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J. Phys. Chem. B 2010, 114, 198–206

Cross-Linked Poly(vinyl alcohol)-Poly(acrylonitrile-co-2-dimethylamino ethylmethacrylate) Based Anion-Exchange Membranes in Aqueous Media Mahendra Kumar, Shalini Singh, and Vinod K. Shahi* Electro-Membrane Processes DiVision, Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, BhaVnagar-364002 (Gujarat), India ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009

Hydroxide anion conducting polymer membranes also termed as anion exchange membranes (AEMs) are recently becoming important materials for electrochemical technology, alkaline fuel cells, and electrolyzers. In this work, the preparation procedure for AEMs based on poly(vinyl alcohol) (PVA) and copolymer of poly(acrylonitrile (PAN)-dimethylamino ethylmethacrylate) (DMAEMA) with strongly basic quaternary ammonium in aqueous media has been reported. This simplified procedure avoids the use of chloromethyl methyl ether (CME), a carcinogen that is harmful to human health, generally used for chloromethylation during AEM preparation. Developed AEMs were extensively characterized by studying physicochemical and electrochemical properties, to assess their suitability for electrodialytic ion separation. These membranes were designed to possess all the required properties of a highly anion conductive membrane such as reasonable water uptake, good ion-exchange capacity (1.18 mequiv g-1), high permselectivity (0.90), along with reasonable conductivity (3.45 mS cm-1) due to quaternary ammonium group functionality. The membrane conductivity values in conjunction with solution conductivity have been used for the estimation of the isoconductivity point, considering the membrane as a combination of the gel phase and integral phase. Electroosmotic studies revealed quite low mass drag and equivalent pore radius (2.7-4.0 Å) of the membrane, which are also desirable properties of an AEM. The excellent electrotransport property of AEM-70 for practical anion separation was concluded from i-v studies. Electrodialytic performance of the AEM-70 membrane revealed its suitability for applications in electromembrane processes. 1. Introduction Traditionally, ion-exchange membranes (IEMs) are classified into anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) depending on the type of ionic groups attached to the membrane matrix.1,2 Several types of polymers, such as poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), copolymer of chloromethylstyrene and divinylbenzene, PVDF-vinylbenzyl chloride, and poly(vinyl alcohol)-poly(1,3-diethyl-1-1vinyl imidazolium bromide), have been used for the preparation of anion-exchange membranes.3-7Also, various AEMs were prepared via chloromethylation of polysulfone, poly(etherimide), and Cardo poly(ether sulfone) based polymer using chloromethyl ether (CME) followed by amination with a tertiary amine.1,8-11 Preparation of these AEMs experienced more complicated procedures: chloromethylation and quaternary amination. Particularly, in the chloromethylation, the used CME is carcinogenic and potentially harmful to human health.3,12 To avoid the use of CME, Sata et al.5,13 reported AEMs based on the copolymer of chloromethylstyrene and divinylbenzene with different amines, such as trimethylamine (TMA) and 4-vinylpyridine. Chloromethylstyrene is very expensive, which dramatically increases the manufacturing costs of the membrane, and subsequent chloromethylation and amination causes the same problem as mentioned above. Furthermore, the above-mentioned AEM preparation methods involve the use of expensive, hazardous chemicals and are based on the solution casting method in suitable solvent. No report is available for AEM * Corresponding author. Tel.: +91-278-2569445. Fax: +91-278-2567562 /2566970. E-mail: [email protected]; [email protected].

preparation in the aqueous media followed by cross-linking to achieve better membrane stability. To avoid the use of CME and simplify the AEM preparation procedure, herein we are reporting a new type of AEM based on PVA and the copolymer of poly(acrylonitrile dimethylamino ethylmethacrylate (PAN-DMAEMA) in aqueous media. In this investigation, PVA was used due to its low cost, water-solubility, and film forming nature, along with hydrophilic properties and high chemical reactive functions favorable for chemical crosslinking.4 Polyacrylonitrile (PAN) is a well-known membrane forming material due to its excellent chemical, thermal, and mechanical stability. Poly(dimethylamino ethylmethacrylate) (PDMAEMA), hydrophilic in nature, contains tertiary amine, essential for introducing strongly basic quaternary ammonium groups.14,15 Cross-linked thin films were quaternized with methyl iodide in methanol at ambient temperature. Developed AEMs were characterized for their physicochemical, electrochemical, and electrodialytic properties to assess their suitability for diversified electromembrane processes. 2. Experimental Section 2.1. Materials. 2-(Dimethylamino)ethylmethacrylate (DMAEMA), azobisisobutyronitrile (AIBN), and acrylonitrile were obtained from Sigma-Aldrich Chemicals and used as received. Poly(vinyl alcohol) (PVA; MW: 125 000, degree of polymerization, 1700; degree of hydrolysis, 88%), methyl iodide, methanol, formaldehyde, Na2SO4, H2SO4, HCl, NaOH, and NaCl etc., of AR grade, were obtained from SD fine chemicals and used without any further purification. In all experiments, double distilled water was used.

10.1021/jp9082079  2010 American Chemical Society Published on Web 11/25/2009

Cross-Linked PVA-PAN-DMAEMA Based AEMs 2.2. Synthesis of PAN-DMAEMA Copolymers and Membrane Preparation. The PAN-co-2-DMAEMA copolymer was synthesized by free radical polymerization. In a typical synthesis procedure, 50 mL of distilled water, 15 g of acrylonitrile, and DMAEMA with different wt % (40-70%) with respect to acrylonitrile were taken into a three-necked round-bottom flask. The flask was then placed into a thermostatic water bath at 60 °C, under a constant flow of nitrogen during the copolymerization procedure. After purging of nitrogen by bubbling, 0.2 g of AIBN was added into the reaction mixture as an initiator. The resulting reaction mixture was heated at 70 °C for 8 h to complete the polymerization process, and thus a clear viscous solution of PAN-co-2-DMAEMA copolymer was obtained. The PVA-PAN-co-2-DMAEMA based AEMs were prepared in three steps: (i) sol-gel process of PAN-co-2-DMAEMA copolymer and PVA; (ii) chemical cross-linking of membrane; (iii) quaternization of -N(CH3)2 into -N+(CH3)3I-. In the first step, PVA (10 wt %) solution was prepared in deionized water at 70 °C under constant stirring. The aqueous solution of synthesized PAN-co-2-DMAEMA copolymer with different content of DMAEMA was mixed with the PVA solution and stirred at room temperature for 24 h to get the cream color gel. The resulting gel was transformed into thin film and dried at ambient temperature for 24 h under an IR lamp followed by drying at 60 °C for another 12 h. Thus obtained membranes were subjected to chemical cross-linking in the second step, in which membranes were refluxed at 60 °C in (HCHO + H2SO4) solution for 2 h. Thus produced membranes were placed in 10% CH3I in methanol at room temperature to quaternize the -N(CH3)2 into -N+(CH3)3I-. The final quarternized membranes were conditioned with water, 1.0 M solution of acid, and base to eliminate any type of impurities present in the matrix. Finally, the membranes were placed into deionized water.16 Thus obtained membranes were designated as AEM-X, where X refers wt % of DMAEMA (varied between 40-70%). 2.3. Membrane Characterization. 2.3.1. FT-IR, CHNS, and SEM Studies. FT-IR spectra of dried membranes were recorded using the ATR technique with a spectrum GX series 49387 spectrometer in the range of 4000-600 cm-1. The elemental analysis was carried out using a Perkin-Elmer-2400 CHNS analyzer. The surface morphology of thoroughly dried membranes was studied using a Leo scanning electron microscope (SEM) after gold sputter coatings on desired samples. 2.3.2. TGA and DSC Analysis. Thermal degradation and stability of the membranes were investigated using a thermogravimetric analyzer (TGA) (Mettler Toledo TGA/SDTA851c with starc software) under a nitrogen atmosphere at a heating rate of 10 °C/min from 50 to 600 °C. Differential scanning calorimetry (DSC) measurements for assessing the glass transition behavior were carried out in a temperature range of 30-400 °C with a heating rate of 5 °C/min. 2.3.3. Water Uptake and Ion-Exchange Capacity (IEC) Measurements. For the determination of volume fraction of water in the membrane matrix, water uptake was evaluated by weight difference of wet and complete dry membranes. Membrane was immersed in distilled water for 24 h, and wet weight was recorded after the removal of surface water by absorbing paper. After drying (under vacuum at 60 °C for 10 h), the weight of the dry membrane (wd) was recorded until a constant weight was obtained. The thickness of wet and dry membranes was determined by using a digital micrometer up to an accuracy of 0.1 µm. The volume fraction of water in the membrane matrix (φw) of the membranes was determined by the following equation17

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φw )

∆w dw wd ∆w + dw dp

(1)

where ∆w is the weight difference between wet and dry membrane and dw and dp are the densities of water and dry membrane, respectively. The ion-exchange capacity (IEC), defined as the ratio between number of exchangeable ionic groups (equivalents) and the weight of dry membrane, was determined by the classical titration method as reported previously.17 Pieces of membrane samples with known dry weight were equilibrated in 1.0 M NaOH solution for converting all ionic sites of the matrix into OH- form. The membranes were then thoroughly washed with double distilled water to remove the last trace of base. Then they were equilibrated in 50 mL of 0.10 M NaCl solution for 24 h to replace the OH- by Cl- ions. Equilibrated solution was titrated against 0.10 M HCl using phenolphthalein as an indicator. 2.3.4. Membrane PermselectiWity and ConductiWity Measurements. Membrane permselectivity was estimated from counterion transport number in the membrane phase obtained from membrane potential measurements. A two-compartment cell, separated by a circular piece of membrane (effective area of 7.0 cm2), was used to measure membrane potential, as reported previously.18 Both the compartments were vigorously stirred by a magnetic stirrer to minimize the effect of boundary layer condition. The potential aroused across the membrane, in equilibration with NaCl solution of unequal concentration, was recorded with the help of a multimeter using saturated calomel electrodes and a salt bridge. Counterion transport number in the membrane phase was estimated by the TMS (Teorell, Meyer, and Sievers) approach.18 Details for the determination of counterion transport number and membrane permselectivity are included in the Supporting Information (S3). Membrane conductivity measurements for the AEMs equilibrated in NaCl solution of different concentrations were carried out using a potentiostat/galvanostat frequency response analyzer (Auto Lab, model PGSTAT 30). The membranes were sandwiched between two in-house made stainless steel circular electrodes (4.0 cm2). Direct current (dc) and sinusoidal alternating currents (ac) were supplied to the respective electrodes for recording the frequency at a scanning rate of 1 µA/s within a frequency range of 106-1 Hz. The membrane resistance was determined from Nyquist plots (Figure S1, Supporting Information) using the Fit and Simulation method.19 2.3.5. Electroosmotic Permeability Measurements. Electroosmotic permeability for different AEMs was measured in a two-compartment membrane cell with an effective membrane area of 20.0 cm2, in equilibration of 0.01 M NaCl solutions.18 Both the compartments were kept under constant agitation by means of a mechanical stirrer. A known potential was applied across the membrane using Ag/AgCl electrodes, and subsequently volume flux was measured by observing the movement of liquid in a horizontally fixed capillary tube of known radius. The current flowing through the system was also recorded with the help of a digital multimeter. Several experiments were performed to obtain reproducible values. 2.3.6. Electrodialytic Performance of AEM. Ion separation efficiency of the developed AEMs was assessed by electrodialysis (ED) by using an in-house prepared ED cell.20 Cationexchange membranes (CEM) prepared by sulfonation of the interpolymer of polyethylene and the styrene-divinylbenzene

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SCHEME 1: Schematic Reaction Route for the Preparation of PAN-DMAEMA

copolymer were used in this investigation (method of preparation, and physicochemical and electrochemical properties are included in the Supporting Information).21 Two pieces of CEM and one piece of prepared AEM (effective area: 8.0 × 10-3 m2) were used to separate the four compartments: catholyte, anolyte, diluted compartment (DC), and concentrated compartment (CC). The schematic diagram of the ED cell is presented in Figure S2 (Supporting Information). The parallel-cum-series flow arrangement was used to monitor each flow stream in its respective compartment. Na2SO4 solution (0.02 M) was recirculated in both electrode wash (EW) compartments, separately. Initially, NaCl solution of known concentration (0.2 M) and volume was fed into the DC, while distilled water was fed through the CC. Peristaltic pumps were used to feed the solutions (500 cm3) in a recirculation mode into the respective compartments with constant flow rate (0.006 m3/h) to maintain the turbulence. Precious metal oxide coated titanium sheets (TiO2 sheet coated with a triple precious metal oxide (titaniumruthenium-platinum), of 6.0 µm thickness, and 8.0 × 10-3 m2 effective area) obtained from Titanium Tantalum Products (TITAN, Chennai, India) were used as the cathode and anode. A DC power supply (Aplab India, model L1285) was used to apply constant potential across the electrodes, and the resulting current variation was recorded as a function of time using a digital multimeter in series. The whole setup was placed at ambient condition (303 K) without any additional temperature control. Changes in conductivities and pH of DC and CC output were regularly monitored by placing the conductivity and pH electrodes in the respective containers during all the experiments. 3. Results and Discussion 3.1. Synthesis of Copolymer and Membrane Preparation. Random PAN-co-2-DMAEMA copolymer was synthesized via the water phase suspension polymerization method. Membrane forming material was prepared by blending the synthesized copolymer with PVA in aqueous media. Material thus obtained was transformed into a thin film on a cleaned glass plate under an IR lamp at ambient temperature and further at 60 °C for 8.0 h in a vacuum oven. Thus obtained water-soluble transparent thin films were cross-linked by HCHO in the presence of acid for 2 h under hot conditions (60 °C). The cross-linking process with formaldehyde is a two-step process, in which formaldehyde reacts with hydroxyl groups of PVA and formed hemiacetal in the first step. This hemiacetal undergoes further reaction with another hydroxyl group and resulted in the acetal formation in the second step. Further, the cross-linked membranes were subjected to quarternization of the -N(CH3)2 into the -N+(CH3)3I- group by CH3I in methanol at ambient temperature.16 Schemes 1 and 2 show the possible reaction route for the synthesis of copolymer and the preparation of membranes. 3.2. FTIR, Elemental Analysis, and Surface Morphology. FTIR spectra of uncross-linked, cross-linked, and quaternized membranes are presented in Figure 1. All three membranes show

Kumar et al. SCHEME 2: Schematic Reaction Route for the Preparation of Anion Exchange Membrane (AEM)

an absorption band at ∼3363-3316 cm-1, due to -OH stretching vibration. The absorption bands at 2922-2949 cm-1 aroused due to -CH2 stretching vibration, while the weak absorption band at 1439-1455 cm-1 corresponded to -OCH2 deformation and wagging vibration of acrylate-based membranes. Peaks corresponding to 1691-1731 cm-1 were due to the >CdO stretching vibration of the acrylate unit. The absorption bands at around 1200 cm-1 were the characteristic bands of -C-O-C- stretching vibration in cross-linked membranes for alicyclic ether ring, which was absent in the uncross-linked membrane. This confirmed that better compatibility between DMAEMA and poly(vinyl alcohol) was achieved by acylation reaction (hemiacetal and acetal formation).22 The bands at 1040-1018 cm-1 were aroused due to the -C-Ostretching of the alicyclic ring. Peaks in the region of 1650-1550, 2246, and 3000-2800 cm-1 indicate the presence of a quaternary ammonium group, and peaks at 3400-3300 cm-1 indicated the quaternary ammonium salt.23 Hence, it was confirmed that

Figure 1. FTIR spectra of anion exchange membranes (AEM): (A) uncross-linked; (B) cross-linked; and (C) quaternized with CH3I.

Cross-Linked PVA-PAN-DMAEMA Based AEMs

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TABLE 1: Membrane Thickness, Elemental Analysis, Volume Fraction of Water in the Membrane Matrix (φw), Ion-Exchange Capacity (IEC), and Surface Charge Concentration (χm) Values for Different AEMs

membrane

thickness (µm)

AEM-40 AEM-50 AEM-60 AEM-70

199 195 196 196

elemental analysis C

H

N

φw

56.07 55.97 52.21 51.50

6.05 7.30 7.72 8.40

4.02 5.75 6.44 8.48

0.220 0.310 0.343 0.362

χm IEC (mmol (meq g-1) dm-3) 0.78 1.02 1.07 1.18

0.28 0.35 0.62 0.99

the quaternization of -N(CH3)2 groups was successfully achieved with methyl iodide in methanol at ambient temperature. The elemental analyses of the developed membranes with varied content of DMAEMA were recorded by CHNS analysis, to estimate nitrogen content in the membrane matrix. Data presented in Table 1 indicate that the N content increased with the increase of DMAEMA content in the membrane matrix. This observation further proved copolymerization of PAN and DMAEMA. Representative SEMs for surface of the AEM-70 membrane: (A) cross-linked and (B) quaternized are presented in Figure S3 (Supporting Information). No cracks, holes, and phase separation were observed, and the membrane seems to be dense and homogeneous in nature. 3.3. Thermal Analysis. The TGA curves of prepared AEMs are depicted in Figure S4 (Supporting Information). All thermograms showed two-step weight loss character. In the first step, the membrane lost water absorbed in the bulk matrix. In the second step, degradation of the quaternary amine group and the membrane matrix occurred.24 Membranes retained about 90% of its weight up to 200 °C. In the first step, the rate of water loss decreased with an increase in PAN-DMAEMA content in the membrane matrix, which may be due to the presence of a higher fraction of water. Beyond 200 °C, rapid weight loss was observed up to 350 °C, due to decomposition of the membrane matrix. Finally, the AEM-70 membrane leaves a maximum char which was about 30%, while others showed a lower value of char (∼25%) of initial weight, respectively. The DSC thermograms of representative membranes in the N2 environment with varied DMAEMA content are presented in Figure S5 (Supporting Information). The first endothermic peak (Tg1) of the membranes corresponded to the values of 86.2, 94, 91.5, and 127.2 °C, which evidenced that Tg is dependent on the DMAEMA content. This observation may be explained due to the increased compatibility, chemical cross-linking of the matrix, and ionic interactions among the functional groups. 3.4. Water Uptake, IEC, and Membrane Permselectivity. The water uptake property of the membranes was evaluated, and the results are presented in Table 1. Data revealed that φw values increased with the DMAEMA content in the membrane matrix. In general, membranes with the same degree of crosslinking and composition absorb the same amount of water, where the density of ionizable groups is the same throughout the membrane matrix.25,26 Observed variation in φw may be explained because of the increased hydrophilic nature of the membrane matrix or ionic group concentration with increasing DMAEMA content. During membrane preparation, phase separation was observed at higher DMAEMA content (beyond 70%), thus incorporation of more than 70% DMAEMA in the membrane matrix is difficult. IEC is an important parameter to assess the ionic charge nature of the membrane in terms of equivalent of ionic functional groups present in the per unit dry weight. IEC also reveals

TABLE 2: Membrane Counterion Transport Number (tm - ), Membrane Permselectivity (Ps), Iso-Concentration (Ciso), m Iso-Conductance (Kiso ), Fraction of Integral Phase (f1), Fraction of the Inter Gel Phase in the Membrane Matrix (f2), and Counterion Diffusion Coefficient in the Membrane Phase (Dim) Values for Different AEMs

membrane

m t-

Ps

AEM-40 AEM-50 AEM-60 AEM-70

0.83 0.85 0.91 0.94

0.72 0.76 0.85 0.90

m Ciso × κiso × 10-3 10-3 (M) (S cm-1)

3.36 3.82 6.02 8.60

0.34 0.40 0.61 0.85

f2

f1

Dim × 10-7 (cm2 s-1)

0.636 0.666 0.719 0.726

0.364 0.334 0.281 0.274

3.19 3.98 4.05 4.30

information about membrane conductivity. The IEC values for all the prepared AEMs (included in Table 1) were found to be 0.78-1.18 meq g-1. IEC values increased with the DMAEMA content in the membrane matrix, which may be due to the increased concentration of available ionic sites. Here it is important to note that tertiary amine groups of DMAEMA were quaternized for anion exchange functional groups.21,25 IEC values along with the water uptake can also be used for the determination of net surface charge density (χm) of the membranes in units of (moles of sites)/(unit volume of wet membrane) (details are given in the S2 section of Supporting Information). The χm values were also found to increase with DMAEMA content in the membrane matrix (Table 1). This further evidenced the increase in ionizable functional groups with increasing content of DMAEMA in the membrane matrix. The permselectivity (Ps) is a measure of the characteristic difference in the membrane permeability for counterions and co-ions. Counterion transport number (tm-) across the membranes was estimated by membrane potential measurement using the TMS approach (the detailed method is given in the S3 section of Supporting Information).18 The tm - and Ps values for different AEMs are also presented in Table 2. Examination of the results clearly shows that a rapid change in the membrane permselectivity was observed, when varying the DMAEMA content from 40 to 75%. With low content of DMAEMA, low Donnan exclusion due to lower surface charge density may be the reason for the observed low permselectivity (0.72). The permselectivity arises due to the nature of the membrane for discrimination between counterions and co-ions. This type of discrimination arises because of the nature and magnitude of the surface charge density that the membrane matrix carries. At high DMAEMA content (70 wt %), relatively high surface charge density on the membrane matrix is responsible for higher membrane permselectivity. Furthermore, for the AEM-70 membrane, about 0.90 permselectivity is comparable for good AEM reported until date.27 3.5. Membrane Conductivity. Membrane resistance (Rm) was measured in equilibration with NaCl solutions (0.01-0.1 M), and membrane conductivity (κm) was estimated by the following eq28,29

κm )

∆x ARm

(2)

where ∆x is the thickness of the wet membrane and A is the effective membrane area. Figure 2(A) shows (κm at different concentration of equilibrating NaCl solution) that membrane conductivity initially increases with concentration and then reaches a limiting point at higher electrolyte concentration. At low external concentration, conductance of the pore solution was higher than the external solution. As the concentration of

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external solution is increased, conductance increases rapidly first and then slowly, until the concentration of the two, external and pore solution, becomes identical.30 As expected, membrane conductivity was also increased with the DMAEMA content in the membrane matrix (Figure 2(B)). This observation can be attributed to: (i) an increase in IEC; (ii) an increase in the water uptake of the membrane matrix; and (iii) an increase in the hydrophilic charged functional groups. It seems that all three factors were responsible for the observed variation of membrane conductivity with DMAEMA content in the membrane matrix. Effect of pH on membrane properties such as membrane conductivity and water content for AEM-70 was also investigated in the pH range 1-12 and presented in Figure 3. It was observed that the membrane showed good properties and stability in equilibration with higher pH (pH > 7). Thus developed membranes are suitable for practical application in the higher pH range. According to the model proposed by Gnusin et al.31 and developed by Zabolosky and Nikonenko,32 an ion-exchange membrane may be considered as a combination of gel phase and integral phase with volume fractions f1 and f2, respectively, where f1 + f2 ) 1. The gel phase represents electroneutral nanoporous medium, consisting of fixed and mobile ions, water, and polymer matrix, while the integral phase represents the inner parts of meso- and microspores as well as voids and cavities.33 The microheterogeneous structure of the membrane matrix is mainly responsible for the concentration-dependent properties such as conductivity, diffusion permeability, and counterion transport number.34 The conductive nature of an ion-exchange membrane may be described by the following equation by taking into account their microheterogeneity29

Kumar et al.

log κm ) f1 log κiso + f2 log κs

(3)

where κiso is the conductivity at the isoconducting point (κm ) κiso), and κs is the solution conductivity. The intercept of the curves drawn in κm-C (solid line) and κ-C (dotted line) (Figure m and iso-concentration 2(A)) was used for the estimation of κiso (Ciso). κmiso and Ciso values increased with the DMAEMA content in the membrane matrix. These values predict the concentration range over which the membranes will be most efficient in the various electrochemical processes. The values of f2 and f1 were calculated from the log κm vs log κs linear plots and are presented in Table 2. The higher f2 values imply the higher fraction of the nonconducting region in the membrane matrix or membrane conductivity. With the increase in volume fraction of the integral phase (f1), Donnan exclusion becomes less effective, due to deterioration in 34 In counterion transport number in the membrane phase (tm -). this case, f1 and f2 values revealed that the volume fraction of conducting gel phase increased, while volume fraction of the integral phase decreased with the increase in DMAEMA content in the membrane matrix. The AEM-70 membrane exhibited high conductivity and permselectivity due to high volume fraction of the conducting integral phase. In Table 3, membrane conductivity for AEM-70 was compared with different AEM either commercially available or earlier reported in the literature, under similar experimental conditions.35-39 Generally, different AEMs recommended for diversified electromembrane processes showed 0.32-2.50 mS cm-1 membrane conductivity, while 3.45 mS cm-1 membrane conductivity of AEM-70 revealed its potentiality for different electromembrane applications.

Figure 2. Variation in membrane conductivity: (A) in equilibration of NaCl solutions of different concentrations and (B) with DMAEMA content in the membrane matrix in equilibration of 0.01 M NaCl solution.

Figure 3. Effect of pH on membrane properties: (A) membrane conductivity and (B) water content for AEM-70 in equilibration with 0.05 M NaCl solution.

Cross-Linked PVA-PAN-DMAEMA Based AEMs

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TABLE 3: Comparison of Reported Membrane Conductivity for AEMs in Equilibration with 0.05 M NaCl Solution

membrane

membrane conductivity (mS cm-1)

reference

AEM ACSb ACMb AEMc AEM/PVDF/SiO2-2.0d AEM-70

0.32 2.50 1.00 1.50 1.50 3.45

ref 35 refs 36 and 37 refs 36 and 37 ref 38 ref 39 this work

a

a Measured in equilibration with 0.5 M NaCl solution (chloromethylated styrene-divinylbenzene copolymer membranes). b Anion-exchange membrane obtained from Tokuyama Soda Co. Ltd. c Measured in equilibration with 0.1 M HCl solution. d Organic-inorganic anion-exchange membrane.

The counterion diffusion coefficient in the membrane phase (Dm i ) presented in the Table 2 was determined using the following equation18,32

Figure 4. Variation of equivalent pore radius (r) with DMAEMA content in the membrane phase in equilibration with 0.01 M NaCl solution.

m

Dim )

RT κiso F2 Q

(4)

where R is the gas constant; T is the temperature; F is the Faraday constant; and Q is the ion exchange capacity of the integral phase, estimated by: Q ) IEC/f1. Dm i values presented in Table 2 also support the observations obtained from membrane permselectivity, conductivity, and ion-exchange capacity studies. Here, we can conclude that counterion diffusion across the ion-exchange membrane depends on its electrochemical properties and electroneutral nanoporous medium consisting of fixed and mobile charges and water (membrane void volume). 3.6. Electro-osmotic Permeability. Study on electro-osmotic transport of mass (solvent) through the ion-exchange membrane is essential to obtain a better understanding of the so-called crossover or electro-osmotic drag process. Additionally, knowledge of transport rates of ions and solvents across the membranes under an applied electric gradient is necessary for intelligent designs of process and membrane.40,41 Electro-osmotic flux across the ion exchange membranes was aroused due to: (i) the presence of charged sites on the membrane matrix and (ii) the existence of an electrical potential at the membranesolution interface called zeta potential.42-44 The electro-osmotic permeability across prepared AEMs in equilibration with NaCl solutions (0.01 M) was measured, and flux was plotted as a function of the applied current in Figure S6 (Supporting Information). In all the cases, a straight line was obtained. Equivalent pore radii (r) of the membranes were estimated from their electroosmotic permeability values using the Katchalsky and Curran approach44 with the help of the following equation

r)

( ) 8ηFβ 0 f 1w

1/2

(5)

where F is the Faraday constant; β implies that every coulomb of electricity will exert a drag effect sufficient to carry β cm3 of water through 1 cm2 of the membrane; η denotes the 0 is the frictional coefficient of viscosity of the permeate; and f1w coefficient between counterion and water in free solution, which 0 ) RT/Di (where Di is the diffusion can be defined as f1w coefficient of the single ion i in the free solution, R is the gas constant, and T is the absolute temperature). The ionic diffusion coefficient (Di) at a given electrolyte concentration was obtained from ionic conductance data.45 Equivalent pore

Figure 5. Current-voltage (i-V) curves for different AEMs in equilibration with 0.1 M NaCl solution.

radius (r) values for different AEMs with varying DMAEMA contents are presented in Figure 4. r values increased with the increase in DMAEMA content in the membrane phase. The measured equivalent pore radius (2.75-4.08 Å) of these membranes suggested their quite dense nature. Incorporation of DMAEMA and functionalization of the membrane matrix lead to an increase of surface charge density and consequently water uptake (Table 1), while simultaneously increasing the equivalent pore radius. The increase in pore radius leads to an increase in mass transfer (solvent) by electro-osmotic drag across the membrane. Further, AEM with the highest DMAEMA content (AEM-70) exhibited high IEC (1.18 meq/g), membrane conductivity (2.53 mS cm-1), and permselectivity (0.90), which enables its applications for diversified electromembrane processes in spite of its slight high electro-osmotic drag. Also, the equivalent pore radius of these membranes suggested their quite dense nature. 3.7. Electrodialysis Performance of Membranes. 3.7.1. Current-Voltage (i-W) Studies. i-V curves for different membranes were studied in equilibration with NaCl solution (0.1 M) and depicted in Figure 5. These curves showed three typical characteristic regions, viz., Ohmic, non-Ohmic, and plateau length regions (defined in Figure S7 of Supporting Information), which reveal the information on the ion transport and concentration polarization phenomenon under the operating conditions across the ion-exchange membrane.46 i-V curves were analyzed to derive the characteristic values (∆V, ∆i, and

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Kumar et al.

TABLE 4: Characteristic Values of i-W Curves for Different AEMs in Equilibration with 0.1 M NaCl Solutions membranes

∆V (V)

∆i (mA cm-2)

Ilim (mA cm-2)

AEM-40 AEM-50 AEM-60 AEM-70

3.40 4.05 6.98 8.58

4.38 5.43 6.40 7.22

5.20 5.90 7.42 8.70

Ilim) reported in Table 4. All the derived properties increased rapidly with the increase in DMAEMA content in the membrane matrix. Also, these values are much higher than those for the pristine membrane. Relatively higher ∆V and ∆i values for membranes with high fixed charge concentration (DMAEMA content) indicate their high electrotransport of counterions from the diffusive boundary layer to the membrane matrix and thus results in higher current for concentration polarization (Ilim). The transport of counterions through an ion-exchange membrane leads to the difference in counterion concentration on the two membrane surfaces, which in turn creates concentration polars ization. Since, tm i > ti (counterion transport number in solution), their flux in the membrane (Jm i ) is always larger than in the m m boundary layer because tm i is proportional to the Ji (Ji ) 47-49 I/F z ). The limiting current corresponding to concentration tm i i polarization may be defined as Ilim, which can be estimated by

Ilim )

ziDiF(Cb - Cm) δ(tim - tis)

Figure 6. Variation in current density with time at 5.0 V constant applied potential during ED (feed of DC: 0.2 M NaCl solution).

(6)

where zi is the electrovalence of the counterion; Di is the diffusion coefficient of counterion in solution; F is the Faraday constant; Cb is bulk concentration of electrolyte solution; Cm is the counterion concentration at the membrane-solution interface; and δ is the thickness of the boundary layer. For membranes with fixed charge concentration, Ilim was low due to depletion of counterion from the membrane-solution interface at low applied current density. Thus prepared AEMs, especially the AEM-70 membrane, showed excellent electrotransport properties and may be used for the practical separation of anions. Furthermore, studies on i-V curves are quite useful for the prediction of suitability of given AEM for achieving desired separation. 3.7.2. ED Performance of Different Composition Membranes. ED experiments (schematic diagram depicted in Figure S2 of Supporting Information) were performed to assess the suitability of the developed AEMs for the electrodialytic separation of salt (NaCl). Experiments were performed at 4.0 V constant applied voltage (below the limiting current density (8.70 mA cm-2)) using NaCl solution (0.2 M) as initial feed of DC, while double distilled water was initially fed through CC and Na2SO4 solution (0.02 M) through both electrode wash compartments, in the recirculation mode of operation. The suitability of developed AEMs for electromembrane applications was assessed in terms of estimating current efficiency and energy consumption for the salt removal experimental conditions. Variations of current (at 4.0 V constant applied voltages) with time during the ED experiment for different membranes are presented in Figure 6. Current initially increases, and after reaching the maxima, it decreased with time. In the beginning of the experiment, the CC offers high electrical resistance because distilled water was used as feed. With the onset of the experiment, ions (Na+ and Cl-) were migrated in opposite direction from DC to CC, which simultaneously reduced the resistance offered by CC. The variation in solution conductivity

Figure 7. Variation of solution conductivity in DC and CC with time at 4.0 V constant applied potential during ED performance of the anion exchange membrane (AEM-70).

Figure 8. Variation of J with electricity passed (Coulombs) for different AEMs at 4.0 V constant applied voltage during ED (feed of DC: 0.2 M NaCl solution).

(for CC and DC, both) presented in Figure 7 for AEM-70 (as a representative case) also supported these findings. Flux for NaCl migration (J) from DC can be obtained by the following relation, considering negligible mass (water) transport through membranes49,50

J)

Va Ct - C0 A ∆t

(7)

where C0 and Ct are the initial and final concentration of NaCl in CC (mol m-3); ∆t is the time allowed for ED (s); Va is the total volume of CC feed (0.50 × 10-3 m3); and A is the effective membrane area (8.0 × 10-3 m2). NaCl fluxes (at 4.0 V constant applied voltage) for the developed AEMs are presented in Figure 8 as a function of electricity passed (Coulombs). In the beginning,

Cross-Linked PVA-PAN-DMAEMA Based AEMs

J. Phys. Chem. B, Vol. 114, No. 1, 2010 205

TABLE 5: Electrodialysis (ED) Performance for AEMs at 4.0 V Applied Potential (Feed of DC: 0.2 M NaCl Solution)

(C)

CE (%)

W (k Whkg-1 of NaCl removed)

5.66 6.42 7.54 7.87

80.14 82.73 89.67 92.64

10.29 9.96 9.19 8.90

Q × 103 membranes AEM-40 AEM-50 AEM-60 AEM-70

flux increased with electricity passed, and after attaining the maxima, it decreased because of progressively lowered NaCl concentration in DC. Ionic flux across the developed AEMs followed the trend: AEM-70 > AEM-60 > AEM-50 > AEM-40. This observation suggested that the rate of electrotransport of ions across the membrane depends on the membrane characteristics (such as fixed charge concentration, IEC, conductivity, and permselectivity) and ED operating conditions. The energy consumption and current efficiency (CE) are important parameters for any electrochemical process to assess the suitability of the membrane for the same. The energy consumption (W, kWh kg-1 of NaCl removed) can be obtained by the following equation50

W(kWhkg-1) )

∫0t VIdt m

(8)

where V is the applied voltage; I is the current; t is the time allowed for the electrochemical process; and m is the weight of salt removed. While the current efficiency (CE), defined as the fraction of Coulombs utilized for the salt removal, may be obtained by

CE(%) )

mnF × 100 MQ

(9)

where F is the Faraday constant; M is the molecular weight of salt; n is the stoichiometric number (n ) 1 in this case); and Q is the electric quantity passed (Coulombs; A s). To evaluate the electrodialytic performance of developed AEMs, energy consumption and CE (%) data, obtained under similar experimental conditions, are presented in Table 5. It was observed that W decreased, while CE increased with the increase in DMAEMA content in the membrane matrix. We also observed that the content of DMAEMA in the membrane matrix leads to the surface functionalization, which was responsible for high membrane permselectivity, conductivity, and fixed charge concentration along with low electro-osmotic drag of mass (solvent) across the membrane, which are the essential electrochemical properties for an efficient AEM. These parameters (W and CE) also depend on the operating conditions of an ED unit. Under optimum operating conditions (after the passage of 7.87 × 103 Coulombs electricity), for the AEM-70 membrane, CE and W were found to be 92.64% and 8.90 kWh kg-1, respectively, which showed the potential of the developed AEM for electrochemical devices. Also, membrane permselectivity, conductivity, and the density of functional groups depend on the DMAEMA content; thus, a definite compromise between the DMAEMA content and the membrane ion-exchange properties is essential to have a highly charged and efficient AEM with the desired properties. 4. Conclusions In this manuscript, we reported an easy method for synthesizing PAN-co-2-DMAEMA copolymer via free radical polymerization. The preparation procedure of PVA-PAN-co-2-

DMAEMA based AEMs occurred in three steps: (i) sol-gel process, (ii) chemical cross-linking, and (iii) quaternization. This is a relatively new method where the tertiary amine group of DMAEMA was converted into quaternary ammonium salt in the preformed polymer film for achieving highly charged AEM. The reported method avoids the use of chloromethyl ether (generally used for chloromethylation to provide the active sites for amination of the polymer matrix) and any solvent (sol-gel preparation in aqueous media). Developed AEMs were characterized for their different physicochemical and electrochemical properties to evaluate their applicability in electromembrane processes. Characterization by TGA and DSC testing revealed an adequate thermal nature, while SEM showed an absence of any cracks, holes, and phase separation in the membrane matrix and their homogeneous dense. These membranes with high water uptake, IEC, conductivity, and permselectivity, with no significant electro-osmotic drag of mass, offer a new dimension for the preparation of AEM, in which the electrochemical properties could be controlled by the DMAEMA content. Among the developed membranes, AEM-70 exhibited higher IEC value (1.18 meq g-1), membrane conductivity (3.45 × 10-3 S cm-1 in equilibration with 0.05 M NaCl), and fixed charge concentration (0.99 mmol dm-3). i-V curve analysis revealed that prepared AEMs (especially AEM-70) showed excellent electrotransport properties and may be used for the practical separation of anions. Electrodialytic studies suggested that the rate of electrotransport of ions across the membrane depends on the DMAEMA content in the membrane matrix and ED operating conditions. We also observed that the content of DMAEMA in the membrane matrix leads to the surface functionalization, which was responsible for high membrane permselectivity, conductivity, and fixed charge concentration along with low electroosmotic drag of mass (solvent) across the membrane, which are the essential electrochemical properties for an efficient AEM. These physicochemical and electrochemical properties represent a promising starting point for creating more highly conducting AEMs. In particular, recent advances in the development of AEMs may encourage the preparation of a variety of anion-exchange materials and new composite membranes for different types of electromembrane applications. Acknowledgment. Financial assistance received from the Department of Science and Technology, New Delhi (Govt. of India), by sponsoring project no. SR/S1/PC/06/2008 is gratefully acknowledged. Instrumental support received from Analytical Science Division, CSMCRI, is also gratefully acknowledged. Supporting Information Available: Figures S1-S7 and parts S1-S3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hibbs, M. R.; Hickner, M. A.; Alam, T. M.; McIntyre, S. K.; Fujimoto, C. H.; Cornelius, C. J. Chem. Mater. 2008, 20, 2566. (2) Wang, L.; Yi, B. L.; Zhang, H. M.; Xing, D. M. J. Phys. Chem. B 2008, 112, 4270. (3) Xu, T.; Liu, Z.; Li, Y.; Yang, W. J. Membr. Sci. 2008, 320, 232. (4) Tang, B.; Wu, P.; Siesler, H. W. J. Phys. Chem. B 2008, 112, 2880. (5) Sata, T.; Yamane, Y.; Matsusaki, K. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 49. (6) Tzanetakis, N.; Varcoe, J.; Slade, R. S.; Scott, K. Electrochem. Commun. 2003, 5, 115. (7) Leburn, L.; Follain, N.; Metayer, M. Electrochim. Acta 2004, 50, 985.

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