Pervaporation performance of quaternized poly(vinyl alcohol) and its crosslinked membranes for the dehydration of ethanol

Journal of Membrane Science 335 (2009) 68–75

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Pervaporation performance of quaternized poly(vinyl alcohol) and its crosslinked membranes for the dehydration of ethanol Qiu Gen Zhang, Qing Lin Liu ∗ , Ai Mei Zhu, Ying Xiong, Lang Ren National Engineering Laboratory for Green & Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Received 13 November 2008 Received in revised form 26 February 2009 Accepted 28 February 2009 Available online 14 March 2009 Keywords: Quaternized poly(vinyl alcohol) Crosslinking Pervaporation Ethanol

a b s t r a c t Quaternized poly(vinyl alcohol) (q-PVA) was synthesized, and crosslinked by glutaraldehyde to reduce the swelling of the q-PVA in an aqueous solution. The structure of the q-PVA and its crosslinked membranes was characterized by scanning electron microscopy, X-ray powder diffraction, and water contact angle measurements. Pervaporation performance of the membranes was studied by the dehydration of 85 wt% ethanol solution at 50 ◦ C. The introduction of quaternary ammonium groups enhanced the hydrophilicity and water permselectivity. The water permeability and permselectivity of the q-PVA membranes increased simultaneously with increasing degree of quaternization (DQ). The crosslinking caused a decrease in swelling and an increase in water permselectivity of the q-PVA membranes. The q-PVA membrane with DQ of 4.035% has a highest water permeability of 12.628 g ␮m m−2 h−1 kPa−1 , and the crosslinked q-PVA membrane with degree of crosslinking of 3.92% has a highest water permselectivity of 75. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation is a membrane separation technology with high selectivity, efficiency and energy saving benefits which make it the method of choice for the separation of mixtures which are heatsensitive, have close boiling points or are azeotropic. Pervaporation has been mostly applied in the following three areas: dehydration of organic solvents, removal of dilute organic compounds from aqueous solutions, and organic–organic mixture separation. Of those, dehydration of organic solvents (alcohols, acids, ethers, ketones, etc.) based on hydrophilic polymer membranes owing to the synergic effect has been well developed and studied by many research groups. Namely, water is both preferentially dissolved and transported in the hydrophilic membranes, such as poly(vinyl alcohol), chitosan, polysulfone, polyimides, polyamides, polyaniline, polyelectrolyte, etc. [1–3]. As mentioned previously, the hydrophilicity of membranes is necessary in pervaporation dehydration of organic solvents. The more hydrophilicity a membrane has, the higher the water sorption selectivity and water permselectivity. To improve the membrane hydrophilicity, many polymer materials were modified using different methods, such as sulfonation, quaternization, grafting, filling and surface-modification. Some of those are sulfonated poly(ether

∗ Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. E-mail addresses: [email protected], [email protected] (Q.L. Liu). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.02.039

ether ketone) [4], quaternized chitosan [5], sulfonated cardo polyetherketone [6], chitosan/silicotungstic acid hydrate membranes [7], maleic anhydride surface-modification of crosslinked chitosan membranes [8], poly(tetrafluoroethylene) by surface grafting polymerization of acrylamide and sodium 4-styrenesulfonate [9]. These modified membranes have higher hydrophilicity and water permselectivity than the incipient membranes. However, an increase of membrane hydrophilicity might make the membrane swelling excessively, leading to an open structure, which results in a decrease of the membrane strength. Crosslinking with an organic chemical reagent, such as glutaraldehyde, poly(acrylic acid), maleic acid, formaldehyde, and fumaric acid [10–16], is an effective way to reduce membrane swelling. And hybridization with a silica precursor R Si(OR)3 is also an alternative [17–22]. These crosslinked membranes and hybrid membranes exhibit low degrees of swelling and high water permselectivity. Poly(vinyl alcohol) (PVA) is one of the most important materials for pervaporation dehydration of organic solvent, and has been studied by many researchers with the aim of enhancing its hydrophilicity. PVA-graft-poly(sodium salt styrene sulfonic acid-co-maleic acid) [23], PVA/zeolite composite [24,25], PVA/sulfosuccinic acid membranes with monovalent metal ion [26], PVA covered with an allyl alcohol or acrylic acid plasmapolymerized layer [27] are just a few examples. In previous work, we successfully prepared two PVA/silica hybrid membranes using ␥-aminopropyl-triethoxysilane and polysilisesquioxane. These hybrid membranes have higher water permselectivity and slightly higher hydrophilicity than the incipient PVA [21,22,28].

Q.G. Zhang et al. / Journal of Membrane Science 335 (2009) 68–75

In this work, quaternized poly(vinyl alcohol) (q-PVA) was synthesized by introduction of ammonium group to enhance the hydrophilicity, and then crosslinked by glutaraldehyde to restrict its swelling in an aqueous solution. The physico-chemical properties of the q-PVA and its crosslinked membranes were investigated. The permeation performance of these membranes was studied by pervaporation dehydration of ethanol and the effects of degree of quaternization and crosslinking were also investigated in detail. 2. Experimental

PVA with polymerization degree of 1750 ± 50 and hydrolysis degree of 98% was supplied by Sinophatm Chemical Reagent Co. Ltd. (China). (2,3-Epoxypropyl) trimethyl ammonium chloride (EPTMAC, purity ≥95%) was purchased from the Shandong Guofeng Fine Chemistry Factory. All other solvents and reagents of analytical grade were purchased from Sinophatm Chemical Reagent Co. Ltd., and used without further purification. 2.2. Synthesis of quaternized poly(vinyl alcohol) Q-PVA was synthesized by a previously described procedure [29]. The components of the reaction solution are shown in Table 1. PVA was dissolved in deionized water and stirred at 90 ◦ C for 3 h, then the hot solution filtered. This was followed by the addition of an amount of KOH solution (2 M) and EPTMAC. The resulting solution was allowed to react at 65 ◦ C and stirred for 4 h after being made up to 100 mL using deionized water. Finally the mixture was precipitated and rinsed to pH 7 by anhydrous ethanol. The precipitate was dried under vacuum at 60 ◦ C, and a white solid q-PVA was thus obtained. The degree of quaternization (DQ) of the q-PVA measured by nitrogen microanalysis (Vario EL III Elemental analyzer, Germany) can be calculated by DQ (%) =

then stirred at 45 ◦ C for 2 h. The resulting q-PVA and its crosslinked membranes are transparent with thickness of 25 ± 1 ␮m. The degree of crosslinking (DC) of the crosslinked q-PVA membranes, which represents the molar percentage ratio of hydroxyl groups crosslinked over all hydroxyl groups in q-PVA during crosslinking at 100% reaction conversion, can be calculated by DC (%) = =

2.1. Materials

X/14 × 100 (100 − 195.5X)/44 + X/14

(1)

where X is the nitrogen content of the q-PVA, 195.5 and 44 are the molecular weights of the q-PVA and PVA repeating units, respectively, and 14 is the atomic weight of nitrogen. 2.3. Preparation of membranes 4 g of q-PVA was dissolved in 96 mL deionized water and stirred at 90 ◦ C for 1.5 h. The resulting solution with 4 wt% q-PVA was cast on poly(methyl methacrylate) plates, and then dried in an oven at 40 ◦ C for 6 h. The dried membranes were subsequently peeled off and the solvent was allowed to completely evaporate under vacuum at 80 ◦ C for 12 h. The same procedure as that for preparing the q-PVA membranes was followed to prepare the crosslinked q-PVA membranes. A 4 wt% solution of q-PVA (DQ = 3.26%) was adjusted to pH 5 using HCl solution and a desired amount of 5 wt% glutaraldehyde aqueous solution was subsequently added. The mixture was

69

2 × nGA nOH 2 × wGA /100 × 100 wq−PVA /[44 × (1 − DQ%) + 195.5 × DQ%]

(2)

where wGA and wq−PVA are the weight concentrations of glutaraldehyde and q-PVA in the reaction solution, respectively. In this study, wq−PVA is 4 wt%, DQ is 3.26%, and wGA is 0.04, 0.08, 0.12, and 0.16 wt%, which corresponds to DC of 0.98, 1.96, 2.94, and 3.92%, respectively.

2.4. Membrane characterization The physical structure of the membranes was investigated by Xray powder diffraction (XRD, Panalytical X pert, Enraf-Nonious Co. Holland) using a Cu K␣ radiation in a range from 5◦ to 45◦ at step size of 0.0167◦ and a scan speed of 0.167◦ s−1 , the resulting XRD data was corrected for Lorenz and polarization factors. The surface and cross-sectional morphologies of the membranes were observed by scanning electron microscopy (SEM, XL30ESEM, Philips). The density of the membranes was measured by a digital microbalance (Mettler Toledo, AB204-S) with density kit at 25 ◦ C. Water contact angles were measured by the sessile drop method using the contact angle meter (SL200B, Shanghai Solon Tech Inc. Ltd.) at 25 ± 1 ◦ C in a relative humidity of 65 ± 2%, as shown in Fig. 1. Membrane samples with thickness of 5 ␮m were prepared by casting q-PVA solution on a glass slide. An initial water droplet (0.5 ␮L), which was extruded out of a steel syringe of 0.7 mm in diameter, dropped onto the surface of a membrane sample. The image of the water droplet was saved and analyzed 10 s later. Water contact angle measurements were repeated five times at different locations on the same membrane surface and the results were averaged.

2.5. Swelling studies The q-PVA and its crosslinked membranes were completely dried at 50 ◦ C under vacuum for 8 h. A membrane was weighed and immersed in an 85 wt% ethanol aqueous solution at 50 ◦ C for 48 h, and then periodically removed, wiped dry, and weighed until a constant weight is observed. Each membrane sample was tested for five times at the same condition, the results were averaged. The

Table 1 Components of the q-PVA reaction solution (5 g of PVA). Sample

Water (mL)

EPTMAC (g)

2 M KOH (mL)

DQ (%)

1 2 3 4 5 6

90 90 90 90 95 85

3 5 7 10 10 10

10 10 10 10 5 15

2.024 2.319 3.260 4.035 2.971 3.336

Fig. 1. Schematic diagram of contact angle measurement with a sessile drop.

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degree of swelling (DS) can be determined by DS (%) =

W − W  s d Wd

× 100

(3)

where Ws and Wd are the mass of the swollen and dry membranes, respectively. After weighing, the swollen membrane was placed into a dry flask, which was connected to a liquid nitrogen cold trap and a vacuum pump. The membrane absorbed liquid was collected under a reduced pressure and the concentration of the absorbed liquid analyzed by gas chromatography. Sorption selectivity (˛sor ) can be calculated by ˛sor =

Swater /Sethanol Fwater /Fethanol

(4)

where, Swater and Sethanol are the mass fractions of water and ethanol in the membrane, respectively; Fwater and Fethanol are those in a water–ethanol mixture, respectively. 2.6. Pervaporation Pervaporation experiments were carried out on the Pervaporation Bench Test Unit (Sulzer Chemtech., Germany) at 50 ◦ C with the pressure on the permeate side maintained at 1 kPa and the feed flow rate of 90 L h−1 . The effective membrane area is 71 cm2 . An 85 wt% ethanol aqueous solution was used as a feed. The penetrant was collected in a liquid nitrogen cold trap, and measured to determine its content by gas chromatography. The permeation performance of the membranes are characterized by the permeability (Q) and water permselectivity (˛), which can be calculated from the following equations, respectively [30]: Ji = Qi = ˛=

Wpermeate Ci,permeate

(5)

A·t vapor

Ji ı

100(pi,feed − ni,permeate ppermeate ) Qwater Qethanol

(6)

(7)

where Ji is the permeation flux of component i, Wpermeate the mass of the permeate, Ci,permeate the mass fractions of i in the permeate, A the area of the membrane in contact with the feed mixture, t the

permeation time. Qi is the normalized permeability of membrane vapor for i, pi,feed the equilibrium partial vapor pressure of i, ni,permeate the mole fraction of i in the permeate, ppermeate the permeate pressure and ı the membrane thickness being normalized to 100 ␮m. 3. Results and discussion 3.1. Synthesis of the q-PVA membranes The q-PVAs with DQ were obtained by adjusting the composition of reaction solution, as listed in Table 1. In the synthesis of q-PVA, KOH was used to open the epoxy bonding and graft the quaternary ammonium group onto the PVA chains, and KOH could exchange Cl− for OH− simultaneously [29]. When an excessive KOH was used, the remnant KOH could form crystals on the q-PVA membrane surfaces, as shown in Fig. 2. The EPTMAC and KOH contents affect the DQ of the q-PVA and the structure of the q-PVA membrane. By increasing the EPTMAC content from samples 1 to 4 under the same KOH content, there should be an increase in the collision between PVA and quaternary ammonium groups to take grafting reaction, and finally leading to an increase in the DQ of q-PVA. Crystals appeared on the membrane surfaces, and decreased in the quantity from samples 1 to 3. The nubby crystals appearing on the surface of sample 4 is probably the oligomers originating from self-condensation of EPTMAC. By changing the KOH content (5, 10 and 15 mL, Table 1) under the same EPTMAC content, it is found that samples 5 and 6 have a lower DQ than sample 4. This indicates that the DQ also increased with increasing KOH content, while an excess of KOH has resulted in a decrease in the DQ. Furthermore, crystals increased rapidly with increasing KOH contents and the largest crystals formed on the surface of sample 6 with the highest KOH content. On the other hand, the SEM cross-sectional images (Fig. 3) suggest that the q-PVA membranes have a compact structure with fewer crystals embedded in the bulk of each membrane. As mentioned above, the synthesis conditions for sample 3 are typical. The resulting q-PVA membrane with a high DQ of 3.26% has a smooth surface. Samples 4 and 6 have higher DQ than sample 3, but they have crystals on the surface of the membranes. To eliminate the effect of crystals on the permeation performance, the q-PVA membranes were immersed into an 85 wt% ethanol solution at 50 ◦ C for 30 min. It is found that the crystals

Fig. 2. Effects of synthesis condition on SEM surface images of the q-PVA membranes.

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Fig. 3. Effects of synthesis condition on SEM cross-sectional images of the q-PVA membranes.

Fig. 4. SEM surface images of the q-PVA membrane after immersed into 85 wt% ethanol aqueous solutions at 50 ◦ C for 30 min.

on the surface of the q-PVA membranes were almost completely dissolved (Fig. 4). 3.2. Characteristics of the q-PVA and its crosslinked membranes The quaternary ammonium groups dispersed in the q-PVA matrix would disorder the q-PVA chains and prevent the formation of crystalline regions, resulting in a decrease in the crystallinity degree of the q-PVA membranes. As shown in XRD spectra (Fig. 5(A)), the intensity of the typical peak of the q-PVA membranes decreased observably with increasing DQ. Fig. 6(A) shows the density of the q-PVA membranes and the water contact angles of membranes. The density of the q-PVA membranes increased slightly with increasing DQ. This may be because the oligomers originated from EPTMAC and KOH crystals in the bulk of the q-PVA membranes. The water contact angle measurements of the q-PVA membranes was observed to decrease with increasing DQ. This suggests that the hydrophilicity of the q-PVA membranes increased with increasing quaternary ammonium groups. As mentioned previously, the q-PVA membranes, having hydrophilicity stronger than a PVA membrane, may swell excessively in an aqueous solution, and finally show a decrease in strength and water permselectivity. To reduce the swelling, the qPVA membrane with the DQ of 3.26% was crosslinked by glutaraldehyde, and effect of the DC on the structure and properties of the crosslinked q-PVA membrane were investigated. The membrane with a DC of 3.92% was put into a water bath at 90 ◦ C and stirred for 3 h to confirm crosslinking. It was found that this crosslinked q-PVA membrane did not completely dissolve. This suggests the crosslinking reaction occurred between glutaraldehyde and q-PVA. Fig. 5(B) shows the XRD spectra of the crosslinked q-PVA membranes, the intensity of the typical peak decreased slightly with increasing DC. The density of the crosslinked q-PVA membranes increased slightly in the range of 1.303–1.328 g cm−3 (Fig. 6(B)) with increasing DC due to the amorphous regions of the q-PVA membrane becoming more compact. Furthermore, the condensation of glutaraldehyde and the hydroxyl groups in q-PVA chains might lead to a decrease of hydroxyl groups in the membrane matrix, and finally resulted in a slight decrease in the hydrophilicity of the crosslinked q-PVA membranes with increasing DC (Fig. 6(B)).

3.3. Swelling studies of the q-PVA and its crosslinked membranes In general to achieve equilibrium swelling solvent molecules absorb onto the surface of membrane during a swelling process, and then diffuse into the pores and all accessible volume. If a polymer is

Fig. 5. XRD spectra of the q-PVA membranes (A), and the crosslinked q-PVA membranes (B).

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Fig. 6. Water contact angles and density of the q-PVA membranes (A), and the crosslinked q-PVA membranes (B).

soluble in a solvent, there is a strong attraction between the polymer and the solvent, and the net interaction between the polymer segments is repulsive. As a result, the coiled chains start to swell as soon as they come into contact with the solvent molecules [31]. The q-PVA with strong hydrophilicity is a water-soluble polymer, and is soluble in water above 80 ◦ C. When the q-PVA membranes are exposed to an aqueous solution, water preferentially absorbs onto the membrane surface, and then enters into the bulk membrane. As a result, q-PVA chains begin to spread out until membrane equilibrium swelling is achieved. With increasing hydrophilicity of the membranes, the interaction between water and the membranes increases, leading to an increase of water molecules absorbing onto the membrane surface and entering into the bulk membrane. Furthermore, a decrease of crystallinity promotes species absorption into the bulk membrane, resulting in an increase in the swelling of the membrane. As shown in Fig. 7(A), the DS of the q-PVA membranes in an 85 wt% ethanol aqueous solution increased progressively with DQ, and ethanol content in the swollen membranes decreased due to an increase in hydrophilicity and amorphous regions with increasing DQ. On the other hand, increasing crosslinking slightly decreased the crystallinity degree and thus enhanced the swelling of the q-PVA membranes. Whereas, the crosslinked q-PVA membranes have lower hydrophilicity than the un-crosslinked counterpart, and crosslinking restricts the mobility of q-PVA chains, whose effect on swelling properties overtakes the effect of a decrease of crystallinity. As a result, increasing crosslinking finally decreases the swelling of the crosslinked q-PVA membrane (Fig. 7(B)). Furthermore, the sorption performance of a membrane is relied on the chemical properties of the membrane, and the inter- and intra-molecular structure of the membrane. For crosslinked q-PVA membranes, crosslinking reduced the hydrophilicity of the q-PVA

Fig. 7. Swelling properties of the q-PVA membranes (A), and the crosslinked q-PVA membranes (B).

membranes (Fig. 6(B)), which is unfavorable for water uptake, however, is favorable for ethanol uptake into the membrane. On the contrary, crosslinking makes the amorphous regions of the qPVA membrane network more compact, and thus hinders ethanol molecules absorption into membrane due to its bigger size. This effect is dominant over the decrease in hydrophilicity, finally resulting in a decrease in ethanol concentration in the swollen membranes, as shown in Fig. 7(B). In pervaporation dehydration of organic solution, polymeric membranes with a strong hydrophilicity generally have high water sorption selectivity, such as PVA [22], chitosan [17] and quaternized chitosan membrane [5]. Fig. 8 shows the q-PVA and its crosslinked membranes having high sorption selectivity. Furthermore, the sorption selectivity of the q-PVA membranes increased observably with increasing DQ, and that of the crosslinked q-PVA membranes also increased with increasing DC. 3.4. Pervaporation properties of the q-PVA and its crosslinked membranes Fig. 9(A) shows pervaporation performance of the q-PVA membranes for separation of an 85 wt% ethanol aqueous solution at 50 ◦ C, water permeability and permselectivity rapidly increased with increasing DQ, while ethanol permeability first increased and then decreased when DQ is above 3.26%. An increase in amorphous regions and thus an increase in the swelling of the q-PVA membranes would lead to an open membrane structure, therefore favor penetrants diffusion through the membranes, finally leading to an increase in permeability for water and ethanol. The diffusion of water is whereas much faster than ethanol, resulting in an remarkable increase in water permselectivity. A significant increase in the

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hydrophilicity of a membrane would promote water adsorption into the membrane. The q-PVA membranes with DQ above 3.26% exhibiting higher hydrophilicity should have a higher water uptake. The absorbed water would inhabit most of the free volume cavities and the free volume accessible for ethanol molecules should thus be decreased, resulting in a decrease in ethanol permeability, as shown in Fig. 9(A). Permeation performance of the crosslinked q-PVA membranes with DQ of 3.26% is displayed in Fig. 9(B). A slight decrease in the crystallinity of the crosslinked q-PVA membranes favors penetrants diffusion through the membranes. Whereas crosslinking induced compact structure of the q-PVA membrane and thus a lower mobility of q-PVA chains restrict penetrants diffusion through the membranes, whose effects on penetrants diffusion through the membranes overtake the effect of a decrease in crystallinity, finally resulting in a decrease in permeability for water and ethanol with increasing DC, as shown in Fig. 9(B). In addition, the crosslinking-induced compactness of the amorphous regions of the q-PVA membranes prefer water diffusion over ethanol diffusion because of size effect, resulting in a rapid increase in water permselectivity with increasing DC (Fig. 9(B)). Diffusion selectivity ˛dif is another important parameter to evaluate permeation performance of membranes, and can be calculated by ˛dif = ˛/˛sor . Fig. 8 shows that ˛dif of the q-PVA and its crosslinked membranes, ˛dif of these membranes is comparatively small, in the range of 1.5–2.2. ˛dif of the q-PVA membranes increased due to an increase of hydrophilicity with increasing DQ, and that of the crosslinked membranes also increased due to a more compact structure of membrane with increasing DC.

Fig. 8. Sorption selectivity (˛sor ) and diffusion selectivity (˛dif ) of the q-PVA membranes (A), and the crosslinked q-PVA membranes (B).

3.5. Comparison of pervaporation performance of PVA-based membranes The pervaporation performance of other PVA-based membranes for the dehydration of ethanol is summarized in Table 2. Compared to other PVA-based membranes, the q-PVA and its crosslinked

Fig. 9. Permeability (Qwater and Qethanol ) and water permselectivity (˛) of the q-PVA membranes (A), and the crosslinked q-PVA membranes (B).

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Table 2 Dehydration of ethanol using PVA-based membranes. Modification method

Fwater

T

ı

Qwater

Qethanol

˛

Refs.

Crosslinking with 7 wt% sulfur–succinic acid Blending with poly(ethylene glycol) and crosslinking with tetraethoxysilane Crosslinking with the sulfated zirconia Crosslinking with GA Plasma grafting by allyl alcohol Plasma grafting by acrylic acid Hybridization with 5 wt% ␥-aminopropyl–triethoxysilane q-PVA with DQ of 3.260% q-PVA with DQ of 3.336% q-PVA with DQ of 4.035% Crosslinked q-PVA (DQ = 3.26%) with DC of 2.94% Crosslinked q-PVA(DQ = 3.26%) with DC of 3.92%

10 15 10 10 4.4 4.4 15 15 15 15 15 15

70 50 50 50 60 60 50 50 50 50 50 50

13–15 ≈2 27 15–25 35 35 18 25 ± 1 25 ± 1 25 ± 1 25 ± 1 25 ± 1

3.045 0.171 5.648 ≈11.277 2.634 3.048 2.960 10.320 11.687 12.623 8.148 7.987

0.037 0.001 0.071 ≈0.318 0.054 0.859 0.047 0.257 0.221 0.217 0.131 0.106

82.1 217.7 80.0 ≈35.5 48.5 3.5 62.6 40.2 52.8 58.3 62.0 75.0

[26] [32] [33] [34] [27] [27] [21] This study This study This study This study This study

membranes have higher water permeability and permselectivity, particularly for water permeability. In this study, the q-PVA membrane with DQ of 4.035% has a highest water permeability of 12.628 g ␮m m−2 h−1 kPa−1 and a water permselectivity of 58.3, the crosslinked q-PVA membrane with DC of 3.92% has a water permeability of 7.987 g ␮m m−2 h−1 kPa−1 and a highest water permselectivity of 75. 4. Conclusions Quaternized poly(vinyl alcohol) (q-PVA) was synthesized by grafting with (2,3-epoxypropyl) trimethylammonium chloride, and then crosslinked by glutaraldehyde to reduce its swelling in an aqueous ethanol solution. These membranes were used for pervaporation dehydration of 85 wt% ethanol solution at 50 ◦ C. The introduction of quaternary ammonium groups was found to reduce the crystallinity of the q-PVA membranes, enhance their hydrophilicity and water permselectivity. With increasing quaternization, the water permeability and permselectivity of the q-PVA membranes increased simultaneously. The q-PVA membrane with degree of quaternization of 4.035% has a water permselectivity of 58.3 and a highest water permeability of 12.628 g ␮m m−2 h−1 kPa−1 . The crosslinked q-PVA membranes exhibit less swelling and higher water permselectivity than the q-PVA membranes. The water permselectivity of the crosslinked q-PVA membranes increased with increasing crosslinking, whereas the water permeability decreased. The crosslinked q-PVA membrane with degree of crosslinking of 3.92% has a water permeability of 7.987 g ␮m m−2 h−1 kPa−1 and a highest water permselectivity of 75.

the permeation flux of i (g m−2 h−1 ) Ji nGA the mole number of glutaraldehyde nOH the mole number of hydroxyl group ni,permeate the mole fraction of i in the permeate ppermeate the permeate pressure (kPa) vapor pi,feed the equilibrium partial vapor pressure of i (kPa) Qi the normalized permeability of membrane for i (g ␮m m−2 h−1 kPa−1 ) Si the mass fractions of i in the membrane (wt%) t the permeation time (h) T the feed temperature (◦ C) wGA the weight concentration of glutaraldehyde in the reaction solution (wt%) wq-PVA the weight concentration of q-PVA in the reaction solution (wt%) mass of the dry membranes (g) Wd Ws mass of the swollen membranes (g) Wpermeate the mass of the permeate (g) X the mass fraction of nitrogen (wt%) Greek letters ˛ the permselectivity ˛dif the diffusion selectivity ˛sor the sorption selectivity ı the membrane thickness (␮m)

References Acknowledgements Financial support on this work from National Nature Science Foundation of China Grant no. 50573063, the Program for New Century Excellent Talents in University and the research fund for the Doctoral Program of Higher Education (no. 2005038401) in preparation of this article is gratefully acknowledged. The authors also wish to thank Dr. Ian Broadwell (Xiamen University) for his technical assistance in editing this manuscript for publication.

Nomenclature A

the effective area of the membrane in contact with the feed mixture (m2 ) Ci,permeate the mass fractions of i in the permeate (wt%) DC the degree of crosslinking (%) DQ the degree of quaternization (%) Fi the mass fractions of i in the feed (wt%)

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