Journal of Membrane Science

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1 Journal of Membrane Science 362 (2010) Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: Novel ultrafiltration membranes prepared from a multi-walled carbon nanotubes/polymer composite Huiqing Wu, Beibei Tang, Peiyi Wu The Key Laboratory of Molecular Engineering of Polymers (Ministry of Education), Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai , People s Republic of China article info abstract Article history: Received 15 June 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords: Multi-walled carbon nanotubes (MWNTs) Brominated polyphenylene oxide (BPPO) Dry-wet phase inversion Hydrophilicity Novel ultrafiltration membranes were prepared by incorporating multi-walled carbon nanotubes (MWNTs) into a matrix of brominated polyphenylene oxide (BPPO) and using triethanolamine (TEOA) as the crosslinking agent. The membranes exhibited not only high permeability and hydrophilicity but also excellent separation performance and chemical stability. Furthermore, the water permeability increased as the weight fraction of MWNTs increased, reaching a maximum of 487 L/m 2 h at 5 wt% of MWNTs, while maintaining a 94% membrane rejection rate to egg albumin. The addition of TEOA into the BPPO/MWNTs casting solution might result in an increase in water permeation rate of membrane if the amount of TEOA exceeded a threshold value; however, the membrane rejection rate was essentially constant despite increasing the molar fraction of TEOA in the casting solution. Using an adequate amount of MWNTs and a proper TEOA/BPPO ratio, it is feasible to make MWNTs/polymer ultrafiltration membranes with both high permeation flux and excellent selectivity Elsevier B.V. All rights reserved. 1. Introduction In recent years, membrane separations have been applied in various industries, such as chemical, food, pharmaceutical, automobile-manufacturing, and metal-finishing industries [1]. As the most popular membranes, polymeric membranes still have an inherent drawback the tradeoff effect between permeability and selectivity, which means that more permeable membranes are generally less selective and vice versa [2,3]. Hybrid membrane incorporating both organic and inorganic components is a convenient and efficient approach to avoid the tradeoff effect [4]. Inorganic materials, such as zeolite, graphite, and silica, have been used to improve the performances of polymeric membranes [5 7]. Carbon nanotubes (CNTs) attracted considerable attention from both academia and industry due to their excellent mechanical, electrical, and thermal properties [8 10]. The rapid mass transport behavior of CNTs has also been gaining interest. Hinds et al. constructed polymer-nanotube composite membranes by incorporating aligned multi-walled carbon nanotubes (MWNTs) across a solid polystyrene film. They observed liquid (alkanes and water) flow rates four to five orders of magnitude greater than the conventional fluid flow predicted by the Hagen Poiseuille equation Corresponding authors. Tel.: ; fax: addresses: bbtang@fudan.edu.cn (B. Tang), peiyiwu@fudan.edu.cn (P. Wu). [11,12]. Holt et al. constructed aligned CNTs in silicon nitride matrix membrane by chemical vapor deposition. They also found that gas and water flow through the carbon nanotubes were at least one order of magnitude greater than expected for flow through a nanoporous material [13,14]. However, these experiments did not provide a practical membrane material because the methods used are expensive, tedious, and difficult to scale up. It may be an appealing way to construct membranes with fast mass transport by dispersing carbon nanotubes in a polymer matrix. Many researchers have been dedicated to this work, and some have achieved positive results. For example, Marand et al. prepared and characterized nanocomposite membranes containing single-walled carbon nanotubes functionalized with long chain alkyl amines inside a polysulfone matrix. Both permeability and diffusivity of the membrane were found to increase with increasing weight fraction of carbon nanotubes [15]. Cong et al. prepared carbon nanotube composite membranes of brominated poly(2,6-diphenyl-1,4-phenylene oxide) for gas separation and found CNTs can be added to the polymeric membranes to improve mechanical strength without affecting gas separation performance [16]. Nowadays, there are more reports on the fluid transport through porous CNTs/polymer membrane. Choi et al. prepared acidified-mwnts/polysulfone (PSf) membranes through an immersed phase inversion process using N-methyl- 2-pyrrolidinone (NMP) as a solvent and water as a coagulant [17,18]. Qiu et al. prepared MWNTs/PSf membranes containing MWNTs functionalized by isocyanate and isophthaloyl chloride /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.memsci

2 H. Wu et al. / Journal of Membrane Science 362 (2010) groups via the reaction between carboxylated carbon nanotubes and 5-isocyanato-isophthaloyl chloride [19]. The incorporation of carbon nanotubes in the mixed matrix membrane was hindered due to the poor interfacial compatibility of carbon nanotubes with the polymer, leading to the formation of nonselective voids in the membrane. Therefore, it is challenging to find an easy and effective method to enhance the compatibility between inorganic and polymeric components. In this research, a novel MWNTs/brominated polyphenylene oxide (BPPO) membrane was prepared for ultrafiltration. The process was initiated via in situ amination with triethanolamine (TEOA) and dry-wet phase inversion using N- methyl-2-pyrrolidinone (NMP) as the solvent and water as the coagulant. TEOA is chosen as the amination agent because it increases the hydrophilicity and anti-fouling property of the membrane due to the molecule s three hydroxyl groups. TEOA also serves to improve the compatibility between the MWNTs and the polymeric component by reacting with both BPPO and the MWNTs. The influences of the weight fraction of MWNTs and the ratio of BPPO to TEOA on the membrane performance and morphology will be investigated. The membranes will be characterized in terms of FTIR, degree of swelling, pure water flux, rejection, contact angle, pore size distribution, SEM, and AFM. 2. Experimental Finally, the membranes were washed with deionized water repeatedly and stored in wet environment. The membrane thickness was approximately 250 m. The preparation of MWNTs/BPPO qua membranes from PPO is schematically shown in Scheme 1. It should be noted that the BPPO qua membrane is meant to represent the membrane aminated with TEOA in the absence of MWNTs, the MWNTs/BPPO membrane denotes the membrane sample containing MWNTs but without TEOA, and the MWNTs/BPPO qua membrane denotes the one which is aminated and crosslinked with TEOA in the presence of MWNTs. To investigate the effect of TEOA on the performance of the MWNTs/polymer membrane, the BPPO qua membrane and MWNTs/BPPO membrane were also prepared by the previously described method Membrane characterization Flux and separation experiments The measurements of pure water flux and protein rejection were performed using a cross-flow membrane module as shown in our recent article [21], and it had a membrane effective area of 60 cm 2. Before measurement, all the membranes were pretreated at a high pressure drop of 0.3 MPa for approximately 10 min. Then, the pure water flux and rejection tests were conducted at an operation pressure of 0.2 MPa. The water flux was calculated in Eq. (1): 2.1. Materials F = V At (1) Ploy(2,6-dimethyl-1,4-phenylene oxide) (PPO) with an intrinsic viscosity of m 3 kg 1 in chloroform at 25 C was obtained from the Institute of Chemical Engineering of Beijing, China. Carboxyl multi-walled carbon nanotubes (MWNTs, purity > 95%, COOH content 3.86 wt%, OD < 8nm, ID 2 5nm, length m), were manufactured by chemical vapor deposition (CVD) and purchased from Chengdu Organic Chemicals Co., Ltd. (Chinese Academy of Sciences). Triethanolamine (TEOA), chlorobenzene, bromine, and N-methyl-2-pyrrolidone (NMP) were all of analytical grade. Egg albumin with an average molecular weight of 45,000 g/mol was used as a probe molecule for rejection tests and supplied by Sinopharm Chemical Reagent Co., Ltd Bromination PPO was dissolved in chlorobenzene to form an approximately 8 wt% solution, and then chlorobenzene-diluted bromine was added. The degree of bromination was controlled by adding a given amount of bromine while the substitution position (benzyl or aryl) was controlled by the temperature [20]. The final solution was precipitated with methanol, washed by deionized water, and dried at 60 C for at least 20 h to obtain the brominated polymer Membrane preparation The brominated PPO (BPPO) was dissolved in NMP to form an approximately 20 wt% homogeneous solution. Next, a given amount of the TEOA NMP solution was added to the polymer solution. The solution was stirred for 30 min at room temperature to accelerate the reaction between TEOA and BPPO. Afterwards, the MWNTs were dispersed throughout the solution. For better dispersion and more sufficient reaction of MWNTs with polymer solution, the mixture was vigorously stirred for 30 min followed by sonication for 90 min. This solution was later used as a casting solution. After removing air bubbles, the casting solution was cast onto a clean glass plate. Subsequently, the glass plate was horizontally immersed into deionized water at a temperature of 30 C for at least 24 h to remove the solvent and solidify the membrane structure. where V is the total volume of permeated pure water, A is the membrane area, and t is the operation time. Deionized water was used for this measurement. The rejection was measured using a 0.5 g/l egg albumin (the average molecular weight is 45,000 g/mol) solution. The isoelectric point of the egg albumin is 4.7, which indicates a net negative charge at a ph of 6 (the operational ph of this experiment). The operation pressure was 0.2 MPa. The concentrations of the permeation and feed solutions were determined by an ultraviolet visible spectrophotometer (Lambda 35, PerkinElmer, USA) at 205 nm. The rejection, R, was calculated in Eq. (2): R = 1 C p C f (2) where C p and C f are the concentrations of the permeation and feed solutions, respectively Contact angle measurement Water was used as the probe liquid for determination of the hydrophilicity at the membrane surface. The static contact angle of water on the surface of a polymer membrane was measured by using OCA15 (Dataphysics Co., Germany) and following the sessile drop method at 25 C and a relative humidity of 65%. Drops were formed using a 10- L Hamilton positive displacement syringe. The average value of the contact angle on each polymer membrane was calculated using at least five different locations on each membrane Determination of the degree of swelling At first, some dried membrane samples were immersed into the solvent (NMP) for 24 h at 30 C. Then, the samples were washed with deionized water repeatedly and dried in vacuum. The degree of swelling was calculated in Eq. (3): D = W 2 W 1 (3) where W 1 and W 2 are the weights of the original and treated membrane samples, respectively.

3 376 H. Wu et al. / Journal of Membrane Science 362 (2010) Scheme 1. Schematic of the MWNTs/BPPO qua membrane prepared from PPO Pore size distribution Pore size distribution of each membrane was determined by the liquid liquid interfacial contact method [22]. The membrane was equilibrated in i-pentanol-saturated water (water phase). At the beginning of the experiment, all pores of the membranes were filled with a liquid (i-pentanol-saturated water). Briefly, water-saturated i-pentanol (i-pentanol phase) was used to displace the water phase in the pre-wetted membrane with constant increment of pressure. The upper section of the measurement cell was filled with the water phase, and the lower section was connected to a reservoir filled with the i-pentanol phase [23]. The interfacial tension of i-pentanol towards water was Ncm 1 (4.8 dyn cm 1 ) according to Fu s measurement [24]. The two sections were connected to the pressure control device. The flux of i-pentanol was measured by micropipette. The pore radius (r) was determined by the Cantor equation and the pore size distribution was expressed as Eq. (4): P ii (P i 1 Q i P i Q i 1 ) f (r) = m {P i 1 i=1 [P (4) i(p i 1 Q i P i Q i 1 )/P i 1 ]} which was derived from the Hagen Poiseuille s equation [18]. Here, r,,, P, and Q are the pore radius, the interfacial tension of water/i-pentanol, the contact angle at the interface of water phase and polymer, the applied pressure, and the permeate flow rate, respectively. As mentioned above, the two phases for liquid liquid replacement were the water saturated with i-pentanol and the i- pentanol saturated with water. Thus, the differences in the contact angles for each membrane were minor. For simplification, it was assumed that the liquid wetted the material completely, i.e., the contact angle was assumed to be zero. To obtain smoothing curves for the flow-pressure data, a smoothed cubic spline method was fitted to the data in Matlab [25] Membrane morphology The surface and cross-section of membrane were observed using a scanning electron microscope (TESCAN 5136MM). Air-dried membrane samples were prepared for SEM imaging by first cryogenically fracturing in liquid nitrogen, then coating the surfaces and cross-sections with a conductive layer of sputtered gold. To better examine the distribution of MWNTs, the MWNTs/BPPO qua membranes were etched on the surface using NMP so that the MWNTs can be exposed. Then, the morphology of the etched membrane was observed with the FE-SEM S The surface roughness of the composite membrane was measured by AFM analysis (Nanoscope IV). Membrane samples were fixed on a specimen holder and 2 m 2 m areas were scanned in the tapping mode in air. The surface roughness was reflected in terms of the average plane Fig. 1. SEM images of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs: (a) cross-sectional SEM image of 1.0 wt% MWNTs, (b) cross-sectional SEM image of 3.0 wt% MWNTs, (c) cross-sectional SEM image of 5.0 wt% MWNTs and (d) surface FE-SEM image of 5.0 wt% MWNTs after etching using NMP.

4 H. Wu et al. / Journal of Membrane Science 362 (2010) Scheme 2. The formula of reaction between BPPO and TEOA. roughness (Ra), root mean square roughness (Rms), and surface area difference (SAD). 3. Results and discussion 3.1. Characterization of the MWNTs/BPPO qua membrane Fig. 1 presents the SEM images of the MWNTs/BPPO qua membranes. It is clear that the MWNTs do not agglomerate or form clusters but disperse well in the polymer matrix. When the TEOA solution is added to the casting solution, a quaternary amination reaction between TEOA and BPPO occurs as shown in Scheme 2. The quaternary ammonium group formed in the polymer chain further interacts with the carboxyl groups on the MWNTs, which is favorable for the dispersion of the MWNTs in the polymer matrix and the subsequent stabilization of the MWNTs/polymer membrane. To validate the interactions among MWNT-COOH, TEOA, and BPPO, the FTIR spectra of MWNTs, the BPPO qua membrane, and the MWNTs/BPPO qua membrane with 5.0 wt% of MWNTs were examined and the results are shown in Fig. 2. InFig. 2a, the band at 1719 cm 1 is attributed to the C O stretching vibration of the carboxyl ( COOH) group on the MWNTs. However, the absorption of the carbonyl (C O) stretching vibration of the MWNTs in the MWNTs/BPPO qua membrane is localized around 1705 cm 1 (Fig. 2c), which reflects a shift downward from the wavenumber (C O) in pure MWNTs. This indicates an interaction between the carbonyl groups on the MWNTs and the polymer. Specifically, the zwitterions containing the quaternary ammonium group ( N + (CH 2 CH 2 OH) 3 ) and MWNT-carboxylate are formed [15,17] when the MWNTs are added to the BPPO TEOA polymer solution (Scheme 3). The stability of the MWNTs/BPPO qua membrane is concluded to be greater than that of the MWNTs-free (BPPO qua ) Fig. 2. The FTIR spectra of the MWNTs and the membrane samples. (a) MWNTs, (b) BPPO qua membrane and (c) MWNTs/BPPO qua membrane with 5.0 wt% of MWNTs. Scheme 3. Schematic of the interaction among MWNTs, TEOA, and BPPO. membrane, which can be proved by the data of degree of swelling. Table 1 shows the degrees of swelling for various weight fractions of MWNTs. Obviously, the degree of swelling of the MWNTs/BPPO qua membrane is higher than that of the BPPO qua membrane (when the content of MWNTs is 0 wt%) and increases when more MWNTs are added. The results suggest that a chemically more stable structure is formed when MWNTs work as the crosslinkages. The pure water fluxes, rejections to egg albumin, contact angles, and surface morphologies of BPPO qua, MWNTs/BPPO, and MWNTs/BPPO qua membranes are summarized in Table 2. The flux through the BPPO/MWNTs membrane is much higher than that of the BPPO qua membrane, whereas its rejection to egg albumin markedly decreases. Consequently, the addition of MWNTs to the polymer membrane effectively improves the water permeation rate at the expense of the retention property. In contrast, the membrane cast from the BPPO TEOA MWNTs system exhibits both high permeability and excellent separation performance. Furthermore, the contact angle at the surface of the MWNTs/BPPO qua membrane is the lowest of the three types of membranes investigated. This suggests that the hydrophilic zwitterions containing the quaternary ammonium group and MWNT-carboxylate are formed, making the membrane surface more hydrophilic. Also, the membrane containing MWNTs has a rougher surface than the MWNTs-free membrane (Table 2). The results above suggest that

5 378 H. Wu et al. / Journal of Membrane Science 362 (2010) Table 1 The degrees of swelling of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs. Weight fraction of MWNTs (%) Degree of swelling (%) Table 2 Properties of the membranes with different components. Membrane samples Pure water flux (L/m 2 h) Rejection (%) Contact angle (degree) Surface morphology BPPO qua membrane MWNTs (5 wt%)/bppo membrane MWNTs (5 wt%)/bppo qua membrane Fig. 3. Effect of weight fraction of MWNTs on the pure water flux and rejection to egg albumin of MWNTs/BPPO qua membrane at 0.2 MPa. greater amounts of MWNTs. Furthermore, the fluxes through all the MWNTs/BPPO qua membranes containing MWNTs are larger than that of the MWNTs-free, while the egg albumin rejection rates of all the MWNTs/BPPO qua membranes remain at a relatively high level despite slight decreases with increasing amounts of MWNTs. Generally, there are two factors favorable for the permeation through the MWNTs/polymer membrane. One is hydrophilicity, and the other is the pore structure of membrane. Firstly, the hydrophilic groups ( COOH and OH) on the surface of MWNTs can improve the hydrophilicity of membrane surface, alleviate the membrane fouling, and enhance the pure water flux. Fig. 4 shows the water contact angles of the MWNTs/BPPO qua membranes with different amounts of MWNTs. The contact angle of membrane decreases with an increase in MWNTs, implying that the hydrophilic MWNTs incorporated to the membrane make the membrane surface more hydrophilic. This increase in hydrophilic- a synergy between MWNTs and TEOA contributes to the better membrane performance Effect of weight fraction of MWNTs on the membrane performance and morphologies To investigate the influence of weight fraction of MWNTs on the membrane performance and morphologies, the MWNTs/BPPO qua membranes containing 1 wt% MWNTs to 7 wt% MWNTs were prepared using the same 8:1 molar ratio of BPPO to TEOA. The BPPO qua membrane without MWNTs was also prepared using the same technique. A plot of pure water flux and rejection vs. weight fraction of MWNTs is shown in Fig. 3. As the amount of MWNTs increases, the pure water flux of the membrane continuously increases. When no MWNTs are present, the flux is 197 L/m 2 h. The flux increases to a maximum of 487 L/m 2 h at 5 wt% MWNTs, but decreases at Fig. 4. Contact angles of the MWNTs/BPPO qua membranes against water as a function of the weight fraction of MWNTs.

6 H. Wu et al. / Journal of Membrane Science 362 (2010) Fig. 5. Pore size distribution of MWNTs/BPPO qua membranes with a MWNTs content of (a) 0 wt%, (b) 1 wt%, (c) 3 wt%, (d) 5 wt%, and (e) 7 wt%, and (f) the pore radius corresponding to the maximum pore size distribution for the five membranes. ity subsequently causes an increase in water flux. Secondly, the adsorption and hydrophilicity effects of carboxyl MWNTs result in an increase in the exchange rate between solvent and nonsolvent during the phase inversion and thus an increase in pore size. The above-mentioned factors together contribute to a higher permeation through the membrane. However, when the MWNTs content exceeds 5 wt%, the density of MWNTs in the membrane is so large that the steric hindrance and electrostatic interactions among the MWNTs and between the MWNTs and the polymer chains bring about the clustering of MWNTs during phase inversion. The high density of MWNTs in the casting solution also leads to an increase in the viscosity of solution [17,19]. This will severely hinder the exchange between solvent and non-solvent during the phase inversion and slow down the precipitation of the membrane [26]. Consequently, a less porous membrane is formed and the flux of the membrane decreases [17,19]. The results above can be confirmed by the pore size distribution of membranes. Fig. 5 specifically presents the pore size distributions of the MWNTs/BPPO qua membranes with 0, 1, 3, 5, and 7 wt% MWNTs, respectively. The pore radius corresponding to the maximum pore size distribution increases with increasing weight percentage to a maximum at 5 wt% and begins to decrease at larger weight percentages. The increase in the pore radius for each weight percentage is: 0 wt%<7wt%<1wt%<3wt%<5wt%(fig. 5f). This order corresponds to the order of the water flux rates. The egg albumin rejection rates of all the MWNTs/BPPO qua membranes stay at a relatively high level despite slight decreases at higher amounts of MWNTs. The decrease is due to an increase in the pore size and stronger electrostatic interactions between egg albumin and membrane, which both are the consequence of an increase in the amount of MWNTs. The SEM images of the cross-sections and surfaces of MWNTs/BPPO qua membranes with different amounts of MWNTs are shown in Figs. 6 and 7, respectively. There is not a pronounced difference in the structures of the cross-sections of the MWNTs/BPPO qua membranes. All have finger-like structures with various pore sizes. Furthermore, as the amount of MWNTs increases, the surface of the MWNTs/BPPO qua membrane becomes rougher and the pores of the cross-section become larger. The MWNTs/BPPO qua membrane with 5 wt% MWNTs has the roughest surface and the largest pore size. For membranes containing more than 5 wt% MWNTs, the surfaces become smoother and the pore sizes of cross-section become smaller. The roughness of the membrane surface can be quantified from the AFM results as presented in Fig. 8 and Table 3. According to Table 3, the order of decreasing roughness using Rms (root mean square height) is: 5wt%>7wt%>3wt%>1wt% 0 wt%. The results from AFM analysis are consistent with the morphologies observed by SEM images. The increased hydrophilicity and viscosity aid in the formation of the microporous MWNTs/BPPO qua membrane [17 19]. As mentioned above, when the membrane contained less than 5 wt% MWNTs, the effect of adsorption and hydrophilicity of carboxyl

7 380 H. Wu et al. / Journal of Membrane Science 362 (2010) Fig. 6. SEM images of the cross-sections of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs: (a) 0 wt% (BPPO qua membrane), (b) 1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt%, (e) 4.0 wt%, (f) 5.0 wt%, (g) 6.0 wt%, and (h) 7.0 wt%. Fig. 7. SEM images of the surfaces of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs: (a) 0 wt% (BPPO qua membrane), (b) 1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt%, (e) 4.0 wt%, (f) 5.0 wt%, (g) 6.0 wt%, and (h) 7.0 wt%. MWNTs accelerates the exchange between solvent and non-solvent during the phase inversion; moreover, MWNTs collocate regularly in the MWNTs/BPPO qua membrane and form nodular structure. So these MWNTs/BPPO qua membranes have nodular structure, larger pore size, and rougher surface. At values above 5 wt% MWNTs, the viscosity of the casting solution continues to increase. The increased viscosity usually hinders the exchange between solvent and nonsolvent, causing a smooth membrane surface with smaller pore size Effect of the molar ratio of BPPO to TEOA on the membrane performance and morphologies MWNTs/BPPO qua membranes with various ratios of the repeated unit of BPPO to TEOA were prepared to study the effect of TEOA on the membrane performance and morphologies. The amount of MWNTs was fixed at 5 wt%. Note that the larger the ratio of BPPO to TEOA, the less TEOA NMP solution was added. Fig. 9 is a plot of the pure water flux and rejection of the MWNTs/BPPO qua membrane vs. the molar ratio of the repeated unit of BPPO to TEOA. The pure water flux decreases as the BPPO/TEOA ratio increases. The flux decreases up until the ratio reaches 10:1, and begins to increase at higher ratios. The observed trend indicates the addition of TEOA in the BPPO/MWNTs casting solution leads to an increase in the water permeation rate of the MWNTs/BPPO qua membrane if the amount of TEOA exceeds a certain amount. Interestingly, the rejection of the MWNTs/BPPO qua membrane almost stays constant when at least the minimum amount of TEOA is added. The increase in the water flux by addition of TEOA can be attributed to the hydrophilicity and the porous structure of the membrane. When the TEOA solution is added to the casting solution, there occurs a quaternary amination reaction and anion exchangers with strong basic N + (CH 2 CH 2 OH) 3 hydrophilic Table 3 Surface roughness values of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs by AFM. Weight fraction of MWNTs (%) Ra (nm) Rms (nm) SAD (%)

8 H. Wu et al. / Journal of Membrane Science 362 (2010) Fig. 8. AFM images of the surfaces of the MWNTs/BPPO qua membranes with different weight fractions of MWNTs: (a) 0 wt% (BPPO qua membrane) and (b) 5.0 wt%. groups are formed. An increase in the amount of TEOA solution results in an increase in the content of hydrophilic groups, and thus the hydrophilicity of membrane increases correspondingly. In addition, the carboxyl groups of the MWNTs can react with quaternary amination groups to form zwitterions (as mentioned above), which favors the dispersion of MWNTs in the polymer matrix. Hence, with a small BPPO/TEOA ratio (i.e., increased TEOA), the dispersion of MWNTs in the membrane is facilitated and the hydrophilicity of membrane increases. This can be proved by examining the contact angle of the MWNTs/BPPO qua membrane (Fig. 10). As the ratio of BPPO to TEOA increases, the contact angle of the membrane increases, suggesting that the hydrophilicity of membrane surface decreases when the amount of TEOA solution is reduced. There- fore, the membrane hydrophilicity and the pure water flux can be enhanced by adding more of the TEOA solution in the casting solution. Furthermore, when the casting solution is coagulated in water, a rapid exchange between solvent and non-solvent due to the strong interaction between the hydrophilic casting solution and water causes the membrane to form larger pores. In general, the greater the hydrophilicity of the casting solution, the bigger the pore size of the membrane. Since the hydrophilicity of the casting solution decreases as the ratio of BPPO to TEOA increases, membranes with small pores are formed at large BPPO/TEOA ratios. Fig. 9. Effect of the ratio of the repeated unit of BPPO to TEOA on the pure water flux and rejection to egg albumin of the MWNTs/BPPO qua membrane at 0.2 MPa. Fig. 10. Contact angles of the MWNTs/BPPO qua membranes against water as a function of the ratio of the repeated unit of BPPO to TEOA.

9 382 H. Wu et al. / Journal of Membrane Science 362 (2010) Fig. 11. SEM images of the cross-sections of the MWNTs/BPPO qua membranes with different ratios of the repeated unit of BPPO to TEOA: (a) 12:1, (b) 11:1, (c) 10:1, (d) 9:1, (e) 8:1, (f) 7:1, and (g) 6:1. However, the dependence of the water flux on the ratio of BPPO to TEOA causes a point of inflection when the BPPO/TEOA ratio reaches 10:1. As discussed previously, TEOA interacts with MWNT- COOH and BPPO (in Scheme 3), which enhances the dispersion of MWNTs in the polymer matrix. A decrease in the amount of TEOA in the casting solution will lead to inadequate dispersion of MWNTs, e.g., non-uniform dispersion, agglomeration, and cluster formation of nanotubes in the polymer matrix. Under such conditions, MWNTs cannot adhere tightly to the polymer matrix so narrow gaps will form in the MWNTs/BPPO qua membrane [16]. Water can easily pass through the gaps, contributing to an increase in the pure water flux. In addition, egg albumin molecules can pass through the gaps if the gaps are large enough. When the ratio of BPPO to TEOA increases up to 12:1, the MWNTs agglomerate to the point where the gaps formed in the membrane are wide enough for egg albumin molecules to pass through. So, the water flux increases and the rejection decreases at a high BPPO/TEOA ratio (low amounts of TEOA). The SEM images are also good explanations for the performance of the MWNTs/BPPO qua membrane (Fig. 11). The pore size in the membrane cross-section increases when the ratio of BPPO to TEOA decreases from 12 (Fig. 11a) to 6 (Fig. 11 g). This suggests that the permeability of membrane is greater when more TEOA is added. Essentially, the membrane cast from the BPPO casting solution containing MWNTs and TEOA has larger pores and higher pure water permeability. 4. Conclusions MWNTs/BPPO qua membranes were prepared by in situ amination with triethanolamine (TEOA), which crosslinked MWNTs and polymer (BPPO). The results of FTIR revealed that the zwitterions containing quaternary ammonium groups and MWNTcarboxylate were formed in the membrane. The membrane cast from the BPPO TEOA MWNTs system exhibited high hydrophilicity, permeability, and chemical stability, and excellent separation performance. The membrane performance and morphologies were remarkably influenced by the weight fraction of MWNTs and the ratio of BPPO to TEOA. As the weight fraction of MWNTs increased, the pure water flux significantly increased to a maximum, then decreased, while the rejection remained fairly constant. The increase in permeability of the membrane was related with the hydrophilicity and adsorption effects of MWNTs. However, increased viscosity of the casting solution at higher MWNTs loading (>5 wt%), retarded the exchange between solvent and non-solvent during phase inversion process, and smoother membrane surfaces and smaller pores appeared as a result. Hence, water flux was severely limited. As the ratio of BPPO to TEOA increased, the water flux decreased at first due to a decrease in the hydrophilicity and pore size. But at a relative higher ratio, more MWNTs agglomeration resulted in the formation of gaps, and these gaps are large enough for both water and egg albumin molecules to pass through. Eventually, water flux increased while the rejection decreased to some extent. Therefore, if a proper amount of MWNTs and TEOA is added, the permeation flux of the MWNTs/polymer membrane can be improved and the selectivity can be simultaneously maintained well. Acknowledgements We gratefully acknowledge the financial support of the National Science of Foundation of China (NSFC) (No , , ), and the National Basic Research Program of China (2005CB623800, 2009CB930000). We are very grateful for the valuable help from Professor Tongwen Xu (University of Science and Technology of China) and Rsimmon (Tulane University). References [1] R. Malaisamy, R. Mahendran, D. Mohan, M. Rajendran, V. Mohan, Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization, J. Appl. Polym. Sci. 86 (2002) [2] B.D. Freeman, Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 32 (1999) 375. [3] F.B. Peng, L.Y. Lu, H.L. Sun, Y.Q. Wang, J.Q. Liu, Z.Y. Jiang, Hybrid organic anorganic membrane: solving the tradeoff between permeability and selectivity, Chem. Mater. 17 (2005) [4] A. Walcarius, Electrochemical applications of silica-based organic inorganic hybrid materials, Chem. Mater. 13 (2001) 3351.

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