Preparation of organic–inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization

Preparation of organic–inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization

Journal of Colloid and Interface Science 287 (2005) 198–206 www.elsevier.com/locate/jcis Preparation of organic–inorganic composite anion-exchange me...

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Journal of Colloid and Interface Science 287 (2005) 198–206 www.elsevier.com/locate/jcis

Preparation of organic–inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization R.K. Nagarale, G.S. Gohil, Vinod K. Shahi ∗ , R. Rangarajan Central Salt & Marine Chemicals Research Institute, Bhavnagar 364002 (Gujarat), India Received 3 November 2004; accepted 24 January 2005 Available online 10 March 2005

Abstract Organic–inorganic composite membranes based on poly(vinyl alcohol)/SiO2 were prepared via an aqueous dispersion polymerization route and anion-exchange groups were introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired content. These membranes were extensively characterized for their surface morphology, thermal stability, water content, and surface-charge properties using SEM, TEM, FTIR, TGA, water uptake, and ion-exchange capacity measurements. Counterion transport numbers across these membranes were estimated from membrane potential data. Membrane conductance measurements were also performed and these data were used for the estimation of values of counterion diffusion coefficients in the membrane phase. Physicochemical and electrochemical properties of these membranes and equivalent pore radius (estimated from electroosmotic flux measurements) were found to be highly dependent on the 4-vinylpyridine (4-VP) content in the membrane phase. It was also observed that for better selectivity and membrane conductivity of anion-exchange membranes complete optimization of the loading of 4-VP in the membrane phase is necessary. Furthermore, among these, membrane with 25% loading with 4-VP exhibited very good selectivity, water content, and ion-exchange capacity along with moderate membrane conductivity, which may be used for their application in electro-driven separation at elevated temperatures or for other electrochemical processes.  2005 Elsevier Inc. All rights reserved. Keywords: Composite membranes; Organic–inorganic composite; Anion-exchange membrane; Membrane conductance; Electroosmosis

1. Introduction Ion-exchange membrane is now widely used in various applied electrochemical devices such as membrane electrolysis, solid polymer electrolyte, and fuel cell storage batteries as well as in the field of separation science, which includes electrodialysis, electro-deionization, etc. [1–4]. For the purpose of these processes under drastic conditions such as high temperatures and strong oxidizing conditions, a more stable ion-exchange membrane should be developed. Perfluorocarbon ion-exchange membranes were successfully applied for different industrial applications, but its high cost is a serious limitation [5]. Several types of polymers * Corresponding author. Fax: +91-278-2567562/2566970.

E-mail addresses: [email protected], [email protected] (V.K. Shahi). 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.01.074

that are being used as homogeneous anion-exchange membranes after chloromethylation and amination with triamine or heterogeneous anion-exchange membrane have been reported [1,6–8]. These include: (i) copolymers of styrene and divinylbenzene [1], (ii) polysulfone [9,10], (iii) styrene and butadiene block copolymers [11], (iv) interpolymers of polyethylene and styrene-divinylbenzene copolymers [12,13], and (v) polyvinyl chloride-glycidyl methacrylatedivinylbenzene-based polymers [14]. During the preparation of these membranes, chloromethyl ether and solvents were used which are hazardous and not ecofriendly in nature. Thus it is desirable to develop an ecofriendly method for the preparation of thermally and dimensionally stable anionexchange membranes in aqueous medium. Organic–inorganic composites offer the possibility for new generation of nanostructured materials with diversified applications such as catalysts [15], electronic or pho-

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tonic devices [16], and sensors [17]. Furthermore, the incorporation of inorganic materials on a nanoscale can enhance the retardancy and mechanical strength of organic polymers [18]. Reports are available, which provide a window for the preparation of organic–inorganic composites using 4-vinylpyridine. Organic–inorganic nanocomposites were used as enzyme bioelectrocatalysts [19]. The synthesis of vinyl polymer–silica nanocomposite material has been also reported in which 4-vinylpyridine was used for the introduction of positively charged functional groups [19]. Preparations of proton-exchange or cation-exchange membranes based on organic–inorganic composite polymers have also been reported [20–22]. But little work has been done on the preparation and application of organic–inorganic composite material in aqueous medium as anion-exchange membrane. It was, therefore, the objective of this work to develop a new type of organic–inorganic composite, viz., thermally stable anion-exchange membranes in aqueous medium by an ecofriendly route. Polyvinyl alcohol–silica composite membranes were prepared and anion-exchange groups were introduced by the chemical grafting of 4-VP in varied compositions. These membranes were characterized by physicochemical and electrochemical studies and their properties were found to be dependent on the content of 4-VP in the membrane matrix.

2. Experimental 2.1. Materials and membranes preparation Polyvinyl alcohol (PVA; MW: 125,000), hydrochloric acid, sodium hydroxide, sodium chloride, ammonium persulfate, and dimethyl sulfate of AR grade were obtained from S. d-fine Chemicals, India. Tetraethylorthosilicate (TEOS), 4-vinylpyridine (4-VP), and divinylbenzene were received from the Aldrich Chemicals and used as obtained. Double-distilled water was used in all experiments. Five grams of PVA was dissolved in 100 ml of hot deionized water under constant stirring to obtain a homogeneous solution. Then the required quantity of the 4-vinylpyridine (monomer), divinylbenzene (4% w/w), and ammonium persulfate (4% w/w) was added and at 70 ◦ C the mixture was kept for 1 h to get the semi-interpenetrating polymer network. Further equivalent amount of the dimethyl sulfate with respect to 4-vinylpyridine was added to quaternize the pyridinium group. The mixture was stirred for 2 h at room temperature to obtain a clear homogeneous solution. Then 2 ml of TEOS was added at room temperature and the mixture was kept under stirred condition for 24 h to get a gel. The resulting gel was cast on a clean glass plate with the desired thickness and dried at room temperature to obtain a film. By varying the amount of the 4-vinylpyridine with respect to PVA, different membranes with 15, 25 28, 33, and 35% loading were prepared. Also, different membranes

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with DVB content ranging between 1 and 10% (w/w) were prepared for the optimization of its content in order to get flexible membranes with good electrochemical properties. Dried membranes were immersed in a solution containing formaldehyde (54.1 g), sodium sulfate (150.0 g), sulfuric acid (125.0 g), and water (470.0 g) for 2 h at 60 ◦ C for effective cross-linking. These membranes were conditioned in a 0.10 M HCl solution and 0.10 M NaOH solutions alternately several times and then equilibrated with the experimental solution before being subjected to physicochemical and electrochemical studies. Different anion-exchange membranes were designated as AEM-X where X is the 4-vinylpyridine content (%) in the membrane phase. 2.2. FTIR studies and CHN analysis The FTIR spectra of different cation-exchange membranes were obtained using spectrum GX series 49387. CHN analysis was carried out with Perkin-Elmer-2400, CHNS/O analyzer. 2.3. Thermal and mechanical strength analysis The degradation process and the thermal stability of the membranes were investigated using thermogravimetric analysis (TGA) (Mettler Toledo TGA/SDTA851 with Star software), under a nitrogen atmosphere using a heating rate of 10 ◦ C/min from 50 to 800 ◦ C. Differential mechanical analysis (DMA) for the mechanical strength of the membranes was carried out using Mettler Toledo DMA 861 with Star software in isothermal condition. 2.4. TEM and SEM studies Surface morphology of the thoroughly dried membrane film was studied by transmission electron microscopy (TEM) (Jeol-1200 EX transmission electron microscope) with a tungsten electron source operating at an accelerating voltage of up to 120 kV. For scanning electron microscopy (SEM), gold sputter coatings were carried out on the desired membrane samples at pressures ranging between 1 and 0.1 Pa. Sample was loaded in the machine, which was operated at 10−2 –10−3 Pa with EHT 15.00 kV with 300 V collector bias using a Leo microscope. SEMs were recorded. 2.5. Water content and ion-exchange capacity (IEC) measurements For the measurement of water content, the membrane samples (3 × 3 cm) were immersed in distilled water for 24 h, their surface was wiped with filter paper, and then wet membrane was weighed. Thickness of wet membrane was determined by means of a digital micrometer and membrane density for wet membrane was determined by dividing the wet membrane weight and volume. Following this, wet membrane was dried at a fixed temperature of 60 ◦ C to

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constant weight. Thickness of dry membrane was also determined in a similar manner and its density was estimated. Water content of the membranes was determined in terms of water concentration in the membrane phase. Ion-exchange capacity was determined by equilibrating the membranes in 1.0 M NaOH solutions for 24 h to ensure that all the charged sites of the membrane were in OH− form. The membrane was then washed with distilled water free of NaOH before equilibration in 0.10 M NaCl for 24 h. Ion-exchange capacity was determined from the increase in basicity by base titration. The total molar number of OH− was obtained and IEC was calculated by dividing this number by the dry membrane weight.

A known potential difference was imposed across the membrane with the help of electronically operated power supply using Ag/AgCl electrodes and ensuing volumetric 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 measured using a digital ammeter in series. Several measurements were performed in order to obtain reproducible values.

2.6. Membrane potential and membrane conductance measurements

Organic–inorganic composite membranes, with molecular or nanometer level dispersion between inorganic and organic polymers by covalent or hydrogen bonding, were prepared using a sol–gel method. Polyvinyl alcohol grafted with the poly(4-vinylpyridine) results in good flexibility and silica of the inorganic part provides better thermal and mechanical stabilities. In the sol–gel process, the reaction of silica precursor and water in the presence of base forms a one-phase solution that goes through a solution-to-gel transition and forms a rigid two-phase system comprising solid silica (SiO2 ) and solvent filled pores. The condensation polymerization reaction of the silica precursor is a bimolecular nucleophilic substitution reaction, preceded by rapid protonation of the OR or OH substituents bonded directly to silicon atom. With time, sufficient numbers of interconnected Si–O–Si bonds are formed in a region and they interact cooperatively to form colloidal particles or a sol. Subsequently, colloidal particles link together to form a three-dimensional network or a gel. The gel was cast on the glass plate and peeled out after drying to get the membrane. The membrane was cross-linked by the formal reaction. The cross-linked membranes after quaternization, were used for further study. Fig. 1 shows the IR spectrum for AEM-25 membrane as a representative. The absorption bands for corresponding aromatic groups were observed near 1651 cm−1 and a C–N stretching band near 1558 cm−1 . The presence of silica gives a strong band around 1000 cm−1 .

Experimental cells used for the measurements of membrane potential [23] had two compartments separated by the membrane in circular shape with 7.0 cm2 area. To minimize the effect of boundary layers on potential, the solutions in both the compartments were vigorously stirred by magnetic stirrers. The potential difference developed across the membrane was recorded with the help of digital multimeter (Systronics, India) using saturated calomel electrodes and salt bridges, which were reproducible up to 0.10 mV. For membrane potential measurements, the ratio of salt concentrations on the higher to lower side (C1 /C2 ) was kept constant at 10.0 while C/Cs = 1.64, where C1 and C2 are the electrolyte concentrations, C = C1 − C2 , and Cs = C1 + C2 /2. Conductance measurements of membranes were carried out separately in NaCl, solutions with concentrations ranging from 0.01 to 0.06 M, using a clip cell as reported earlier [24]. The membranes equilibrated in the experimental solution were sandwiched between both electrodes and secured in place by means of a set of screws. Using potentiostatic two-electrode mode with alternating current (AC) membrane conductance measurements were performed. Membrane conductivity (C m ) was estimated from total cell conductivity (Ccell ) with the membrane equilibrated in desired electrolyte solutions and electrolyte conductivity (Csol ) measured without membrane by the relationship [1/C m = 1/Ccell − 1/Csol ]. The membrane conductance was measured with the help of a digital conductivity meter (Century, Model CC601, conductance range 0 to 200 mS, frequency 1–50 kHz, AC current) up to ±0.01 mS reproducibility.

3. Result and discussion 3.1. Membrane preparation

2.7. Electroosmotic permeability measurements Electroosmotic permeability for different organic–inorganic composite membranes was measured in a two-compartment membrane cell [23] with effective membrane area of 20.0 cm2 , in equilibrium with 0.02 mol dm−3 NaCl solutions. Both compartments were kept in a state of constant agitation by means of magnetic (or mechanical) stirrer.

Fig. 1. FTIR spectra of AEM-25.

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Table 1 Physicochemical properties of the different organic–inorganic composite membranes with 15 µm thickness V a

τa

N

IEC (meq/g)

0.36 0.73 1.02 1.12 1.83

1.396 1.473 1.639 2.271 2.652

0.569 0.980 1.012 1.049 1.057

0.363 0.495 0.503 0.512 0.514

Membranes

CHN analysis data (%) C

H

AEM-15 AEM-25 AEM-28 AEM-33 AEM-35

56.03 55.97 53.42 54.83 53.87

7.64 7.83 7.94 8.23 8.59

a Dimensionless.

Fig. 4. TGA of AEM-25 and AEM-35 membranes.

Fig. 2. TEM micrograph of AEM-25.

(A)

(B)

Fig. 3. SEM images of the composite membranes. (A) AEM-25; (B) AEM-35.

Different anion-exchange membranes with varied contents of 4-vinylpyridine were prepared and they were also analyzed by CHN analysis, to determine the amount of N atom in the membrane matrix. CHN analysis data are presented in Table 1, which shows the increase in the N content in the membrane with the loading of 4-vinylpyridine. 3.2. TEM, SEM, and TGA studies Membrane prepared with condensation polymerization of silica precursors by base catalyst was opaque. A representative TEM for AEM-25 is shown in Fig. 2 for a base-catalyzed membrane before cross-linking in which dispersion of the silica clusters at the nanometer level could be observed. Fig. 3 shows SEMs of the surface of AEM-25 and AEM-35 composite membranes with varied 4-VP content. The effect of 4-VP content on the membrane morphology was clearly observed in these SEMs. With the increase in 4-VP content in the membrane matrix, increase in the membrane poros-

ity was observed despite a small reduction in silica content. Further, from all the SEM views, no phase separation of the membrane surface could be observed, suggesting that the synthesized polymeric films were homogeneous in nature and hence formed dense membrane. The thermal stability of the membranes was illustrated by their TGA studies. The TGA curves measured under flowing nitrogen are presented in Fig. 4 for representative composite membranes AEM-25 and AEM-35. Curves were fitted using two main degradation stages arising from desolvation and thermal oxidation of the polymer matrix. The first weight loss occurred below 100 ◦ C, and was attributed to the loss of absorbed water molecules in the membrane matrix. The second weight loss region (300–400 ◦ C) corresponds to decomposition of main chain of PVA. Furthermore, both membranes resulted in similar types of TGA curves, with a varied amount of absorbed water. Also these membranes were kept in boiling water for a prolong time and no weight loss or dimensional changes were observed. 3.3. Water content and IEC properties Water content in terms of water concentration in the membrane phase was determined by means of the following equation [25], Cwm =

(Wh − Wd )ρm , W h Mw

(1)

where Cwm designates the concentration of water in the membrane, Wh the wet membrane weight, Wd the dry membrane weight, ρm the density of wet membrane, and Mw the molar mass of water (18 g mol−1 ). Variation of water content with the loading of 4-VP is presented in Fig. 5A. It can be observed that an increase in 4-VP content in the membrane phase initially leads to a substantial increase in the water content and after about 25–30% loading, Cwm attained a limiting value. In general, membranes having same degree of cross-linking and composition absorb the same amount of water, where the density of ionizable groups is the same throughout the membrane matrix [26,27]. An increase in hydrophilic species such as ionic group concentration with the

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Fig. 5. Variation of (A) water content concentration (Cw ); (B) membrane fixed concentration (Xm ), with 4-VP content in the membrane matrix.

loading of 4-VP was responsible for the observed increase in Cwm . At quite higher loading, the water content of membranes was very high and further loading of 4-VP resulted in swelling of the membranes. The membrane void porosity τ (volume of free water within membrane per unit volume of wet membrane) can be obtained by following equation [28], V , (2) 1 + V where V designates the volume increase of the membrane upon absorption of the water per unit of dry membrane volume, which may be estimated by using following equation,

τ=

(Wh − Wd )ρd V = , ρw Wd

(3)

where ρd is the density of dry membrane and ρw is the density of water, which enters into the membrane. Membrane porosities (τ ) and increase in volume (V ) values are also presented in Table 1 for different types of composite membranes. From data, it can be seen that membrane porosity and swelling parameters are increasing with the loading of 4-VP in the membrane matrix. Ion-exchange capacity indicates the density of ionizable hydrophilic groups in the membrane matrix, which are responsible for the ionic conductivity in the ion-exchange membrane. The IEC values for the different membranes are also presented in Table 1, which increased with the 4VP content in the membrane matrix. Ion-exchange capacity arises due to the presence of quaternary nitrogen atoms and thus it was proportional to the 4-VP content in the membrane matrix. Also it can be used for the determination of fixedion concentration (X m ) of the membrane in units of (moles of sites)/(unit volume of wet membrane) which is related to the ion-exchange capacity by X m = τ (IEC)ρd /V ,

(4)

where IEC is expressed in equivalents per gram of dry membrane. X m values for different types of membrane are also depicted in Fig. 5B. Initially, the X m value increased slowly up to about 25% loading of 4-VP and beyond this

m ) on the external Fig. 6. Dependence of counterion transport number (t− NaCl concentration for different types of membranes.

it increased rapidly. From these observations it was obvious that up to about 25% loading of 4-VP, membranes behaved as anion-exchange membrane with a lesser degree of swelling and thus water concentration. With the increase in the loading of 4-VP (above 25%), a high degree of membrane swelling and water content were observed, which restricts their dimensional stability and applicability as anionexchange membranes despite the higher ion-exchange capacity. 3.4. Counterion transport number in the membrane phase When electrolyte solutions of unequal concentration are separated by a membrane, an electrical potential difference develops across the membrane due to the tendency of oppositely charged ions to move with different mobilities. The magnitude of the membrane potential depends on the electrical characteristics of the membrane in addition to the nature and concentration of the equilibrating electrolyte solutions [29,30]. Membrane potential (E m ) data were recorded using different composite membranes in NaCl solutions (mean concentration ranging between 0.01 and 0.10 mol dm−3 while the concentration ratio for both sides was kept at 10) and used for the estimation of counterion transport number m ) using the TMS approach [31] in the membrane phase (t− for univalent electrolyte by  m  RT a1 ln , E m = 2t− (5) −1 F a2 where a1 and a2 are activities of electrolyte solutions contacting two surfaces of the membrane, R is the gas constant, T is the absolute temperature, and F is the Faraday m values presented in Fig. 6 at varied mean constant. The t− concentrations of NaCl solution ranging between 0.01 and 0.10 mol dm−3 for different types of membranes. For each m values decreased with mean concentype of membrane t− tration of NaCl solution. At higher equilibrating electrolyte concentration, these membranes exhibited low selectivity for the counterion because of two factors: (i) suppression in the dissociation of functional group at higher electrolyte concentration and/or (ii) reduced Donnan exclusion of co-ions. It

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m with the loading of 4-VP in the membrane matrix at Fig. 7. Variation of t−

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m and (B) κ m with the DVB content in the memFig. 8. Variation of (A) t− brane phase in equilibration with 0.010 mol dm−3 NaCl solution.

different NaCl concentrations.

seems both factors together are responsible for the reduction in membrane selectivity at higher concentrations. Furthermore, these membranes exhibited good anion selectivity for m values with certain composite membranes. Variation of t− the content of 4-VP in the membrane phase is presented in Fig. 7. Initially membrane selectivity increases with the 4VP content in the membrane phase (up to 25%) and with further increases in the loading of 4-VP, a fast decline in the membrane anion selectivity was observed. This deterioration in the membrane selectivity may be attributed to the increase in porosity and swelling properties of the membrane with the loading of 4-VP. Also AEM-25 exhibited very good anion m = 0.92) for the counterion, which is comselectivity (t− parable to the available commercial membranes and may be suitable for the practical application as anion-exchange membranes. 3.5. Optimization of DVB content in the membrane phase In order to optimize the DVB content in the membrane phase, different membranes with varied DVB content and 25% loading of 4-VP were prepared. Counterion transm ) and specific membrane conductivity (κ m ) port number (t− of these membranes were determined in equilibration with 0.010 mol dm−3 NaCl solution. Resulting data are presented m values of these membranes were seen to in Fig. 8. That t− increase with the increase in DVB content in the membrane phase may be because of the formation of quite dense polymeric matrix due the higher degree of cross-linking, which may affect the increase in the Donnan exclusion. Also increase in the cross-linking in the membrane matrix restricted the free movement of the counterion, which may be responsible for the reduction in specific membrane conductivity with DVB content. Furthermore, membranes became brittle with the increase in the DVB content. The mechanical strength of these membranes was studied by differential mechanical analysis and the results are presented in Fig. 9. No deformation was observed for the membranes up to 4% (w/w) DVB content, while clear deformation in the membrane can be seen with 6% (w/w) DVB content. Based on these studies, it was concluded that AEM-25 with 4% (w/w)

Fig. 9. DMA recorded at 50 M Pa and 200 Hz frequency of the AEM-25 membrane with (A) 1%, (B) 4%, and (C) 6% (w/w) DVB content.

DVB content is the best composition for obtaining the best flexible membrane with good electrochemical properties. 3.6. Membrane conductance studies and counterion diffusion coefficient in the membrane phase Membrane conductance was measured for different composite membranes equilibrated with NaCl solutions of concentrations ranging between 0.0002 and 0.0200 mol dm−3 . The variation of specific membrane conductivity κ m with the NaCl concentration is shown in Fig. 10 for different types of composite membranes. The dotted line represents the solution conductivity. As seen in Fig. 10, the κ m value increases with the equilibrating solution concentration and attains a limiting value at higher electrolytic concentrations in all cases. Furthermore, variation of κ m with the loading of 4-VP in the membrane phase is depicted in Fig. 11. Membrane conductivity increased rapidly with the loading of 4-VP because of high ionogenic molality and porosity at a high degree of 4-VP loading. It seems that both these two factors together were responsible for the increase in membrane conductivity. We can have further information regarding counterion diffusion coefficient in the membrane phase by membrane

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Table 2 m ) and Cl− diffusion coefficient Isoconcentration (Ciso ), isoconductance (κiso values (D m − ) in the membrane phase for different membranes Cl

Membranes AEM-15 AEM-25 AEM-28 AEM-33 AEM-35

Ciso × 103 (mol dm−3 )

m × 104 κiso (S cm−1 )

D m − × 108

0.63 1.33 1.89 2.59 3.16

0.500 0.911 1.178 1.651 1.925

1.76 2.36 2.83 2.99 3.02

Cl

(cm2 s−1 )

Fig. 10. Variation of specific conductance for membrane and solution with concentration of NaCl solution.

Fig. 12. Variation of electroosmotic flux (Jv ) with the applied current. Fig. 11. Dependence of the κ m on the loading of 4-VP at different concentrations of NaCl solution.

conductivity. An ion-exchange membrane must be considered as a microheterogeneous system. According to the model proposed by Gnusin et al. [32] and recently developed by the Zabolotsky and Nikonenko [33], the ionexchange membranes may be considered as a combination of a gel phase with a relative uniform distribution of functional groups and a hydrophilic part of the polymer matrix chains, on the one hand, and an electroneutral solution filling the interstices between the elements of the gel phase, named the integral phase, on the other hand. It was shown [34] that the microheterogeneous structure of the membrane material is the main factor determining the concentration dependence of membrane transport properties such as electrical conductivity, diffusion permeability, and transport numbers. Near m the conductivity of the memthe isoconductance point κiso m brane phase (κ ) and the solution phase (κ) become equal. m is obtained from the intercept of the curves drawn for κiso membrane conductivity and solution conductivity in Fig. 10. Concentrations of counterions in the membrane phase (Ciso ) m for different composite membranes are presented in and κiso Table 2. Both these values increased with the loading of 4VP in the membrane phase. IEC and water content (Cwm ) also increased with the loading of 4-VP, indicating the increase in hydrophilic nature of the membrane with the loading of 4-VP. m ) of the counterions in the The diffusion coefficient (D− membrane phase can also be calculated from the magnitude of the membrane conductivity at the isoconductance point

using the equation [33] m D− =

m RT κiso , F2 Q

(6)

where Q is the ion-exchange capacity of the joint gel phase m for different compos(equiv/cm3 ). Estimated values of D− ite membranes are also presented in Table 2. It also follows m , which supports the explanation a trend similar to that of κiso of increment in membrane porosity with loading of 4-VP. Moreover, from these studies it is obvious that for better selectivity and membrane conductivity of anion-exchange membranes, one has to make a compromise between the two parameters and optimize the loading of 4-VP in the membrane phase. 3.7. Electroosmotic studies Electroosmotic flux across the membrane arises on account of (i) the presence of charged sites on the membrane matrix and/or (ii) the existence of an electrical potential at the membrane–solution interface (zeta potential) [23,35, 36]. These properties along with the pore radius of the membrane matrix govern the electroosmotic flux across the ion-exchange membranes. Determination of the equivalent pore radius from electroosmotic flux of the solvent is proposed here. Electroosmotic permeability across the composite membranes was measured and flux is plotted as function of the applied current in Fig. 12. In all cases straight lines were obtained. Electroosmotic permeability, β, which implies that every coulomb of electricity will exert a drag effect

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Table 3 Electroosmotic permeability (β) and equivalent pore radius (r) for different organic–inorganic composite anion-exchange membranes in equilibrium with NaCl solution of 0.02 mol dm−3 concentration Membranes

β × 104 (cm3 /coulomb)

r (nm)

AEM-15 AEM-25 AEM-28 AEM-33 AEM-35

0.41 1.32 1.73 1.94 2.17

1.28 2.29 2.62 2.78 2.94

sufficient to carry β cm3 of water through 1 cm2 of the membrane was estimated from the slopes of these lines. Equivalent pore radius (r) of composite membranes was estimated from β values using the Katchalsky and Curran approach [35] with the help of the equation   8ηFβ 1/2 , r= (7) 0 f1w where η denotes the viscosity coefficient of permeates and 0 is the frictional coefficient between counterion (Cl− ) f1w 0 = and water in free solution, which can be defined as f1w RT /D− . D− is the diffusion coefficient of the counterion in the solution and was obtained at a given electrolyte concentration from ionic conductance data [37]. Equivalent pore radius estimated from Eq. (7) for different types of composite membranes with varying 4-VP content is presented in Table 3. It varied between 1.28 and 2.94 nm depending on 4-VP content. Increases in the pore radius of these composite membranes also support the observation of a high degree of membrane swelling and porosity at high 4-VP content. Increase in pore radius leads to the reduction in Donnan exclusion and thus membrane permselectivity. Thus a definite compromise among 4-VP content, membrane selectivity, and conductivity is essential for a thermally stable anion-exchange membrane with the desired properties. 4. Conclusions Organic–inorganic composite membranes based on PVASiO2 composite were prepared by the aqueous dispersion polymerization (sol–gel) method and anion selectivity was introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired contents. In all these composite membranes, the inorganic content is fixed, which contributes toward the reduction of swelling properties of PVA. While 4-VP, which is essential for the introduction of ionogenic functional groups, contributes to the increase in the swelling properties of the membrane matrix. Characterization by TGA, IEC, water content, and surface morphology revealed that these membranes are thermally stable with high IEC values and water contents in the membrane phase. It was also observed that the content of 4-VP in the membrane matrix contributes to the increase in

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membrane conductivity and water content. A rapid decline in membrane selectivity was observed at higher 4-VP contents. This may be because of a larger pore radius at higher 4-VP contents, which leads to the reduction in Donnan exclusion of co-ion and thus membrane selectivity. Counterion diffusion coefficient value and equivalent pore radius data estimated from membrane conductivity and electroosmotic flux measurements also support these observations. Thus it is possible to have an anion-exchange membrane by an ecofriendly method in aqueous medium without the use of solvent and hazardous chemicals with desired properties as well as thermal and dimensional stabilities by the optimization of 4-VP and inorganic content in the membrane matrix. Furthermore, these membranes, especially AEM25, exhibited very good selectivity, water content, and ionexchange capacity along with moderate membrane conductivity, which may be used for their application in electrodriven separation at elevated temperatures or for other electrochemical processes. Acknowledgments The authors are grateful to Dr. P.K. Ghosh, Director, CSMCRI, Bhavnagar, for his keen interest and encouragements. Financial assistance from the Department of Science & Technology, Government of India, for funding Project No. SR/S1/PC-15/2003 is also gratefully acknowledged. References [1] H. Strathman, in: W.S.W. Ho, K.K. Sirakar (Eds.), Membrane Handbook, Van Nostrand–Reinhold, New York, 1992. [2] T. Sata, J. Membr. Sci. 167 (2000) 1. [3] A. Elattar, A. Elmidaoui, N. Pismenskaia, C. Gavach, G. Pourcelly, J. Membr. Sci. 167 (1998) 249. [4] V.K. Shahi, R. Prakash, G. Ramachandraiah, R. Rangarajan, D. Vasudevan, J. Colloid Interface Sci. 216 (1999) 179. [5] G.-J. Hwang, H. Ohya, J. Membr. Sci. 140 (1998) 195. [6] T. Sata, T. Yamaguchi, K. Matsusaki, J. Phys. Chem. 99 (1995) 12875. [7] P.V. Vyas, P. Ray, G.S. Trivedi, S.K. Adhikary, R. Rangarajan, J. Colloid Interface Sci. 246 (2002) 366. [8] R.K. Nagarale, G.S. Gohil, V.K. Shahi, R. Rangarajan, Colloids Surf. A (2004), in press. [9] M.-S. Kang, Y.-J. Choi, I.-J. Choi, T.-H. Yoon, S.-H. Moon, J. Membr. Sci. 216 (2003) 39. [10] J.A. Kerrs, J. Membr. Sci. 185 (2001) 3. [11] J.M. Yang, M.C. Wang, Y.G. Hsu, C.H. Chang, J. Membr. Sci. 128 (1997) 133. [12] P.K. Narayanan, S.K. Adhikary, W.P. Harkare, K.P. Govindan, Indian patent 160880 (1987). [13] R.K. Nagarale, G.S. Gohil, V.K. Shahi, G.S. Trivedi, R. Rangarajan, J. Colloid Interface Sci. 277 (2004) 162. [14] M.-S. Kang, Y.-J. Choi, S.-H. Moon, AIChE J. 49 (12) (2003) 3213. [15] C.-W. Chen, T. Serizawa, M. Akashi, Chem. Mater. 11 (1999) 1381. [16] D.B. Mitzi, Chem. Mater. 13 (2001) 3283. [17] T. Vossmeyer, B. Guse, I. Besnard, R.E. Bauer, K. Mullen, A. Yasuda, Adv. Mater. 14 (2002) 238. [18] M.J. Percy, V. Michailidou, S.P. Armes, Langmuir 19 (2003) 2072. [19] V. Soukharev, N. Mano, A. Heller, J. Am. Chem. Soc. 126 (2004) 8368.

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