Investigation of Proton Conductivity of Cation-Exchanged, 

Sulfonated Poly(b-Styrene-b-Isobutylene-b-Styrene) 

Membranes 

by Eugene Napadensky 

ARL-TR-4940 September 2009 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-4940 September 2009 

Investigation of Proton Conductivity of Cation-Exchanged, 

Sulfonated Poly(b-Styrene-b-Isobutylene-b-Styrene) 

Membranes 

Eugene Napadensky 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
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Investigation of Proton Conductivity of Cation-Exchanged, Sulfonated Poly(bStyrene-b-Isobutylene-b-Styrene) Membranes 

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Eugene Napadensky 

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AH84 

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U.S. Army Research Laboratory 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

In this study, membranes made with highly sulfonated (81%–97%) poly(styrene-b-isobutylene-b-styrene) polymer were 

exchanged with a variety of cations: Cu, Ba, Zn, Cs, Ag, Ca, and Mg. Absorption levels of the cation-exchanged materials was 

determined for water, methanol, and an equimolar water/methanol mixture, measured using a gravimetric technique. Proton 

conductivity of the membranes was measured in a dry state, as well as swollen in water, methanol, and the equimolar water 

/methanol mixture. Water-swollen membranes showed proton conductivity in the range of 2  10–3 to 1.2  10–2 S/cm, 

depending on the cation used for exchange. Conductivity increased proportionally to amount to the water molecules present in 

the membranes. Different cations promoted different levels of water absorption, with Cs and Ag having the highest amounts of 

water and conductivity, presumably because these two elements had only one valence electron compared to others used in this 

study. 

15. SUBJECT TERMS

ionomers, cations, proton conductivity, solvent sorption, FTIR 

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Eugene Napadensky 

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iii

Contents 

List of Figures iv

List of Tables v

1. Introduction 1

2. Experimental 2

2.1 Materials ..........................................................................................................................2

2.2 Membrane Preparation ....................................................................................................2

2.3 Proton Conductivity ........................................................................................................3

2.4 Solvent Sorption ..............................................................................................................3

2.5 Spectroscopic Characterization .......................................................................................4

3. Results and Discussion 4

3.1 Solvent Sorption ..............................................................................................................4

3.2 Conductivity ....................................................................................................................6

3.2.1 Effect of Solvent Composition ............................................................................7

3.2.2 Lambda Effects ....................................................................................................8

3.2.3 FTIR Band Shifts ...............................................................................................11

4. Conclusion 14

5. References 15

Distribution List 17 
iv

List of Figures 

Figure 1. Sulfonation reaction scheme. ...........................................................................................2

Figure 2. Lambda methanol vs. lambda water relationship for cation-exchanged S-SIBS. ...........6

Figure 3. Conductivity as function of methanol content.................................................................8

Figure 4. Dependence of conductivity on lambda for water-swollen membranes. ........................8

Figure 5. Representative FTIR spectra of water/methanol-swollen SIBS-88-Cs. ..........................9

Figure 6. Dependence of conductivity on lambda values for water- and water/methanolswollen membranes. .................................................................................................................11

Figure 7. FTIR band shifts as induced by various solvents. .........................................................12

Figure 8. Shift in asymmetric stretching vibration band of the sulfonate group is related to 

electronegativity of cation. .......................................................................................................12

Figure 9. Solvent induced shift in 1160 cm–1 FTIR band from dry state. .....................................13

Figure 10. Relationship between proton conductivity and position of 1127 cm–1 FTIR band. ....14
v

List of Tables 

Table 1. Conterions used for S-SIBS exchange. .............................................................................3

Table 2. Solvent sorption results. ....................................................................................................5

Table 3. Conductivity in various swelling solvents. .......................................................................7

Table 4. Deconvoluted lambdas for water/methanol-swollen samples. .......................................10
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

Research in ionic polymers has been gaining popularity in the scientific community. Their 

potential use for novel fuel cell membranes, sensors, batteries, and ether electrochemical 

applications has promoted much work in the field. Most of the efforts are aimed at developing a 

less expensive version of Nafion* (a perflourosulfonate ionomer) for proton conductive, direct 

methanol fuel cells (DMFC). 

Different types of ionic materials are being researched by various organizations throughout the 

world including poly(styrene-co-acrylonitrile) (1), poly(ethylenetetrafluoroethylene-graftpolystyrenesulfonic acid) (2), sulfonated trifluorostyrene (2), sulfonated poly(ether ketone 

ketone) (3), sulfonated poly[bis(3-methylphenoxy) phosphazene] (4), poly(vinylbenzyl 

phosphonic acid) (5). Most of the materials are comprised of polymeric backbones incorporating 

sulfonic acid (and, in some cases, phosphoric) groups that offer proper environment for 

conglomeration of water molecules around the ionic groups. As the amount of water clustered 

around the ionic groups exceeds a threshold level (percolation limit), the clusters create an 

interconnected network, allowing charge transfer through the membrane (6). This could include 

transfer of protons between anode and cathode for use as a fuel cell’s membranes or transfer of 

various molecules/ions for various purification and sensing applications. 

Sulfonated polystyrene-b-polyisobutylene-b-polystyrene (S-SIBS) is one example of such an 

ionic polymer. The sulfonic groups are attached to styrene molecules, creating polar polymer 

sections covalently bonded to aliphatic polyisbutylene. Thermodynamic incompatibility forces 

those two types of polymers to separate into different (hydrophobic – hydrophilic) domains. 

When the acidic groups in this (or similar) polymer are further modified with organic or 

inorganic cations, many properties undergo significant changes. It was shown that properties 

such as thermal stability, chemical agent resistance, and solvent resistance can be significantly 

altered by such modification (7, 8). 

In this work, the impact produced by exchanging acidic hydrogen of a sulfonic group in an 

S-SIBS polymer with various cations was examined. Specifically, proton conductivity was 

evaluated to gauge potential usefulness of these polymers for a DMFC membrane. In addition, 

membrane sorption of water and methanol was evaluated. An attempt was made to find patterns 

that allow predictions in how cation-substituted S-SIBS polymers will perform in membrane 

applications. 

 *

Nafion is a registered trademark of DuPont. 
2

2. Experimental 

2.1 Materials 

Poly(styrene-b-isobutylene-b-styrene) block copolymer (SIBS) was provided by Kuraray Co., 

Ltd., Tsukuba Research Laboratories, with the reported properties: 30.84 weight-percent 

styrene, 0.95 specific gravity, Mw = 71,920 g/mol, Mn = 48,850 g/mol, and polydispersity index 

= 1.47. Other chemicals used in this study included magnesium perchlorate (Acros Organics), 

barium chloride (J. T. Baker), calcium chloride (EM Science, 90%), copper (II) chloride (Acros 

Organics, 99%), zinc chloride (EM Science, 98%), silver nitrate (Acros Organics), cesium 

chloride (Aldrich), water (J. T. Baker, HPLC grade), acetone (Warner-Graham Co., reagent 

grade), ethanol (VWR, reagent grade), toluene (EM Science, 99.5%), hexanol (J. T. Baker), and 

methanol (J. T. Baker, HPLC grade). 

2.2 Membrane Preparation

Poly(styrene-b-isobutylene-b-styrene) block copolymer was sulfonated (figure 1) at the U.S. 

Army Research Laboratory with acetyl sulfate to the following various levels: 81%, 88% and 

97%. The percent sulfonation refers to the percent of styrene groups in the polymer that were 

modified with the sulfonic acid group as verified by titration and elemental analysis. The 

sulfonation procedure is described in detail elsewhere (9). 

(CH3C O )2O  +  H2SO4 C H3C O2H + C H3C O2SO3H

0 °C

C H2C l2

+  C H3C O2SO3 C H * H 2 * C H

n

C H * 2 * C H

40° C n

C H2C l2

+  C H3C O2H

SO 3H

Figure 1. Sulfonation reaction scheme. 

Membranes were prepared by solution casting of sulfonated SIBS using a mixed solvent of 

toluene/hexanol (85/15, w/w) at a polymer concentration of 2.5% (w/v). Solvent was allowed to 

evaporate at ambient conditions for ~1 week, and resulting films were dried/annealed in a 

vacuum oven at 50 °C for an additional 2 weeks to remove any residual hexanol. This resulted 

in flexible membranes roughly 100 μm thick.
3

Exchange of the acidic hydrogen with various cations was accomplished by immersing 

membranes into a 1 molar aqueous solution of an appropriate salt for at least 1 h. Subsequently, 

membranes were washed with copious amounts of deionized water, dehydrated using acetone or 

ethanol, and dried between cotton cloth under weight (1–2 lb) at ambient conditions. Table 1 

lists sample name, percent sulfonation of each membrane, salt used for cation exchange, and 

valence of exchanging cation. 

Table 1. Conterions used for S-SIBS exchange. 

Sample Sulfonation 

(%) 

Salt Used Ion Type 

SIBS-97-H 97 NA +1 

SIBS-97-Cu 97 CuCl2 +2

SIBS-97-Ba 97 BaCl2 +2

SIBS-88-Zn 88 ZnCl2 +2

SIBS-88-Cs 88 CsCl +1

SIBS-88-Cu 88 CuCl2 +2

SIBS-88-Ag 88 AgNO3 +1

SIBS-88-Ba 88 BaCl2 +2

SIBS-81-Ca 81 CaCl2 +2

SIBS-81-Mg 81 Mg(ClO4)2 +2

2.3 Proton Conductivity 

Proton conductivity values of the membranes were measured using Solartron Electrochemical 

Impedance Spectrometer “SI-1287” described elsewhere (10). Membrane resistance was 

measured at AC frequencies from 100 Hz to 1 MHz at room temperature. Proton conductivity 

was measured perpendicular to the plane of the membrane with two 1.22-cm2

 stainless steel 

electrodes encapsulated inside a sealable Teflon* drum to prevent solvent evaporation. 

Measurements were conducted at least three times to verify repeatability; reported values were 

the averages of those results. Conductivity of each membrane was measured in a dry state as 

well as swollen in water, methanol, and 50/50 molar water/methanol solution. Swollen membranes 

were allowed to equilibrate in the appropriate liquid for over 24 h at room temperature to ensure 

equilibrium levels of solvent absorption. Swollen membrane thickness (value required for 

calculation of conductivity) was measured prior to impedance measurements (11). 

2.4 Solvent Sorption 

Amount of solvent absorbed into membranes was determined using the commonly utilized 

gravimetric sorption method (1, 10, 12). Films were cut into circular samples, 15 mm in 

diameter, weighed (Wd), and placed into a vials containing selected solvent (water, methanol, or 

 *

Teflon is a registered trademark of E.I. DuPont de Nemours & Co. 
4

equimolar water/methanol mixture) at room temperature. After 24 h, swollen polymer samples 

were removed from the vials, excess solvent was dabbed off the surface, and samples were 

weighed again (Ww). The difference in weight corresponded to the amount of solvent absorbed 

into the polymer during this time. Weight percent uptake can be calculated using the following 

equation: 

( )100 = w d

d

W –W Wt%

W . (1)

In addition, in cases when pure water or methanol (MeOH) were used as the swelling solvent, a 

lambda (λ) value (defined here as mols of solvent per mol of sulfonic group) (13) can be easily 

calculated using the following equation: 

3

mols_of_solvent λ= mols_of_SO . (2) 

Mols of sulfonic groups are calculated based on sulfonation level, known weight ratio of 

polyisobutylene block to polystyrene block (6.9 to 3.1) and molecular weight and amount of 

cations incorporated into the polymer (with assumption of 100% exchange level with acidic 

hydrogens). 

Lambda selectivity (relative ratio of water to methanol molecules per given number of ionic 

groups) was also calculated. It represented the extent of preference of absorption of water over 

the methanol by the ionic clusters in the polymer. 

2.5 Spectroscopic Characterization 

Vibration infrared (Fourier-transform infrared [FTIR]) spectra of dry and solvent-swollen 

polymeric films were collected using a Nicolet 6700 FTIR spectrometer equipped with a Golden 

Gate/Diamond crystal attachment. Spectra were collected from 4000 to 600 cm–1, with a 4 cm–1

resolution, and 32 scans per sample. All spectra were corrected by background subtraction and 

collected immediately prior to the actual sample run. 

3. Results and Discussion 

3.1 Solvent Sorption 

Results of solvent absorption measurements (table 2) showed that in an ion-exchanged 

sulfonated SIBS, the amount of pure water absorbed was significantly higher than pure 

methanol. The amount of equimolar water/methanol solution absorbed by the membrane fell 

between those two extremes. The only exception was the acid form of the polymer. For SIBS97-H, the amount of water/methanol mixture absorbed was higher than either of the two pure 
5

Table 2. Solvent sorption results. 

 Weight Percent Uptake Lamda 

(mols Solvent/mols SO3) 

Solvent Water 

(%) 

Mixa

(%) 

Methanol 

(%) 

Water Methanol Selectivity 

97-H 396 580 327 96.2 44.7 2.2 

97-Cu 98 50 22 25.5 3.2 8.0 

97-Ba 26 10 5 7.3 0.8 9.2 

88-Zn 88 31 28 24.7 4.4 5.6 

88-Cs 213 57 6 71.7 1.1 63.1 

88-Cu 90 50 26 25.3 4.1 6.2 

88-Ag 145 14 6 46.8 1.1 43.0 

88-Ba 27 12 5 8.1 0.8 9.6 

81-Ca 105 25 7 30.8 1.2 26.7 

81-Mg 126 46 24 36.4 3.9 9.3 a

Mix refers to equimolar blend of water and methanol. 

solvents. Similar behavior was observed for Nafion-117 (also a polymer with ionic SO3H clusters) 

by other researchers (14). 

Results shown in this table suggested that interactions of a given solvent with ionic clusters were 

ion dependent. Observe Ba2+ and Cu2+ ions with different sulfonation levels. Lambdas of SIBS 

with different sulfonation levels exchanged with the same type of cation were almost identical 

for Cu2+ (25.3 and 25.5) and reasonably close for Ba2+ (8.1 and 7.3). The proximity of two ions 

on the periodic table did result, in certain cases, in a similarity of properties. Observe the Mg2+, 

Ca2+, Ba2+ set. All three were group 2 ions. A pattern of decreasing lambdas can be seen for 

both water (36.4, 30.8, and 8.1) and methanol (3.9, 1.2, and 0.8). The decrease, however, was 

not proportional when evaluating lambda’s selectivity (ratio of lambdas commonly used to 

compare relative sorption properties in a normalized fashion). Selectivity for Ca2+ was actually 

much higher than for Ba2+ or Mg2+. 

Similarly, although Cu2+ and Zn2+ ions were located in different but adjacent groups of the same 

period, identical lambdas for water (25.3 and 24.7) and methanol (4.1 and 4.4) were shown. 

This, however, was not observed for the Cs+

-Ba2+ combination (71.3 and 8.1). The most obvious 

reason why it would not be similar to the Cu2+-Zn2+ pair was the difference in valence. Cs is a 

+1 ion, and Ba is a +2 ion; this affected various properties. 

Another example of different valence effects can be observed by examining Cu2+ (a +2 ion)- and 

Ag+ (a +1 ion)-exchanged SIBS. Even though they were in the same group, water lambda for 

Ag+

 (46.8) ion was much higher than Cu2+ (8.1), contrary to the pattern observed for the group 2 

elements Mg2+, Ca2+, and Ba2+ (all +2 ions), where lambdas decreased as elements proceeded
6

farther down the periodic table. The significance of these observations was that ions with onevalence electron allowed larger clusters of water to aggregate (as much as twice or three times 

compared to ions with two-valence electrons). Single-charge cations presumably enabled low 

levels of crosslinking, which, in turn, allowed the polymer to swell to a higher degree. 

Interestingly, samples exchanged with one-valence electron ions, while exhibiting very high 

affinity for creating large water clusters as illustrated by large lambdas (46–71 molecules per 

sulfonic group), did not behave similarly when exposed to methanol. Methanol clusters for these 

cations were almost the smallest observed in this study—1.1 molecules per sulfonic group. 

Plotting methanol lambda vs. water lambda (figure 2) showed no observable pattern other than 

the valence difference just mentioned. The values seemed to change independent of each other. 

This signified that there were two different mechanisms of interaction between ionic domains 

and these two solvents. This was important since it meant that we could potentially control these 

entities independently of each other and possibly find ions (or possibly a bend of two or more 

ions) that produced desired effects for a given application. 

Figure 2. Lambda methanol vs. lambda water relationship for cation-exchanged 

S-SIBS. 

3.2 Conductivity 

The real impedance was determined from the x-intercept of imaginary vs. real impedance data 

(Z’ vs. Z”). Depending on the shape produced by the plot, a linear or circular fit was used to 

determine the x-intercept (R). For linear fits, an attempt was made to use a frequency range that 

produced a linear region on the Z-vs.-Hz curve, usually corresponding to a frequency from 104

 to 

106

 Hz. Table 3 lists conductivity K in siemens per centimeter (S/cm) values as calculated based 

on sample thickness L (cm), sample area A (1.22 cm2

), and resistance R (ohms).

0

10

20

30

40

50

60

70

80

012 3 4 5

Lambda Methanol

Lambda Water

97 Cu

88/97 Ba

88 Zn

88 Cs

88 Cu

88 Ag

81 Ca

81 Mg
7

Table 3. Conductivity in various swelling solvents. 

Conductivity 

(S/cm) 

Lamda 

(mols Solvent/mols SO3) 

Solvent Dry Water Mix Methanol Water Methanol 

97-H 0.0053 0.042 0.021 0.0420 96.2 44.7 

97-Cu 0.0000 0.009 0.002 0.0002 25.5 3.2 

97-Ba 0.0001 0.002 0.000 0.0000 7.3 0.8 

88-Zn 0.0000 0.008 0.002 0.0000 24.7 4.4 

88-Cs 0.0000 0.013 0.008 0.0000 71.7 1.1 

88-Cu 0.0000 0.007 0.002 0.0000 25.3 4.1 

88-Ag 0.0000 0.012 0.001 0.0000 46.8 1.1 

88-Ba –0.0001 0.001 0.000 –0.0006 8.1 0.8 

81-Ca 0.0000 0.007 0.002 0.0000 30.8 1.2 

81-Mg 0.0000 0.010 0.002 0.0000 36.4 3.9 

Nafiona — 0.025 — — 22 — 

a

Nafion (registered trademark of E.I. DuPont de Nemours & Co.) data is taken from DeLuca and Elabd (10). 



L K =

R A . (3) 

In cases of methanol-swollen and dry membranes (except for SIBS-97-H), membranes were 

practically unconductive. This was judged based on very small (and, in some cases, negative) 

values for calculated conductivity as well as the general shape and position of the impedance 

data curves. 

3.2.1 Effect of Solvent Composition 

Examining the effect of swelling solvent on the conductivity of the different ion-exchanged 

membranes (figure 3), in most cases, there was a dramatic drop in conductivity when equimolar 

mixture of water/methanol (stociometricly proper mixture for electrochemical reaction in 

DMFC) was used as a swelling solvent. One notable exception was the Cs+

-exchanged 

membrane. While conductivity was smaller for a pure water-swollen membrane, the drop in 

conductivity was less than for other cations tested in this study. This deviation from the 

behaviors of all other cations has been verified by repeated experiments and may warrant a more 

detailed investigation into its causes. 

Overall, inclusion of tested ions had a detrimental effect on the conductivity of water-swollen 

membranes. Conductivity of cation-containing membranes is roughly 25% of the value obtained 

for the nonexchanged acid form of SIBS, or 50% of the value reported for Nafion-117 (10). 

However, lower methanol cross-over (not yet evaluated) might provide overweighing benefits 

that will produce an overall more efficient DMFC system.
8

-0.001

0.001

0.003

0.005

0.007

0.009

0.011

0.013

0.015

0 0.5 1 1.5

MeOH mole fraction

Conductivity (S/cm)

88-Cu

88-Ag

88-Ba

88-Zn

88-Cs

81-Ca

81-Mg

97-Ba

97-Cu

Figure 3. Conductivity as function of methanol content. 

3.2.2 Lambda Effects 

It was well documented that conductivity in ionic-polymeric membranes was heavily dependent 

on levels of water absorbed into the membrane (2, 12). In order to examine the effects different 

cations in exchanged ionic membranes have on conductivity, we need to determine if changes in 

conductivity followed changes in lambda or if there were some additional factors affecting it. As 

shown in the plot of water lambda vs. conductivity (figure 4), conductivity was directly 

proportional to the log of lambda. This result implied that the effect various cations had on 

proton conductivity was limited to the amount of water molecules allowed to aggregate into the 

ionic clusters. Lambda and proton conductivity of the membrane increased as more water 

molecules were attracted to the clusters. 

Figure 4. Dependence of conductivity on lambda for water-swollen membranes.

y = 0.0052Ln(x) - 0.0091

R2 = 0.9542

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

1 10 100

Log Lambda (mol water/mol acid) 

Conductivity (K)

88-Cu

97-Ba

88-Ba

88-Ag

88-Zn

88-Cs

97-Cu

81-Ca

81-Mg
9

However, this pattern was only valid for ionic S-SIBS membranes exchanged with cations; 

neither acid form of SIBS-97-H nor Nafion-117 fell on this line. In order to better illustrate the 

cation data, those two data points were not shown but were located at 112  0.042 and 22 

0.025, respectively (off scale). 

In order to evaluate the relationship of water content to proton conductivity for membranes 

swollen in an equimolar methanol-water mixture, it was necessary to determine relative 

quantities of water and methanol sorbed into the membranes. This was attempted by FTIR 

analysis. Spectra of dry samples and those swollen in water, methanol, and mixed solvent were 

collected using conditions just described. The following peaks were selected as representative 

entities of interest: 1020 cm–1-methanol, 1366 cm–1-isobutylene, and 3396 cm–1-water. Some of 

these bands were indicated with arrows in a representative FTIR spectrum shown in figure 5. 

1400 1350 1300 1250 1200 1150 1100 1050 1000 950

1034

1020

1160 1127

1366

Figure 5. Representative FTIR spectra of water/methanol-swollen SIBS-88-Cs. 

Unfortunately, the methanol peak was flanked by sulfonic group peaks on both sides, making it 

impossible to estimate the areas. As a result, it was decided to use a somewhat less-accurate 

peak height as an indicator of relative concentrations of the corresponding molecule. The 

isobutylene peak was chosen as an internal standard since it was relatively unobscured in the 

spectra. The O-H stretching signal at 3396 cm–1 was chosen to represent water quantity in the 

sample. While, technically, methanol also produced vibrations in that part of spectra, no 

significant difference in intensity of the peak between dry and methanol-swollen samples was 

found from the raw data. Thus, it will not be a significant source of error. 

FTIR peak heights obtained from dry spectra of cation-exchanged membranes were subtracted 

from FTIR peak heights for the same swollen membrane and pure water and pure methanol 

(3396 and 1020 cm–1). The resulting values were normalized by dividing them by the height of 
10

internal standard peak at 1366 cm–1 (isobutylene). This provided an FTIR signal ratio of “moles 

of solvent per moles of isobutylene.” Real values for moles of solvent per moles of isobutylene 

can be calculated from experimental data collected by the gravimetric sorption method. When 

compared to corresponding FTIR signal ratios, a calibration between signal level and solvent 

absorbed can be archived. By evaluating FTIR spectra of the swollen membrane in mixed 

solvent, normalizing water (3396 cm–1) and methanol (1020 cm–1) peaks to “isobutylene” levels 

(1366 cm–1), and comparing peak heights to the calibration obtained earlier, the quantity of 

individual solvents present in a given membrane was estimated. 

When the amount of water and methanol estimated to be present from the FTIR spectra was 

converted to total weight and compared to experimental data obtained by the gravimetric uptake 

method, FTIR results provided values that, in most cases, were 20%–100% larger. Presumably, 

this discrepancy is caused by the use of peak height instead of peak area in the calculations. In 

order to improve accuracy of estimation, data obtained by FTIR will be used to calculate mass 

ratios of methanol to water. These ratios, coupled with experimentally gained weight uptake of 

the solvent mixture, will then be used to estimate actual amounts of water and methanol in 

swollen membranes shown in table 4. 

Table 4. Deconvoluted lambdas for water/methanol-swollen samples. 

Solvent K 

(S/cm) 

Water 

(λ) 

MeOH 

(λ) 

97-H 0.021 112.1 16.2 

97-Cu 0.002 10.9 1.2 

97-Ba 0.000 1.7 0.6 

88-Zn 0.002 6.6 1.2 

88-Cs 0.009 16.9 1.3 

88-Cu 0.003 13.1 0.6 

88-Ag 0.001 4.0 0.3 

88-Ba 0.000 2.8 0.4 

81-Ca 0.003 6.1 0.7 

81-Mg 0.003 10.2 1.7 

While certainly not as accurate as direct measurements, the estimated lambdas followed the trend 

found in the pure water-conductivity relationship, as can be seen in figure 6. The plot 

incorporated lambda-vs.-conductivity data for membranes swollen in pure water (blue) and in the 

equimolar water/methanol mixture (red). For the water/methanol mixture, values plotted 

represented only the water contribution of absorbed solvent.
11

Figure 6. Dependence of conductivity on lambda values for water- and water/methanolswollen membranes. 

All data fell on the straight line again, confirming that conductivity was largely governed by the 

quantity of water in the membrane. The trend line intercepted the x-axis at ~3, indicating that, in 

this system, any sort of conductivity will require water molecule levels of more than three per 

each sulfonic group in order to participate in proton conductivity. 

3.2.3 FTIR Band Shifts 

Figure 7 shows the effect various solvents and cations impose on the shift of the asymmetric 

stretching vibrations band of the sulfonate group located around 1160 cm–1 (9). The plot shows 

four different points for each cation representing different conditions of the sample: dry, swollen 

in methanol, swollen in water/methanol mix, and swollen in pure water. It was apparent that the 

band shifted to a higher and higher position as the sample’s state changed from dry, to methanolswollen, to swollen in the water/methanol mixture, and finally (a highest value), swollen in 

water. This shift in IR band position generally indicated more restricted movement of the bands, 

which, in turn, implied a higher level of interaction between solvent and sulfonic groups. It did 

not, however, have any apparent correlation to affinity for a particular solvent or proton 

conductivity. 

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

1 10 100

Log Lambda (mol water/ mol acid)

Conductivity (K)

Water lamda for

pure water swolen

membranes

Water lamda for

water/methanol

swolen membranes
12

1140 

1150 

1160 

1170 

1180 

1190 

 

Dry Methanol Mix Water 

97-H 97 Ba 88 Ba 88 Zn 88 Cs 88 Cu 97 Cu 88 Ag 81 Ca 

 

81 Mg

V (1/cm)

Figure 7. FTIR band shifts as induced by various solvents. 

Band position in cation-exchanged samples showed an interesting shift from acid form (SIBS-97-H), 

the magnitude which can be correlated to electronegativity of the incorporated cation (figure 8). 

To demonstrate this, exact position of 1160 cm–1 peak for the acid form of the polymer was 

subtracted from positions of the same peak obtained from FTIR spectra of cation-exchanged 

samples, normalizing them. Dry samples were normalized to dry samples of the acid form, 

methanol-swollen samples were normalized to methanol-swollen acid form, polymer spectra, 

etc., to minimize solvent effects from consideration. The band-shifting effect was more 

pronounced in dry polymers and increased linearly as elecronegativity of the cation decreased 

(except for Ag). 

 

-25 

-20 

-15 

-10 

-5 

0 

5 

10 

15 

20 

25 

30 

0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 

Electronegativity 

Mix MeOH 

Dry Water 

Cs Ba Ca Mg Zn Cu Ag H 

Shift V (/cm) from H form H form

 

 

 

Figure 8. Shift in asymmetric stretching vibration band of the sulfonate group is 

related to electronegativity of cation.
13

Introduction of solvents into the membranes also had an effect on the asymmetric stretching 

vibration of the sulfonate group. As sample state changed from dry to swollen, the band at 

1160 cm–1 shifted to a higher field compared to spectra of the dry sample. The magnitude of the 

shift was linearly proportional to the electronegativity of the cation attached to the sulfonic group 

(figure 9). Since the shift was related to the strength of interactions between sulfonic group 

cation and the surrounding solvent, electronegativity having a strong effect on interactions with 

polar solvents like water and methanol was not surprising. This did not correlate with lambdas 

obtained during gravimetric portion of the experiments because swelling levels were limited by a 

crosslinking effect that constrained the membrane and was observed even in +1 cations. 

-5

0

5

10

15

20

25

0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 

Electronegativity

Shift V (/cm) from Dry form

Mix 

MeOH 

water

Cs Ba Ca Mg Zn Cu Ag H 

Figure 9. Solvent induced shift in 1160 cm–1 FTIR band from dry state. 

There was a correlation that could estimate the level of proton conductivity K of the membranes 

based on very small shifts in FTIR spectrum of water-swollen, cation-exchanged samples. Two 

affected FTIR bands were in plane bending vibrations of the sulfonate anion attached to the 

aromatic ring 1127 cm–1 and symmetric stretching vibrations of the sulfonate group at 1034 cm–1

(9). 

In response to levels of solvent absorbed into the membrane, the peak shifted (shown in figure 10 

for the 1127-cm–1 band). This corresponded to an observed increase in proton conductivity and 

could be used as an indicator of a highly swollen state of the membrane. Since the level of water 

was proportional to proton conductivity in the system, this observation may be utilized as a 

prescreening tool for the prediction of proton conductivity. Note that this correlation existed only 

for polymers swollen in pure water. Methanol and water/methanol-swollen polymers produced 

band shifts disproportionally large for observed levels of exhibited conductivity. Also, the acid 

form of the polymer did not follow the same trend line and was not even shown on the plot as 

being dramatically off the scale (SIBS-97-H FTIR peak was 1126.36 cm–1, and conductivity was 

0.042 S/cm).
14

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

1125.5 1126 1126.5 1127 1127.5 1128

FTIR Peak Position (cm-1)

K (S/cm) 

Figure 10. Relationship between proton conductivity and position of 

1127 cm–1 FTIR band. 

4. Conclusion 

Novel, cation-exchanged ionic polymer membranes were synthesized. Their interactions with 

water and methanol were evaluated using the gravimetric method. It was observed that levels of 

solvent absorbed into the membranes varied depending on the cation used, with a valence 

difference resulting in the most dramatic changes. While the amount of water absorbed by 

cation-containing membranes decreased by 50%–95% (as compared to acid form), decreases in 

the amount of methanol absorbed by the same membranes were significantly higher (90%–98%, 

an order of magnitude). This might significantly reduce methanol crossover in DMFCs, thus 

reducing catalyst poisoning on the cathode side and increasing life of the cell (15). Proton 

conductivity of water-swollen membranes was measured and varied from 2  10–3 to 1.2  

10–2 S/cm, depending on the cation. Conductivity for those materials correlated to water content 

in the membranes and, in all other ways, was independent of specific ions used. Proton 

conductivity in water/methanol-swollen membranes was also dependent on water content in the 

membrane and followed the same trend. 

While exchanging acidic hydrogens in sulfonated SIBS membranes with cations did make the 

membranes stronger due to crosslinking effects, swollen membranes still exhibited weakness 

detrimental to the performance as DMFC. Further work is needed to address these issues, such 

as physical encapsulations into a strong but porous substrate, stronger crosslinking, or a plasma 

treatment to create surface-level crosslinks (16).
15

5. References 

1. Silva, A. L. A.; Takase, I.; Pereira, R. P.; Rocco, A. M. Poly(styrene-co-acrylonitrile) Based 

Proton Conductive Membranes. European Polymer Journal 2008, 44 (5), 1462–1474. 

2. Peckham, T. J.; Schmeissert, J.; Holdcroft, S. Relationships of Acid and Water Content to 

Proton Transport in Statistically Sulfonated Proton Exchange Membranes: Variation of 

Water Content via Control of Relative Humidity. Journal of Physical Chemistry B 2008, 

112 (10), 2848–2858. 

3. Swier, S.; Chun, Y. S.; Gasa, J.; et al. Sulfonated poly(ether ketone ketone) Ionomers as 

Proton Exchange Membranes. Polymer Engineering and Science 2005, 45 (8), 1081–1091. 

4. Guo, Q.; Pintauro, P. N.; Tang, H.; O’Connor, S. Sulfonated and Crosslinked 

Polyphosphazene-Based Proton-Exchange Membranes. J. of Membrane Science 1999, 154, 

175. 

5. Jiang, F.; Kaltbeitzel, A.; Fassbender, B.; et al. Effect of Polymer Composition and Water 

Content on Proton Conductivity in Vinyl Benzyl Phosphonic Acid-4-Vinyl Pyridine 

Copolymers. Macromolecular Chemistry and Physics 2008, 209 (24), 2494–2503. 

6. Elabd, Y. A.; Napadensky, E.; Sloan, J. M.; Crawford, D. M.; Walker, C. W. Triblock 

Copolymer Ionomer Membranes Part I. Methanol and Proton Transport. J. of Membrane 

Science 2003, 217 (1–2), 227–242. 

7. Suleiman, D.; Napadensky, E.; Sloan, J. M.; Crawford, D. M. Thermogravimetric 

Characterization of Highly Sulfonated Poly (styrene-isobutylene-styrene) Block 

Copolymers: Effects of Sulfonation and Counter-Ion Substitution. Thermochemica Acta

2007, 460 (1–2), 35–40. 

8. Hamad, F. A.; Chowdhury, G.; Matsuura, T. Sulfonated Polyphenylene OxidePolyethersulfone Thin-Film Composite Membranes - Effect of Counterions on the Gas 

Transport Properties. J. of Membrane Science 2001, 191 (1–2), 71–83. 

9. Elabd, Y. A.; Napadensky, E. Sulfonation and Characterization of poly (styreneisobutylene-styrene) Triblock Copolymers at High Ion-Exchange Capacities. Polymer 2004, 

45 (9), 3037–3043. 

10. DeLuca, N. W.; Elabd, Y. A. Nafion((R))/poly(vinyl alcohol) Blends: Effect of 

Composition and Annealing Temperature on Transport Properties. J. of Membrane Science 

2006, 282 (1–2), 217–224.
16

11. Elabd, Y. A.; Walker, C. W.; Beyer, F. L. Triblock Copolymer Ionomer Membranes Part II. 

Structure Characterization and Its Effects on Transport Properties and Direct Methanol Fuel 

Cell Performance. J. of Membrane Science 2004, 231 (1–2), 181–188. 

12. Gromadzki, D.; Cernoch, P.; Janata, M.; et al. Morphological Studies and Ionic Transport 

Properties of Partially Sulfonated Diblock Copolymers. European Polymer Journal 2006, 

42 (10), 2486–2496. 

13. Lee, C. H.; Park, H. B.; Lee, Y. M.; et al. Importance of Proton Conductivity Measurement 

in Polymer Electrolyte Membrane for Fuel Cell Application. Industrial & Engineering 

Chemistry Research 2005, 44 (20), 7617–7626. 

14. Hallinan, D. T.; Elabd, Y. A. Diffusion and Sorption of Methanol and Water in Naflon 

Using Time-Resolved Fourier Transform Infrared-Attenuated Total Reflectance 

Spectroscopy. Journal of Physical Chemistry B 2007, 111 (46), 13221–13230. 

15. Kjaer, J.; Yde-Anderson, S. Solid State Direct Methanol Fuel Cells. Proc. Intersoc. Energy 

Convers. Eng. Conf. 26 1991, 542. 

16. Pappas, D.; Bujanda, A.; Demaree, J. D.; Hirvonen, J. K.; et al. Surface Modification of 

Polyamide Fibers and Films Using Atmospheric Plasmas. Surface & Coatings Technology

2006, 201, 4384–4388. 
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