polyelectrolyte complex blend membrane for pervaporation dehydration of isopropanol

polyelectrolyte complex blend membrane for pervaporation dehydration of isopropanol

Journal of Membrane Science 343 (2009) 53–61 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.co...

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Journal of Membrane Science 343 (2009) 53–61

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Poly(vinyl alcohol)/polyelectrolyte complex blend membrane for pervaporation dehydration of isopropanol Qiang Zhao, Jinwen Qian ∗ , Quanfu An, Meihua Zhu, Minjie Yin, Zhiwei Sun Department of Polymer Science and Engineering, 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 9 April 2009 Received in revised form 6 July 2009 Accepted 7 July 2009 Available online 14 July 2009 Keywords: PVA Pervaporation Isopropanol dehydration Blend membrane Polyelectrolyte complex

a b s t r a c t Poly(vinyl alcohol) (PVA) was blended with soluble polyelectrolyte complex (PEC) made from poly(diallyldimethylammonium chloride) (PDDA) and sodium carboxymethyl cellulose (CMCNa). Crystallinity, thermal transition, and thermal stability of the PVA/PEC blends were characterized by using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and thermal gravity analysis (TGA), respectively. Surface morphology, cross-section and phase structure of the blend membranes were examined by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Surface hydrophilicity and swelling behavior of the blend membranes were examined by water contact angle (CA) and swelling tests. Blend membranes were subjected to isopropanol dehydration, and effects of blend composition, feed composition and feed temperature on pervaporation performance are discussed in terms of phase structures of blend membranes. A performance of J = 1.35 kg/m2 h, ˛ = 1002, was obtained for blend membrane containing 50 wt% PEC in dehydrating 10 wt% water–isopropanol at 70 ◦ C. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Membrane technologies are gaining increasing interest from both the scientific and applied points of view [1], and membrane separation is one of the most studied membrane technologies [2]. Gas separation [3,4], ultra-filtration [5–8], nano-filtration [9], reverse osmosis [10], membrane distillation [11], and pervaporation [12–14] constitute the main scope of membrane separations. Membrane materials are of crucial importance for successful membrane separation technologies. Poly(vinyl alcohol) (PVA) is one of the most studied membrane materials due to its outstanding membrane forming ability, easy processing and abundant availability [15]. PVA membranes have been utilized in enzyme immobilization [16,17], gas separation [18], fuel cells [19] and pervaporation dehydration of organics [20–29]. However, due to the semi-crystalline character of PVA, permeation flux of PVA membranes in pervaporation dehydration is not satisfied. Strategies such as polymer blending [30,31] and organic–inorganic hybridization [32–37] were adopted to overcome this problem. Jiang and co-workers [38–40] designed a series of graphite

∗ Corresponding author. Tel.: +86 0571 87953780. E-mail address: [email protected] (J.W. Qian). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.07.009

filled PVA membranes and utilized them in separating benzene/cyclohexane mixtures and dehydrating organics. Huang and co-workers [41] blended chitosan with PVA and the membranes showed improved permeation flux. Aminabhavi and co-workers [42–45] blended poly(methyl methacrylate), sodium alginate and polyaniline with PVA and the selectivity of the blend membranes was improved as compared with pristine PVA membranes. These studies revealed useful illuminations on improving the pervaporation performance of PVA membranes. However, it is still quite necessary to further improve the permeation flux of PVA membranes. Recently, we reported a new method for fabricating novel homogeneous polyelectrolyte complex (PEC) membranes, whose permeation flux in organics dehydration was very promising [46–49]. These PEC membranes are composed of needle-shaped polyelectrolyte complex aggregates (PEC aggregates) and there are hydroxyl groups on these PEC aggregates. Hydrogen bond interactions should exist between PVA and PEC, due to which PVA is compatible with PEC. Hence, it is expected that the crystallinity of PVA membrane can be reduced, resulting in a lower mass transfer toward water and consequently higher fluxes. In this work, PVA was blended with soluble PEC to improve the pervaporation performance of PVA membranes. The relationship between structural characters of the PVA/PEC blend membranes and their pervaporation performances is also within the scope of this work.

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2. Experimental 2.1. Materials Poly(vinyl alcohol) (PVA1788) was obtained from Beijing First Chemical Plant. Poly(diallyldimethylammonium chloride) (PDDA) (Mw = 100 000 g/mol, 20% aqueous solution) was purchased from Aldrich. Sodium carboxymethyl cellulose (CMCNa, degree of substitution: 0.85) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The intrinsic viscosity of CMCNa in 0.01 M NaCl aqueous solution at 30 ◦ C is 1198.3 mL/g. Polysulfone ultrafiltration membranes were obtained from Development Centre of Water Treatment Technology, Hangzhou, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) are analytical reagents. Deionized water with a resistance of 18 M cm was used in all experiments. Polyelectrolyte complexes with PDDA and CMCNa (PDDA–CMCNa PECs) were prepared according to our previously reported method [46,47]. The PDDA–CMCNa PEC used in this study has a PDDA monomer to CMCNa monomer mole ratio of 0.28 (MPDDA :MCMCNa = 0.28), and was referred to as PEC0.28. Both the chemical structure and composition of PEC0.28 were characterized previously [46,47]. We chose to blend PDDA–CMCNa PEC with PVA because CMCNa is a readily available, commercial material.

2.2. Membrane preparation A specified amount of PDDA–CMCNa PEC and PVA was dissolved in aqueous NaOH at 60 ◦ C. The pH value of the solution was adjusted to 8 and the concentration of the casting solution was 4 wt%. The blend solution was cast on polysulfone ultra-filtration membranes and dried at 60 ◦ C for 12 h to make composite membranes for pervaporation dehydration. The overall thickness of the composite membrane was ca. 55 ± 0.5 ␮m, and the thickness of the blend membrane layer was ca. 8 ± 0·3 ␮m. PVA blend membranes containing 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt% PEC0.28 were named PVA/PEC9010, PVA/PEC8020, PVA/PEC7030, PVA/PEC6040 and PVA/PEC5050, respectively. The composite PVA/PEC blend membranes cannot be fractured in liquid nitrogen because their polysulfone supporting membranes contain non-woven fabrics. Thus, free-standing PVA/PEC blend membranes are prepared for FESEM cross-sectional examination. The fabrication of these free-

standing membranes is done by casting the same PVA/PEC blend solution onto glass plate and peering them off after being dried at 60 ◦ C for 12 h. Fig. 1 shows the chemical structures of PDDA–CMCNa PEC and PVA as well as the interaction between them. It can be seen that hydrogen bond interactions exist between the hydroxyl groups on CMCNa and PVA chains. 2.3. Characterizations Wide angle X-ray diffraction (WAXD) was performed on an X-ray diffractometer (XD-98, Philips X light pipe), in which X-rays were generated by a Cu K source and the angle of diffraction varied from 2◦ to 60◦ . Performing differential scanning calorimetry (DSC) on a PerkinElmer Pyris 1 DSC under a nitrogen atmosphere, samples were heated from 60 ◦ C to 120 ◦ C at a rate of 10 ◦ C/min. Performing thermal gravity analysis (TGA) on a PerkinElmer Pyris 1 TGA, samples were heated from 50 ◦ C to 600 ◦ C at a rate of 20 ◦ C/min. Surface, and cross-section morphology of the membranes were observed by a field emission scanning electron microscopy (FESEM, Hitachi S4800). Samples were coated with gold before FESEM examination. Atomic force microscopy (AFM) (tapping mode) was performed on a Seiko SPI3800N station (Seiko Instruments Inc.). Silicon tips (NSG10, NT-MDT) with a resonance frequency of ca. 330 kHz were used. Dynamic water contact angle (tested at 25 ◦ C) was obtained by the sessile drop method using a contact angle meter (OCA 20, Dataphysics Instruments GmbH, Germany). Equilibrium swelling degree (ESD) of the membranes was calculated through the equation: ESD = (m1 − m0 )/m0 , where m0 is the weight of the dry membrane before swelling and m1 is the weight of the swelled membrane in the equilibrium state. 2.4. Pervaporation experiments Pervaporation was performed on the same equipment as reported previously [50]. The downstream pressure was measured by a piezometer and maintained at about 180 Pa by a vacuum pump. The feed composition was maintained by circulation, and the feed temperature was maintained within an accuracy of 0.3 ◦ C by an electric control thermometer. The permeates were condensed by liquid nitrogen and their composition was determined by a 102G gas chromatograph (Shanghai, China). Permeation flux (J) and separa-

Fig. 1. Chemical structures of PDDA–CMCNa PEC, PVA, and the interaction between them.

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Fig. 2. WAXD of PVA/PEC blend membranes, PVA, and PEC0.28.

tion factor (˛) were calculated according to the following equations:

J=

˛=

g S × t PH2 O /PIP FH2 O /FIP

Above, g is the permeate weight collected in liquid nitrogen traps during the operation time t, and S is the membrane area (18.09 cm2 ). FH2 O and FIP are the weight fractions of water and isopropanol in the feed, and PH2 O and PIP are those in the permeates, respectively. Data of pervaporation performance were repeated three times and averaged. 3. Results and discussion 3.1. Characterizations of PVA/PEC blend membranes WAXD curves of PVA/PEC9010, PVA/PEC7030, PVA/PEC5050, PVA and PEC0.28 are shown in Fig. 2. The sharp band at 2 = 20◦ for PVA is due to the semi-crystalline character of PVA [33,35,39]. PEC0.28 is non-crystalline because PEC0.28 is composed of ionic cross-linked PEC aggregates [47,49]. Moreover, the band intensity at 2 = 20◦ for blend membranes decreases with increasing PEC0.28 content. As shown in Fig. 1, PVA interacts with the ionic cross-linked PEC aggregates via hydrogen bonds. This interaction hinders the ordered packing of PVA chains, resulting in reduced crystallinity of the blend membranes. DSC curves of PVA/PEC9010, PVA/PEC8020, PVA/PEC7030, PVA/PEC5050 and PVA are shown in Fig. 3. It can be seen that the glass transition temperature (Tg ) of PVA is about 60 ◦ C. In literature, the Tg of PVA varies from 53 ◦ C [42] to 80 ◦ C [51]. With increasing the PEC0.28 content, Tg of the blend membranes gradually increases, and reaches ca. 72 ◦ C for PVA/PEC5050. It is known that PDDA–CMCNa PEC has no Tg due to its ionic cross-linking character [47]. The mobility of PVA chains in the blend membranes is depressed due to the hydrogen bond interaction between PVA and PEC aggregates as shown in Fig. 1. This interaction increases with increasing PEC0.28 content, due to which Tg of the blend membranes increases with increasing PEC0.28 content. TGA curves of PVA/PEC9010, PVA/PEC7030, PVA/PEC5050, PVA and PEC0.28 are shown in Fig. 4. The initial decomposition temperatures of blend membranes gradually decrease with increasing PEC0.28 content due to the lower decomposition temperature of PEC0.28 components (247 ◦ C). Weight losses for PVA/PEC9010, PVA/PEC7030 and PVA/PEC5050 before their decomposition are

Fig. 3. DSC curves of PVA/PEC9010, PVA/PEC8020, PVA/PEC7030, PVA/PEC5050, and PVA.

7.2 wt%, 9.1 wt% and 10.5 wt%, respectively. The weight loss is attributed to desorption of small molecules and structure water in the blend membranes [47–49], which are thermally stable under the pervaporation temperature between 40 ◦ C and 70 ◦ C. Fig. 5 shows the surface morphologies of PVA/PEC0.28 blend membranes, PVA, and PEC0.28. The surface of PVA membrane is glabrous (Fig. 5a), and Fig. 5b–d shows that the dense surfaces of PVA/PEC9010, PVA/PEC7030 and PVA/PEC5050 gradually become coarse. Fig. 5e and f shows that the surface of PEC0.28 membrane is composed of needle-shaped nano-structures, referred to as PEC aggregates. Our previous studies show that these PEC aggregates are formed as a result of ionic interaction between PDDA and CMCNa during the preparation of the PDDA–CMCNa PEC [47,49]. It has also been proved that PEC aggregates were the basic building blocks of PEC membranes and that they are stable during the common physical processes such as dissolving and precipitation [47,49]. Due to the nano-scale sizes of PEC aggregates, surfaces of PVA/PEC blend membranes gradually became rough with increasing PEC0.28 content. However, the shapes and sizes of PEC aggregates cannot be seen clearly from Fig. 5 because these nano-structures are tightly embodied in the dense PVA matrix. Fig. 6 shows the FESEM micrographs of dilute PVA/PEC5050 solution (0.05 wt% and 0.02 wt%). Needle-shaped PEC aggregates with sizes ca. 50–100 nm in width and 100–300 nm in length are clearly shown in Fig. 6 and this confirms the existence of PEC aggregates in PVA/PEC blend

Fig. 4. TGA curves of PEC, PVA/PEC5050, PVA/PEC7030, PVA/PEC9010 and PVA, respectively.

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Fig. 5. Surface morphology of (a) PVA membrane, (b) PVA/PEC9010, (c) PVA/PEC7030, (d) PVA/PEC5050, (e and f) PEC0.28 membrane.

membrane. The distribution of PEC aggregates in PVA matrix is further examined by AFM (Fig. 7). Fig. 7a and e shows that both the phase morphologies of PVA and PEC0.28 membranes are homogeneous. Fig. 7b–d shows that “sea-island” like two-phase structure is observed for PVA/PEC9010, PVA/PEC7030 and PVA/PEC5050, wherein the “islands” are evenly distributed. These “islands” are PEC aggregates dispersed in PVA matrix, with sizes ranging from 100 nm to 500 nm depending on the PEC0.28 content. Fig. 7f also shows that the size of a single PEC aggregate is 50–100 nm in width and 100–300 nm in length. The cross-section of the membranes is shown in Fig. 8. The thickness of the PVA/PEC5050 layer cast on the polysulfone sub-

strate is ca. 8 ␮m (Fig. 8a), and the cross-section of PVA membrane is glabrous (Fig. 8b). The cross-section of the PEC0.28 membrane (Fig. 8f) is also composed of needle-shaped PEC aggregates, whose ridges are sharp. The size of PEC aggregates in Fig. 8f seems larger than that observed by AFM in Fig. 7f because AFM observations were performed on PEC aggregates from dilute solution of PEC0.28. Fig. 8c–e shows that the cross-section of the blend membranes gradually becomes rough, and particles sized 200–500 nm are revealed in the cross-section of PVA/PEC7030 and PVA/PEC5050. Moreover, the ridges of these particles are blunt, indicating that PEC aggregates are tightly embodied in PVA matrix. Figs. 5–8 all show that the blend membranes have a two-phase structure, in

Fig. 6. FESEM micrographs of PVA/PEC5050 dilute solutions: (a) 0.05 wt% and (b) 0.02 wt%.

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Fig. 7. AFM phase morphology of membranes (a) PVA, (b) PVA/PEC9010, (c) PVA/PEC7030, (d) PVA/PEC5050, (e) PEC0.28, (f) PEC aggregates obtained by drying 0.05 wt% PEC0.28 solution.

which the domain size of PEC0.28 phase increases with increasing PEC0.28 content in the blend. Fig. 9 shows the dynamic contact angle of PVA/PEC0.28 blend membranes, PVA and PEC0.28. The contact angle of all membranes in Fig. 9 decreases with increasing time due to the wetting of the membrane and this is a widely observed phenomenon for hydrophilic surfaces [52]. PEC0.28 membrane is more hydrophilic than PVA membrane due to the charged character of PEC0.28 aggregates [47,49]. Moreover, the water contact angle of these membranes after ca. 75 s gradually decreases with increasing PEC0.28 content. As indicated by AFM, this change is consistent with PEC phase size increasing concurrently with PEC0.28 content in blend membranes. Fig. 10 shows that the equilibrium swelling degree (ESD) of blend membranes in 10 wt% water–isopropanol at 40 ◦ C decreases with increasing PEC0.28 content, and ESD of PVA and PEC0.28 membrane are 35.6 wt% [36,44] and 8.5 wt%, respectively. Polyelectrolyte complexes have very low solubility in isopropanol due to their ionic cross-linking and charged character [53]. Thus, more water and less isopropanol should swell into the blend membranes with increasing PEC0.28 content. Here, the blend membranes were immersed in aqueous isopropanol containing as high as 90 wt% isopropanol, due to which it is reasonable that the overall ESD of the blend membranes decreased with increasing PEC0.28 content. Based on the WAXD, DSC, FESEM, and AFM characterizations performed on the blend membranes, structural characteristics of PVA/PEC blend membranes are summarized as follow. Needle-shaped PEC aggregates were dispersed in PVA matrix and came into contact with each other to form larger PEC phase with increasing PEC0.28 content. As a result, surface

hydrophilicity of the blend membranes increases, and ESD of the blend membranes in 10 wt% water–isopropanol decreases. Furthermore, the interaction between PVA chains and PEC aggregates via hydrogen bonds increases with increasing PEC0.28 content in the blend. Hence, the mobility and ordered packing of the PVA chains are reduced and result in the increased Tg and the reduced crystallinity of the blends. 3.2. Pervaporation performance of blend membranes The effect of PEC0.28 content on pervaporation performance of blend membranes in dehydrating10 wt% water–isopropanol at 40 ◦ C is given in Fig. 11. It can be seen that both the permeation flux and water content in permeate increase with increasing PEC0.28 content (anti trade-off) [35]. As shown in Figs. 9 and 10, the hydrophilicity of the blend membranes increases and their ESD in 10 wt% water–isopropanol decreases with increasing PEC0.28 content. As a result, both the permeation flux and selectivity increase with increasing PEC0.28 content in the blend membranes. Notably, permeation flux and water content in permeate for PVA/PEC5050 in dehydrating 10 wt% water–isopropanol at 40 ◦ C are 0.55 kg/m2 h and 99.42 wt% (equals ˛ = 1542), respectively. Both are highly improved as compared with pristine PVA membrane (J = 0.17, ˛ = 61.6). Fig. 12 shows the effect of water content in feed on the pervaporation performance of PVA/PEC5050 at 40 ◦ C. The permeation flux increased with increasing water content in feed. The water content in permeate was kept above 99.15 wt% when water content in feed was below 20 wt%, after which it decreased rapidly. This

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Fig. 8. Cross-section of membranes (a) PVA/PEC5050 on a polysulfone ultra-filtration membrane, (b) PVA, (c) PVA/PEC9010, (d) PVA/PEC7030, (e) PVA/PEC5050, (f) PEC0.28. Note: Membranes in b–f are free-standing.

phenomenon has been widely observed. A common explanation is that the increasing permeation flux is due to the increasing driving force, and the decreasing selectivity is due to excessive swelling of the membrane at higher water content in the feed [33,35,42]. Fig. 13 shows the effect of feed temperature on PVA/PEC7030, PVA/PEC5050 and PVA membranes in dehydrating 10 wt%

water–isopropanol. It can be seen that water content in permeate for PVA/PEC5050 is stable with temperature, while that for PVA/PEC7030 slightly decreases with increasing temperature. However, the water content in permeate for pristine PVA membrane greatly decreases from 93.1 wt% to 80.1 wt% with increasing feed

Fig. 9. Dynamic water contact angle of blend membranes, PVA membrane, and PEC0.28 membrane.

Fig. 10. Effect of PEC0.28 content in blend membranes on their equilibrium swelling degree in 10 wt% water–isopropanol at 40 ◦ C.

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Table 1 Pervaporation performances of PVA-based membranes in dehydrating 10 wt% water–isopropanol. Membranes

70 70 60

1.35 0.75 0.644

1002 810 5000+

This work [20] [41]

PVA/PEC5050 PVA membrane 50 wt% TEOSb /PVA 10 wt% Na+ MMATc /PVA 5 wt% APTEOSd /PVA 10 wt% silicalite-1/PVA 50 wt% NaAlge /PVA 40 wt% PANIf /PVA

30 30 30 30 30 30 30 30

0.32 0.095 0.012 0.075 0.026 0.069 0.034 0.069

1491 77 424 2241 1580 2241 119 564

This work [45] [21] [32] [33] [34] [44] [45]

b c d e f

Fig. 12. Effect of water in feed on pervaporation performance of PVA/PEC5050: permeation flux (open); water content in permeate (solid). The feed temperature is 40 ◦ C.

temperature. The decreasing selectivity of PVA membranes with increasing feed temperature is a common phenomenon [29,30,41] and is ascribed to the increasing chain mobility with increasing feed temperature [41]. For PVA/PEC blend membranes, DSC results (Fig. 3) show that Tg of the blends increase with increasing PEC0.28 content. That is, the thermal mobility of PVA chains decreases with increasing PEC content. This thermal behavior results in more sta-

Separation Ref factor (˛H2 O/IP )

PVA/PEC5050 PERVAP2510a 75 wt% Chitosan/PVA

a

Fig. 11. Effect of PEC0.28 content in the blend membranes on the pervaporation performance in dehydrating 10 wt% water–isopropanol at 40 ◦ C: permeation flux (open); water content in permeate (solid).

Temperature Permeation flux (kg/m2 h) (◦ C)

PERVAP2510: commercial membrane from Sulzer Chemtech, Germany. TEOS: tetraethylorthosilicate. Na+ MMAT: sodium montmorillonite. APEOS: ␥-aminopropyl-triethoxysilane. NaAlg: sodium alginate. PANI: polyaniline.

ble selectivity of PVA/PEC5050 with increasing feed temperature. Moreover, the ionic cross-linking of PEC aggregate also contributes to this phenomenon because pristine PEC membrane shows stable selectivity with increasing feed temperature [47]. Furthermore, permeation flux for PVA/PEC5050 (1.35 kg/m2 h) at 70 ◦ C is twice as large as that of pristine PVA membrane (0.63 kg/m2 h), and water content in permeate for PVA/PEC5050 (99.11 wt%) is even larger than that for PVA membrane (80.1 wt%). The increase in both permeation flux and selectivity is due to the cooperative effect of the increased hydrophilicity, decreased crystallinity of blend membranes and their decreased ESD in 10 wt% water–isopropanol [54]. Table 1 summarizes the pervaporation performance of PVAbased membranes in isopropanol dehydration. It can be seen that permeation flux of PVA/PEC5050 membrane is considerably improved without compromising its selectivity. For example, performance of J = 1.35 kg/m2 h, ˛ = 1002 at 70 ◦ C and J = 0.32 kg/m2 h, ˛ = 1491 at 30 ◦ C are obtained with PVA/PEC5050 in dehydrating 10 wt% water–isopropanol. The improved flux is due to the lower crystallinity and higher hydrophilicity of PVA/PEC0.28 blend membranes as compared with pristine PVA membranes. Moreover, the insolubility of PECs in isopropanol depressed the swelling of the blend membranes in the feed, also improving the selectivity of blend membranes.

Fig. 13. Effect of feed temperature on pervaporation performance of blend membranes and PVA membranes in dehydrating 10 wt% water–isopropanol.

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4. Conclusions Blend membranes of PVA and PDDA–CMCNa PEC (PVA/PEC) containing 10–50 wt% PEC0.28 were made by the solution blending method. WAXD and DSC show that the crystallinity of the blends decreases, and Tg of the blends increases with increasing PEC0.28 content in the blends. This observation results from both the ordered packing as well as the mobility of PVA chains in blends being reduced due to hydrogen bonding between PVA and PEC0.28. FESEM and AFM show that needle-shaped PEC aggregates are evenly dispersed in the PVA matrix to form a two-phase structure. Due to this structural character, surface hydrophilicity of blend membranes increases while swelling degree of blend membranes in 90 wt% isopropanol–water decreases with increasing PEC0.28 content in the blend membranes. Both the permeation flux and water content in permeate in dehydrating 10 wt% water–isopropanol at 40 ◦ C increase with increasing PEC0.28 content in blend membranes. This is due to the cooperative effect of the increased surface hydrophilicity and depressed swelling degree of the blend membranes. Permeation fluxes for PVA/PEC5050, PVA/PEC7030 and PVA in dehydrating 10 wt% water–isopropanol all increase with increasing feed temperature (40–70 ◦ C). Meanwhile, water content in permeate for PVA decreases while that for PVA/PEC5050 is maintained around 99 wt%. The stable selectivity for PVA/PEC5050 is due to the reduced mobility of PVA chains caused by hydrogen bonding between PVA and PEC. The best performance of blend membranes in dehydrating 10 wt% water–isopropanol at 70 ◦ C is obtained with PVA/PEC5050 (J = 1.35 kg/m2 h and ˛ = 1002). This performance is satisfied from both the permeation flux and selectivity points of view. Thus, blending PVA with soluble PEC is a promising strategy for improving the pervaporation performance of PVA membranes. Acknowledgements Financial support from the NNSFC (50633030, 20876134 and 20606028) is greatly appreciated. We thank Master J.M. Pahlas in University of Georgia and C.H. Han in Zhejiang University for editing this manuscript. References [1] M. Mulder, Basic Principles of Membrane Technology, second ed., Kluwer Academic Publishers, Netherlands, 1998. [2] M. Ulbricht, Advanced functional polymer membranes, Polymer 47 (2006) 2217–2262. [3] N.W. Ockwig, T.M. Nenoff, Membranes for hydrogen separation, Chem. Rev. 107 (2007) 4078–4110. [4] T.S. Chung, L.Y. Jiang, Y. Li, Santi Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483–507. [5] T.N. Shah, H.C. Foley, A.L. Zydney, Development and characterization of nanoporous carbon membranes for protein ultrafiltration, J. Membr. Sci. 295 (2007) 40–49. [6] B. Kwon, J. Molek, A.L. Zydney, Ultrafiltration of PEGylated proteins: fouling and concentration polarization effects, J. Membr. Sci. 319 (2008) 206–213. [7] A. Mehta, A.L. Zydney, Effect of spacer arm length on the performance of chargemodified ultrafiltration membranes, J. Membr. Sci. 313 (2008) 304–314. [8] D.R. Latulippe, K. Ager, A.L. Zydney, Flux-dependent transmission of supercoiled plasmid DNA through ultrafiltration membranes, J. Membr. Sci. 294 (2007) 169–177. [9] Y.H. See-Toh, M. Silva, A. Livingston, Controlling molecular weight cut-off curves for highly solvent stable organic solvent nanofiltration (OSN) membranes, J. Membr. Sci. 324 (2008) 220–232. [10] T.H. Chong, F.S. Wong, A.G. Fane, The effect of imposed flux on biofouling in reverse osmosis: role of concentration polarisation and biofilm enhanced osmotic pressure phenomena, J. Membr. Sci. 325 (2008) 840–850. [11] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [12] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162–179. [13] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5–37.

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