Coion exclusion properties of polyphosphazeneion-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-cross-

linked sulfonated phosphazene polymers, such as

poly[(3-methylphenoxy)(phenoxy)phosphazene],

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-polymer-

electrolyte fuel cell applications, such as low/moder-

ate swelling (33% in water at 308C), high proton

conductivity (0.082 S/cm for water-equilibrated ®lms

at 658C), low water diffusivity (1.2�10ÿ7 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,

Journal of Membrane Science 162 (1999) 135±143

*Corresponding author. Tel.: +1-504-865-5872; fax: +1-504-

865-6744; e-mail: peter.pintauro@tulane.edu1Present address: International Paper Company, Inc., Odenton,

MD.

0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 1 3 2 - 5

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 cation-

exchange membranes with SOÿ3 ®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 measure-

ment of the equilibrium uptake of positive and nega-

tive 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 multi-

component 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 space-

charge uptake models [7,8]. Similarly, the effect of

external salt concentration on monovalent and divalent

equilibrium absorption has been quanti®ed in Neo-

septa (manufactured by Tokuyama Soda Co.), Sele-

mion (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 pur-

chased from Technically, Inc., Andover, MA. The

polymer, with an average molecular weight of

2�106 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 708Cfor 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 cross-

linked 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 pre-

vious 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 mem-

brane) were computed (where the Clÿ ion concentra-

tion is equal to the Na� concentration in the leach

solutions).

Small-angle X-ray scattering (SAXS) measure-

ments were carried out on fully hydrated PBMP

membranes with a Rigaku 12-kw rotating anode dif-

fractometer, 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 water-

swollen PBMP membrane sample changed by less

than 0.5% during the time period required to obtain

small-angle X-ray scans.

136 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 con-

centration 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 membra-

ne'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 mem-

brane ®xed-charge concentration is homogeneous

(this point was veri®ed by measuring � on different

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 IEC�1.2; at lower

ion-exchange capacities there was an insuf®cient

number of ®xed charges to enhance greatly the hydro-

philicity of the ®lms and crosslinking had little effect

on reducing membrane swelling. For those PBMP

membranes examined in this study, only the cross-

linked 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 concen-

tration. 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).

L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 137

Fig. 3(a) and (b), where the coion concentration in

sulfonated PBMP membranes (crosslinked and non-

crosslinked) and Na®on 117 are plotted as a function

of the external salt concentration. The 1.4 IEC cross-

linked 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 quan-

tity 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 mem-

brane 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 cross-

linked 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 con-

centrations.

As noted above, our rationale in crosslinking the

sulfonated PBMP ®lms was to reduce swelling and

increase the effective concentration of ®xed mem-

brane charges, thereby reducing coion intrusion. A

careful examination of the data in Fig. 3 reveals that

we have accomplished this goal. The coion concen-

tration in a crosslinked membrane was always lower

than that in its corresponding (similar IEC) non-cross-

linked counterpart for the same external salt concen-

tration (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 concen-

tration 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/cross-

linked 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-

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).

138 L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143

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 mem-

branes have been shown to control coion exclusion

[12] and in¯uence membrane performance in chlor-

alkali cells [13]. In the present study, small-angle

X-ray diffraction data were collected on crosslinked

(0.6�IEC�1.4) and non-crosslinked (0.6�IEC�1.6)

polyphosphazene ®lms (fully hydrated and in the H�

form) and interpreted in terms of an ionic cluster-

network 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� (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 mem-

branes, the Bragg spacing increased with increasing

IEC, decreased with polymer crosslinking (for mem-

branes with similar IEC) and was generally larger than

that for water-equilibrated Na®on 117 (where

d�5.5 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 center-

to-center distance between two clusters was regulated

by (and was a single universal function of) the mem-

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).

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.

L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 139

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-

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 mem-

brane 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 follow-

ing equation [13],

D � d6�V

��1��V�� �1=3

� 6NpVp

�

� �1=3

(3)

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 mem-

brane 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 SOÿ3 ion-

exchange site (68�10ÿ30 m3 [13]). The quantity �V/

(1��V) in Eq. (3) is, by de®nition, the membrane

porosity. The number of ion-exchange sites per clus-

ter, Np, was obtained from the dry polymer density, the

membrane IEC, and the extent of polymer swelling,

Np � NA�p�IEC��1��V�� �

d3 (4)

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 polypho-

sphazene 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 d�5.5 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 mem-

branes increased with increasing IEC (due to an

increase in swelling with the number of polymer

ion-exchange sites) and decreased with polymer cross-

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.

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.

140 L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143

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 concentra-

tion, 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 swel-

ling 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]

� � eNA�pd�IEC�2 � �1��V�

2

9�

1��V

�V

� �2" #1=3

(5)

where e is the electronic charge (1.6�10ÿ19 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 propor-

tional to the cluster size. In other words, the magnitude

of the electrostatic SOÿ3 /coion repulsion force within

the clusters of a PBMP membrane and the depth of

penetration of this force into the solution away from

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)

0.1 0.3 0.5 0.7 1.0

Non-crosslinked PBMP

0.60 IEC 5.17 5.20 4.90 5.02 4.84

1.22 IEC 7.25 7.09 7.08 7.02 7.00

1.40 IEC 7.94 7.90 7.87 7.84 7.81

Crosslinked PBMP

0.6 IEC 4.19 4.17 4.11 4.08 4.03

1.22 IEC 6.30 6.22 6.18 6.14 6.09

1.40 IEC 7.39 7.40 7.36 7.24 7.27

Nafion 117 5.12 5.12 5.13 5.13 5.08

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.

L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 141

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 con-

tained in the cluster's ®xed-charge/volume ratio,

SOÿ3 charges per cluster

cluster volume

� � � cluster wall surface area

cluster volume� � � 6

D: (6)

The experimentally measured coion exclusion

(given as a percentage of the external Clÿ concentra-

tion that is blocked from absorbing into the mem-

brane) 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 increas-

ing the polymer's IEC with additional crosslinking to

limit/decrease membrane swelling.

4. Conclusions

Equilibrium uptake studies of aqueous NaCl solu-

tions in crosslinked and non-crosslinked cation-

exchange 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. Mem-

brane 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 poly-

phosphazene membranes, coion intrusion decreased

with membrane ®xed-ion concentration and crosslink-

ing. 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 polyphospha-

zene membranes in terms of a cluster-network struc-

ture. 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 concentra-

tion was an accurate measure of the coion rejection

capabilities of the polyphosphazene membranes rela-

tive to Na®on. It was found, however, that coion

exclusion in crosslinked and non-crosslinked poly-

phosphazene 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 ion-

exchange membranes, J. Membr. Sci. 119 (1996) 155.

[2] R. Graves, P.N. Pintauro, Polyphosphazene membranes. II.

Solid-state photocrosslinking of poly[(alkylphenoxy)(phenox-

y)phosphazene] films, J. Appl. Polym. Sci. 68 (1998) 827.

[3] H. Tang, P.N. Pintauro, Q. Guo, S. O'Connor, Polypho-

sphazene membranes III. Solid-state characterization and

properties of sulfonated poly[bis(3-methylphenoxy) phospha-

zene], 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 mem-

branes, 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.

142 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.

[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, Polyphos-

phazene 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. Eisen-

berg, H.L. Yeager (Eds.), Perfluorinated ionomer membranes,

ACS symp. series, 180, 1982, pp. 283±307.

L. Jones et al. / Journal of Membrane Science 162 (1999) 135±143 143