Polyelectrolyte complex (PEC) modified by poly(vinyl alcohol) and their blend membranes for pervaporation dehydration

Journal of Membrane Science 378 (2011) 233–242

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Polyelectrolyte complex (PEC) modified by poly(vinyl alcohol) and their blend membranes for pervaporation dehydration Meihua Zhu, Jinwen Qian ∗ , Qiang Zhao, Quanfu An, Yihu Song, Qiang Zheng MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 24 February 2011 Received in revised form 30 April 2011 Accepted 6 May 2011 Available online 13 May 2011 Keywords: Polyelectrolyte complex Poly(vinyl alcohol) Blend membrane Mechanical property Pervaporation

a b s t r a c t Soluble polyelectrolyte complex (PEC), made of poly(2-methacryloyloxy ethyl trimethylammonium chloride)/sodium carboxymethyl cellulose (PDMC/CMCNa), was modified by poly(vinyl alcohol) (PVA) with the content from 0 to 50 wt% to improve the PEC mechanical properties. It was found that the best mechanical properties of PEC/PVA blend containing 30 wt% PVA (PEC/PVA7030) were obtained. The tensile strength, modulus and elongation at break are about 1.5, 1.4 and 3.6 times of the pristine PEC. The prepared PEC/PVA blend membranes were subjected to the dehydration of 10 wt% water–isopropanol, displaying very good PV performance as the PVA content in PEC/PVA blend membranes is ≤30 wt%, especially for the selectivity of the PEC/PVA7030 blend membranes with and without polysulfone substrate, respectively. The results of the solution viscosity, differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), dynamic water contact angle (DWCA), equilibrium swelling degree (ESD) and field emission scanning electron microscopy (FESEM), and the mechanical behavior as well as pervaporation performance of PEC/PVA blends and their membranes all display special characters as the PEC/PVA blend contains 30 wt% PVA. These characters are interpreted in term of the compatibility of PEC and PVA. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The water removal from aqueous–organic mixtures is an important issue in industry. The pervaporation (PV) dehydration of organics has been concerned in both academic and industry all along [1–3]. One expects a membrane with high PV performances both in flux and selectivity, good membrane forming properties, as well as high mechanical properties. The membrane mechanical properties are of importance for successful membrane separation technologies. Many works have been done [4–16]. Among them more efforts focused on improving the swelling behavior and mechanical properties of common PV membranes such as, poly(vinyl alcohol) (PVA), chitosan (CS) and sodium alginate (SA) [8–16] through blending or crosslinking methods. The blending, however, is a comparatively simple way and widely adopted usually. Chen and Hyder [15] prepared CS–PVA blend PV membranes and found that the tensile strength was 69 MPa, higher than 57 MPa and 49 MPa of both component membranes of CS and PVA, respectively, while the strain at break was 11% and closed to 9% and 15% of both component membranes. This is due to the hydrogen bonding interaction between functional groups of –NH2 (amino

∗ Corresponding author. Tel.: +86 0571 87953780; fax: +86 0571 87953780. E-mail address: [email protected] (J.W.Qian). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.007

group) in CS and –OH (hydroxyl group) in PVA. Aminabhavi and co-workers [16] prepared silicalite-1 loaded NaAlg and PVA mixed matrix membrane to improve the separation performance of pristine NaAlg and PVA. Nevertheless, the mechanical properties of both pristine membranes of NaAlg and PVA were also enhanced as small amount of silicalite-1 was incorporated in them. For example the tensile strength of NaAlg membrane incorporated 10 wt% silicate-1 was almost twice the pristine NaAlg membrane and its swelling degree decreased. This is because the free volume spaces in the blend membranes were occupied by silicate-1, suppressing the swelling of NaAlg membrane and increasing the tensile strength. Recently we reported a type of novel homogeneous polyelectrolyte complex (PEC) membranes fabricated by soluble PECs solids via casting solutions, displaying excellent PV performances in dehydration of organics [17–21]. However, the mechanical properties of homogenous PEC are insufficient, especially for the toughness. The elongations at break of the PEC membranes modified by inorganic SiO2 [22] and MWCNT [23] even at the optimal contents are still low, about 2.4 and 3.4%, respectively, though tensile strengths of them rise. We supposed that if there are some –COO− , –OH or –NH2 in a PEC sample, the PVA containing –OH could be chosen to improving the tensile strength and elongation at break of the PEC, due to both of the hydrogen bonding interaction among these functional groups and the good elongation at break of polymeric PVA [24]. Thus, the commercial PVA may be

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a good candidate to improve the mechanical properties of PEC membranes. In this study, soluble PEC composed of sodium carboxymethyl cellulose (CMCNa) and poly(2-methacryloyloxy ethyl trimethylammonium chloride) (PDMC) (CMCNa–PDMC), with two functional groups of –COO− and –OH was prepared, and modified by PVA through PVA-solid/PEC-solution blend method [25]. Both relationships of mechanical property-PVA content and PV performance-PVA content for PEC/PVA blend membranes were examined. Moreover, PEC/PVA blend membranes without polysulfone ultrafiltration substrate have been fabricated, also displaying very good membrane-formation and high PV performances. 2. Experimental

c was used to evaluate the intrinsic viscosity [] of the blend. Differential scanning calorimetry (DSC) was performed on a PerkinElmer Pyris 1 DSC under a nitrogen atmosphere, and samples were heated from 40 ◦ C to 150 ◦ C at a heating rate of 10 ◦ C/min. 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 3◦ to 70◦ . Dynamic water contact angle (DWCA) was obtained by the sessile drop method using a contact angle meter (OCA 20, Dataphysics Instruments GmbH, Germany). The surface morphology of membranes was observed by a field emission scanning electron microscopy (FESEM, Hitachi S4800). Samples were coated with gold before FESEM examination.

2.1. Materials

2.4. Universal testing machine (UTM)

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. Poly(2methacryloyloxy ethyl trimethylammonium chloride) (PDMC, Mw: 300,000 g/mol) was obtained from HenYi Chemical Company Shanghai, China. PVA1788 (degree of polymerization of 1700, 88% hydrolyzed) was obtained from Beijing First Chemical Plant, China. Deionized water with a resistance of 18 M cm was used in all experiments. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were analytical reagents. Polysulfone (PSF) ultrafiltration membrane as a substrate was provided by Development Centre of Water Treatment Technology, Hangzhou, China. According to the method in previous study [19], CMCNa–PDMC polyelectrolyte complex (PEC) was fabricated at 0.005 M HCl concentration by adding PDMC aqueous solution dropwise into CMCNa aqueous solution through a burette at a steady rate under vigorous stir till precipitates happened at the bottom of the container. The PEC precipitates were washed with deionized water and dried at 60 ◦ C with the ionic complex degree (ICD) of 0.46.

PEC/PVA blend films for mechanical tests were cut into 30 × 10 mm2 (length by width) strips, and deposited at 40 ◦ C for 24 h. Stretching tests of the blend films were performed on a UTM (SANS CMT4204, Shenzhen, China) with a stretching rate of 2 mm min−1 . Films of gauge length of 20 mm and width of 10 mm were stretched at room temperature (25 ± 1 ◦ C) and relative humidity of 50 ± 2%. Five measurements were performed for each sample at least and averaged. 2.5. Pervaporation experiment Pervaporation (PV) experiment of PEC/PVA blend membranes was conducted on a laboratory made apparatus reported in our previous work [26]. The pressure of the downstream side was maintained at 180 Pa by a vacuum pump during the PV process. Permeates were condensed in a cold trap by liquid nitrogen and the compositions were determined by a 102 G gas Chromatograph (Shanghai, China). The permeation flux (J) and selectivity (˛) were calculated according to the following equations: J=

2.2. Preparation of PEC/PVA blends and blend membranes PEC(CMCNa–PDMC0.46)/PVA blends were fabricated as follows: a specified amount of PEC was dissolved in 0.07 M NaOH aqueous solution at room temperature and precipitated with acetone to obtain the water-soluble PEC, and then 0.5 g water-soluble PEC dried was dissolved in 20–25 ml deionized water (according to the amount of PVA). 0.056–0.5 g PVA solid was dissolved in PEC solutions at 75 ◦ C under stir for 10 h to obtain PEC/PVA casting solutions with various PVA contents from 10 to 50 wt% in PEC/PVA blends. The casting solutions above were kept for 3 days at room temperature and then cast on a Teflon sheet, dried at 40 ◦ C for 24 h, then peeled off carefully for the measurements of DSC, equilibrium swelling degree (ESD), mechanical property and the PV performance of blend membranes (with about 30 ␮m thickness). For the experiments of dynamic water contact angle (DWCA) and PV performance of PEC/PVA membranes with the substrate, the PEC/PVA casting solutions were cast on PSF substrate, and the thickness of top PEC/PVA layer is about 2 ␮m. 2.3. Characterization Viscosity measurement of PEC/PVA blends was carried out with an Ubbelohde dilution viscometer at 30 ◦ C. Diluted blend solutions were obtained from the corresponding casting blend solutions diluted with deionized water, and initial concentration of all the diluted blend solutions is 2 × 10−4 g/mL. An extrapolation procedure according to the Huggins viscosity equation sp/C = [] + kH []2

˛=

g (S × t) (PH2 O /PIP ) (FH2 O /FIP )

where, g is the permeate weight collected during the operation time t, and S is the membrane area (18.09 cm2 ). FH2 O and FIP are the mass fractions of water and isopropanol in the feed, and PH2 O and PIP are those in permeate, respectively. The water in permeate (PH2 O ) is proportional to ˛ when the FH2 O and FIP are fixed· Data of PV performance were repeated three times and averaged. 3. Results and discussion 3.1. Viscosity behavior of diluted blend solutions Fig. 1 shows the diluted viscosity behavior of PEC/PVA blends, pristine PEC and PVA in deionized water at 30 ◦ C. From Fig. 1a, it can be seen that the specific reduced viscosity (sp /c) ∼ concentration (c) curves of PEC/PVA blends with PVA content of 0–50 and 100 wt% (i.e., PEC, PEC/PVA9010, PEC/PVA8020, PEC/PVA7030, PEC/PVA6040, PEC/PVA5050, and PVA) are linear very well. According to the Huggins viscosity equation sp/c = [] + kH []2 c, an extrapolation procedure was applied to obtain the intrinsic viscosities [] of seven samples. The values of [] are 4838, 4737, 4287, 4155, 3390, 2503 and 90 mL/g for PEC/PVA blends with PVA content of 0, 10, 20, 30, 40, 50 and 100 wt%, respectively. Fig. 1b shows the variation of the [] with the PVA content of 0–50 and 100 wt%. The [] of blends are higher than those calculated from the

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

8000

b

6000

6000 5000

sp

η /c (mL/g)

7000

7500

a

PEC PEC/PVA9010 PEC/PVA8020 PEC/PVA7030 PEC/PVA6040 PEC/PVA5050

[η] (mL/g)

9000

235

4000

4500

3000

1500

3000 0 2000 0.00

0.04

0.08

0.12

0.16

0

0.20

20

40

60

80

100

PVA content (wt%)

3

10 c (g/mL)

Fig. 1. (a) Specific reduced viscosity (sp /c) ∼ concentration (c) curves of PEC (), and PEC/PVA blends with PVA content (wt%) of 10 (), 20 (), 30 (), 40 (), 50 () in deionized water at 30 ◦ C; (b) variation of the intrinsic viscosity of PEC/PVA blends with PVA content.

additivity law (dot line), i.e., displaying a positive deviation. Generally the positive deviation represents the good compatibility between two polymers mentioned elsewhere [14,27–29]. It indicates that PEC and PVA are compatible due to some hydrogen bonding interaction between them, as shown in Fig. 2. Moreover, the maximum positive deviation of the [] of PEC/PVA blends is located at nearly 30 wt% PVA content, indicating that the best compatibility happens at this blending ratio.

in the PEC/PVA7030 blend is the hardest, or the strongest hydrogen bonding interaction and the best compatibility between PEC and PVA exists in this case. Fig. 4 shows WAXD curves of PEC/PVA9010, PEC/PVA8020, PEC/PVA7030, PEC/PVA6040, PEC/PVA5050, and PVA. The sharp band at 2 = 20◦ for PVA is due to the semi-crystalline character of PVA [31–33]. The band intensity of the peak decreased with increasing PVA content from 10 wt% to 30 wt%, and then increased gradually as the PVA content is higher than 30 wt%. The lowest band intensity is at 30 wt% PVA because the strongest hydrogen interaction between PEC and PVA hinders the ordered packing of PVA chains and results in reducing the crystallinity of the PVA in PEC/PVA7030 blend. This result is also in good agreement with the rule of viscosity in solutions and DSC data in solids. Fig. 5 shows FESEM surface morphologies of PEC/PVA blend membranes and pristine PEC membrane with two magnifications of 5 K and 20 K (inserts). Fig. 5a displays a typical needle-shaped nano-structure of a given PEC [18,20,21,23], which presents randomly homogeneous distribution. However, blending 10 wt% PVA

3.2. Characterizations of blends and blend membranes Fig. 3 shows DSC curves of PEC, PEC/PVA9010, PEC/PVA8020, PEC/PVA7030, PEC/PVA6040, PEC/PVA5050 and PVA, and the variation of the glass transition temperature (Tg ) of them with PVA content. It can be seen that pristine PEC has no Tg due to its ionic cross-linking character [17,18], and the Tg of PVA is about 63 ◦ C [30]. The Tg of PEC/PVA blends increases gradually and then decreases close to the Tg of PVA with increasing PVA. The maximum Tg is ca. 73 ◦ C for PEC/PVA7030 blend. It indicates that the chains movement

Poly(vinylalcohol) O H

O

O

O

H

H

H O

OH HO

C

O-Na +

O O

O HO

O C O O

CMCNa-PDMC (PEC)

H3C H3C

CH3

N+ CH2 CH2

CH2 O CH2

Hydrogenbond

O

Ionicbond

C O C

OH

CH2

n

Fig. 2. A schematic diagram of the interaction between the PEC and PVA.

n

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

74

a

1 PEC 2 PEC/PVA9010 3 PEC/PVA8020 4 PEC/PVA7030 5 PEC/PVA6040 6 PEC/PVA5050 7 PVA

b

72 70 7

Tg (ºC)

Endotherm

236

6 5 4

68 66

3 2 1

64 62

40

60

80

100

120

140

160

0

20

40

60

80

100

PVA content (wt%)

Temperature (ºC)

Fig. 3. (a) DSC curves of PEC, PEC/PVA9010, PEC/PVA8020, PEC/PVA7030, PEC/PVA6040, PEC/PVA5050, and PVA; (b) variation of the glass transition temperature of PEC/PVA blends with PVA content.

into PEC the needle-shaped structures of the blend membrane are affected significantly and the sizes of needle-shaped aggregates decrease and change greatly (Fig. 5b). For 20 wt% PVA the aggregates become more homogeneous dispersion (Fig. 5c), indicating good compatibility of PEC and PVA. The PEC needle-shaped aggregates get fuzzy at 25 wt% PVA (Fig. 5d), and further are mazed, swelled but more homogeneous dispersion as the PVA content was 30 wt% PVA (Fig. 5e). This should be attributed to the increase of compatibility between PEC and PVA. However, a rediscovery and non-homogeneous distribution of the PEC needle-shaped aggregates are observed at PVA contents of 35 wt% and 40 wt% PVA, indicating decreasing the compatibility between PEC and PVA gradually (Fig. 5f and g). No needle-shaped aggregates happen and a comparatively smooth surface appears for the PEC/PVA blend membrane with 50 wt% PVA (Fig. 5h) finally. It is probably due to the more PVA wrapping up the PEC and making the blend membrane approach to a pure PVA membrane in the morphology. Thus, the PEC/PVA7030 membrane is only a blend membrane with swelled and homogenous PEC aggregates, presenting compact and dense morphology. Fig. 6 shows dynamic water contact angles (DWCA) of blend membranes with different PVA content. From Fig. 6a it can be seen that the DWCA of all membranes decreases with increasing time due to the wetting of their hydrophilic surfaces, and the DWCA of

pure PEC and PVA membranes are smallest and biggest, respectively. The DWCA of PEC/PVA blend membranes increases with increasing PVA. That is, the surface hydrophillity of blend membranes is poorer than that of pristine PEC membrane. This is because the hydrogen bonding interaction between PEC and PVA depresses the hydrophilicity of –COONa and –OH groups in PEC aggregates. Fig. 6b gives the WCA of all membranes at 150 s. Interestingly, it can be seen that the increase of the WCA with increasing PVA is discontinuous at 30 wt% PVA content. This could be attributed to the surface morphology difference of PEC/PVA blend membranes. That is, in the former the surfaces are composed of the needle-shaped aggregates of strongly hydrophilic PEC, while the surfaces in the latter are mainly composed of weakly hydrophilic PVA. Fig. 7 shows the variation of the equilibrium swelling degree (ESD) of PEC/PVA blend membranes in 10 wt% water–isopropanol at 40 ◦ C with increasing PVA. From Fig. 7 it can be seen that the ESD of PEC is ca. 36% and the ESD of PVA is ca. 35%, which is consistent with the value reported in the literature [34]. Interestingly, there is a minimum ESD of PEC/PVA blend, ca. 25%, for the PEC/PVA blend with 30 wt% PVA. It indicates the PEC/PVA7030 blend membrane has more dense structure, quite agreeing with the FESEM image of the compact and dense morphology in Fig. 5e. 3.3. Mechanical property of PEC/PVA blend membranes

3000

PEC/PVA9010 PEC/PVA8020 PEC/PVA7030 PEC/PVA6040 PEC/PVA5050 PVA

Intensity (arbitary)

2500 2000 1500 1000 500 0 10

20

30 40 2θ degrees

50

Fig. 4. WAXD of PEC/PVA blends and PVA.

60

70

Fig. 8a shows the stress–strain curves of PEC/PVA blend membranes and pristine PEC membrane. Fig. 8b gives detailed values of the tensile strength, elongation at break and Young’s modulus of PEC/PVA blend membranes with different PVA contents. From Fig. 8b it is clearly seen that the mechanical properties increase with increasing PVA up to 30 wt%. The maximum values of the tensile strength, elongation at break and Young’s modulus are 49.4 MPa, 15.12%, and 1.55 GPa, or 1.5, 3.6 and 1.4 times of the pristine PEC membrane, respectively. Actually, mechanical properties of PEC/PVA blending membranes in Fig. 8 all raised compared to pristine PEC membrane. Further, the tensile strengths are all larger than the tensile strength, 30.95 MPa of stock PVA obtained at the same testing conditions. These results indicates again that PEC and PVA are compatible and the best compatibility is at 30 wt% PVA content. It is worth noting that the elongation at break rises to 15.12% for PEC/PVA7030 membrane which is much larger than 2.4% and 3.5% obtained from modified PEC (PDDA–CMCNa) membranes by rigid SiO2 and MWCNT [22,23], respectively. Also, the elongation at break of PEC/PVA7030 membrane was superior to common

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

237

Fig. 5. FESEM surface morphologies of (a) PEC, (b) PEC/PVA9010, (c) PEC/PVA8020, (d) PEC/PVA7525, (e) PEC/PVA7030, (f) PEC/PVA6535, (g) PEC/PVA6040 and (h) PEC/PVA5050 membranes with the magnification of 5 K. The magnified insertions of (a)–(h) are all with the magnification of 20 K.

polymer PV membrane materials such as cellulose [4], chitosan [35] and cellulose acetate [36] though it was lower than 33.91% of PVA measured by us. The good mechanical properties of PEC/PVA7030 blend membrane are due to the best compatibility between PEC and PVA caused by the most hydrogen bonds in the PEC/PVA7030 blend.

A formula is proposed to calculate the ratio of hydrogen bonding sites (RH) on both structure units of PVA and PEC in PEC/PVA blends with various PVA contents. RH =

0 )×N (WPVA /MPVA PVA 0 0 (WPEC /(2MCMCNa + MPDMC )) × NPEC

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

90

a

1 2 3 4 5 6 7

80

WCA (º)

70

PEC PEC/PVA9010 PEC/PVA8020 PEC/PVA7030 PEC/PVA6040 PEC/PVA5050 PVA

60

7

50 40

6 5 4 3

30

2 1

0

40

80

120

160

Time (s)

b 60

WCA (º)

238

50

40

30 0

20

40

60

80

100

PVA content (wt%)

Fig. 6. (a) Dynamic water contact angle (DWCA) of PEC/PVA blend membranes; (b) variation of the dynamic WCA of blend membranes with PVA content at 150 s.

image with swelled needle-shaped aggregates, homogenous distribution and dense morphology.

ESD (%)

45 40

3.4. PV performance of PEC/PVA blend membranes

35

Fig. 9 shows the PV performances of PEC/PVA blend membranes with different PVA contents from 0 to 100 wt%. On the whole, both

30 25 20 15 0

20

40

60

80

100

PVA content (wt%) Fig. 7. Variation of the equilibrium swelling degree (ESD) of PEC/PVA blend membranes in 10 wt% water–isopropanol at 40 ◦ C with PVA content.

0 , where WPVA and WPEC are the weight fraction of PVA and PEC; MPVA 0 0 MCMCNa and MPDMC represent the unit molecular weight of PVA, CMCNa and PDMC, respectively; NPVA and NPEC are the hydrogen bonding sites on both structure units of PVA and PEC, which is equal to 1 and 7 as shown in Fig. 2. Table 1 gives the RH values of PEC/PVA blends with various PVA contents. It can be clearly seen that the RH increases from 0.26 to 2.30 with increasing PVA from 10 to 50 wt% in the PEC/PVA blends. The RH value of 0.99 is just for 30 wt% PVA content. It means that the hydrogen bonding sites in both PEC and PVA are equal theoretically, displaying matching hydrogen bonding number. In other words, there is strongest hydrogen bonding interaction in the PEC/PVA7030 blend with 30 wt% PVA content. This endows the best compatibility and other crucial characters for PEC/PVA7030 blend and its membrane, such as the maximum values of the intrinsic viscosity [], the glass transition temperature Tg and mechanical property; the minimum values of WAXD band intensity and the equilibrium swelling degree (ESD); the discontinuous phenomenon of the dynamic water contact angle (DWCA); and the best FESEM

Table 1 The ratio of hydrogen bonding sites (RH) on the structure unit of PVA and PEC for PEC/PVA blends with various PVA contents. PVA content (wt%) RH

10 0.26

20 0.57

30 0.99

40 1.53

50 2.30

Fig. 8. (a) Stress–strain curves of PEC/PVA blend membranes and PEC; (b) effect of PVA content on the tensile strength, elongation at break and Young’s modulus of PEC/PVA blend membranes. The modulus is calculated according to the incipient slope of each curve (with the elongation region from 0.5 to 2%).

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

40 C o 50 C o 60 C o 70 C

12 90

Flux (kg/m h)

98 96

9

2

Water in permeate (wt%)

100

o

a

94 92

80 6

3

70

0

60

Water in permeate (wt%)

100

239

90 88 0

5

10

15

20

25

30

35

40

Water content in feed (wt%) 0

20

40

60

80

100

PVA content (wt%)

Fig. 11. Effect of feed concentration on water in permeate (solid) and permeation flux (hollow) of PEC/PVA7030 blend membrane at 40 ◦ C (circle) and 70 ◦ C (square).

4.0

b

o

70 C o 60 C o 50 C o 40 C

3.5

100

2.4

2

98 2.0

2

Flux (kg/m h)

2.0 1.5 1.0

96 J Water in permeate

1.6

94

0.5

92 1.2 0

20

40

60

80

100 90

PVA content (wt%)

0

Fig. 9. PV performance of PEC/PVA blend membranes in the dehydration of 10 wt% water–isopropanol at different PVA contents: (a) water in permeate and (b) permeation flux.

94 1.5 92 1.0 90 50

60

70

Temperature (ºC) Fig. 10. Operating temperature effect on water in permeate (solid) and permeation flux (hollow) of PEC/PVA7030 (circle) and PEC/PVA5050 (square) membranes in the dehydration of 10 wt% water–isopropanol.

40

50

60

100

2.0

98

1.6

96

1.2

94

0.8

92

2

Flux (J) kg/m h

2

Flux (kg/m h)

96 2.0

30

2.4

Water in permeate (wt%)

98

20

Fig. 12. Stability of pervaporation performances of PEC/PVA7030 blend membrane in dehydration 10 wt% water–isopropanol at 70 ◦ C. Permeation flux (hollow) and water in permeate (solid).

100

2.5

10

Operation time (h)

3.0

40

Water in permeate (wt%)

2.5

Water in permeate (wt%)

Flux (kg/m h)

3.0

90

0.4 40

50

60

70

Temperature (ºC) Fig. 13. Effect of operating temperature effect on water in permeate (solid) and permeation flux (hollow) of PEC/PVA7030 blend membrane with (square) and without (circle) PSF substrate in the dehydration of 10 wt% water–isopropanol.

240

M. Zhu et al. / Journal of Membrane Science 378 (2011) 233–242

Fig. 14. Optical photos of PEC/PVA7030 membrane without PSF substrate before (a) and after (b) PV experiment.

water in permeate (or selectivity) (Fig. 9a) and permeation flux (Fig. 9b) decrease with increasing PVA in PEC/PVA blend membranes. However, the detailed PV characters are exceptional, the water in permeate maintains constant or slightly rises while the permeation flux becomes minimum at 30 wt% PVA content in the PEC/PVA blend membrane. Noticeably, the change of the permeation flux performance is quite in agreement with that of the ESD. This is because PEC/PVA7030 blend membrane presents much denser swelled needle-shaped structure compared other blend membranes, as shown in Fig. 5. Based on special characters of PEC/PVA7030 blend membrane mentioned above, especially for the maximal mechanical properties and the very good PV selectivity as well as the best compatibility between PEC and PVA, PEC/PVA7030 blend membrane was subjected to further investigated in pervaporation. Fig. 10 shows the operating temperature effect on PV performance of PEC/PVA7030 blend membrane. It can be seen that the permeation flux increases and the water in permeate maintains constant or a little bit rises with increasing temperature, displaying anti-“trade-off” phenomenon [37,38]. In fact, the anti“trade-off” phenomenon of all blend membranes with PVA content ≤30 wt% exists, similar to PEC membranes [18–20]. However, no anti-“trade-off” phenomenon of the PEC/PVA5050 blend membranes with large PVA content reveals, which is the same as pure PVA membrane, decreasing the selectivity with increasing the operating temperature [39,40]. So, there have two types of temperature dependences of the selectivity of PEC/PVA blend membranes. It should be attributed to the different blend membrane structures, caused mainly by the needle-shaped aggregates of PEC (≤30 wt% PVA) and mainly by the PVA (>30 wt% PVA) respectively in PEC/PVA blend membranes. Fig. 11 shows the effect of water content in feed on PV performance of PEC/PVA7030 blend membrane. From Fig. 11, it can be seen that the permeation flux increases and the water in permeate decreases slowly with increasing water content in feed at both operating temperatures 40 ◦ C and 70 ◦ C. This phenomenon has been widely observed. However, there is almost no decrease of the water in permeate as the water content in feed is ≤15 wt%. It is better than the PEC membranes [20,21], in which the water in permeate decreases slightly. Furthermore, for the feed with 30 wt% water, the water in permeate of the PEC/PVA7030 membrane still maintains above 98% and the permeation flux highly reaches up to10 kg/m2 h at 70 ◦ C, and about 96% and 3.4 kg/m2 h at 40 ◦ C. No such promising result can be observed for pristine PEC membrane. It is because hydrophilicity of PEC is depressed by the relatively hydrophobic PVA, and the strong hydrogen bonding interaction between PVA and PEC in the PEC/PVA7030 blend mem-

brane. Fig. 12 shows the persistence stability of PV performance of PEC/PVA7030 blend membrane for 60 h running in dehydration of 10 wt% water–isopropanol at 70 ◦ C. It can be seen from Fig. 12, that the permeate flux and water in permeate maintain nearly the same with their initial value after 60 h continuous operating at 70 ◦ C, and displays very good PV performances, 2.12 kg/m2 h and 99.57%, respectively. This good stability should be attributed to both of the inherently ionic cross-linking structure in the PEC and the best hydrogen bonding interaction in the PEC/PVA7030 blend. Due to the best mechanical properties of PEC/PVA7030 blend, its PV membrane without PSF substrate was prepared successfully and subjected to PV process. Fig. 13 shows the PV performances of PEC/PVA7030 blend membrane without substrate with the operation temperature. For comparison, the PV performances of PEC/PVA7030 blend membrane with PSF substrate are also cited from Fig. 9. From Fig. 13, it can be seen that the trends of the PV performance of two blend membranes with and without substrate are the same. The water in permeate maintains constant and the permeation flux increases with increasing operation temperature. Moreover, the water in permeates of the blend membrane without substrate are higher than that of the blend membrane with substrate and vice versa for the permeation fluxes. This higher water in permeate and lower permeation flux are due to the larger thickness of the PEC/PVA membrane without substrate, about 30 ␮m, compared with that of ca 2 ␮m PEC/PVA top layer in the blend membrane with substrate. Even then, the permeation flux of PEC/PVA7030 blend membrane without PSF substrate can reach 0.857 kg/m2 h and 0.987 kg/m2 h at 60 ◦ C and 70 ◦ C, respectively, in dehydration of 10 wt% water–isopropanol, which is quite high compared with many other conventional membranes without substrate. Although blending with PVA inevitably could reduce the permeation flux of the soluble PEC membrane, the permeation flux is still higher than that of the two-ply Chitosan and sodium alginate membrane without substrate [41], 0.554 kg/m2 h, and also higher than that of common polymer blend PV membranes without substrate such as chitosan/PVA7525 [13] and chitosan/hydroxyethylcellulose (9/1) [42] at the same PV conditions. Fig. 14 shows both photographs of the PEC/PVA7030 blend membrane before and after PV experiment (running 10 h). It can be seen from Fig. 14 that the PEC/PVA7030 blend membrane has good mechanical strength. 4. Conclusions PEC(CMCNa–PDMC)/PVA blends and its blend membranes with PVA content from 0 to 50 wt% were prepared via PVA-solid/

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PEC-solution blending and casting solution method respectively, and characterized by various technologies. The measurement results indicate that a variety of special characters reveal for the PEC/PVA blend or membrane with 30 wt% PVA content. The maximum solution viscosity, the highest Tg and the lowest band intensity of WAXD of the PEC/PVA7030 blend in the solution and in the solid respectively indicate the best compatible between the PEC and PVA. As a result, the surface morphology of FESEM transfers from the needle-shaped aggregates to the flat structure and passes through a stage of the swelled needle-shaped or fuzzy aggregates and its homogenous distribution in the PEC/PVA7030 blend membrane. Dynamic water contact angles (DWCA) suggest that the surface hydrophillity of blend membranes decreases quickly and then slowly before and after 30 wt% PVA content, due to the surface structures controlled mainly by the strongly hydrophilic PEC needle-shaped aggregates and by the weakly hydrophilic PVA, respectively. The PEC/PVA7030 blend membrane displays the best mechanical properties. The tensile strength, Young’s modulus and elongation at break are about 1.5, 1.4 and 3.6 times of the pristine PEC membrane because the best compatibility between PEC and PVA and the optimum hydrogen bond matching theoretically in the PEC/PVA7030 blend. The PV results indicate that PEC/PVA blend membranes display very good PV performance as the PVA content is ≤30 wt%, especially for the high selectivity and the keeping of the anti-“trade-off” phenomenon existed in the pristine PEC membranes. Further, due to much denser swelled needleshaped structure, the water in permeate and the permeate flux in the dehydration of 10 wt% water–isopropanol at 70 ◦ C reach up to 99.57% and 2.12 kg/m2 h, and ∼100% and 0.987 kg/m2 h for the PEC/PVA7030 blend membranes with and without polysulfone substrate, respectively.

Acknowledgment This work was supported by Natural Science Foundation of China (Grant Nos.: 20876134, 50633030) and Key Program of Natural Science Foundation of China (Grant No.: 90101024).

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