Irdham Kusumawardhana 1

Journal of Colloid and Interface Science 336 (2009) 260–267

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

PTFE/polyamide thin-film composite membranes using PTFE films modified with ethylene diamine polymer and interfacial polymerization: Preparation and pervaporation application Chung-Hao Yu, Irdham Kusumawardhana, Juin-Yih Lai, Ying-Ling Liu * R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, #200, Chung-Pei Road, Chungli, Taoyuan 32023, Taiwan

a r t i c l e

i n f o

Article history: Received 11 February 2009 Accepted 21 March 2009 Available online 5 April 2009 Keywords: Plasma polymerization Interfacial polymerization Composite membrane Poly(tetrafluoroethylene)

a b s t r a c t Plasma polymerization of ethylene diamine (EDA) on PTFE film surfaces is applied to modify PTFE surfaces to become hydrophilic and to incorporate amino groups onto PTFE surfaces. The surface-modified PTFE films are utilized as substrates for interfacial polymerization of EDA and trimesoyl chloride to prepare PTFE/polyamide thin-film composite (TFC) membranes. The effect of plasma power for plasma polymerization on the morphology and performances of the PTFE/PA TFC membranes are examined and discussed. The presence of amino groups on the PTFE substrates provides chemical linkages between PTFE and PA layers in interfacial polymerization to make the PTFE/PA TFC membranes are stable for pervaporation separations. A high permeation flux of 1910 g/h m2 and a separation factor of 290 are observed with the PTFE/PA TFC membranes for pervaporation dehydration on a 70 wt% isopropanol aqueous solution at 70 °C. This approach explores a new method to prepare PTFE-based TFC membranes via interfacial polymerizations. The prepared TFC membranes could be potentially utilized in pervaporation and nanofiltration separations. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Thin-film composite (TFC) membranes are attractive in membrane separations. One major approach to prepare TFC membranes is introduction of the selective layer to the top surfaces of porous substrates through interfacial polymerization [1–9]. In addition to reverse osmosis and nanofiltration, TFC membranes made from interfacial polymerization have also been applied to pervaporation in liquid–liquid separations. One example is polysulfone/polyimide TFC membranes and their application in pervaporation dehydration on ethanol aqueous solutions [9]. Another example is poly(acrylonitril)/polyamide TFC membranes, which were utilized in the pervaporation dehydration on isopropanol (IPA)/water mixtures [10,11]. However, the separation performances of the reported TFC membranes are not impressive enough due to the poor stability of the membranes. Improvements on the stability of TFC membranes could be done with uses of high performance polymers as the substrates of TFC membranes. For examples, poly(ether ether ketone) [12] and polyetherimide [13] were utilized as the substrates in preparation of TFC membranes basing on their good thermal stability and high chemical resistance. Another polymer being used as the porous support of TFC membranes was poly(propylene) [14,15], which exhibits high durability, chemical resistance, and sustainability in pH variation. * Corresponding author. Fax: +886 3 2654199. E-mail address: [email protected] (Y.-L. Liu). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.052

Poly(tetrafluoroethylene), which possesses excellent chemical resistance, good thermal stability, and environment sustainability, is attractive in the applications of membrane separations. However, surface modifications on PTFE are requested for the applications due to its surface inertness and hydrophobicity. To extend the application scopes of PTFE in pervaporation separations, some of our previous efforts have been reported [16,18,17]. In 2006 we reported the first PTFE TFC membrane using porous PTFE films as substrates and chitosan as the separation layers [18]. In 2008, we further applied the interfacial polymerization technique to prepare PTFE/polyamide TFC membranes [17]. It was demonstrated that incorporation of amine groups to PTFE surfaces is necessary for polyamide interfacial polymerization to provide chemical linkages between the PA selective layer and the PTFE substrate. In that work we performed surface-initiated polymerization and post chemical modification to incorporate the amine groups to PTFE surfaces. However, the synthetic routes and steps were complicated and the modification efficiency was not high. Plasma polymerization has been widely used in surface modifications due to its simplicity and eases of operations [19–21]. Therefore, in this work we try to employ plasma polymerization into the preparation route of the PTFE/PA TFC membranes to replace the complicated surface-initiated polymerizations. Since amine groups are desired to be incorporated onto PTFE surface after plasma polymerization, ethylene diamine (EDA) is utilized as the monomer in plasma polymerization. The effect of plasma power for plasma polymerization on the morphology and

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

performances of the resulting PTFE/PA TFC membranes are examined and discussed. The EDA plasma-polymerized PTFE films are successful for the preparation of PTFE/PA TFC membranes and the membranes exhibits high performances in the pervaporation dehydration on an IPA aqueous solution. 2. Experimental 2.1. Materials PTFE films with a thickness of 500 lm and pore sizes of 0.2– 0.4 lm were received from Yu-Min-Tai Co., Ltd., Taiwan. Ethylene diamine (EDA) was purchased from Acros Chemical Co. Trimesoyl chloride (TMC) from Aldrich Chemical Co. was used as received. 2.2. Surface modification on PTFE with EDA by plasma polymerization PTFE surfaces were pre-treated with hydrogen plasma using a capacity coupling electrodes (Dressler HF-Technik GmbH, Germany; model CESAR-1310) under the conditions of a hydrogen mass flow rate of 10 standard cubic centimeter per minute (sccm), a radio frequency of 13.56 MHz, a power of 50 W, a pressure of 0.3 torr, and a reaction time of 180 s [22]. Afterwards, the system was evacuated to a pressure of 0.03 torr. EDA vapor was introduced to the system at 80 °C under vacuum. Plasma polymerization of EDA on PTFE film surfaces was conducted with various plasma powers for a polymerization time of 120 min. The flow rate of EDA was 13 sccm. The modified PTFE films were washed with distilled water to remove out the physically absorbed EDA monomers and polymeric segments, and then dried under vacuum to result in the PTFE–PEDA-X films, where X denotes to the plasma power (in watt) utilized for EDA polymerization. 2.3. Preparation of PTFE/PA TFC membranes via interfacial polymerization [17] PTFE–PEDA films were immersed in a 5 wt% EDA aqueous solution for 30 min. The films were taken out. After sweeping off the EDA solution on the film surfaces, the films were put into a TMC solution (1 wt% in toluene) for 3 min. The films were taken out, dried under ambient environment, and heat-treated at 70 °C for 1 h. Finally the samples were washed with methanol and dried under ambient environment to give the PTFE/PA-X TFC membranes, where X denotes to the plasma power (in watt) utilized for EDA polymerization in preparation of PTFE–PEDA-X films.

261

and the downstream (permeate side) pressure is 667–1067 Pa. While the system was in steady-state (usually after a pre-operation in 2 h), the data was taken in 1 h period of continuing operation. The compositions of feeding solutions and permeates were determined with a gas chromatography (China Chromatography GC8700T). The separation factor (awater/organic compound) was calculated from equation of

awater=organic compound ¼ ðY water =Y organic compound Þ=ðX water =X organic compound Þ; where Y and X are the concentrations of permeate and feeding solutions, respectively, and the subscription (water and organic compound) indicates the species. Permeation flux was determined by measuring the weight of permeate liquid through the membrane at given time. Data was obtained from the average of measuring results from four pieces of separate membranes. 3. Results and discussion 3.1. EDA plasma polymerization on PTFE surfaces Amino-functionalized PTFE surfaces were successfully utilized in preparation of PTFE/PA TFC membranes via interfacial polymerization in the previous work [17]. To simplify the process of aminofunctionalization on PTFE surfaces, plasma polymerization using EDA as a monomer is utilized in this work (Fig. 1). After plasma polymerization of EDA, some EDA polymer is deposited on PTFE film surfaces. The prepared PTFE–PEDA films possess amino groups on their surfaces and are reactive toward compounds with acyl chloride groups (like TMC). Fig. 2 shows the SEM micrographs of pristine PTFE and PTFE–PEDA films prepared at various plasma powers. PEDA polymer covering on PTFE surfaces was observed. The EDA deposition yields are 0.077, 0.186, 0.216, and 0.211 mg/ cm2 for PTFE–PEDA films prepared at a plasma power of 50, 100, 150, and 200 W, respectively. Increase in the plasma powers from 50 to 150 W increases the amounts of EDA polymers deposited on PTFE surfaces from 0.077 to 0.216 mg/cm2. However, etching effect becomes more significant for the sample prepared at high plasma power of 200 W, as a relatively low EDA deposition yield of 0.211 mg/cm2 and a very uneven surface are observed with PTFE–PEDA-200 (Fig. 2e). FTIR characterization on the PTFE/PEDA films provides supports to the presence of PEDA on the modified PTFE surfaces with the absorptions of NAH vibrations at 3353,

2.4. Instrumental analysis ATR–FTIR analysis on film and membrane surfaces were performed with a Perkin Elmer Spectrum One FTIR equipped with a multiple internal reflectance apparatus and a ZnSe prism as an internal reflection element. Scanning electron micrographs (SEM) were recorded with a Hitachi S-3000N SEM. Water contact angles were measured with an angle-meter (Automatic Contact Angle Meter, Model CA-VP, Kyowa Interface Science Co., Ltd., Japan) at room temperature. Distilled water (5 lL) was dropped on the sample surface at ten different sites. The average of ten measured values for a sample was taken as its water contact angle. 2.5. Pervaporation dehydration operation Pervaporation dehydration operation was conducted with a conventional process [16]. The effective area of the used membrane is 6.7 cm2. The temperature of the feeding solution is 70 °C

Fig. 1. Preparation route and chemical structure of PTFE/PA TFC membranes.

262

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

Fig. 2. Effect of plasma powers of EDA polymerization on the surface morphology (1K) of PTFE–PEDA films: (a) pristine PTFE (b) PTFE–PEDA-50, (c) PTFE–PEDA-100, (d) PTFE–PEDA-150, and (e) PTFE–PEDA-200. Plasma polymerization time: 120 min.

3262, 3175, and 1585 cm1 (Fig. 3). The intensities of absorptions relating to NAH groups also increase with increasing the plasma power in EDA polymerization, indicating high plasma power in EDA polymerization is effective to increase the PEDA amounts deposited on PTFE surfaces. This result is coincident to the data of the EDA deposition yields. The absorption at about 3300 cm1 is attributed to the presence of AOH groups, which formed from the reaction between the radicals generated on the PTFE film surface in plasma treatment and the oxygen in air. The presence of

amine and hydroxyl groups results in the reductions of water contact angles of PTFE–PEDA film surfaces from 120° to 45°. The surface densities of amine groups on PTFE–PEDA films are also measured using 4-nitrobenzaldehyde as a testing agent according to the reported method [23,24]. The surface densities of amine groups of the PTFE/PDEA films slightly increase with increases in the plasma powers for EDA plasma polymerization, as values of about 5000 and 5800 groups/nm2 are measured with PTFE– PEDA-50 and PTFE–PEDA-200, respectively. On the other hand,

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

Fig. 3. ATR–FTIR spectra of pristine PTFE and PEDA modified PTFE films under various plasma power and a plasma polymerization time of 120 min.

the measured surface densities of the amine groups for the PTFE– PEDA films are relatively high comparing to the values (around 2–5 amine groups/nm2) reported in literatures [23,24]. The formed PEDA film on PTFE surface might not very dense. The amine groups of the whole PEDA layer could react with the testing agent to increase the measured amine density. The high surface densities of amine groups could also be due to the porous structure of the PTFE films using in plasma polymerization. The amine groups might not only exist on the top but also on the pore surfaces of PTFE films. The quantitative analysis on the amine groups of PTFE–PEDA films demonstrates that PTFE–PEDA films possess certain amounts of amine groups on their surfaces, so as to be effective and suitable for uses in interfacial polymerization to prepare PTFE/PA TFC membranes. 3.2. Preparation of PTFE/PA TFC membranes via interfacial polymerization As the PTFE–PEDA films possess amine groups on their surfaces, in a control experiment PTFE–PEDA-150 was immersed into the TMC solution (not sequentially in EDA solution), to result in the sample of PTFE–PEDA-150/TMC. TMC would react with the amine groups of PTFE–PEDA-150 and formed amide structures on the PTFE–PEDA-150/TMC film surface. The amide structures were characterized by FTIR analysis with the absorption peaks of amide I (C@O) at 1650 cm1 and amide II (NAH) at 1540 cm1(Fig. 4a). However, as the deposited PEDA did not fully cover the PTFE surface (especially at the pore portion), the amide layer could only form at the PEDA-covered portions of PTFE–PEDA-150. Therefore, the formed PA layer in PTFE–PEDA-150/TMC does not fully cover the PTFE film surface (Fig. 4b). Some open pores could still be observed on the surface of PTFE–PEDA-150/TMC. The pores in PTFE– PEDA/TMC result in failure of PTFE–PEDA-150/TMC in pervaporation dehydration operation. Therefore, a polyamide layer for effective separation is incorporated to PTFE–PEDA film surfaces through interfacial polymerization using EDA and TMC as monomers (Fig. 1). The resulting membranes are coded as PTFE/PA, since the PEDA part between PTFE substrate and PA selective layer is relatively minor. High compatibility and strong linkages between the formed PA layers and the PTFE substrates are established with the occurrence of the

263

reaction between the amine groups of PTFE–PEDA and the acyl chloride groups of TMC. The presence of PA layers in the PTFE/PA TFC membranes was demonstrated by FTIR analysis (Fig. 5) with the absorption peaks of amide I (C@O) at 1643 cm1 and amide II (NAH) at 1547 cm1. However, the intensities of the amide absorptions are relatively low for PTFE/PA-50 membrane, indicating the plasma power of 50 W is not high enough to generate certain amounts of EDA polymer on PTFE surface, as the EDA deposition yield of PTFE–PEDA-50 is only 0.077 mg/cm2. Increases in the plasma powers for EDA polymerization increase the deposited amounts of PEDA on PTFE film surfaces, so as to enhance the efficiency of interfacial polymerization of EDA and TMC and to warrant the formation of PA layers on PTFE surfaces. However, the amide absorption intensity observed for PTFE/PA-200 is not as strong as that for PTFE/PA-150. As mentioned earlier, etching effect becomes significant for EDA plasma polymerization under the plasma power of 200 W. The uneven surface of PTFE–PEDA-200 makes the interfacial polymerization become less effective. Fig. 6 collects the SEM micrographs of PTFE/PA TFC membranes. Not like PTFE–PEDA/TMC, interfacial polymerization of EDA and TMC formed dense PA layers on PTFE surfaces. The PA layers fully cover the PTFE film surfaces in the PTFE/PA membranes. The deposited EDA layer, although not fully covering the PTFE surfaces, change the PTFE surfaces from hydrophobic to be hydrophilic and provide effective affinity to the EDA molecules. Therefore, PTFE–PEDA films were wetted with the EDA solution. A layer of absorbed EDA solution forms on PTFE–PEDA film surfaces after immersing in EDA solution, just like the cases of using hydrophilic substrates in interfacial polymerization. The PA layers covering on the surfaces of PTFE/PA TFC membranes were also observed with the cross-sectional SEM micrographs of the membranes. The thicknesses of the PA layers are of about 1.0–1.4 lm, which are comparable to the data reported in the literature [17]. A noticeable feature is that the PA layers of PTFE/PA-50 and PTFE/PA-100 membranes do not tightly integrate with the PTFE substrates. Some space exists between the PA layer and the PTFE substrate. However, the space dimensions are gradually unobvious for PTFE/PA-150 and PTFE/ PA-200 membranes, in which the PA layers tightly bind to the PTFE substrates through the amide linkages between PTFE–PEDA and PA. The small space between the PA layer and PTFE substrate of PTFE/PA-200 might result from the uneven surface of PTFE/PEDA200. The results could indicate the successful formation of chemical linkages between the PA layer and PTFE substrates in the PTFE/ PA-150 and PTFE/PA-200 membranes. However, since the deposited PEDA layers in PTFE–PEDA-50 and PEDA-100 do not fully cover the PTFE surfaces (Fig. 2), no sufficient amine groups on PTFE– PEDA surfaces could react with TMC in the uncovered portion (most in the pore area). Therefore, no effective chemical linkages formed between PA layer and PTFE substrate in the portions. Some vacuum spaces then appear at the interfaces of PA and PTFE in PTFE–PEDA-50 and PEDA-100 membranes. 3.3. Pervaporation dehydration test on a 70 wt% IPA aqueous IPA is a cleaning agent widely used in modern semiconductor and electronic industries, where recycling of wasted IPA is essential from environmental and economical point of view. Dehydration on IPA aqueous solutions is one of the major targets for pervaporation processes [25–31]. The concentrations of the feeding IPA aqueous solutions in pervaporation should be lower than the azeotropic concentration of IPA and water (87.8 wt%). Therefore, the stability of membranes against swelling in water becomes critical. The PTFE/PA TFC membranes have shown potential of application in pervaporation dehydration on the 70 wt% IPA aqueous solution in our previous work [17]. The performance of the PTFE–PEDA based TFC membranes prepared in this work on per-

264

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

Fig. 4. (a) ATR–FTIR spectra of PTFE–PEDA-150 and PTFE–PEDA-150 reacting with TMC; (b) SEM micrographs of PTFE–PEDA-150/TMC and its precursors of pristine PTFE and PTFE–PEDA-150.

Fig. 5. ATR–FTIR spectra of PTFE/PA thin composite membranes PTFE/PA-50, PTFE/ PA-100, PTFE/PA-150, and PTFE/PA-200.

vaporation dehydration is also examined using a 70 wt% IPA aqueous solution as the feeding solution. The untreated PTFE films do not show effective selectivity for the feeding solution (70 wt% IPA aqueous solution) as they are porous. Overflow permeation fluxes (>10,000 g/h m2) and water concentrations of 28–30 wt% in the permeates are measured with the porous PTFE films. On the other hand, formation of PTFE/PA TFC membranes improves the performances of the membranes in pervaporation dehydration. The results are shown in Fig. 7. The plasma powers using in EDA polymerization on PTFE film surfaces show a significant effect on the pervaporation performance. Increases in the plasma powers result in a decrease in the permeation fluxes, but an increase in the water concentrations in the permeate side, i.e. the selectivity factors. The water concentrations in the permeate side found with PTFE/PA-50 and PTFE/PA-100 membranes are quite low, indicating their poor selectivity in IPA/water separation. The poor selectivity of these two membranes could be due to the poor linkages between the PTFE substrates and the PA selective layers, as the results read from the SEM micrographs. A dramatic increase in the water selectivity is found with the PTFE/PA-150 and PTFE/PA-200 membranes. The permeation flux and water concentration in permeate found with PTFE/PA-200 membrane are 1910 g/h m2 and 99.2 wt% (selective factor: 290) at 70 °C, respectively. The perfor-

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

Fig. 6. SEM micrographs of PTFE/PA TFC membranes: (a–d) top surface and (e–h) cross-sectional views.

265

266

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

Fig. 7. Effect of plasma powers of EDA polymerization on the pervaporation dehydration performances of PTFE/PA TFC membranes. Feed: 70 wt% IPA aqueous solution at 70 °C.

Table 1 Pervaporation dehydration performances of PTFE/PA thin-film composite membranes and other membranes in literature on isopropanol (IPA) aqueous solutions. Thin-film composite membranes

IPA in TempePermeation Separation Refs. feed rature (°C) flux factor (wt%) (g/h m2)

PTFE/PA-200

70

70

PTFE-g-PAm/PA

70

70

PVA/APTEOS hybrida PVA/TEOS hybridb Chitosan/GPTMS hybrid Chitosan/PTFE compositec (Glutaraldehyde + H2SO4) crosslinked chitosan Hexamethylene diisocyanate crosslinked chitosan Diamine modified polyimide Polyetherimide Poly(diallyldimethylammonium chloride)/sodium carboxymethyl cellulose Polyamide-imide/ polyetherimide dual layer hollow fiber

70 70 70 70 70

a b c

177

This work [18]

50 30 70 70 50

1910 ± 88 1720 ± 150 678 85 1730 1730 1000

290

276 31 694 775 800

[23] [24] [26] [17] [27]

70

30

400

190

[28]

85 85 90

100 60 40

1382 15 1130

1774 1823 1144

[29] [30] [31]

85

60

765

1944

[32]

PVA: poly(vinyl alcohol), APTEOS: c-aminopropyl-triethoxysilane. GPTMS: c-glycidylpropyl-trimethoxysilane. PTFE: poly(tetrafluoroethylene).

mance is comparable to or even better than the results reported to the pervaporation dehydration on a 70 wt% IPA aqueous solution in the literature (Table 1). Some data reported to membranes working on IPA solutions in high concentrations (85–90 wt%) is also presented in Table 1 for comparisons. It is noteworthy that plasma polymerization of EDA on PTFE is an effective approach to make a hydrophilic and amine-terminated PTFE surface, which is suitable for uses in interfacial polymerization in preparation of PTFE/ PA TFC membranes. The membranes are capable of being used in pervaporation dehydration on a 70 wt% IPA aqueous solution. Comparing to the previous work, in which the hydrophilic and amine-terminated PTFE surfaces were obtained with surface-initiated polymerization and post modification, the plasma polymerization approach is relatively convenient, simple, and mass productive. Moreover, the pervaporation dehydration performance of the TFC membranes is also somewhat enhanced with employing the plasma polymerization approach.

Fig. 8. Stability of PTFE/PA-200 membrane in pervaporation dehydration operation examined with (a) FTIR–ATR and (b) SEM (feed: 70 wt% IPA aqueous solution at 70 °C).

The stability of PTFE/PA-200 TFC membrane under pervaporation operation is also examined with both ATR–FTIR analysis and SEM (Fig. 8) [17]. No significant changes appear with the ATR–FTIR spectra and the relative absorption peak areas of the amide groups, indicating the polyamide layer is stable under pervaporation operation. The SEM observations on the membranes also support this conclusion. As shown in Fig. 8b, no obvious differences in membrane morphology appear on the SEM micrographs of PTFE/PA200 TFC membrane after pervaporation operation. 4. Conclusions Plasma polymerization of EDA on PTFE film surfaces is an effective approach to introduce hydrophilicity and amine-functionalization to PTFE film surfaces. The modified PTFE films are suitably utilized in preparation of PTFE/PA TFC membranes via the interfacial polymerization technique. The prepared PTFE/PA-200 membrane (EDA polymerization under a plasma power of 200 W and a polymerization time of 120 min) exhibits attractive performances in pervaporation dehydration on a 70 wt% IPA aqueous solution with a permeation flux of 1910 g/h m2 and a water concentration in permeate side of 99.2 wt%. The developed approach

C.-H. Yu et al. / Journal of Colloid and Interface Science 336 (2009) 260–267

is effective in preparation of other PTFE-based TFC membranes and the prepared TFC membranes could also be potentially applied to other applications like nanofiltration. Acknowledgments The authors appreciate the financial support on this work from the Ministry of Education, Taiwan (The Center-of-Excellence Program 2008–2010) and from Chung Yuan Christian University (Taiwan) under the project of Toward Sustainable Green Technology (Grant No. CYCU-97-CR-CE). References [1] A. Bhattacharya, P. Ray, H. Brahmbhatt, K.N. Vyas, S.V. Joshi, C.V. Devmurari, J.J. Trivedi, J. Appl. Polym. Sci. 102 (2006) 3575. [2] B.H. Jeong, E.M.V. Hoek, Y. Yan, A. Subramani, X. Huang, G. Hurwitz, A.K. Ghosh, A. Jawor, J. Membr. Sci. 294 (2007) 1. [3] J.E. Hwang, J. Jegal, K.H. Lee, J. Appl. Polym. Sci. 86 (2002) 2847. [4] A.L. Ahmad, B.S. Ooi, J.P. Choudhury, Sep. Sci. Technol. 39 (2004) 1815. [5] S.Y. Lee, H.J. Kim, E. Patel, S.J. Im, J.H. Kim, B.R. Min, Polym. Adv. Technol. 18 (2007) 562. [6] J. Jegal, S.G. Min, K.H. Lee, J. Appl. Polym. Sci. 86 (2002) 2781. [7] V. Freger, Langmuir 19 (2003) 4791. [8] Y. Song, P. Sun, L.L. Henry, B. Sun, J. Membr. Sci. 251 (2005) 67. [9] J.H. Kim, K.H. Lee, S.Y. Kim, J. Membr. Sci. 169 (2000) 81.

267

[10] S.H. Huang, C.L. Li, C.C. Hu, H.A. Tsai, K.R. Lee, J.Y. Lai, Desalination 200 (2006) 387. [11] S.H. Huang, C.J. Hsu, D.J. Liaw, C.C. Hu, K.R. Lee, J.Y. Lai, J. Membr. Sci. 307 (2008) 73. [12] L. Li, B. Wang, H. Tan, T. Chen, J. Xu, J. Membr. Sci. 269 (2006) 84. [13] S. Verissimo, K.V. Peinemann, J. Bordado, J. Membr. Sci. 279 (2006) 166. [14] A.P. Korikov, P.B. Kosaraju, K.K. Sirkar, J. Membr. Sci. 279 (2006) 588. [15] P.B. Kosaraju, K.K. Sirkar, J. Membr. Sci. 321 (2008) 155. [16] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, J. Membr. Sci. 274 (2006) 47. [17] Y.L. Liu, C.H. Yu, K.R. Lee, J.Y. Lai, J. Membr. Sci. 287 (2007) 230. [18] Y.L. Liu, C.H. Yu, J.Y. Lai, J. Membr. Sci. 315 (2008) 106. [19] G.H. Yang, E.T. Kang, K.G. Neoh, J. Polym. Sci. A: Polym. Chem. 38 (2000) 3498. [20] N. Inagaki, K. Narushima, K. Kuwabara, K. Tamura, J. Adhes. Sci. Technol. 19 (2005) 1189. [21] N. Inagaki, K. Narushim, N. Tuchida, N. Miyazaki, J. Polym. Sci. B: Polym. Phys. 42 (2004) 3727. [22] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Polymer 46 (2005) 6976. [23] J.H. Moon, J.W. Shin, S.Y. Kim, J.W. Park, Langmuir 12 (1996) 4621. [24] J. Kim, D. Jung, Y. Park, Y. Kim, D.W. Moon, T.G. Lee, Appl. Surf. Sci. 253 (2007) 4112. [25] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, J. Membr. Sci. 318 (2008) 5. [26] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Lai, J. Membr. Sci. 251 (2005) 233. [27] R.Y.M. Huang, R. Pal, G.Y. Moon, J. Membr. Sci. 160 (1999) 17. [28] M.G.M. Nawawi, R.Y.M. Huang, J. Membr. Sci. 124 (1999) 53. [29] X. Qiao, Y.S. Chung, AIChE J. 52 (2006) 3462. [30] Y. Wang, L. Jiang, T. Matsuura, T.S. Chung, S.H. Goh, J. Membr. Sci. 318 (2008) 217. [31] Q. Zhao, J. Qian, Q. An, Z. Gui, H. Jin, M. Yin, J. Membr. Sci. 329 (2009) 175. [32] Y. Wang, S.H. Goh, T.S. Chung, P. Na, J. Membr. Sci. 326 (2009) 222.