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Coion exclusion properties of polyphosphazene ion-exchange membranes
Journal of Membrane Science 162 (1999) 135±143
Coion exclusion properties of polyphosphazene ion-exchange membranes Leslie Jones1, Peter N. Pintauro*, Hao Tang Department of Chemical Engineering, Tulane University, New Orleans, LA 70118, USA Received 11 December 1998; received in revised form 26 March 1999; accepted 29 March 1999
Abstract Equilibrium NaCl uptake studies in crosslinked and non-crosslinked cation-exchange membranes composed of sulfonated poly[bis(3-methylphenoxy)phosphazene] have been performed at 258C for external salt solutions ranging in concentration from 0.1 to 1.0 M. Membrane swelling, membrane ®xed-ion concentration, and membrane coion (anion) concentration were determined for polyphosphazene membranes with ion-exchange capacities of 0.60, 1.22, and 1.40 mmol/g and for a Na®on 117 per¯uorosulfonic acid membrane. For the polyphosphazene membranes, coion intrusion decreased with increasing membrane ®xed-ion concentration and with polymer crosslinking, but was higher than that with Na®on. Small angle X-ray diffraction was used to model the micro-morphology of the polyphosphazene membranes in terms of a cluster-network structure. Neither the dry polymer ion-exchange capacity nor the wet membrane ®xed-ion concentration were accurate measures of the coion exclusion capability of the polyphosphazene membranes, relative to Na®on. It was found, however, that coion exclusion in crosslinked and non-crosslinked polyphosphazene membranes and in Na®on 117 correlated linearly with the ®xed-charge/volume ratio of the ionic clusters. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Coion uptake; Water swelling; Ion-exchange membranes; Ionic clusters; Small-angle X-ray diffraction
1. Introduction We have reported previously on the fabrication and preliminary characterization of cation-exchange membranes composed of crosslinked and non-crosslinked sulfonated phosphazene polymers, such as poly[(3-methylphenoxy)(phenoxy)phosphazene],
*Corresponding author. Tel.: +1-504-865-5872; fax: +1-504865-6744; e-mail: [email protected] 1 Present address: International Paper Company, Inc., Odenton, MD.
poly[(4-methylphenoxy)(phenoxy)phosphazene], and poly[bis(3-methylphenoxy)phosphazene] [1±3]. Crosslinked polymer membranes prepared from sulfonated poly[bis(3-methylphenoxy)phosphazene] (henceforth abbreviated as PBMP) were found to have many attractive properties for solid-polymerelectrolyte fuel cell applications, such as low/moderate swelling (33% in water at 308C), high proton conductivity (0.082 S/cm for water-equilibrated ®lms at 658C), low water diffusivity (1.210ÿ7 cm2/s at 658C), and good chemical/mechanical stability, and may be an attractive alternative to per¯uorosulfonic acid membrane materials [4]. In the present study,
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we have turned our attention to the anion/cation separation properties of these membranes and have measured the coion exclusion properties of a series of crosslinked and non-crosslinked PBMP cationexchange membranes with SOÿ 3 ®xed charge sites. The primary means by which an ion-exchange membrane regulates anion/cation permeation is by the selective absorption of counterions vs. coions at the membrane/solution interface. Thus, the measurement of the equilibrium uptake of positive and negative ions from external electrolytic solutions of varying concentration is a crucial test of any new ion-exchange membrane. Numerous studies have been carried out over the years to measure directly and/or model the equilibrium uptake of anions and cations in ion-exchange membranes, such as those composed of per¯uorinated ionomers (these membranes have important electrochemical applications). Cation uptake from external single salt solutions and multicomponent salt mixtures by DuPont's Na®on1 per¯uorosulfonic acid cation-exchange membranes, for example, has been examined thoroughly by collecting experimental data [5,6] and by developing spacecharge uptake models [7,8]. Similarly, the effect of external salt concentration on monovalent and divalent equilibrium absorption has been quanti®ed in Neosepta (manufactured by Tokuyama Soda Co.), Selemion (Asahi Glass Co.), and Aciplex (manufactured by Asahi Chemical Industry Co.) cation-exchange membranes [9,10]. 2. Experimental The base polymer for membrane fabrication, poly[bis(3-methylphenoxy)phosphazene], was purchased from Technically, Inc., Andover, MA. The polymer, with an average molecular weight of 2106 daltons, was dissolved in dichloroethane and sulfonated with SO3, as described elsewhere [1,3]. Membranes were cast from a solution of 5% polymer in N,N-dimethylacetamide and then dried at 708C for 72 h. For crosslinked membranes, 15 mol% benzophenone photo-initiator was dissolved in the casting solution and the dried ®lms were exposed to UV light (365 nm wavelength and 2.8 mW/cm2 intensity) for 20 h [2,11]. The thickness of crosslinked and non-crosslinked membranes was 100
and 200 mm, respectively. The ion-exchange capacity (IEC) of fully-dried non-crosslinked and crosslinked membranes was 0.60, 1.22, and 1.40 mmol/g. Coion uptake experiments were performed with all six PBMP membranes and with a Na®on 117 ®lm (0.909 mmol/g ion-exchange capacity). The Na®on membrane was pretreated by boiling for 1 h in 6 M HNO3 followed by boiling in deionized and distilled water for 1 h. The equilibrium membrane coion concentration was found by: (i) soaking a membrane sample of known dry weight (0.1±0.2 g) and dry volume in 100 ml of a given NaCl solution (between 0.1 and 1.0 M) for 24 h, (ii) measuring the wet membrane weight and volume after wiping excess electrolyte from the ®lm's surfaces (this wiping method has been used successfully in previous studies [7,8]), (iii) leaching Na and Clÿ ions from a membrane sample by soaking for 24 h in two 50 ml aliquots of deionized and distilled water, and (iv) determining the total moles of sodium ions in the combined leach solutions by atomic absorption spectrophotometry (Perkin Elmer Model 5000). From these experimental measurements, the dry membrane density (g/cm3), wet membrane density (g/cm3 of wet membrane), membrane porosity (cm3 of solution/cm3 of wet membrane), membrane ®xed-ion concentration (mmol/cm3 of wet membrane), the dry/ wet membrane weight ratio, and the membrane Clÿ concentration (mmol/cm3 of solution in the membrane) were computed (where the Clÿ ion concentration is equal to the Na concentration in the leach solutions). Small-angle X-ray scattering (SAXS) measurements were carried out on fully hydrated PBMP membranes with a Rigaku 12-kw rotating anode diffractometer, using Ni-®ltered Cu-Ka radiation and a Rigaku scintillation counter detector. The beam was collimated by two slits of widths 0.16 and 0.12 mm. The sample-to-detector distance was 200 mm and the scanning rate was 0.018/min. Sulfonated polymer ®lms were sealed thermally in a bag composed of oriented polypropylene to prevent water evaporation. The polypropylene material was ``transparent'' to the small-angle X-ray beam and the weight of a waterswollen PBMP membrane sample changed by less than 0.5% during the time period required to obtain small-angle X-ray scans.
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3. Results and discussion 3.1. Fixed-ion concentration The ion-exchange capacity (IEC) of a membrane, with units of moles of ®xed charges per gram of dry polymer, is not particularly useful for characterizing membrane performance because it does not take into account the swelling properties of the polymer. The addition of hydrophilic ®xed charges to a normally hydrophobic base-polymer causes the membrane to absorb polar solvents such as water, thus expanding the polymer matrix and diminishing the effective strength (i.e., concentration) of the ion-exchange groups. A more appropriate measure of the true concentration of ®xed-charge groups in a membrane during its actual use is the ®xed-ion concentration, with units of mol/cm3 of wet membrane. A membrane's IEC and ®xed-ion concentration (denoted as ) are related by, IEC m
(1)
where is the dry/wet membrane weight ratio and m is the wet membrane density. The use of Eq. (1) presumes that all ®xed-charge groups in the membrane are accessible for ion-exchange and that the membrane ®xed-charge concentration is homogeneous (this point was veri®ed by measuring on different
137
sections of the same membrane). Fig. 1(a) and (b) shows values for sulfonated PBMP membranes (non-crosslinked and crosslinked) and Na®on 117 as a function of the external NaCl in which the membranes were equilibrated. The values of increased moderately with increasing external NaCl concentration because the membranes swelled less in a high concentration salt solution (swelling data for PBMP and Na®on membranes are shown in Fig. 2(a) and (b)). The lower ®xed-ion concentrations for the non-crosslinked PBMP ®lms as compared to their crosslinked counterparts can also be attributed to polymer swelling. The effect of polymer crosslinking on was most pronounced for an IEC1.2; at lower ion-exchange capacities there was an insuf®cient number of ®xed charges to enhance greatly the hydrophilicity of the ®lms and crosslinking had little effect on reducing membrane swelling. For those PBMP membranes examined in this study, only the crosslinked 1.4 mmol/g IEC membrane had a ®xed-ion concentration greater than that in Na®on 117. 3.2. Membrane coion concentration The membrane coion concentration should increase with increasing external salt concentration and decrease with increasing membrane ®xed-ion concentration. This behavior is generally observed in
Fig. 1. Variation in membrane fixed-ion concentration with external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
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Fig. 2. Variation in membrane swelling with external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
Fig. 3(a) and (b), where the coion concentration in sulfonated PBMP membranes (crosslinked and noncrosslinked) and Na®on 117 are plotted as a function of the external salt concentration. The 1.4 IEC crosslinked PBMP membrane did not follow the predicted trend since its ®xed-charge concentration was greater than that for Na®on but it did not exclude coions as effectively as Na®on. We have used membrane coion concentration units of mmol/cm3 of solution in Fig. 3 because this quantity better re¯ects the true magnitude of the aqueous salt concentration within the membrane, as compared to a coion concentration based on the wet membrane volume (i.e., concentration units of mmol/cm3 of wet membrane). Since coions only enter into the membrane polymer matrix via pores and microvoids where aqueous solution is present, one should not include the volume of the inert polymer matrix when describing/ quantifying the concentration of sorbed salt. For example, at an external concentration of 0.5 M NaCl, the ordering of coion concentration (with units of mmol/cm3 of solution) in non-crosslinked and crosslinked PBMP membranes and Na®on 117 was 0.6 IEC>1.22 IEC>1.4 IEC, which is consistent with that anticipated from the magnitude of the ®xed-ion concentrations.
As noted above, our rationale in crosslinking the sulfonated PBMP ®lms was to reduce swelling and increase the effective concentration of ®xed membrane charges, thereby reducing coion intrusion. A careful examination of the data in Fig. 3 reveals that we have accomplished this goal. The coion concentration in a crosslinked membrane was always lower than that in its corresponding (similar IEC) non-crosslinked counterpart for the same external salt concentration (e.g., 12% lower for a 0.6 IEC membrane, 8% lower for a 1.22 IEC membrane, and 26% lower for a 1.4 IEC membrane, when the external NaCl concentration was 0.5 M). As can be seen in Fig. 3, none of the PBMP membranes was as effective in excluding coions as Na®on 117. It is also apparent from the ®xed-ion concentration data in Fig. 1 and the coion uptake results in Fig. 3, that neither nor the IEC is an accurate measure of a PBMP membrane's capability of blocking coion absorption, relative to Na®on. For example, the coion rejection of the 1.4 IEC/crosslinked PBMP membrane in a 0.5 M NaCl solution (with a ®xed-charge concentration 1.42 mmol/cm3 of wet membrane) is slightly lower than that for a 0.909 IEC Na®on ®lm (with a ®xed-ion concentration of 1.21 mmol/cm3). To further investigate this problem, the micro-morphologies of Na®on and PBMP mem-
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Fig. 3. Equilibrium membrane coion (Clÿ) concentration as a function of the external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
branes were quanti®ed and related to anion uptake, as discussed below. 3.3. Correlation of coion exclusion and membrane micro-structure Ionic clusters in Na®on cation-exchange membranes have been shown to control coion exclusion [12] and in¯uence membrane performance in chloralkali cells [13]. In the present study, small-angle X-ray diffraction data were collected on crosslinked (0.6IEC1.4) and non-crosslinked (0.6IEC1.6) polyphosphazene ®lms (fully hydrated and in the H form) and interpreted in terms of an ionic clusternetwork morphology. A diffraction peak was visible in the X-ray data for all of the PBMP ®lms, as shown in Figs. 4 and Fig. 5. The Bragg spacing for each X-ray peak, which was associated with the distance between the centers of two ionic clusters, was determined using Bragg's Law, 2d sin
branes with similar IEC) and was generally larger than that for water-equilibrated Na®on 117 (where d5.5 nm [13]). The Bragg spacings for all of the PBMP membranes as well as for Na®on 117, when plotted against the % membrane swelling in water, fall on a single curve, as shown in Fig. 6. Thus, the centerto-center distance between two clusters was regulated by (and was a single universal function of) the mem-
(2)
where is the X-ray wavelength (0.1542 nm for Cu Ka), d is the Bragg spacing (with units of nm), and 2 is the Bragg angle. For the polyphosphazene membranes, the Bragg spacing increased with increasing IEC, decreased with polymer crosslinking (for mem-
Fig. 4. Small angle X-ray diffraction scans of fully hydrated, sulfonated (non-crosslinked) PBMP membranes. (Ð ± Ð) 0.60 IEC; (± ± ±) 0.80 IEC; (Ð Ð Ð) 1.20 IEC; (ÐÐÐ) 1.60 IEC.
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network'' model of Gierke and Hsu [13], where the ionic clusters are assumed to be spherical in shape and distributed on a simple cubic lattice network. In prior applications of this micro-structure model to Na®on membranes [13], it was assumed that the distance between clusters remained constant but the diameter of the clusters increased/decreased when the membrane swelled/shrank in salt solutions of different concentration. From the distance between clusters and equilibrium membrane swelling data, the diameter of a spherical cluster was calculated using the following equation [13], 1=3 6Np Vp 1=3 6V Dd (3) 1 V Fig. 5. Small angle X-ray diffraction scans of fully hydrated, sulfonated and crosslinked PBMP membranes. (Ð ± Ð) 0.60 IEC; (± ± ±) 1.22 IEC; (ÐÐÐ) 1.40 IEC.
brane water content, which, in turn, varied with IEC, type of polymer (PBMP vs. Na®on) and presence/ absence of crosslinks. To further quantify the inter-relationship between the cluster micro-morphology and coion uptake, the size and wall-charge density of ionic clusters in PBMP membranes were calculated. We used the ``cluster-
Fig. 6. Increase in spacing between ionic clusters in sulfonated PBMP and Nafion membranes with increasing membrane swelling. All membranes were in the H form and equilibrated in water. () non-crosslinked PBMP; (r) crosslinked PBMP; (&) Nafion.
where D is the cluster diameter (with units of m), d is the distance between clusters (the Bragg spacing, with units of m), V is the volume increase of the membrane upon absorption of an aqueous solution per cm3 of dry membrane, Np is the number of ion-exchange sites in a cluster, and Vp is the volume of a SOÿ 3 ionexchange site (6810ÿ30 m3 [13]). The quantity V/ (1V) in Eq. (3) is, by de®nition, the membrane porosity. The number of ion-exchange sites per cluster, Np, was obtained from the dry polymer density, the membrane IEC, and the extent of polymer swelling, NA p IEC 3 (4) d Np 1 V where NA is Avogadro's number, p is the dry polymer density (with units of g/m3), and IEC has units of mol/ g of dry membrane. Cluster diameters in polyphosphazene and Na®on 117 membranes equilibrated in various NaCl solutions at 258C are listed in Table 1 (the Na®on results were computed from Eqs. (3) and (4), using d5.5 nm and the swelling data in Fig. 2). All of the membranes sorbed less electrolyte when immersed in higher salt concentration solutions (see Fig. 2) and, accordingly, the cluster diameters decreased with increasing external salt concentration. The variation in cluster size was smaller than the decrease in membrane swelling due to the 1/3 power dependence of D on swelling (i.e., porosity) in Eq. (3). The size of the clusters in the polyphosphazene membranes increased with increasing IEC (due to an increase in swelling with the number of polymer ion-exchange sites) and decreased with polymer cross-
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Table 1 Diameter (in nm) of ionic clusters in sulfonated/non-crosslinked and sulfonated/crosslinked poly[bis(3-methylphenoxy)phosphazene] membranes External NaCl concentration (M)
Non-crosslinked PBMP 0.60 IEC 1.22 IEC 1.40 IEC Crosslinked PBMP 0.6 IEC 1.22 IEC 1.40 IEC Nafion 117
0.1
0.3
0.5
0.7
1.0
5.17 7.25 7.94
5.20 7.09 7.90
4.90 7.08 7.87
5.02 7.02 7.84
4.84 7.00 7.81
4.19 6.30 7.39 5.12
4.17 6.22 7.40 5.12
4.11 6.18 7.36 5.13
4.08 6.14 7.24 5.13
4.03 6.09 7.27 5.08
linking. Only clusters in the crosslinked, 0.60 IEC polyphosphazene membrane were smaller than those in Na®on 117 (the smaller cluster size was attributed to the low ®xed-charge concentration in combination with polymer crosslinking). At a given salt concentration, the ratio of macroscopic swelling for a PBMP ®lm (crosslinked or non-crosslinked) and a Na®on membrane (see Fig. 2) was always much smaller than the ratio of cluster volumes, indicating fewer ionic clusters in the polyphosphazene membranes. From the cluster size, macroscopic membrane swelling data, and the IEC of the dry polymer, the surface charge density in an individual cluster (, with units of
C/m2) was calculated using [13] " #1=3 eNA p d IEC 2 1 V 2 2 1 V 9 V
(5)
where e is the electronic charge (1.610ÿ19 C). One would anticipate that the absorption of mobile anions (coions) into a cluster is directly dependent on the cluster's wall charge density and inversely proportional to the cluster size. In other words, the magnitude of the electrostatic SOÿ 3 /coion repulsion force within the clusters of a PBMP membrane and the depth of penetration of this force into the solution away from
Fig. 7. Correlation between % coion exclusion and the charge/volume ratio of ionic clusters in PBMP and Nafion membranes. (^) 0.60 IEC non-crosslinked; (r) 0.60 IEC crosslinked; (~) 1.22 IEC non-crosslinked; (*) 1.22 IEC crosslinked; (*) 1.40 IEC non-crosslinked; (&) 1.40 IEC crosslinked; () Nafion 117. Solid line is a linear least-squares fit of all data points.
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the cluster wall are dependent on the surface charge density of ion-exchange sites (, from Eq. (5)). The size (diameter) of a cluster relative to the electric ®eld penetration depth, on the other hand, would determine whether coions could absorb into the center region of a cluster. Both and the cluster diameter D are contained in the cluster's ®xed-charge/volume ratio, SOÿ 3 charges per cluster cluster volume cluster wall surface area 6 : cluster volume D
(6)
The experimentally measured coion exclusion (given as a percentage of the external Clÿ concentration that is blocked from absorbing into the membrane) correlated well with the charge/volume ratio of ionic clusters (i.e., 6/D), as shown in Fig. 7. All of the data (including the Na®on results) except the 1.22 IEC/crosslinked PBMP absorption results lie close to a linear least squares straight line correlation. Attempts to correlate the coion uptake data with only the cluster's surface charge density or the cluster volume were unsuccessful. As one would expect, the results show that membrane rejection of coions could be improved by increasing the ®xed-charge-to-volume ratio of the clusters, which can be achieved by increasing the polymer's IEC with additional crosslinking to limit/decrease membrane swelling. 4. Conclusions Equilibrium uptake studies of aqueous NaCl solutions in crosslinked and non-crosslinked cationexchange membranes composed of sulfonated poly[bis(3-methylphenoxy)phosphazene] (PBMP) have been performed at 258C for external salt solutions ranging in concentration from 0.1 to 1.0 M. Three different ion-exchange capacity PBMP polymers were examined: 0.60, 1.22, and 1.40 mmol/dry g. Membrane swelling, membrane ®xed-ion concentration, and membrane coion (anion) concentration were determined for the polyphosphazene membranes and compared to similar data collected with a Na®on 117 per¯uorosulfonic acid membrane. For the polyphosphazene membranes, coion intrusion decreased with membrane ®xed-ion concentration and crosslinking. None of the PBMP membranes rejected Clÿ ions
as effectively as Na®on, although coion intrusion with a 1.4 IEC/crosslinked PBMP ®lm was only slightly greater than that for Na®on. Small angle X-ray diffraction was used to model the micro-morphology of fully-hydrated polyphosphazene membranes in terms of a cluster-network structure. As expected, the size of ionic clusters in the PBMP ®lms increased with increasing IEC and decreased with crosslinking (for the same IEC). The cluster size (diameter) in all of the PBMP membranes except the 0.60 IEC/crosslinked ®lm was greater than that in Na®on. At a given salt concentration, the ratio of macroscopic swelling for a PBMP (crosslinked or non-crosslinked) ®lm and a Na®on membrane was always much smaller than the ratio of cluster volumes, indicating fewer ionic clusters in the polyphosphazene membranes. Neither the dry polymer ion-exchange capacity nor the wet membrane ®xed-ion concentration was an accurate measure of the coion rejection capabilities of the polyphosphazene membranes relative to Na®on. It was found, however, that coion exclusion in crosslinked and non-crosslinked polyphosphazene membranes and in Na®on correlated linearly with the ®xed-charge/volume ratio of the ionic clusters. Acknowledgements This work was funded by the Of®ce of Naval Research and by the National Science Foundation (Grant no. CTS-9632079). References [1] R. Wycisk, P.N. Pintauro, Sulfonated polyphosphazene ionexchange membranes, J. Membr. Sci. 119 (1996) 155. [2] R. Graves, P.N. Pintauro, Polyphosphazene membranes. II. Solid-state photocrosslinking of poly[(alkylphenoxy)(phenoxy)phosphazene] films, J. Appl. Polym. Sci. 68 (1998) 827. [3] H. Tang, P.N. Pintauro, Q. Guo, S. O'Connor, Polyphosphazene membranes III. Solid-state characterization and properties of sulfonated poly[bis(3-methylphenoxy) phosphazene], J. Appl. Polym. Sci. 71 (1999) 387. [4] Q. Guo, P.N. Pintauro, H. Tang, S. O'Connor, Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes, J. Membr. Sci. 154 (1999) 175. [5] H.L. Yeager, A. Steck, Ion exchange selectivity and metal ion separation with a perfluorinated cation exchange polymer, Anal. Chem. 51 (1979) 862.
L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 [6] H.L. Yeager, Cation exchange selectivity, in: A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated ionomer membranes, ACS symp. series, 180, 1982, pp. 41±64. [7] J.R. Bontha, P.N. Pintauro, Water orientation and ion hydration effects during multicomponent salt partitioning in a nafion 117 cation exchange membrane, Chem. Eng. Sci. 49 (1994) 3835. [8] P.N. Pintauro, R. Tandon, L. Chao, W. Xu, R. Evilia, Equilibrium partitioning of monovalent/divalent cation-salt mixtures in nafion cation-exchange membranes, J. Phys. Chem. 99 (1995) 12915. [9] H. Miyoshi, M. Chubachi, M. Yamagami, T. Kataoka, Characteristic coefficients for equilibrium between solution and neosepta or selemion cation exchange membranes, J. Chem. Eng. Data 37 (1992) 120.
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[10] H. Miyoshi, M. Chubachi, M. Yamagami, T. Kataoka, Characteristic coefficients of cation-exchange membranes for bivalent cations in equilibrium between the membrane and solution, J. Chem. Eng. Data 39 (1994) 595. [11] R. Wycisk, P.N. Pintauro, W. Wang, S. O'Connor, Polyphosphazene membranes. I. Solid-state photocrosslinking of poly[(4-ethylphenoxy)(phenoxy)phosphazene], J. Appl. Polym. Sci. 59 (1996) 1607. [12] S. Capeci, P.N. Pintauro, D.N. Bennion, The molecular-level interpretation of salt uptake and anion transport in nafion membranes, J. Electrochem. Soc. 136 (1989) 2876. [13] T.D. Gierke, W.Y. Hsu, The cluster network model of ion clustering in perfluorosulfonated membranes, in: A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated ionomer membranes, ACS symp. series, 180, 1982, pp. 283±307.
Coion exclusion properties of polyphosphazene ion-exchange membranes Leslie Jones1, Peter N. Pintauro*, Hao Tang Department of Chemical Engineering, Tulane University, New Orleans, LA 70118, USA Received 11 December 1998; received in revised form 26 March 1999; accepted 29 March 1999
Abstract Equilibrium NaCl uptake studies in crosslinked and non-crosslinked cation-exchange membranes composed of sulfonated poly[bis(3-methylphenoxy)phosphazene] have been performed at 258C for external salt solutions ranging in concentration from 0.1 to 1.0 M. Membrane swelling, membrane ®xed-ion concentration, and membrane coion (anion) concentration were determined for polyphosphazene membranes with ion-exchange capacities of 0.60, 1.22, and 1.40 mmol/g and for a Na®on 117 per¯uorosulfonic acid membrane. For the polyphosphazene membranes, coion intrusion decreased with increasing membrane ®xed-ion concentration and with polymer crosslinking, but was higher than that with Na®on. Small angle X-ray diffraction was used to model the micro-morphology of the polyphosphazene membranes in terms of a cluster-network structure. Neither the dry polymer ion-exchange capacity nor the wet membrane ®xed-ion concentration were accurate measures of the coion exclusion capability of the polyphosphazene membranes, relative to Na®on. It was found, however, that coion exclusion in crosslinked and non-crosslinked polyphosphazene membranes and in Na®on 117 correlated linearly with the ®xed-charge/volume ratio of the ionic clusters. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Coion uptake; Water swelling; Ion-exchange membranes; Ionic clusters; Small-angle X-ray diffraction
1. Introduction We have reported previously on the fabrication and preliminary characterization of cation-exchange membranes composed of crosslinked and non-crosslinked sulfonated phosphazene polymers, such as poly[(3-methylphenoxy)(phenoxy)phosphazene],
*Corresponding author. Tel.: +1-504-865-5872; fax: +1-504865-6744; e-mail: [email protected] 1 Present address: International Paper Company, Inc., Odenton, MD.
poly[(4-methylphenoxy)(phenoxy)phosphazene], and poly[bis(3-methylphenoxy)phosphazene] [1±3]. Crosslinked polymer membranes prepared from sulfonated poly[bis(3-methylphenoxy)phosphazene] (henceforth abbreviated as PBMP) were found to have many attractive properties for solid-polymerelectrolyte fuel cell applications, such as low/moderate swelling (33% in water at 308C), high proton conductivity (0.082 S/cm for water-equilibrated ®lms at 658C), low water diffusivity (1.210ÿ7 cm2/s at 658C), and good chemical/mechanical stability, and may be an attractive alternative to per¯uorosulfonic acid membrane materials [4]. In the present study,
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we have turned our attention to the anion/cation separation properties of these membranes and have measured the coion exclusion properties of a series of crosslinked and non-crosslinked PBMP cationexchange membranes with SOÿ 3 ®xed charge sites. The primary means by which an ion-exchange membrane regulates anion/cation permeation is by the selective absorption of counterions vs. coions at the membrane/solution interface. Thus, the measurement of the equilibrium uptake of positive and negative ions from external electrolytic solutions of varying concentration is a crucial test of any new ion-exchange membrane. Numerous studies have been carried out over the years to measure directly and/or model the equilibrium uptake of anions and cations in ion-exchange membranes, such as those composed of per¯uorinated ionomers (these membranes have important electrochemical applications). Cation uptake from external single salt solutions and multicomponent salt mixtures by DuPont's Na®on1 per¯uorosulfonic acid cation-exchange membranes, for example, has been examined thoroughly by collecting experimental data [5,6] and by developing spacecharge uptake models [7,8]. Similarly, the effect of external salt concentration on monovalent and divalent equilibrium absorption has been quanti®ed in Neosepta (manufactured by Tokuyama Soda Co.), Selemion (Asahi Glass Co.), and Aciplex (manufactured by Asahi Chemical Industry Co.) cation-exchange membranes [9,10]. 2. Experimental The base polymer for membrane fabrication, poly[bis(3-methylphenoxy)phosphazene], was purchased from Technically, Inc., Andover, MA. The polymer, with an average molecular weight of 2106 daltons, was dissolved in dichloroethane and sulfonated with SO3, as described elsewhere [1,3]. Membranes were cast from a solution of 5% polymer in N,N-dimethylacetamide and then dried at 708C for 72 h. For crosslinked membranes, 15 mol% benzophenone photo-initiator was dissolved in the casting solution and the dried ®lms were exposed to UV light (365 nm wavelength and 2.8 mW/cm2 intensity) for 20 h [2,11]. The thickness of crosslinked and non-crosslinked membranes was 100
and 200 mm, respectively. The ion-exchange capacity (IEC) of fully-dried non-crosslinked and crosslinked membranes was 0.60, 1.22, and 1.40 mmol/g. Coion uptake experiments were performed with all six PBMP membranes and with a Na®on 117 ®lm (0.909 mmol/g ion-exchange capacity). The Na®on membrane was pretreated by boiling for 1 h in 6 M HNO3 followed by boiling in deionized and distilled water for 1 h. The equilibrium membrane coion concentration was found by: (i) soaking a membrane sample of known dry weight (0.1±0.2 g) and dry volume in 100 ml of a given NaCl solution (between 0.1 and 1.0 M) for 24 h, (ii) measuring the wet membrane weight and volume after wiping excess electrolyte from the ®lm's surfaces (this wiping method has been used successfully in previous studies [7,8]), (iii) leaching Na and Clÿ ions from a membrane sample by soaking for 24 h in two 50 ml aliquots of deionized and distilled water, and (iv) determining the total moles of sodium ions in the combined leach solutions by atomic absorption spectrophotometry (Perkin Elmer Model 5000). From these experimental measurements, the dry membrane density (g/cm3), wet membrane density (g/cm3 of wet membrane), membrane porosity (cm3 of solution/cm3 of wet membrane), membrane ®xed-ion concentration (mmol/cm3 of wet membrane), the dry/ wet membrane weight ratio, and the membrane Clÿ concentration (mmol/cm3 of solution in the membrane) were computed (where the Clÿ ion concentration is equal to the Na concentration in the leach solutions). Small-angle X-ray scattering (SAXS) measurements were carried out on fully hydrated PBMP membranes with a Rigaku 12-kw rotating anode diffractometer, using Ni-®ltered Cu-Ka radiation and a Rigaku scintillation counter detector. The beam was collimated by two slits of widths 0.16 and 0.12 mm. The sample-to-detector distance was 200 mm and the scanning rate was 0.018/min. Sulfonated polymer ®lms were sealed thermally in a bag composed of oriented polypropylene to prevent water evaporation. The polypropylene material was ``transparent'' to the small-angle X-ray beam and the weight of a waterswollen PBMP membrane sample changed by less than 0.5% during the time period required to obtain small-angle X-ray scans.
L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143
3. Results and discussion 3.1. Fixed-ion concentration The ion-exchange capacity (IEC) of a membrane, with units of moles of ®xed charges per gram of dry polymer, is not particularly useful for characterizing membrane performance because it does not take into account the swelling properties of the polymer. The addition of hydrophilic ®xed charges to a normally hydrophobic base-polymer causes the membrane to absorb polar solvents such as water, thus expanding the polymer matrix and diminishing the effective strength (i.e., concentration) of the ion-exchange groups. A more appropriate measure of the true concentration of ®xed-charge groups in a membrane during its actual use is the ®xed-ion concentration, with units of mol/cm3 of wet membrane. A membrane's IEC and ®xed-ion concentration (denoted as ) are related by, IEC m
(1)
where is the dry/wet membrane weight ratio and m is the wet membrane density. The use of Eq. (1) presumes that all ®xed-charge groups in the membrane are accessible for ion-exchange and that the membrane ®xed-charge concentration is homogeneous (this point was veri®ed by measuring on different
137
sections of the same membrane). Fig. 1(a) and (b) shows values for sulfonated PBMP membranes (non-crosslinked and crosslinked) and Na®on 117 as a function of the external NaCl in which the membranes were equilibrated. The values of increased moderately with increasing external NaCl concentration because the membranes swelled less in a high concentration salt solution (swelling data for PBMP and Na®on membranes are shown in Fig. 2(a) and (b)). The lower ®xed-ion concentrations for the non-crosslinked PBMP ®lms as compared to their crosslinked counterparts can also be attributed to polymer swelling. The effect of polymer crosslinking on was most pronounced for an IEC1.2; at lower ion-exchange capacities there was an insuf®cient number of ®xed charges to enhance greatly the hydrophilicity of the ®lms and crosslinking had little effect on reducing membrane swelling. For those PBMP membranes examined in this study, only the crosslinked 1.4 mmol/g IEC membrane had a ®xed-ion concentration greater than that in Na®on 117. 3.2. Membrane coion concentration The membrane coion concentration should increase with increasing external salt concentration and decrease with increasing membrane ®xed-ion concentration. This behavior is generally observed in
Fig. 1. Variation in membrane fixed-ion concentration with external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
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Fig. 2. Variation in membrane swelling with external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
Fig. 3(a) and (b), where the coion concentration in sulfonated PBMP membranes (crosslinked and noncrosslinked) and Na®on 117 are plotted as a function of the external salt concentration. The 1.4 IEC crosslinked PBMP membrane did not follow the predicted trend since its ®xed-charge concentration was greater than that for Na®on but it did not exclude coions as effectively as Na®on. We have used membrane coion concentration units of mmol/cm3 of solution in Fig. 3 because this quantity better re¯ects the true magnitude of the aqueous salt concentration within the membrane, as compared to a coion concentration based on the wet membrane volume (i.e., concentration units of mmol/cm3 of wet membrane). Since coions only enter into the membrane polymer matrix via pores and microvoids where aqueous solution is present, one should not include the volume of the inert polymer matrix when describing/ quantifying the concentration of sorbed salt. For example, at an external concentration of 0.5 M NaCl, the ordering of coion concentration (with units of mmol/cm3 of solution) in non-crosslinked and crosslinked PBMP membranes and Na®on 117 was 0.6 IEC>1.22 IEC>1.4 IEC, which is consistent with that anticipated from the magnitude of the ®xed-ion concentrations.
As noted above, our rationale in crosslinking the sulfonated PBMP ®lms was to reduce swelling and increase the effective concentration of ®xed membrane charges, thereby reducing coion intrusion. A careful examination of the data in Fig. 3 reveals that we have accomplished this goal. The coion concentration in a crosslinked membrane was always lower than that in its corresponding (similar IEC) non-crosslinked counterpart for the same external salt concentration (e.g., 12% lower for a 0.6 IEC membrane, 8% lower for a 1.22 IEC membrane, and 26% lower for a 1.4 IEC membrane, when the external NaCl concentration was 0.5 M). As can be seen in Fig. 3, none of the PBMP membranes was as effective in excluding coions as Na®on 117. It is also apparent from the ®xed-ion concentration data in Fig. 1 and the coion uptake results in Fig. 3, that neither nor the IEC is an accurate measure of a PBMP membrane's capability of blocking coion absorption, relative to Na®on. For example, the coion rejection of the 1.4 IEC/crosslinked PBMP membrane in a 0.5 M NaCl solution (with a ®xed-charge concentration 1.42 mmol/cm3 of wet membrane) is slightly lower than that for a 0.909 IEC Na®on ®lm (with a ®xed-ion concentration of 1.21 mmol/cm3). To further investigate this problem, the micro-morphologies of Na®on and PBMP mem-
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139
Fig. 3. Equilibrium membrane coion (Clÿ) concentration as a function of the external NaCl concentration. (a) Sulfonated and non-crosslinked PBMP membranes (b) sulfonated and crosslinked PBMP membranes. (*) 0.60 IEC; (~) 1.22 IEC; (^) 1.40 IEC; (!) Nafion 117 (0.909 IEC).
branes were quanti®ed and related to anion uptake, as discussed below. 3.3. Correlation of coion exclusion and membrane micro-structure Ionic clusters in Na®on cation-exchange membranes have been shown to control coion exclusion [12] and in¯uence membrane performance in chloralkali cells [13]. In the present study, small-angle X-ray diffraction data were collected on crosslinked (0.6IEC1.4) and non-crosslinked (0.6IEC1.6) polyphosphazene ®lms (fully hydrated and in the H form) and interpreted in terms of an ionic clusternetwork morphology. A diffraction peak was visible in the X-ray data for all of the PBMP ®lms, as shown in Figs. 4 and Fig. 5. The Bragg spacing for each X-ray peak, which was associated with the distance between the centers of two ionic clusters, was determined using Bragg's Law, 2d sin
branes with similar IEC) and was generally larger than that for water-equilibrated Na®on 117 (where d5.5 nm [13]). The Bragg spacings for all of the PBMP membranes as well as for Na®on 117, when plotted against the % membrane swelling in water, fall on a single curve, as shown in Fig. 6. Thus, the centerto-center distance between two clusters was regulated by (and was a single universal function of) the mem-
(2)
where is the X-ray wavelength (0.1542 nm for Cu Ka), d is the Bragg spacing (with units of nm), and 2 is the Bragg angle. For the polyphosphazene membranes, the Bragg spacing increased with increasing IEC, decreased with polymer crosslinking (for mem-
Fig. 4. Small angle X-ray diffraction scans of fully hydrated, sulfonated (non-crosslinked) PBMP membranes. (Ð ± Ð) 0.60 IEC; (± ± ±) 0.80 IEC; (Ð Ð Ð) 1.20 IEC; (ÐÐÐ) 1.60 IEC.
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network'' model of Gierke and Hsu [13], where the ionic clusters are assumed to be spherical in shape and distributed on a simple cubic lattice network. In prior applications of this micro-structure model to Na®on membranes [13], it was assumed that the distance between clusters remained constant but the diameter of the clusters increased/decreased when the membrane swelled/shrank in salt solutions of different concentration. From the distance between clusters and equilibrium membrane swelling data, the diameter of a spherical cluster was calculated using the following equation [13], 1=3 6Np Vp 1=3 6V Dd (3) 1 V Fig. 5. Small angle X-ray diffraction scans of fully hydrated, sulfonated and crosslinked PBMP membranes. (Ð ± Ð) 0.60 IEC; (± ± ±) 1.22 IEC; (ÐÐÐ) 1.40 IEC.
brane water content, which, in turn, varied with IEC, type of polymer (PBMP vs. Na®on) and presence/ absence of crosslinks. To further quantify the inter-relationship between the cluster micro-morphology and coion uptake, the size and wall-charge density of ionic clusters in PBMP membranes were calculated. We used the ``cluster-
Fig. 6. Increase in spacing between ionic clusters in sulfonated PBMP and Nafion membranes with increasing membrane swelling. All membranes were in the H form and equilibrated in water. () non-crosslinked PBMP; (r) crosslinked PBMP; (&) Nafion.
where D is the cluster diameter (with units of m), d is the distance between clusters (the Bragg spacing, with units of m), V is the volume increase of the membrane upon absorption of an aqueous solution per cm3 of dry membrane, Np is the number of ion-exchange sites in a cluster, and Vp is the volume of a SOÿ 3 ionexchange site (6810ÿ30 m3 [13]). The quantity V/ (1V) in Eq. (3) is, by de®nition, the membrane porosity. The number of ion-exchange sites per cluster, Np, was obtained from the dry polymer density, the membrane IEC, and the extent of polymer swelling, NA p IEC 3 (4) d Np 1 V where NA is Avogadro's number, p is the dry polymer density (with units of g/m3), and IEC has units of mol/ g of dry membrane. Cluster diameters in polyphosphazene and Na®on 117 membranes equilibrated in various NaCl solutions at 258C are listed in Table 1 (the Na®on results were computed from Eqs. (3) and (4), using d5.5 nm and the swelling data in Fig. 2). All of the membranes sorbed less electrolyte when immersed in higher salt concentration solutions (see Fig. 2) and, accordingly, the cluster diameters decreased with increasing external salt concentration. The variation in cluster size was smaller than the decrease in membrane swelling due to the 1/3 power dependence of D on swelling (i.e., porosity) in Eq. (3). The size of the clusters in the polyphosphazene membranes increased with increasing IEC (due to an increase in swelling with the number of polymer ion-exchange sites) and decreased with polymer cross-
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Table 1 Diameter (in nm) of ionic clusters in sulfonated/non-crosslinked and sulfonated/crosslinked poly[bis(3-methylphenoxy)phosphazene] membranes External NaCl concentration (M)
Non-crosslinked PBMP 0.60 IEC 1.22 IEC 1.40 IEC Crosslinked PBMP 0.6 IEC 1.22 IEC 1.40 IEC Nafion 117
0.1
0.3
0.5
0.7
1.0
5.17 7.25 7.94
5.20 7.09 7.90
4.90 7.08 7.87
5.02 7.02 7.84
4.84 7.00 7.81
4.19 6.30 7.39 5.12
4.17 6.22 7.40 5.12
4.11 6.18 7.36 5.13
4.08 6.14 7.24 5.13
4.03 6.09 7.27 5.08
linking. Only clusters in the crosslinked, 0.60 IEC polyphosphazene membrane were smaller than those in Na®on 117 (the smaller cluster size was attributed to the low ®xed-charge concentration in combination with polymer crosslinking). At a given salt concentration, the ratio of macroscopic swelling for a PBMP ®lm (crosslinked or non-crosslinked) and a Na®on membrane (see Fig. 2) was always much smaller than the ratio of cluster volumes, indicating fewer ionic clusters in the polyphosphazene membranes. From the cluster size, macroscopic membrane swelling data, and the IEC of the dry polymer, the surface charge density in an individual cluster (, with units of
C/m2) was calculated using [13] " #1=3 eNA p d IEC 2 1 V 2 2 1 V 9 V
(5)
where e is the electronic charge (1.610ÿ19 C). One would anticipate that the absorption of mobile anions (coions) into a cluster is directly dependent on the cluster's wall charge density and inversely proportional to the cluster size. In other words, the magnitude of the electrostatic SOÿ 3 /coion repulsion force within the clusters of a PBMP membrane and the depth of penetration of this force into the solution away from
Fig. 7. Correlation between % coion exclusion and the charge/volume ratio of ionic clusters in PBMP and Nafion membranes. (^) 0.60 IEC non-crosslinked; (r) 0.60 IEC crosslinked; (~) 1.22 IEC non-crosslinked; (*) 1.22 IEC crosslinked; (*) 1.40 IEC non-crosslinked; (&) 1.40 IEC crosslinked; () Nafion 117. Solid line is a linear least-squares fit of all data points.
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the cluster wall are dependent on the surface charge density of ion-exchange sites (, from Eq. (5)). The size (diameter) of a cluster relative to the electric ®eld penetration depth, on the other hand, would determine whether coions could absorb into the center region of a cluster. Both and the cluster diameter D are contained in the cluster's ®xed-charge/volume ratio, SOÿ 3 charges per cluster cluster volume cluster wall surface area 6 : cluster volume D
(6)
The experimentally measured coion exclusion (given as a percentage of the external Clÿ concentration that is blocked from absorbing into the membrane) correlated well with the charge/volume ratio of ionic clusters (i.e., 6/D), as shown in Fig. 7. All of the data (including the Na®on results) except the 1.22 IEC/crosslinked PBMP absorption results lie close to a linear least squares straight line correlation. Attempts to correlate the coion uptake data with only the cluster's surface charge density or the cluster volume were unsuccessful. As one would expect, the results show that membrane rejection of coions could be improved by increasing the ®xed-charge-to-volume ratio of the clusters, which can be achieved by increasing the polymer's IEC with additional crosslinking to limit/decrease membrane swelling. 4. Conclusions Equilibrium uptake studies of aqueous NaCl solutions in crosslinked and non-crosslinked cationexchange membranes composed of sulfonated poly[bis(3-methylphenoxy)phosphazene] (PBMP) have been performed at 258C for external salt solutions ranging in concentration from 0.1 to 1.0 M. Three different ion-exchange capacity PBMP polymers were examined: 0.60, 1.22, and 1.40 mmol/dry g. Membrane swelling, membrane ®xed-ion concentration, and membrane coion (anion) concentration were determined for the polyphosphazene membranes and compared to similar data collected with a Na®on 117 per¯uorosulfonic acid membrane. For the polyphosphazene membranes, coion intrusion decreased with membrane ®xed-ion concentration and crosslinking. None of the PBMP membranes rejected Clÿ ions
as effectively as Na®on, although coion intrusion with a 1.4 IEC/crosslinked PBMP ®lm was only slightly greater than that for Na®on. Small angle X-ray diffraction was used to model the micro-morphology of fully-hydrated polyphosphazene membranes in terms of a cluster-network structure. As expected, the size of ionic clusters in the PBMP ®lms increased with increasing IEC and decreased with crosslinking (for the same IEC). The cluster size (diameter) in all of the PBMP membranes except the 0.60 IEC/crosslinked ®lm was greater than that in Na®on. At a given salt concentration, the ratio of macroscopic swelling for a PBMP (crosslinked or non-crosslinked) ®lm and a Na®on membrane was always much smaller than the ratio of cluster volumes, indicating fewer ionic clusters in the polyphosphazene membranes. Neither the dry polymer ion-exchange capacity nor the wet membrane ®xed-ion concentration was an accurate measure of the coion rejection capabilities of the polyphosphazene membranes relative to Na®on. It was found, however, that coion exclusion in crosslinked and non-crosslinked polyphosphazene membranes and in Na®on correlated linearly with the ®xed-charge/volume ratio of the ionic clusters. Acknowledgements This work was funded by the Of®ce of Naval Research and by the National Science Foundation (Grant no. CTS-9632079). References [1] R. Wycisk, P.N. Pintauro, Sulfonated polyphosphazene ionexchange membranes, J. Membr. Sci. 119 (1996) 155. [2] R. Graves, P.N. Pintauro, Polyphosphazene membranes. II. Solid-state photocrosslinking of poly[(alkylphenoxy)(phenoxy)phosphazene] films, J. Appl. Polym. Sci. 68 (1998) 827. [3] H. Tang, P.N. Pintauro, Q. Guo, S. O'Connor, Polyphosphazene membranes III. Solid-state characterization and properties of sulfonated poly[bis(3-methylphenoxy) phosphazene], J. Appl. Polym. Sci. 71 (1999) 387. [4] Q. Guo, P.N. Pintauro, H. Tang, S. O'Connor, Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes, J. Membr. Sci. 154 (1999) 175. [5] H.L. Yeager, A. Steck, Ion exchange selectivity and metal ion separation with a perfluorinated cation exchange polymer, Anal. Chem. 51 (1979) 862.
L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 [6] H.L. Yeager, Cation exchange selectivity, in: A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated ionomer membranes, ACS symp. series, 180, 1982, pp. 41±64. [7] J.R. Bontha, P.N. Pintauro, Water orientation and ion hydration effects during multicomponent salt partitioning in a nafion 117 cation exchange membrane, Chem. Eng. Sci. 49 (1994) 3835. [8] P.N. Pintauro, R. Tandon, L. Chao, W. Xu, R. Evilia, Equilibrium partitioning of monovalent/divalent cation-salt mixtures in nafion cation-exchange membranes, J. Phys. Chem. 99 (1995) 12915. [9] H. Miyoshi, M. Chubachi, M. Yamagami, T. Kataoka, Characteristic coefficients for equilibrium between solution and neosepta or selemion cation exchange membranes, J. Chem. Eng. Data 37 (1992) 120.
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[10] H. Miyoshi, M. Chubachi, M. Yamagami, T. Kataoka, Characteristic coefficients of cation-exchange membranes for bivalent cations in equilibrium between the membrane and solution, J. Chem. Eng. Data 39 (1994) 595. [11] R. Wycisk, P.N. Pintauro, W. Wang, S. O'Connor, Polyphosphazene membranes. I. Solid-state photocrosslinking of poly[(4-ethylphenoxy)(phenoxy)phosphazene], J. Appl. Polym. Sci. 59 (1996) 1607. [12] S. Capeci, P.N. Pintauro, D.N. Bennion, The molecular-level interpretation of salt uptake and anion transport in nafion membranes, J. Electrochem. Soc. 136 (1989) 2876. [13] T.D. Gierke, W.Y. Hsu, The cluster network model of ion clustering in perfluorosulfonated membranes, in: A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated ionomer membranes, ACS symp. series, 180, 1982, pp. 283±307.