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Effect of molecular weight of polymeric additives on formation, permeation properties and hypochlorite treatment of asymmetric polyacrylonitrile membranes
Journal of Membrane Science 243 (2004) 45–57
Effect of molecular weight of polymeric additives on formation, permeation properties and hypochlorite treatment of asymmetric polyacrylonitrile membranes Bumsuk Jung a,∗ , Joon Ki Yoon a,b , Bokyung Kim a , Hee-Woo Rhee b a
Polymer Hybrids Research Centers, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea b Department of Chemical and Biomolecular Engineering, Sogang University, Shinsu-Dong 1, Mapo-Ku, Seoul 121-742, South Korea Received 5 February 2004; accepted 2 June 2004 Available online 17 August 2004
Abstract Asymmetric polyacrylonitrile membranes were prepared via phase inversion process from casting solution composed of synthesized polyacrylonitrile (PAN), dimethylsulfoxide (DMSO), and poly(vinylpyrrolidone) (PVP) of different molecular weights. The effect of molecular weight of PVP in the casting solution on the morphology, water flux was investigated. The effect of hypochlorite treatment on PVP which was entrapped in pore was investigated. As the higher molecular weight of PVP is added, the thicker top layer of the asymmetric membrane is. The top layer of the membrane and the suppression of macrovoid formation strongly depend on the molecular weight of PVP. The addition with different molecular weight of PVPs also changes pure water permeability. It is also found that the substantial amount of PVP is entrapped in the pores, the water flux decreases. The effect of pore-filling depends on the molecular weight of the additive. Using sodium hypochlorite solution, the entrapped PVP was able to be partially removed. The effectiveness of the selective bleach is pronounced for a higher molecular weight of PVP, which results in higher water permeability, compared to untreated membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Additive; Microporous and porous membranes; Morphology; Membrane characterization; Hypochlorite treatment
1. Introduction Since Loeb and Souriajan first introduced phase inversion method [1], much investigation has been made for understanding the mechanism of formation of asymmetric membranes. Phase inversion is the most extensively used technique for the preparation of asymmetric membranes, which is that cast solution film on a substrate is immersed and is precipitated in water bath. During the process, the solvent in the casting solution film is exchanged with nonsolvent and phase separation occurs in the film. It usually turns out to be a characteristic morphology of asymmetric membrane showing a dense top layer and porous sub layer [2]. A so-called asymmetric membrane have been widely used for gas separation and liquid separation, because the thin top layer plays a role of selective barrier film, and the porous
∗
Corresponding author. Fax: +82-2-958-6869. E-mail address: [email protected] (B. Jung).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.011
sub layer, in which includes macrovoids, pores and micropores, offers good mechanical strength. It is well known that the formation of symmetric membrane depends on kinetic parameters such as exchange rate between solvent and nonsolvent, kinetics of phase separation, as well as on thermodynamic parameters such as phase diagrams, polymer–solvent interactions, solvent–nonsolvent interactions and interfacial stability. Thus, the material selection such as polymers, solvents, and nonsolvents is very important for the fabrication of asymmetric membranes, according to its applications [3]. Polyacrylonitrile (PAN) is one of popular membrane materials for water treatment, because of sufficiently chemical stability, hydrophilicity and good solubility to common solvents. Due to the highly hydrophilic properties among other membrane materials such as polysulfone, polyethersulfone, polyethylene, polypropylene, it has known as low fouling membranes for aqueous filtration and has already commercialized [4]. Compared to other polymer materials, PAN has also good resistance against chlorine, and cleaning agents such as sodium hypochlorite, sodium hydroxide. Neverthe-
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less, the applications of PAN membrane have been sometimes hindered by the brittleness and pore collapse when it is dried [5]. It has been well known that the morphology control can be made by adding small amount of additives. The addition of ionic salts such as LiCl, ZnCl2 and organic acid such as acetic acid, propionic acid causes macrovoid formation. For example, the addition of LiCl cause lager macrovoids, and the higher the amount of LiCl is added, the larger macrovoid is formed [6]. It is generally accepted that the macrovoids are suppressed as organic acids are added, because the acids form acid–base complex with basic polar solvents such as NMP, DMF, DMAc [7]. Further to the inorganic low molecular additive, polymer additives are widely used for the structure control of membranes such as polyvinylpyrroidone (PVP), polyethylene glycol (PEG) for the fabrication of ultrafiltration and microfiltration membranes [8]. PVP, which is known as a nontoxic material widely used for biomedical applications, is one of good polymer additives, because PVP is miscible with membrane materials and is quite well soluble in water as well as solvents. In fact, it has been widely studied that the effect of PVP addition to polymer solution on the formation of phase inversion membrane is accounted for promotion of micropores. During the phase inversion process, it is assumed that the hydrophilic additive, PVP is dissolved out by water and the sites where PVP exist become micropores. Besides the formation of micropores, it has been generally accepted that the porosity increases and the macrovoid formation disappears as adding PVP to the casting solution [9]. For instance, Xu et al. studied the effect of PVP for different molecular weight on morphology of polyetherimide hollow fiber membrane. They found that the higher molecular weight of PVP was added, the bigger pore was made [10]. On the other hands, D. Wang et al. also found similar result that the decrease in the concentration of PVP results in lower water flux and higher rejection [11]. Nevertheless, it is still uncertain how the addition of PVP makes an effect not only on the morphology, but also on the water flux. In addition, it has not comprehensibly manifested how much amount of PVP is washed out during the membrane formation, because there is still small amount of PVP exist in the pore wall as well as the membrane matrix [12]. The substantial amounts of PVP in asymmetric membranes are effectively removed by sodium hypochlorite treatment. The mechanism of the selective removal of PVP is known as follows. Oxidative reaction of PVP with sodium hypochlorite causes ring opening of the pyrrolidone ring of the PVP molecule. Because the high molecular weight of PVP is decomposed as lower molecular weight polymers which can be washed out of the membrane matrix easily. A detailed experiment was performed by Roesink [19] and Wienk et al. [20]. Their experiments gave more clarity on the mechanism of the reaction of PVP with sodium hypochlorite in hydrophobic polymeric membranes such as the ultrafiltration membranes of poly(ethersulfone) (PES) and the microfiltration membranes of polyetherimide (PEI). Very recently,
Qin and coworkers [21,22] reported that there was an optimum hypochlorite concentration around 500 mg/L for the treatment to achieve a high flux PAN membrane, and showed that the hypochlorite treatment causes a larger surface pore and a higher water flux as well as a lower retention. In this paper, the asymmetric membranes are prepared using polymer solutions composed of the PAN and PVP and solvent, DMSO by classical phase inversion method. By adding the different molecular weight of PVP to the solutions, the effect of molecular weight of PVP on morphological and transport properties of the asymmetric membranes such as water flux and rejections will be investigated and discussed. Quantitative analyses are to be performed, using FT-IR, in order to measure the residual amount of PVP entrapped in the final membranes. Lastly, it will be investigated how the characteristics of water permeation changes when the residual PVP is selectively and partially decomposed through the hypochlorite treatment.
2. Experimental 2.1. Materials Acrylonitrile (AN) was purchased from Aldrich. AN was purified by passing through column (Aldrich) that was packed with inhibitor-remover for removing MEHQ. Ammonium persulfate using initiator was received from Aldrich, and then used without further purification. Deionized water purified with MilliQ system (Millipore) was used throughout experiment. dimethylsulfoxide (DMSO) from Baker was used without purification. 2.2. Synthesis and characterization of PAN Polyacrylonitrile (PAN) was synthesized by suspension polymerization. Deionized water (1500 ml) was poured into a three-neck round bottom flask. The reaction vessel was purged with nitrogen for an hour to remove all of dissolved oxygen, and then purified AN (84.8 g) was added to the reaction vessel under nitrogen atmosphere. An initiator solution of ammonium persulfate (APS) (2.2 g) dissolved in deionized water (20 ml) was dropped to the reaction vessel with mild stirring (200 rpm). The polymerization was allowed to proceed for 24 h before the reaction was terminated. The reaction temperature was kept at 55 ◦ C. The precipitated PAN was filtered and purified by washing with deionized water. For further purification of the synthesized PAN, re-precipitation was performed in methanol and then was dried in a vacuum oven at 60 ◦ C. 1 H NMR (Varian 200 MHz) was used for the characterization of synthesized polymers. The calibration to determine the molecular weight of the synthesized polymers was performed, using monodisperse polystyrene (PS) standards (Shodex standard S-series, Showa Denko). The eluted carrier solvent was DMF (Mallinckrodt, HPLC grade) with
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LiCl (0.05 mol/L, Aldrich) and flow rate was 1 mL/min. The molecular weights (Mw ) obtained by GPC were corrected by considering the difference of hydrodynamic volumes in DMF between PS and PAN. Molecular weight of the synthesized PAN characterized by GPC was 180,000 g/mol and polydisperse index was 1.62.
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2.5. Hypochlorite treatment of membranes Aqueous NaOCl (5000 ppm) solution was prepared in 100 ml beaker. The membranes were immersed in the NaOCl solution for 2 h and then were placed in deionized distilled water for 5 h.
2.3. Preparation and filtration of asymmetric membrane 3. Results and discussion The ultrafiltration membranes were prepared by phase inversion method in water as a coagulant. The various compositions of PAN/PVP were dissolved in DMSO. The membrane was prepared from 12 wt.% of PAN solutions with three different molecular weight of PVP (10K, 46K and 360K). The three PVPs were added in 12 wt.% of PAN solution. The dope solution was poured onto a glass plate, and was spread with a Doctor’s blade to be 250 m thick. The glass plate was immediately immersed in water bath at room temperature. The obtained membranes were kept in water bath for overnight to coagulate, and then the membranes were washed with deionized water before filtration experiment. The membranes were cut into disks with diameter of 43 mm for setup in a filtration cell (Amicon Co. Ltd., Type 8050). Pure water flux experiments were performed by applying pressure from 0.5 to 2 kg/cm2 . Rejection was determined by a standard dextran (Mw = 160K, Sigma). Using low concentration feed of dextran (0.5 wt.%), the solute rejection experiments were performed under a low pressure of 0.5 kg/cm2 . The permeated samples were collected for few minutes and the concentration of permeates were determined, using refractive index detector (RI 2000-F, Younglin). Molecular weight cut-off (MWCO) was determined by solute rejections of dextrans (Mw = 103–106 g/mol, Sigma–Aldrich). Using low concentration feed of dextran (1 wt.%), the solute rejection experiments were performed under a low pressure of 5 kPa/cm2 . The permeated samples were collected for further GPC analysis. Fractional rejection and MWCO were determined [2]. 2.4. Scanning electron microscope and FT-IR The morphology of membranes was observed with scanning electron microscope (S-2500C, Noran Instruments, Inc., USA). Samples were freeze–dried under vacuum before fracturing under cryogentic condition using nitrogen. Quantitative analysis of PAN and PVP were performed by FT-IR (DruSampl-IR-II, SensIR Technologies). The quantitative calibration of PVP and PAN was carried out, using PAN/PVP blend films prepared with different compositions, because each functional group has different oscillation strength. By evaluating the peak areas of carbonyl (–CO) and nitrile (–CN) groups in blend films, the master curve of concentration of PVP and PAN was obtained. The residual amount of PAN and PVP were calculated from the master curve.
3.1. The effect of molecular weight of PVP on morphology Fig. 1 shows SEM photographs of cross-sectional images of PAN membranes. The membranes were prepared from 12 wt.% of PAN solutions by adding three different molecular weight of PVP (10K, 46K and 360K), respectively. It can be seen that the membranes have an asymmetrical structure consisting of a dense top layer and a porous sub layer that was occupied by cellular morphologies enclosed in the polymer matrix, as well as finger-like macrovoids. As can be seen in Fig. 1, it is commonly found that the top layers are thicker as more PVP is added, and the number of macrovoids gradually disappears, regardless of the molecular weight of additive. When the same amount of PVP is added, the top layer is thicker as higher molecular weight of PVP is added. The morphological changes on the addition of lower molecular PVP (10K) are rather unperceivable, but when higher molecular weights of PVP (46K and 360K) are added, the changes of morphology are quite noticeable. In particular, it is quite obvious that the morphology changes added with higher molecular weight (360K) of PVP (Fig. 1(c)). The top layers grow even over the half of overall thickness of membrane. In fact, intensive studies on the effect of water-soluble additives to a dope solution on the morphology of asymmetric membranes have been performed [13]. The effect of PVP on the formation of membranes was in-depth investigated by Boom et al. [14] and on the performance of ultrafiltration membranes by Lafrenière et al. [15]. Furthermore, Marchese et al. suggested that when PVP is entrapped by membrane materials, PVP should increase hydrophilicity, and results in important change in the performance of ultrafiltration membrane such as solute rejection and fouling [16]. As for ionic PAN membranes reported by Kobayashi et al., the PAN dope solutions containing more hydrophilic polymers, the kinetics of phase separation was retarded and the top layer was thicker [17]. A similar result has shown by Jung that hydrophilic PAN is added to PAN solution, the hydrophilicity and the thickness of top layer increases [18]. In spite of arguments, it has been known that the addition of PVP in the casting solution could control the morphology of asymmetric membranes and the water permeability of the membrane for similar pore size and distribution increases due to the residual PVP in pore or to the dissolution of PVP during the formation and the washing process [13].
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For a quantitative analysis, the thicknesses of top layers were measured. As shown in Fig. 2, the growth of top layer simply increases as PVP is added. It should be noted that the top-layer thicknesses is not varied at low concentration of PVP, but the distinct changes of the top layer thickness at high concentration of PVP are
found. Compared to the same of amount of PVP (8 wt.%), the molecular weight effect on the growth of top layer is quite noticeable. The top layer made with molecular weight 360K of PVP is almost is four times as thick as the PAN membrane made with molecular weight 10K of PVP.
Fig. 1. SEM photographs of membranes made from 12 wt.% of PAN solutions containing: (a) PVP (Mw : 10K), (b) PVP (Mw : 46K), (c) PVP (Mw : 360K). The concentration of PVP added is noted on the top of each photograph.
B. Jung et al. / Journal of Membrane Science 243 (2004) 45–57
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Fig. 1. (Continued )
It has been known that the formation of skin and macrovoid are influenced by the molecular weight of additives, because the low molecular weight of PVP is highly soluble than higher molecular weight. It can be said that the low molecular weight of PVP can be easily washed out quickly with the solvent during the formation of membrane as well as cleaning process of the membrane slowly. Thus, it seems that the thickness of top layers and the finger-like
macrovoids are in relation to the solubility of the additive, PVP. It is very important to know how much of PVP remains in the resulting membranes during the membrane formation in terms of molecular weight of PVP. In order to assess the amount of residual PVP in the membrane, IR spectra analyses were performed because PAN and PVP have characteristic functional groups of nitrile (CN) stretching band (2240 cm−1 ) and carbonyl (CO) band (1640 cm−1 )
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Fig. 1. (Continued ).
which are caused by the added PVP, respectively. As can be seen in Fig. 3, the residual amount of PVP (360K) is able to characterize with the two distinct bands. As the amount of PVP added in the polymer solution increases, the peak area of carbonyl band (1640 cm−1 ) also increases in the resulting membranes due to the undissolved PVP during the formation of membrane. For instance, when 8 wt.% of
PVP is added, the high molecular weight of PVP (360K) contains 26 mol% of PVP in all solid content, and then remain 12.8 mol% of PVP in the resultant membrane. It can be estimated that about 50% of PVP is washed out during the membrane formation. For the moderate molecular weight of PVP (46K), initially the polymer solution film contains 74 mol% of PVP and then 1.53 mol% of PVP is
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Fig. 2. The thickness of top layer as a function of the concentration of PVP.
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Fig. 4. Pure water flux changes with the concentration of added different molecular weight of PVP.
membrane formation. In particular for the high molecular weight of PVP (360K) shows that relatively higher amount of PVP are located at the membrane and it could also imagine that the remaining PVP takes a room in the pores and PAN matrix. The amounts of residual PVP for the different concentration and the different molecular weight of PVPs were listed in Table 1. 3.2. The water flux and molecular weight cut-off (MWCO)
Fig. 3. FT-IR spectra of PAN membrane prepared by polymer solutions containing different concentration of PVP (Mw : 360K).
left in the final membrane. In other words, 98% of PVP was dissolved into the nosolvent bath through the membrane formation and the washing. Such a washing-out effect is very noticeable for the low molecular weight of PVP (10K). Initially, 93 mol% of PVP in the polymer solution leaves only 0.33 mol% of PVP in the membrane. About 99.6 mol% of the low molecular weight of PVP (10K) was washed out. Thus, it can be said that as the molecular weight of the additive increases, the PVP would rather reside in the resulting membranes than washes out of the membrane during the
It is important to see how the morphology, such as the thicknesses of top layers and macrovoids, makes effect on the water fluxes of the membranes. The thickness of top layer is of important in membrane filtration, because it has been widely accepted that the flux decreases as the thickness of top layer increases since the water flux is strongly dependent upon the top layer resistance as long as the sub layer is composed of the fingers and open and cross-connected pores. Besides the morphology, it would be expected that the residual PVP in the membranes plays a crucial role in the permeation of water. Thus, it is very important to know how the different molecular weights of residual PVP affect the permeability of water, since there has still been argued that the entrapped PVP offers hydrophilic to hydrophobic membranes and reduces the water flux due to the swelling of the pore-filled PVP [20]. Fig. 4 shows the water flux changes with the concentration of additive and the different molecular weights of PVP. The
Table 1 Water flux, the residual amount of PVP, MWCO for the membranes prepared with different molecular weight of PVP Concentration of added PVP (wt.%) 0 1 2 3 5 8
PVP Mw : 10K
PVP Mw : 46K
PVP Mw : 360K
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
205 234 301 392 443 444
160 139 137 114 262 418
– 0.067 0.183 0.232 0.285 0.33
205 266 273 221 218 180
160 172 142 90 70 65
– 0.70 1.01 1.08 1.29 1.53
205 139 89 72 79 44
160 150 60 31 24 4.2
– 6.61 9.98 10.1 12.6 12.8
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Table 2 Water flux, the residual amount of PVP, rejection for hypochlorite treatment with different molecular weight of PVP Concentration of PVP added (wt.%)
PVP Mw : 10K Water flux (kg/(m2 h))
(%)
Residual PVP (mol%)
Water flux (kg/(m2 h))
(%)
Residual PVP (mol%)
Water flux (kg/(m2 h))
Rejectiona (%)
Residual PVP (mol%)
0 1 2 3 5 8
205 439 423 537 512 540
83 43 21 22 23 25
– 0.077 0.081 0.098 0.116 0.141
205 433 558 479 546 508
83 46 50 48 42 43
– 0.61 0.69 0.89 0.99 1.00
205 347 354 370 336 352
83 62 57 54 48 52
– 5.56 8.38 9.41 11.2 11.4
a
Rejectiona
PVP Mw : 46K
PVP Mw : 360K
Rejectiona
Rejections were measured using dextran Mw : 160K.
pure water flux of the membrane made with 12 wt.% of the additive-free PAN solution exhibits 205 kg/(m2 h), whereas the flux of the membrane prepared with the three different molecular weight of PVP (10K, 46K and 360K) show different permeation fluxes. At first, for a lower molecular weight of PVP (10K), the water flux gradually increase and reach a plateau after the added concentration of PVP is 5 wt.%. The pure water flux is twice as much as that of the membrane made with 12 wt.% of the additive-free PAN solution. Although the skin thickness increases with the addition of PVP, the water flux increases. For the moderate molecular weight of PVP (46K), a hump of flux change with the concentration of PVP is found. Before 2 wt.% of PVP added the water flux increase, but after this points the water flux declines. Even for 8 wt.% of PVP added the pure water flux is slight lower than that of the additive-free PAN membrane. Lastly, it would be interesting that when the high molecular weight of PVP (360K) is added the water flux gradually decreases with the concentration of PVP. Even when the high molecular weight of additive are dissolved up to 8 wt.% of PVP concentration, the water flux is four times as low as the water flux of the additive-free PAN membrane. The sizes of pore (MWCO) for the membranes were measured by fractional rejection experiment with monodisperse dextrans as a permeation standard. By analyzing the distribution of chromatograms of GPC, the values of MWCO for the membranes were evaluated. The water flux and MWCO are also summarized in Table 1.
without changing the structure of pores [12]. On the other hands, Xu et al. reported that the hypochlorite treatment causes to lower the solute rejection as well as to enlarge the size of pores in the blend membrane of polyetherimide and PVP [10]. Further, Qin et al. found that an optimum concentration and condition of the hypochlorite treatment for the blend membranes of poysulfone and PVP as well as the blend of cellulose acetate and PVP [21,22] (Table 2). In order to test the selective bleaching of PVP with 5000 ppm of NaOCl, the treatment was performed with time. As can be seen Fig. 5, the removals of PVP were monitored by IR spectrum (1640 cm−1 ) with the treatment time. As time goes, it can be observed that the intensity carbonyl band of IR reduces, due to the selective decomposition of PVP by NaOCl without attacking the nitrile group (2240 cm−1 ) of PAN. As shown inset denotes in Fig. 5, the quantitative analyses of the selective bleach of PVP with time were calculated with the two areas of IR bands. Within a few hours, the most of PVP was decomposed and reached a plateau. In fact, the treatment of hypochlorite rate is dependent upon the concentration of NaOCl and the molecular weight of PVP. The comparison of IR spectra between before and after the hypochlorite treatments are illustrated in Fig. 6. For a same amount of PVP (5 wt.%)
3.3. Characterization of residual PVP after hypochlorite treatments It is very essential to understand how the permeation characteristics changes when the residual PVP is effectively removed. For the effective removal of PVP, hypochlorite treatments were performed for 2 h, the decompositions of PVP were estimated using FT-IR. It has known that the reaction occurs between hypochlorite and pyrrolidone ring of PVP, and caused chain scission of PVP molecules. After all, the bleached PVP in the membrane matrix dissolve out. According to Wienk et al., the effect of treatment of hypochlorite on the blend membrane of polyethersulfone and PVP blend membranes is that the swelling of PVP can be removed
Fig. 5. FT-IR spectra of PAN membrane made with PVP 360K after the selective bleaching with time for 5 wt.% of PVP (Mw : 360K) added. The inset plot denotes the residual concentration of PVP (Mw : 360K) with time.
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Fig. 6. FT-IR spectra change of PAN membranes before and after hypochlorite treatment for different molecular weight of PVP added. The solid and the dotted line denote before and after the hypochlorite treatment, respectively.
added, it can be seen that peak area of carbonyl (CO) band (1640 cm−1 ) increases with molecular weight of PVP without much variation of the peak area of nitrile (2240 cm−1 ). It can be also seen that the peak area of carbonyl (CO) band (1640 cm−1 ) are reduced for the three different molecular weights of PVP added after the hypochlorite treatments. Due to the partial decomposition of PVP within 2-h treatment and the cleaning process in 24 h, the PVP are still remains in the membranes in form of shorter chains, which could affect the permeation properties. Using the areas of the two bands in IR spectrum, the residual amounts of PVP in the resulting membranes before and after hypochlorite treatments were quantitatively calculated. As shown in Fig. 7, the residual amount of PVP increase as the addition of PVP increase, and as the higher molecular weight of PVP are added the more amount of PVP are resided. Interesting results can be found after the hypochlo-
Fig. 7. Comparison of the residual amount of PVP in resulting membranes, before and after hypochlorite treatment. The filled and the unfilled symbols denote before and after the hypochlorite treatment with arrows and the symbols denote: (䊐) PVP (Mw : 10K), (䊊) PVP (Mw : 46K) and () PVP (Mw : 360K), respectively.
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Fig. 8. Pure water flux changes with the concentration of added different molecular weight of PVP after the hypochlorite treatment. The inset plot denotes the increment of water flux compared with untreated membranes.
rite treatment. Although it is not obvious to see the effect of hypochlorite treatment in the low molecular weight of PVP (10K) system, because only small fraction of PVP initially exists and the decomposed PVP becomes quite shorter ones so that can be easily dissolved in water. It is perceivable that hypochlorite treatment is more effect to high molecular weight of PVP. The low and moderate molecular weight of PVP (10K and 46K) were removed by 0.2, 0.53 mol% of residual PVP, respectively, but the high molecular weight of PVP (360K) is eliminated by 1.4 mol% during the hypochlorite treatments. 3.4. Effect of the hypochlorite treatments on permeation property and morphology To investigate the effect of the hypochlorite treatment on water flux, water permeation test was performed. Fig. 8
Fig. 9. The change of rejection with the concentrations added different molecular weight of PVP, before and after the hypochlorite treatment. The filled and the unfilled symbols denote before and after the hypochlorite treatment with arrows and the symbols denote: (䊐) PVP (Mw : 10K), (䊊) PVP (Mw : 46K) and () PVP (Mw : 360K), respectively.
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shows the pure water flux changes after the hypochlorite treatment, and the amount of flux enhancement is also illustrated in the inset plot as a function of PVP added. As can be seen in Fig. 8, the water flux by the hypochlorite treatment is quite noticeable. The water flux of all the membranes
increases after the hypochlorite treatment. It should be noted that the flux of the membranes made by two low molecular weights (10K and 46K) shows a similar water flux. Compared with untreated membranes (see Fig. 4), the flux change as a function of the PVP concentration shows differ-
Fig. 10. SEM photographs of membrane surfaces made with: (a) PVP (Mw : 360K) before and after the hypochlorite treatment and (b) the different molecular weight of PVPs (5 wt.%) (figure full scale: 1 m × 1 m).
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Fig. 10. (Continued ).
ent trends. In particular, it seems that the flux of the 360K of PVP membranes is almost constant (350 kg/(m2 h)) regardless of the PVP concentrations and is still lower than the membrane made by the two low molecular weights (10K and 46K). The water flux enhancement by the hypochlorite treatment is illustrated in the inset of Fig. 8. The compari-
son between before and after the hypochlorite treatment is made by means of the ratio of the water flux before treatment to the water flux after treatment. For the 10K PVP systems, the flux change is very small, and the flux enhancement becomes double for 46K systems. For the 360K systems, the flux gradually increases and the enhancement is
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over seven times higher than the flux of untreated membrane when 8 wt.% of PVP is added, because higher amount of PVP was bleached out. The solute rejections were estimated and the comparison of rejection between before and after hypochlorite treatment has made. As can be seen in Fig. 9, for a lower molecular weight of PVP (10K) the rejection gradually decreases near 2 wt.% of PVP added membrane and reach a plateau where the rejection is 20. For the molecular weight of PVP (46K), a downhill of rejection with PVP concentration is found. Before 2 wt.% of PVP added, the rejection decreases, but after this points the water flux gradually increase. When 8 wt.% of PVP is added the rejection is almost identical to that of the additive-free PAN membrane. For the rejection of higher molecular weight of PVP used, the values of rejection slowly increase with the added amount. After the hypochlorite treatment, the rejection of low molecular weight of PVP (10K) is not varied, but the rejection of the rest of two systems (46K and 360K) decreases. It seems that the hypochlorite process affects the size and numbers of pores although PVP is partially decomposed. One may question why 10K PVP shown in Fig. 9, rejection decreases up to 2 wt.% of PVP added and then it becomes constant, but the flux shown in Fig. 8 also increases up to 2 wt.% of PVP added and the changes between before and after hypochlorite have hardly found, which are different from higher molecular weight of PVP (360K). It can be explained as follows. At first, it can imagine that two types of PVP are located in the membrane, i.e. inside pores and in the matrix (PAN). The hypochlorite can attack easier to pore-side PVP rather than to those in the matrix. The oxidation reaction to decompose PVP by hypochlorite partially takes place within a given time. For the higher molecular weight of PVP systems, the pores in the membranes were plugged by the long chain of PVP which causes to block and to resist the passage of water and permeates. After hypochlorite treatment, the relatively high concentration of PVP still resides in the pores, but the long chain were decomposed into shorter chains as well as much of them were washed out. Thus, the pore-blocking effect would be reduced by the hypochlorite treatment. However, for the small molecular weight of PVP systems a small fraction of PVP exists, because much of them had washed out during the membrane formation, compared to high molecular weight system. Thus, the effect of hypochlorite treatment such as chain break, which causes higher solubility to water, is not noticeable, because the chain is too short and less to block the pores or to resist the passage of the permeate like dextran. It is very important whether a new morphological change happens after hypochlorite treatment for different molecular weight and different residual amount of PVP, the high resolution SEM pictures were taken. Fig. 10(a) shows the surface structure of membranes made with molecular weight 360K of PVP, before and after hypochlorite treatment. Before the hypochlorite treatment, a fused nodular structure is well de-
veloped. It is obvious that the size of the nodules decreases as more PVP is added. The argument of nodular structure formation is still under debate, but as suggested by Smolders et al. that the spherical structure is presumably related to the clustering of entanglement of polymer molecules during spinodal decomposition [24]. It seems that the added PVP possibly governs the mechanism of the spinodal decomposition and suppress the nodular formation. After the hypochlorite treatment, it appears that the fused nodules disappear, and the surface is much smoother than untreated membrane, because the fused nodules made of PAN and PVP together were selectively and partially decomposed by NaOCl. In addition, it is hardly to find the phenomena such as the new formation of pores or the enlargement of pores as specifically demonstrated by Qin and coworkers [21–23], because the full decomposition of PVP were not performed here. The morphology changes through the hypochlorite treatment can be expected when the membranes were treated in a higher concentration of NaOCl and a longer treatment time. Thus, it seems that the entrapped PVP is fully swollen in pores and hinders permeation of water and dextran particles, but the permeation can be enhanced by the partial decomposition of PVP after the hypochlorite treatment without variation of pores, because the carbonyl band of IR can be still observed. The decomposition of PVP by NaOCl depends on the concentration of residual PVP as well as their molecular weight. As can be seen Fig. 10(b), due to the low residual concentration and low molecular weight of PVP (10K and 46K), the morphological change by hypochlorite treatment are not found. However, the treatment of hypochlorite for the high residual concentration and high molecular weight of PVP (360K) are so effective to enhance the water flux.
4. Conclusions The effect of the addition of different molecular weight of PVP to polyacrylonitrile solution on the fundamental characteristics of membrane such as morphology, water flux, and rejection were investigated. It is found that the top layers are thicker as more PVP is added, and the number of macrovoids like fingers gradually disappears. When the same amount of PVP is added, the thickness of top layer increase as higher molecular weight of PVP is added. The water flux depends on the structure of membrane as well as on the pore-filled PVP. It was also found that as the higher molecular weight of PVP and the more amount of PVP were added, the more amount of PVP were remained. After hypochlorite treatment, the selective removal of residual PVP in the PAN membranes was more effective, when higher molecular weight of PVP was filled in pores. It should be noted that due the water flux was enhanced, to the effective removal of PVP without changing the micropores. Due to the decomposition of PVP by NaOCl, the membrane surface is smoother.
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Acknowledgements Authors thank the financial support under the program of Green Korea 21 by Korea Institute of Science and Technology.
References [1] G.S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser. 38 (1963) 117. [2] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1991. [3] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, B.J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1. [4] E.R. Cornelissen, T. van den Boomgaard, H. Strathmann, Physicochemical aspects of polymer selection for ultrafiltration and microfiltration membranes, Colloids Surf. A: Physicochem. Eng. Aspects 138 (1998) 283. [5] N. Scharnagl, H. Buschatz, Polyacrylonitrile (PAN) membranes for ultra- and microfiltration, Desalination 139 (2001) 191. [6] H.J. Lee, J. Won, H. Lee, Y.S. Kang, Solution properties of poly(amic acid)—NMP containing LiCl and their effects on membrane morphologies, J. Membr. Sci. 196 (2002) 267. [7] M.J. Han, Effect of propionic acid in the casting solution on the characteristics of phase inversion polysulfone membranes, Desalination 121 (1999) 31. [8] J.H. Kim, K.H. Lee, Effect of PEG additive on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153. [9] I.M. Wienk, R.M. Boom, M.A.M. Beerlage, A.M.W. Bulte, C.A. Smolders, H. Strathmann, Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers, J. Membr. Sci. 113 (1996) 361. [10] Z.L. Xu, T.S. Chung, Y. Huang, Effect of polyvinylpyrrolidone molecular weights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes, J. Appl. Polym. Sci. 74 (1999) 2220.
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[11] D. Wang, K. Li, W.K. Teo, Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes, J. Membr. Sci. 163 (1999) 211. [12] I. Wienk, Ultrafiltration Membranes from a Polymer Blend, Ph.D. Thesis, University of Twente, 1993. [13] 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. [14] R.M. Boom, I.M. Wienk, T. van den Boomgaard, C.A. Smolders, Microstructures in phase inversion membranes. Part 2. The role of a polymeric additive, J. Membr. Sci. 73 (1992) 277. [15] L.Y. Lafrenière, F.D.F. Talbot, T. Matsuura, S. Sourirajan, Effect of polyvinylpyrrolidone additive on the performance of polyethersulfone ultrafiltration membranes, Ind. Eng. Chem. Res. 26 (1987) 2385. [16] J. Marchese, M. Ponce, N.A. Ochoa, P. Prádanos, L. Palacio, A. Hernández, Fouling behaviour of polyethersulfone UF membranes made with different PVP, J. Membr. Sci. 211 (2003) 1. [17] T. Kobayashi, T. Miyamoto, T. Nagai, N. Fujii, Polyacrylonitrile ultrafiltration membranes containing negatively charged groups for permeation and separation of dextran and dextransulfate, J. Appl. Polym. Sci. 52 (1994) 1519. [18] B. Jung, Preparation of hydrophilic polyacrylonitrile blend membranes for ultrafiltration, J. Membr. Sci. 229 (2004) 129. [19] H.D.W. Roesink, Microfiltration membrane development and module design, Ph.D. Thesis, University of Twente, 1989. [20] I.M. Wienk, E.E.B. Meuleman, Z. Borneman, Th. van den Boomgaard, C.A. Smolders, Chemical treatment of membranes of a polymer blend: Mechanism of the reaction of hypochlorite with poly(vinyl pyrrolidone), J. Polym. Sci. Part A: Polym. Chem. 33 (1995) 49. [21] J.-J. Qin, Y.-M. Cao, Y.-Q. Li, Y. Li, M.-H. Oo, H. Lee, Hollow fiber ultrafiltration membranes made from blends of PAN and PVP, Sep. Purif. Technol., in press. [22] J.-J. Qin, F.-S. Wong, Hypochlorite treatment of hydrophilic hollow fiber ultrafiltration membranes for high fluxes, Desalination 146 (2002) 307. [23] J.-J. Qin, Y. Li, L.-S. Lee, H. Lee, Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360K/NMP/water, J. Membr. Sci. 218 (2003) 173. [24] C.A. Smolders, A.J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phase-inversion membranes. Part 1. Formation of macrovoids, J. Membr. Sci. 73 (1992) 259.
Effect of molecular weight of polymeric additives on formation, permeation properties and hypochlorite treatment of asymmetric polyacrylonitrile membranes Bumsuk Jung a,∗ , Joon Ki Yoon a,b , Bokyung Kim a , Hee-Woo Rhee b a
Polymer Hybrids Research Centers, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea b Department of Chemical and Biomolecular Engineering, Sogang University, Shinsu-Dong 1, Mapo-Ku, Seoul 121-742, South Korea Received 5 February 2004; accepted 2 June 2004 Available online 17 August 2004
Abstract Asymmetric polyacrylonitrile membranes were prepared via phase inversion process from casting solution composed of synthesized polyacrylonitrile (PAN), dimethylsulfoxide (DMSO), and poly(vinylpyrrolidone) (PVP) of different molecular weights. The effect of molecular weight of PVP in the casting solution on the morphology, water flux was investigated. The effect of hypochlorite treatment on PVP which was entrapped in pore was investigated. As the higher molecular weight of PVP is added, the thicker top layer of the asymmetric membrane is. The top layer of the membrane and the suppression of macrovoid formation strongly depend on the molecular weight of PVP. The addition with different molecular weight of PVPs also changes pure water permeability. It is also found that the substantial amount of PVP is entrapped in the pores, the water flux decreases. The effect of pore-filling depends on the molecular weight of the additive. Using sodium hypochlorite solution, the entrapped PVP was able to be partially removed. The effectiveness of the selective bleach is pronounced for a higher molecular weight of PVP, which results in higher water permeability, compared to untreated membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Additive; Microporous and porous membranes; Morphology; Membrane characterization; Hypochlorite treatment
1. Introduction Since Loeb and Souriajan first introduced phase inversion method [1], much investigation has been made for understanding the mechanism of formation of asymmetric membranes. Phase inversion is the most extensively used technique for the preparation of asymmetric membranes, which is that cast solution film on a substrate is immersed and is precipitated in water bath. During the process, the solvent in the casting solution film is exchanged with nonsolvent and phase separation occurs in the film. It usually turns out to be a characteristic morphology of asymmetric membrane showing a dense top layer and porous sub layer [2]. A so-called asymmetric membrane have been widely used for gas separation and liquid separation, because the thin top layer plays a role of selective barrier film, and the porous
∗
Corresponding author. Fax: +82-2-958-6869. E-mail address: [email protected] (B. Jung).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.011
sub layer, in which includes macrovoids, pores and micropores, offers good mechanical strength. It is well known that the formation of symmetric membrane depends on kinetic parameters such as exchange rate between solvent and nonsolvent, kinetics of phase separation, as well as on thermodynamic parameters such as phase diagrams, polymer–solvent interactions, solvent–nonsolvent interactions and interfacial stability. Thus, the material selection such as polymers, solvents, and nonsolvents is very important for the fabrication of asymmetric membranes, according to its applications [3]. Polyacrylonitrile (PAN) is one of popular membrane materials for water treatment, because of sufficiently chemical stability, hydrophilicity and good solubility to common solvents. Due to the highly hydrophilic properties among other membrane materials such as polysulfone, polyethersulfone, polyethylene, polypropylene, it has known as low fouling membranes for aqueous filtration and has already commercialized [4]. Compared to other polymer materials, PAN has also good resistance against chlorine, and cleaning agents such as sodium hypochlorite, sodium hydroxide. Neverthe-
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less, the applications of PAN membrane have been sometimes hindered by the brittleness and pore collapse when it is dried [5]. It has been well known that the morphology control can be made by adding small amount of additives. The addition of ionic salts such as LiCl, ZnCl2 and organic acid such as acetic acid, propionic acid causes macrovoid formation. For example, the addition of LiCl cause lager macrovoids, and the higher the amount of LiCl is added, the larger macrovoid is formed [6]. It is generally accepted that the macrovoids are suppressed as organic acids are added, because the acids form acid–base complex with basic polar solvents such as NMP, DMF, DMAc [7]. Further to the inorganic low molecular additive, polymer additives are widely used for the structure control of membranes such as polyvinylpyrroidone (PVP), polyethylene glycol (PEG) for the fabrication of ultrafiltration and microfiltration membranes [8]. PVP, which is known as a nontoxic material widely used for biomedical applications, is one of good polymer additives, because PVP is miscible with membrane materials and is quite well soluble in water as well as solvents. In fact, it has been widely studied that the effect of PVP addition to polymer solution on the formation of phase inversion membrane is accounted for promotion of micropores. During the phase inversion process, it is assumed that the hydrophilic additive, PVP is dissolved out by water and the sites where PVP exist become micropores. Besides the formation of micropores, it has been generally accepted that the porosity increases and the macrovoid formation disappears as adding PVP to the casting solution [9]. For instance, Xu et al. studied the effect of PVP for different molecular weight on morphology of polyetherimide hollow fiber membrane. They found that the higher molecular weight of PVP was added, the bigger pore was made [10]. On the other hands, D. Wang et al. also found similar result that the decrease in the concentration of PVP results in lower water flux and higher rejection [11]. Nevertheless, it is still uncertain how the addition of PVP makes an effect not only on the morphology, but also on the water flux. In addition, it has not comprehensibly manifested how much amount of PVP is washed out during the membrane formation, because there is still small amount of PVP exist in the pore wall as well as the membrane matrix [12]. The substantial amounts of PVP in asymmetric membranes are effectively removed by sodium hypochlorite treatment. The mechanism of the selective removal of PVP is known as follows. Oxidative reaction of PVP with sodium hypochlorite causes ring opening of the pyrrolidone ring of the PVP molecule. Because the high molecular weight of PVP is decomposed as lower molecular weight polymers which can be washed out of the membrane matrix easily. A detailed experiment was performed by Roesink [19] and Wienk et al. [20]. Their experiments gave more clarity on the mechanism of the reaction of PVP with sodium hypochlorite in hydrophobic polymeric membranes such as the ultrafiltration membranes of poly(ethersulfone) (PES) and the microfiltration membranes of polyetherimide (PEI). Very recently,
Qin and coworkers [21,22] reported that there was an optimum hypochlorite concentration around 500 mg/L for the treatment to achieve a high flux PAN membrane, and showed that the hypochlorite treatment causes a larger surface pore and a higher water flux as well as a lower retention. In this paper, the asymmetric membranes are prepared using polymer solutions composed of the PAN and PVP and solvent, DMSO by classical phase inversion method. By adding the different molecular weight of PVP to the solutions, the effect of molecular weight of PVP on morphological and transport properties of the asymmetric membranes such as water flux and rejections will be investigated and discussed. Quantitative analyses are to be performed, using FT-IR, in order to measure the residual amount of PVP entrapped in the final membranes. Lastly, it will be investigated how the characteristics of water permeation changes when the residual PVP is selectively and partially decomposed through the hypochlorite treatment.
2. Experimental 2.1. Materials Acrylonitrile (AN) was purchased from Aldrich. AN was purified by passing through column (Aldrich) that was packed with inhibitor-remover for removing MEHQ. Ammonium persulfate using initiator was received from Aldrich, and then used without further purification. Deionized water purified with MilliQ system (Millipore) was used throughout experiment. dimethylsulfoxide (DMSO) from Baker was used without purification. 2.2. Synthesis and characterization of PAN Polyacrylonitrile (PAN) was synthesized by suspension polymerization. Deionized water (1500 ml) was poured into a three-neck round bottom flask. The reaction vessel was purged with nitrogen for an hour to remove all of dissolved oxygen, and then purified AN (84.8 g) was added to the reaction vessel under nitrogen atmosphere. An initiator solution of ammonium persulfate (APS) (2.2 g) dissolved in deionized water (20 ml) was dropped to the reaction vessel with mild stirring (200 rpm). The polymerization was allowed to proceed for 24 h before the reaction was terminated. The reaction temperature was kept at 55 ◦ C. The precipitated PAN was filtered and purified by washing with deionized water. For further purification of the synthesized PAN, re-precipitation was performed in methanol and then was dried in a vacuum oven at 60 ◦ C. 1 H NMR (Varian 200 MHz) was used for the characterization of synthesized polymers. The calibration to determine the molecular weight of the synthesized polymers was performed, using monodisperse polystyrene (PS) standards (Shodex standard S-series, Showa Denko). The eluted carrier solvent was DMF (Mallinckrodt, HPLC grade) with
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LiCl (0.05 mol/L, Aldrich) and flow rate was 1 mL/min. The molecular weights (Mw ) obtained by GPC were corrected by considering the difference of hydrodynamic volumes in DMF between PS and PAN. Molecular weight of the synthesized PAN characterized by GPC was 180,000 g/mol and polydisperse index was 1.62.
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2.5. Hypochlorite treatment of membranes Aqueous NaOCl (5000 ppm) solution was prepared in 100 ml beaker. The membranes were immersed in the NaOCl solution for 2 h and then were placed in deionized distilled water for 5 h.
2.3. Preparation and filtration of asymmetric membrane 3. Results and discussion The ultrafiltration membranes were prepared by phase inversion method in water as a coagulant. The various compositions of PAN/PVP were dissolved in DMSO. The membrane was prepared from 12 wt.% of PAN solutions with three different molecular weight of PVP (10K, 46K and 360K). The three PVPs were added in 12 wt.% of PAN solution. The dope solution was poured onto a glass plate, and was spread with a Doctor’s blade to be 250 m thick. The glass plate was immediately immersed in water bath at room temperature. The obtained membranes were kept in water bath for overnight to coagulate, and then the membranes were washed with deionized water before filtration experiment. The membranes were cut into disks with diameter of 43 mm for setup in a filtration cell (Amicon Co. Ltd., Type 8050). Pure water flux experiments were performed by applying pressure from 0.5 to 2 kg/cm2 . Rejection was determined by a standard dextran (Mw = 160K, Sigma). Using low concentration feed of dextran (0.5 wt.%), the solute rejection experiments were performed under a low pressure of 0.5 kg/cm2 . The permeated samples were collected for few minutes and the concentration of permeates were determined, using refractive index detector (RI 2000-F, Younglin). Molecular weight cut-off (MWCO) was determined by solute rejections of dextrans (Mw = 103–106 g/mol, Sigma–Aldrich). Using low concentration feed of dextran (1 wt.%), the solute rejection experiments were performed under a low pressure of 5 kPa/cm2 . The permeated samples were collected for further GPC analysis. Fractional rejection and MWCO were determined [2]. 2.4. Scanning electron microscope and FT-IR The morphology of membranes was observed with scanning electron microscope (S-2500C, Noran Instruments, Inc., USA). Samples were freeze–dried under vacuum before fracturing under cryogentic condition using nitrogen. Quantitative analysis of PAN and PVP were performed by FT-IR (DruSampl-IR-II, SensIR Technologies). The quantitative calibration of PVP and PAN was carried out, using PAN/PVP blend films prepared with different compositions, because each functional group has different oscillation strength. By evaluating the peak areas of carbonyl (–CO) and nitrile (–CN) groups in blend films, the master curve of concentration of PVP and PAN was obtained. The residual amount of PAN and PVP were calculated from the master curve.
3.1. The effect of molecular weight of PVP on morphology Fig. 1 shows SEM photographs of cross-sectional images of PAN membranes. The membranes were prepared from 12 wt.% of PAN solutions by adding three different molecular weight of PVP (10K, 46K and 360K), respectively. It can be seen that the membranes have an asymmetrical structure consisting of a dense top layer and a porous sub layer that was occupied by cellular morphologies enclosed in the polymer matrix, as well as finger-like macrovoids. As can be seen in Fig. 1, it is commonly found that the top layers are thicker as more PVP is added, and the number of macrovoids gradually disappears, regardless of the molecular weight of additive. When the same amount of PVP is added, the top layer is thicker as higher molecular weight of PVP is added. The morphological changes on the addition of lower molecular PVP (10K) are rather unperceivable, but when higher molecular weights of PVP (46K and 360K) are added, the changes of morphology are quite noticeable. In particular, it is quite obvious that the morphology changes added with higher molecular weight (360K) of PVP (Fig. 1(c)). The top layers grow even over the half of overall thickness of membrane. In fact, intensive studies on the effect of water-soluble additives to a dope solution on the morphology of asymmetric membranes have been performed [13]. The effect of PVP on the formation of membranes was in-depth investigated by Boom et al. [14] and on the performance of ultrafiltration membranes by Lafrenière et al. [15]. Furthermore, Marchese et al. suggested that when PVP is entrapped by membrane materials, PVP should increase hydrophilicity, and results in important change in the performance of ultrafiltration membrane such as solute rejection and fouling [16]. As for ionic PAN membranes reported by Kobayashi et al., the PAN dope solutions containing more hydrophilic polymers, the kinetics of phase separation was retarded and the top layer was thicker [17]. A similar result has shown by Jung that hydrophilic PAN is added to PAN solution, the hydrophilicity and the thickness of top layer increases [18]. In spite of arguments, it has been known that the addition of PVP in the casting solution could control the morphology of asymmetric membranes and the water permeability of the membrane for similar pore size and distribution increases due to the residual PVP in pore or to the dissolution of PVP during the formation and the washing process [13].
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For a quantitative analysis, the thicknesses of top layers were measured. As shown in Fig. 2, the growth of top layer simply increases as PVP is added. It should be noted that the top-layer thicknesses is not varied at low concentration of PVP, but the distinct changes of the top layer thickness at high concentration of PVP are
found. Compared to the same of amount of PVP (8 wt.%), the molecular weight effect on the growth of top layer is quite noticeable. The top layer made with molecular weight 360K of PVP is almost is four times as thick as the PAN membrane made with molecular weight 10K of PVP.
Fig. 1. SEM photographs of membranes made from 12 wt.% of PAN solutions containing: (a) PVP (Mw : 10K), (b) PVP (Mw : 46K), (c) PVP (Mw : 360K). The concentration of PVP added is noted on the top of each photograph.
B. Jung et al. / Journal of Membrane Science 243 (2004) 45–57
49
Fig. 1. (Continued )
It has been known that the formation of skin and macrovoid are influenced by the molecular weight of additives, because the low molecular weight of PVP is highly soluble than higher molecular weight. It can be said that the low molecular weight of PVP can be easily washed out quickly with the solvent during the formation of membrane as well as cleaning process of the membrane slowly. Thus, it seems that the thickness of top layers and the finger-like
macrovoids are in relation to the solubility of the additive, PVP. It is very important to know how much of PVP remains in the resulting membranes during the membrane formation in terms of molecular weight of PVP. In order to assess the amount of residual PVP in the membrane, IR spectra analyses were performed because PAN and PVP have characteristic functional groups of nitrile (CN) stretching band (2240 cm−1 ) and carbonyl (CO) band (1640 cm−1 )
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Fig. 1. (Continued ).
which are caused by the added PVP, respectively. As can be seen in Fig. 3, the residual amount of PVP (360K) is able to characterize with the two distinct bands. As the amount of PVP added in the polymer solution increases, the peak area of carbonyl band (1640 cm−1 ) also increases in the resulting membranes due to the undissolved PVP during the formation of membrane. For instance, when 8 wt.% of
PVP is added, the high molecular weight of PVP (360K) contains 26 mol% of PVP in all solid content, and then remain 12.8 mol% of PVP in the resultant membrane. It can be estimated that about 50% of PVP is washed out during the membrane formation. For the moderate molecular weight of PVP (46K), initially the polymer solution film contains 74 mol% of PVP and then 1.53 mol% of PVP is
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Fig. 2. The thickness of top layer as a function of the concentration of PVP.
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Fig. 4. Pure water flux changes with the concentration of added different molecular weight of PVP.
membrane formation. In particular for the high molecular weight of PVP (360K) shows that relatively higher amount of PVP are located at the membrane and it could also imagine that the remaining PVP takes a room in the pores and PAN matrix. The amounts of residual PVP for the different concentration and the different molecular weight of PVPs were listed in Table 1. 3.2. The water flux and molecular weight cut-off (MWCO)
Fig. 3. FT-IR spectra of PAN membrane prepared by polymer solutions containing different concentration of PVP (Mw : 360K).
left in the final membrane. In other words, 98% of PVP was dissolved into the nosolvent bath through the membrane formation and the washing. Such a washing-out effect is very noticeable for the low molecular weight of PVP (10K). Initially, 93 mol% of PVP in the polymer solution leaves only 0.33 mol% of PVP in the membrane. About 99.6 mol% of the low molecular weight of PVP (10K) was washed out. Thus, it can be said that as the molecular weight of the additive increases, the PVP would rather reside in the resulting membranes than washes out of the membrane during the
It is important to see how the morphology, such as the thicknesses of top layers and macrovoids, makes effect on the water fluxes of the membranes. The thickness of top layer is of important in membrane filtration, because it has been widely accepted that the flux decreases as the thickness of top layer increases since the water flux is strongly dependent upon the top layer resistance as long as the sub layer is composed of the fingers and open and cross-connected pores. Besides the morphology, it would be expected that the residual PVP in the membranes plays a crucial role in the permeation of water. Thus, it is very important to know how the different molecular weights of residual PVP affect the permeability of water, since there has still been argued that the entrapped PVP offers hydrophilic to hydrophobic membranes and reduces the water flux due to the swelling of the pore-filled PVP [20]. Fig. 4 shows the water flux changes with the concentration of additive and the different molecular weights of PVP. The
Table 1 Water flux, the residual amount of PVP, MWCO for the membranes prepared with different molecular weight of PVP Concentration of added PVP (wt.%) 0 1 2 3 5 8
PVP Mw : 10K
PVP Mw : 46K
PVP Mw : 360K
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
Water flux (kg/(m2 h))
MWCO (Da)
Residual PVP (mol%)
205 234 301 392 443 444
160 139 137 114 262 418
– 0.067 0.183 0.232 0.285 0.33
205 266 273 221 218 180
160 172 142 90 70 65
– 0.70 1.01 1.08 1.29 1.53
205 139 89 72 79 44
160 150 60 31 24 4.2
– 6.61 9.98 10.1 12.6 12.8
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Table 2 Water flux, the residual amount of PVP, rejection for hypochlorite treatment with different molecular weight of PVP Concentration of PVP added (wt.%)
PVP Mw : 10K Water flux (kg/(m2 h))
(%)
Residual PVP (mol%)
Water flux (kg/(m2 h))
(%)
Residual PVP (mol%)
Water flux (kg/(m2 h))
Rejectiona (%)
Residual PVP (mol%)
0 1 2 3 5 8
205 439 423 537 512 540
83 43 21 22 23 25
– 0.077 0.081 0.098 0.116 0.141
205 433 558 479 546 508
83 46 50 48 42 43
– 0.61 0.69 0.89 0.99 1.00
205 347 354 370 336 352
83 62 57 54 48 52
– 5.56 8.38 9.41 11.2 11.4
a
Rejectiona
PVP Mw : 46K
PVP Mw : 360K
Rejectiona
Rejections were measured using dextran Mw : 160K.
pure water flux of the membrane made with 12 wt.% of the additive-free PAN solution exhibits 205 kg/(m2 h), whereas the flux of the membrane prepared with the three different molecular weight of PVP (10K, 46K and 360K) show different permeation fluxes. At first, for a lower molecular weight of PVP (10K), the water flux gradually increase and reach a plateau after the added concentration of PVP is 5 wt.%. The pure water flux is twice as much as that of the membrane made with 12 wt.% of the additive-free PAN solution. Although the skin thickness increases with the addition of PVP, the water flux increases. For the moderate molecular weight of PVP (46K), a hump of flux change with the concentration of PVP is found. Before 2 wt.% of PVP added the water flux increase, but after this points the water flux declines. Even for 8 wt.% of PVP added the pure water flux is slight lower than that of the additive-free PAN membrane. Lastly, it would be interesting that when the high molecular weight of PVP (360K) is added the water flux gradually decreases with the concentration of PVP. Even when the high molecular weight of additive are dissolved up to 8 wt.% of PVP concentration, the water flux is four times as low as the water flux of the additive-free PAN membrane. The sizes of pore (MWCO) for the membranes were measured by fractional rejection experiment with monodisperse dextrans as a permeation standard. By analyzing the distribution of chromatograms of GPC, the values of MWCO for the membranes were evaluated. The water flux and MWCO are also summarized in Table 1.
without changing the structure of pores [12]. On the other hands, Xu et al. reported that the hypochlorite treatment causes to lower the solute rejection as well as to enlarge the size of pores in the blend membrane of polyetherimide and PVP [10]. Further, Qin et al. found that an optimum concentration and condition of the hypochlorite treatment for the blend membranes of poysulfone and PVP as well as the blend of cellulose acetate and PVP [21,22] (Table 2). In order to test the selective bleaching of PVP with 5000 ppm of NaOCl, the treatment was performed with time. As can be seen Fig. 5, the removals of PVP were monitored by IR spectrum (1640 cm−1 ) with the treatment time. As time goes, it can be observed that the intensity carbonyl band of IR reduces, due to the selective decomposition of PVP by NaOCl without attacking the nitrile group (2240 cm−1 ) of PAN. As shown inset denotes in Fig. 5, the quantitative analyses of the selective bleach of PVP with time were calculated with the two areas of IR bands. Within a few hours, the most of PVP was decomposed and reached a plateau. In fact, the treatment of hypochlorite rate is dependent upon the concentration of NaOCl and the molecular weight of PVP. The comparison of IR spectra between before and after the hypochlorite treatments are illustrated in Fig. 6. For a same amount of PVP (5 wt.%)
3.3. Characterization of residual PVP after hypochlorite treatments It is very essential to understand how the permeation characteristics changes when the residual PVP is effectively removed. For the effective removal of PVP, hypochlorite treatments were performed for 2 h, the decompositions of PVP were estimated using FT-IR. It has known that the reaction occurs between hypochlorite and pyrrolidone ring of PVP, and caused chain scission of PVP molecules. After all, the bleached PVP in the membrane matrix dissolve out. According to Wienk et al., the effect of treatment of hypochlorite on the blend membrane of polyethersulfone and PVP blend membranes is that the swelling of PVP can be removed
Fig. 5. FT-IR spectra of PAN membrane made with PVP 360K after the selective bleaching with time for 5 wt.% of PVP (Mw : 360K) added. The inset plot denotes the residual concentration of PVP (Mw : 360K) with time.
B. Jung et al. / Journal of Membrane Science 243 (2004) 45–57
Fig. 6. FT-IR spectra change of PAN membranes before and after hypochlorite treatment for different molecular weight of PVP added. The solid and the dotted line denote before and after the hypochlorite treatment, respectively.
added, it can be seen that peak area of carbonyl (CO) band (1640 cm−1 ) increases with molecular weight of PVP without much variation of the peak area of nitrile (2240 cm−1 ). It can be also seen that the peak area of carbonyl (CO) band (1640 cm−1 ) are reduced for the three different molecular weights of PVP added after the hypochlorite treatments. Due to the partial decomposition of PVP within 2-h treatment and the cleaning process in 24 h, the PVP are still remains in the membranes in form of shorter chains, which could affect the permeation properties. Using the areas of the two bands in IR spectrum, the residual amounts of PVP in the resulting membranes before and after hypochlorite treatments were quantitatively calculated. As shown in Fig. 7, the residual amount of PVP increase as the addition of PVP increase, and as the higher molecular weight of PVP are added the more amount of PVP are resided. Interesting results can be found after the hypochlo-
Fig. 7. Comparison of the residual amount of PVP in resulting membranes, before and after hypochlorite treatment. The filled and the unfilled symbols denote before and after the hypochlorite treatment with arrows and the symbols denote: (䊐) PVP (Mw : 10K), (䊊) PVP (Mw : 46K) and () PVP (Mw : 360K), respectively.
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Fig. 8. Pure water flux changes with the concentration of added different molecular weight of PVP after the hypochlorite treatment. The inset plot denotes the increment of water flux compared with untreated membranes.
rite treatment. Although it is not obvious to see the effect of hypochlorite treatment in the low molecular weight of PVP (10K) system, because only small fraction of PVP initially exists and the decomposed PVP becomes quite shorter ones so that can be easily dissolved in water. It is perceivable that hypochlorite treatment is more effect to high molecular weight of PVP. The low and moderate molecular weight of PVP (10K and 46K) were removed by 0.2, 0.53 mol% of residual PVP, respectively, but the high molecular weight of PVP (360K) is eliminated by 1.4 mol% during the hypochlorite treatments. 3.4. Effect of the hypochlorite treatments on permeation property and morphology To investigate the effect of the hypochlorite treatment on water flux, water permeation test was performed. Fig. 8
Fig. 9. The change of rejection with the concentrations added different molecular weight of PVP, before and after the hypochlorite treatment. The filled and the unfilled symbols denote before and after the hypochlorite treatment with arrows and the symbols denote: (䊐) PVP (Mw : 10K), (䊊) PVP (Mw : 46K) and () PVP (Mw : 360K), respectively.
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shows the pure water flux changes after the hypochlorite treatment, and the amount of flux enhancement is also illustrated in the inset plot as a function of PVP added. As can be seen in Fig. 8, the water flux by the hypochlorite treatment is quite noticeable. The water flux of all the membranes
increases after the hypochlorite treatment. It should be noted that the flux of the membranes made by two low molecular weights (10K and 46K) shows a similar water flux. Compared with untreated membranes (see Fig. 4), the flux change as a function of the PVP concentration shows differ-
Fig. 10. SEM photographs of membrane surfaces made with: (a) PVP (Mw : 360K) before and after the hypochlorite treatment and (b) the different molecular weight of PVPs (5 wt.%) (figure full scale: 1 m × 1 m).
B. Jung et al. / Journal of Membrane Science 243 (2004) 45–57
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Fig. 10. (Continued ).
ent trends. In particular, it seems that the flux of the 360K of PVP membranes is almost constant (350 kg/(m2 h)) regardless of the PVP concentrations and is still lower than the membrane made by the two low molecular weights (10K and 46K). The water flux enhancement by the hypochlorite treatment is illustrated in the inset of Fig. 8. The compari-
son between before and after the hypochlorite treatment is made by means of the ratio of the water flux before treatment to the water flux after treatment. For the 10K PVP systems, the flux change is very small, and the flux enhancement becomes double for 46K systems. For the 360K systems, the flux gradually increases and the enhancement is
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B. Jung et al. / Journal of Membrane Science 243 (2004) 45–57
over seven times higher than the flux of untreated membrane when 8 wt.% of PVP is added, because higher amount of PVP was bleached out. The solute rejections were estimated and the comparison of rejection between before and after hypochlorite treatment has made. As can be seen in Fig. 9, for a lower molecular weight of PVP (10K) the rejection gradually decreases near 2 wt.% of PVP added membrane and reach a plateau where the rejection is 20. For the molecular weight of PVP (46K), a downhill of rejection with PVP concentration is found. Before 2 wt.% of PVP added, the rejection decreases, but after this points the water flux gradually increase. When 8 wt.% of PVP is added the rejection is almost identical to that of the additive-free PAN membrane. For the rejection of higher molecular weight of PVP used, the values of rejection slowly increase with the added amount. After the hypochlorite treatment, the rejection of low molecular weight of PVP (10K) is not varied, but the rejection of the rest of two systems (46K and 360K) decreases. It seems that the hypochlorite process affects the size and numbers of pores although PVP is partially decomposed. One may question why 10K PVP shown in Fig. 9, rejection decreases up to 2 wt.% of PVP added and then it becomes constant, but the flux shown in Fig. 8 also increases up to 2 wt.% of PVP added and the changes between before and after hypochlorite have hardly found, which are different from higher molecular weight of PVP (360K). It can be explained as follows. At first, it can imagine that two types of PVP are located in the membrane, i.e. inside pores and in the matrix (PAN). The hypochlorite can attack easier to pore-side PVP rather than to those in the matrix. The oxidation reaction to decompose PVP by hypochlorite partially takes place within a given time. For the higher molecular weight of PVP systems, the pores in the membranes were plugged by the long chain of PVP which causes to block and to resist the passage of water and permeates. After hypochlorite treatment, the relatively high concentration of PVP still resides in the pores, but the long chain were decomposed into shorter chains as well as much of them were washed out. Thus, the pore-blocking effect would be reduced by the hypochlorite treatment. However, for the small molecular weight of PVP systems a small fraction of PVP exists, because much of them had washed out during the membrane formation, compared to high molecular weight system. Thus, the effect of hypochlorite treatment such as chain break, which causes higher solubility to water, is not noticeable, because the chain is too short and less to block the pores or to resist the passage of the permeate like dextran. It is very important whether a new morphological change happens after hypochlorite treatment for different molecular weight and different residual amount of PVP, the high resolution SEM pictures were taken. Fig. 10(a) shows the surface structure of membranes made with molecular weight 360K of PVP, before and after hypochlorite treatment. Before the hypochlorite treatment, a fused nodular structure is well de-
veloped. It is obvious that the size of the nodules decreases as more PVP is added. The argument of nodular structure formation is still under debate, but as suggested by Smolders et al. that the spherical structure is presumably related to the clustering of entanglement of polymer molecules during spinodal decomposition [24]. It seems that the added PVP possibly governs the mechanism of the spinodal decomposition and suppress the nodular formation. After the hypochlorite treatment, it appears that the fused nodules disappear, and the surface is much smoother than untreated membrane, because the fused nodules made of PAN and PVP together were selectively and partially decomposed by NaOCl. In addition, it is hardly to find the phenomena such as the new formation of pores or the enlargement of pores as specifically demonstrated by Qin and coworkers [21–23], because the full decomposition of PVP were not performed here. The morphology changes through the hypochlorite treatment can be expected when the membranes were treated in a higher concentration of NaOCl and a longer treatment time. Thus, it seems that the entrapped PVP is fully swollen in pores and hinders permeation of water and dextran particles, but the permeation can be enhanced by the partial decomposition of PVP after the hypochlorite treatment without variation of pores, because the carbonyl band of IR can be still observed. The decomposition of PVP by NaOCl depends on the concentration of residual PVP as well as their molecular weight. As can be seen Fig. 10(b), due to the low residual concentration and low molecular weight of PVP (10K and 46K), the morphological change by hypochlorite treatment are not found. However, the treatment of hypochlorite for the high residual concentration and high molecular weight of PVP (360K) are so effective to enhance the water flux.
4. Conclusions The effect of the addition of different molecular weight of PVP to polyacrylonitrile solution on the fundamental characteristics of membrane such as morphology, water flux, and rejection were investigated. It is found that the top layers are thicker as more PVP is added, and the number of macrovoids like fingers gradually disappears. When the same amount of PVP is added, the thickness of top layer increase as higher molecular weight of PVP is added. The water flux depends on the structure of membrane as well as on the pore-filled PVP. It was also found that as the higher molecular weight of PVP and the more amount of PVP were added, the more amount of PVP were remained. After hypochlorite treatment, the selective removal of residual PVP in the PAN membranes was more effective, when higher molecular weight of PVP was filled in pores. It should be noted that due the water flux was enhanced, to the effective removal of PVP without changing the micropores. Due to the decomposition of PVP by NaOCl, the membrane surface is smoother.
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Acknowledgements Authors thank the financial support under the program of Green Korea 21 by Korea Institute of Science and Technology.
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