- Email: [email protected]
A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration
Journal of Membrane Science 320 (2008) 8–12
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration Qiang Zhao, Jin-Wen Qian ∗ , Quan-Fu An, Qian Yang, Peng Zhang Institute of Polymer Science, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 17 December 2007 Received in revised form 10 April 2008 Accepted 11 April 2008 Available online 22 April 2008 Keywords: Polyelectrolyte complex Membrane Pervaporation Isopropanol dehydration
a b s t r a c t Polyelectrolyte complexes (PECs) of sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA) were prepared in dilute hydrochloric acid (HCl) aqueous solution and obtained in its solid form. Element analysis and FT-IR showed that the composition of PECs could effectively be tuned by the concentration of HCl in parent polyelectrolyte solution. The PECs were then dissolved in 0.1 mol/L aqueous NaOH and subsequently cast onto polysulfone ultra-filtration membrane. This composite membrane, which has a unique homogeneous PECs separation layer, was subjected to pervaporation test for the first time and gave a performance of J = 3.0 kg/m2 h, ˛ = 960 for 10 wt% water–isopropanol feed at 75 ◦ C. Meanwhile, performance of the PECs membrane displays good stability and unique dependence on feed temperature. These findings, together with its ultra-high performance, are primarily explained by the structure characteristic of PECs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Pervaporation is an efficient membrane-based technique for separating azeotropic, close-boiling, or aqueous organic mixtures and the past decade have witnessed substantial progress and exciting breakthroughs in both the fundamental and application aspects of pervaporation [1]. Among the applications of pervaporation, dehydration of organics is probably the best known and developed. Materials used for pervaporation dehydration include naturally occurring polymers like chitosan [2], synthetic hydrophilic polymers like poly(vinyl alcohol) [3], inorganic filled polymer hybrids [4,5], polyelectrolyte and its complexes or multi-layers [6–10]. For membranes used in dehydration, a crucial issue is to control their swelling degree in aqueous feed under pervaporation condition [11,12]. Polyelectrolyte complexes (PECs) were inherently ionic cross-linked and highly hydrophilic, making it an ideal membrane candidate for dehydration. However, PECs were generally infusible and insoluble in common solvents and it is notoriously difficult to process them. Obviously, this difficulty in processing solid PECs greatly limited its application in membrane separation field because it is difficult to fabricate homogeneous PECs membranes through traditional solvent method or thermal method. Some effort also paid in how to utilize PECs in pervaporation, Richau et al. [13,14] introduced interfacial reaction method, in which one polyelec-
∗ Corresponding author. Tel.: +86 571 87953780. E-mail address: [email protected] (J.-W. Qian). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.04.040
trolyte solution was firstly cast onto micro-porous substrate and another polyelectrolyte solution was subsequently cast or spin cast onto the surface of the former polyelectrolyte layer. The membrane obtained in this way, which is macroscopically inhomogeneous along its cross-section, was usually termed as two-ply polyelectrolyte complex membrane and found to be good in selectivity due to depressing its swelling degree through ionic cross-linking structure [15]. It is obvious that this interfacial reaction method artfully avoids processing solid PECs. However, there are still some shortcomings need to overcome. First, the ionic cross-linking degree of PECs formed at interface was not controlled and in most cases both polyelectrolyte were fully neutralized. In this case, permeability of the two-ply polyelectrolyte complex membrane to water was reduced and resulted in low permeate flux [16]. Also, the interfacial reaction method targeted at a continuous layer of PECs at the interface while actually PECs at the interface may be not continuous since the formation of PECs is a precipitation process and causes phase separation. In this case, instead of improvement, selectivity of the membrane should be negatively influenced. It can be seen that both these two problems were caused by the inability of fabricating homogeneous PECs membrane with controlled crosslinking degree. So, suppose one can first prepare PECs in solution with controlled ionic cross-linking degree and then dissolve it and made homogeneous membrane, these problems may expect to be overcome. Start from this idea, our aim in this communication is to propose a facile route for fabricating novel homogeneous polyelectrolyte complex membrane (HPECM) and to control its ionic cross-linking
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 1. Protocol of fabricating CMCNa–PDDA PECs and its HPECM.
degree. By doing this, we also expect to improve the pervaporation performance of isopropanol dehydration with this type of HPECM. 2. Experimental 2.1. Materials Sodium carboxymethyl cellulose (CMCNa) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Its viscosity in 0.1 M sodium hydroxide (NaOH) aqueous solution at 30 ◦ C was 625.1 mL/g. Before use, CMCNa was dried to a constant mass in vacuum oven at 60 ◦ C. Poly(diallyldimethylammonium chloride) (PDDA) (Mw = 70,000 g/mol, 20% aqueous solution) was purchased from Aldrich and used without further purification. Polysulfone ultra-filtration membrane was obtained from Development Centre of Water Treatment Technology, Hangzhou, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were analytical reagents. Deionized water with a resistance of 18 M cm was used in all experiments. 2.2. Fabrication of CMCNa–PDDA HPECM CMCNa and PDDA were chosen as starting materials for fabricating their HPECMs and the route was shown in Fig. 1, from which two characteristics can be seen. First, the obtained CMCNa–PDDA PECs have residue unionized carboxyl groups in them and it was exactly these unionized carboxyl groups, which make CMCNa–PDDA PECs soluble in 0.1 M NaOH. Second, compositions of CMCNa–PDDA PECs can easily be controlled by tuning HCl concentration in both component polyelectrolyte solutions. Experimentally, two HCl concentrations, 0.005 and 0.009 M, were chosen in this study and detailed description for synthesizing CMCNa–PDDA PECs was as follows. 400 mL CMCNa and PDDA were first dissolved in 0.005 M HCl with their monomer unit mole concentrations both kept at 0.01 M. The PDDA solution was added into the CMCNa solution drop-wise through a buret at a steady rate under vigorous stir. Turbidity occurred immediately in CMCNa solutions upon the adding of PDDA, indicating the yield of insoluble PECs. This turbidity increased as more PDDA solution was added and a macroscopical phase separation occurred when the endpoint was reached, after which the adding of PDDA solution was stopped. It should be noted here that at the concentration of 0.005 M HCl, this macroscopical phase separation happened at the addition of 148 mL PDDA. At the concentration of 0.009 M HCl, macroscopical phase separation was not as obvious as that of 0.005 M HCl and happened at the addition of 78 mL PDDA. After the stir was stopped, a white loosen precipitate appeared at the bottom of the container. The precipitate was then filtrated and soaked in 200 mL deionized water for 2 h to remove free ions and component polyelectrolyte adhered
9
to PECs, during which deionized water was changed for three times till the conductance of deionized water was unchanged. The precipitate was then dried at 60 ◦ C in vacuum for 24 h till constant mass and stored in nitrogen atmosphere for further use. Further, for fabrication of CMCNa–PDDA HPECM, 0.22 g PECs obtained above was dissolved in 10 mL aqueous NaOH. To avoid the existence free NaOH as much as possible, [NaOH] for PECs casting solution (2.2 wt%) is important. In this study, [NaOH] was determined from equation: [NaOH] = [CMCNa] × (1 − MPDDA :MCMCNa ), where [CMCNa] was mole concentration of CMCNa monomer and was calculated from the mass concentration of PECs and its MPDDA :MCMCNa value. In this way, theoretically there were no free NaOH in the PECs solution. Actually, it was found that pH value of the obtained casting solution was in the range of 8–9, confirming that there were not much free NaOH ions in the casting solution. It also should be noted that the dissolve of PECs is purely due to the ionization of carboxyl groups in them and the ionic interaction between CMCNa and PDDA was not broken. This is because when parent polyelectrolyte CMCNa and PDDA was first dissolved in 0.1 M NaOH and then adding the PDDA solution into CMCNa, PECs precipitate could also be obtained, indicating that cross-linking between CMCNa and PDDA is stable at a 0.1 M NaOH. Also, when CMCNa–PDDA PEC was formed in neutral water, i.e., there were no unionized carboxyl acid groups in the obtained solid PEC, the PEC was found to be insoluble in aqueous NaOH, proving that ionic interaction between CMCNa and PDDA was stable in aqueous NaOH. The concentrated solution of PECs was filtrated and cast on polysulfone ultra-filtration membrane, which was dried at 60 ◦ C for 4 h followed by another 4 h at 60 ◦ C in vacuum to remove any residue solvent. The membranes prepared above have an average overall thickness of about 52 ± 0.5 m and were stored in nitrogen atmosphere before use. PECs prepared in 0.005 M HCl was named as PECs2.6 and PECs prepared in 0.009 M HCl was named as PECs5.5 and their membranes were named as HPECM2.6 or HPECM5.5, respectively. The number of 2.6 and 5.5 here represent monomer mole ratio of CMCNa to PDDA in their solid PECs and the values were obtained from element analysis. 2.3. Pervaporation experiment Pervaporation experiments were conducted on the same equipment as reported previously [17]. Each pervaporation was carried out in a steady state, which means both the operation condition and membrane structure were stable. The downstream pressure was measured by a piezometer and maintained at about 180 Pa by a vacuum pump, the feed temperature was maintained within an accuracy of 0.3 ◦ C by an electric control thermometer, the feed composition was maintained stable by circulation. The permeate was condensed by liquid nitrogen and determined by a 102G gas chromatograph (made in Shanghai, China). Permeate flux (J), separation factor (˛) were calculated according to the following equations: J= ˛=
g S × t PH2 O /PIP FH2 O /FIP
where g is the permeate weight collected in liquid nitrogen traps during the operation time t; S is the membrane area (18.09 cm2 ); where FH2 O and FIP are the weight fractions of water and isopropanol in the feed and PH2 O and PIP are those in permeate, respectively. Finally, recording of pervaporation performance data was done after the stable state of mass transport was reached, which means that the equilibrium swelling of the membrane was reached and its pervaporation performance was stable at a steady operation condition. Also, pervaporation performance at
10
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 3. Effect of water in feed on pervaporation performance at 40 ◦ C, flux (hollow), water concentration in permeate (solid), HPECM2.6 (circle), HPECM5.5 (triangle).
3.2. Pervaporation performance of HPECM Fig. 2. FT-IR spectrum of PECs2.6, PECs5.5, HPECM5.5 and CMCNa, respectively.
the same operation condition was repeated three times and averaged. 3. Results and discussion 3.1. Characterization of CMCNa–PDDA Fig. 2 shows the FT-IR spectrum of PECs2.6, PECs5.5, HPECM5.5 and CMCNa, respectively. Since (C O) in unionized carboxylic group has absorption at 1720 cm−1 and it shift to 1600–1610 cm−1 upon ionization, this shift can be used to follow the state of carboxyl group in CMCNa, PECs and HPECM [18]. In the curve for CMCNa, absorption was observed only in 1610 cm−1 and this is because carboxyl groups in CMCNa were fully ionized. In curves for PECs5.5 and PECs2.6, absorptions revealed at both 1600 and 1720 cm−1 . This is because PECs2.6 and PECs5.5 were prepared in acidic media and the acid concentration is lower than monomer concentration of parent polyelectrolyte. So, there are both ionized carboxyl groups and unionized carboxyl groups in the result PECs, which confirmed the design of our route. It also can be seen that intensity ratio of 1720–1600 cm−1 for PECs5.5 is larger than that for PECs2.6 and this is because more carboxyl groups was protonated in higher acid concentration. In the curve for HPECM5.5, intensity of absorption at 1720 cm−1 was greatly reduced and absorption at 1600 cm−1 became much stronger. This proved the ionization of unionized carboxyl groups in PECs5.5 by 0.1 M NaOH solvent in Fig. 1. To further confirm the composition of PECs, element analysis was operated on Flash EA1112 (Thermofinnigan, Italy) to determine the composition of PECs and the result was given in Table 1. It can be seen that composition of PECs, which was expressed by mole ratio of CMCNa monomer unit to PDDA monomer unit (MCMCNa :MPDDA ), can be effectively tuned by the HCl concentration in both component polyelectrolyte solution. PECs5.5 has larger CMCNa content in it because ionization degree of CMCNa in 0.009 M HCl is lower than that in 0.005 M HCl. Hence, it can be deduced that PECs5.5 has lower ionic cross-linking density than that in PECs2.6.
Fig. 3 shows the effect of water content in feed on pervaporation isopropanol dehydration performance of HPECM2.6 and HPECM5.5 at 40 ◦ C. It can be seen that HPECM5.5 shows higher permeate flux and lower water content in permeate than HPECM2.6. This is because HPECM5.5 has larger number of ionized carboxyl groups and lower ionic cross-linking density than HPECM2.6. It also can be seen from Fig. 3 that flux increases with increasing water content in feed from 5 to 20 wt% and water content in permeate is maintained above 99%. For HPECM5.5, permeate flux is up to more than 2.0 kg/m2 h at 20 wt% water and this high value for flux is probably due to the hydrophilic character caused by the ionized carboxyl group in both HPECMs. The stable and high value of water concentration in permeate is due to the ionic cross-linking structure of both HPECMs. Fig. 4 shows the effect of feed temperature on pervaporation performance of HPECM5.5 and HPECM2.6 for 10 wt% water–isopropanol. It can be seen from Fig. 4 that HPECM5.5 also shows higher flux and lower water content in permeate than HPECM2.6. This phenomenon is also observed in Fig. 3 and the reason for this is believed to be the same. It also can be seen from Fig. 4 that permeate flux increases steady with increasing feed temperature from 40 to 75 ◦ C, during which water content in the permeate was maintained both high and stable. The increasing permeate flux is commonly due to the increasing driving force and this phenomenon was widely observed in pervaporation. However on the other hand, water content in permeate was usually
Table 1 Composition of PECs determined by element analysis PECs
[HCl] (M)
C (wt%)
N (wt%)
H (wt%)
MCMCNa :MPDDA
2.6 5.5
0.005 0.009
49.61 46.85
2.004 1.047
6.89 5.85
2.6:1 5.5:1
Fig. 4. Effect of feed temperature on pervaporation performance for dehydration of 10 wt% water–isopropanol, flux (hollow), water in permeate (solid), HPECM2.6 (circle), HPECM5.5 (triangle).
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 5. Durability of HPECM5.5 at two feed temperatures: flux (hollow), water in permeate (solid), 40 ◦ C (circle), 70 ◦ C (triangle). (HPECM5.5 was immersed in 10 wt% water–isopropanol mixture at 40 ◦ C after its first pervaporation performance was finished, and then its pervaporation performance was periodically tested.)
found to decrease with increasing feed temperature and it was less common that its value was maintained both high and stable with increasing feed temperature. Considering the structure characteristics of HPECMs, the ionic cross-linking in HPECMs is believed to be the reason for this less common phenomenon. Stable water content with increasing feed temperature can also be achieved by chemical cross-linking, which however sacrifices permeate flux. What unique here is that HPECMs give both high water content in permeate and permeate flux, i.e., a permeate flux of 3.0 kg/m2 h and a separation factor of 960 for HPECM5.5 at 75 ◦ C. To the best of our knowledge, normally for common polymeric membranes, permeate flux for isopropanol dehydration is below 1.5 kg/m2 h at the comparable operation condition [19–22]. So, performance of HPECM5.5 shows substantial superiority among the already reported performances of polymeric membranes [1]. 3.3. Durability of HPECM It is well known that relaxation is an intrinsic characteristic of common polymeric materials and may result in the falling of its pervaporation performance, making the durability of a common polymeric membrane in pervaporation questionable. To overcome this problem, many works have been done, most of which are crosslinking by chemical agents [23,24]. Since PECs are inherently ionic cross-linked, HPECMs are expected to show good durability. Fig. 5 shows the durability of pervaporation performance for HPECM5.5 at two feed temperatures. It can be seen from Fig. 5 that permeate flux at both 40 and 70 ◦ C just slightly decrease in 30 days and so does water content in permeate, i.e., the permeate flux decreased 5.5% and water concentration in permeate decreased 1.5% compared with their initial value after one month at 70 ◦ C. This decrease may due to the unavoidable contamination of membrane surface, which may reduce surface hydrophilicity. We consider that the durability of HPECMs is much better compared with other pervaporation membrane materials studied by us though the similar test of durability is rare in literature [9,25]. In addition, reproducibility of pervaporation performance of HPECMs at a fixed operation condition, as shown by the error range of each data point in Figs. 3–5, is very good. 4. Conclusions By synthesizing CMCNa–PDDA PECs in acidic media and dissolving it in alkali solvent, we succeed in fabricating pervaporation membranes with a homogeneous PECs separating layer and con-
11
trolled ionic cross-linking degree for the first time. The composition and ionic cross-linking degree of HPECM could be controlled by tuning the acid concentration in parent polyelectrolyte solution. This membrane, which is inherently different from the traditional “twoply” polyelectrolyte complex membrane prepared by interfacial reaction method, showed excellent pervaporation performance and durability. The cross-linking structure, highly hydrophilic character and homogeneity of PECs membrane are believed to be the reasons for its high pervaporation performance and durability. It also should be noted that this route of fabricating homogeneous PECs membrane may not be limited to CMCNa and PDDA only. Theoretically, any pair of weak poly acid and strong cationic polyelectrolyte could be subjected to this route. Thus, application of polyelectrolyte complex in the field of membrane separation science could be expanded with this facile method. Acknowledgement Financially support from the NNSFC (20574059 and 50633030) is greatly appreciated. References [1] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162–179. [2] Y.L. Liu, C.H. Yu, K.R. Lee, J.Y. Lai, Chitosan poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes, J. Membr. Sci. 287 (2007) 230–236. [3] A.S. Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815–823. [4] M. Sairam, M.B. Patil, R.S. Veerapur, S.A. Patil, T.M. Aminabhavi, Novel dense poly(vinyl alcohol)–TiO2 mixed matrix membranes for pervaporation separation of water–isopropanol mixtures at 30 ◦ C, J. Membr. Sci. 281 (2006) 95–102. [5] Y.L. Liu, C.Y. Hsu, Y.H. Su, J.Y. Lai, Chitosan–silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol–water solutions, Biomacromolecules 6 (2005) 368–373. [6] C.L. Hu, R.L. Guo, B.L.X.C. Ma, H. Wu, Z.Y. Jiang, Development of novel mordenitefilled chitosan–poly(acrylic acid) polyelectrolyte complex membranes for pervaporation dehydration of ethylene glycol aqueous solution, J. Membr. Sci. 293 (2007) 142–150. [7] G.J. Zhang, H.H. Yan, S.L. Ji, Z.Z. Liu, Self-assembly of polyelectrolyte multilayer pervaporation membranes by a dynamic layer-by-layer technique on a hydrolyzed polyacrylonitrile, J. Membr. Sci. 292 (2007) 1–8. [8] M.H. Jochen, L. Wolfgang, L. Dieter, K. Lunkwitz, Pervaporation separation of water/alcohol mixtures using composite membranes based on polyelectrolyte multilayer assemblies, J. Membr. Sci. 184 (2001) 233–243. [9] A. Toutianoush, L. Krasemann, B. Tieke, Polyelectrolyte multilayer membranes for pervaporation separation of alcohol/water mixtures, Colloids Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 881–889. [10] B. Tiekea, F. van Ackern, L. Krasemann, A. Toutianoush, Ultrathin self-assembled polyelectrolyte multilayer membranes, Eur. Phys. J. E 5 (2001) 29–39. [11] T. Uragami, H. Matsugi, T. Miyata, Pervaporation characteristics of organicinorganic hybrid membranes composed of poly(vinyl alcohol-co-acrylic acid) and tetraethoxysilanem for water–ethanol separation, Macromolecules 38 (2005) 8840–9446. [12] S.D. Xiao, X.S. Feng, R.Y.M. Huang, Investigation of sorption properties and pervaporation behaviors under different operating conditions for trimesoyl chloride-crosslinked PVA membranes, J. Membr. Sci. 302 (2007) 36–44. [13] K. Richau, H.H. Schwarz, R. Apostel, D. Paul, Dehydration of organics by pervaporation with polyelectrolyte complex membranes: some considerations concerning the separation mechanism, J. Membr. Sci. 113 (1996) 31–41. ´ s, K. Richau, Surface and permeability properties of [14] H.H. Schwarz, J. Lukaˇ membranes from polyelectrolyte complexes and polyelectrolyte surfactant complexes, J. Membr. Sci. 218 (2003) 1–9. [15] G.Y. Moon, R. Pal, R.Y.M. Huang, Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol, J. Membr. Sci. 156 (1999) 175–227. [16] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 167 (2000) 275–289. [17] P. Zhang, J.W. Qian, Y. Yang, Y.X. Bai, Q.F. An, W.D. Yan, Swelling behavior of palygorskite-polyacrylamide hybrid membrane in xylene mixtures and its pervaporation performance for separating the xylene isomers, J. Membr. Sci. 288 (2007) 280–289.
12
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
[18] J.S. Maciel, D.A. Silva, H.C.B. Paula, R.C.M. Paula, Chitosan carboxymethyl cashew gum polyelectrolyte complex: synthesis and thermal stability, Eur. Polym. J. 41 (2005) 2726–2733. [19] S.K. Choudhari, A.A. Kittur, S.S. Kulkarni, M.Y. Kariduraganavar, Development of novel blocked diisocyanate crosslinked chitosan membranes for pervaporation separation of water–isopropanol mixtures, J. Membr. Sci. 302 (2007) 197–206. [20] S.G. Adoor, L.S. Manjeshwar, B.V.K. Naidu, M. Sairam, T.M. Aminabhavi, Poly(vinyl alcohol)/poly(methyl methacrylate) blend membranes for pervaporation separation of water + isopropanol and water + 1,4-dioxane mixtures, J. Membr. Sci. 280 (2006) 594–602. [21] M. Sairam, B.V.K. Naidu, S.K. Nataraj, B. Sreedhar, T.M. Aminabhavi, Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran, J. Membr. Sci. 283 (2006) 65–73.
[22] S.G. Adoor, M. Sairam, L.S. Manjeshwar, K.V.S.N. Raju, T.M. Aminabhavi, Sodium montmorillonite clay loaded novel mixed matrix membranes of poly(vinyl alcohol) for pervaporation dehydration of aqueous mixtures of isopropanol and 1,4-dioxane, J. Membr. Sci. 285 (2006) 182–195. [23] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Laia, Crosslinked organic–inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol–water mixtures with a long-term stability, J. Membr. Sci. 251 (2005) 233–238. [24] W.Y. Won, X.S. Feng, D. Lawless, Separation of dimethyl carbonate methanol water mixtures by pervaporation using crosslinked chitosan membranes, Sep. Purif. Technol. 31 (2003) 129–140. [25] C.Y. Tu, Y.L. Liu, R.L. Lee, J.Y. Lai, Hydrophilic surface-grafted poly(tetrafluoroethylene) membranes using in pervaporation dehydration processes, J. Membr. Sci. 274 (2006) 47–55.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration Qiang Zhao, Jin-Wen Qian ∗ , Quan-Fu An, Qian Yang, Peng Zhang Institute of Polymer Science, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 17 December 2007 Received in revised form 10 April 2008 Accepted 11 April 2008 Available online 22 April 2008 Keywords: Polyelectrolyte complex Membrane Pervaporation Isopropanol dehydration
a b s t r a c t Polyelectrolyte complexes (PECs) of sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA) were prepared in dilute hydrochloric acid (HCl) aqueous solution and obtained in its solid form. Element analysis and FT-IR showed that the composition of PECs could effectively be tuned by the concentration of HCl in parent polyelectrolyte solution. The PECs were then dissolved in 0.1 mol/L aqueous NaOH and subsequently cast onto polysulfone ultra-filtration membrane. This composite membrane, which has a unique homogeneous PECs separation layer, was subjected to pervaporation test for the first time and gave a performance of J = 3.0 kg/m2 h, ˛ = 960 for 10 wt% water–isopropanol feed at 75 ◦ C. Meanwhile, performance of the PECs membrane displays good stability and unique dependence on feed temperature. These findings, together with its ultra-high performance, are primarily explained by the structure characteristic of PECs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Pervaporation is an efficient membrane-based technique for separating azeotropic, close-boiling, or aqueous organic mixtures and the past decade have witnessed substantial progress and exciting breakthroughs in both the fundamental and application aspects of pervaporation [1]. Among the applications of pervaporation, dehydration of organics is probably the best known and developed. Materials used for pervaporation dehydration include naturally occurring polymers like chitosan [2], synthetic hydrophilic polymers like poly(vinyl alcohol) [3], inorganic filled polymer hybrids [4,5], polyelectrolyte and its complexes or multi-layers [6–10]. For membranes used in dehydration, a crucial issue is to control their swelling degree in aqueous feed under pervaporation condition [11,12]. Polyelectrolyte complexes (PECs) were inherently ionic cross-linked and highly hydrophilic, making it an ideal membrane candidate for dehydration. However, PECs were generally infusible and insoluble in common solvents and it is notoriously difficult to process them. Obviously, this difficulty in processing solid PECs greatly limited its application in membrane separation field because it is difficult to fabricate homogeneous PECs membranes through traditional solvent method or thermal method. Some effort also paid in how to utilize PECs in pervaporation, Richau et al. [13,14] introduced interfacial reaction method, in which one polyelec-
∗ Corresponding author. Tel.: +86 571 87953780. E-mail address: [email protected] (J.-W. Qian). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.04.040
trolyte solution was firstly cast onto micro-porous substrate and another polyelectrolyte solution was subsequently cast or spin cast onto the surface of the former polyelectrolyte layer. The membrane obtained in this way, which is macroscopically inhomogeneous along its cross-section, was usually termed as two-ply polyelectrolyte complex membrane and found to be good in selectivity due to depressing its swelling degree through ionic cross-linking structure [15]. It is obvious that this interfacial reaction method artfully avoids processing solid PECs. However, there are still some shortcomings need to overcome. First, the ionic cross-linking degree of PECs formed at interface was not controlled and in most cases both polyelectrolyte were fully neutralized. In this case, permeability of the two-ply polyelectrolyte complex membrane to water was reduced and resulted in low permeate flux [16]. Also, the interfacial reaction method targeted at a continuous layer of PECs at the interface while actually PECs at the interface may be not continuous since the formation of PECs is a precipitation process and causes phase separation. In this case, instead of improvement, selectivity of the membrane should be negatively influenced. It can be seen that both these two problems were caused by the inability of fabricating homogeneous PECs membrane with controlled crosslinking degree. So, suppose one can first prepare PECs in solution with controlled ionic cross-linking degree and then dissolve it and made homogeneous membrane, these problems may expect to be overcome. Start from this idea, our aim in this communication is to propose a facile route for fabricating novel homogeneous polyelectrolyte complex membrane (HPECM) and to control its ionic cross-linking
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 1. Protocol of fabricating CMCNa–PDDA PECs and its HPECM.
degree. By doing this, we also expect to improve the pervaporation performance of isopropanol dehydration with this type of HPECM. 2. Experimental 2.1. Materials Sodium carboxymethyl cellulose (CMCNa) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Its viscosity in 0.1 M sodium hydroxide (NaOH) aqueous solution at 30 ◦ C was 625.1 mL/g. Before use, CMCNa was dried to a constant mass in vacuum oven at 60 ◦ C. Poly(diallyldimethylammonium chloride) (PDDA) (Mw = 70,000 g/mol, 20% aqueous solution) was purchased from Aldrich and used without further purification. Polysulfone ultra-filtration membrane was obtained from Development Centre of Water Treatment Technology, Hangzhou, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were analytical reagents. Deionized water with a resistance of 18 M cm was used in all experiments. 2.2. Fabrication of CMCNa–PDDA HPECM CMCNa and PDDA were chosen as starting materials for fabricating their HPECMs and the route was shown in Fig. 1, from which two characteristics can be seen. First, the obtained CMCNa–PDDA PECs have residue unionized carboxyl groups in them and it was exactly these unionized carboxyl groups, which make CMCNa–PDDA PECs soluble in 0.1 M NaOH. Second, compositions of CMCNa–PDDA PECs can easily be controlled by tuning HCl concentration in both component polyelectrolyte solutions. Experimentally, two HCl concentrations, 0.005 and 0.009 M, were chosen in this study and detailed description for synthesizing CMCNa–PDDA PECs was as follows. 400 mL CMCNa and PDDA were first dissolved in 0.005 M HCl with their monomer unit mole concentrations both kept at 0.01 M. The PDDA solution was added into the CMCNa solution drop-wise through a buret at a steady rate under vigorous stir. Turbidity occurred immediately in CMCNa solutions upon the adding of PDDA, indicating the yield of insoluble PECs. This turbidity increased as more PDDA solution was added and a macroscopical phase separation occurred when the endpoint was reached, after which the adding of PDDA solution was stopped. It should be noted here that at the concentration of 0.005 M HCl, this macroscopical phase separation happened at the addition of 148 mL PDDA. At the concentration of 0.009 M HCl, macroscopical phase separation was not as obvious as that of 0.005 M HCl and happened at the addition of 78 mL PDDA. After the stir was stopped, a white loosen precipitate appeared at the bottom of the container. The precipitate was then filtrated and soaked in 200 mL deionized water for 2 h to remove free ions and component polyelectrolyte adhered
9
to PECs, during which deionized water was changed for three times till the conductance of deionized water was unchanged. The precipitate was then dried at 60 ◦ C in vacuum for 24 h till constant mass and stored in nitrogen atmosphere for further use. Further, for fabrication of CMCNa–PDDA HPECM, 0.22 g PECs obtained above was dissolved in 10 mL aqueous NaOH. To avoid the existence free NaOH as much as possible, [NaOH] for PECs casting solution (2.2 wt%) is important. In this study, [NaOH] was determined from equation: [NaOH] = [CMCNa] × (1 − MPDDA :MCMCNa ), where [CMCNa] was mole concentration of CMCNa monomer and was calculated from the mass concentration of PECs and its MPDDA :MCMCNa value. In this way, theoretically there were no free NaOH in the PECs solution. Actually, it was found that pH value of the obtained casting solution was in the range of 8–9, confirming that there were not much free NaOH ions in the casting solution. It also should be noted that the dissolve of PECs is purely due to the ionization of carboxyl groups in them and the ionic interaction between CMCNa and PDDA was not broken. This is because when parent polyelectrolyte CMCNa and PDDA was first dissolved in 0.1 M NaOH and then adding the PDDA solution into CMCNa, PECs precipitate could also be obtained, indicating that cross-linking between CMCNa and PDDA is stable at a 0.1 M NaOH. Also, when CMCNa–PDDA PEC was formed in neutral water, i.e., there were no unionized carboxyl acid groups in the obtained solid PEC, the PEC was found to be insoluble in aqueous NaOH, proving that ionic interaction between CMCNa and PDDA was stable in aqueous NaOH. The concentrated solution of PECs was filtrated and cast on polysulfone ultra-filtration membrane, which was dried at 60 ◦ C for 4 h followed by another 4 h at 60 ◦ C in vacuum to remove any residue solvent. The membranes prepared above have an average overall thickness of about 52 ± 0.5 m and were stored in nitrogen atmosphere before use. PECs prepared in 0.005 M HCl was named as PECs2.6 and PECs prepared in 0.009 M HCl was named as PECs5.5 and their membranes were named as HPECM2.6 or HPECM5.5, respectively. The number of 2.6 and 5.5 here represent monomer mole ratio of CMCNa to PDDA in their solid PECs and the values were obtained from element analysis. 2.3. Pervaporation experiment Pervaporation experiments were conducted on the same equipment as reported previously [17]. Each pervaporation was carried out in a steady state, which means both the operation condition and membrane structure were stable. The downstream pressure was measured by a piezometer and maintained at about 180 Pa by a vacuum pump, the feed temperature was maintained within an accuracy of 0.3 ◦ C by an electric control thermometer, the feed composition was maintained stable by circulation. The permeate was condensed by liquid nitrogen and determined by a 102G gas chromatograph (made in Shanghai, China). Permeate flux (J), separation factor (˛) were calculated according to the following equations: J= ˛=
g S × t PH2 O /PIP FH2 O /FIP
where g is the permeate weight collected in liquid nitrogen traps during the operation time t; S is the membrane area (18.09 cm2 ); where FH2 O and FIP are the weight fractions of water and isopropanol in the feed and PH2 O and PIP are those in permeate, respectively. Finally, recording of pervaporation performance data was done after the stable state of mass transport was reached, which means that the equilibrium swelling of the membrane was reached and its pervaporation performance was stable at a steady operation condition. Also, pervaporation performance at
10
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 3. Effect of water in feed on pervaporation performance at 40 ◦ C, flux (hollow), water concentration in permeate (solid), HPECM2.6 (circle), HPECM5.5 (triangle).
3.2. Pervaporation performance of HPECM Fig. 2. FT-IR spectrum of PECs2.6, PECs5.5, HPECM5.5 and CMCNa, respectively.
the same operation condition was repeated three times and averaged. 3. Results and discussion 3.1. Characterization of CMCNa–PDDA Fig. 2 shows the FT-IR spectrum of PECs2.6, PECs5.5, HPECM5.5 and CMCNa, respectively. Since (C O) in unionized carboxylic group has absorption at 1720 cm−1 and it shift to 1600–1610 cm−1 upon ionization, this shift can be used to follow the state of carboxyl group in CMCNa, PECs and HPECM [18]. In the curve for CMCNa, absorption was observed only in 1610 cm−1 and this is because carboxyl groups in CMCNa were fully ionized. In curves for PECs5.5 and PECs2.6, absorptions revealed at both 1600 and 1720 cm−1 . This is because PECs2.6 and PECs5.5 were prepared in acidic media and the acid concentration is lower than monomer concentration of parent polyelectrolyte. So, there are both ionized carboxyl groups and unionized carboxyl groups in the result PECs, which confirmed the design of our route. It also can be seen that intensity ratio of 1720–1600 cm−1 for PECs5.5 is larger than that for PECs2.6 and this is because more carboxyl groups was protonated in higher acid concentration. In the curve for HPECM5.5, intensity of absorption at 1720 cm−1 was greatly reduced and absorption at 1600 cm−1 became much stronger. This proved the ionization of unionized carboxyl groups in PECs5.5 by 0.1 M NaOH solvent in Fig. 1. To further confirm the composition of PECs, element analysis was operated on Flash EA1112 (Thermofinnigan, Italy) to determine the composition of PECs and the result was given in Table 1. It can be seen that composition of PECs, which was expressed by mole ratio of CMCNa monomer unit to PDDA monomer unit (MCMCNa :MPDDA ), can be effectively tuned by the HCl concentration in both component polyelectrolyte solution. PECs5.5 has larger CMCNa content in it because ionization degree of CMCNa in 0.009 M HCl is lower than that in 0.005 M HCl. Hence, it can be deduced that PECs5.5 has lower ionic cross-linking density than that in PECs2.6.
Fig. 3 shows the effect of water content in feed on pervaporation isopropanol dehydration performance of HPECM2.6 and HPECM5.5 at 40 ◦ C. It can be seen that HPECM5.5 shows higher permeate flux and lower water content in permeate than HPECM2.6. This is because HPECM5.5 has larger number of ionized carboxyl groups and lower ionic cross-linking density than HPECM2.6. It also can be seen from Fig. 3 that flux increases with increasing water content in feed from 5 to 20 wt% and water content in permeate is maintained above 99%. For HPECM5.5, permeate flux is up to more than 2.0 kg/m2 h at 20 wt% water and this high value for flux is probably due to the hydrophilic character caused by the ionized carboxyl group in both HPECMs. The stable and high value of water concentration in permeate is due to the ionic cross-linking structure of both HPECMs. Fig. 4 shows the effect of feed temperature on pervaporation performance of HPECM5.5 and HPECM2.6 for 10 wt% water–isopropanol. It can be seen from Fig. 4 that HPECM5.5 also shows higher flux and lower water content in permeate than HPECM2.6. This phenomenon is also observed in Fig. 3 and the reason for this is believed to be the same. It also can be seen from Fig. 4 that permeate flux increases steady with increasing feed temperature from 40 to 75 ◦ C, during which water content in the permeate was maintained both high and stable. The increasing permeate flux is commonly due to the increasing driving force and this phenomenon was widely observed in pervaporation. However on the other hand, water content in permeate was usually
Table 1 Composition of PECs determined by element analysis PECs
[HCl] (M)
C (wt%)
N (wt%)
H (wt%)
MCMCNa :MPDDA
2.6 5.5
0.005 0.009
49.61 46.85
2.004 1.047
6.89 5.85
2.6:1 5.5:1
Fig. 4. Effect of feed temperature on pervaporation performance for dehydration of 10 wt% water–isopropanol, flux (hollow), water in permeate (solid), HPECM2.6 (circle), HPECM5.5 (triangle).
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
Fig. 5. Durability of HPECM5.5 at two feed temperatures: flux (hollow), water in permeate (solid), 40 ◦ C (circle), 70 ◦ C (triangle). (HPECM5.5 was immersed in 10 wt% water–isopropanol mixture at 40 ◦ C after its first pervaporation performance was finished, and then its pervaporation performance was periodically tested.)
found to decrease with increasing feed temperature and it was less common that its value was maintained both high and stable with increasing feed temperature. Considering the structure characteristics of HPECMs, the ionic cross-linking in HPECMs is believed to be the reason for this less common phenomenon. Stable water content with increasing feed temperature can also be achieved by chemical cross-linking, which however sacrifices permeate flux. What unique here is that HPECMs give both high water content in permeate and permeate flux, i.e., a permeate flux of 3.0 kg/m2 h and a separation factor of 960 for HPECM5.5 at 75 ◦ C. To the best of our knowledge, normally for common polymeric membranes, permeate flux for isopropanol dehydration is below 1.5 kg/m2 h at the comparable operation condition [19–22]. So, performance of HPECM5.5 shows substantial superiority among the already reported performances of polymeric membranes [1]. 3.3. Durability of HPECM It is well known that relaxation is an intrinsic characteristic of common polymeric materials and may result in the falling of its pervaporation performance, making the durability of a common polymeric membrane in pervaporation questionable. To overcome this problem, many works have been done, most of which are crosslinking by chemical agents [23,24]. Since PECs are inherently ionic cross-linked, HPECMs are expected to show good durability. Fig. 5 shows the durability of pervaporation performance for HPECM5.5 at two feed temperatures. It can be seen from Fig. 5 that permeate flux at both 40 and 70 ◦ C just slightly decrease in 30 days and so does water content in permeate, i.e., the permeate flux decreased 5.5% and water concentration in permeate decreased 1.5% compared with their initial value after one month at 70 ◦ C. This decrease may due to the unavoidable contamination of membrane surface, which may reduce surface hydrophilicity. We consider that the durability of HPECMs is much better compared with other pervaporation membrane materials studied by us though the similar test of durability is rare in literature [9,25]. In addition, reproducibility of pervaporation performance of HPECMs at a fixed operation condition, as shown by the error range of each data point in Figs. 3–5, is very good. 4. Conclusions By synthesizing CMCNa–PDDA PECs in acidic media and dissolving it in alkali solvent, we succeed in fabricating pervaporation membranes with a homogeneous PECs separating layer and con-
11
trolled ionic cross-linking degree for the first time. The composition and ionic cross-linking degree of HPECM could be controlled by tuning the acid concentration in parent polyelectrolyte solution. This membrane, which is inherently different from the traditional “twoply” polyelectrolyte complex membrane prepared by interfacial reaction method, showed excellent pervaporation performance and durability. The cross-linking structure, highly hydrophilic character and homogeneity of PECs membrane are believed to be the reasons for its high pervaporation performance and durability. It also should be noted that this route of fabricating homogeneous PECs membrane may not be limited to CMCNa and PDDA only. Theoretically, any pair of weak poly acid and strong cationic polyelectrolyte could be subjected to this route. Thus, application of polyelectrolyte complex in the field of membrane separation science could be expanded with this facile method. Acknowledgement Financially support from the NNSFC (20574059 and 50633030) is greatly appreciated. References [1] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162–179. [2] Y.L. Liu, C.H. Yu, K.R. Lee, J.Y. Lai, Chitosan poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes, J. Membr. Sci. 287 (2007) 230–236. [3] A.S. Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815–823. [4] M. Sairam, M.B. Patil, R.S. Veerapur, S.A. Patil, T.M. Aminabhavi, Novel dense poly(vinyl alcohol)–TiO2 mixed matrix membranes for pervaporation separation of water–isopropanol mixtures at 30 ◦ C, J. Membr. Sci. 281 (2006) 95–102. [5] Y.L. Liu, C.Y. Hsu, Y.H. Su, J.Y. Lai, Chitosan–silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol–water solutions, Biomacromolecules 6 (2005) 368–373. [6] C.L. Hu, R.L. Guo, B.L.X.C. Ma, H. Wu, Z.Y. Jiang, Development of novel mordenitefilled chitosan–poly(acrylic acid) polyelectrolyte complex membranes for pervaporation dehydration of ethylene glycol aqueous solution, J. Membr. Sci. 293 (2007) 142–150. [7] G.J. Zhang, H.H. Yan, S.L. Ji, Z.Z. Liu, Self-assembly of polyelectrolyte multilayer pervaporation membranes by a dynamic layer-by-layer technique on a hydrolyzed polyacrylonitrile, J. Membr. Sci. 292 (2007) 1–8. [8] M.H. Jochen, L. Wolfgang, L. Dieter, K. Lunkwitz, Pervaporation separation of water/alcohol mixtures using composite membranes based on polyelectrolyte multilayer assemblies, J. Membr. Sci. 184 (2001) 233–243. [9] A. Toutianoush, L. Krasemann, B. Tieke, Polyelectrolyte multilayer membranes for pervaporation separation of alcohol/water mixtures, Colloids Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 881–889. [10] B. Tiekea, F. van Ackern, L. Krasemann, A. Toutianoush, Ultrathin self-assembled polyelectrolyte multilayer membranes, Eur. Phys. J. E 5 (2001) 29–39. [11] T. Uragami, H. Matsugi, T. Miyata, Pervaporation characteristics of organicinorganic hybrid membranes composed of poly(vinyl alcohol-co-acrylic acid) and tetraethoxysilanem for water–ethanol separation, Macromolecules 38 (2005) 8840–9446. [12] S.D. Xiao, X.S. Feng, R.Y.M. Huang, Investigation of sorption properties and pervaporation behaviors under different operating conditions for trimesoyl chloride-crosslinked PVA membranes, J. Membr. Sci. 302 (2007) 36–44. [13] K. Richau, H.H. Schwarz, R. Apostel, D. Paul, Dehydration of organics by pervaporation with polyelectrolyte complex membranes: some considerations concerning the separation mechanism, J. Membr. Sci. 113 (1996) 31–41. ´ s, K. Richau, Surface and permeability properties of [14] H.H. Schwarz, J. Lukaˇ membranes from polyelectrolyte complexes and polyelectrolyte surfactant complexes, J. Membr. Sci. 218 (2003) 1–9. [15] G.Y. Moon, R. Pal, R.Y.M. Huang, Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol, J. Membr. Sci. 156 (1999) 175–227. [16] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 167 (2000) 275–289. [17] P. Zhang, J.W. Qian, Y. Yang, Y.X. Bai, Q.F. An, W.D. Yan, Swelling behavior of palygorskite-polyacrylamide hybrid membrane in xylene mixtures and its pervaporation performance for separating the xylene isomers, J. Membr. Sci. 288 (2007) 280–289.
12
Q. Zhao et al. / Journal of Membrane Science 320 (2008) 8–12
[18] J.S. Maciel, D.A. Silva, H.C.B. Paula, R.C.M. Paula, Chitosan carboxymethyl cashew gum polyelectrolyte complex: synthesis and thermal stability, Eur. Polym. J. 41 (2005) 2726–2733. [19] S.K. Choudhari, A.A. Kittur, S.S. Kulkarni, M.Y. Kariduraganavar, Development of novel blocked diisocyanate crosslinked chitosan membranes for pervaporation separation of water–isopropanol mixtures, J. Membr. Sci. 302 (2007) 197–206. [20] S.G. Adoor, L.S. Manjeshwar, B.V.K. Naidu, M. Sairam, T.M. Aminabhavi, Poly(vinyl alcohol)/poly(methyl methacrylate) blend membranes for pervaporation separation of water + isopropanol and water + 1,4-dioxane mixtures, J. Membr. Sci. 280 (2006) 594–602. [21] M. Sairam, B.V.K. Naidu, S.K. Nataraj, B. Sreedhar, T.M. Aminabhavi, Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran, J. Membr. Sci. 283 (2006) 65–73.
[22] S.G. Adoor, M. Sairam, L.S. Manjeshwar, K.V.S.N. Raju, T.M. Aminabhavi, Sodium montmorillonite clay loaded novel mixed matrix membranes of poly(vinyl alcohol) for pervaporation dehydration of aqueous mixtures of isopropanol and 1,4-dioxane, J. Membr. Sci. 285 (2006) 182–195. [23] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Laia, Crosslinked organic–inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol–water mixtures with a long-term stability, J. Membr. Sci. 251 (2005) 233–238. [24] W.Y. Won, X.S. Feng, D. Lawless, Separation of dimethyl carbonate methanol water mixtures by pervaporation using crosslinked chitosan membranes, Sep. Purif. Technol. 31 (2003) 129–140. [25] C.Y. Tu, Y.L. Liu, R.L. Lee, J.Y. Lai, Hydrophilic surface-grafted poly(tetrafluoroethylene) membranes using in pervaporation dehydration processes, J. Membr. Sci. 274 (2006) 47–55.