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Formation and characterization of highly crosslinked anion-exchange membranes
Journal of Membrane Science 217 (2003) 117–130
Formation and characterization of highly crosslinked anion-exchange membranes Ashok K. Pandey a , A. Goswami a,∗ , Debasis Sen b , S. Mazumder b , Ronald F. Childs c a b
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Deparment of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4M1 Received 21 October 2002; accepted 14 February 2003
Abstract Highly crosslinked/hyperbranched anion-exchange membranes have been prepared by anchoring poly(vinylbenzyl chloride) (PVBCl) within the pores of poly(propylene) microporous base membranes by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). The resulting PVBCl-filled precursor membranes were converted to anion-exchange membranes by reacting these with (i) excess of DABCO followed by alkylation with ␣,␣ -dibromo-p-xylene (DBX) (membrane A), and (ii) with excess of tetraethylenepentamine (TEPA) (membrane B). A third membrane C was synthesized by alkylating membrane B with DBX. The chemical analyses indicated that these anion-exchange membranes consist of highly crosslinked/hyperbranched anionic gels within the pores of host microporous membranes. These anion-exchange membranes were characterized in terms of water-uptake capacities, ion-exchange capacities and thermal stability. The physical structures of the membranes were examined by small angle X-rays scattering (SAXS) analysis. The study of SAXS profiles of the dry and water equilibrated membrane A samples indicated that microstructure of anionic gel within the pores of membrane was changed significantly on water equilibration. However, no significant change in the SAXS profile was observed in wet samples of membranes B and C with respect to their dry samples. Thus, the crosslinking generated in membrane A was flexible and very rigid in membranes B and C. The self-diffusion coefficient of I− ions and transport numbers of Cl− ions were measured to examine the effects of crosslinking on transport properties of the membranes. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Anion-exchange membrane; Pore-filled method; Highly rigid crosslinking; Small angle X-rays scattering (SAXS); Self-diffusion coefficient
1. Introduction Pore-filled anion-exchange membranes are prepared by incorporating the polyelectrolyte gel within ∗ Corresponding author. Fax: +91-22-25505150/25505151. E-mail addresses: [email protected] (A.K. Pandey), [email protected] (A. Goswami), debasis@apsara. barc.ernet.in (D. Sen), [email protected] (S. Mazumder), [email protected] (R.F. Childs).
pores of a robust microporous host membrane [1–6]. The microporous host membrane provides containment and mechanical strength to the ionic gel and mitigates the impact of osmotic forces. The appropriate chemistry of the ionic gel, the guest component, leads to good separation characteristics of resulting membrane. There are two possible routes for preparing strongly basic pore-filled anion-exchange membranes. These are: (i) in situ polymerization of suitable monomer with a crosslinker within pores of
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00084-X
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a host microporous membrane [1–5], and (ii) in situ crosslinking of preformed polymer with a suitable crosslinker within pores of a microporous membrane [7,8]. Most work to date on these systems has involved poly(4-vinylpyridine) (P4VP) or poly(vinylbenzyl chloride) (PVBCl) polymers anchored within the pores of a host microporous membrane. The fixed positive charge sites in the membranes are obtained by subjecting P4VP and PVBCl-filled membranes to alkylation or amination reactions, respectively. Both the routes have been found to be effective in producing membranes in which a well-defined polyelectrolyte is firmly anchored within the pores. These membranes give excellent performance in ultra-low pressure water softening [8,9], and in diffusion dialysis for acid recovery [10]. The extent of pore-filling in both the routes is dependent on the solubility of the monomer or preformed polymer in the solvent used for the incorporation step. Typically, the high solubility of monomers as compared to polymers means that membranes with a high degree of gel incorporation can be prepared by the in situ polymerization method. However, high incorporations using the in situ polymerization route typically require substantial amount of a crosslinking agent DVB. This, in turn, leads to significant decrease in the ion-exchange capacity of the resulting pore-filled ion-exchange membranes. Recently, a method involving simultaneous amination-crosslinking of brominated poly(2,6-dimethyl1,4-phenylene oxide) (PPO) has been reported for the preparation of crosslinked anion-exchange membranes [11]. This method involves use of trimethylamine and ethylenediamine for generating fixed positive charge sites and crosslinks in the membrane, respectively. The nitrogen atoms involved in crosslinking are still basic and can be alkylated to form quaternary ammonium groups. Another possible approach would be to use a crosslinker that generates crosslinks with fixed positive charge sites [7]. These approaches allow a high degree of crosslinking to be introduced without sacrificing fixed positive charged site density in the membrane. In this article, we report the preparation of the three highly crosslinked/hyperbranched anion-exchange membranes by in situ crosslinking of PVBCl polymer with 1,4-diazabicyclo[2.2.2]octane (DABCO) in the pores of poly(propylene) microporous host
membrane. The subsequent chemical modifications involving amination and alkylation have been used to produce positively charge crosslinks in the pore-filled anion-exchange membrane. The resulting membranes have been chemically and physically characterized. Transport numbers of Cl− and self-diffusion coefficients of I− ions in the membranes were measured to examine the transport properties of membranes.
2. Experimental 2.1. Materials Poly(vinylbenzyl chloride) (60/40 mixture of 3- and 4-isomers with average molecular weight Mn ca. 55,000 and Mw ca. 100,000), diazabicyclo[2.2.2]octane (98% purity), ␣,␣ -dibromo-p-xylene (DBX) (97% purity) and tetraethylenepentamine (technical grade) were procured from Aldrich. These chemicals were used without any further purification. The linear chains of PVBCl used in the present work contained 80% benzyl chloride units and 20% styrene units [7]. The host microporous substrates used for the preparation of pore-filled anion-exchange membranes were poly(propylene) microfiltration membranes. The specifications of this poly(propylene) microporous membrane are: average pore-diameter 0.26 m, thickness 63 ± 2 m and porosity 60 vol.%. The radiotracer 131 I used for measuring the self-diffusion coefficients of I− ions in the membrane samples was obtained from the Board of Radiation and Isotope Technology, Mumbai, India. 2.2. Preparation of pore-filled anion-exchange membranes 2.2.1. Preparation of PVBCl-filled precursor membranes A known amount of PVBCl was dissolved in a mixed solvent consisting of 75 vol.% N,N -dimethylformamide (DMF) and 25 vol.% of tetrahydrofuran (THF). A solution of the crosslinker DABCO (concentration determined by the crosslinking ratio required) in DMF was prepared separately. Appropriate volumes of solution containing PVBCl and DABCO were rapidly mixed and total 2.5 ml of resulted mixture was applied on a weighed piece of the poly(propylene)
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microporous membrane (12 cm × 12 cm) placed on a glass plate. The concentration of PVBCl in the mixed pore-filling solution was 0.3 g/ml and ratio of DABCO to PVBCl was adjusted to 12 ± 1 mol%. The excess of solution on the top of poly(propylene) membrane was removed by a gentle application of a Teflon roller and a Teflon gasket was placed around the membrane sample. Finally, the filled poly(propylene) membrane with Teflon gasket around it was sandwiched between two glass plates for approximately 16 h at room temperature to allow gelling (crosslinking) to occur. The pore-filled membranes thus formed were washed with methanol and THF to remove un-crosslinked PVBCl and DABCO. 2.2.2. Preparation of membrane A The PVBCl-filled precursor membrane was reacted with an excess of DABCO. This reaction was carried out by immersing the precursor membrane (12 cm × 12 cm) in 50 ml of a DMF solution containing 2 wt.% of DABCO. The reaction was carried out at 70 ◦ C for 16–18 h. The unreacted DABCO present in the resulting membrane was removed by washing it with excess of water and methanol. The DABCO-reacted precursor membrane was alkylated with ␣,␣ -dibromo-p-xylene to obtain membrane A. This reaction was carried out by keeping the membrane in 50 ml solution of 2 wt.% DBX in DMF for 16–18 h at 70 ◦ C. 2.2.3. Preparation of membrane B The membrane B was prepared by treating PVBCl-filled precursor membrane with a DMF solution of tetraethylenepentamine (TEPA) (10 wt.%) for 16–18 h at 70 ◦ C. 2.2.4. Preparation of membrane C The membrane C was formed by alkylation of membrane B with DBX. The experimental conditions used for this reaction are the same as described for the membrane A, i.e. immersing membrane B in 50 ml of a solution of 2 wt.% DBX in DMF for 16–18 h at 70 ◦ C. 2.3. Characterization of anion-exchange membranes The final membranes were washed with DMF, methanol, an excess of water and 0.1 mol/l HCl to remove soluble components present in the membrane sample before being dried in a vacuum and
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weighed. The membrane samples were conditioned by soaking in a 0.5 mol/l aqueous solution of NaCl for 2–3 days and washed with water before use. The anion-exchange membranes were converted to I− or SO4 2− ionic forms by equilibrating these with relevant sodium salt solution (0.5 mol/l) for 24 h. The mass-gain of the membrane samples was determined as the wt.% of anionic gel introduced in the pores of membrane per unit weight of nascent membrane. The weight of the gel introduced in the pores of the poly(propylene) microporous membranes was obtained from the weight difference of the final vacuum dried membrane sample and nascent membrane sample used for obtaining same membrane. The thickness of dry membrane samples were measured by using a digital micrometer (Mitutoy) with an accuracy of 0.001 mm. The thickness of wet samples were determined by pycnometer as described elsewhere [7]. The water-uptake capacity of the membranes, defined as the amount of water absorbed per unit dry weight of the membrane, was obtained from the difference in weight of wet membrane and vacuum dried membrane sample at room temperature. The transport numbers of Cl− ions for membranes A and C were determined by EMF method described by Kontturi et al. [12]. The rotating cell assembly with Ag/AgCl electrodes situated close to and on either side of the rotating membrane was used to measure EMF as a function of rotating speed. The rotating diffusion cell was filled with 1 mol/l NaCl solution, and non-rotating compartment was filled with 0.5 mol/l NaCl solution. The EMF was measured as a function of the rotation speed from 7.85 rad/s to 62.8 rad/s (75 to 600 rpm). The EMF at zero concentration polarization was obtained by extrapolation of the measured EMF as a function of 1/(rotation speed)1/2 . The equation described by Kontturi et al. [12] was used to obtain Cl− transport numbers from EMF at zero concentration polarization. 2.4. Elemental analyses The contents of C, H, N and S in the membrane samples with SO4 2− counterions were estimated by using an Elemental Analyzer (EA 1110 model, C.E. Instruments). The amounts of covalently bonded chlorine and bromine in the membrane samples were determined by neutron activation analyses (NAA) of
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membranes in their SO4 2− or I− form so as to provide discrimination between covalent and ionic chlorine/bromine. The amount of counterions (Cl− or I− ) in salt equilibrated membrane samples was also determined by NAA. These data were used to calculate the fixed positive charge sites, and hence ion-exchange capacity of the membrane samples. Acid exchangeable ion-exchange capacities of the membranes were obtained by neutron activation analyses of Cl− ions in the membrane samples equilibrated with HCl solution of 0.2 mol/l concentration. For the neutron activation analyses, known amounts of the membrane and standard samples (KI, KCl and KBr) were irradiated for the appropriate time (15–30 min) in the APSARA reactor at BARC, Mumbai, India. The radioactivity (counts/s) generated in the membrane samples were monitored by counting 1642 keV ␥-ray of 38 Cl and 443 keV ␥-ray of 128 I with a 120 cm3 HPGe detector coupled to a PC-based multichannel analyzer. 2.5. Thermal analyses The thermal analyses of the membrane samples were carried out in flowing argon gas atmosphere using differential thermal analysis (DTA), thermogravimetry analysis (TGA) and differential thermogravimetry analysis (DTGA). The thermograms were recorded at a heating rate of 3 and 5 ◦ C/min in a thermoanalyzer (Setaram, France) using alumina as the reference material for DTA. 2.6. Small angle X-ray scattering Small angle X-ray scattering (SAXS) measurements of the wet (water equilibrated) and dry membrane samples with Cl− counterions were carried out with a rotating anode based 12 KW Rigaku SAXS machine equipped with a three-slit system with soller slits. Intensities were measured by a scintillation counter with a pulse height analyzer. Ni-filtered Cu K␣ (λ = 0.1541 nm) radiation was used as incident X-ray source. The scattered intensities were recorded as a function of scattering vector q (=4π sin θ /λ, where 2θ is the scattering angle and λ is the incident X-ray wavelength). The intensities were corrected for sample absorption and for smearing effects of collimating slits.
2.7. Measurements of self-diffusion coefficients The measurements of self-diffusion coefficients of I− ions in the various anion-exchange membranes were carried out using 2.5 cm × 2.5 cm pieces of samples. A membrane sample in the I− -form was loaded with 131 I− radiotracer ions by equilibrating it in 25 ml of the vigorously stirred solution of KI (0.01 mol/l) spiked with known amount of radiotracer. The membrane sample was equilibrated for 4 h before taking out for monitoring the amount of the radiotracer loaded in it. The radiotracer ions diffusing out from the membrane into equilibrating salt solution was measured as a function of time in order to determine the self-diffusion coefficient of I− ions in the membrane. The desorption experiments were done by immersing the radiotracer loaded membrane sample into 25 ml aqueous solution of 0.5 mol/l KI at 27 ◦ C. The constant stirring (≈52.4 rad/s (≈500 rpm)) was used to minimize the concentration gradient of radiotracer ions in the equilibrating solution. The amount of radiotracer ions diffusing out of the membrane sample was monitored by taking out 100 l samples of equilibrating salt solution at regular time intervals and monitoring the radioactivity by ␥-counting using a well-type NaI(Tl) detector connected to a single channel analyzer.
3. Results and discussion The precursor membranes used for constructing the massively crosslinked anion-exchange membranes were prepared by the introduction and immobilization of PVBCl within pores of a poly(propylene) host membrane. An in situ crosslinking of PVBCl polymer chains with DABCO was used for immobilizing PVBCl into the pores of support membrane [7]. The degree of crosslinking was controlled by adjusting the amounts of DABCO and PVBCl in the solution used for pore-filling. The amounts of PVBCl and DABCO incorporated into the support membrane were computed from the ratio of DABCO to PVBCl in the pore-filling solution and total weight of the material anchored in the membrane. The nitrogen content calculated from the expected weight of DABCO in the membrane based upon the concentration of DABCO in the filling
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Scheme 1. Preparation of PVBCl-filled precursor membrane.
solution was found to be in good agreement (±3%) with experimental nitrogen content. As observed in our previous work [7], the extent of fixed positive charge sites generated during initial crosslinking was found to correspond to the measured nitrogen content indicating that the DABCO molecules were direacted and formed crosslinks between PVBCl chains in the membrane as shown in Scheme 1. The initial degree of crosslinking, defined as ratio of moles of direacted crosslinker to moles of benzyl chloride units of PVBCl in the precursor membrane, was found to be 12 ± 2 mol% which is in agreement with degree of crosslinking expected from the ratio of DABCO to PVBCl in pore-filling solution. Benzyl chloride units in the precursor membrane, left after initial crosslinking of PVBCl with DABCO, were converted to quaternary ammonium groups to generate fixed positive charge sites. The reactions in the conversion of benzyl chloride units to quaternary ammonium groups were selected so as to form positively charged crosslink sites between poly(vinylbenzyl) chains, which would not only ensure high degree of crosslinking without sacrificing ion-exchange capacity but also a higher degree of pore-filling in the membranes. Two possible routes chosen were based on the reaction of precursor mem-
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brane with: (i) excess of DABCO to form membrane A, and (ii) excess of tetraethylenepentamine to form membrane B and subsequently membrane C. The nature of the reaction of DABCO with the benzyl chloride units of PVBCl is dependent upon the mole ratio of DABCO to benzyl chloride units [7,13]. The DABCO is predominately direacted when there is an excess of benzyl chloride units available. This fact was used for initial crosslinking of PVBCl with DABCO to form precursor membranes. If mole ratio of DABCO to PVBCl units is greater than 1, then most of the DABCO molecules remain either monoreacted or unreacted, i.e. one of the tertiary nitrogen of DABCO reacts with PVBCl and other tertiary nitrogen remains free. This free tertiary nitrogen of DABCO molecules attached to poly(vinylbenzyl) (PVB) chains, can be further reacted with di-alkylating reagents like dibromo-p-xylene to form PVB–DABCO–xylene–DABCO–PVB crosslinking chains. The membrane A was prepared by utilizing this approach. The chemical reaction involved in the preparation of membrane A from the precursor membrane is shown in the Scheme 2. The extent of chemical reactions involved in the preparation of membrane A was determined by elemental analyses of vacuum dried membrane samples (2 cm × 2 cm) after each step. The elemental analyses of membrane A are summarized in Table 1. The amount of fixed charge sites in the PVBCl-filled precursor membrane, which is in close agreement with the nitrogen content introduced during initial crosslinking, is given in the column 2 of Table 1. The amount of covalently bonded chlorine in the structure I of Scheme 2 was obtained by NAA of the precursor membrane with I− counterions (see column 3, Table 1). In order to estimate the amount of chlorine substitution by DABCO, the DABCO-reacted membrane (containing I− or SO4 2− counterions) was subjected to chlorine analysis in a similar fashion as described for precursor membrane. The presence of negligible amount of covalently bonded chlorine in the DABCO-reacted membrane, as shown in the column 4 of Table 1, indicated nearly quantitative substitution of chlorine with DABCO. From the knowledge of amount of nitrogen introduced in the formation of precursor membrane and amount of chlorine displaced by DABCO, the expected nitrogen content of DABCO-reacted membrane was calculated for mono-reaction with DABCO. The
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Scheme 2. Preparation of membrane A from PVBCl-filled precursor membrane.
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Table 1 Elemental analyses of the anion-exchange membranes Membrane identification
Fixed charge sites after initial crosslinkinga (mmol/cm3 )
Cl in precursor membraneb (mmol/cm3 )
Cl left after aminationc (mmol/cm3 )
Expected N from bonded Cld (mmol/cm3 )
Experimental N-content after aminatione (mmol/cm3 )
Quaternary N (fixed charge sites)f (mmol/cm3 )
Br-content in DBX-reacted membraneg (mmol/cm3 )
A B C
0.37 ± 0.02 0.36 ± 0.02 0.36 ± 0.02
1.06 ± 0.04 1.10 ± 0.03 1.05 ± 0.04
0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01
2.49 5.86 5.61
2.38 ± 0.09 4.17 ± 0.15 4.08 ± 0.18
1.78 ± 0.03 0.70 ± 0.03 2.14 ± 0.03
Not detected – 0.02 ± 0.01
a
Generated during initial crosslinking to anchor PVBCl within pores of microporous membrane (precursor membrane, Scheme 1). The total nitrogen content of the precursor membrane was in close agreement with the amount of fixed positive charge sites (±3%). b Covalently bonded Cl corresponding to benzyl chloride units in precursor membrane (structure I of Schemes 2 and 3). c Residual covalently bonded chlorine in the membranes after reaction with DABCO or TEPA (structure II of Schemes 2 and 3). d Total calculated nitrogen content for 1:1 reaction of benzyl chloride units with DABCO/TEPA (=moles nitrogen introduced during crosslinking + (moles of substituted Cl × no. of nitrogen atoms in the aminating reagent)). e Total nitrogen content of final membrane (after amination) as estimated by elemental analyses (structure II of Schemes 2 and 3 for membranes A, B and C). f Fixed charge in the membrane as estimated by elemental analyses of exchangeable Cl− and I− counterions. g Total amount of DBX-reacted in the membrane C was 1.51 mmol/cm3 .
close agreement between expected and experimental nitrogen contents (columns 5 and 6 of Table 1), suggests that mono-reaction of the residual benzyl chloride units in the membrane with excess of DABCO occurred as shown in structure II of Scheme 2. The extent of quaternization of free tertiary nitrogen of DABCO attached to benzyl chloride units, shown in the structure II of Scheme 2, with DBX was measured by estimating the fixed charge sites (counterions) after equilibrating the membrane samples with the appropriate salt solutions. The comparison of total nitrogen content of structure II of Scheme 2 and fixed positive charge sites of structure III of Scheme 2 indicates that the 74% of the total nitrogen of DABCO in membrane A exist as quaternary ammonium groups (Table 1). This means that the remaining 26% of the tertiary nitrogen of DABCO did not undergo quaternization with DBX. The absence of bromine in membrane A indicated that DBX molecules were direacted in membrane A to form crosslinks between free ends of two DABCO molecules attached to PVB chains. Thus, the membrane A has two types of crosslinks. First type of crosslinks is short chain of PVB–DABCO–PVB formed during anchoring of PVBCl in the pores of poly(propylene) microporous host membrane. The other is due to formation PVB–DABCO–xylene–DABCO–PVB chains during the conversion of PVBCl-filled precursor
membrane into anion-exchange membranes. The possible chemical composition of membrane A based on chemical analyses is shown in structure III of Scheme 2. The other route examined to form highly crosslinked anion-exchange membranes was based on the amination of precursor membrane with excess of TEPA, membrane B. This membrane was subjected to chemical analyses in a similar fashion as described for membrane A. The total nitrogen content of membrane B was found to be less than that expected from the mono-substitution of the remaining benzyl chlorides with TEPA (columns 5 and 6, Table 1). The presence of negligible amount of covalently bonded chlorine in TEPA-reacted precursor membrane (column 4, Table 1) suggests that the substitution of chlorine was almost complete. Hence, the difference in the expected and experimental nitrogen contents suggests that some of TEPA molecules are direacted with available benzyl chloride units in the precursor membrane as shown in the structure II of Scheme 3. Comparison of columns 2 and 7 for membrane B in Table 1, also shows that fixed positive charge sites were generated during the reaction of precursor membrane with TEPA molecules. Thus, direaction of TEPA with benzyl chloride units also involves reactions leading to quaternization of some of the nitrogen atom of TEPA chains. Thus, rigid crosslinks comprising chain
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Scheme 3. Preparation of membranes B and C from PVBCl-filled precursor membrane.
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of PVB–N(TEPA)–PVB were also introduced during the conversion of precursor membrane to membrane B as shown in the Scheme 3. Membrane B possesses substantial amount of un-quaternized nitrogen as shown in structure II, Scheme 3. In order to enhance the number of ion-exchange sites, membrane B was further alkylated with DBX to form membrane C. The chemical analysis of this membrane, summarized in Table 1, shows considerable enhancement the ion-exchange sites of membrane C as compared to its precursor membrane B. The presence of negligible amounts of bromine in the neutron activation analysis of membrane C indicated that the DBX molecules were largely direacted and formed additional crosslinks between two nitrogen atoms of TEPA molecules attached to PVBCl in the membrane. However, the amount of fixed positive charge sites generated during quaternization of membrane B with DBX was found to be only ≈50% of total moles of DBX-reacted with the membrane B. The amount of DBX in the membrane was obtained from the increase in weight during the quaternization. Therefore, it appears that most of DBX molecules formed one positively charge crosslink site and other side of DBX was attached to a secondary or primary amine site in the membrane C as shown in the structure III of Scheme 3. The crosslinking present in membrane C is expected to be highly rigid.
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3.1. Properties of the anion-exchange membranes The three types of the anion-exchange membranes developed in the present work were characterized in terms of mass-gain (weight of gel loaded per unit weight of the host membrane), thickness, ion-exchange capacity and water-uptake capacity (Table 2). The thickness of all the membranes (63 ± 2 m) was found to be same as the thickness of microporous polypropylene host membrane used for pore-filling. No significant changes in the other dimensions (length or breadth) of the membrane were observed, regardless of degree of loading (mass-gain) and chemical composition of the membrane. The membranes were found to absorb water readily. The water-uptake capacities of these membranes were measured as the weight of water sorbed in the membrane over the weight of the dry membrane. The water-uptake capacity of membrane B was found to be higher than that of membrane C as would be expected based on the degree of loading (mass-gain) (Table 2). This trend was not observed in comparing membranes A and C, despite the ion-exchange capacities of both membranes being nearly identical. It can be seen in Table 2 that the water-uptake and fixed charge ion-exchange capacities of membranes A and C are comparable to commercially available anion-exchange membranes [14,15]. As evident from Table 2, the acid exchangeable ion-exchange capacities were consider-
Table 2 Properties of anion-exchange membranes Membrane identification
Ion-exchange capacity
Water-uptake capacity in
Totala (mmol/g)
Fixed charge (mmol/g)
Cl− form (wt.%)
I− form (wt.%)
Anion-exchange membrane developed in the present work A 184 62 ± 1 B 149 63 ± 1 C 235 63 ± 1
2.53 2.68 3.32
2.0 0.9 2.1
36.5 53.9 43.0
30.3 54.2 35.6
Commercially available anion-exchange membranesb Selemion-DSV (Asahi Glass) – Neosepta-AFX (Tokuyama Soda) – Neosepta-AFN (Tokuyama Soda) – Neosepta-AMX (Tokuyama Soda) – Neosepta-ACS (Tokuyama Soda) – ADS XL 10 (Morgane-Solay) –
– – – – – –
2.0 1.3 1.9 1.3 1.6 1.27
40 32 32 20 21 32
a b
Mass-gain (wt.%)
Thickness of wet membrane (m)
– – – – – 150
Acid exchangeable ion-exchange capacity. Data taken from [14] for DSV, AFX, AFN, AMX and ACS and from [15] for ADS XL 10.
– – – – –
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ably higher than fixed charge ion-exchange capacities in all the membranes. This is due to the fact that all the nitrogen sites present in the membrane could not be alkylated to form quaternary ammonium groups. Thus, these un-quaternized nitrogen sites in the membranes were still basic and were responsible for higher acid exchangeable ion-exchange capacities of the membranes. The transport numbers of Cl− ions (tCl ) for membranes A and C were determined by EMF method [12] and found to be 0.94 and 0.96, respectively. The higher transport numbers of these membranes suggest that they would be well suited for electro-chemical applications. 3.2. Small angle X-rays scattering In order to examine the physical structures of the anionic gels anchored in the microporous poly(propylene) host membranes, the membranes A, B and C were examined using small angle X-rays scattering (SAXS) analysis [16]. SAXS measurements of membrane samples (in Cl− form) were carried out in both dry and wet (water equilibrated) states. The variations of scattered intensities of X-ray I(q) as a function of scattering vector q (=4π sin θ /λ, where 2θ is the scattering angle and λ is the incident X-ray wavelength) are shown in Figs. 1 and 2 for the membranes A and B, respectively. It can be seen in Figs. 1 and 2 that the SAXS intensities I(q) in double logarithmic scale are almost linear as a function of q. Therefore, I(q) from the membrane samples follows a power law relation with q [16]. As the membranes are highly crosslinked, it is expected that the pore interfaces are not exactly smooth. Hence, to extract real space information in the present case, the membranes were viewed as a surface fractal object. The term ‘fractal’ originates from the fact that the structures follow some self-similarity over a wide length scale. However, it should be noted that most of the real objects are self-affine rather than self similar with direction dependent rescaling factor, and in an average sense they behave as fractal object. SAXS intensity I(q) from a surface fractal is given by [17]: Is (q) ∝ q −1 [1 + (qξs )2 ](ds −5)/2 × sin[(ds − 1) arctan(qξs )]
(1)
Fig. 1. SAXS profiles of dry (䊊) and wet (䊉) samples of membrane A.
where ξ is the upper cut-off of the fractal and ds is the fractal dimension. It is evident from Fig. 1 that the SAXS profile changes significantly in the water equilibrated membrane A sample as compared to its dry
Fig. 2. SAXS profiles of dry ( ) and wet (䉱) samples of membrane B.
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Table 3 SAXS analysis of the membranes Membrane identification
A B
Upper cut-off, ξ (nm)
Fractal dimension, ds
Average pore radius (nm)
Dry
Wet
Dry
Wet
Dry
Wet
5 20
11 20
2.1 ± 0.2 2.0 ± 0.2
2.2 ± 0.2 2.0 ± 0.2
3.85 5.4
4.78 5.4
sample. The sharper nature of the profile for the wet sample indicates that there is significantly change in the microstructure of anionic gel in the pores of membrane A equilibrated with water. However, no significant change in SAXS profile was observed in the water equilibrated and dry forms of membrane B (Fig. 2). Similar behavior was observed in the SAXS profiles of wet and dry samples of membrane C as those found for membrane B. This suggests that the very rigid crosslinking in the anionic gel anchored in membranes B and C prevented the change in the microstructure of polymer chains with water. In contrast, the crosslinking in the membrane A appears to be more flexible. For membrane samples A and B, Eq. (1) has been fitted to their SAXS profiles. In case of membrane C, the SAXS signal was not very prominent. This was attributed to very small pore size in the membrane C as evident by very high degree of gel incorporation (see Table 2). The pore size distribution ρ(r) for a surface fractal can be written as: ρ(r) ∝ r −(ds +1)
(2)
The average pore size r can be obtained from Eq. (2) as: ∞ ρ(r)r dr r = 0 ∞ 0 ρ(r) dr However, considering the upper cut-off of the fractals, the integration limit in present case has been approximated to ξ π/1 ρ(r)r dr r = ξ (3) π/1 ρ(r) dr where the lower limit of the integration has been chosen as (π /1) because from the nature of the SAXS profiles it appears that the same trends continues at least up to q equal to 1 nm−1 . However, the exact
value of the lower limit of integration should be the lower cut-off of the fractals that could not be obtained in this case due to statistical ambiguity of the data beyond 0.5 nm−1 . The estimated parameters obtained from SAXS analyses of membranes A and B are tabulated in Table 3. The pore size of the membranes A and B were found to be in order of degree of gel incorporation in the host poly(propylene) membrane. 3.3. Thermal analysis Thermal stability of the membranes was examined by the differential thermal analysis, thermogravimetric analysis and differential thermogravimetric analysis. The results of the thermal analyses are summarized in Table 4. The first endothermic peaks in samples of membranes A, B and C were assigned to water loss from the samples. The first endothermic peak in DTA of the poly(propylene) host membrane was found to be at 167 ◦ C. This endothermic peak was assigned to crystalline melting point (Tm ) of the poly(propylene) host membrane as no mass loss was observed in the TGA at the same temperature range. The reported Tm of crystalline poly(propylene) (167–170 ◦ C) is found to be in close agreement with the temperature assigned to Tm of the poly(propylene) host membrane [18]. Comparison of the DTA of the host membrane (first endothermic peak) with the gel-filled anion-exchange membranes (A, B and C) (second endothermic peak) indicates the shifting of the onset of this peak to higher temperatures (≈153 ◦ C), however, the maximum of the peaks in all the membranes (host, A, B and C) was found to be at same 167 ◦ C. The results suggest that the anion-exchange membranes prepared in the present work are physically stable upto 153 ◦ C. The anion-exchange membranes (A, B and C) were found to degrade at lower temperatures (216–250 ◦ C) than the poly(propylene) host membrane (383 ◦ C).
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Table 4 Thermal analysis of membranes Membrane identification
Peak identification
Process
Base membrane
1 (endo) 2 (exo)
B
DTA (◦ C)
TGA (◦ C)
DTGA (◦ C)
Peak onset
Peak maximum
Peak end
Peak onset
Peak end
Tm Degradation
132 383
167 450
189 475
– 383
– 475
– 450
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
35 153 255
78 167
105 190
35 97 – – Continuous mass loss
72 –
C
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
46 153 246
104 167
135 182
46 135 – – Continuous mass loss
99 –
A
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
45 155 216
85 167
139 182
45 146 – – Continuous mass loss
72 –
3.4. Self-diffusion coefficients of I− ions in the anion-exchange membrane A non-stationary radiotracer diffusion method was used for obtaining the self-diffusion coefficients of I− in anion-exchange membranes [19]. The amount of radiotracer ions (131 I− ) diffusing out of the membrane sample kept in contact with equilibrating solution as a function of time was analyzed by using following equation [19]: 8 −Dπ 2 tk ∗ n(tk ) = n 1 − exp π2 L2 2 −9Dπ tk 1 + exp + . . . (4) 9 L2 where n(tk ) and n∗ are the amounts of radiotracer ions in the equilibrating solution at fixed time t = tk and ∞, respectively, D the self-diffusion coefficient of the ions in the membrane and L the thickness of the membrane. The value of D was obtained by a nonlinear least-squares fit of Eq. (4) with both n∗ and D as free parameters. The plot of n(tk )/n∗ versus time tk for 131 I− ions in the membranes A, B and C is shown in Fig. 3. The values of D obtained in this analysis are (1.23 ± 0.04) × 10−7 , (2.91 ± 0.09) × 10−7 and (0.22 ± 0.01) × 10−7 cm2 /s for membranes A, B and C, respectively. The diffusion coefficient of Br− ions in AMS ion-exchange membrane was measured by Heintz
et al. using Maxwell–Stefan formalism [20]. The average value of D for Br− in AMS anion-exchange membrane (Tokuyama Corp., Japan) was found to 4.6 × 10−7 cm2 /s. Since the aqueous self-diffusion coefficients of Br− and I− are not very different [21] (Br− , 2.08×10−5 cm2 /s; I− , 2.04×10−5 cm2 /s), the mobility of ions as indicated by their D values in AMS and anion-exchanges membranes developed in the present work can be compared. It appears from this comparison that mobility of the ions in membranes A and B are in same order as that in AMS anion-exchange membrane. However, the mobility of ions in membrane C is 20 times less than that in AMS membrane. The D values of I− in the anion-exchange membranes A, B and C appear to follow the order of pore-radii rather than water-uptake capacities in these membranes. This seems to suggest that the diffusion of ions in the membranes is affected by narrow channels in the ion-diffusion path and not by the free volume available in the pores of the membrane. The physical structure of these diffusion channels is fixed by crosslinking. In accordance with SAXS observation, the physical dimensions of diffusion channels do not change in membranes B and C with water-uptake. Therefore, the possible reason of lower self-diffusion coefficient of I− in the membrane C as compared to membrane A, which has same ion-exchange capacity as membrane C, may be related to a higher tortuosity due to rigid crosslinking in the membrane C. The
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Fig. 3. Variation of fractions of the radiotracer I− ions in the equilibrating salt solution in contact with membranes A, B and C as a function of time t. The n(tk ) and n∗ represent the amount of 131 I− radiotracer ions at time tk and at equilibrium (t = ∞), respectively. The membranes A, B and C are represented by ( ), (䊊) and (䊐), respectively.
effect of lower water-uptake in the membrane A on self-diffusion coefficient is probably nullified by the more flexible crosslinking that allows the formation of more open diffusion channels on water equilibration.
4. Conclusions The present work demonstrates the possibility of making highly crosslinked/hyperbranched anion-exchange membrane with high ion-exchange and water-uptake capacities. The chemical and physical characterization of the membranes indicated that the longer crosslink chains (PVB–DABCO–xylene– DABCO–PVB in membrane A) are more flexible and allow the polymer chains to undergo change in their microstructure on equilibration with water within the pores of membrane. The shorter crosslinks (PVB–N(TEPA)–PVB in membranes B and C) were
found to highly rigid and prevent any physical changes of anionic gel within the pores of membrane. The transport number of Cl− ions and self-diffusion coefficients of I− in membranes seem to suggest that physical structure of the membranes developed in the present work plays an important role in transport process rather than the water-uptake capacity of the membrane. The anionic gel anchored in the membrane was found to thermally stable upto 200–250 ◦ C. However, the lower crystalline melting point temperature (Tm ) of the poly(propylene) host membrane makes these anion-exchange membrane physically stable up to ∼150 ◦ C. The properties of the anion-exchange membranes developed in the present work should allow sieving of small ions from the larger ions. The method of generating highly rigid crosslinking may be suitable for imprinting membrane by efficient fixing of geometry of template ions.
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Acknowledgements Authors are thankful to Dr. S.B. Manohar, Head, Radiochemistry Division and Dr. A.V.R. Reddy, Head, Nuclear Chemistry Section for their keen interest in the present work. Authors are also thankful to Drs. V.K. Manchanda and S.K. Mukherjee for their help in the elemental and thermal analyses of the membranes. References [1] A.M. Mika, R.F. Childs, M. West, J.N.A. Lott, Poly(4vinylpyridine)-filled microfiltration membranes: physicochemical properties and morphology, J. Membr. Sci. 136 (1997) 221–232. [2] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. McCarry, D.R. Gagnon, Porous polyelectrolyte-filled membranes: effect of crosslinking on flux and separation, J. Membr. Sci. 135 (1997) 81–92. [3] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. McCarry, D.R. Gagnon, A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity, J. Membr. Sci. 108 (1995) 37–56. [4] M. Ulbricht, H.-H. Schwarz, Novel high performance photo-graft composite membranes for separation of organic liquids by pervaporation, J. Membr. Sci. 136 (1997) 25–33. [5] K.L. Thunhorst, R.D. Noble, C.N. Bowman, Transport of ionic species through functionalized poly(vinylbenzyl chloride) membranes, J. Membr. Sci. 128 (1997) 183– 193. [6] V. Kapur, J.C. Charkoudian, S.B. Kesseler, J.L. Anderson, Hydrodynamic permeability of hydrogels stabilized within porous membranes, Ind. Eng. Chem. Res. 35 (1996) 3179– 3185. [7] A.K. Pandey, R.F. Childs, M. West, J.N.A. Lott, B.E. McCarry, J.M. Dickson, Formation of pore-filled ion-exchange membranes with in situ crosslinking: poly(vinylbenzyl ammonium salt)-filled membranes, J. Polym. Sci. A: Polym. Chem. 39 (2001) 807–820. [8] (a) R.F. Childs, A.M. Mika, A.K. Pandey, C. McCrory, S. Mouton, J.M. Dickson, Nanofiltration using pore-filled membranes: effect of polyelectrolyte composition on performance, Sep. Purif. Technol. 22–23 (2001) 507–517; (b) A.M. Mika, A.K. Pandey, R.F. Childs, Ultra-low-pressure water softening with pore-filled membranes, Desalination 140 (2001) 265–275.
[9] A.M. Mika, R.F. Childs, J.M. Dickson, Ultra-low pressure water softening: a new approach to membrane construction, Desalination 121 (1999) 149–158. [10] D.M. Stachera, R.F. Childs, A.M. Mika, J.M. Dickson, Acid recovery using diffusion dialysis with poly(4-vinylpyridine)filled microporous membranes, J. Membr. Sci. 148 (1998) 119–127. [11] X. Tongwen, F.F. Zha, Fundamental studies on a new series of anion exchange membranes: effect of simultaneous amination-crosslinking processes on membranes ion-exchange capacity and dimensional stability, J. Membr. Sci. 199 (2002) 203–210. [12] K. Kontturi, A. Savonen, M. Vuoristo, Study of adsorption and ion-exchange properties of some porous membranes, Acta Chem. Scand. 48 (1994) 1–11. [13] K. Rengan, R. Engel, Ammonium cascade molecules, J. Chem. Soc. Chem. Commun. (1992) 757–758. [14] M. Hichour, F. Persin, J. Molenat, J. Sandeaux, C. Gavach, Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes, Desalination 122 (1999) 53–62. [15] R. Delimi, J. Sandeaux, R. Sandeaux, C. Gavach, Properties of an anion-exchange membrane in contact with aqueous solutions of sodium chloride and sodium benzenecarboxylate or benzenesulfonate, J. Membr. Sci. 103 (1995) 83–94. [16] (a) D. Sen, S. Mazumder, A. Sequeria, A.K. Ghosh, V. Ramachandhran, M.S. Hanra, B.M. Misra, Fractal morphology in porous membranes: a small-angle X-ray scattering investigation, J. Macromol. Sci. Phys. B38 (1999) 341–348; (b) D. Sen, S. Mazumder, P. Sengupta, A.K. Ghosh, V. Ramachandhran, Small-angle X-ray scattering study of porous polysulfone and poly(vinyl pyrrolidone)/polysulfone blend membranes, J. Macromol. Sci. Phys. B39 (2000) 235–243. [17] D.F.R. Mildner, P.L. Hall, Small-angle scattering from porous solids with fractal geometry, J. Phys. D: Appl. Phys. 19 (1986) 1535–1545. [18] K. Scott, Handbook of Industrial Membranes, Elsevier, Oxford, UK, 1995, p. 197. [19] A. Goswami, A. Acharya, A.K. Pandey, Study of self-diffusion of monovalent and divalent cations in Nafion-117 ion-exchange membrane, J. Phys. Chem. B105 (2001) 9196–9201. [20] A. Heintz, E. Wiedemann, J. Ziegler, Ion exchange diffusion in electromembranes and its description using the Maxwell–Stefan formalism, J. Membr. Sci. 137 (1997) 121– 132. [21] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, second ed., Butterworths, London, 1965, p. 314.
Formation and characterization of highly crosslinked anion-exchange membranes Ashok K. Pandey a , A. Goswami a,∗ , Debasis Sen b , S. Mazumder b , Ronald F. Childs c a b
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Deparment of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4M1 Received 21 October 2002; accepted 14 February 2003
Abstract Highly crosslinked/hyperbranched anion-exchange membranes have been prepared by anchoring poly(vinylbenzyl chloride) (PVBCl) within the pores of poly(propylene) microporous base membranes by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). The resulting PVBCl-filled precursor membranes were converted to anion-exchange membranes by reacting these with (i) excess of DABCO followed by alkylation with ␣,␣ -dibromo-p-xylene (DBX) (membrane A), and (ii) with excess of tetraethylenepentamine (TEPA) (membrane B). A third membrane C was synthesized by alkylating membrane B with DBX. The chemical analyses indicated that these anion-exchange membranes consist of highly crosslinked/hyperbranched anionic gels within the pores of host microporous membranes. These anion-exchange membranes were characterized in terms of water-uptake capacities, ion-exchange capacities and thermal stability. The physical structures of the membranes were examined by small angle X-rays scattering (SAXS) analysis. The study of SAXS profiles of the dry and water equilibrated membrane A samples indicated that microstructure of anionic gel within the pores of membrane was changed significantly on water equilibration. However, no significant change in the SAXS profile was observed in wet samples of membranes B and C with respect to their dry samples. Thus, the crosslinking generated in membrane A was flexible and very rigid in membranes B and C. The self-diffusion coefficient of I− ions and transport numbers of Cl− ions were measured to examine the effects of crosslinking on transport properties of the membranes. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Anion-exchange membrane; Pore-filled method; Highly rigid crosslinking; Small angle X-rays scattering (SAXS); Self-diffusion coefficient
1. Introduction Pore-filled anion-exchange membranes are prepared by incorporating the polyelectrolyte gel within ∗ Corresponding author. Fax: +91-22-25505150/25505151. E-mail addresses: [email protected] (A.K. Pandey), [email protected] (A. Goswami), debasis@apsara. barc.ernet.in (D. Sen), [email protected] (S. Mazumder), [email protected] (R.F. Childs).
pores of a robust microporous host membrane [1–6]. The microporous host membrane provides containment and mechanical strength to the ionic gel and mitigates the impact of osmotic forces. The appropriate chemistry of the ionic gel, the guest component, leads to good separation characteristics of resulting membrane. There are two possible routes for preparing strongly basic pore-filled anion-exchange membranes. These are: (i) in situ polymerization of suitable monomer with a crosslinker within pores of
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00084-X
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a host microporous membrane [1–5], and (ii) in situ crosslinking of preformed polymer with a suitable crosslinker within pores of a microporous membrane [7,8]. Most work to date on these systems has involved poly(4-vinylpyridine) (P4VP) or poly(vinylbenzyl chloride) (PVBCl) polymers anchored within the pores of a host microporous membrane. The fixed positive charge sites in the membranes are obtained by subjecting P4VP and PVBCl-filled membranes to alkylation or amination reactions, respectively. Both the routes have been found to be effective in producing membranes in which a well-defined polyelectrolyte is firmly anchored within the pores. These membranes give excellent performance in ultra-low pressure water softening [8,9], and in diffusion dialysis for acid recovery [10]. The extent of pore-filling in both the routes is dependent on the solubility of the monomer or preformed polymer in the solvent used for the incorporation step. Typically, the high solubility of monomers as compared to polymers means that membranes with a high degree of gel incorporation can be prepared by the in situ polymerization method. However, high incorporations using the in situ polymerization route typically require substantial amount of a crosslinking agent DVB. This, in turn, leads to significant decrease in the ion-exchange capacity of the resulting pore-filled ion-exchange membranes. Recently, a method involving simultaneous amination-crosslinking of brominated poly(2,6-dimethyl1,4-phenylene oxide) (PPO) has been reported for the preparation of crosslinked anion-exchange membranes [11]. This method involves use of trimethylamine and ethylenediamine for generating fixed positive charge sites and crosslinks in the membrane, respectively. The nitrogen atoms involved in crosslinking are still basic and can be alkylated to form quaternary ammonium groups. Another possible approach would be to use a crosslinker that generates crosslinks with fixed positive charge sites [7]. These approaches allow a high degree of crosslinking to be introduced without sacrificing fixed positive charged site density in the membrane. In this article, we report the preparation of the three highly crosslinked/hyperbranched anion-exchange membranes by in situ crosslinking of PVBCl polymer with 1,4-diazabicyclo[2.2.2]octane (DABCO) in the pores of poly(propylene) microporous host
membrane. The subsequent chemical modifications involving amination and alkylation have been used to produce positively charge crosslinks in the pore-filled anion-exchange membrane. The resulting membranes have been chemically and physically characterized. Transport numbers of Cl− and self-diffusion coefficients of I− ions in the membranes were measured to examine the transport properties of membranes.
2. Experimental 2.1. Materials Poly(vinylbenzyl chloride) (60/40 mixture of 3- and 4-isomers with average molecular weight Mn ca. 55,000 and Mw ca. 100,000), diazabicyclo[2.2.2]octane (98% purity), ␣,␣ -dibromo-p-xylene (DBX) (97% purity) and tetraethylenepentamine (technical grade) were procured from Aldrich. These chemicals were used without any further purification. The linear chains of PVBCl used in the present work contained 80% benzyl chloride units and 20% styrene units [7]. The host microporous substrates used for the preparation of pore-filled anion-exchange membranes were poly(propylene) microfiltration membranes. The specifications of this poly(propylene) microporous membrane are: average pore-diameter 0.26 m, thickness 63 ± 2 m and porosity 60 vol.%. The radiotracer 131 I used for measuring the self-diffusion coefficients of I− ions in the membrane samples was obtained from the Board of Radiation and Isotope Technology, Mumbai, India. 2.2. Preparation of pore-filled anion-exchange membranes 2.2.1. Preparation of PVBCl-filled precursor membranes A known amount of PVBCl was dissolved in a mixed solvent consisting of 75 vol.% N,N -dimethylformamide (DMF) and 25 vol.% of tetrahydrofuran (THF). A solution of the crosslinker DABCO (concentration determined by the crosslinking ratio required) in DMF was prepared separately. Appropriate volumes of solution containing PVBCl and DABCO were rapidly mixed and total 2.5 ml of resulted mixture was applied on a weighed piece of the poly(propylene)
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microporous membrane (12 cm × 12 cm) placed on a glass plate. The concentration of PVBCl in the mixed pore-filling solution was 0.3 g/ml and ratio of DABCO to PVBCl was adjusted to 12 ± 1 mol%. The excess of solution on the top of poly(propylene) membrane was removed by a gentle application of a Teflon roller and a Teflon gasket was placed around the membrane sample. Finally, the filled poly(propylene) membrane with Teflon gasket around it was sandwiched between two glass plates for approximately 16 h at room temperature to allow gelling (crosslinking) to occur. The pore-filled membranes thus formed were washed with methanol and THF to remove un-crosslinked PVBCl and DABCO. 2.2.2. Preparation of membrane A The PVBCl-filled precursor membrane was reacted with an excess of DABCO. This reaction was carried out by immersing the precursor membrane (12 cm × 12 cm) in 50 ml of a DMF solution containing 2 wt.% of DABCO. The reaction was carried out at 70 ◦ C for 16–18 h. The unreacted DABCO present in the resulting membrane was removed by washing it with excess of water and methanol. The DABCO-reacted precursor membrane was alkylated with ␣,␣ -dibromo-p-xylene to obtain membrane A. This reaction was carried out by keeping the membrane in 50 ml solution of 2 wt.% DBX in DMF for 16–18 h at 70 ◦ C. 2.2.3. Preparation of membrane B The membrane B was prepared by treating PVBCl-filled precursor membrane with a DMF solution of tetraethylenepentamine (TEPA) (10 wt.%) for 16–18 h at 70 ◦ C. 2.2.4. Preparation of membrane C The membrane C was formed by alkylation of membrane B with DBX. The experimental conditions used for this reaction are the same as described for the membrane A, i.e. immersing membrane B in 50 ml of a solution of 2 wt.% DBX in DMF for 16–18 h at 70 ◦ C. 2.3. Characterization of anion-exchange membranes The final membranes were washed with DMF, methanol, an excess of water and 0.1 mol/l HCl to remove soluble components present in the membrane sample before being dried in a vacuum and
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weighed. The membrane samples were conditioned by soaking in a 0.5 mol/l aqueous solution of NaCl for 2–3 days and washed with water before use. The anion-exchange membranes were converted to I− or SO4 2− ionic forms by equilibrating these with relevant sodium salt solution (0.5 mol/l) for 24 h. The mass-gain of the membrane samples was determined as the wt.% of anionic gel introduced in the pores of membrane per unit weight of nascent membrane. The weight of the gel introduced in the pores of the poly(propylene) microporous membranes was obtained from the weight difference of the final vacuum dried membrane sample and nascent membrane sample used for obtaining same membrane. The thickness of dry membrane samples were measured by using a digital micrometer (Mitutoy) with an accuracy of 0.001 mm. The thickness of wet samples were determined by pycnometer as described elsewhere [7]. The water-uptake capacity of the membranes, defined as the amount of water absorbed per unit dry weight of the membrane, was obtained from the difference in weight of wet membrane and vacuum dried membrane sample at room temperature. The transport numbers of Cl− ions for membranes A and C were determined by EMF method described by Kontturi et al. [12]. The rotating cell assembly with Ag/AgCl electrodes situated close to and on either side of the rotating membrane was used to measure EMF as a function of rotating speed. The rotating diffusion cell was filled with 1 mol/l NaCl solution, and non-rotating compartment was filled with 0.5 mol/l NaCl solution. The EMF was measured as a function of the rotation speed from 7.85 rad/s to 62.8 rad/s (75 to 600 rpm). The EMF at zero concentration polarization was obtained by extrapolation of the measured EMF as a function of 1/(rotation speed)1/2 . The equation described by Kontturi et al. [12] was used to obtain Cl− transport numbers from EMF at zero concentration polarization. 2.4. Elemental analyses The contents of C, H, N and S in the membrane samples with SO4 2− counterions were estimated by using an Elemental Analyzer (EA 1110 model, C.E. Instruments). The amounts of covalently bonded chlorine and bromine in the membrane samples were determined by neutron activation analyses (NAA) of
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membranes in their SO4 2− or I− form so as to provide discrimination between covalent and ionic chlorine/bromine. The amount of counterions (Cl− or I− ) in salt equilibrated membrane samples was also determined by NAA. These data were used to calculate the fixed positive charge sites, and hence ion-exchange capacity of the membrane samples. Acid exchangeable ion-exchange capacities of the membranes were obtained by neutron activation analyses of Cl− ions in the membrane samples equilibrated with HCl solution of 0.2 mol/l concentration. For the neutron activation analyses, known amounts of the membrane and standard samples (KI, KCl and KBr) were irradiated for the appropriate time (15–30 min) in the APSARA reactor at BARC, Mumbai, India. The radioactivity (counts/s) generated in the membrane samples were monitored by counting 1642 keV ␥-ray of 38 Cl and 443 keV ␥-ray of 128 I with a 120 cm3 HPGe detector coupled to a PC-based multichannel analyzer. 2.5. Thermal analyses The thermal analyses of the membrane samples were carried out in flowing argon gas atmosphere using differential thermal analysis (DTA), thermogravimetry analysis (TGA) and differential thermogravimetry analysis (DTGA). The thermograms were recorded at a heating rate of 3 and 5 ◦ C/min in a thermoanalyzer (Setaram, France) using alumina as the reference material for DTA. 2.6. Small angle X-ray scattering Small angle X-ray scattering (SAXS) measurements of the wet (water equilibrated) and dry membrane samples with Cl− counterions were carried out with a rotating anode based 12 KW Rigaku SAXS machine equipped with a three-slit system with soller slits. Intensities were measured by a scintillation counter with a pulse height analyzer. Ni-filtered Cu K␣ (λ = 0.1541 nm) radiation was used as incident X-ray source. The scattered intensities were recorded as a function of scattering vector q (=4π sin θ /λ, where 2θ is the scattering angle and λ is the incident X-ray wavelength). The intensities were corrected for sample absorption and for smearing effects of collimating slits.
2.7. Measurements of self-diffusion coefficients The measurements of self-diffusion coefficients of I− ions in the various anion-exchange membranes were carried out using 2.5 cm × 2.5 cm pieces of samples. A membrane sample in the I− -form was loaded with 131 I− radiotracer ions by equilibrating it in 25 ml of the vigorously stirred solution of KI (0.01 mol/l) spiked with known amount of radiotracer. The membrane sample was equilibrated for 4 h before taking out for monitoring the amount of the radiotracer loaded in it. The radiotracer ions diffusing out from the membrane into equilibrating salt solution was measured as a function of time in order to determine the self-diffusion coefficient of I− ions in the membrane. The desorption experiments were done by immersing the radiotracer loaded membrane sample into 25 ml aqueous solution of 0.5 mol/l KI at 27 ◦ C. The constant stirring (≈52.4 rad/s (≈500 rpm)) was used to minimize the concentration gradient of radiotracer ions in the equilibrating solution. The amount of radiotracer ions diffusing out of the membrane sample was monitored by taking out 100 l samples of equilibrating salt solution at regular time intervals and monitoring the radioactivity by ␥-counting using a well-type NaI(Tl) detector connected to a single channel analyzer.
3. Results and discussion The precursor membranes used for constructing the massively crosslinked anion-exchange membranes were prepared by the introduction and immobilization of PVBCl within pores of a poly(propylene) host membrane. An in situ crosslinking of PVBCl polymer chains with DABCO was used for immobilizing PVBCl into the pores of support membrane [7]. The degree of crosslinking was controlled by adjusting the amounts of DABCO and PVBCl in the solution used for pore-filling. The amounts of PVBCl and DABCO incorporated into the support membrane were computed from the ratio of DABCO to PVBCl in the pore-filling solution and total weight of the material anchored in the membrane. The nitrogen content calculated from the expected weight of DABCO in the membrane based upon the concentration of DABCO in the filling
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Scheme 1. Preparation of PVBCl-filled precursor membrane.
solution was found to be in good agreement (±3%) with experimental nitrogen content. As observed in our previous work [7], the extent of fixed positive charge sites generated during initial crosslinking was found to correspond to the measured nitrogen content indicating that the DABCO molecules were direacted and formed crosslinks between PVBCl chains in the membrane as shown in Scheme 1. The initial degree of crosslinking, defined as ratio of moles of direacted crosslinker to moles of benzyl chloride units of PVBCl in the precursor membrane, was found to be 12 ± 2 mol% which is in agreement with degree of crosslinking expected from the ratio of DABCO to PVBCl in pore-filling solution. Benzyl chloride units in the precursor membrane, left after initial crosslinking of PVBCl with DABCO, were converted to quaternary ammonium groups to generate fixed positive charge sites. The reactions in the conversion of benzyl chloride units to quaternary ammonium groups were selected so as to form positively charged crosslink sites between poly(vinylbenzyl) chains, which would not only ensure high degree of crosslinking without sacrificing ion-exchange capacity but also a higher degree of pore-filling in the membranes. Two possible routes chosen were based on the reaction of precursor mem-
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brane with: (i) excess of DABCO to form membrane A, and (ii) excess of tetraethylenepentamine to form membrane B and subsequently membrane C. The nature of the reaction of DABCO with the benzyl chloride units of PVBCl is dependent upon the mole ratio of DABCO to benzyl chloride units [7,13]. The DABCO is predominately direacted when there is an excess of benzyl chloride units available. This fact was used for initial crosslinking of PVBCl with DABCO to form precursor membranes. If mole ratio of DABCO to PVBCl units is greater than 1, then most of the DABCO molecules remain either monoreacted or unreacted, i.e. one of the tertiary nitrogen of DABCO reacts with PVBCl and other tertiary nitrogen remains free. This free tertiary nitrogen of DABCO molecules attached to poly(vinylbenzyl) (PVB) chains, can be further reacted with di-alkylating reagents like dibromo-p-xylene to form PVB–DABCO–xylene–DABCO–PVB crosslinking chains. The membrane A was prepared by utilizing this approach. The chemical reaction involved in the preparation of membrane A from the precursor membrane is shown in the Scheme 2. The extent of chemical reactions involved in the preparation of membrane A was determined by elemental analyses of vacuum dried membrane samples (2 cm × 2 cm) after each step. The elemental analyses of membrane A are summarized in Table 1. The amount of fixed charge sites in the PVBCl-filled precursor membrane, which is in close agreement with the nitrogen content introduced during initial crosslinking, is given in the column 2 of Table 1. The amount of covalently bonded chlorine in the structure I of Scheme 2 was obtained by NAA of the precursor membrane with I− counterions (see column 3, Table 1). In order to estimate the amount of chlorine substitution by DABCO, the DABCO-reacted membrane (containing I− or SO4 2− counterions) was subjected to chlorine analysis in a similar fashion as described for precursor membrane. The presence of negligible amount of covalently bonded chlorine in the DABCO-reacted membrane, as shown in the column 4 of Table 1, indicated nearly quantitative substitution of chlorine with DABCO. From the knowledge of amount of nitrogen introduced in the formation of precursor membrane and amount of chlorine displaced by DABCO, the expected nitrogen content of DABCO-reacted membrane was calculated for mono-reaction with DABCO. The
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Scheme 2. Preparation of membrane A from PVBCl-filled precursor membrane.
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Table 1 Elemental analyses of the anion-exchange membranes Membrane identification
Fixed charge sites after initial crosslinkinga (mmol/cm3 )
Cl in precursor membraneb (mmol/cm3 )
Cl left after aminationc (mmol/cm3 )
Expected N from bonded Cld (mmol/cm3 )
Experimental N-content after aminatione (mmol/cm3 )
Quaternary N (fixed charge sites)f (mmol/cm3 )
Br-content in DBX-reacted membraneg (mmol/cm3 )
A B C
0.37 ± 0.02 0.36 ± 0.02 0.36 ± 0.02
1.06 ± 0.04 1.10 ± 0.03 1.05 ± 0.04
0.03 ± 0.01 0.01 ± 0.01 0.02 ± 0.01
2.49 5.86 5.61
2.38 ± 0.09 4.17 ± 0.15 4.08 ± 0.18
1.78 ± 0.03 0.70 ± 0.03 2.14 ± 0.03
Not detected – 0.02 ± 0.01
a
Generated during initial crosslinking to anchor PVBCl within pores of microporous membrane (precursor membrane, Scheme 1). The total nitrogen content of the precursor membrane was in close agreement with the amount of fixed positive charge sites (±3%). b Covalently bonded Cl corresponding to benzyl chloride units in precursor membrane (structure I of Schemes 2 and 3). c Residual covalently bonded chlorine in the membranes after reaction with DABCO or TEPA (structure II of Schemes 2 and 3). d Total calculated nitrogen content for 1:1 reaction of benzyl chloride units with DABCO/TEPA (=moles nitrogen introduced during crosslinking + (moles of substituted Cl × no. of nitrogen atoms in the aminating reagent)). e Total nitrogen content of final membrane (after amination) as estimated by elemental analyses (structure II of Schemes 2 and 3 for membranes A, B and C). f Fixed charge in the membrane as estimated by elemental analyses of exchangeable Cl− and I− counterions. g Total amount of DBX-reacted in the membrane C was 1.51 mmol/cm3 .
close agreement between expected and experimental nitrogen contents (columns 5 and 6 of Table 1), suggests that mono-reaction of the residual benzyl chloride units in the membrane with excess of DABCO occurred as shown in structure II of Scheme 2. The extent of quaternization of free tertiary nitrogen of DABCO attached to benzyl chloride units, shown in the structure II of Scheme 2, with DBX was measured by estimating the fixed charge sites (counterions) after equilibrating the membrane samples with the appropriate salt solutions. The comparison of total nitrogen content of structure II of Scheme 2 and fixed positive charge sites of structure III of Scheme 2 indicates that the 74% of the total nitrogen of DABCO in membrane A exist as quaternary ammonium groups (Table 1). This means that the remaining 26% of the tertiary nitrogen of DABCO did not undergo quaternization with DBX. The absence of bromine in membrane A indicated that DBX molecules were direacted in membrane A to form crosslinks between free ends of two DABCO molecules attached to PVB chains. Thus, the membrane A has two types of crosslinks. First type of crosslinks is short chain of PVB–DABCO–PVB formed during anchoring of PVBCl in the pores of poly(propylene) microporous host membrane. The other is due to formation PVB–DABCO–xylene–DABCO–PVB chains during the conversion of PVBCl-filled precursor
membrane into anion-exchange membranes. The possible chemical composition of membrane A based on chemical analyses is shown in structure III of Scheme 2. The other route examined to form highly crosslinked anion-exchange membranes was based on the amination of precursor membrane with excess of TEPA, membrane B. This membrane was subjected to chemical analyses in a similar fashion as described for membrane A. The total nitrogen content of membrane B was found to be less than that expected from the mono-substitution of the remaining benzyl chlorides with TEPA (columns 5 and 6, Table 1). The presence of negligible amount of covalently bonded chlorine in TEPA-reacted precursor membrane (column 4, Table 1) suggests that the substitution of chlorine was almost complete. Hence, the difference in the expected and experimental nitrogen contents suggests that some of TEPA molecules are direacted with available benzyl chloride units in the precursor membrane as shown in the structure II of Scheme 3. Comparison of columns 2 and 7 for membrane B in Table 1, also shows that fixed positive charge sites were generated during the reaction of precursor membrane with TEPA molecules. Thus, direaction of TEPA with benzyl chloride units also involves reactions leading to quaternization of some of the nitrogen atom of TEPA chains. Thus, rigid crosslinks comprising chain
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Scheme 3. Preparation of membranes B and C from PVBCl-filled precursor membrane.
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of PVB–N(TEPA)–PVB were also introduced during the conversion of precursor membrane to membrane B as shown in the Scheme 3. Membrane B possesses substantial amount of un-quaternized nitrogen as shown in structure II, Scheme 3. In order to enhance the number of ion-exchange sites, membrane B was further alkylated with DBX to form membrane C. The chemical analysis of this membrane, summarized in Table 1, shows considerable enhancement the ion-exchange sites of membrane C as compared to its precursor membrane B. The presence of negligible amounts of bromine in the neutron activation analysis of membrane C indicated that the DBX molecules were largely direacted and formed additional crosslinks between two nitrogen atoms of TEPA molecules attached to PVBCl in the membrane. However, the amount of fixed positive charge sites generated during quaternization of membrane B with DBX was found to be only ≈50% of total moles of DBX-reacted with the membrane B. The amount of DBX in the membrane was obtained from the increase in weight during the quaternization. Therefore, it appears that most of DBX molecules formed one positively charge crosslink site and other side of DBX was attached to a secondary or primary amine site in the membrane C as shown in the structure III of Scheme 3. The crosslinking present in membrane C is expected to be highly rigid.
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3.1. Properties of the anion-exchange membranes The three types of the anion-exchange membranes developed in the present work were characterized in terms of mass-gain (weight of gel loaded per unit weight of the host membrane), thickness, ion-exchange capacity and water-uptake capacity (Table 2). The thickness of all the membranes (63 ± 2 m) was found to be same as the thickness of microporous polypropylene host membrane used for pore-filling. No significant changes in the other dimensions (length or breadth) of the membrane were observed, regardless of degree of loading (mass-gain) and chemical composition of the membrane. The membranes were found to absorb water readily. The water-uptake capacities of these membranes were measured as the weight of water sorbed in the membrane over the weight of the dry membrane. The water-uptake capacity of membrane B was found to be higher than that of membrane C as would be expected based on the degree of loading (mass-gain) (Table 2). This trend was not observed in comparing membranes A and C, despite the ion-exchange capacities of both membranes being nearly identical. It can be seen in Table 2 that the water-uptake and fixed charge ion-exchange capacities of membranes A and C are comparable to commercially available anion-exchange membranes [14,15]. As evident from Table 2, the acid exchangeable ion-exchange capacities were consider-
Table 2 Properties of anion-exchange membranes Membrane identification
Ion-exchange capacity
Water-uptake capacity in
Totala (mmol/g)
Fixed charge (mmol/g)
Cl− form (wt.%)
I− form (wt.%)
Anion-exchange membrane developed in the present work A 184 62 ± 1 B 149 63 ± 1 C 235 63 ± 1
2.53 2.68 3.32
2.0 0.9 2.1
36.5 53.9 43.0
30.3 54.2 35.6
Commercially available anion-exchange membranesb Selemion-DSV (Asahi Glass) – Neosepta-AFX (Tokuyama Soda) – Neosepta-AFN (Tokuyama Soda) – Neosepta-AMX (Tokuyama Soda) – Neosepta-ACS (Tokuyama Soda) – ADS XL 10 (Morgane-Solay) –
– – – – – –
2.0 1.3 1.9 1.3 1.6 1.27
40 32 32 20 21 32
a b
Mass-gain (wt.%)
Thickness of wet membrane (m)
– – – – – 150
Acid exchangeable ion-exchange capacity. Data taken from [14] for DSV, AFX, AFN, AMX and ACS and from [15] for ADS XL 10.
– – – – –
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ably higher than fixed charge ion-exchange capacities in all the membranes. This is due to the fact that all the nitrogen sites present in the membrane could not be alkylated to form quaternary ammonium groups. Thus, these un-quaternized nitrogen sites in the membranes were still basic and were responsible for higher acid exchangeable ion-exchange capacities of the membranes. The transport numbers of Cl− ions (tCl ) for membranes A and C were determined by EMF method [12] and found to be 0.94 and 0.96, respectively. The higher transport numbers of these membranes suggest that they would be well suited for electro-chemical applications. 3.2. Small angle X-rays scattering In order to examine the physical structures of the anionic gels anchored in the microporous poly(propylene) host membranes, the membranes A, B and C were examined using small angle X-rays scattering (SAXS) analysis [16]. SAXS measurements of membrane samples (in Cl− form) were carried out in both dry and wet (water equilibrated) states. The variations of scattered intensities of X-ray I(q) as a function of scattering vector q (=4π sin θ /λ, where 2θ is the scattering angle and λ is the incident X-ray wavelength) are shown in Figs. 1 and 2 for the membranes A and B, respectively. It can be seen in Figs. 1 and 2 that the SAXS intensities I(q) in double logarithmic scale are almost linear as a function of q. Therefore, I(q) from the membrane samples follows a power law relation with q [16]. As the membranes are highly crosslinked, it is expected that the pore interfaces are not exactly smooth. Hence, to extract real space information in the present case, the membranes were viewed as a surface fractal object. The term ‘fractal’ originates from the fact that the structures follow some self-similarity over a wide length scale. However, it should be noted that most of the real objects are self-affine rather than self similar with direction dependent rescaling factor, and in an average sense they behave as fractal object. SAXS intensity I(q) from a surface fractal is given by [17]: Is (q) ∝ q −1 [1 + (qξs )2 ](ds −5)/2 × sin[(ds − 1) arctan(qξs )]
(1)
Fig. 1. SAXS profiles of dry (䊊) and wet (䊉) samples of membrane A.
where ξ is the upper cut-off of the fractal and ds is the fractal dimension. It is evident from Fig. 1 that the SAXS profile changes significantly in the water equilibrated membrane A sample as compared to its dry
Fig. 2. SAXS profiles of dry ( ) and wet (䉱) samples of membrane B.
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Table 3 SAXS analysis of the membranes Membrane identification
A B
Upper cut-off, ξ (nm)
Fractal dimension, ds
Average pore radius (nm)
Dry
Wet
Dry
Wet
Dry
Wet
5 20
11 20
2.1 ± 0.2 2.0 ± 0.2
2.2 ± 0.2 2.0 ± 0.2
3.85 5.4
4.78 5.4
sample. The sharper nature of the profile for the wet sample indicates that there is significantly change in the microstructure of anionic gel in the pores of membrane A equilibrated with water. However, no significant change in SAXS profile was observed in the water equilibrated and dry forms of membrane B (Fig. 2). Similar behavior was observed in the SAXS profiles of wet and dry samples of membrane C as those found for membrane B. This suggests that the very rigid crosslinking in the anionic gel anchored in membranes B and C prevented the change in the microstructure of polymer chains with water. In contrast, the crosslinking in the membrane A appears to be more flexible. For membrane samples A and B, Eq. (1) has been fitted to their SAXS profiles. In case of membrane C, the SAXS signal was not very prominent. This was attributed to very small pore size in the membrane C as evident by very high degree of gel incorporation (see Table 2). The pore size distribution ρ(r) for a surface fractal can be written as: ρ(r) ∝ r −(ds +1)
(2)
The average pore size r can be obtained from Eq. (2) as: ∞ ρ(r)r dr r = 0 ∞ 0 ρ(r) dr However, considering the upper cut-off of the fractals, the integration limit in present case has been approximated to ξ π/1 ρ(r)r dr r = ξ (3) π/1 ρ(r) dr where the lower limit of the integration has been chosen as (π /1) because from the nature of the SAXS profiles it appears that the same trends continues at least up to q equal to 1 nm−1 . However, the exact
value of the lower limit of integration should be the lower cut-off of the fractals that could not be obtained in this case due to statistical ambiguity of the data beyond 0.5 nm−1 . The estimated parameters obtained from SAXS analyses of membranes A and B are tabulated in Table 3. The pore size of the membranes A and B were found to be in order of degree of gel incorporation in the host poly(propylene) membrane. 3.3. Thermal analysis Thermal stability of the membranes was examined by the differential thermal analysis, thermogravimetric analysis and differential thermogravimetric analysis. The results of the thermal analyses are summarized in Table 4. The first endothermic peaks in samples of membranes A, B and C were assigned to water loss from the samples. The first endothermic peak in DTA of the poly(propylene) host membrane was found to be at 167 ◦ C. This endothermic peak was assigned to crystalline melting point (Tm ) of the poly(propylene) host membrane as no mass loss was observed in the TGA at the same temperature range. The reported Tm of crystalline poly(propylene) (167–170 ◦ C) is found to be in close agreement with the temperature assigned to Tm of the poly(propylene) host membrane [18]. Comparison of the DTA of the host membrane (first endothermic peak) with the gel-filled anion-exchange membranes (A, B and C) (second endothermic peak) indicates the shifting of the onset of this peak to higher temperatures (≈153 ◦ C), however, the maximum of the peaks in all the membranes (host, A, B and C) was found to be at same 167 ◦ C. The results suggest that the anion-exchange membranes prepared in the present work are physically stable upto 153 ◦ C. The anion-exchange membranes (A, B and C) were found to degrade at lower temperatures (216–250 ◦ C) than the poly(propylene) host membrane (383 ◦ C).
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Table 4 Thermal analysis of membranes Membrane identification
Peak identification
Process
Base membrane
1 (endo) 2 (exo)
B
DTA (◦ C)
TGA (◦ C)
DTGA (◦ C)
Peak onset
Peak maximum
Peak end
Peak onset
Peak end
Tm Degradation
132 383
167 450
189 475
– 383
– 475
– 450
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
35 153 255
78 167
105 190
35 97 – – Continuous mass loss
72 –
C
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
46 153 246
104 167
135 182
46 135 – – Continuous mass loss
99 –
A
1 (endo) 2 (endo) 3 (exo)
Water loss Tm Degradation
45 155 216
85 167
139 182
45 146 – – Continuous mass loss
72 –
3.4. Self-diffusion coefficients of I− ions in the anion-exchange membrane A non-stationary radiotracer diffusion method was used for obtaining the self-diffusion coefficients of I− in anion-exchange membranes [19]. The amount of radiotracer ions (131 I− ) diffusing out of the membrane sample kept in contact with equilibrating solution as a function of time was analyzed by using following equation [19]: 8 −Dπ 2 tk ∗ n(tk ) = n 1 − exp π2 L2 2 −9Dπ tk 1 + exp + . . . (4) 9 L2 where n(tk ) and n∗ are the amounts of radiotracer ions in the equilibrating solution at fixed time t = tk and ∞, respectively, D the self-diffusion coefficient of the ions in the membrane and L the thickness of the membrane. The value of D was obtained by a nonlinear least-squares fit of Eq. (4) with both n∗ and D as free parameters. The plot of n(tk )/n∗ versus time tk for 131 I− ions in the membranes A, B and C is shown in Fig. 3. The values of D obtained in this analysis are (1.23 ± 0.04) × 10−7 , (2.91 ± 0.09) × 10−7 and (0.22 ± 0.01) × 10−7 cm2 /s for membranes A, B and C, respectively. The diffusion coefficient of Br− ions in AMS ion-exchange membrane was measured by Heintz
et al. using Maxwell–Stefan formalism [20]. The average value of D for Br− in AMS anion-exchange membrane (Tokuyama Corp., Japan) was found to 4.6 × 10−7 cm2 /s. Since the aqueous self-diffusion coefficients of Br− and I− are not very different [21] (Br− , 2.08×10−5 cm2 /s; I− , 2.04×10−5 cm2 /s), the mobility of ions as indicated by their D values in AMS and anion-exchanges membranes developed in the present work can be compared. It appears from this comparison that mobility of the ions in membranes A and B are in same order as that in AMS anion-exchange membrane. However, the mobility of ions in membrane C is 20 times less than that in AMS membrane. The D values of I− in the anion-exchange membranes A, B and C appear to follow the order of pore-radii rather than water-uptake capacities in these membranes. This seems to suggest that the diffusion of ions in the membranes is affected by narrow channels in the ion-diffusion path and not by the free volume available in the pores of the membrane. The physical structure of these diffusion channels is fixed by crosslinking. In accordance with SAXS observation, the physical dimensions of diffusion channels do not change in membranes B and C with water-uptake. Therefore, the possible reason of lower self-diffusion coefficient of I− in the membrane C as compared to membrane A, which has same ion-exchange capacity as membrane C, may be related to a higher tortuosity due to rigid crosslinking in the membrane C. The
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Fig. 3. Variation of fractions of the radiotracer I− ions in the equilibrating salt solution in contact with membranes A, B and C as a function of time t. The n(tk ) and n∗ represent the amount of 131 I− radiotracer ions at time tk and at equilibrium (t = ∞), respectively. The membranes A, B and C are represented by ( ), (䊊) and (䊐), respectively.
effect of lower water-uptake in the membrane A on self-diffusion coefficient is probably nullified by the more flexible crosslinking that allows the formation of more open diffusion channels on water equilibration.
4. Conclusions The present work demonstrates the possibility of making highly crosslinked/hyperbranched anion-exchange membrane with high ion-exchange and water-uptake capacities. The chemical and physical characterization of the membranes indicated that the longer crosslink chains (PVB–DABCO–xylene– DABCO–PVB in membrane A) are more flexible and allow the polymer chains to undergo change in their microstructure on equilibration with water within the pores of membrane. The shorter crosslinks (PVB–N(TEPA)–PVB in membranes B and C) were
found to highly rigid and prevent any physical changes of anionic gel within the pores of membrane. The transport number of Cl− ions and self-diffusion coefficients of I− in membranes seem to suggest that physical structure of the membranes developed in the present work plays an important role in transport process rather than the water-uptake capacity of the membrane. The anionic gel anchored in the membrane was found to thermally stable upto 200–250 ◦ C. However, the lower crystalline melting point temperature (Tm ) of the poly(propylene) host membrane makes these anion-exchange membrane physically stable up to ∼150 ◦ C. The properties of the anion-exchange membranes developed in the present work should allow sieving of small ions from the larger ions. The method of generating highly rigid crosslinking may be suitable for imprinting membrane by efficient fixing of geometry of template ions.
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Acknowledgements Authors are thankful to Dr. S.B. Manohar, Head, Radiochemistry Division and Dr. A.V.R. Reddy, Head, Nuclear Chemistry Section for their keen interest in the present work. Authors are also thankful to Drs. V.K. Manchanda and S.K. Mukherjee for their help in the elemental and thermal analyses of the membranes. References [1] A.M. Mika, R.F. Childs, M. West, J.N.A. Lott, Poly(4vinylpyridine)-filled microfiltration membranes: physicochemical properties and morphology, J. Membr. Sci. 136 (1997) 221–232. [2] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. McCarry, D.R. Gagnon, Porous polyelectrolyte-filled membranes: effect of crosslinking on flux and separation, J. Membr. Sci. 135 (1997) 81–92. [3] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. McCarry, D.R. Gagnon, A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity, J. Membr. Sci. 108 (1995) 37–56. [4] M. Ulbricht, H.-H. Schwarz, Novel high performance photo-graft composite membranes for separation of organic liquids by pervaporation, J. Membr. Sci. 136 (1997) 25–33. [5] K.L. Thunhorst, R.D. Noble, C.N. Bowman, Transport of ionic species through functionalized poly(vinylbenzyl chloride) membranes, J. Membr. Sci. 128 (1997) 183– 193. [6] V. Kapur, J.C. Charkoudian, S.B. Kesseler, J.L. Anderson, Hydrodynamic permeability of hydrogels stabilized within porous membranes, Ind. Eng. Chem. Res. 35 (1996) 3179– 3185. [7] A.K. Pandey, R.F. Childs, M. West, J.N.A. Lott, B.E. McCarry, J.M. Dickson, Formation of pore-filled ion-exchange membranes with in situ crosslinking: poly(vinylbenzyl ammonium salt)-filled membranes, J. Polym. Sci. A: Polym. Chem. 39 (2001) 807–820. [8] (a) R.F. Childs, A.M. Mika, A.K. Pandey, C. McCrory, S. Mouton, J.M. Dickson, Nanofiltration using pore-filled membranes: effect of polyelectrolyte composition on performance, Sep. Purif. Technol. 22–23 (2001) 507–517; (b) A.M. Mika, A.K. Pandey, R.F. Childs, Ultra-low-pressure water softening with pore-filled membranes, Desalination 140 (2001) 265–275.
[9] A.M. Mika, R.F. Childs, J.M. Dickson, Ultra-low pressure water softening: a new approach to membrane construction, Desalination 121 (1999) 149–158. [10] D.M. Stachera, R.F. Childs, A.M. Mika, J.M. Dickson, Acid recovery using diffusion dialysis with poly(4-vinylpyridine)filled microporous membranes, J. Membr. Sci. 148 (1998) 119–127. [11] X. Tongwen, F.F. Zha, Fundamental studies on a new series of anion exchange membranes: effect of simultaneous amination-crosslinking processes on membranes ion-exchange capacity and dimensional stability, J. Membr. Sci. 199 (2002) 203–210. [12] K. Kontturi, A. Savonen, M. Vuoristo, Study of adsorption and ion-exchange properties of some porous membranes, Acta Chem. Scand. 48 (1994) 1–11. [13] K. Rengan, R. Engel, Ammonium cascade molecules, J. Chem. Soc. Chem. Commun. (1992) 757–758. [14] M. Hichour, F. Persin, J. Molenat, J. Sandeaux, C. Gavach, Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes, Desalination 122 (1999) 53–62. [15] R. Delimi, J. Sandeaux, R. Sandeaux, C. Gavach, Properties of an anion-exchange membrane in contact with aqueous solutions of sodium chloride and sodium benzenecarboxylate or benzenesulfonate, J. Membr. Sci. 103 (1995) 83–94. [16] (a) D. Sen, S. Mazumder, A. Sequeria, A.K. Ghosh, V. Ramachandhran, M.S. Hanra, B.M. Misra, Fractal morphology in porous membranes: a small-angle X-ray scattering investigation, J. Macromol. Sci. Phys. B38 (1999) 341–348; (b) D. Sen, S. Mazumder, P. Sengupta, A.K. Ghosh, V. Ramachandhran, Small-angle X-ray scattering study of porous polysulfone and poly(vinyl pyrrolidone)/polysulfone blend membranes, J. Macromol. Sci. Phys. B39 (2000) 235–243. [17] D.F.R. Mildner, P.L. Hall, Small-angle scattering from porous solids with fractal geometry, J. Phys. D: Appl. Phys. 19 (1986) 1535–1545. [18] K. Scott, Handbook of Industrial Membranes, Elsevier, Oxford, UK, 1995, p. 197. [19] A. Goswami, A. Acharya, A.K. Pandey, Study of self-diffusion of monovalent and divalent cations in Nafion-117 ion-exchange membrane, J. Phys. Chem. B105 (2001) 9196–9201. [20] A. Heintz, E. Wiedemann, J. Ziegler, Ion exchange diffusion in electromembranes and its description using the Maxwell–Stefan formalism, J. Membr. Sci. 137 (1997) 121– 132. [21] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, second ed., Butterworths, London, 1965, p. 314.