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polymer blend membranes
Journal of Membrane Science 284 (2006) 406–415
Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes Jae-Hyun Choi a,b , Jonggeon Jegal a,∗ , Woo-Nyon Kim b a
Membrane and Separation Research Center, Korea Research Institute of Chemical Technology, Post Office Box 107, Yuseong, Daejeon 305-606, South Korea b Department of Chemical and Biological Engineering, Applied Rheology Center, Korea University, Anam-dong, Seoul 136-701, South Korea Received 10 May 2006; received in revised form 8 August 2006; accepted 10 August 2006 Available online 22 August 2006
Abstract Multi-walled carbon nanotubes (MWNTs)/polysulfone (PSf) blend membranes were prepared by a phase inversion process, using N-methyl-2pyrrolidinone (NMP) as a solvent and water as a coagulant. Before making the blend membranes, MWNTs were first treated with strong acid to make them well dispersed in organic solvents such as NMP for the preparation of homogeneous MWNTs/PSf blend solutions. The prepared MWNTs/PSf blend membranes were then characterized using the several analytical methods such as a Fourier transform infrared (FTIR) spectroscopy, a contact angle goniometer, a scanning electron microscopy (SEM) and permeation tests. Because of the hydrophiic MWNTs, the surface of the MWNTs/PSf blend membranes appeared to be more hydrophilic than a just PSf membrane. The carboxylic acid functional groups developed by the treatment with acid on the surface of MWNTs seemed to act to increase hydrophilicity of the blend membranes. The morphology and permeation properties of the blend membranes were also found to be dependent on the amounts of MWNTs used. The pore size of the blend membranes increased along with the contents of MWNTs up to 1.5%, then decreased, and at 4.0% of MWNTs, it became even smaller than PSf membrane. The PSf membrane with 4.0% of MWNTs showed higher flux and rejection than the PSf membrane without MWNTs. © 2006 Elsevier B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Polysulfone; Composite membrane; Phase inversion; Hydrophilicity; Permeability
1. Introduction There are several ways to prepare porous polymeric membranes, such as sintering, track etching and phase inversion process [1]. The majority of polymer membranes used for microand ultra-filtration of liquids are prepared by phase inversion process, in which a film of concentrated polymer solution is cast on a suitable substrate and subsequently immersed in a nonsolvent bath where the exchange of solvent by coagulant and phase separation occurs in the casting film. Even though the phase inversion process is the most widely used technique for preparing various types of polymeric membranes, many aspects in the process are not clearly clarified, including the complete analysis of skin layer formation at the initial moment of phase inversion. However, it is known that asymmetric membrane
∗
Corresponding author. Tel.: +82 42 860 7637; fax: +82 42 861 4151. E-mail address: [email protected] (J. Jegal).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.013
depends on two dominating factors, such as kinetic parameters and thermodynamic parameters [2–4]. Therefore, selection of membrane materials, such as polymers, solvent, non-solvent, is important in the fabrication of membrane having desirable membrane structure and properties. Among several polymeric materials, polysulfone (PSf) has been widely used as a polymer for commercial microfiltration and ultrafiltration membranes. Moreover, PSf has been used for support material of nanofiltration and reverse osmosis membranes because of its excellent balance in the chemical, thermal and mechanical properties. However, application of the PSf membrane often limited because of its hydrophobic nature. Because of its hydrophobic properties, performance of PSf membranes results in low water flux and serious membrane fouling. To overcome these problems, many research groups are trying to enhance performance of PSf membranes. For example, polyvinylpyrrolidone (PVP) has been widely used as a polymer additive to provide hydrophilicity and to increase membrane permeability [5–7]. And it is also easy to make the hydrophilic
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surface of membrane by plasma treatment [8,9]. Kim et al. reported that poly(1-vinylpyrrolidone-co-acrylonitrile) copolymer and their blends with PSf were used for the fabrication of ultrafiltration membranes having hydrophilic properties [10]. Recently, hybrid materials were used to achieve high permeability and selectivity in membrane applications [11–13]. Garcia-Valls and co-workers reported that activated carbon–PSf composite membranes were synthesized by an immersion–precipitation method. Their results showed that permeability for composite membranes was influenced by activated carbon particle size and loading on the membrane [13]. Based on this, we prepared multi-walled carbon nanotubes (MWNTs)/PSf blend membranes by the immersion phase inversion process, using water as a coagulant. The typical advantages of surface modified MWNTs as an additive are high conductivity and hydrophilicity. With the decreasing price of the MWNTs (less than $1.0 for 1 kg of MWNTs), MWNTs is expected to become one of the additives to give membranes special functionalities. It would also be possible to give conductivity to the membrane as well as to modify the morphologies of the membrane. However, in this study, the effect of carbon nanotubes (CNTs) loading on the performance of the PSf membrane was mainly investigated. UV/visible spectroscopy, rheometer, Fourier transform infrared (FTIR) spectroscopy, contact angle goniometer, and scanning electron microscope (SEM) have been used to characterize solution property and the prepared membrane. The permeation performance of the membranes is being discussed through the tests with various feed solutions (pure water, 1000 ppm aqueous solutions of PEO 100,000 or PVP 55,000). 2. Experimental 2.1. Materials Polysulfone (PSf, Udel® P 3500, Amoco, Marietta, OH) was used as a membrane material. Carbon nanotubes (multi-walled carbon nanotubes, MWNTs) manufactured by CVD process (supplied by Iljin nanotech, Korea) whose purity of greater than 95% were used for the modification of a PSf membrane. The MWNTs in tubular shape, composed of six-membered carbon rings, were like a rolled graphite sheet with 10–20 nm of outer diameter, 10–50 m of the length, and 4.3 ± 2.3 nm of inner diameter. The MWNTs were surface modified with strong acids, concentrated nitric (HNO3 ) and sulfuric acids (H2 SO4 ) (1:3 in vol.%), to make them easy to be dispersed in the organic solvents [14]. One gram of raw MWNTs was added to 150 ml of the acid mixture in a round-bottomed flask, and refluxed at 80 ◦ C for 1 h. On cooling, the mixture was washed with distilled water until nearly neutrality water was obtained. The collection was then freeze dried under vacuum at −80 ◦ C for further use. N-Methyl2-pyrrolidinone (NMP) was purchased from Junsei Co. and used as a solvent without further purification. Polyvinylpyrrolidone (PVP) with molecular weight of 55,000 g/mol and poly(ethylene oxide) (PEO) with molecular weight of 100,000 g/mol bought from Aldrich Co. were used for permeation tests.
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2.2. Membrane preparation Twenty grams of PSf was dissolved in 80 g of NMP at 60 ◦ C with constant stirring to prepare a PSf solution with a composition of 20/80 (PSf/NMP). And 2.3 g of modified MWNTs was dispersed into 97.7 g of NMP to prepare a MWNT solution. For better dispersion of the MWNTs in NMP, the solution was sonicated for 80 s using a high-power sonic tip (sonicskorea, SKB-2000, 2 kW). Then proper amounts of the both solutions were taken and mixed with stirring at room temperature to prepare PSf/MWNTs blend solutions with different compositions. For all the blend solutions, PSf/MWNTs mixture to NMP ratio was 15/85, but the ratio of PSf to MWNTs was varied: 99.5/0.5, 99.0/1.0, 98.5/1.5, 98/2.0, and 96/4. Hereafter, the contents of the MWNTs with respect to the PSf will be used through out this paper. After which, the casting solutions were kept at room temperature for at least 24 h to remove air bubbles from the solutions. The casting solutions were then cast with 150 m thickness on a polyester non-woven fabric using a Baker Applicator. The nascent membranes were immersed into a water bath conditioned at 20 m for 24 h. 2.3. Charaterization By using several analytical methods such as UV/visible spectroscopy, contact angle goniometer, rheometer, Fourier transform infrared (FTIR) spectroscopy, and field emission scanning electron microscopy (FESEM), the prepared MWNTs/PSf blend membranes were characterized. To evaluate the extent of dispersion of the surface modified MWNTs in different solvents, a UV/visible spectroscopy (UV-2550, Shimadzu) was used. The viscosities of the casting solutions were measured using a Brookfield Digital Rheometer (model: DV-III, Brookfield Eng. Bab.) at 25 ◦ C. The hydrophilicity of the surface of the blend membranes as a function of different contents of MWNTs was evaluated based on the dynamic water contact angle of the membrane using contact angle goniometer (SEO 300A). The surface and cross-section morphology of the blend membranes were studied with a FESEM (model: XL 30, Philips Co., USA). 2.4. Capillary flow porometry The pore size and pore size distribution of the PSf membranes with or without MWNTs were studied, using a capillary flow porometery (model CFP-1500-AEL, Porous Materials Inc., NY, USA). The short procedure was as a follow: the loaded sample (membrane) was soaked in a wetting liquid, a low surface tension liquid (σ = 15.9 × 10−3 N/m) named Galwick, to fill spontaneously the pores of the sample, and then gas pressure was applied to one side of the sample. The gas pressure was increased slowly until the liquid was removed from the pores, forming gas flow through pores, which then increased with further increase in pressure [15–16]. At a certain pressure, the largest pore was emptied first and the gas flow started, which gave the bubble point
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3. Results and discussion 3.1. Preparation of MWCNT/PSf blend membranes
Fig. 1. Schematic diagram of ultrafiltration experimental set-up.
pressure. On further increase of the pressure, smaller pores were then emptied and gas flow increased. The flow rate was determined as a function of pressure and used to calculate the desired pore characteristics by the Washburn equation [17]: D=
4σ cos θ P
where D is the pore diameter, σ the surface tension of the liquid, θ the contact angle of the liquid, and P is the applied gas pressure. From the measured gas pressure and flow rates, the pore size and pore size distribution were determined. 2.5. Permeation test The common method to characterize the MWNTs/PSf blend membrane was to measure its performance in terms of pure permeability, flux and solute rejection of different feed solutions, using a general UF test set-up (Fig. 1). The concentration of all the feed solutions used in this experiment was 1000 ppm and an upstream pressure was controlled by using back-pressure regulators (1–4 bar). The permeation tests were conducted at 25 ◦ C, and the effective membrane area was 18.8 cm2 . Before measuring the flux, each membrane was subjected at 4 bar for about 2 h to avoid the compaction effect of the membrane. A flux was measured by weighing permeates penetrated through the membranes per unit time, and a solute rejection was calculated from the concentrations of the feed solution and permeate using the following equation. Rejection (%) =
The MWNTs/PSf blend membranes were prepared by the immersion phase inversion process, using water as a coagulant. Many organic solvents can be used for this method: Nmethyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), etc., for the preparation of PSf solutions. These solvents should be well miscible with non-solvents that will be used for the phase inversion of the PSf solutions via the solvent–non-solvent exchange process. One of the common non-solvents, so called coagulants, is water. Beyond water, sometimes alcohols such as ethanol and propanol also can be used as coagulants. In this study NMP was chosen as a solvent due to its strong interaction with polymer and miscibility with water. In order to fabricate the MWNTs/PSf blend membranes, it is necessary to make a homogeneous MWNTs solution in NMP before blending them with the PSf solution in NMP. However, MWNTs as synthesized are strongly hydrophobic and generally have very low solubility in all solvents. To overcome these problems by introducing hydrophilic functional groups into the surface of the MWNTs, they were treated with strong acid such as concentrated H2 SO4 and HNO3 . Acid-treated MWNTs are known to have carboxyl groups on their surfaces, showing good dispersion in polar organic solvents [18–19]. Fig. 2 shows the absorption spectra of UV/visible spectrophotometry of the surface modified MWNTs solutions in various polar organic solvents. It is well known that the more stable suspension of CNTs in solvents shows increased absorption in a UV/visible spectrophotometry [20–21]. It was found, based on the explanation in the previous paper, from Fig. 2 that NMP was the best solvent for the dispersion of the surface modified MWNTs, and the second was DMSO, the third water, the last one DMF. Even in DMF, the surface modified MWNTs were so well dispersed that the MWNTs were not precipitated after 10 days of keeping at room temperature without agitation.
Cf − Cp × 100 Cf
where Cf and Cp are the concentrations of the feed solution and permeate, respectively. The Cf and Cp were measured by using a high-performance liquid chromatograph (HPLC) (model Waters 501) that was attached to a differential refractometer R401 as a detector.
Fig. 2. UV–vis spectra of MWNTs dispersed in different solvents.
J.-H. Choi et al. / Journal of Membrane Science 284 (2006) 406–415 Table 1 Solubility parameters of various solvents Solvents
NMP DMSO DMF Water
Solubility parameters δd (MPa0.5 )
δp (MPa0.5 )
δh (MPa0.5 )
δt (MPa0.5 )
18 18.4 17.4 15.5
12.3 16.4 13.7 16
7.2 10.2 11.3 42.4
22.9 26.6 24.8 47.9
The dispersion behavior of the nanotubes in solvents can be explained with the solvent solubility parameter (see Table 1). Ham et al. reported that the unfunctionalized carbon nanotubes (CNT) were well dispersed in the solvents with dispersive component (δd ) values of 17–18 MPa0.5 [22–23]. However, they precipitated in the solvents with high polar component (δp ) values or hydrogen-bonding component (δh ) values, because of their hydrophobic nature. So, for the dispersion of unfunctionalized CNT, NMP was good, but DMSO, DMF and water were bad. However, as modified MWNTs were used in this experiment, the solubility tendency became different. They were also very well dispersed in those solvents (DMSO, DMF, and water) with high values of polar component and hydrogen-bonding component. From this result, it was confirmed that the modification with strong acids developed polar, hydrogen-bonding possible functional groups such as carboxylic acid on the surface of the MWNTs. Therefore, it might be concluded that the solubility of the modified MWNTs in solvents may be determined by the combination of the hydrophobicity originated from the main structure of the MWNTs and the hydrophilicity attributed to the carboxylic groups developed on the surface of the MWNTs. So, in this study, NMP was found to be the best solvent for the dispersion of the modified MWNTs. The certain amounts of modified MWNTs dispersed in NMP (2.3/97.7 = MWNTs/NMP) were then added into the 20 wt.% of PSf solution in NMP with stirring. The contents of the MWNTs with respect to PSf of the blend solutions were 0.5, 1.0, 1.5, 2.0, and 4 wt.%. After blending, to figure out the solution properties of the blend solutions that might affect the characteristics of the resulting membranes, solution viscosities were measured using Brookfield viscometer. Table 2 shows the viscosities of the MWNTs/PSf blend solution as
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a function of the amount of MWNTs added. With increasing contents of MWNTs, the viscosity increased. Until 2 wt.%, viscosity increased gradually along with MWNTs, but at 4 wt.%, the viscosity jumped steeply, becoming 473.2 cP. From these data, it was found that the rheological percolation threshold of MWNTs/PSf blend solutions was between 2 and 4 wt.% due to the combined nanotubes–polymer network, rather than the nanotube–nanotube network. That is to say, the rheological properties in carbon nanotube–polymer blends dominate the entangled polymer network and the combined nanotubes–polymer network, and nanotube–nanotube network can be considered as electrical conductivity percolation threshold [24–26]. Similar effects have been reported for melt mixed poly(ethylene terephthalate)/MWNTs blends [27], polycarbonate/MWNTs blends [24] and polypropylene/MWNTs blends [28]. However, to form MWNTs/PSf blend membranes by phase inversion process, the rheological property with nanotubes– polymer interaction is more important than nanotube–nanotube network. In general, the higher solution viscosity indicates that kinetically, the overall diffusion between components in the phase inversion system can be suppressed because of the increase in the rheological hindrance or a delayed exchange between solvent and non-solvent [2]. The data reveals that the overall diffusion can be inhibited kinetically by MWNTs in a solution because of the increased viscosity, and therefore, can affect the exchange between solvent and non-solvent during the phase inversion process. These kinds of variation of the viscosities of the MWNTs/PSf blend solutions appeared to affect on the characteristics of the resulting membranes that will be explained in the following section. 3.2. Characterization Fig. 3 exhibits the FTIR spectra of the modified MWNTs, the surfaces of the PSf membrane and the blend membranes
Table 2 Viscosities of the MWNTs/PSf blend solutions used as casting solutions in this study as a function of the contents of MWNTs with respect to the contents of PSf, in which the ratio of the PSf/NMP was fixed at 15/85 in wt.% Casting solutions
1 2 3 4 5 6
Composition (wt.%) PSf
NMP
15 15 15 15 15 15
85 85 85 85 85 85
Contents of MWNTs respect to PSf (wt.%)
Viscosity (cP)
0 0.5 1.0 1.5 2.0 4.0
311.3 327.2 337.4 356.0 365.3 473.2
Fig. 3. The FTIR spectra of the acid treated MWNTs and the MWNTs/PSf blend membranes with different contents of MWNTs: (a) acid-treated MWNTs; (b) just PSf membrane; (c) the blend membrane with 1.0 wt.% of MWNTs; (d) the blend membrane with 4.0 wt.% of MWNTs.
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Table 3 Frequencies and assignments of the peaks of the FTIR spectra of Fig. 2 Peak (cm−1 )
Assignment
3435.78 2857.75–2967.05 1715.18
–OH stretching vibrations CH stretching vibrations C O stretching vibrations in ketone or carboxylic acid groups Conjugation of C O with C C bonds or interaction between localized C C bonds and carboxylic acids and ketones S O asymmetric stretching vibrations C–O–C symmetric stretching vibrations S O symmetric stretching vibrations
1640.2–1684.45
1294.09 1236.65 1146.97
with 1 and 4 wt.% of MWNTs. Table 3 summarizes the assignments of the main IR bands of the spectra of Fig. 3. As observed from FTIR spectrum of Fig. 3a, the modified MWNTs show –COOH and –OH functional groups. The peak at 1715 and 3435 cm−1 are in correspondence to C O and –OH stretching, respectively, indicating the existence of carboxyl groups in the modified CNTs. The peaks between 2857 and 2970 cm−1 are associated with the C–H stretching vibration, and the bands of 1640–1685 cm−1 region are likely due to conjugation of C O with C C bonds or interaction between localized C C bonds and carboxylic acids and ketones [29]. Fig. 3b presents ATR–FTIR spectra of the PSf membrane. The bands at 1294 and 1146 cm−1 can be attributed to the stretching vibrations of S O asymmetric and S O symmetric, respectively. The peak at 1236 cm−1 can be assigned to the symmetric C–O–C stretching vibration. These peaks appear too from the spectra of MWNTs/PSf blend membranes (Fig. 3c and d). However, the spectra of blend membranes are different from that of the original PSf membrane, showing C O bond of carboxyl groups around 1680 cm−1 by the added MWNTs. The surface hydrophilicity of the blend membranes was also found to be affected by the MWNTs added into the PSf membrane. Fig. 4 shows the contact angles of the surfaces of the blend membranes as a function of the contents of the MWNTs.
The contact angle decreased with increasing content of MWNTs. This result suggests that during phase inversion process in water, the hydrophilic MWNTs migrated to the membrane surface, making the membrane surface hydrophilic. 3.3. Membrane morphology Figs. 5 and 6 show the FESEM photographs of the surfaces and the fractures of the MWNTs/PSf blend membranes, respectively. As one can see from Fig. 5, by the MWNTs, the PSf membrane surface became rougher with increased pore size. The PSf membrane with 1.5 wt.% MWNTs looks to have the highest surface roughness and the largest pore size. These pictures suggest that the modified MWNTs with hydrophilic functional groups make the PSf membrane to have nodular structure, with increased pore size and rough surface. This result might be explained by the fast exchange of solvent and non-solvent in the phase inversion process due to the hydrophilic MWNTs [3]. However when the content of MWNTs increased further, especially when it was 4 wt.%, the surface structure started to become smooth again. This is maybe due to the increased viscosity of the MWNTs/PSf blend solution [5]. As explained above, the viscosity of the blend solutions increased along with the contents of MWNTs. The increased viscosity usually retards the exchange of solvent and non-solvent, making smooth membrane surface. Conclusively speaking, in this case, two factors (increased hydrophilicity and increased viscosity by the added hydrophilic MWNTs) acted at same time for the formation of the microporous blend membranes. When the content of the added MWNTs was less than 4 wt.%, increased hydrophilicity of the solution did major role to form a nodular structure, but when it was more than 4 wt.%, increased viscosity of the solution was the major factor to make a smooth membrane surface. On the other hand, there was not a distinct difference in the structures of the fractures of the blend membranes, all showing finger like structure (Fig. 6), except for the different numbers of the MWNTs positioned on the surface layer. With increasing contents of MWNTs, more MWNTs were found from the surface layer, making the membrane surface hydrophilic and supporting the decreasing contact angle in Fig. 4. 3.4. Membrane performance
Fig. 4. Contact angles of the surface of the MWNTs/PSf blend membranes against water as a function of the contents of MWNTs used.
The permeation properties of the blend membranes with different contents of MWNTs were characterized, using water, and 1000 ppm aqueous solutions of PEO 100,000 and PVP 55,000 as feed solutions. Figs. 8–10 show the results of the permeation tests. Fig. 8 shows the water flux as a function of operating pressure when pure water was used as a feed, no fouling problems involved, only affected by the pore size and porosity of the membranes used. The order of the flux according to the contents of MWNTs of the blend membranes was 1.5% > 1.0% > 2.0% > 0.5% > 0.0% > 4.0%. This result can be explained by the pore sizes of the membranes that appeared to be very dependent on the contents of the MWNTs, with the results from FESEM and capillary flow porometry (CFP) (Figs. 5 and 7). As explained in Section 3.3, the FESEM pic-
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Fig. 5. FESEM photographs of the surfaces of the MWNTs/PSf blend membranes with different contents of MWNTs: (a) just PSf membrane; (b) 0.5 wt.% of MWNTs; (c) 1.0 wt.% of MWNTs; (d) 1.5 wt.% of MWNTs; (e) 2.0 wt.% of MWNTs; (f) 4.0 wt.% of MWNTs.
tures of the membranes revealed that the pore size of the membranes is dependent on the contents the MWNTs used, maximum roughness and pore sizes at 1.5 wt.% of MWNTs. This FESEM result was also supported by the data obtained from the CFP. From Fig. 7, it was found that the average pore size of the membranes increased with MWNTs to 1.5 wt.% then decreased. The order of the pore size with the contents of MWNTs was: 4.0% < 0.0% < 0.5% < 2.0% < 1.0% < 1.5%. This order of pore size is very well matched with the order of the water flux through the membranes, suggesting that the pure water flux was determined mainly by the pore sizes of the membranes. Considering the pure water flux and the average pore diameters of the blend membranes with different contents of MWNTs, it became clearer that the pore size of the blend membranes increased gradually with the content of MWNTs up to 1.5 wt.%, and then decreased, probably due to the combined effect of increased viscosity and lowered thermodynamic stability of the
blend solutions by the addition of hydrophilic MWNTs. In the phase inversion process, for the formation of microporous membranes, the higher the solution viscosity, solvent’s outdiffusion from the cast solution is favored over non-solvent’s indiffusion into the solution, a membrane with smaller pore sizes is formed [5]. More understanding about permselective properties of the blend membranes can be obtained from the permeation tests with 1000 ppm aqueous solutions of PEO 100,000 and PVP 55,000. From Fig. 9a, showing the flux as a function of operating pressures when 1000 ppm aqueous solution of PEO 100,000 was used as a feed, it is found from the much lower flux values compared with the pure water flux shown in Fig. 8 that serious membrane fouling is involved. In other words, PEO 100,000 molecules are cumulated on the membrane surface with some of them in the pore, blocking the pores and inhibiting the water flow through the pores. One interesting point found here is that the blend membrane with 4 wt.% MWNTs that showed lower pure water flux than
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Fig. 6. FESEM photographs of the cross-sections of the MWNTs/PSf blend membranes with different contents of MWNTs: (a) just PSf membrane; (b) 1.0 wt.% of MWNTs; (c) 2.0 wt.% of MWNTs; (d) 4.0 wt.% of MWNTs.
the pure PSf membranes, because of the smaller pore sizes, shows higher flux in this case. That is probably due to the improve hydrophilicity of the blend membrane by the MWNTs as explained in Section 2.3. The improved hydrophilicity is found to have protected the membrane somewhat from the
membrane fouling, improving the flux. Generally speaking, the membrane with better hydrophilicity shows lower membrane fouling, resulting in higher flux. So, all the flux obtained from the blend membranes is found to be higher than that of pure PSf membrane.
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Fig. 7. The average pore diameter and the size distribution of pores on the surface of the MWNTs/PSf blend membranes.
On the other hand, the order of the flux was different from the pure water flux. For example, when pure water was used, the membrane with 1.5 wt.% MWNTs showed the highest flux, but in this case, the membrane with 0.5 wt.% MWNTs showed the highest flux. To explain this, the fouling pattern has to be considered [30–31]. For large pores, it would relatively easy for the PEO 100,000 to get into the pore inside, plugging pores, but when pore is small, the molecules of PEO 100,000 are not easy to get into the pores, and just cumulating on the surface of the membrane. In other hand, when pore size is bigger, more pores are possible to be blocked effectively by the PEO 100,000 plugging pores. Another fact that affected on the membrane fouling was the surface roughness shown in Fig. 5. Usually, the surface with higher roughness has been known to be easier to be fouled by the accumulation of foulant. So, smoother surface, less fouling is involved [32].
Fig. 8. Pure water flux of the MWNTs/PSf blend membranes as a function of operating pressure.
Fig. 9. Permeation properties of the MWNTs/PSf blend membranes as a 1000 ppm aqueous solution of PEO 100,000 was used as a feed solution: (a) flux and (b) rejection.
Because of such kind of fouling behavior, the membrane with 1.5 wt.% MWNTs composed with relatively large pores was fouled more and showed less flux than the membrane with smaller pores such as the membranes of the MWNTs of 0.5 and 1.0 wt.%. The rejection shown in Fig. 9b reveals a typical behavior, reflecting the flux result. The membrane showed high flux exhibits low rejection, and vice versa, except for the membrane with 4 wt.% of MWNTs. From this result, it became clear that the higher flux of that membrane with smaller pores than the just PSf membrane is due to the its improved surface hydrophilicity. In this case, the rejection increased with operating pressure. This might be explained by the membrane fouling with PEO 100,000. The molecular size of PEO 100,000 is high enough to give high rejection such as over 95% of rejection. So, the transport of the solute through the membrane became relatively less sensitive to the operating pressure than the water (solvent). As a result, the concentration of the solute of permeate obtained at high operating pressure became lower than that of permeate obtained at low operating pressure, resulting in the increased rejection. The more compaction of the fouling layer at higher operating pressure probably inhibited more the transport of the
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water with the aid of concentration polarization effect (increased concentration of the solute near the membrane by the selective permeation of water through the membrane), resulting in the decreased rejection of PVP 55,000 at higher operating pressure. Membrane fouling was involved in this case too, but the degree of the fouling is much less than the case of PEO 100,000. This can be confirmed from the flux value positioned in between the pure water flux and the flux of PEO 100,000 solution. 4. Conclusions The MWNTs/PSf blend membranes can be prepared easily by the phase inversion method by using surface modified MWNTs. The surface modified MWNTs can be prepared by the treatment of the MWNTs with strong acids such as sulfuric acid and nitric acid, and they were easy to be dispersed in polar organic solvent such as NMP, DMSO and DMF. The stability of the surface modified MWNTs in such organic solvents was found to be good enough to be used for the formation of blend membranes with other polymers such as PSf. The MWNTs turned out to be a good modifier for the formation of functional microporous PSf membranes, controlling the hydrophilicity of the membrane surface, adjusting the pore size and porosity. The permselective properties of the MWNTs/PSf blend membranes were appeared to be very dependent on the contents of the MWNTs used. By using proper amount of MWNTs, it was possible to increase the flux and the solute rejection at the same time for the PSf microporous membranes. Fig. 10. Permeation properties of the MWNTs/PSf blend membranes as a 1000 ppm aqueous solution of PVP 55,000 was used as a feed solution: (a) flux and (b) rejection.
solute than the solvent through the membrane, causing higher rejection at higher operating pressure. From these results, it can be concluded that by blending the surface modified hydrophilic MWNTs with PSf, it is possible to control the surface hydrophilicity and the pore size of the PSf membrane. This conclusion can also be confirmed by the results from the permeation tests with a 1000 ppm aqueous solution of PVP 55,000 shown in Fig. 10. When PVP 55,000 was used as a solute of the feed solution, overall flux of the membranes was improved, but the solute rejection decreased substantially from the case of PEO 100,000. This is definitely due to the smaller molecular sizes of the PVP 55,000. So, except the rejection behavior with operating pressure, everything appeared to be in the same trend as the case of PEO 100,000. For PVP 55,000, the rejection appeared to be decreased with operating pressure. This is probably related to the molecular size of the solute: the molecular size of PVP 55,000 is much smaller than that of PEO 100,000 so that the rejection of PVP 55,000 is only about less than 60%. So, PVP 55,000 is much easier to pass through the membrane than the PEO 100,000. As a result, the increment of the solute that passed through the membrane by the increased operating pressure became larger than that of
Acknowledgement This research was supported by a grant (code #: 05K150101210) from ‘Center for Nanostructured Materials Technology under 21st Century Frontier R&D Programs’ of the Ministry of Science and Technology, Korea. References [1] P. van de Witte, P.J. Dijkstra, J.W.A. van den berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1. [2] M. Mulder, Basic Principles of Membrane Technology, Kluwer, London, 1996. [3] I.M. Wienk, R.M. Boom, M.A.M. Beerlage, et al., Recent advances in the formation of phase inversion membranes made from amorphous or semicrystalline polymers, J. Membr. Sci. 113 (1996) 361. [4] R.M. Boom, I.M. Wienk, T. van den Boomgaard, C.A. Smolders, Microstructure in phase inversion membranes. Part 2. The role of a polymer additive, J. Membr. Sci. 3 (1992) 277. [5] M.J. Han, S.T. Nam, Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane, J. Membr. Sci. 202 (2002) 55. [6] K.W. Lee, B.K. Seo, S.T. Nam, M.J. Han, Trade-off between thermodynamic enhancement and kinetic hindrance during phase inversion in the preparation of polysulfone membranes, Desalination 159 (2003) 289. [7] A.L. Ahmad, M. Sarif, S. Ismail, Development of an integrally skinned ultrafiltration membrane for wastewater treatment: effect of different formulations of PSf/NMP/PVP on flux and rejection, Desalination 179 (2005) 257.
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Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes Jae-Hyun Choi a,b , Jonggeon Jegal a,∗ , Woo-Nyon Kim b a
Membrane and Separation Research Center, Korea Research Institute of Chemical Technology, Post Office Box 107, Yuseong, Daejeon 305-606, South Korea b Department of Chemical and Biological Engineering, Applied Rheology Center, Korea University, Anam-dong, Seoul 136-701, South Korea Received 10 May 2006; received in revised form 8 August 2006; accepted 10 August 2006 Available online 22 August 2006
Abstract Multi-walled carbon nanotubes (MWNTs)/polysulfone (PSf) blend membranes were prepared by a phase inversion process, using N-methyl-2pyrrolidinone (NMP) as a solvent and water as a coagulant. Before making the blend membranes, MWNTs were first treated with strong acid to make them well dispersed in organic solvents such as NMP for the preparation of homogeneous MWNTs/PSf blend solutions. The prepared MWNTs/PSf blend membranes were then characterized using the several analytical methods such as a Fourier transform infrared (FTIR) spectroscopy, a contact angle goniometer, a scanning electron microscopy (SEM) and permeation tests. Because of the hydrophiic MWNTs, the surface of the MWNTs/PSf blend membranes appeared to be more hydrophilic than a just PSf membrane. The carboxylic acid functional groups developed by the treatment with acid on the surface of MWNTs seemed to act to increase hydrophilicity of the blend membranes. The morphology and permeation properties of the blend membranes were also found to be dependent on the amounts of MWNTs used. The pore size of the blend membranes increased along with the contents of MWNTs up to 1.5%, then decreased, and at 4.0% of MWNTs, it became even smaller than PSf membrane. The PSf membrane with 4.0% of MWNTs showed higher flux and rejection than the PSf membrane without MWNTs. © 2006 Elsevier B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Polysulfone; Composite membrane; Phase inversion; Hydrophilicity; Permeability
1. Introduction There are several ways to prepare porous polymeric membranes, such as sintering, track etching and phase inversion process [1]. The majority of polymer membranes used for microand ultra-filtration of liquids are prepared by phase inversion process, in which a film of concentrated polymer solution is cast on a suitable substrate and subsequently immersed in a nonsolvent bath where the exchange of solvent by coagulant and phase separation occurs in the casting film. Even though the phase inversion process is the most widely used technique for preparing various types of polymeric membranes, many aspects in the process are not clearly clarified, including the complete analysis of skin layer formation at the initial moment of phase inversion. However, it is known that asymmetric membrane
∗
Corresponding author. Tel.: +82 42 860 7637; fax: +82 42 861 4151. E-mail address: [email protected] (J. Jegal).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.013
depends on two dominating factors, such as kinetic parameters and thermodynamic parameters [2–4]. Therefore, selection of membrane materials, such as polymers, solvent, non-solvent, is important in the fabrication of membrane having desirable membrane structure and properties. Among several polymeric materials, polysulfone (PSf) has been widely used as a polymer for commercial microfiltration and ultrafiltration membranes. Moreover, PSf has been used for support material of nanofiltration and reverse osmosis membranes because of its excellent balance in the chemical, thermal and mechanical properties. However, application of the PSf membrane often limited because of its hydrophobic nature. Because of its hydrophobic properties, performance of PSf membranes results in low water flux and serious membrane fouling. To overcome these problems, many research groups are trying to enhance performance of PSf membranes. For example, polyvinylpyrrolidone (PVP) has been widely used as a polymer additive to provide hydrophilicity and to increase membrane permeability [5–7]. And it is also easy to make the hydrophilic
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surface of membrane by plasma treatment [8,9]. Kim et al. reported that poly(1-vinylpyrrolidone-co-acrylonitrile) copolymer and their blends with PSf were used for the fabrication of ultrafiltration membranes having hydrophilic properties [10]. Recently, hybrid materials were used to achieve high permeability and selectivity in membrane applications [11–13]. Garcia-Valls and co-workers reported that activated carbon–PSf composite membranes were synthesized by an immersion–precipitation method. Their results showed that permeability for composite membranes was influenced by activated carbon particle size and loading on the membrane [13]. Based on this, we prepared multi-walled carbon nanotubes (MWNTs)/PSf blend membranes by the immersion phase inversion process, using water as a coagulant. The typical advantages of surface modified MWNTs as an additive are high conductivity and hydrophilicity. With the decreasing price of the MWNTs (less than $1.0 for 1 kg of MWNTs), MWNTs is expected to become one of the additives to give membranes special functionalities. It would also be possible to give conductivity to the membrane as well as to modify the morphologies of the membrane. However, in this study, the effect of carbon nanotubes (CNTs) loading on the performance of the PSf membrane was mainly investigated. UV/visible spectroscopy, rheometer, Fourier transform infrared (FTIR) spectroscopy, contact angle goniometer, and scanning electron microscope (SEM) have been used to characterize solution property and the prepared membrane. The permeation performance of the membranes is being discussed through the tests with various feed solutions (pure water, 1000 ppm aqueous solutions of PEO 100,000 or PVP 55,000). 2. Experimental 2.1. Materials Polysulfone (PSf, Udel® P 3500, Amoco, Marietta, OH) was used as a membrane material. Carbon nanotubes (multi-walled carbon nanotubes, MWNTs) manufactured by CVD process (supplied by Iljin nanotech, Korea) whose purity of greater than 95% were used for the modification of a PSf membrane. The MWNTs in tubular shape, composed of six-membered carbon rings, were like a rolled graphite sheet with 10–20 nm of outer diameter, 10–50 m of the length, and 4.3 ± 2.3 nm of inner diameter. The MWNTs were surface modified with strong acids, concentrated nitric (HNO3 ) and sulfuric acids (H2 SO4 ) (1:3 in vol.%), to make them easy to be dispersed in the organic solvents [14]. One gram of raw MWNTs was added to 150 ml of the acid mixture in a round-bottomed flask, and refluxed at 80 ◦ C for 1 h. On cooling, the mixture was washed with distilled water until nearly neutrality water was obtained. The collection was then freeze dried under vacuum at −80 ◦ C for further use. N-Methyl2-pyrrolidinone (NMP) was purchased from Junsei Co. and used as a solvent without further purification. Polyvinylpyrrolidone (PVP) with molecular weight of 55,000 g/mol and poly(ethylene oxide) (PEO) with molecular weight of 100,000 g/mol bought from Aldrich Co. were used for permeation tests.
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2.2. Membrane preparation Twenty grams of PSf was dissolved in 80 g of NMP at 60 ◦ C with constant stirring to prepare a PSf solution with a composition of 20/80 (PSf/NMP). And 2.3 g of modified MWNTs was dispersed into 97.7 g of NMP to prepare a MWNT solution. For better dispersion of the MWNTs in NMP, the solution was sonicated for 80 s using a high-power sonic tip (sonicskorea, SKB-2000, 2 kW). Then proper amounts of the both solutions were taken and mixed with stirring at room temperature to prepare PSf/MWNTs blend solutions with different compositions. For all the blend solutions, PSf/MWNTs mixture to NMP ratio was 15/85, but the ratio of PSf to MWNTs was varied: 99.5/0.5, 99.0/1.0, 98.5/1.5, 98/2.0, and 96/4. Hereafter, the contents of the MWNTs with respect to the PSf will be used through out this paper. After which, the casting solutions were kept at room temperature for at least 24 h to remove air bubbles from the solutions. The casting solutions were then cast with 150 m thickness on a polyester non-woven fabric using a Baker Applicator. The nascent membranes were immersed into a water bath conditioned at 20 m for 24 h. 2.3. Charaterization By using several analytical methods such as UV/visible spectroscopy, contact angle goniometer, rheometer, Fourier transform infrared (FTIR) spectroscopy, and field emission scanning electron microscopy (FESEM), the prepared MWNTs/PSf blend membranes were characterized. To evaluate the extent of dispersion of the surface modified MWNTs in different solvents, a UV/visible spectroscopy (UV-2550, Shimadzu) was used. The viscosities of the casting solutions were measured using a Brookfield Digital Rheometer (model: DV-III, Brookfield Eng. Bab.) at 25 ◦ C. The hydrophilicity of the surface of the blend membranes as a function of different contents of MWNTs was evaluated based on the dynamic water contact angle of the membrane using contact angle goniometer (SEO 300A). The surface and cross-section morphology of the blend membranes were studied with a FESEM (model: XL 30, Philips Co., USA). 2.4. Capillary flow porometry The pore size and pore size distribution of the PSf membranes with or without MWNTs were studied, using a capillary flow porometery (model CFP-1500-AEL, Porous Materials Inc., NY, USA). The short procedure was as a follow: the loaded sample (membrane) was soaked in a wetting liquid, a low surface tension liquid (σ = 15.9 × 10−3 N/m) named Galwick, to fill spontaneously the pores of the sample, and then gas pressure was applied to one side of the sample. The gas pressure was increased slowly until the liquid was removed from the pores, forming gas flow through pores, which then increased with further increase in pressure [15–16]. At a certain pressure, the largest pore was emptied first and the gas flow started, which gave the bubble point
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3. Results and discussion 3.1. Preparation of MWCNT/PSf blend membranes
Fig. 1. Schematic diagram of ultrafiltration experimental set-up.
pressure. On further increase of the pressure, smaller pores were then emptied and gas flow increased. The flow rate was determined as a function of pressure and used to calculate the desired pore characteristics by the Washburn equation [17]: D=
4σ cos θ P
where D is the pore diameter, σ the surface tension of the liquid, θ the contact angle of the liquid, and P is the applied gas pressure. From the measured gas pressure and flow rates, the pore size and pore size distribution were determined. 2.5. Permeation test The common method to characterize the MWNTs/PSf blend membrane was to measure its performance in terms of pure permeability, flux and solute rejection of different feed solutions, using a general UF test set-up (Fig. 1). The concentration of all the feed solutions used in this experiment was 1000 ppm and an upstream pressure was controlled by using back-pressure regulators (1–4 bar). The permeation tests were conducted at 25 ◦ C, and the effective membrane area was 18.8 cm2 . Before measuring the flux, each membrane was subjected at 4 bar for about 2 h to avoid the compaction effect of the membrane. A flux was measured by weighing permeates penetrated through the membranes per unit time, and a solute rejection was calculated from the concentrations of the feed solution and permeate using the following equation. Rejection (%) =
The MWNTs/PSf blend membranes were prepared by the immersion phase inversion process, using water as a coagulant. Many organic solvents can be used for this method: Nmethyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), etc., for the preparation of PSf solutions. These solvents should be well miscible with non-solvents that will be used for the phase inversion of the PSf solutions via the solvent–non-solvent exchange process. One of the common non-solvents, so called coagulants, is water. Beyond water, sometimes alcohols such as ethanol and propanol also can be used as coagulants. In this study NMP was chosen as a solvent due to its strong interaction with polymer and miscibility with water. In order to fabricate the MWNTs/PSf blend membranes, it is necessary to make a homogeneous MWNTs solution in NMP before blending them with the PSf solution in NMP. However, MWNTs as synthesized are strongly hydrophobic and generally have very low solubility in all solvents. To overcome these problems by introducing hydrophilic functional groups into the surface of the MWNTs, they were treated with strong acid such as concentrated H2 SO4 and HNO3 . Acid-treated MWNTs are known to have carboxyl groups on their surfaces, showing good dispersion in polar organic solvents [18–19]. Fig. 2 shows the absorption spectra of UV/visible spectrophotometry of the surface modified MWNTs solutions in various polar organic solvents. It is well known that the more stable suspension of CNTs in solvents shows increased absorption in a UV/visible spectrophotometry [20–21]. It was found, based on the explanation in the previous paper, from Fig. 2 that NMP was the best solvent for the dispersion of the surface modified MWNTs, and the second was DMSO, the third water, the last one DMF. Even in DMF, the surface modified MWNTs were so well dispersed that the MWNTs were not precipitated after 10 days of keeping at room temperature without agitation.
Cf − Cp × 100 Cf
where Cf and Cp are the concentrations of the feed solution and permeate, respectively. The Cf and Cp were measured by using a high-performance liquid chromatograph (HPLC) (model Waters 501) that was attached to a differential refractometer R401 as a detector.
Fig. 2. UV–vis spectra of MWNTs dispersed in different solvents.
J.-H. Choi et al. / Journal of Membrane Science 284 (2006) 406–415 Table 1 Solubility parameters of various solvents Solvents
NMP DMSO DMF Water
Solubility parameters δd (MPa0.5 )
δp (MPa0.5 )
δh (MPa0.5 )
δt (MPa0.5 )
18 18.4 17.4 15.5
12.3 16.4 13.7 16
7.2 10.2 11.3 42.4
22.9 26.6 24.8 47.9
The dispersion behavior of the nanotubes in solvents can be explained with the solvent solubility parameter (see Table 1). Ham et al. reported that the unfunctionalized carbon nanotubes (CNT) were well dispersed in the solvents with dispersive component (δd ) values of 17–18 MPa0.5 [22–23]. However, they precipitated in the solvents with high polar component (δp ) values or hydrogen-bonding component (δh ) values, because of their hydrophobic nature. So, for the dispersion of unfunctionalized CNT, NMP was good, but DMSO, DMF and water were bad. However, as modified MWNTs were used in this experiment, the solubility tendency became different. They were also very well dispersed in those solvents (DMSO, DMF, and water) with high values of polar component and hydrogen-bonding component. From this result, it was confirmed that the modification with strong acids developed polar, hydrogen-bonding possible functional groups such as carboxylic acid on the surface of the MWNTs. Therefore, it might be concluded that the solubility of the modified MWNTs in solvents may be determined by the combination of the hydrophobicity originated from the main structure of the MWNTs and the hydrophilicity attributed to the carboxylic groups developed on the surface of the MWNTs. So, in this study, NMP was found to be the best solvent for the dispersion of the modified MWNTs. The certain amounts of modified MWNTs dispersed in NMP (2.3/97.7 = MWNTs/NMP) were then added into the 20 wt.% of PSf solution in NMP with stirring. The contents of the MWNTs with respect to PSf of the blend solutions were 0.5, 1.0, 1.5, 2.0, and 4 wt.%. After blending, to figure out the solution properties of the blend solutions that might affect the characteristics of the resulting membranes, solution viscosities were measured using Brookfield viscometer. Table 2 shows the viscosities of the MWNTs/PSf blend solution as
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a function of the amount of MWNTs added. With increasing contents of MWNTs, the viscosity increased. Until 2 wt.%, viscosity increased gradually along with MWNTs, but at 4 wt.%, the viscosity jumped steeply, becoming 473.2 cP. From these data, it was found that the rheological percolation threshold of MWNTs/PSf blend solutions was between 2 and 4 wt.% due to the combined nanotubes–polymer network, rather than the nanotube–nanotube network. That is to say, the rheological properties in carbon nanotube–polymer blends dominate the entangled polymer network and the combined nanotubes–polymer network, and nanotube–nanotube network can be considered as electrical conductivity percolation threshold [24–26]. Similar effects have been reported for melt mixed poly(ethylene terephthalate)/MWNTs blends [27], polycarbonate/MWNTs blends [24] and polypropylene/MWNTs blends [28]. However, to form MWNTs/PSf blend membranes by phase inversion process, the rheological property with nanotubes– polymer interaction is more important than nanotube–nanotube network. In general, the higher solution viscosity indicates that kinetically, the overall diffusion between components in the phase inversion system can be suppressed because of the increase in the rheological hindrance or a delayed exchange between solvent and non-solvent [2]. The data reveals that the overall diffusion can be inhibited kinetically by MWNTs in a solution because of the increased viscosity, and therefore, can affect the exchange between solvent and non-solvent during the phase inversion process. These kinds of variation of the viscosities of the MWNTs/PSf blend solutions appeared to affect on the characteristics of the resulting membranes that will be explained in the following section. 3.2. Characterization Fig. 3 exhibits the FTIR spectra of the modified MWNTs, the surfaces of the PSf membrane and the blend membranes
Table 2 Viscosities of the MWNTs/PSf blend solutions used as casting solutions in this study as a function of the contents of MWNTs with respect to the contents of PSf, in which the ratio of the PSf/NMP was fixed at 15/85 in wt.% Casting solutions
1 2 3 4 5 6
Composition (wt.%) PSf
NMP
15 15 15 15 15 15
85 85 85 85 85 85
Contents of MWNTs respect to PSf (wt.%)
Viscosity (cP)
0 0.5 1.0 1.5 2.0 4.0
311.3 327.2 337.4 356.0 365.3 473.2
Fig. 3. The FTIR spectra of the acid treated MWNTs and the MWNTs/PSf blend membranes with different contents of MWNTs: (a) acid-treated MWNTs; (b) just PSf membrane; (c) the blend membrane with 1.0 wt.% of MWNTs; (d) the blend membrane with 4.0 wt.% of MWNTs.
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Table 3 Frequencies and assignments of the peaks of the FTIR spectra of Fig. 2 Peak (cm−1 )
Assignment
3435.78 2857.75–2967.05 1715.18
–OH stretching vibrations CH stretching vibrations C O stretching vibrations in ketone or carboxylic acid groups Conjugation of C O with C C bonds or interaction between localized C C bonds and carboxylic acids and ketones S O asymmetric stretching vibrations C–O–C symmetric stretching vibrations S O symmetric stretching vibrations
1640.2–1684.45
1294.09 1236.65 1146.97
with 1 and 4 wt.% of MWNTs. Table 3 summarizes the assignments of the main IR bands of the spectra of Fig. 3. As observed from FTIR spectrum of Fig. 3a, the modified MWNTs show –COOH and –OH functional groups. The peak at 1715 and 3435 cm−1 are in correspondence to C O and –OH stretching, respectively, indicating the existence of carboxyl groups in the modified CNTs. The peaks between 2857 and 2970 cm−1 are associated with the C–H stretching vibration, and the bands of 1640–1685 cm−1 region are likely due to conjugation of C O with C C bonds or interaction between localized C C bonds and carboxylic acids and ketones [29]. Fig. 3b presents ATR–FTIR spectra of the PSf membrane. The bands at 1294 and 1146 cm−1 can be attributed to the stretching vibrations of S O asymmetric and S O symmetric, respectively. The peak at 1236 cm−1 can be assigned to the symmetric C–O–C stretching vibration. These peaks appear too from the spectra of MWNTs/PSf blend membranes (Fig. 3c and d). However, the spectra of blend membranes are different from that of the original PSf membrane, showing C O bond of carboxyl groups around 1680 cm−1 by the added MWNTs. The surface hydrophilicity of the blend membranes was also found to be affected by the MWNTs added into the PSf membrane. Fig. 4 shows the contact angles of the surfaces of the blend membranes as a function of the contents of the MWNTs.
The contact angle decreased with increasing content of MWNTs. This result suggests that during phase inversion process in water, the hydrophilic MWNTs migrated to the membrane surface, making the membrane surface hydrophilic. 3.3. Membrane morphology Figs. 5 and 6 show the FESEM photographs of the surfaces and the fractures of the MWNTs/PSf blend membranes, respectively. As one can see from Fig. 5, by the MWNTs, the PSf membrane surface became rougher with increased pore size. The PSf membrane with 1.5 wt.% MWNTs looks to have the highest surface roughness and the largest pore size. These pictures suggest that the modified MWNTs with hydrophilic functional groups make the PSf membrane to have nodular structure, with increased pore size and rough surface. This result might be explained by the fast exchange of solvent and non-solvent in the phase inversion process due to the hydrophilic MWNTs [3]. However when the content of MWNTs increased further, especially when it was 4 wt.%, the surface structure started to become smooth again. This is maybe due to the increased viscosity of the MWNTs/PSf blend solution [5]. As explained above, the viscosity of the blend solutions increased along with the contents of MWNTs. The increased viscosity usually retards the exchange of solvent and non-solvent, making smooth membrane surface. Conclusively speaking, in this case, two factors (increased hydrophilicity and increased viscosity by the added hydrophilic MWNTs) acted at same time for the formation of the microporous blend membranes. When the content of the added MWNTs was less than 4 wt.%, increased hydrophilicity of the solution did major role to form a nodular structure, but when it was more than 4 wt.%, increased viscosity of the solution was the major factor to make a smooth membrane surface. On the other hand, there was not a distinct difference in the structures of the fractures of the blend membranes, all showing finger like structure (Fig. 6), except for the different numbers of the MWNTs positioned on the surface layer. With increasing contents of MWNTs, more MWNTs were found from the surface layer, making the membrane surface hydrophilic and supporting the decreasing contact angle in Fig. 4. 3.4. Membrane performance
Fig. 4. Contact angles of the surface of the MWNTs/PSf blend membranes against water as a function of the contents of MWNTs used.
The permeation properties of the blend membranes with different contents of MWNTs were characterized, using water, and 1000 ppm aqueous solutions of PEO 100,000 and PVP 55,000 as feed solutions. Figs. 8–10 show the results of the permeation tests. Fig. 8 shows the water flux as a function of operating pressure when pure water was used as a feed, no fouling problems involved, only affected by the pore size and porosity of the membranes used. The order of the flux according to the contents of MWNTs of the blend membranes was 1.5% > 1.0% > 2.0% > 0.5% > 0.0% > 4.0%. This result can be explained by the pore sizes of the membranes that appeared to be very dependent on the contents of the MWNTs, with the results from FESEM and capillary flow porometry (CFP) (Figs. 5 and 7). As explained in Section 3.3, the FESEM pic-
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Fig. 5. FESEM photographs of the surfaces of the MWNTs/PSf blend membranes with different contents of MWNTs: (a) just PSf membrane; (b) 0.5 wt.% of MWNTs; (c) 1.0 wt.% of MWNTs; (d) 1.5 wt.% of MWNTs; (e) 2.0 wt.% of MWNTs; (f) 4.0 wt.% of MWNTs.
tures of the membranes revealed that the pore size of the membranes is dependent on the contents the MWNTs used, maximum roughness and pore sizes at 1.5 wt.% of MWNTs. This FESEM result was also supported by the data obtained from the CFP. From Fig. 7, it was found that the average pore size of the membranes increased with MWNTs to 1.5 wt.% then decreased. The order of the pore size with the contents of MWNTs was: 4.0% < 0.0% < 0.5% < 2.0% < 1.0% < 1.5%. This order of pore size is very well matched with the order of the water flux through the membranes, suggesting that the pure water flux was determined mainly by the pore sizes of the membranes. Considering the pure water flux and the average pore diameters of the blend membranes with different contents of MWNTs, it became clearer that the pore size of the blend membranes increased gradually with the content of MWNTs up to 1.5 wt.%, and then decreased, probably due to the combined effect of increased viscosity and lowered thermodynamic stability of the
blend solutions by the addition of hydrophilic MWNTs. In the phase inversion process, for the formation of microporous membranes, the higher the solution viscosity, solvent’s outdiffusion from the cast solution is favored over non-solvent’s indiffusion into the solution, a membrane with smaller pore sizes is formed [5]. More understanding about permselective properties of the blend membranes can be obtained from the permeation tests with 1000 ppm aqueous solutions of PEO 100,000 and PVP 55,000. From Fig. 9a, showing the flux as a function of operating pressures when 1000 ppm aqueous solution of PEO 100,000 was used as a feed, it is found from the much lower flux values compared with the pure water flux shown in Fig. 8 that serious membrane fouling is involved. In other words, PEO 100,000 molecules are cumulated on the membrane surface with some of them in the pore, blocking the pores and inhibiting the water flow through the pores. One interesting point found here is that the blend membrane with 4 wt.% MWNTs that showed lower pure water flux than
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Fig. 6. FESEM photographs of the cross-sections of the MWNTs/PSf blend membranes with different contents of MWNTs: (a) just PSf membrane; (b) 1.0 wt.% of MWNTs; (c) 2.0 wt.% of MWNTs; (d) 4.0 wt.% of MWNTs.
the pure PSf membranes, because of the smaller pore sizes, shows higher flux in this case. That is probably due to the improve hydrophilicity of the blend membrane by the MWNTs as explained in Section 2.3. The improved hydrophilicity is found to have protected the membrane somewhat from the
membrane fouling, improving the flux. Generally speaking, the membrane with better hydrophilicity shows lower membrane fouling, resulting in higher flux. So, all the flux obtained from the blend membranes is found to be higher than that of pure PSf membrane.
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Fig. 7. The average pore diameter and the size distribution of pores on the surface of the MWNTs/PSf blend membranes.
On the other hand, the order of the flux was different from the pure water flux. For example, when pure water was used, the membrane with 1.5 wt.% MWNTs showed the highest flux, but in this case, the membrane with 0.5 wt.% MWNTs showed the highest flux. To explain this, the fouling pattern has to be considered [30–31]. For large pores, it would relatively easy for the PEO 100,000 to get into the pore inside, plugging pores, but when pore is small, the molecules of PEO 100,000 are not easy to get into the pores, and just cumulating on the surface of the membrane. In other hand, when pore size is bigger, more pores are possible to be blocked effectively by the PEO 100,000 plugging pores. Another fact that affected on the membrane fouling was the surface roughness shown in Fig. 5. Usually, the surface with higher roughness has been known to be easier to be fouled by the accumulation of foulant. So, smoother surface, less fouling is involved [32].
Fig. 8. Pure water flux of the MWNTs/PSf blend membranes as a function of operating pressure.
Fig. 9. Permeation properties of the MWNTs/PSf blend membranes as a 1000 ppm aqueous solution of PEO 100,000 was used as a feed solution: (a) flux and (b) rejection.
Because of such kind of fouling behavior, the membrane with 1.5 wt.% MWNTs composed with relatively large pores was fouled more and showed less flux than the membrane with smaller pores such as the membranes of the MWNTs of 0.5 and 1.0 wt.%. The rejection shown in Fig. 9b reveals a typical behavior, reflecting the flux result. The membrane showed high flux exhibits low rejection, and vice versa, except for the membrane with 4 wt.% of MWNTs. From this result, it became clear that the higher flux of that membrane with smaller pores than the just PSf membrane is due to the its improved surface hydrophilicity. In this case, the rejection increased with operating pressure. This might be explained by the membrane fouling with PEO 100,000. The molecular size of PEO 100,000 is high enough to give high rejection such as over 95% of rejection. So, the transport of the solute through the membrane became relatively less sensitive to the operating pressure than the water (solvent). As a result, the concentration of the solute of permeate obtained at high operating pressure became lower than that of permeate obtained at low operating pressure, resulting in the increased rejection. The more compaction of the fouling layer at higher operating pressure probably inhibited more the transport of the
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water with the aid of concentration polarization effect (increased concentration of the solute near the membrane by the selective permeation of water through the membrane), resulting in the decreased rejection of PVP 55,000 at higher operating pressure. Membrane fouling was involved in this case too, but the degree of the fouling is much less than the case of PEO 100,000. This can be confirmed from the flux value positioned in between the pure water flux and the flux of PEO 100,000 solution. 4. Conclusions The MWNTs/PSf blend membranes can be prepared easily by the phase inversion method by using surface modified MWNTs. The surface modified MWNTs can be prepared by the treatment of the MWNTs with strong acids such as sulfuric acid and nitric acid, and they were easy to be dispersed in polar organic solvent such as NMP, DMSO and DMF. The stability of the surface modified MWNTs in such organic solvents was found to be good enough to be used for the formation of blend membranes with other polymers such as PSf. The MWNTs turned out to be a good modifier for the formation of functional microporous PSf membranes, controlling the hydrophilicity of the membrane surface, adjusting the pore size and porosity. The permselective properties of the MWNTs/PSf blend membranes were appeared to be very dependent on the contents of the MWNTs used. By using proper amount of MWNTs, it was possible to increase the flux and the solute rejection at the same time for the PSf microporous membranes. Fig. 10. Permeation properties of the MWNTs/PSf blend membranes as a 1000 ppm aqueous solution of PVP 55,000 was used as a feed solution: (a) flux and (b) rejection.
solute than the solvent through the membrane, causing higher rejection at higher operating pressure. From these results, it can be concluded that by blending the surface modified hydrophilic MWNTs with PSf, it is possible to control the surface hydrophilicity and the pore size of the PSf membrane. This conclusion can also be confirmed by the results from the permeation tests with a 1000 ppm aqueous solution of PVP 55,000 shown in Fig. 10. When PVP 55,000 was used as a solute of the feed solution, overall flux of the membranes was improved, but the solute rejection decreased substantially from the case of PEO 100,000. This is definitely due to the smaller molecular sizes of the PVP 55,000. So, except the rejection behavior with operating pressure, everything appeared to be in the same trend as the case of PEO 100,000. For PVP 55,000, the rejection appeared to be decreased with operating pressure. This is probably related to the molecular size of the solute: the molecular size of PVP 55,000 is much smaller than that of PEO 100,000 so that the rejection of PVP 55,000 is only about less than 60%. So, PVP 55,000 is much easier to pass through the membrane than the PEO 100,000. As a result, the increment of the solute that passed through the membrane by the increased operating pressure became larger than that of
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