Synthesis and Characterization of Multiblock Sulfonated Poly(arylenethioethersulfone) Copolymers for...

Synthesis and Characterization of Multiblock Partially Fluorinated

Hydrophobic Poly(arylene Ether Sulfone)-Hydrophilic Disulfonated

Poly(arylene Ether Sulfone) Copolymers for Proton Exchange Membranes

Yu Chen, Jarrett R. Rowlett, Chang Hyun Lee, Ozma R. Lane, Desmond J. VanHouten,

Mingqiang Zhang, Robert B. Moore, James E. McGrath

Department of Chemistry and Macromolecules and Interfaces Institute, Macromolecular Science and Engineering Program,

Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

Correspondence to: J. E. McGrath (E-mail: [email protected])

Received 1 November 2012; accepted 31 January 2013; published online

DOI: 10.1002/pola.26618

ABSTRACT: Hydrophobic-hydrophilic sequence multiblock

copolymers, based on alternating segments of phenoxide ter-

minated fully disulfonated poly(arylene ether sulfone) (BPS100)

and fluorine-terminated poly(arylene ether sulfone) (6FBPS0)

were synthesized and evaluated for application as proton

exchange membranes. By utilizing mild reaction conditions the

ether–ether interchange reactions were minimized, preventing

the randomization of the multiblock copolymers. Tough, duc-

tile, transparent membranes were solution cast from the block

copolymers and were characterized with regard to intrinsic vis-

cosity, morphology, water uptake, and proton conductivity.

The conductivity values of the 6FBPS0-BPSH100 membranes

were compared to Nafion 212 and a partially fluorinated sulfo-

nated poly(arylene ether sulfone) random copolymer

(6F40BP60). The nanophase separated morphology was con-

firmed by transmission electron microscopy and small angle

X-ray scattering, and enhanced proton conductivity at reduced

relative humidity was observed with longer block lengths.

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000: 000–000, 2013

KEYWORDS: hydrophilic-hydrophobic copolymer; membranes;

multiblock copolymer; nano-phase separated; partially fluori-

nated; phase separation; poly(ether sulfones); proton exchange

membrane

INTRODUCTION Polymer electrolyte membrane fuel cells(PEMFCs) have attracted substantial attention as potentialpower sources for automobiles, portable, and stationary appli-cations due to their high power density, excellent energyconversion efficiency, quiet operation and very low pollutionemission.1 Proton exchange membranes (PEMs) in PEMFCsplay a critical role in transporting hydrated protons and pro-vide a barrier to fuel crossover from the anode to the cath-ode.2,3 A successful PEM material must meet multiple stringentrequirements, such as high proton conductivity, good mechani-cal strength, high oxidative and hydrolytic stability, low fuel andoxidant permeability, ease of fabrication, and excellent watermanagement under low relative humidity (RH) cycling.4,5 Per-fluorosulfonic acid polymers, such as NafionV

R

, are the state-of-the-art PEM materials commercially available due to their goodchemical stability, excellent mechanical properties and high pro-ton conductivity.6 However, they have several drawbacks, suchas decreased conductivity performance at elevated temperature(>80 �C), high methanol permeability and high cost.

Significant effort has been devoted to the development ofsulfonated aromatic statistical copolymers, such as sulfo-

nated poly(arylene ether sulfone)s (BPSHs), poly(ether etherketone)s, poly(arylene ether nitrile)s, polyimides, and poly-benzimidazoles as PEM materials.7–13 Even though theproton conductivity of some sulfonated aromatic statisticalcopolymer membranes was comparable to NafionV

R

underfully hydrated conditions, the values decreased remarkablyat lower RH. An increase in ion exchange capacity (IEC) ofthe copolymers was helpful, but the high IEC copolymerstested were found to swell excessively and lose mechanicalstrength when the percolation limit was reached.14 The pro-ton conductivity performance, water management anddimensional stability of sulfonated aromatic copolymer mem-branes are challenges that must be overcome by carefuldesign of chemical structure and architectural morphology.

Sulfonated aromatic multiblock copolymers exhibiting ananophase separation of lamellar hydrophilic and hydropho-bic phases have been synthesized.15,16 The hydrophilicdomains afford proton transporting channels and signifi-cantly improved the proton conductivity, especially underpartially hydrated conditions.16–19 Significant improvementsin proton conductivity, water management and dimensional

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stability have been reported for the multiblock copolymersystems.16,20–27

Lee et al.22 synthesized and characterized several BPS100-BPS0 non-fluorinated multiblock copolymers, Similarly Yuet al.26 developed a series of BisSF-BPSH fluorinated multi-block copolymers which achieved satisfactory proton con-ductivity in fully and partially hydrated states. Nakabayashiet al.28 reported the synthesis and characterization of severalsulfonated multiblock poly(ether sulfone) copolymers byusing decafluorobiphenyl (DFBP) as a chain extender, butreported some difficulties with maintaining precise controlof the block length and sequence.

In this article, we report the synthesis and characterizationof partially fluorinated hydrophobic poly(arylene ether sul-fone)-hydrophilic sulfonated poly(arylene ether sulfone)(6FBPS0-BPSH100) multiblock copolymers. The 6FBPS0-BPSH100 multiblock copolymers were directly synthesizedbased on fluorine-terminated partially fluorinated hydropho-bic poly(arylene ether sulfone) (6FBPS0) telechelic oligomersand phenoxide-terminated hydrophilic sulfonated poly(ary-lene ether sulfone) telechelic oligomers (BPS100) via nucleo-philic substitution coupling reactions. 13C nuclear magneticresonance (NMR) spectroscopic results confirmed that themild reaction conditions utilized minimized possible ether–ether exchange reactions. The proton conductivity of multi-block 6FBPS0-BPSH100 copolymer membranes has beenproven to outperform that of the analogous random copoly-mer and Nafion 212 both under fully and partially hydratedconditions.

EXPERIMENTAL

Materials4-Fluorophenyl sulfone (DFDPS) was purchased from Aldrichand used as received. 4,40-hexafluoroisopropylidenediphenol(6F-BPA), received from Ciba, sublimed and then recrystal-lized from toluene. Monomer grade 4,40-biphenol (BP) wasprovided by Solvay, and dried under vacuum at 80 �C priorto use. 3,30-Disulfonated-4,40-dichlorodiphenylsulfone(SDCDPS) was synthesized from DCDPS and purified accord-ing to our procedure reported elsewhere.29,30 N-Methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide, cyclohexane,and toluene were purchased from Aldrich and distilled overcalcium hydride before use. Potassium carbonate (K2CO3)was purchased from Aldrich and dried under vacuum at 180�C prior to use. Acetone and methanol were purchased fromFisher Scientific and used without further purification.

Synthesis of the 6FBPS0 Hydrophobic OligomerA typical polymerization procedure for the hydrophobic 10kg/mol 6FBPS0 oligomer was as follows: 6F-BPA (3.500 g,10.41 mmol) was added to a three-neck 100 mL round bot-tom flask equipped with a mechanical stirrer, a condenser, anitrogen inlet and a Dean-Stark trap. DMAc (24 mL) wasadded to the flask and the mixture was stirred to obtain aclear solution. Then K2CO3 (1.655 g, 11.96 mmol) wasadded, followed by toluene (12 mL). The reaction bath wasfirst set to 140 �C for 4 h to dehydrate the system. The bath

temperature was raised to 160 �C to remove the toluene,then cooled to 80 �C and DFBPS (2.797 g, 11.00 mmol) wasadded. The bath temperature was raised to 135 �for 10 h,then cooled to room temperature and then the reaction mix-ture was vacuum filtered to remove the salt. The oligomersolution was precipitated into methanol (1000 mL), stirredovernight and then dried in vacuo at 110 �C for at least 48h. The yield of 6FBPS0 oligomer was 5.27 g (90%).

Synthesis of the BPS100 Hydrophilic OligomerThe synthesis of 10 kg/mol BPS100 was as follows: A three-neck round bottom flask, equipped with a mechanical stirrer,a condenser, a nitrogen inlet and a Dean-Stark trap, wascharged with BP (1.590 g, 8.54 mmol), SDCDPS (4.435 g,9.03 mmol), and DMAc (28 mL). The mixture was stirreduntil dissolved, then K2CO3 (1.36 g, 9.85 mmol) and toluene(14 mL) were added. The reaction bath was heated to 140�C for 4 h to azeotropically remove water. The bath tempera-ture was slowly raised to 180 �C for the controlled removalof toluene and allowed to proceed at 180 �C for 72 h. Theviscous solution mixture was cooled to room temperature,filtered to remove the salt and then coagulated in acetone(1000 mL). The precipitated was stirred overnight in acetoneand then dried in vacuo at 160 �C for 48 h. The yield ofBPS100 oligomer was 4.56 g (85%).

Synthesis of the Multiblock CopolymerA typical synthesis of a 10k–10k 6FBPS0-BPS100 multiblockcopolymer was conducted as follows: BPS100 10k (5.000 g,0.500 mmol), K2CO3 (0.150 g, 1.087 mmol), NMP (44 mL),and cyclohexane (15 mL) were added to a three-necked 100-mL flask equipped with a mechanical stirrer, a condenser, anitrogen inlet and a Dean-Stark trap. The reaction bath washeated to 120 �C for 6 h to remove water from the system.After removing cyclohexane, the reaction bath was cooled to90 �C, and 6FBPS0 10k oligomer (5.000 g, 0.500 mmol) wasadded. The bath temperature was raised to 145 �C and heldat this temperature for 48 h. The viscous reaction mixturewas precipitated into acetone (1000 mL), affording a fibrouscopolymer. The product was stirred in acetone for 12 h, indeionized (DI) water at 90 �C for 12 h, and dried in vacuo at120 �C for 24 h. The yield of 6FBPS0-BPS100 multiblock co-polymer was 9.1 g (91%).

Membranes Casting and AcidificationThe salt form 6FBPS0-BPS100 copolymer was dissolved inDMAc (�7%, w/v) and filtered through a 0.45 lm Teflon sy-ringe filter. The filtered solution was then cast onto a dry,clean glass substrate and dried for 24 h under an infraredlamp at �45 �C. The membranes were treated via two meth-ods. Method 1, the membranes were annealed under vacuumat 230 �C, which is about 25 �C higher than the Tg of6FBPS0 blocks. For method 2, the membranes were dried ina vacuum oven at 120 �C for 24 h. All of the membraneswere in the salt form and were converted to the acid formby boiling in 0.5 M sulfuric acid solution for 2 h, followed byboiling in deionized water for 2 h.

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Characterization1H NMR and 19F NMR spectroscopic analyses were con-ducted on a Varian INOVA spectrometer operating at 400MHz. The spectra of BPS100 oligomers and 6FBPS0-BPS100multiblock copolymers were obtained in DMSO-d6 solution atroom temperature. The spectra of the 6FBPS0 hydrophobicoligomers were obtained from CDCl3 solution. 13C NMR anal-yses were conducted on a Varian Unity spectrometer withDMSO-d6 as solvent.

X-ray scattering (SAXS) experiments were performed using aRigaku S-Max 3000 3 pinhole SAXS system, equipped with arotating anode emitting X-ray with a wavelength of 0.154 nm(Cu Ka). The sample-to-detector distance was 1603 m, andq-range was calibrated using a silver behenate standard. Two-dimensional (2D) SAXS patterns were obtained using a fullyintegrated 2D multiwire, proportional counting, gas-filled de-tector, with an exposure time of 1 h. SAXS data was correctedfor sample thickness, sample transmission and backgroundscattering. All SAXS data were analyzed using the SAXSGUIsoftware package to obtain radically integrated SAXS intensityversus scattering vector q, where q ¼ (4p/k)sin(y/2), y is thescattering angle and k is the wavelength of X-ray.

The bulk morphologies of the membranes were character-ized by transmission electron microscopy (TEM). Electrondensity contrast between hydrophilic and hydrophobic seg-ments within the membrane samples was enhanced by quan-titatively exchanging the acidic protons on the sulfonic acidmoieties with cesium ions from a CsOH solution. The Csstained membranes were then embedded in epoxy and ultra-microtomed into 50–70 nm thin sections with a diamondknife. Transmission electron micrographs were obtainedusing a Philips EM 420 transmission electron microscope(TEM) operating at an accelerating voltage of 96 kV.

Glass transition temperatures (Tgs) of 6FBPS0-BPS100 mem-branes (salt form) were determined by differential scanningcalorimetry (DSC) with a TA Instruments DSC Q-1000 at aheating rate of 10 �C/min under nitrogen. Prior to DSC char-acterization, all the samples were thermally treated withMethod 1.

Intrinsic viscosity (IV) of BPS100 oligomers and 6FBPS0-BPS100 multiblock copolymers were obtained from a sizeexclusion chromatography (SEC) equipped with a Waters1515 isocratic HPLC pump, a Waters autosampler, a WatersHR5–HR4–HR3 column set, a Waters 2414 refractive-indexdetector, and a Viscotek 270 viscometric detector. NMP (con-taining 0.05 M LiBr) at 60 �C was used as the mobile phase.Intrinsic viscosities of 6FBPS0 oligomers were obtained froman Alliance Waters 2690 Separations Module with a ViscotekT60A dual viscosity detector and laser refractometerequipped with a Waters HR 0.5 þ HR 2 þ HR 3 þ HR fourstyragel column set. Chloroform was used as the mobilephase at 30 �C

Uniaxial load tests were performed using an Instron 5500Runiversal testing machine equipped with a 200-lb load cellat room temperature and 40–50% RH. The crosshead

displacement speed was 5 mm/min and gauge lengths wereset to 25 mm. A dogbone die was used to punch specimens50-mm long with a minimum width of 4 mm. Film speci-mens were prepared by Method 1 and then equilibrated at44% RH and 30 �C for 48 h prior to testing. All specimenswere mounted in manually tightened grips. Tensile modulusfor each specimen was calculated based on the stress andelongation values for the specimen at the first data point ator above 2% elongation. All the membranes were tested inthe salt form.Measurement of Proton ConductivityThe ohmic resistance (R, X) of the PEM membrane wasmeasured using a custom made test cell via four-point probealternating current (a.c.) impedance spectroscopy in the fre-quency range from 3 MHz to 10 Hz under two different con-ditions [liquid water at 30 �C and RH from 30 to 95% at 80�C].31 The resistance value of each membrane was obtainedby averaging at least five measurements at each temperatureand humidity. The wiring system for measuring the ohmicresistance was shielded from the platinum electrodes to thepotentiometer (a combined system of an electrochemicalinterface, Solartron 1287, and an impedance/gain-phase ana-lyzer, Solartron 1252A) to avoid electromagnetic noise. Theproton conductivity (r, S/cm) was obtained from the follow-ing equation (eq 1):

r ¼ LR � S (1)

where r (S/cm) is proton conductivity, L (cm) is the distancebetween the two electrodes, R (X) is the resistance of themembrane, and S (cm2) is the proton transport surface areatested. For measurements of proton conductivity in liquidwater, each membrane was fully hydrated by soaking in DIwater at room temperature for 24 h. For determining protonconductivity under partially hydrated conditions, membraneswere equilibrated in a humidity-temperature controlled oven(ESPEC, SH-240) at the desired RH level and 80 �C for atleast 40 min before each measurement.

Water Uptake and Swelling RatiosThe water uptake of all membranes was determined gravi-metrically. First, the membranes were equilibrated in DIwater at room temperature for 2 days after acidification. Wetmembranes were removed from the DI water, blotted dry toremove surface droplets, and quickly weighed. The mem-branes were then dried at 120 �C under vacuum for 24 hand weighed again. The water uptake of the membranes wascalculated according to the following equation (eq 2), whereWdry and Wwet refer to the mass of the dry and wet mem-brane, respectively.

Water Uptake ð%Þ ¼ Wwet �Wdry

Wdry� 100 (2)

The volume swelling ratios of the membranes were deter-mined from the dimensional changes from wet to dry state.Membranes were equilibrated in DI water, and dimensions inthe wet state were measured using a micrometer. The dried



dimensions were obtained after drying the wet membrane at120 �C in a vacuum oven for 24 h.

RESULTS AND DISCUSSION

Synthesis and Characterization of Oligomers andMultiblock CopolymersA of Scheme 1 shows the synthesis of fluorine terminated6FBPS0 hydrophobic oligomers via copolymerization of 6F-BPA and an excess of DFDPS. The molecular weight and endgroup functionality of the oligomers were precisely controlledby off-setting the molar feed ratios of monomers according tothe Carothers equation. The molar feed ratio of DFDPS to 6F-BPA was greater than 1 to provide fluorine telechelic function-ality of the oligomers and provide target number-average mo-lecular weights ranging from 4 to 17 kg/mol.

Figure 1 shows the 19F NMR spectrum of a 6FBPS0-10koligomer. The peak at �63.5 ppm (integration value 110.2)was attributed to the fluorine in the chain. The peak at�104.9 ppm (integration value 2.0) was attributed to theend-group aromatic fluorine atoms of the 6FBPS0 oligomer.The two kinds of fluorine atoms in the 6FBPS0 oligomershave a mole ratio of 6n/2, where n is the number of repeatunits. Using this equation and the 19F NMR integrationresults, the number of repeat units was calculated, multipliedby the repeat unit molecular weight, and with the additionof the molecular weight of the endgroups the Mn of 6FBPS0oligomers was obtained.

Phenoxide terminated disulfonated poly(arylene ether sul-fone) (BPS100) hydrophilic oligomers were synthesized bycopolymerization of SDCDPS and excess BP monomers, asshown in B of Scheme 1.

The Mn of the hydrophilic oligomers was determined by 1HNMR. Figure 2 shows the spectrum of BPS100, where pro-tons from the terminal BP moieties were assigned to foursmall peaks at 6.80, 7.05, 7.40, and 7.55 ppm, while thepeaks at 7.10 and 7.65 were assigned to the protons of theBP moieties in the BPS100 oligomer backbone. By comparingthe integrations of the protons on the end-group BP (peak a)and the protons on the SDCDPS moieties (peak d), the Mn ofeach BPS100 oligomer was determined.

The molecular weights measured from NMR, and IV obtainedfrom SEC of 6FBPS0 and BPS100 oligomers are summarizedin Table 1. Linear relationship between the log Mn and log IVfor both oligomers were observed, confirming the successfulcontrol of the molecular weights for both hydrophobic andhydrophilic oligomers, as Figure 3 shows.

As shown in C of Scheme 1, a series of 6FBPS0-BPS100 mul-tiblock copolymers were successfully synthesized via nucleo-philic aromatic substitution reactions. The stoichiometry ofthe coupling reaction was based on a 1.1:1 molar ratio ofthe hydrophobic (6FBPS0)/hydrophilic (BPS100) oligomersto control the IEC value at 1.5 mequiv./g. Although the feedratio of the oligomers is generally higher than 1:1, all themultiblock copolymers yielded high intrinsic viscosities andwere cast into mechanically tough membranes. The multi-block copolymers are identified as 6FBPS0-BPS(H)100 Ak-Bk, where A and B represent the block length of 6FBPS0 andBPS(H)100, respectively, and H represents the acid form ofthe copolymer.

The progress of the copolymer synthesis was monitored by1H NMR. After 48 h of reaction time the 1H NMR spectrumconfirmed the disappearance of the aromatic peaks due to

SCHEME 1 Synthesis of (A) 6FBPS0 Hydrophobic Oligomer, (B) BPS100 Hydrophilic Oligomer, and (C) 6BPS0-BPS100 Multiblock

Copolymer.


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the phenoxide end group of the hydrophilic oligomer, asshown in Figure 4, suggesting relatively high molecularweight was achieved.

During the synthesis of the 6FBPS0-BPS100 multiblockcopolymers, the possible nucleophilic ether–ether chaininterchange side reaction is a concern, as it would result in arandomized chain structure rather than the precise desiredmultiblock structure. A comparison of the 13C NMR spectraof the 6F60BP40 random copolymer (bottom) and 6FBPS0-BPS100 10k-10k multiblock copolymer (top) is shown in Fig-ure 5. The spectrum of 6F60BP40 shows multiplet peaks,consistent with a random sequence of each moiety on themain chain. In contrast, the spectrum of 6FBPS0-BPS10010k-10k shows only narrow, sharp, singlet peaks, confirmingthe presence of long-range ordered hydrophobic and hydro-philic segments and the minimization of the ether–etherchain interchange reactions.

Thermal Treatment of the 6FBPS0-BPSH100 MembranesWater management is one of the most important critical prop-erties for a PEM. Lee et al.22,32 reported that the water uptakeof the membranes increased with the block length of the mul-tiblock copolymers. High water uptake and swelling ratios oflonger block length multiblock copolymers reduced the me-chanical strength of the membranes and the durability ofmembrane electrode assemblies (MEAs). To overcome thisdrawback, we adopted new thermal treatments of the salt-form membranes in order to enhance the self-assembly mor-phology development (nanophase separation) during film for-mation. It was hypothesized this would reduce the watersorption without compromising IEC and high proton conduc-tivity. After acidification, the non-annealed membranesshowed the typical trend of water uptake increasing withincreasing hydrophilic block length. However, as can be seenin Table 2, annealing at 25 �C above the Tg of the 6FBPS0

FIGURE 1 19F NMR of 6FBPS0 oligomer.

FIGURE 2 1H NMR of phenoxide terminated BPS100.



block, water sorption was significantly reduced for all acidi-fied membranes. Furthermore, the water uptake of theannealed samples was independent of copolymer blocklength, which suggests that the annealing treatment is a feasi-ble and efficient approach to reduce the loss of mechanicaland dimensional stabilities for membranes containing verylong hydrophilic blocks under fully hydrated conditions, pos-sibly by enhancing the self-assembly of rubbery and glassynanophases.

DSCThe DSC thermogram of the 6FBPS0-BPS100 12k-12k multiblockcopolymer is shown in Figure 6. The result is from the secondheating cycle with a 10 �C/min heating rate to 350 �C. A sharptransition at 205 �C was assigned to the Tg of 6FBPS0 blocks.Another broad transition, over a temperature range of 250–300�C, is related to the relaxation of the ionic BPS100 blocks. Theobservation of the Tg for hydrophobic block segments indicatesthe existence of a nanophase separated morphology.

SAXSFigure 7 shows the SAXS profiles of the 6FBPS0-BPS100 multi-block copolymers with various block lengths cast from DMAc

and annealed at 230 �C. Scattering maxima were observed inthe profiles of 6FBPS0-BPS100 multiblock copolymers, whilethe profile of the 6F40BP60 random copolymer was feature-less. The observed scattering maxima in the SAXS profiles of6FBPS0-BPS100 multiblock copolymers were attributed to thenanophase separation between hydrophobic and hydrophilicdomains. The scattered intensity of 6FBPS0-BPS100 copoly-mers was enhanced with an increase in block length, suggest-ing the longer hydrophilic block facilitated an enhanced mor-phology development. The SAXS profiles of 6FBPS0-BPS100multiblock copolymers also confirmed the presence of the or-dered periodic microstructures and the dependence of orderedperiodic microstructures on the block length. The morphologyof the long-range ordered periodic microdomain structure in ablock copolymer can be determined from the relative positionof q values of higher-order peaks to the first-order peak. For alamellar structure, the SAXS profile shows a series peaks withthe q value at qmax, 2qmax 3qmax where qmax is the scatteringvector of the main scattering maximum.33 Two peaks wereclearly observed in each SAXS profile of 6FBPS0-BPS100 10k-10k and 15k-15k. The first order peak was at qmax and the sec-ond order one was at 2qmax, which suggests the presence of anordered lamellar structure. The lack of the second order peakin the SAXS profile of 6FBPS0-BPS100 5k-5k indicates that theshorter block lengths do not form a well-ordered lamellarstructure. The SAXS profiles of 6FBPS0-BPS100 multiblockcopolymers confirmed that enhanced phase separation yieldedmore distinct and ordered ionic domains. It was observed thatthe main scattering peak systemically shifted to the lower qvalues as the block length increased, indicating the interdo-main distance increased with the longer block lengths.

TEMOrdered nanophase separation and the dependence of thenanophase separation on the block length were observed in thehydrophobic–hydrophilic multiblock copolymers.21,34 Thenanophase separated morphology provides co-continuoushydrophilic channels across the membrane. As the block lengthof 6FBPS0-BPS100 multiblock copolymers increased, the co-continuous hydrophilic domains increased and facilitated bet-ter proton transport. Figure 8 shows TEM images of the6FBPS0-BPS100 multiblock copolymer membranes with differ-ent block lengths. The bright and dark regions in the imagescorresponded to non-stained hydrophobic domains and ce-sium-stained ion electron-rich hydrophilic domains, respec-tively. The nanophase separation between hydrophilic andhydrophobic domains was observed in the 6FBPS0-BPSH multi-block copolymer membranes. Furthermore, as the block lengthincreased, an enhanced degree of phase separation and a moreevident lamellar structure was observed, which supports theconclusion drawn from the SAXS profiles shown in Figure 7.

Tensile TestingThe tensile properties of 6FBPS0-BPS100 multiblock copoly-mer membranes were determined, using a BPS32 randomcopolymer as a control. As shown in Figure 9, all the6FBPS0-BPS100 membranes showed tensile strength about50 MPa, which was about 20 MPa lower than that of BPS32random copolymer. This difference may indicate a side effect

TABLE 1 Molecular Weight Characterization of the

Hydrophobic And Hydrophilic Telechelic Oligomers

Target Hydrophobic Blocks Hydrophilic Blocks

Mn (g/mol) Mna (g/mol) IVb (dL/g) Mn

c (g/mol) IVd (dL/g)

6,000 5,800 0.09 6,000 0.15

8,000 7,700 0.10 8,100 0.20

10,000 10,400 0.12 10,300 0.25

12,000 11,700 0.13 12,200 0.27

14,000 14,100 0.14 14,200 0.31

a Measured by 19F NMR.b Measured by GPC with THF as mobile phase at 25 �C.c Measured from 1H NMR.d Measured by GPC with 0.05 M LiBr/NMP as mobile phase at 60 �C.

FIGURE 3 ln g VS. ln Mn plot of 6FBPS0 and BPS100

oligomers.


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of the anisotropy behavior of the 6FBPS0-BPS100 multiblockcopolymer membranes. In addition, all of the 6FBPS0-BPS100 multiblock copolymer membranes showed betterelongation to failure performance than the BPS32 randomcopolymer, which may be explained by the influence of co-continuous 6FBPS0 domains. The increased block length alsoincreased the elongation before failure, which was a markedimprovement over the performance of the BPS32 randomcontrol polymer sample.

Properties of 6FBPS0-BPSH100 Multiblock CopolymerMembranesThe fundamental properties of the 6FBPS0-BPSH100 multi-block copolymer membranes are summarized in Table 3. A6F40BP60 random copolymer and Nafion 212 were alsoincluded as controls. The IV data and the tough ductile mem-brane behavior confirmed high molecular weight 6FBPS0-BPS100 multiblock copolymers were obtained.

Excellent proton conductivity was observed for all 6FBPS0-BPSH100 multiblock copolymer membranes compared toNafion 212 and the random copolymer 6F40BP60H.

Real-situ conditions for a PEMFC frequently include reducedRH, thus PEMs that have high proton conductivity at low RHare desirable. Figure 10 shows proton conductivity at 80 �Cas a function of RH for 6FBPS0-BPSH100 6k-6k, 10k-10kand 15k-15k, where the conductivities of a Nafion 212 mem-brane are also shown for comparison.

Across the entire RH range, 6FBPS0-BPSH100 block copoly-mers outperformed Nafion 212 over 45 min period. The per-formance of the multiblock copolymers under the partiallyhydrated state improved as block length increased. This per-formance improvement is related to the formation of longerco-continuous hydrophilic domains in the copolymer. Protonsare transported along with the water molecules through theenhanced co-continuous hydrophilic channels. It is suggested

FIGURE 5 13C NMR of 6FBPS0-BPS100 10k-10k multiblock copolymer (top) and 6F60BP40 random copolymer (bottom).

FIGURE 4 1H NMR of of 6FBPS0-BPS100 multiblock copolymer.



that enhanced nanophase separated lamellar hydrophilic do-main morphology, as confirmed by SAXS and TEM, loweredthe morphological barrier for proton transport and improvedthe proton transport at low RH.

Swelling-Deswelling Properties of 6FBPS0-BPSH100Multiblock CopolymersThe mechanical strength of a PEM is strongly related to theweight-based water sorption in the fully hydrated state. Thevolume-based water swelling, which directly influences thedimensional changes of a PEM, strongly impacts the durabil-ity of a MEA. When a membrane shows significant in-planeswelling, considerable stress is created over the wet-drycycles which may damage the membrane–electrode interfa-ces and leading to premature failure of the MEA. It has beenfound that the 6FBPS0-BPSH100 multiblock copolymer mem-branes exhibited attractive dimension swelling behaviorwhich may make them promising as alternative PEMs.

The dimensional swelling behaviors of 6FBPS0-BPSH100 mul-tiblock copolymers with 6F40BP60H and Nafion 112 as com-parison are shown in Figure 11. The multiblock copolymersexhibited anisotropic swelling behavior and the 6F40BP60Hand Nafion 112 showed isotropic swelling behavior. In-planeswelling (x- and y-directions) of 6FBPS0-BPSH100 multiblockcopolymer membranes stayed the same or decreased slightly,while through-plane (z-direction) swelling increased withblock length. This behavior is a result of the development of

enhanced, ordered morphology as block length increases. Thelow in-plane swelling of 6FBPS0-BPSH100 multiblock copoly-mer membranes is expected to provide much lower interface

FIGURE 8 TEM images of 6FBPS0-BPSH100 multiblock copoly-

mer membranes (left: 6FBPS0-BPSH100 10K- 10K, right:

6FBPS0-BPSH100 15K-15K), length of scale bar= 100 nm.

TABLE 2 Effect of Annealing on the Water Uptake of 6FBPS0-

BPSH100 Block Copolymers

6FBPS0-BPS100a

Water Uptake (%)

6k-6k 8k-8k 10k-10k 12k-12k 15k-15k

Method 1 48 55 55 58 60

Method 2 44 95 185 250 340

a All membranes are in acid form.

FIGURE 6 DSC thermogram of 6FBPS0-BPS100 12k-12k multi-

block copolymer.

FIGURE 7 SAXS profiles of 6FBPS0-BPS100 multiblock copoly-

mer and 6F40BP60 random copolymer membranes.

FIGURE 9 Room temperature stress-strain curves of 6FBPS0-

BPS100 multiblock copolymers with a BPS32 random copoly-

mer as control.


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stress resulting in improved MEA stability by reducing prema-ture membrane electrode failure.

CONCLUSIONS

A series of 6FBPS0-BPS100 hydrophobic–hydrophilic multi-block copolymers were successfully synthesized based onthe coupling of a fluorine terminated poly(arylene ether sul-fone) (6FBPS0) hydrophobic oligomers and phenoxide termi-nated disulfonated poly(arylene ether sulfone) (BPS100)hydrophilic oligomers. IV of 6FPAEB-BPS100 copolymersshowed that high molecular weight copolymers wereobtained. The regularly sequenced structure of 6FBPS0-BPS100 multiblock copolymers was confirmed by 13C NMR.The possible ether-ether interchange side reactions wereminimized by utilization of relatively mild reaction tempera-tures. DSC thermograms of 6FBPS0-BPS100 multiblockcopolymers confirmed the existence of two thermal transi-tions suggesting the nanophase separation in copolymers.Water uptake was significantly reduced upon annealing attemperature greater than the Tg of the 6FBPS0 block and thewater uptake dependence on block length diminishedentirely. Multiblock copolymers with longer block lengths

showed improved performance with regard to proton con-ductivity. This was presumably due to a more distinct nano-phase separation and better connectivity among the ionicdomains, as evidenced by morphology characterizations fromTEM and SAXS profiles. The anisotropic swelling behavior of6FBPS0-BPSH100 multiblock copolymers should result inmuch lower stress at the interface and better stability of theMEA to prevent membrane electrode failure.

ACKNOWLEDGMENTS

The authors thank Giner Electrochemical Systems, LLC, theDepartment of Energy (DE-EE0000471). The SAXS experi-ments are based upon work supported by the National ScienceFoundation (DMR-0923107).

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TABLE 3 Characterization of 6FBPS0-BPSH100 Block Copolymers with Equal Block Length

6FBPS0-BPSH100 (1.1:1) IV (dL/g)a IEC (mequiv./g)b Water Uptake (wt %) Proton Conductivityc (S/cm)

4k-4k 0.68 1.55 40 0.14

6k-6k 0.82 1.53 48 0.17

8k-8k 0.83 1.50 55 0.17

10k-10k 1.05 1.55 55 0.18

12k-12k 1.17 1.60 58 0.19

15k-15k 1.25 1.56 60 0.20

Nafion 212 N/A 1.00 22 0.12

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a Measured by SEC with 0.05M LiBr/NMP as mobile phase at 60 �C.b Measured from 1H NMR.c Measured in liquid water at 30 �C.

FIGURE 10 Effect of block length on the proton conductivity of

6FBPS0-BPSH 100 multiblock copolymers (measured at 80 �C).

FIGURE 11 Comparison of dimensional swelling data for

6FBPS0-BPSH100 multiblock, copolymers, 6F40BP60H and

Nafion 112.



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