9240 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251Published on Web 07/07/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Interfacial Interactions in Aprotic Ionic Liquid Based Protonic Membrane

and Its Correlation with High Temperature Conductivity and Thermal

Properties

Mayur K. Mistry, Surya Subianto, Namita Roy Choudhury,* and Naba K. Dutta

IanWark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095,Australia

Received October 6, 2008. Revised Manuscript Received June 9, 2009

Novel supported liquid membranes (SLMs) have been developed by impregnating Nafion and Hyflon membraneswith ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI-BTSI). These supported liquidmembranes were characterized in terms of their ionic liquid uptake behavior, leaching of ionic liquid by water, thermalstability, mechanical properties, glass transition temperature, ion exchange capacity, and proton conductivity. Ingeneral, modified membranes are more flexible than unmodified samples due to the plasticization effects of the ionicliquid. However, these supported liquid membranes exhibit a significant increase in their operational stability andproton conductivity over unmodified membranes. We also demonstrate that proton conductivity of these supportedliquidmembranes allows conduction of protons in anhydrous conditions with conductivity increasing with temperature.Conductivity of up to 3.58mS cm-1 has been achieved at 160 �C in dry conditions, making thesematerials promising forvarious electrochemical applications.

Introduction

Nafion provides excellent thermal, chemical, and mechanicalstability along with high proton conductivity for application inelectrochemical devices such as fuel cells and batteries, whichhas contributed to its widespread usage in the area of membranesand separators. This membrane has a unique phase separatedmorphology due to the aggregation of sulfonic acid groups fromfluorinated phase to form ionic domains. It is through these ionicdomains that proton conductivity takes place in proton exchangemembranes. This is highly dependent on the hydration state of thepolymer. At temperatures above 80 �C, Nafion tends to dehy-drate, which results in a collapse of the ionic domains and hence asignificant decrease in proton conductivity. Nafion’s conductivityis dependent on water as the protic solvent, which limits itsoperation to below 80 �C.1 This has prompted significant researchin the area of alternate proton conducting membranes, whichcan operate in an anhydrous environment at temperatures above100 �C. In order to achieve anhydrous conductivity at hightemperatures, water needs to be replaced with an alternativeproton solvent. Incorporating a dipolar solvent with low vaporpressure can overcome this limitation of Nafion. Ionic liquids(ILs) show high ionic conductivity, excellent thermal and electro-chemical stability, as well as negligible vapor pressure, whichmake them attractive to incorporate into Nafion membranes toimprove anhydrous conductivity at high temperatures.

Recently, Schmidt et al.2 studied Nafion membranes impreg-nated with various imidazolium (1-hexyl-3-methyl imidazolium/HMI, 1-butyl-3-methyl-imidazolium/BMI) and pyrrolidium(1-butyl-1-methyl-pyrrolidium/BMPyr) based ionic liquids bear-ing hydrophobic (tris(pentafluoroethyl)trifluorophosphate/FAP,bis(trifluoromethylsulfonyl)imide/BTSI, hexafluoro phosphate/PF6)

andhydrophilic (tetrafluoroborate/BF4) anions.Nafion117 impreg-nated with HMI-FAP showed conductivity up to 1 mS cm-1 at120 �C at anhydrous conditions. Doyle et al.3 have preparedNafion based IL membranes that are thermally stable upto temperatures over 200 �C and obtained conductivities of0.1 S cm-1 at 180 �C by incorporating 1-butyl-3-methyl imida-zolium trifluoromethane sulfonate (BMITf) into perfluorinatedionomer membranes with varying equivalent weight (EW). It hasbeen observed that high proton conductivity can be achievedwhen sufficient IL absorbs into the membrane, which can bemore readily achieved with lower equivalent weight membranes.Bennett et al.4 also observed an increase in conductivity ofIL swollen Nafion membranes with an increase in IL uptake.The results reveal that the cation of the IL interacts with themembrane by displacing the protons of Nafion away fromthe exchange sites, with uptake of IL being dependent on thesize of the counterion. Other works have reported the dopingof polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP)polymer with various room temperature ILs.5,6 PVDF-HFPdoped withN-ethylimidazole bis(trifluoromethanesulfonyl)imideshowed excellent thermal stability up to 300 �C with conductivityof the order of 10-2 S cm-1 at 140 �C. The conductivity in theseIL membranes is very stable, and results demonstrate thatconductivity does not change over a period of several days.

In view of the potential of the IL for making anhydrousmembranes, in this work, we have chosen to prepare IL mem-branes from the hydrophobic IL 1-butyl-3-methylimidazoliumbis-(trifluoromethylsulfonyl)imide (BMI-BTSI) and perfluorinatedmembranes. These supported liquid membranes can also findapplication in the area of fuel cells. Thus, in this work, we aim to

*Corresponding author. E-mail: Namita.choudhury@unisa.edu.au.(1) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104(10), 4535–4585.(2) Schmidt, C.; Glueck, T.; Schmidt-Naake, G. Chem. Eng. Technol. 2008, 31

(1), 13–22.

(3) Doyle, M.; Choi, S. K.; Proulx, G. J. Electrochem. Soc. 2000, 147(1), 34–37.(4) Bennett, M. D.; Leo, D. J.; Wilkes, G. L.; Beyer, F. L.; Pechar, T. W.

Polymer 2006, 47(19), 6782–6796.(5) Fernicola, A.; Panero, S.; Scrosati, B.; Tamada, M.; Ohno, H. Chem-

PhysChem 2007, 8(7), 1103–1107.(6) Fortunato, R.; Afonso, C. A. M.; Reis, M. A. M.; Crespo, J. G. J. Membr.

Sci. 2004, 242(1-2), 197–209.

DOI: 10.1021/la901330y 9241Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

develop a fundamental understanding of interactions betweenmembranes and ILs. The focus of this research is to probe thespecific interactions between ILs and fluoropolymer membranesand todetermine the effects of the ILon the structure,morphology,and anhydrous conductivity of the membrane. This particularionic liquid was chosen for its high thermal stability, excellentproton conductivity, and low vapor pressure. The presence of thefluorinated anion makes this ionic liquid hydrophobic in nature,which is expected to improve compatibility between the ionicliquid and fluorinated membrane. While most previous publica-tions have focused onNafion as the perfluorinatedmembrane, wehave prepared novel IL membranes based on perfluorinatedmembrane Hyflon Ion. Hyflon Ion7-9 has a similar chemicalstructure to Nafion but has a shorter side chain length, whichresults in a lower EW (Table 1). Other differences in Hyfloninclude a higher glass transition temperature (Tg),

10 better thermalstability, and higher ionic conductivity over Nafion.8 Here, weinvestigate the effect of EWand side chain length on the uptake ofIL within both Nafion and Hyflon membranes, and their impacton the morphology and anhydrous proton conductivity.

Experimental Section

Materials. Nafion 112 with an equivalent weight of 1100 g/SO3H was purchased from Sigma-Aldrich, and Hyflon with anequivalent weight of 890 g/SO3Hwas provided by Solvay Solexis,Milan, Italy. All membranes were converted to sulfonic acidform using a reported initialization procedure.11 Membraneswere purified by boiling in 3% hydrogen peroxide solution for2 h followed by 1 M H2SO4 for a further 2 h and finally MilliQ water for another 2 h. The ionic liquid 1-butyl-3-methylimida-zolium bis(trifluoromethylsulfonyl)imide (BMI-BTSI) witha purity of 99.9% was purchased from Solvent Innovation,Germany and was used without further purification. Methanoland Milli Q water were used as solvents.

Sample Preparation. All membranes were thoroughly driedin a vacuum oven to ensure all traces of water were removedbefore modification. IL impregnated membranes were preparedby soaking a weighed piece of the perfluorinated membrane in asolution of the ionic liquid, which was diluted with methanol.Membranes were soaked in an ionic liquid/methanol solutionover a period of several days at two different temperatures. Uponremoving membranes from the liquid after a defined time, theywere blottedwith kimwipes and dried at 80 �Cunder vacuum.Themass of the absorbed IL was determined gravimetrically.

Leaching Test. The leaching test was carried out by immer-singmembranes impregnated with ionic liquid inMilli Q water atroom temperature. The membranes were removed from waterevery 24 h and dried in a vacuumoven until a constantweight wasachieved. Loss of ionic liquid from themembranewas determinedgravimetrically. Proton conductivity measurements were carriedout on leached samples to determine the drop in conductivity incomparison to nonleached membranes.

Ion Exchange Capacity. Ion exchange capacity (IEC) wasdetermined by soaking the membranes in a 0.1MNaOH solutionfor 24 h and titrating the solution with standardized 0.1 M HClusing phenolphthalein indicator. The IEC was calculated basedon the difference in the volume of HCl required to neutralize thesolution compared to the blank with no sample present, and was

calculated according to eq 1.

IEC ðmeq g-1Þ ¼ VblankðHClÞ-VsampleðHClÞ� �CHCl

msampleð1Þ

The titration was performed three times for each sample, and theaverage value was taken for the IEC.

Spectroscopic Characterization. Photoacoustic-Fouriertransform infrared spectroscopy (PA-FTIR)wasperformedusinga Nicolet Magna spectrometer (model 750) equipped with aMTEC (model 300) photoacoustic cell. Carbon black was usedas reference. Helium was used due to its high thermal conductiv-ity, and a gas flow rate of 10 cm3 s-1 was used. The resolution of8 cm-1, 256 scans, and amirror velocity of 0.158 cm s-1 were usedfor all measurements.

X-ray photoelectron spectroscopy was carried out using aKratos Axis Ultra DLD spectrometer using a monochromaticAl KR (1486.6 eV) irradiation source operated at 130 W. Thesupported liquid membranes were characterized at a photoelec-tron takeoff angle of 90� to the cross section of the sample.Energies of 160 eV for the survey spectra and 20 eV for highresolution spectra were used for all elemental spectral regions.Charge correction was performed by fixing the hydrocarboncomponent of the C 1s peak to 284.5 eV. The spectra weredeconvoluted using the CasaXPS software package.

Thermal Analysis. Thermogravimetric analysis (TGA) ofthe samples was conducted using a TGA 2950 thermal analyzer(TA Instruments, DE) using a conventional mode (dynamicheating) under either nitrogen or oxygen atmosphere at aflow rate of 50 mL min-1. The mass of the samples was keptbetween 10 and 12 mg. The sample was heated from 30 to 900 �Cat a heating rate of 10 �C min-1 up to 500 �C in nitrogen, andbetween 500 and 900 �C in oxygen. The onset of degradation, theweight loss, and the residue remaining at 900 �C were evaluated.Differential scanning calorimetry (DSC) was performed using aTA 2920 DSC instrument (TA Instruments, DE) with a heatingrate of 10 �C min-1 under a nitrogen atmosphere. The samplemass was kept between 5 and 10 mg. The unit was fitted with aliquid nitrogen cooling accessory (LNCA). Dry samples werecooled from room temperature down to -50 �C, kept at thetemperature isothermally for 5min, and then heated up to 300 �C.Dynamic Mechanical Analysis. Dynamic mechanical prop-

erties of the samples were measured using a DMA 2980 instru-ment (TA Instruments, DE) in tension mode. Samples wereheated from -100 to 175 �C at a frequency of 1 Hz, at 0.08%strain amplitude with a programmed heating rate of 3 �C min-1.

Proton Conductivity. An ESPEC SH-240 (ESPEC Corp,Japan) temperature/humidity chamber and a conductivity cellwere used for the measurement of membrane conductivity

Table 1. Chemical Structure and Composition of Supported Liquid

Membranes

(7) Arcella, V.; Troglia, C.; Ghielmi, A. Ind. Eng. Chem. Res. 2005, 44(20), 7646–7651.(8) Ghielmi, A.; Vaccarono, P.; Troglia, C.; Arcella, V. J. Power Sources 2005,

145(2), 108–115.(9) Arico, A. S.; Baglio, V.; Di Blasi, A.; Antonucci, V.; Cirillo, L.; Ghielmi, A.;

Arcella, V. Desalination 2006, 199(1-3), 271–273.(10) Merlo, L.; Ghielmi, A.; Cirillo, L.; Gebert,M.; Arcella, V. J. Power Sources

2007, 171(1), 140–147.(11) Daiko, Y.; Klein, L. C.; Kasuga, T.; Nogami,M. J.Membr. Sci. 2006, 281(1

þ 2), 619–625.

9242 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251

Article Mistry et al.

(σ) under conditions of variable temperature and controlledhumidity. Each sample used was approximately 12 � 10 mm2,and σ was calculated using eq 2:

σ ¼ l

Rhwð2Þ

where l is the distance (cm) between the two Pt electrodes, h andw are the thickness (cm) and width (cm) of the membranerespectively, andR (Ω) is the resistance of themembrane obtainedfrom the complex impedance plot measured by using a Solartron1260 instrument (Solartron Analytical, U.K.).

Small Angle X-ray Scattering (SAXS). Small angle X-rayscattering (SAXS) data were collected on a S3-MICRO SAXS/WAXS instrument (HECUS GmbH, Graz, Austria) which con-sists of a GeniX microfocus X-ray source (Xenocs, Grenoble,France) (sealed Cu KR source, power 50 W, brilliance ∼109

photons/s/mm2/mrad2/% bandwidth) integrated with FOX2Dsingle-bounce multilayer optics providing a point focusingbeamdelivery systemwith lowdivergence (<1mrad). The samplewas loaded in a 2 mm diameter quartz capillary (Charles SupperCo.) and mounted in the single position heater block. Simulta-neous SAXS/WAXS data was collected as a function of tempera-ture using the instrument control software. The instrument isequipped with two one-dimensional (1D) position sensitivedetectors (HECUS 1D-PSD-50 M system); each detector is50 mm long (spatial resolution 54 μm/channel, 1024 channels)and covers the SAXS Q-range (0.003 < Q < 0.6 A-1) and theWAXS Q-range (1.2 < Q < 1.9 A-1).

Results and Discussion

Structure Analysis Using PA-FTIR. FTIR spectroscopywas used to confirm the incorporation of IL into the polymermatrix and also probe specific interactions between IL andthe polymer. Figure 1 shows the representative IR spectra ofthe pure BMI-BTSI, unmodified Hyflon, and the Hyflonsupported liquid membrane. The spectrum of the pure BMI-BTSI shows various characteristic peaks associated with theionic liquid structure. The first two bands at 3137 and2928 cm-1 represent the stretching of -CH groups of theimidazole ring of the IL as does the signal for the CdC doublebonds of the imidazole ring around 1572 cm-1. Bands at1056, 790, and 736 cm-1 correspond to SdO, C-S, and -CF3

stretching vibrations, respectively, which relate to the structureof the anion. The spectrum of umodified Hyflon is characterized

by the symmetric stretching vibration of the C-O-C group at966 cm-1 and the symmetric stretching vibration of the sulfonicacid group around 1062 cm-1. The broad band at 1218 cm-1 isrelated to the-CF2- stretching vibration of the fluorinatedmainchain of the polymer. The band at 3427 cm-1 is attributed tostrongly boundwaterwithin themembrane, which is very difficultto remove.12 In the spectrum of the Hyflon impregnated with IL,all the characteristic peaks from the pure IL and unmodifiedmembrane are present, indicating IL’s ability to permeatethrough the polymer membrane. It is observed that the sulfonicacid peak at 1062 cm-1 in the unmodified Hyflon has shifted inthe case of the Hyflon IL membrane to 1051 cm-1. It has beenreported that the sulfonic acid peak shifts to lower wave-numbers as the size of the counterion is increased and as thecontent of water within the membrane is increased.4 This shiftcorresponds to a decrease in the polarization of the S-O dipoleresulting from increased separation between the counterion andexchange site.4 This increased separation can be caused byincreased size of the counterion as protons from sulfonic acidgroups are partially replaced by the cation of the ionic liquid. Inthis case, the membranes were in a dry state so a shift in thesulfonic acid peak to lower wavenumbers can be attributed to theincreased size of the counterion, indicating presence of interactionbetween IL and the membrane. IR spectra of blank Nafion andNafion IL membranes show the same features to correspondingHyflon samples, which is due to their similar structure.Ionic Liquid Uptake/Leaching Test. Both Nafion and

Hyflon membranes were impregnated with ionic liquid atroom temperature and at 75 �C. The amount of absorbedionic liquid increases with temperature with membranes impreg-nated at 75 �C, showing a higher ionic liquid uptake than thatof membranes impregnated with ionic liquid at room tempera-ture. The amount of ionic liquid absorbed by membranes isdisplayed in Table 2. At room temperature, Nafion and Hyflononly absorbed 10 and 7.6% ionic liquid, respectively; however,this increases to 17% when both membranes are impregnatedat higher temperature. The higher uptake of ionic liquid athigher temperatures can be mainly attributed to the increasedmobility and swelling of the perfluorinated membranes athigher temperatures that facilitates diffusion of the ionic liquid

Figure 1. Infrared spectra of ionic liquid, Hyflon, and Hyflon IL membranes.

(12) Wang, Y.; Kawano, Y.; Aubuchon, S. R.; Palmer, R. A. Macromolecules2003, 36(4), 1138–1146.

DOI: 10.1021/la901330y 9243Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

molecules into the polymer. The absorption of IL in themembrane is thus dependent on the availability and accessibilityof the ionic groups. The clustering of sulfonic acid groups inthe membrane can limit the accessibility of SO3

- groups toion exchangewith the cation of the ionic liquid. The impregnationof membranes at two different temperatures clearly revealsthat at higher temperature higher uptake of ionic liquid results.As the temperature is increased, the ionic clusters begin todissociate, making more sulfonic acid groups accessible andavailable for ion exchange with the cation of the ionic liquid.In addition, the size of the cation is very large and bulky incomparison to protons, and this also limits the absorption ofionic liquid within membranes.

An increase in thickness of the membranes was observedafter uptake of IL. Schmidt et al.2 demonstrated that the impreg-nation kinetics are mainly governed by the anion of the ionicliquid and especially by its bulkiness and hydrophilicity. Theleaching test was carried out to determine the amount of ionicliquid that may be leached out from the membrane. This testdoes not simulate real operating conditions of such membranesbut helps to evaluate the long-term stability of these supportedliquid membranes. Membranes were leached in water and driedin a vacuum oven until a constant weight was achieved. Theamount of ionic liquid leached out from each membrane variesand depends on the amount of ionic liquid initially absorbedand its hydrophobicity. The percentage (%) losses of ionic liquidfrom the membranes are shown in Table 2. Results from leachingtest show that both Nafion and Hyflon membranes are capableof retaining a significant amount (∼50%) of ionic liquid withinthe polymer. The amount of IL remaining within the membranesis a result of specific interactions between IL and sulfonic acidgroups immobilizing IL within the polymer membrane. Hydro-phobic anions such as bis(trifluoromethylsulfonyl)imide (BTSI)may even permeate into the hydrophobic, perfluorinated segmentof the membrane, where they are shielded from water and alsoremain immobilized within the polymer matrix.13 The amountof ionic liquid leached out from membranes is lower for Hyflon(IEC=1.22meq g-1) than forNafion (IEC=0.94meq g-1) dueto its higher ion exchange capacity. Hyflon has a greater numberof sulfonic acid groups per gram of dry polymer, which allowsmore ionic liquid to ion exchange with sulfonic acid groups.From leaching experiments, it is clear that a significant amountof absorbed ionic liquid (<50%) is removed bywashing in water.The high amount of ionic liquid that is leached out fromthe membranes is due to the fact that not all of the ionicliquid interacts with the membrane through electrostatic interac-tions, as only part of the sulfonic acid groups in the polymer areaccessible for ion exchange. Much of the ionic liquid is thusonly physically residing within the membrane and is not boundthrough interactions, and hence is leached out. However, ∼50%IL also remains immobilized within the membrane through anion exchange process.

Thermal Analysis: Thermal Stability of Supported Liquid

Membranes.TGAwas used to determine the thermal stability ofpure ionic liquid, unmodified membrane, and membrane impreg-nated with IL. Figure 2 shows the TGA curve of the pure ionicliquidwith a one-stepdecomposition profile and exhibits excellentthermal stability up to 380 �C. The ummodified Hyflon mem-brane shows a small weight loss between 30 and 290 �C, which isattributed to residual water within the membrane that is tightlybound through hydrogen bonding to sulfonic acid groups. Suchwater can be removed by drying thoroughly in vacuum oven foran extended period of time.Decomposition in unmodifiedHyflonbetween 290 and 400 �C is related to desulfonation followed byside chain decomposition between 400 and 470 �C. The largeweight loss seen between 470-580 �C is due to main chaindecomposition of Hyflon. Nafion 112 also shows a very similarTGA curve to that of Hyflon (not shown), due to their structuralsimilarity. In the Hyflon membrane impregnated with IL, there isno weight loss between 30 and 290 �C, indicating that thehydrophilicity of the membrane has been reduced significantly.This result indicates supported liquid membranes are hydropho-bic in nature and there is no water present within the system. TheHyflon/IL membrane shows excellent thermal stability up to370 �C with no significant weight loss. It is evident that thedecomposition due to desulfonation of sulfonic acid groupshas been suppressed and shifted to higher temperatures in thecase ofHyflon/IL. It iswell-known that theNafion in the sodiatedform shows a higher thermal stability than Nafion in theprotonated form due to increased radius of the counterion.This observation confirms that there is some specific interactionbetween sulfonic acid sites of the polymer and IL, which hasincreased the thermal stability of the membrane. The Hyflon/ILleached membrane shows a very similar decomposition profile tothat of the Hyflon/IL but shows a slight decrease in thermalstability by 25 �C. This shows that after thorough leaching someIL remains immobilized within the membrane as desulfonationhas still been suppressed in this leached sample. The experimental

Table 2. Quantity of Ionic Liquid Leached out and Proton Conductivity Values of Leached Samples

conductivityat 160 �C (mS/cm)

sampleionic liquiduptake (%) T (�C)

% loss afterleaching test

remaining ionic liquidin membrane (%)

beforeleaching

afterleaching

Nafion/IL 10 10 25 3.8 6.2 0.96 0.86Nafion/IL 17 17 75 10 7 3.58 1.01Hyflon/IL 7 7.6 25 2.6 5 0.84 0.59Hyflon/IL 17 17 75 8.9 8.1 1.77 0.75

Figure 2. TGA curves of pure BMI-BTSI, umodified Hyflon,Hyflon/IL, and Hyflon/IL leached membrane.

(13) Bennett, M. D.; Leo, D. J. Sens. Actuators, A 2004, 115(1), 79–90.

9244 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251

Article Mistry et al.

results of Nafion/IL samples show a similar trend to that ofHyflon/IL samples.Swelling of the Membranes. TGA was used to evaluate

the water uptake characteristics of bothmodified and unmodifiedmembranes as shown in Figure 3. Before TGA measurements,membranes were swollen in Milli Q water until equilibriumwas reached and the surface of the membrane was blottedwith kimwipes to remove any excess water. Both unmodifiedNafion and Hyflon show significant (>20%) water uptake dueto the hydrophilic nature of sulfonic acid groups. Unmodifiedmembranes show a two-step water desorption profile withthe majority of the water loss occurring below 100 �C. Afterthis initial loss, relatively small weight changes are observed dueto release of tightly bound water within the membrane. Mem-branes impregnated with IL show significantly (<5%) decreasedwater uptake in comparison to unmodified membranes. Thereduced swelling capability of IL impregnated membranesundoubtedly reveals reduced hydrophilicity. This can be ex-plained by the incorporation of the hydrophobic anions,which have miscibility with the fluorinated backbone, and byexchange of protons of sulfonic acid groups of unmodifiedmembranes for the cations of the ionic liquid.Interaction Study by Differential Scanning Calorimetry

(DSC).Thermodynamic characteristics such asmelting (Tm) andglass transition (Tg) temperature of ionic liquids were measuredbyDSC.The results are shown inFigure 4.Uponheating the ionicliquid from -100 �C at a rate of 10 �C min-1, a clear Tg wasobserved at -87.2 �C. The other phase transition observedincludes a cold crystallization temperature at -35.3 �C and aTm of -0.59 �C, which is consistent with reported literaturevalues.14,15 DSC was also used to probe the specific interactionsbetween the fluoropolymer and IL. Figure 5 shows the thermaltransition of the supported liquid membranes (SLMs) from bothNafion and Hyflon. The unmodified Nafion membrane shows alarge endothermic peak at 125 �C. This endothermic peak can beattributed to structural changes in ionic clusters, as they changefrom an ordered to disordered state. This peak, related to thecluster transitions of Nafion, is also observed using DMA(discussed later). Tadano et al.16,17 assigned this endothermicpeak to an order-disorder transition inside the ionic clusters.This behavior can be associated with the molecular rearrange-ments inside the polar phase of the base polymer.18 There is a

second endothermic peak centered at 214 �C, which can beattributed to the melting of the crystalline regions and is some-times referred to as the R transition, and this is related to thefluoro main chain of the polymer. Unmodified Hyflon showsa very similar DSC trace to that of Nafion with an endothermicpeakat 140 �C related to cluster transitions and a shoulder presentat 215 �C, which is attributed to the R transition.19 Hyflonhas a relatively higher cluster transition temperature thanNafion, which is due to the shorter side chain of the Hyflonmembrane. The enthalpies related to the cluster transitionshave been calculated for both Nafion and Hyflon and are248 and 138 J g-1, respectively. The enthalpies for R transitionswere calculated to be ∼40 and ∼39 J g-1 for both Nafion andHyflon, respectively. The enthalpy values are very similar, asthe fluoro main chain is the same in both polymers. In thecase of IL impregnated membranes, no cluster or R transitionis observed from the DSC trace. This observation confirmsthat the IL is interacting with the sulfonic acid groups throughionic interactions. As the radius of the counterion is increased,the electrostatic interaction between sulfonate groups and coun-terion becomes weaker and the corresponding clusters are lessstable. Therefore, lower thermal energy is required to promotean ionic rearrangement. This observation confirms the interac-tion phenomena seen from FTIR measurements, TGA, andIEC calculations, and suggests that the cation of the IL hasreplaced the protons of sulfonic acid groups through a cationexchange process.Dynamic Mechanical Analysis (DMA): Viscoelastic

Properties. DMA was used to determine the ionic clusterrelaxation temperature, Tc, and the influence of the ionic liquid

Figure 3. TGA curves of umodified Hyflon, Nafion 112, Hyflon,and Nafion impregnated with IL all in a hydrated state.

Figure 4. DSC of pure ionic liquid showing the thermal transi-tions.

Figure 5. DSC curves of both unmodified and modified mem-branes.

(14) Jin, H.; O’Hare, B.; Dong, J.; Arzhantsev, S.; Baker, G. A.; Wishart, J. F.;Benesi, A. J.; Maroncelli, M. J. Phys. Chem. B 2008, 112(1), 81–92.(15) Hunt, P. A.; Gould, I. R.; Kirchner, B. Aust. J. Chem. 2007, 60(1), 9–14.(16) Hirasawa, E.; Yamamoto, Y.; Tadano, K.; Yano, S.Macromolecules 1989,

22(6), 2776–2780.(17) Tadano, K.; Hirasawa, E.; Yamamoto, H.; Yano, S.Macromolecules 1989,

22(1), 226–233.(18) Su, S.; Mauritz, K. A. Macromolecules 1994, 27(8), 2079–86.

(19) De Almeida, S. H.; Kawano, Y. J. Therm. Anal. Calorim. 1999, 58(3), 569–577.

DOI: 10.1021/la901330y 9245Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

on the viscoelastic properties of the membranes. The tan δcurves for unmodified and supported liquid membranes areshown in Figure 6A. For all the membranes, Tc was identifiedfrom the peak of the tan δ curve. Unmodified Nafion andHyflonhave Tc’s of 118 and 143 �C, respectively. The variation incluster transition between Nafion and Hyflon is attributed tothe difference in side chain length. It is widely known thatabove Tc the network of hydrophilic clusters, made from sulfonicacid groups, becomes extremely mobile before the clusteredstructure finally collapses.20 The mobility of the backboneand cluster network in both unmodified membranes is less incomparison to the supported liquid membranes and is demon-strated by a larger value of tan δ. In the case of the NafionSLM, the cluster transition temperature Tc has decreasedfrom 118 to 110 �C and the tan δ peak has become broaderwith significant decreased intensity compared to the unmodifiedsamples. The Hyflon SLM also shows a similar decrease in Tc

from 143 to 128 �C. The shift in Tc between unmodified andmodifiedmembranes is in the range between 10 and 15 �Cand canbe explained in terms of the size of the counterion. In unmodifiedmembranes, the counterions are protons, which are relativelysmall in size, but once membranes are impregnated with ionicliquid, protons are replaced by the cation of the ionic liquidthrough an ion exchange process. The cation of the ionic liquidis much larger and bulkier than protons, which affects interac-tions between ionic clusters. As the size of the counterionincreases, the larger counterions significantly decrease thestrength of the electrostatic interactions and the bulky, organiccounterions can effectively plasticize the membrane, resulting inreduced cluster relaxation temperature.21 Figure 6B shows thestorage modulus curves as a function of temperature for allmembranes. The moduli for all the samples decrease with an

increase in temperature as the membranes pass from the glassy tothe rubbery state. The storage moduli of the SLMs were, in allcases, determined to be lower than that of the correspondingunmodified membrane. Modified membranes were softer andmore flexible than pristine membranes due to the plasticizationeffect of the ionic liquid. The plasticization effect of the absorbedionic liquid can be attributed to the incorporation of cations withflexible alkyl chains and partial disruption of the hydrogen bondsgenerated by sulfonic acid groups in hydrophilic clusters byhydrophobic bulky anions.2

Ion Exchange Capacity (IEC). Ion exchange capacityvalues for Nafion and Hyflon were calculated to be 0.94 and1.22meqg-1, respectively.The IECs for IL impregnatedmembraneswere considerably lower than those of unmodified membranes asshown in Table 3. This can be explained in terms of a cationexchange process occurring between the ionic liquid and sulfonicacid exchange sites. The cation of the IL replaces protons of thesulfonic acid groups, which prevents them from taking part in theproton exchange reaction when determining the IEC. IECs forHyflon/IL7, Nafion /IL10, Hyflon/IL17, and Nafion/IL17 mem-branes were calculated to be 0.36, 0.31, 0.14, and 0.06 meq g-1,respectively, as shown in Table 3. The hypothesis that the sulfonicacid groups are deprotonated and their protons replacedby the ILcations is also supported by FTIR studies. The reduction in IECof IL impregnated membranes is essentially caused by thereplacement of protons by IL cations. A proposed mechanismof the ion exchange process is shown schematically in Figure 7.Asprotons are displaced, they associate with anions of the IL, andthrough a combined hopping and diffusion mechanism protonsare transported through the IL membrane. The amount ofabsorbed IL per sulfonic acid group (λ) has been calculated andis shown in Table 3.Interaction Study by X-ray Photoelectron and Infrared

Spectroscopy. X-ray photoelectron spectroscopy (XPS) wascarried out on unmodified and leached membranes to confirmthe nature of interaction of ionic liquid with polymer afterthorough leaching. Figure 8A shows representative survey scansof the unmodified Hyflon membrane and Hyflon/IL leachedmembrane. The signals present are those of fluorine, oxygen,carbon, sulfur, and nitrogen, which are from the polymer andionic liquid constituents. The survey scan also shows smallamounts of silica contamination which is due to sample prepara-tion andhandling.High resolution spectra for individual elements(O 1s and S 2p) are also obtained and are shown inFigure 8B. TheC 1s spectra of unmodified membranes show several peaksrelated to the different carbon configurations in the polymer.A peak at 284.5 eV is assigned to carbon in the C-CF or C-Cconfiguration,while thepeak at∼291 eV is ascribed toCF2. TheF1s spectrum shows one peak which can be deconvoluted into twocomponents, the peak at 689 eV is attributed toCF2, and a secondpeak at 688 eV is related to OCF. The O 1s core level spectrum ofunmodified Hyflon membrane (Figure 8B) has been fitted withtwo peaks at binding energies (BEs) of 532 and 534 eV, indicatingthe fact that oxygen in Hyflon has two different binding states.The first peak at 532 eV can be attributed to the oxygen in thesulfonate group, and the second peak at 534.8 eV is related to thatof ether oxygen.22 O 1s peak values obtained from experimentaldata are consistentwith literature values as reportedbyHoffmannet al.23 Figure 8C shows the O 1s spectrum of the leached Hyflonsupported liquid membrane, which is fitted with three peaks. The

Figure 6. Curves of tan δ (A) and storage modulus (B) forunmodified and modified membranes.

(20) Alberti, G.; Casciola,M.; Massinelli, L.; Bauer, B. J.Membr. Sci. 2001, 185(1), 73–81.(21) Page, K. A.; Cable, K. M.; Moore, R. B. Macromolecules 2005, 38(15),

6472–6484.

(22) Chen, C.; Levitin, G.; Hess, D.W.; Fuller, T. F. J. Power Sources 2007, 169(2), 288–295.

(23) Hoffmann, E. A.; Fekete, Z. A.; Korugic-Karasz, L. S.; Karasz, F. E.;Wilusz, E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42(3), 551–556.

9246 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251

Article Mistry et al.

first two peaks at 532 and 534 eV are identical to the spectra of theunmodified Hyflon. Previous XPS studies on the ionic liquid1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imidereport only one sharp peak at 532.3 eV,24,25 which has overlappedwith the oxygen peak from sulfonate groups. The O 1s spectrumof the leached Hyflon supported liquid membrane shows thepresence of a new peak at 536 eV, which is not present in theneat polymer. This indicates that the incorporation of ionicliquid into Hyflon has changed the electron binding energy ofoxygen in the sulfonate groups of the polymer. We can attributethe change in bindings states of oxygen to the electrostaticinteractions between the cation of the ionic liquid and sulfonategroups of Hyflon membrane. A similar observation was reporteddue to the electrostatic interactions between the sodium counter-ion and sulfonate group26 and the appearance of new peaks in thehigh resolutionO 1s spectrumof poly(sodium 4-styrenesulfonate)at 536 eV. The S 2p spectrum of unmodified Hyflon membraneis shown in Figure 8D. Due to the high noise to signal ratio, S 2pspectra could not be peak fitted. The spectra shows one peakcentered around 168.7 eV and is attributed to sulfur from sulfonicacid groups in Hyflon. It has been reported in the literaturethat the binding energy of sulfur from sulfonic acid groups is inthe range of 168.3-171.0 eV.27 The S 2p spectrum of thecorresponding leached sample is shown in Figure 8E. Whencomparing the S 2p spectrum of the unmodified membrane(Figure 8D) to the S 2p spectrum of the corresponding leachedmembrane (Figure 8E), a shift in binding energy in the presenceof ionic liquid is observed. The change in binding energy valuecan occur only if there is any interaction between sulfonate groupsand the cation of ionic liquid.28 High resolution XPS results ofunmodified Nafion and Nafion/IL leached membranes showsimilar results to that of Hyflon membranes. Recently, interfacialinteractions of ionomer with a hydroxyl functional polymer havebeen specifically investigated by FTIR.29 In the present case, we

have used photoacoustic FTIR to further prove interactionsbetween the ionic liquid and polymer. Figure 9A shows theinfrared spectra of representative Nafion unleached and leachedmembranes. The main peaks attributed to the IL component areat 3137 and 2928 cm-1 and represent the stretching of -CHgroups of the imidazole ring. These peaks are present in bothunleached and leached membranes, which indicates that theionic liquid has been immobilized within the polymer. In thespectrum of leached membranes, there is a decrease in intensityas some of the excess ionic liquid, which is not bound to thepolymer, is washed out. The remaining ionic liquid in themembrane, after leaching, is bound to the polymer throughelectrostatic interactions. Figure 9B shows the spectra of the ionicliquid, unmodified Nafion, and Nafion/IL leached samples.There is one main absorption band noteworthy here to revealinteractions in leached samples between the ionic liquid andpolymer. The absorption band at 1055 cm-1 in the unmodifiedNafion spectrum is related to symmetrical stretching of sulfonategroups. This band is present in the spectrum of the neat ionicliquid but appears at 1057 cm-1 and also corresponds to stretch-ing of sulfonate groups from the anion of the ionic liquid. In thecase of the Nafion/IL leached spectrum, this band has shifted to1048 cm-1. Gomes Lage et al.30 have investigated the interactionsof Nafion membranes substituted with different counterions.They reported that the stretching band of the sulfonate groupshifted under the influence of the cation and shifted to lowerwavenumbers as the radius of the cation increased. The shift ofthe band related to sulfonate groups occurs in the case of theNafion/IL leached membrane at lower wavenumbers due toreplacement of protons with the cation of the ionic liquid. Thecation of the ionic liquid is much larger in size in comparisonto a proton, and hence, we observe a shift in absorption bandof the sulfonate groups. The peak at 1696 cm-1 is related to thepresence of water in the unmodified Nafion membrane. Thispeak is not observed in the spectrum of the ionic liquid SLMsamples due to its hydrophobic nature. This indicates that theincorporation of ionic liquid into the membrane has restrictedaccess of water present within the ionic domains of the polymermembrane due to ion exchange between the cation of the ionicliquid and protons in the ionic domains. The shift of the banddue to sulfonate groups supports the hypothesis of interactionsbetween the polymer and ionic liquid.31,32

Structure Analysis by Small-Angle X-ray Scattering.

SAXS has been done in order to examine whether any structuralchange has occurred due to leaching that may influence con-ductivity. Figure 10 shows a representative SAXS profile forunmodified Hyflon, Hyflon/IL, and Hyflon/IL leached samples.The SAXS profile of the unmodified Hyflon membrane clearlyshows the ionomer peak centered around q ∼ 0.2 A�-1, a broadpeak due to the crystalline region at around 0.03 A�-1, andan upturn in intensity at q < 0.02 A�-1. The ionomer peak atq= 0.2 A�-1 in unmodified Hyflon corresponds to a d spacing ofapproximately 31.4 A�. The SAXS spectrum of Nafion (notshown) shows an ionomer peak around 0.22 A-1 which resultsin a d spacing of 28.6 A. Since Nafion has a longer side chain, itforms larger ionic domains than Hyflon does and results in asmaller intercluster spacing as each ionic domain is in closerproximity to the next one due to their larger size. This result

Table 3. IEC, EW, Swelling, and λ Values for Unmodified and

Modified Membranes

sampleIEC

meq g-1)EW

(g SO3H-1) swelling (%)

λ (mmol ILmmol SO3H

-1)

Nafion 0.94 1100 17.25Hyflon 1.22 890 21Nafion/IL10 0.31 0.13 0.26Nafion/IL17 0.06 0 0.45Hyflon/IL7 0.36 2.25 0.16Hyflon/IL17 0.14 0 0.38

Figure 7. Proposed mechanism of cation exchange process be-tween IL and sulfonic acid exchange sites.

(24) Caporali, S.; Bardi, U.; Lavacchi, A. J. Electron Spectrosc. Relat. Phenom.2006, 151(1), 4–8.(25) Kolbeck, C.; Killian,M.;Maier, F.; Paape, N.;Wasserscheid, P.; Steinruck,

H.-P. Langmuir 2008, 24(17), 9500–9507.(26) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers The

Scienta ESCA3000 Database; John Wiley and Sons: 1992.(27) Yen, C.-Y.; Lee, C.-H.; Lin, Y.-F.; Lin, H.-L.; Hsiao, Y.-H.; Liao, S.-H.;

Chuang, C.-Y.; Ma, C.-C. M. J. Power Sources 2007, 173(1), 36–44.(28) Lin, Y.-F.; Yen, C.-Y.; Ma, C.-C. M.; Liao, S.-H.; Hung, C.-H.; Hsiao, Y.-

H. J. Power Sources 2007, 165(2), 692–700.(29) Shelat, K. J.; Dutta, N.K.; Choudhury, N. R.Langmuir 2008, 24(10), 5464–

5473.

(30) Gomes Lage, L.; Gomes Delgado, P.; Kawano, Y. Eur. Polym. J. 2004, 40(7), 1309–1316.

(31) Schaefer, T.; Di Paolo, R. E.; Franco, R.; Crespo, J. G. Chem. Commun.2005, 20, 2594–2596.

(32) Luczak, J.; Joskowska, M.; Hupka, J. Physicochem. Probl. Miner. Process.2008, 42, 223–236.

DOI: 10.1021/la901330y 9247Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

has also been recently observed by Kreuer et al. who comparedshort side chain and long side chain perfluorosulfonic acidionomers with different IECs through SAXS.33 The broad peakat 0.03 A-1 is associated with the crystallites in the fluorocarbonmatrix and corresponds to a d spacing of 209.4 A, which is themean distance between crystalline regions. This broad peak

related to crystalline regions has shifted to 0.053 A-1 in the caseof the Hyflon/IL membrane and indicates that crystallitesare now closer to each other due to the incorporation of IL.The distance between crystallites has been reduced from 209.4 Ain unmodified Hyflon to 118.6 A in the Hyflon/IL membrane.The IL is likely to be incorporated at least partially into thefluorinated matrix due to the fluorinated anion of the IL whichhas more compatibility with the matrix. This incorporation of ILresults in partial homogenization of the membrane, and due to

Figure 8. (A) Survey spectrum of unmodified Hyflon membrane, (B,C) high resolution spectra of O 1s, and (D,E) S 2p of unmodified andleached Hyflon membranes.

(33) Kreuer, K. D.; Schuster, M.; Obliers, B.; Diat, O.; Traub, U.; Fuchs, A.;Klock, U.; Paddison, S. J.; Maier, J. J. Power Sources 2008, 178(2), 499–509.

9248 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251

Article Mistry et al.

this effect the ionomer peak is not clearly seen in this sample. Ascrystallites become closer together, the density increases and isseen by an increase in the intensity of the crystalline region. Thisobservation indicates that the IL interacts with both ionicdomains and fluorocarbon matrix of Hyflon, where the anion isdirected toward the hydrophobic matrix and the cation ionexchanges with sulfonic acid sites within the ionic clusters. Afterleaching IL membranes, excess IL is washed out and there is asmall shift in the crystalline peak from 0.053 to 0.048 A-1. Thisobservation suggests that the majority of the leached IL is fromionic domains which are not bound through electrostatic inter-actions with sulfonic acid groups. Only a small amount of IL isremoved from the crystalline regions of Hyflon, as water cannoteasily penetrate into the hydrophobic matrix. As IL is removedfrom the fluorocarbon matrix, the distance between crystallineregions becomes larger from 118.6 A in Hyflon/IL to 130.9 A in

Hyflon/IL leached membranes. This results in a reduced densityof crystallites and is observed by a decrease in intensity in leachedsamples. Due to excess IL being removed, the membrane is nolonger homogenized and the ionomer peak ismore pronounced inleached membranes. It is observed that the ionomer peak hasshifted from 0.2 A-1 in unmodified Hyflon to 0.24 A-1 inHyflon/IL membrane. This corresponds to a decrease in themean intercluster distance from 31.4 to 26.2 A and is due to thecation of IL being complexed with sulfonate groups. The cationof the IL is considerably larger and bulkier in comparison toprotons and explains the decrease in the intercluster distance ofionic domains in samples impregnated with IL. A closer look atthe position of the ionomer peak in Hyflon/IL membrane incomparison to Hyflon/IL leached membrane reveals that there isno shift in the ionomer peak. This observation indicates that theIL is strongly bound to sulfonic acid groups through electrostatic

Figure 9. Infrared spectra of (A) Nafion unleached and leached samples and (B) ionic liquid, Nafion, and Nafion/IL leached.

DOI: 10.1021/la901330y 9249Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

interactions and is not removed through leaching, which indicatesthat IL is complexedwith sulfonic acid sites and proves their long-term stability.Proton Conductivity. The proton conductivities of unmodi-

fied and modified membranes in anhydrous conditions areplotted as a function of temperature and displayed in Figure 11.The conductivity of unmodified membranes decreases withincreasing experimental temperature as the mobile water phasenecessary for proton conduction in these systems is evaporated,leading to a collapse of the percolated cluster network structure.In our previous work,34 we have reported the conductivity ofNafion 117 to be 99 mS cm-1 at 100 �C and 90% RH whichdrops to 8 mS cm-1 at 100 �C and 30% RH. The conductivityof unmodified Nafion decreases from about 1.07 mS cm-1 at60 �C to 0.018mS cm-1 at 140 �C in anhydrous conditions. Theseresults clearly show the hydration dependence of Nafion toprovide high proton conductivity. Unmodified Hyflon withshort side chains shows a similar trend to Nafion with aconductivity of 0.07mS cm-1 at 60 �C that decreases to0.011 mS cm-1 at 100 �C. After 100 �C with no humidity, theHyflon membrane became resistive and showed no conductivity.In the case of membranes impregnated with IL, there is amonotonous increase in conductivity with temperature up to160 �C.Nafion/IL17membrane shows the highest conductivity of3.6 mS cm-1 at 160 �C, while at the same temperature Hyflon/IL17membranehas a conductivity of 1.8mS cm-1. The differencein conductivity values can be attributed to the amount ofabsorbed IL per sulfonic acid group in each system. The higherconductivity of Nafion supported liquid membranes over corre-sponding Hyflon membranes is also related to the length of theside chain. A longer side chain allows more segmental mobility ofsulfonic acid groups and results in more sulfonic acid groupsaggregating together to form larger ionic domains. The short sidechain of Hyflon reduces the segmental mobility of sulfonic acidgroups, and therefore, less acidic groups aggregate and formsmaller ionic domains in comparison toNafion. Holdcroft et al.35

have recently investigated the effect of side chain length in blockcopolymers and also reported similar observations. Nafion/IL17membrane has 0.45 mmol of IL per SO3H group in comparison toHyflon/IL17, which only has 0.38 mmol of IL per SO3H group.

FromTGA results, all ILmembranes have shown excellent thermalstability up to 350 �C; and conductivity results show its continuedincrease with temperature. Figure 10 also shows the conductivity ofleached samples for both Nafion and Hyflon. The conductivity ofleached membranes is relatively lower than that of membraneswhich have not been leached; however, it remains considerablyhigher than that of unmodified membranes. For Nafion membraneimpregnated with ionic liquid, the conductivity drops from3.6 mS cm-1 at 160 �C to 1.0 mS cm-1 after 10% of the initialionic liquid content was leached. The conductivity of Hyflon/IL17membrane decreases from 1.77mS cm-1 at 160 �C to 0.75mS cm-1

on leaching, and only 8.1% ionic liquid remains within the mem-brane. We assume proton transport in unleached samples occursthrough a combined diffusion and hopping mechanism with excessIL contributing to diffusion of protons through the membrane. Inleached membranes, proton transport is predominately through ahopping mechanism, which is consistent with proton conductivityresults as leached samples show a reduced conductivity. Duringleaching, excess IL which is not bound through interfacial interac-tions to the polymer and contributes to conductivity is washed out.This results in a decreased conductivity of the membrane, as there isa reduced amount of IL to facilitate diffusion of protons. Eventhough excess IL has been leached out, there is still sufficient ILbound to the polymer to promote charge transfer which increaseswith temperature. It has been reported that interaction betweenpolymer and solvent becomes more favorable with increasingtemperature if evaporation of the solvent is suppressed and occursdue to capillary condensation.36 To compare and contrast theSLM conductivity, activation energy was estimated using theArrhenius equation. A plot of lnσ against inverse temperature (T)

Figure 10. SAXS spectra of unmodified Hyflon, Hyflon/IL, andHyflon/IL leached.

Figure 11. Anhydrous proton conductivity of unmodified andmodified membranes up to 160 �C.

(34) Mistry, M. K.; Choudhury, N. R.; Dutta, N. K.; Knott, R.; Shi, Z.;Holdcroft, S. Chem. Mater. 2008, 20(21), 6857–6870.(35) Rubatat, L.; Shi, Z.; Diat, O.; Holdcroft, S.; Frisken, B. J.Macromolecules

2006, 39(2), 720–730.(36) Park, M. J.; Downing, K. H.; Jackson, A.; Gomez, E. D.; Minor, A. M.;

Cookson, D.; Weber, A. Z.; Balsara, N. P. Nano Lett. 2007, 7(11), 3547–3552.

9250 DOI: 10.1021/la901330y Langmuir 2009, 25(16), 9240–9251

Article Mistry et al.

enables the energy of activation to be calculated from the slope ofthe plot. The increase in conductivity of the IL impregnatedmembranes closely follows the Arrhenius law as shown inFigure 12, and a plot of σ versus 1/T exhibits linear plots. It alsoappears that the conductivity of theHyflon/ILmembrane is moretemperature sensitive than the Nafion/IL membrane, as a largervariation in conductivity is observedwith increase in temperature.The activation energies of Hyflon/IL and Nafion/IL membraneswere calculated to be 24.9 and 19.1 kJ mol-1, respectively,and corroborate well with reported literature values of Nafionand other IL based systems.3

High Temperature Stability. It is well-known that Nafion inits acid form starts to turn brown in color upon heat treat-ment of the membrane at temperatures above 100 �C. However,Nafion in the sodium form is much more thermally stable incomparison to the acid form and shows no discoloration whenexposed to high temperatures.19 It has been reported that thethermal stability ofNafion increases with the atomic radius of thecation, which can be explained by reduction of water content anda stronger interaction between sulfonate groups and the cationswith higher ionic radius.30 The onset temperature of the thermaldegradation of Nafion in the sodiated form is about ∼150 �Chigher than that observed for Nafion in the acid form, which isdue to the larger atomic radius of sodium over a proton. Duringproton conductivity measurements, it was observed that unmo-dified Nafion turned brown after exposure at 160 �C. Therefore,IEC measurement was performed on the unmodified samplesafter the proton conductivity test at 160 �C, in which membranesturned brown color. Interestingly, the IEC (0.92 meq g-1) did notchange significantly. Furthermore, brown membranes werecleaned according to the standard cleaning procedure for Nafionby boiling in hydrogen peroxide for 1 h and then in 1 M sulfuricacid for another hour and finally inMilli Qwater for 1more hour.It was observed that the brown color of the membrane disap-peared after cleaning and the membrane returned to a clear ortransparent appearance. These results confirm that the browncolor of the membrane after heat treatment is not due todesulfonation. Earlier work37 also reports that Nafion turnsbrown after heat treatment due to the fact that reactive organicgas components within the surrounding air may undergo chemi-cal changes when exposed to the superacid catalytic activity ofNafion, which may combine to form larger compounds that areliquid or solid in nature. Over time, these organic residues willbuild up a deposit on the Nafion, causing the original color togradually change from translucent to yellow, then brown, and

then even black. These chemical reactions in the air surroundingNafion membranes are stimulated by exposure to light and toelevated temperature.37 However, the Nafion membrane impreg-nated with ionic liquid remained transparent after conductivitymeasurements at 160 �C, which can be seen in Figure 13. Thisobservation can be explained in terms of protons of sulfonic acidgroups being partially replaced by the bulky cation of the ionicliquid, which reduces the superacid catalytic activity of Nafionsince it is no longer in the acid form.ModifiedNafionmembranesshow no discoloration after heat treatment, as adventitiouscompounds in the atmosphere can no longer be acid-catalyzedby modified membranes. These results confirm that the ionexchange process between the protons of sulfonic acid groupsand the cation of the ionic liquid has occurred, which prevents thesuperacid catalytic activity of Nafion. The cation of the ionicliquid is very large and bulky in comparison to protons and hasseemed to stabilize theC-S bond. TGA results also show that thedesulfonation process has been shifted to higher temperatures inthe case of membranes impregnated with ionic liquid. Discolora-tion of unmodified Nafion was more apparent after DMA at200 �C, which is also shown in Figure 13. In the case of the ionicliquid membrane, there was no discoloration even after 200 �C,which further confirms that Nafion is not in its acid form afterimpregnation of ionic liquid within the membrane. ModifiedHyflon membranes also showed similar results as observed inNafion IL membranes.

Conclusion

In summary, we have prepared novel supported liquid

membranes using Nafion and Hyflon membranes, which have

been impregnated with 1-butyl-3-methylimidazolium bis-

(trifluoromethylsulfonyl)imide (BMI-BTSI). The thermal sta-

bility of supported liquid membranes has improved signifi-

cantly in comparison to unmodified membranes and is more

flexible due to plasticization effects of the ionic liquid. Results

from PA-FTIR, IEC, XPS, SAXS, and DSC measurements

indicate that the ionic liquid specifically interacts with sulfonic

acid sites of the polymer through an ion exchange process. The

conductivity of all IL impregnated membranes increases with

increasing temperature up to 160 �C in anhydrous conditions.

Proton conductivity values of 3.58 mS cm-1 at 160 �C have

been obtained for Nafion/IL membranes, which is more than 2

orders in magnitude higher than those for unmodified Nafion.

Unmodified membranes turn brown after exposure to tem-

peratures above 160 �C, whereas impregnated membranes

remain transparent, also confirming that all the SLMs exhibit

high thermal and long-term stability.From this study, it can be concluded that Hyflon/IL was the

most promising system. In particular, Hyflon impregnated

Figure 12. Arrhenius plot of ionic liquidmembranes as a functionof temperature.

Figure 13. Pictures of unmodified and modified Nafion mem-branes showing discoloration of unmodified Nafion after conduc-tivity test at 160 �C and DMA after 200 �C.

(37) Grot, W. Experimental Methods. In Fluorinated Ionomers; WilliamAndrew Publishing: Norwich, NY, 2008; pp 167-189.

DOI: 10.1021/la901330y 9251Langmuir 2009, 25(16), 9240–9251

Mistry et al. Article

with 17% ionic liquid showed the best results in terms ofthermal stability, proton conductivity, and amount of ILretained. The short side chain of Hyflon reduces the protonconductivity due to formation of smaller ionic domains incomparison to the long side chain of Nafion membranes butincreases Tg. Due to Hyflon’s higher Tg, it is more thermallystable than Nafion, which allows higher operational tem-peratures without degradation. Hyflon/IL membrane showsan exponential trend in conductivity with increasing tempera-ture as well as excellent thermal stability, making it more

suitable for high temperature electrochemical application inanhydrous conditions.

Acknowledgment. The authors gratefully acknowledge thefinancial support of the Australian Research Council through aDiscovery Grant to carry out this work and to the AustralianInstitute of Nuclear Science and Engineering (AINSE) for fund-ing the SAXS work through AINSE award. Thanks are also dueto Dr. R. Knott, ANSTO for his help in SAXS experiment andSolvay Solexis for providing Hyflon samples.