Journal of Membrane Science 428 (2013) 516–522

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Journal of Membrane Science

0376-73

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journal homepage: www.elsevier.com/locate/memsci

Effects of neutralization with Et3Al on structure and properties in sulfonatedstyrenic pentablock copolymers

Jae-Hong Choi a, Carl L. Willis b, Karen I. Winey a,n

a Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USAb Kraton Performance Polymers, Inc, Houston, TX 77084, USA

a r t i c l e i n f o

Article history:

Received 6 June 2012

Received in revised form

25 October 2012

Accepted 27 October 2012Available online 5 November 2012

Keywords:

Pentablock copolymer

Neutralization

Morphology

X-ray scattering

Water vapor transport rate

88/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.memsci.2012.10.051

esponding author. Tel.: þ1 215 898 0593; fax

ail address: winey@seas.upenn.edu (K.I. Wine

a b s t r a c t

The effect of neutralization with Et3Al on morphologies in a sulfonated pentablock copolymer solution and

cast membrane was investigated using small-angle X-ray scattering (SAXS) and transmission electron

microscopy (TEM). The sulfonated pentablock copolymer in a non-polar solvent mixture (cyclohexane/

heptane) persists as spherical micelles after neutralization where the micelle core of sulfonated

polystyrene (SS) incorporates Al (3þ) ions and the corona is solvated hydrogenated isoprene (HI) and

t-butylstyrene (tBS). Membranes cast from this micellar solution exhibit a bicontinuous microphase

separated morphology with interconnected SS microdomains. The aluminum-neutralized membrane with

a continuous transport channel shows excellent water vapor transport rate (WVTR) (22 kg/(m2 day)),

which is comparable to the WVTR of membranes in the sulfonic acid form. The strong interaction between

Al(3þ) ions and the sulfonic acid group in the aluminum-neutralized membrane results in significantly

lower water uptake (�30 wt%) that provides much-improved mechanical stability in the wet state.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polymers with sulfonated groups have been widely used asfunctional materials for applications including polymer electro-lyte membrane (PEM) fuel cells [1–10] and reverse osmosis (RO)membranes [11–14] due to their unique combination of mechan-ical strength and ion transport. Sulfonated block copolymers haveattracted considerable attention for these diverse applicationsbecause one can design and control their nanoscale, self-assembled morphologies to improve desired properties includingmechanical stability, water uptake and ionic conductivity. Formany applications, there are a number of demanding require-ments [15–17]: high ion or water transport properties, goodchemical and mechanical stability, and low reactant permeability.Recently, researchers have tried to synthesize new sulfonatedblock copolymer membranes and to understand structure–property relationships of these materials to control their mor-phology and optimize performance [2,5–10].

Previously, in efforts to create new materials that achieve targetedmembrane properties, we presented a novel sulfonated pentablockcopolymer, poly(t-butylstyrene-b-hydrogenated isoprene-b-sulfo-nated styrene-b-hydrogenated isoprene-b-t-butylstyrene) (tBS-HI-SS-HI-tBS). In this sulfonated pentablock copolymer, the tBS endblock provides good mechanical strength in both dry and wet states

ll rights reserved.

: þ1 215 573 2128.

y).

and the HI block gives additional toughness to avoid severe brittle-ness in the dry state, which is a critical attribute in membranes. TheSS middle block, which is selectively sulfonated to a desired ionexchange capacity (IEC), provides high hydrophilicity and enableshigh ion and water transport. In two previous studies we havereported the effect of sulfonation level on the spherical micellarmorphology of these sulfonated pentablock copolymers in cyclohex-ane/heptane solutions [18] and the implications of the microphaseseparation of these solutions on membrane morphology and proper-ties [19].

Sulfonated pentablock copolymer membranes with a highsulfonation levels (1.5 and 2.0 mequiv./g IECs) exhibit bicontin-uous microphase separated morphologies with interconnected SSmicrodomains that enable good water vapor transport rate [19].Also, when exposed to liquid water, the extent of water uptakeand the increase in primary spacing of the membranes are greaterat higher sulfonation levels, suggesting that absorbed waterplasticizes the hydrophilic SS microdomains. The plasticizedhydrophilic microdomains lead to deterioration of membraneperformance for reverse osmosis, desalination, and humidifica-tion/dehumidification applications due to reduced mechanicalstrength. Others have reported that one method to minimizethe amount of water uptake in membranes is to neutralize thesulfonic acid groups with monovalent and multivalent metalcounterions to cross-link the membranes [16,20–22].

In the present study, we investigate the effects of neutralizationwith Et3Al on morphologies of both a sulfonated pentablockcopolymer solution in a non-polar solvent mixture and a membrane

J.-H. Choi et al. / Journal of Membrane Science 428 (2013) 516–522 517

cast from this micellar solution. The solution and membranemorphologies are determined by small-angle X-ray scattering(SAXS) and transmission electron microscopy (TEM). Previously,Geise et al. reported the effects of aluminum-neutralization onmembrane properties including permeability, selectivity, andmechanical and transport properties of these sulfonated pentablockcopolymers [23]. In this study, we mainly focus on a fundamentalunderstanding of the effect of aluminum-neutralization on self-assembled morphologies of both solutions and membranes. Thesedetermined morphologies are correlated to both transport andmechanical properties of membranes. Additionally, it is difficult toobtain the equilibrium morphologies in these sulfonated blockcopolymers due to the strong interaction parameter and thedifficulty in preparing equilibrium morphologies due to the highsensitivity to processing conditions. Thus, thorough studies of theself-assembled morphologies in these pentablock copolymer solu-tions and membranes are essential for constructing structure–property relationships.

2. Experimental

2.1. Materials

The sulfonated pentablock copolymers in this study wereprepared and provided by Kraton Polymers LLC. The pentablockcopolymer of tBS–HI–S–HI–tBS was synthesized via anionic poly-merization. After polymerization the polyisoprene blocks werehydrogenated (HI). The molecular weight of the unsulfonatedpentablock copolymer is approximately 15–10–28–10–15 kg/mol[14,19]. The middle styrene block of the pentablock copolymerwas selectively sulfonated to an ion exchange capacity (IEC) of2.0 milliequivalents of sulfonic acid per gram of dry polymer(mequiv./g). The structure of the studied pentablock copolymerand detailed synthetic procedures have been described elsewhere[19,24,25]. The concentration of sulfonated pentablock copolymersolutions is �10 wt% in a mixed solvent of cyclohexane andheptane. (approximately 28:72 by weight).

The neutralization was conducted by adding triethylaluminum(Et3Al) to the stirred sulfonated pentablock copolymer in acyclohexane/heptane solution in an inert atmosphere. Anexotherm (�20 1C) was observed upon addition of 1 mol ofneutralizing agent (Et3Al) per equivalent of sulfonic acid (1 mol/equivalent of –SO3H) to the sulfonated pentablock copolymersolution [23,26]. Because three sulfonated groups are needed toneutralize a single Al3þ ion [27], this neutralization in solutionhas been referred to as ionic crosslinking as depicted in Scheme 1.There was no significant increase in viscosity and no change invisual appearance upon neutralization of the micellar solution.The aluminum leaching experiment with aqueous acid alsoshowed that there was no significant amount of excess aluminumin the membrane [23]. It should be noted that because this metal

Scheme 1. Neutralization with Et3Al of the sulfonated polystyrene monomeric

units in the midblock of the tBS–HI–SS–HI–tBS pentablock copolymer. Dashed line

(---) indicates a partially ionic bond.

compound may be a hazardous substance and reacts vigorouslywith oxygen, the metal compound must be handled in dispersedform, or as a solution, in an inert solvent or diluents in theabsence of oxygen [26].

The acid-form and aluminum-neutralized pentablock copoly-mer membranes were prepared by hand casting from theirsolutions in cyclohexane/heptane. Both solutions were cast ontoa silicanized glass plate and then solvents were evaporated atroom temperature and 50% relative humidity. The measured filmthickness of both the acid-form and aluminum-neutralized mem-branes is �25 mm.

2.2. Small-angle X-ray scattering

About 1 ml of each micellar solution was loaded into acapillary tube (�1 mm diameter) and the capillary tube wasflame sealed. Cyclohexane was also loaded into a capillary andstudied using SAXS, so that the incoherent scattering from thesolvent could be subtracted from the block copolymer solutions.Small-angle X-ray scattering was performed on the membranesand solutions. The Cu X-rays were generated from a Nonius FR591 rotating-anode generator operated at 40 kV and 85 mA. Thebright, highly collimated beam was obtained via Osmic Max-Fluxoptics and pinhole collimation in an integral vacuum system. Thescattering data were collected with a Bruker Hi-Star two-dimen-sional detector with a sample to detector distance of 150 cm.Using the Datasqueeze software [28], 2-D scattering patternswere converted to 1-D plots with azimuthal angle integration.The scattering intensity was corrected for the primary beamintensity. The corrected scatterings from the cyclohexane capil-lary and an empty cell were subtracted from the solution andmembrane data, respectively. Note that there is no difference inscattering patterns between using cyclohexane and using mixtureof cyclohexane/heptane as a background.

To characterize the microphase separated morphology ofmembranes exposed to liquid water, a small piece of membraneand deionized (DI) water were loaded into a capillary tube (1 mmdiameter) and flame-sealed. The swelling of the membranes wascharacterized as a function of time. The scattering from a capillaryfilled with DI water was substracted from the pentablock copo-lymer membrane scattering.

The scattering data of solutions were modeled as modified hardspheres where micelles are treated as monodisperse hard spheresdistributed with liquidlike order in a uniform matrix [29]. Themodel parameters include the radius of the micelle core (R), theradius of closest approach (RCA) that limits the spatial correlationbetween micelles, and the number density of micelles (n).The parameter RCA is also size of the micelle with a core ofSS and a corona of tBS and HI swollen by the non-polar solventmixture. This model uses the Percus–Yevick [30] total correlationfunction to account for correlations between all micelles in thesystem.

2.3. Transmission electron microscopy (TEM)

The aluminum-neutralized pentablock copolymer solutionwas diluted to 0.5 wt % by adding cyclohexane to the originalsolutions (10 wt %). TEM samples were prepared by placing asmall droplet of solution on a carbon-coated copper grid andsolvent was rapidly evaporated at 80 1C in a vacuum oven toprevent changes in micelle shape and size. The TEM specimenswere dried under these conditions for 2 days before imaging. Thealuminum-neutralized membrane was sectioned at �60 1C usinga Reichert-Jung ultramicrotome with a diamond knife to anominal thickness of 40–70 nm. The dried TEM specimens froma dilute solution and ultrathin sections of the membranes were

Fig. 1. Transmission electron micrograph of aluminum-neutralized IEC¼2.0

pentablock copolymer solution (0.5 wt % dilute solution) deposited on a carbon

support grid. Dark domains indicate the micelle cores.

J.-H. Choi et al. / Journal of Membrane Science 428 (2013) 516–522518

examined in a JEOL 2010 F field emission transmission electronmicroscope. Images were recorded at an accelerating voltage of200 kV.

2.4. Water vapor transport rate (WVTR) measurement

The water vapor transport rate was measured by the invertedcup method (ASTM E96) at 23 1C and 50% RH in an environmentalchamber controlling humidity and temperature. The cup was filledwith DI water and the membrane was placed over the cup. The cupwas then inverted so that the water directly contacted the mem-brane. The cup was weighed after various periods of time todetermine the weight of evaporated water. The weight changeprovides a water vapor transport rate (kg/(m2 day)). Because wateraccumulation at the membrane/air interface during the test canlead to inaccurate measurements, a low speed fan was installedbelow the inverted cup to promote the evaporation of water at themembrane/air interface.

2.5. Water uptake measurement

The membranes were dried under vacuum at room tempera-ture and the dry mass of the membrane measured. The membranewas then soaked in de-ionized water for 24 h and the wet massweighed again. The water uptake of each sample was calculatedas follows:

Water uptake %ð Þ ¼Wwet�Wdry

Wdry� 100 ð1Þ

where Wwet and Wdry are the mass of the wet and dry membrane,respectively.

2.6. Tensile property measurement

Tensile property measurements were performed on a tensiletester (MTS EM System 6430 with load frame 2208) equippedwith a custom-built chamber for wet-state measurement.Mechanical property data were measured for both dry and wetmembranes. In this study, dry membranes are as-cast membranesthat were equilibrated at room conditions after casting and wetmembranes are membranes that were equilibrated in liquid DIwater for 24 h prior to the measurement. Detailed conditions forthe experiments were described elsewhere [23].

3. Results and discussion

3.1. Morphological characterization of micellar solutions

Fig. 1 shows a TEM micrograph of aluminum-neutralizedpentablock micelles after rapidly drying from a dilute solution.Because the contrast is mainly provided by the difference inelectron densities, the micelle core containing the charged group(–[SO3]3Al) appears dark in TEM. The neutralized solution showsspherical micelles with a core diameter of �20 nm. The acid-formprecursor solution also contains spherical micelles in a non-polarsolvent mixture as reported previously [18].

Fig. 2 shows small angle X-ray scattering profiles of acid-formand aluminum-neutralized IEC¼2.0 sulfonated pentablock copo-lymers in a non-polar solvent mixture. Both systems exhibitscattering profiles consistent with spherical micellar structures.The primary scattering peak in the aluminum-neutralized solu-tion shifts to slightly lower q relative to the acid-form pentablockcopolymer solution without any other significant changes in SAXSprofile. This indicates that the neutralized solution retains itsspherical micellar structure and the distance between cores

increases slightly. This scattering result is consistent with theabsence of a significant increase in viscosity of the micellar solutionupon neutralization. Because ionic crosslinking has occurred selec-tively in the SS cores of the micelles upon neutralization, there is nosignificant change in the micelle size or micellar volume fraction tochange the viscosity. Previously, we reported that increasing themicellar volume fraction increased the solution viscosity in theacid-form pentablock copolymers [18].

Quantitative information about the size and separation ofspherical micelles in these pentablock copolymer solutions canbe obtained by fitting the data to a modified hard sphere model[29]. The experimental data show good agreement with thismodel. The discrepancy at q is attributed primarily to sizepolydispersity of the micelles or broad interfaces between coreand solvated corona in the solutions, whereas the model assumesmonodisperse spheres and sharp interfaces. Table 1 summarizesthe fitting parameters for acid-form and aluminum-neutralizedsolutions. The size of micelle core found by TEM is consistent withthe X-ray scattering result. The diameter of micelle core (2R)increases slightly (�5%) upon neutralization, while the closestapproach distance between cores (2RCA) increases �20%. Neu-tralization of the SS core increases the incompatibility betweencore (ionic) and corona (non-ionic) and this appears to cause theHI and tBS chains in the corona to become significantly morestretched and thereby increases RCA.

Fig. 3 shows morphological changes that occur when sulfonatedpentablock copolymer solution is neutralized with Et3Al, based onX-ray scattering and TEM results. The pentablock copolymer(IEC¼2.0) in a non-polar solvent (a mixed solvent of cyclohexaneand heptane) forms spherical micelles both before and after solutionneutralization. The neutralized micelles have cores with diametersof �21 nm containing SS and Al (3þ) ions. Upon neutralization theHI–tBS blocks expand to increase the corona thickness.

3.2. Morphological characterization of membranes

Fig. 4 shows the through-plane X-ray scattering profiles foras-received IEC¼2.0 sulfonated pentablock copolymer precursor

Fig. 3. Schematic of the spherical micelles and the size changes in sulfonated

pentablock copolymer solution upon neutralization. Spherical micelles in neutra-

lized solution contain a dense core of SS and Al(3þ) ions and a corona of HI–tBS

swollen by solvent. The tBS, HI, SS blocks and Al ions are shown in green, red, blue

and yellow, respectively. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

Fig. 4. Through-plane X-ray scattering intensity as a function of scattering vector q

for as received acid-form and aluminum-neutralized membranes.

Fig. 2. X-ray scattering intensity as a function of scattering vector q for (a) acid-form IEC¼2.0 sulfonated pentablock copolymer solution and (b) aluminum-neutralized

solution. Experimental data (square) and the model fit (solid line) for monodisperse spherical micelles.

Table 1Fitting parameters for Kinning–Thomas model for acid-form and aluminum-

neutralized IEC¼2.0 sulfonated pentablock copolymer solutions. 2R, 2RCA, and n

represent the diameter of the micelle core, the closest approach distance between

cores, and the number of micelles per unit volume, respectively.

Sample 2R (nm) 2RCA (nm) n (10�6/ nm3)

Acid-form 20.4 42.2 9.71

Aluminum-neutralized 21.4 51.0 4.58

J.-H. Choi et al. / Journal of Membrane Science 428 (2013) 516–522 519

(acid-form) and neutralized membranes cast from 10 wt% cyclo-hexane/heptane solutions. Both as-received pentablock copolymermembranes show broad scattering features and their primaryscattering spacings (d¼2p/q1, q1 is the position of the primarypeak) are 24.7 and 23.5 nm for the acid-form and aluminum-neutralized materials, respectively. These sizes are somewhatlarger (10–20%) than the micelle core diameters (2R in Table 1).These membranes show anisotropic 2-D scattering patterns for in-plane X-ray scattering and the primary scattering spacings aresmaller perpendicular the membrane, even smaller than themicellar cores in the acid-form and aluminum-neutralized penta-block solutions. From our previous study of these acid-form

pentablock copolymer membranes, this suggests that bothas-received membranes exhibit bicontinuous morphology withinterconnected SS microdomains [19].

The TEM micrograph of the aluminum-neutralized pentablockcopolymer membrane, Fig. 5, reveals disordered interconnectedmicrodomains. The dark microdomains in the image correspondto the SS microdomains with Al. The measured interdomainspacing of the SS microdomains from the TEM image is �19–22 nm, which is consistent with X-ray scattering result. A similarbicontinuous morphology in IEC¼2.0 acid-form pentablock copo-lymer membrane has previously been reported [19]. Thus, boththe acid-form and aluminum-neutralized membranes exhibitbicontinuous morphology with interconnected SS microdomainsbased on X-ray scattering and TEM results.

X-ray scattering was performed on the membranes aftersoaking in water for 1 and 80 days as shown in Fig. 6. Themembranes remain microphase separated which is consistentwith water selectively entering the SS microdomains. The extentof swelling as indicated by the shift of the primary peak to lowerscattering angle is much more pronounced for the acid-formpentablock copolymer membrane. The ratio of the primary spa-cing in the swollen state (d) to the primary spacing in the drystate (as-received) (d0) was calculated. The ratio of d/d0 indicatesthe relative increment of spacing in a wet state, which alsocorresponds to how much water was absorbed into the hydro-philic SS microdomains. There is a 53% increase in microdomain

Fig. 5. Transmission electron micrograph of aluminum-neutralized IEC¼2.0

pentablock copolymer membrane. This is an unstained specimen and dark

microdomains indicate the SS microdomains with Al.

Fig. 6. X-ray scattering results for as-received and swollen IEC¼2.0 pentablock

copolymers in acid-form and aluminum-neutralized membranes. The swollen

membranes were immersed in water for 1 and 80 days.

J.-H. Choi et al. / Journal of Membrane Science 428 (2013) 516–522520

spacing for the acid-form membrane when exposed to water for1 day, while there is only an 8% increase for the neutralizedmembrane, Table 2. The acid-form membranes absorb morewater into the SS microdomains and this leads to plasticization

of the hydrophilic SS microdomains, such that the hydrogenbonds between sulfonic acid centers in the dry state are destroyedwhen exposed to water [19]. This results in a greater increase inSS microdomain spacing in the acid-form membrane whenexposed to liquid water. On the other hand, water only slightlyplasticizes the ion-containing microdomains in aluminum-neutralized membranes. The ionic interactions are retained evenin the wet state, because Al(3þ) ions have a stronger interactionwith the oxygen center in the sulfonic acid than with the oxygenin water.

The absorption of water reaches a maximum within one day,as indicated by no change in the primary spacing of thealuminum-neutralized membrane when equilibrated with liquidwater up to 80 days. In contrast, the acid-form pentablockcopolymer membrane continues to evolve after 1 day in water.These X-ray scattering results of membranes suggest that thealuminum-neutralized membrane exhibits different morphologi-cal behavior when exposed to water compared to the membranein the acid form. This will be correlated to water transport andmechanical properties of these membranes in the next section.

3.3. Structure–property relationships in membranes

Table 2 summarizes transport and mechanical properties foracid-form and aluminum-neutralized IEC¼2.0 sulfonated penta-block copolymer membranes. The swelling data from X-rayscattering presented above is consistent with the water uptakeproperty of these membranes. The neutralized membrane takesup just �29% by weight when immersed in water, while the acid-form membrane absorbs 140% by weight. A decrease in wateruptake in aluminum-neutralized membranes can be attributed tothe chemical cross-linking of the SS blocks. Similar behaviorhas been reported in other sulfonated membranes [20,22]. Thelower water uptake in the aluminum-neutralized membrane pro-vides better dimensional stability which is a critical factor formembranes in reverse osmosis, desalination, and humidification/dehumidification device applications, because sagging mem-branes in the presence of water vapor and liquid water impedeperformance.

The water vapor transport rates (WVTR) are comparable forthe acid-form and neutralized membranes (Table 2), indicatingthat both membranes have continuous SS microdomains forwater transport. This is consistent with a bicontinuous morphol-ogy with interconnected SS microdomains inferred from the X-rayscattering and TEM data. The small decrease in WVTR from 24 to22 kg/(m2 day) after aluminum-neutralization can be attributedto lower water uptake in the neutralized membrane whenexposed to liquid water [14,23]. Note that the wet neutralizedmembrane has smaller SS microdomains than the wet acid-formmembrane, but still exhibits similar water transport rates. Thisindicates that the effect of the size of the SS microdomains onwater transport property is much less significant than the effectof connectivity of the SS microdomains for transport in thissulfonated pentablock copolymer system. By comparison, wepreviously found a WVTR of �2.5 kg/(m2 day) for acid-formpentablock copolymer membranes (IEC¼0.4–1.0 mequiv./g) withdiscrete SS microdomains [19].

Aluminum neutralization has a significant effect on mechan-ical properties of the membranes, Table 2. In the wet state, theacid-form membrane shows a significantly lower modulus thanthe dry acid-form membrane and has no yield point. This isattributed to the softening of the membrane resulting from thehydrolysis of hydrogen bonds between sulfonic acid groups inSS microdomains when exposed to water. Importantly, thealuminum-neutralized membrane retains mechanical strengthin the wet state. This indicates that water only slightly plasticizes

Table 2Summary of properties for IEC¼2.0 sulfonated pentablock copolymer in acid-form and aluminum-neutralized membranes. The d-spacings for the dry and wet states

correspond to the calculated primary spacings from X-ray scattering results for as-received membranes and membranes swollen in water for 1 day, respectively. The error

of mechanical properties is roughly715% of the reported values [23].

Sample Structure Properties

d-Spacing

(dry) (nm)

d-Spacing

(wet) (nm)

Water

uptake

(wt %)

WVTR (kg/

(m2 day))

Modulus

(dry) (MPa)

Modulus

(wet) (MPa)

Yield stress

(dry) (MPa)

Yield stress

(wet) (MPa)

Tensile stress at

break (dry) (MPa)

Tensile stress at

break (wet) (MPa)

Acid-form 24.7 37.4 140 24 455 21.4 12.4 no yield 16.5 8.27

Aluminum-

neutralized

23.5 25.6 29 22 365 648 8.27 8.27 9.65 8.27

J.-H. Choi et al. / Journal of Membrane Science 428 (2013) 516–522 521

the SS microphases, which is consistent with reduced wateruptake in aluminum-neutralized membranes relative to theacid-form membrane. Also, note that the wet neutralized mem-brane is stiffer (higher modulus) than the dry neutralized mem-brane, which can be attributed to the topological constraints ofhighly cross-linked neutralized membranes in the wet state. Thepolymer chains of SS block are already extended (expanded chainconformation) in the swollen state, which results in a highermodulus under stress than the dry neutralized membrane.

4. Conclusion

We examined the effect of neutralization of IEC¼2.0 sulfonatedpentablock copolymer with Et3Al on the solution morphology innon-polar solvents (cyclohexane/heptane) and the membrane mor-phology and properties. The aluminum-neutralized solution retainsspherical micelles where the micelle contains a core of SS and Al(3þ) ions and a corona of solvated HI-tBS. The aluminum-neutralized membrane with continuous SS microdomains showsexcellent water transport properties that are comparable to theacid-form membrane. The strong interaction between Al(3þ) ionsand the oxygen center in the sulfonic acid (cross-linking) in thealuminum-neutralized membrane restricts water uptake and pro-vides much improved mechanical properties and better dimen-sional stability. The aluminum-neutralized membrane with theseimproved properties could have an important role in variousmembrane-based applications. This study demonstrates that theunderstanding of the structure–property relationships is a key partof the development of new membranes with improved properties.

Acknowledgements

This work was supported by Kraton Performance Polymers,Inc. The authors thank Kraton Polymer scientists for usefuldiscussions. We acknowledge the financial support of the U.S.Army Research Office under grant no. W911NF-07-1-0452 IonicLiquids in Electro-Active Devices (ILEAD) MURI. We acknowledgethe use of facilities at the University of Pennsylvania funded inpart by the MRSEC Program of the National Science Foundation(#DMR11-20901).

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