PVA blend membranes for dehydration of isopropanol

PVA blend membranes for dehydration of isopropanol

Journal of Membrane Science 361 (2010) 182–190 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 361 (2010) 182–190

Contents lists available at ScienceDirect

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

Preparation method and pervaparation performance of polyelectrolyte complex/PVA blend membranes for dehydration of isopropanol Meihua Zhu a , Jinwen Qian a,∗ , Qiang Zhao a , Quanfu An a , Jing Li b a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou Zhejiang Province 310027, China b Department of Chemistry and Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA

a r t i c l e

i n f o

Article history: Received 26 February 2010 Received in revised form 25 May 2010 Accepted 28 May 2010 Available online 4 June 2010 Keywords: Blend membrane Pervaporation Preparation method Polyelectrolyte complex PVA

a b s t r a c t Poly(diallyldimethylammonium chloride) (PDDA)/sodium polyacrylate (PAANa) polyelectrolyte complex (PEC) was fabricated by adding PDDA dilute aqueous solution dropwise into PAANa dilute aqueous solution with 0.007 M HCl. PEC/PVA(polyvinyl alcohol) blend liquids and their blend membranes were prepared by two blend methods of (a) PVA-solution/PEC-solution blending commonly used and (b) PVA-solid/PEC-solution blending proposed. Both blend liquids obtained were studied by viscometry and particle size analysis meter. Surface morphologies of PEC/PVA blend membranes cast from blend liquids were characterized by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM). Dynamic water contact angle and equilibrium swelling degree (ESD) of blend membranes in water–isopropanol feed were measured. Pervaporation (PV) dehydration of blend membranes for water–isopropanol feed was investigated. It was found that the structure and property of blend liquids, the surface property and ESD of blend membranes prepared by (b) are quite different from those prepared by (a). The stability and PV performance of PEC/PVA blend membranes prepared by (b) is much better than that prepared by (a). It is attributed to the preparation methods, which led to the difference in the interaction in blend liquids and the transition of fluid property and finally the change of the compatibility between PEC and PVA in blend membranes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Currently, a hot research of pervaporation (PV) is still to develop new materials and new formation technology of PV membranes [1–6]. Dehydration of aqueous–organic mixtures is the main application of PV, especially for dehydration of alcohols [7]. Recently, we developed a type of polyelectrolyte complexes (PECs) membranes for dehydration of alcohols with high PV performance especially for super water permeation [8–12]. However, the basic study of PEC and cost reduction as well as improving the film-forming of PEC membranes are needed. A simple way is to make its blend membrane by blending with a common polymer. Polyvinyl alcohol (PVA) membranes have been studied [13–17] and applied for the dehydration of alcohols in industry [15–17] because of the good film-forming property, chemical resistance property and low cost. Also, PVA was used to improve certain properties of other polymers or membranes [18–21]. For example, Feng and coworkers [22] prepared sericin/PVA blend membranes to overcome the fragility of pure sericin membranes, and found that sericin/PVA blend membranes for pervaporation separation of ethanol/water

∗ Corresponding author. Tel.: +86 0571 87953780; fax: +86 0571 87953780. E-mail address: [email protected] (J. Qian). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.05.058

mixtures are in general superior to membranes made from either sericin or PVA alone. As was well known, the performance of blend material is strongly dependent on its composition. There are many works to study the dependence of PV performance of blend membrane on the composition [23–32]. Jiang and co-workers [30] prepared chitosan (CS)/poly(acrylic acid) (PAA) blend membranes with different compositions by a solution-blend method, and found that the best PV performance for the dehydration of ethylene glycol aqueous solution is the CS/PAA blend membrane with the composition of 60/40 mass ratio. It should be noted that properties and performance of a given polymer blend are also strongly dependent on the preparation method at the same composition [33–38]. Wang et al. [33] discussed the influence of preparation methods on structure and properties of PA6/PA66 blends through comparing the melt-mixing and in situ blending, and found the opposite trends of changes in mechanical properties for these two blends. Kannan et al. [36] found changing the addition sequence of nanoclay into TPU/PP blends with fixed composition, the thermomechanical properties changed, and Umeda and Uchida [37] discovered that when different external electric field was applied in preparing some blend membrane, the proton conductivity of this membrane is different. Kusworo et al. [38] studied the shear rate effect in polyimide/polyethersulfone (PI/PES)-zeolite 4A mixed matrix membrane casting and found gas separation performance

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of membranes with same composition was dependent on different shear rates by applying varying casting speeds during fabrication. All experimental results suggest that the preparation method of a PV membrane made of multiple polymers should be concerned. To the best of our knowledge, few works were reported about the influence of preparation methods of blend membranes on their PV performances. In this work, we choose the soft PVA to blend with rigid PEC to prepare optimum PEC/PVA blend membranes, keeping virtues of both polymers, and focus on the role of the preparation method for PEC/PVA blend liquids and blend membranes. Experimental results indicated that preparation method influences greatly the liquid property of PEC/PVA blends and also the structure, morphology and PV performance of PEC/PVA blend membranes. 2. Experimental 2.1. Materials Sodium polyacrylate (PAANa) (molecular weight ≥3 × 107 g/mol) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Poly(diallyldimethylammonium chloride) (PDDA) (Mw = 10, 0000–20, 0000 g/mol, 20 wt% aqueous solution) was purchased from Aldrich and used without further purification. PVA1788 (degree of polymerization of 1700, 88% hydrolyzed) was obtained from Beijing First Chemical Plant. Deionized water with a resistance of 18 M cm was used in all experiments. Polysulfone ultrafiltration membranes were provided by Development Centre of Water Treatment Technology, Hangzhou, China. Polyelectrolyte complexes (PDDA/PAANa) were prepared according to our previously reported method [10]. In the process of preparation, we controlled the concentration of HCl aqueous solution and then obtained PEC with ionic cross-linking degree of 0.3 calculated, which was referred to as PDDA/PAANa 0.3. 2.2. Preparation method The PEC/PVA casting liquid or its blend membrane was prepared by two preparation methods. One is PVA-solution/PEC-solution blend method (a), the other is PVA-solid/PEC-solution blend method (b). In method (b), a specified amount of PEC (PDDA/PAANa 0.3) was dissolved in 0.25 M NaOH aqueous solution at room temperature and precipitated with acetone to obtain the water-soluble PEC, and then 1.05 g water-soluble PEC dried dissolved in 42 mL deionized water under stir at room temperature to obtain PEC solution. Then, 0.45 g PVA solid was dissolved in PEC solution under stir at 75 ◦ C for 10 h to obtain PEC/PVA blend liquid with concentration of 3.57% (m/v) and composition of PEC/PVA = 70/30, denoted as b-75. In method (a), solutions of PEC and PVA were prepared separately. The PEC solution was made the same way as in method (b) with 1.05 g water-soluble PEC dried dissolved completely in 37.5 mL deionized water, the PVA solution was prepared by dissolving 5 g PVA in 50 mL deionized water under stir at 75 ◦ C for 10 h to obtain a homogenous PVA solution with concentration of 10% (m/v). After cooling the PVA solution to room temperature (25 ◦ C), the mixing of PEC solution and 4.5 mL PVA solution was performed at 25 ◦ C for 10 h under stir to obtain PEC/PVA blend liquid with total concentration of 3.57% (m/v) and the composition of PEC/PVA = 70/30, denoted the blend liquid as a-25. For comparison, two additional blend liquids were prepared by the solution-blend method, denoted as a-50 and a-75, which were prepared at blend temperatures of 50 ◦ C and 75 ◦ C, respectively. Four blend liquids of a-25, a-50, a-75 and b-75 above were kept for 3 days at room temperature before being cast on polysulfone ultrafiltration membranes and dried at 40 ◦ C for 24 h to make composite membranes for

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the measurement of the morphology and pervaporation dehydration. The thickness of these membranes is between 4.5 and 5.5 ␮m. 2.3. Viscosity measurement Viscosity measurement of PEC/PVA diluted blend liquids was conducted with an Ubbelohde type viscometer at 30 ◦ C. Diluted blend liquids were obtained from the corresponding casting blend liquids diluted with deionized water. The Huggins viscosity equation sp /c = [] + kH []2 c was used to evaluate the intrinsic viscosity [] of PEC/PVA blends and the liquids property thermodynamically, kH via the extrapolation procedure. Three measurements were carried out at each concentration and averaged values were taken. 2.4. Particle size analysis Particle size and its distribution in four PEC/PVA diluted blend liquids with the concentration of 2 × 10−4 g/mL were measured by a Particle Size Analyzer (Brookhaven Instruments Corporation) at 25 ◦ C. 2.5. Equilibrium swelling degree (ESD) Blend liquids were cast onto a Teflon sheet. After 24 h at 40 ◦ C, dried sample membranes were peeled off and immersed in the given feed solution at designed temperature. The swollen membrane was taken out at certain time intervals, blotted with filter paper to remove the feed solution on the surface, weighed and then recycled until the attainment of the swelling equilibrium. The percentage mass swelling degree at equilibrium (ESD) was determined using the following expression: ESD (%) =

Me − Md × 100 Md

where Md and Me are masses of the membrane dried and at the swelling equilibrium, respectively. 2.6. Contact angle Dynamic water contact angle (tested at 25 ◦ C) was obtained by the sessile drop method using a contact angle meter (OCA 20, Data physics Instruments GmbH, Germany). 2.7. Surface morphology 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. Surface morphology of membranes was examined with a field emission scanning electron microscope (FESEM, Hitachi, S4800). Before FESEM examination, samples were coated with gold. 2.8. Pervaporation experiment Pervaporation (PV) was carried out on the same equipment as reported in our previous work [39]. The pressure of the downstream side was maintained at 180 Pa by a vacuum pump during the process of PV measurement. Permeates were condensed in a cold trap by liquid nitrogen and their composition was determined by a 102 G gas Chromatograph (Shanghai, China). Permeation flux (J) and separation factor (˛) were calculated according to the following equations: J=

g S × t

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Fig. 2. ln r /c ∼ c curves of PEC/PVA blend liquids with the solution-blend method (a-25, a-50 and a-75) and the solid-blend method (b-75). Fig. 1. sp /c ∼ c curves of PEC/PVA blend liquids with the solution-blend method (a-25, a-50 and a-75) and the solid-blend method (b-75).

˛=

PH2 O /PIP FH2 O /FIP

where 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 mass fractions of water and isopropanol in the feed, and PH2 O and PIP are those in permeate, respectively. Data of pervaporation performance were repeated three times and averaged. 3. Results and discussion 3.1. Viscosity behavior of diluted blend liquids Viscosity data of a given polymer in diluted medium can provide more information on the solvency, the interaction between polymer and solvent and the shape of the dissolved or suspended polymer particles [6,40–44]. Fig. 1 shows the reduced specific viscosity and the concentration, sp /c ∼ c, curves of PEC/PVA blends in water with two preparation methods, i.e. the solid-blend method (b-75) and solution-blend method (a-25, a-50 and a-75). From Fig. 1, it can be seen that the viscosity data of the blend liquid for each preparation method are quite fit to Huggins viscosity equation, which is the most common one used. Based on the Huggins equation, the intrinsic viscosities [] of PEC/PVA blends from the intercepts of four lines are 1000, 2108, 4091 and 4800 mL/g for preparation methods of a-25, a-50, a-75 and b-75, respectively. The [] calculated according to the addition law of PEC/PVA blend with the composition of 70/30 is 4254 mL/g (based on the [] of pristine PEC and PVA being 6039 and 90 mL/g, respectively). It means that the [] of PEC/PVA blend prepared by the solid-blend method is larger than the [] calculated, while the [] of PEC/PVA blends prepared by the solution-blend method are all lower than the [] calculated but increase with increasing the blend temperature. Further, Huggins constants kH from the slopes of lines are 117.5,15, 0.744 and 0.375 for preparation methods of a25, a-50, a-75 and b-75, respectively. That is, largest [] and smallest kH are for the solid-blend method (b-75), smallest [] and largest kH are for the solution-blend method (a-25), and the [] decreased and the kH increased with decreasing blend temperature in the solution-blend method. [] and kH are the most common parameters to represent the polymer solution structures, the chain dimension, chain shape and

the interaction between the polymer and the solvent used, respectively [45,46]. Large [] and small kH denote that the polymer chains expand and have strong interaction with the solvent, small [] and large kH represents shrinkage and aggregated chains of a given polymer in solutions [46]. Thus, it can be considered that chains of PVA and PEC expand in the solid-blend method (b-75) but shrink or aggregate in the solution-blend method (a-75, a-50 and a-25) especially for a-25 with lowest blend temperature. According to the viscosity assessment of the compatibility for two polymers [47–49], the measured [] of the blend being larger than the calculated [] represents the compatibility between two polymers, the compatibility of PEC/PVA blend from the solid-blend method (b-75) is much better than that from the solution-blend method (a-75, a-50 and a-25) even at the same blend temperature, 75 ◦ C. Thus, the compatibility of PEC/PVA blend decreases in the order of b-75, a-75, a-50 and a-25. Kraemer viscosity equation is another common one, ln r /c = [] + kK []2 c, which fits viscosity data for dilute polymer solutions and also for dilute suspensions [50]. The parameter kK is negative for polymer solutions and positive for suspensions of rigid particles. This issue is also mentioned by [51]. Fig. 2 shows the variation of logarithm of relative viscosity with the concentration, ln r /c ∼ c, of PEC/PVA blends in water with two different preparation methods. From Fig. 2, viscosity data for each preparation method are also quite fit for Kraemer viscosity equation, and the kK is negative for the solid-blend method (b-75) and positive for the solution-blend method (a-75, a-50 and a-25). Further, kK becomes more positive in the order of a-75, a-50 and a-25. It means that PEC/PVA blend liquids from the solid-blend method (b-75) is indeed a polymer solution, while PEC/PVA blend liquids from the solution-blend method (a-75, a-50 and a-25) are suspensions in practice. That is, PEC becomes large aggregated particles in the solution-blend method and the aggregation extent increases with decreasing the blend temperature, a-75, a-50 and a-25 in order. In other words, the compatibility between PEC and PVA is the best in the solid-blend method (b-75) but decreased with a-75, a-50 and a-25 in order in the solution-blend method. The poor compatibility is probably attributed to strong hydrated shells [52] formed around PVA chains and PEC aggregates in the solution-blend method, compared to the solid-blend method. The hydrated shells will be compressed and even destroyed at high blend temperature. Thus, the interaction or the compatibility between PEC and PVA increases, making expanded chains, small PEC aggregates, large [] (i.e., low chains density), small kH and negative kK , and displaying the character of polymer solutions. Contrarily, the hydrated shells of PEC and PVA in the solution-

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Fig. 3. Method effects of the particle size distribution of PEC/PVA blends (solid lines) and pristine PEC (dotted lines) in liquids, with the concentration of 2 × 10−4 g/mL, prepared by two different preparation methods or at different blend temperatures. Solid lines: (1) b-75, (2) a-75, (3) a-50 and (4) a-25; dotted lines: (a) 75 ◦ C, (b) 50 ◦ C and (c) 25 ◦ C.

blend method, especially for low blend temperature (a-25), are full and the interaction or the compatibility between PEC and PVA decreases, which results in less expanded PVA chains, large PEC aggregates, small [] (i.e., high chains density), large kH and positive kK , and displaying the character of suspensions. Fig. 3 shows the method dependence of the particle size distribution of PEC/PVA in liquids prepared by the solid-blend method and the solution-blend method, and also the particle size distribution of the pristine PEC in liquids prepared at different temperatures. From Fig. 3 it can be obtained that the particle size polydispersity of PEC/PVA in solutions or suspensions prepared by the solid-blend method (b-75) and the solution-blend method (a-75, a-50 and a-25) are 0.378, 0.447, 0.456 and 0.497, respectively, and the average particles size are 477.4, 894.4, 1224.3 and 4643.3 nm, respectively. Clearly, the small particle sizes and narrow size distribution were observed for the solid-blend method compared with the solution-blend method (a-75, a-50 and a-25). However, in the solution-blend method, both average particle sizes and particle size polydispersity of PEC/PVA decrease from 4643.3 to 894.4 nm and from 0.497 to 0.447, respectively, as the blend temperature increases from 25 ◦ C to 75 ◦ C. To understand the reason, particle size analysis of pristine PEC (PDDA/PAANa 0.3) liquids prepared at

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25 ◦ C, 50 ◦ C and 75 ◦ C was performed and the results were shown by dotted lines in Fig. 3. It can be obtained that the particle sizes of pristine PEC in liquids prepared at 25 ◦ C, 50 ◦ C and 75 ◦ C are 4209, 3617.9 and 3049.3 nm, respectively, and the size distribution becomes narrow with increasing blend temperature. It means that decreases of the particle sizes and size distributions of PEC/PVA in the solution-blend method were also caused by increasing blend temperature. That is, increasing the blend temperature can make large PEC aggregates to be small ones, and then results in small particle sizes of PEC/PVA blend with increasing solution-blend temperature. Noticeably, the sizes of PEC/PVA blend particles are much smaller than that of the pristine PEC at the same temperature of 50 ◦ C and 75 ◦ C except for the room temperature (25 ◦ C). For example, both particles of PEC/PVA (a-75) and PEC prepared at 75 ◦ C are 894.4 and 3049.3 nm, respectively. This suggests that there is another factor to decrease the particle size of PEC/PVA in blend liquids except for the preparation method (the blend sequence and blend temperature). This factor should be the addition of PVA. Fig. 4 shows PVA content influences the particle size distribution and the average particle size of PEC/PVA blends in solutions or suspensions, prepared by the solid-blend method at 75 ◦ C. From Fig. 4, it can be seen clearly that particle sizes and particle size distributions of PEC/PVA decrease greatly with increasing PVA content up to 30 wt%. This should be attributed to the increase of the interaction between PVA chains and PEC aggregates, weakening the interaction between PEC aggregates and making smaller PEC/PVA particle sizes. For instance, the average particle size of PEC/PVA is only about 500 nm with 30 wt% PVA in PEC/PVA. So, the increase of the interaction between PVA and PEC plays an important role in making small particle size and narrow particle size distribution (homogenous solutions or suspensions) of PEC/PVA blends. Now, we are interested in why a great difference of particle sizes and particle size polydispersity exists for the two preparation methods, 477.4 nm and 0.378 in the solid-blend method and 894.4 nm and 0.447 in the solution-blend method though the blend temperature (75 ◦ C) and the PVA content (30 wt%) are exactly the same in both cases. In the solid-blend method, PVA solid dissolves and large PEC aggregates disintegrate synchronously due to high preparation temperature. Thus, the interaction between the firstborn PVA chain and the small PEC aggregate firstborn could be produced easily and effectively, compared to in the solutionblend method, in which the PVA chains were formed already. In other words, PVA chains are like as naked chains in the solid-blend method, but are hydrated by a full solvent shell or with a strong hydrated shell in the solution-blend method. The later weakens

Fig. 4. PVA content effect of (a) the particle size distribution (lines: (1) PDDA/PAANa 0.3, (2–6) blends of PDDA/PAANa 0.3 and PVA with 10, 20, 30, 40, 50 wt%, respectively) and (b) the average particle size of PEC/PVA blends in liquids, with the concentration of 2 × 10−4 g/mL, prepared by the solid-blend method at 75 ◦ C (b-75).

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Fig. 5. AFM phase morphology of PEC/PVA blend membranes prepared by the solution method (a-25, a-50 and a-75) and the solid method (b-75). (PEC/PVA concentrations are all 3.57% (m/v).)

the interaction between PEC and PVA, making poor compatibility between them and displaying large particle sizes. 3.2. Surface properties of blend membranes As was well known, the solution structure and property of a given polymer will usually influence the structure and property of the polymer bulk made via its polymer solution [44,53–57]. Fig. 5 shows AFM phase morphology of PEC/PVA blend membranes prepared by the solution-blend method and the solid-blend method. From Fig. 5 it can be observed that there exist different globular structures or different compatibility of PEC/PVA blends from different preparation methods. The globular patterns are about 70, 60 and 40 nm in height and the root-mean-square (RMS) surface roughness are ca. 27.07, 19.15 and 16.11 nm for PEC/PVA blend membranes prepared by the solution-blend method, a-25, a-50 and a-75, while the globular pattern of the membrane prepared by the solid-blend method, b-75, is 30 nm in height and the RMS surface roughness is only 12.18 nm. It indicates that the blend membrane prepared by the solid-blend method is more homogeneous than those prepared by the solutionblend method even at the same blend temperature of 75 ◦ C (a-75 and b-75), displaying more blurry globular structures and more

unclear interface between PEC and PVA in the solid-blend method. There are some works to study the compatibility of polymer blends and the morphology-structure of their blend membranes [36–38] by FESEM. Fig. 6 shows surface morphologies of PEC/PVA blend membranes prepared by the solution-blend method (a-25) and the solid-blend method (b-75), respectively. From Fig. 6, it can be seen that a mass of small globular patterns (ca. 1 ␮m width and 2 ␮m length) emerge on the surface with a clear interface for the blend membrane prepared by the solution-blend method. Oppositely, a small quantity of large globular patterns (ca. 3 ␮m width and 4 ␮m length) appears on the surface of the blend membrane prepared by the solid-blend method. These large globular patterns swelled with a dark interface are due to the strong hydrogen bonds between PEC and PVA, as shown in Fig. 7, to enhance the compatibility between PEC and PVA. Combing the viscosity behavior and the particle size analysis of diluted blend liquids of PEC/PVA blends, the increase of the compatibility between PEC and PVA in the blend membrane prepared by the solid-blend method or by increasing blend temperature in the solution-blend method is quite consistent to the change of blend liquids properties, transferring from the suspension to the solution, which makes the chain dimension bigger, the particle size smaller and more compatibility in PEC/PVA blend membranes.

Fig. 6. SEM surface morphology of PEC/PVA blend membranes with a-25 and b-75. (PEC/PVA concentrations are all 3.57% (m/v).)

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Fig. 7. A scheme diagram of the interaction between the PEC (PDDA/PAANa) and PVA.

3.3. Pervaporation performance of blend membranes The PV performances of PEC/PVA blend membranes prepared by two methods here are also examined. Fig. 8 shows the variation of PV performance of PEC/PVA blend membranes prepared by the solution-blend method (a-25) and the solid-blend method (b-75) with the operation time in dehydrating 10 wt% water–isopropanol at 50 ◦ C and 70 ◦ C. From Fig. 8, it is seen clearly that the permeation flux (J) and the separation factor (˛) of PEC/PVA blend membranes prepared by the solid-blend method are quite stable with the operating time even at feed temperature of 70 ◦ C, compared to those by the solution-blend method. For the PEC/PVA blend membrane prepared by the solution-blend method, J decreases quickly and ˛ fluctuates and decreases with the operation time even up to the operation time of 10 h. Poor stability of PV performances should be attributed to the blend membrane cast by the PEC/PVA suspen-

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sion in the solution-blend method. Noticeably, both J and ˛ of the blend membrane prepared by the solid-blend method are all higher than that prepared by the solution-blend method. This is probably attributed to the high surface hydrophilicity and low swelling of the blend membrane prepared by the solid-blend method. Fig. 9 shows the dynamic water contact angle and equilibrium swelling degree (ESD) in 10 wt% H2 O/isopropanol at 40 ◦ C for PEC/PVA blend membranes prepared by the solution-blend method and the solidblend method. From Fig. 9a and b, it can be seen indeed that a lowest dynamic water contact angle and also a lowest ESD are observed for the PEC/PVA blend membrane prepared by the solid-blend method. Fig. 10 shows the influence of water content in feed on the PV performance of blend membranes after run for 10 h in dehydrating 10 wt% water–isopropanol. From Fig. 10, it is seen that the permeation fluxes increase and the separation factors decrease with increasing water content in feed, displaying ordinary rules. However, compared to the blend membrane prepared by the solution-blend method, the permeation flux of the blend membrane prepared by the solid-blend method is low and less dependent on the water content in feed, and the separation factor is high and also less dependent on the water content in feed. The reason should be the same as that mentioned above, low ESD or high hydrophilicity of the blend membrane prepared by the solid-blend method. That is, the blend membrane prepared by the solid-blend method has dense structure inside and the surface hydrophilic structure. Fig. 11 shows the dependence of the PV performance on the operating temperature after each blend membrane run for 10 h in dehydrating 10 wt% water–isopropanol. From Fig. 11, it can be seen that the permeation fluxes increase and the separation factors increase at the beginning and then decrease with increasing operating temperature for the three blend membranes prepared by the

Fig. 8. Effect of the operating time on pervaporation performance of blend membranes prepared by the solution-blend method a-25 and the solid-blend method b-75 in dehydrating 10 wt% H2 O–isopropanol at feed temperature of 50 ◦ C (a and b) and 70 ◦ C (c and d).

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Fig. 9. (a) Dynamic water contact angle and (b) equilibrium swelling degree (ESD) in 10 wt% H2 O/isopropanol at 40 ◦ C for PEC/PVA blend membranes prepared by the solution-blend method (a-25, a-50 and a-75) and the solid-blend method (b-75).

Fig. 10. Effect of the water content in feed on pervaporation performance of blend membranes prepared by the solution-blend method a-25 and the solid-blend method b-75 in dehydrating 10 wt% H2 O–isopropanol at 50 ◦ C.

solid-blend method (b-75) and the solution-blend method (a-75, a-25). The increase of the permeation flux of blend membranes with increasing operating temperature is normal, but then the flux of the blend membrane prepared by the solid-blend method (b-75) increases faster than that by the solution-blend method (a75 and a-25). Indeed, the permeation activation energy is largest, 23.4 kJ/mol for the b-75 membrane and lower, 20.2 and 15.5 kJ/mol for a-75 and a-25 membranes, respectively according to Arrhenius plots in Fig. 12. The increase and then the decrease of the separation factor of these three blend membranes with increasing operat-

ing temperature are the synergic result of PEC and common PVA polymer because PEC membranes usually displays anti-trade-off phenomenon [11]. Furthermore, both permeation flux and separation factor of the blend membrane prepared by the solid-blend method are the best, compared to the blend membranes prepared by the solution-blend method, especially at high blend temperature of 75 ◦ C. It should be also ascribed to the dense structure inside and the surface hydrophilic structure of the blend membrane prepared by the solid-blend method. It can be seen also from Fig. 11 that the good dense structures could be maintained at least

Fig. 11. Effect of the operating temperature on pervaporation performance of blend membranes prepared by the solution-blend method (a-25, a-75) and the solid-blend method (b-75) in dehydrating 10 wt% H2 O–isopropanol. (Data were recorded and averaged after 10 h operating in PV process.)

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Nomenclature PEC PV m

v

Fig. 12. Arrhenius plots (ln J vs. 1000/T) of b-75, a-75 and a-25 membranes for dehydrating 10 wt% water–isopropanol.

up to 60 ◦ C for the blend membrane prepared by the solid-blend method.

[] r sp c kH kK Md Me J g t S ESD FH2 O , FIP PH2 O , PIP

polyelectrolyte complex pervaporation weight of solute (g) volume of solution (mL) intrinsic viscosity (mL/g) relative viscosity specific viscosity concentration of solute (g/mL) Huggins’ constant Kraemer’s constant mass of the membrane mass of the membrane at the swelling equilibrium permeation flux (g/m2 h) permeate weight (g) operation time (h) effective membrane area (m2 ) swelling degree at equilibrium mass fractions of water and isopropanol in the feed mass fractions of water and isopropanol in permeate

Greek symbols ˛ separation factor

4. Conclusions Blend liquids and their blend membranes of PDDA–PAANa PEC (ICD = 0.3) and PVA (containing 30 wt%) were prepared by the PVAsolid/PEC-solution blend method and PVA-solution/PEC-solution blend method. Viscosity parameters of [], kH and kK and the particle size and size distribution show that PEC/PVA blend liquids display a transition gradually from a typical polymer solution to a typical suspension with the preparation method from the solid-blend to solution-blend. AFM and FESEM show that the RMS surface roughness and surface morphology are quite different for the blend membranes prepared by the solid-blend method and solution-blend method. Pervaporation results show good stability and high pervaporation performance of the blend membrane prepared by the solid-blend method, compared to blend membranes prepared by the solution-blend method. The best performances of blend membrane prepared by the solid-blend method for dehydrating 10 wt% water–isopropanol at 70 ◦ C were obtained with the permeation flux, J = 2.36 kg/m2 h and the separation factor ˛ = 978. All differences in the structure–property of blend liquids, the surface morphology and PV performance of blend membranes are attributed to the preparation method which influences the chain size and chain shape of PVA and the aggregation of PEC in the blend liquids, and further changes the compatibility or the interaction between PEC and PVA in blend membranes. PEC/PVA blend by using the solid-blend method is a promising strategy for high pervaporation performance, good film-forming property and reducing cost of PEC membranes. Acknowledgements 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). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.memsci.2010.05.058.

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