced to 0.05 MPa and the stable pure water flux (Jw1) was measured. Next, the feed solution was displaced by 1 g/L of BSA solution (in 0.01 M PBS, pH 7.4) and the
Fig. 2. Schematic diagram of thermal-induced surface crosslinking on PES membranes.
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steady flux under 0.05 MPa (Jp) was recorded. To investigate the flux recovery properties of the protein-filtered membrane, the membrane was washed for 30 min by pure water flushing, and then the steady-state pure water flux was remeasured (Jw2). The relative flux reduction (RFR) and the flux recovery ratio (FRR) were calculated according to the following equations: Jp J 100%; FRR ¼ w2 100% RFR ¼ 1 J w1 Jw1
3. Results and discussion 3.1. Effects of TMPTMA and PEGDA concentrations on mass gain PEG is a hydrophilic and biocompatible polymer, and often used as a surface modifier of porous polymer membranes. In previous reports, to tether firmly PEG chains onto membrane, reactive groups had to be introduced to membrane surface, and thus destructive surface treatments (such as UV, plasma, electron beam, g-ray irradiation, etc.) were commonly required in advance [17,18]. In the present work, simple surface crosslinking process was employed for surface modification of PES membranes, and PEGDA containing terminating double bonds was used as the main crosslinking monomer. TMPTMA is a trifunctional compound and often used as a crosslinker of printing ink and rubber etc. Both PEGDA and TMPTMA are multifunctional monomers and they are able to crosslink itself or each other at a high temperature. Here, in order to accelerate the crosslinking of PEGDA during surface modification, TMPTMA was used as an assistant crosslinker. The dependence of the mass gain (MG) of PES membranes on crosslinking time in different TMPTMA-added concentration is illustrated in Fig. 3. It is found that, in various TMPTMA concentrations, the MG increases obviously with the increase of crosslinking time. Moreover, the higher the TMPTMA concentration, the more quickly the MG value increases. It is worth noting that in a low TMPTMA concentration (e.g. 0 or 5 g/L), the MG increases almost linearly with the crosslinking time. But in a relatively higher TMPTMA concentration (e.g. 15 g/L), the value of MG ascends with the crosslinking time at first, and then tends to level off when the reaction time exceeds 25 min. When the crosslinking time reaches 30 min, the MG value in the reaction system with 10 g/L of TMPTMA is approximately 151 mg/cm2, which is very close to that in the system with 15 g/L of TMPTMA. These results indicate that the addition of TMPTMA promotes obviously the progress of crosslinking reaction due to the high content of double bonds in TMPTMA. However, the accelerating
Fig. 3. Effect of crosslinking time on the mass gain of PES membranes in various TMPTMA concentrations.
Fig. 4. Effect of PEGDA concentration on the mass gain of PES membranes.
effect is not obvious at the end stage when the crosslinking time is longer than 30 min. Based on the results above, 10 g/L was selected as the TMPTMA concentration, and the investigating crosslinking time was locked at 30 min in the following discussion. PEGDA concentration is another important factor to affect the MG value in the surface modification of PES membranes. Fig. 4 presents the effect of PEGDA concentration on the MG of PES membranes. As can be seen, with the increase of PEGDA concentration from 50 to 150 g/L, the MG value increases almost linearly from 72 to 253 mg/cm2. The increase of the MG value with PEGDA concentration is reasonable because more PEGDA is absorbed into the porous PES membranes during dipping process in a relatively higher PEGDA concentration. The absorbed PEGDA crosslinks to form a hydrophilic layer under the initiation of BPO, and thus the membranes become weightier. When the PEGDA concentration increases further, the rise of MG value tends to slow down. The MG value in the reaction system with 300 g/L of PEGDA is only a little bigger than that in the system with 250 g/L of PEGDA. This phenomenon may be due to a saturated content of PEGDA being reached at the concentration of approximate 250 g/L. Higher concentration of PEGDA solution contributes little to the uptake amount of PEGDA. From the above-mentioned results, it is concluded that the MG value can be conveniently modulated at the range of 0–350 mg/cm2 by varying the crosslinking time and the PEGDA concentration.
Fig. 5. FT-IR/ATR spectra of the unmodified and modified PES membranes: (A) pristine PES membrane; (B)–(D) the modified PES membranes with the mass gain of 115.4, 151.0, 345.6 mg/cm2, respectively.
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Fig. 6. SEM images of the unmodified and modified PES membranes: (A) pristine PES membrane; (B)–(D) the modified PES membranes with the mass gain of 115.4, 253.1, 345.6 mg/cm2, respectively. 1 denotes the separation surface, 2 is the cross-section.
3.2. FT-IR/ATR and SEM characterization of PES membranes Fig. 5 shows the FT-IR/ATR spectra of the pristine and modified PES membranes. Compared to the spectrum of the pristine membrane, a new absorption at about 1728 cm1 is observed in those of the modified membranes. Obviously, this peak is ascribed to the C O stretching vibration in the crosslinked PEGDA and TMPTMA. Moreover, the absorption becomes stronger with the increase of MG value, which indicates that a thicker PEGDA layer is formed. In addition, the peak at about 1150 cm1 seems to become stronger with the MG value, which may be attributed to the C–O–C stretching vibration in PEG chains. These results prove the
presence of the crosslinking layers from PEGDA and TMPTMA on the membrane surfaces. The surface and cross-sectional SEM images of the membranes before and after surface modification are presented in Fig. 6. It can be seen that the pristine PES membrane (A) exhibits a typical microfiltration membrane structure with a porous surface and a network cross-section. The surface pore size is in the range of 0.2– 0.4 mm. After a surface modification, the coverage of crosslinking layer reduces the pore size on membrane surface. With the increase of mass gain (B, C and D), both pore size and number gradually decrease. When the MG value reaches 345.6 mg/cm2, nearly no pore is observed on membrane surface. No obvious
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Fig. 7. Dynamic declines of water contact angle with drop time for the pristine and modified PES membranes with various mass gains.
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Fig. 8. Initial water flux (Jw1), BSA solution flux (Jp) and recovery flux (Jw2) for the unmodified and modified PES membranes with different mass gains.
difference is found in the cross-section morphologies between the pristine PES membrane (A2) and the modified ones with lower MGs (B2, C2). However, the cross-section of the modified membrane with a higher MG (D2) seems to become tighter, suggesting that the modification might be able to reach membrane bulk. 3.3. Membrane hydrophilicity and permeability Water contact angle measurements were performed to evaluate the hydrophilicity of the membrane surfaces. Fig. 7 shows the dynamic changes of water contact angle with drop time for the pristine and modified PES membranes with various mass gains. The initial water contact angle for the pristine membrane is about 898, and the contact angle attenuates with time due to the permeation of water drop into membrane pore. The contact angle declines to 668 at the drop time of 185 s. Compared to the unmodified membranes, the initial contact angles of the modified membranes decrease gradually with the increase of mass gain. When the mass gain reach 345.6 mg/cm2, the initial contact angle declines to about 378, which indicates that a highly hydrophilic surface is obtained via this surface crosslinking approach. In addition, it is worth noting that the attenuation of contact angle with time becomes more rapidly when the mass gain of membrane increases. For the membrane with mass gain of 345.6 mg/cm2, the water contact angle decreases to about 68 within the time of 30 s. This result shows that the hydrophilic surface modification is able to touch the pores inside the membranes, which is in agreement with the phenomenon observed by SEM. The results of the initial water flux (Jw1), BSA solution flux (Jp) and recovery flux (Jw2) for the unmodified and modified PES membranes are illustrated in Fig. 8. It can be seen that, with the increase of mass gain, all of Jw1, Jp and Jw2 increase significantly at first, and then decrease obviously when the mass gain exceeds 151.0 mg/cm2. In a relatively lower range, the increase of mass gain raises the membrane hydrophilicity and decreases the resistance of water permeation through membrane, and thus the fluxes go up. However, too high mass gain is disadvantageous to the improvement of membrane permeability due to the excessive coverage of crosslinking layer on membrane pores. That is to say, rather than being dominated by a single aspect, the flux of the modified membranes is affected by the trade-off between membrane structure and surface hydrophilicity. Therefore, a moderate mass gain should be controlled to maximize the membrane permeability.
Fig. 9. Relative flux reduction (RFR) and flux recovery ratio (FRR) for the unmodified and modified PES membranes with different mass gains.
In order to evaluate the anti-fouling abilities of the pristine and modified PES membranes, the anti-fouling factors, including the relative flux reduction (RFR) and the flux recovery ratio (FRR) after washing, were calculated according to the flux data Jw1, Jp and Jw2. The calculated results are shown in Fig. 9. Obviously, it can be found that the RFR value decreases and the FRR value increases with the increase of mass gain. For the pristine PES membrane (mass gain is 0), the RFR and FRR values are 75.5% and 50.7%, respectively. After a surface modification, the RFR and FRR values for the membrane with a mass gain of 151.0 mg/cm2 become 50.8% and 77.8%, respectively. These results allow us to conclude that the modified membranes are less susceptible to fouling than the pristine one. Moreover, the modified membranes have greater flux recoveries after cleaning, which shows that the fouling is more reversible due to the hydrophilic character of the modified membranes. In addition, it is noted that the RFR and FRR values vary only slightly when the mass gain is greater than 151.0 mg/ cm2. This result indicates that the excessive crosslinked PEGDA coating contributes less to the anti-fouling abilities of the modified PES membranes. 4. Conclusion A hydrophilic surface modification of PES porous membranes was successfully performed by a convenient thermal-induced crosslinking process. The addition of trifunctional monomer,
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TMPTMA, into the reaction liquid (PEGDA solution) promoted the crosslinking rate on membrane surface. The mass gain of PES membranes could be conveniently modulated by the crosslinking time and PEGDA concentration. The pore size and number on the surfaces of modified membranes decreased with the increase of mass gain due to the coverage of crosslinking layer. The hydrophilicity of PES membranes was improved significantly by the surface crosslinking process. When a moderate mass gain was obtained (about 150 mg/cm2), both the permeability and antifouling ability of PES membrane could be optimized nicely.
Acknowledgement The financial support from the Tackling Key Problem of Shanxi Science & Technology Bureau (grant no. 2004K07-G12) is gratefully acknowledged.
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