DTIC ADA581390: Synthesis and Structure-Conductivity Relationship of Polystyrene-block-Poly(vinyl benzyl trimethylammonium) for Alkaline Anion Exchange Membrane Fuel Cells

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Synthesis and structure-conductivity relationship of polystyrene- W911NF-10-1-0520
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611103
6. AUTHORS 5d. PROJECT NUMBER
Tsung-Han Tsai, Ashley M. Maes, Melissa A. Vandiver, Craig Versek,
Sönke Seifert, Mark Tuominen, Matthew W. Liberatore, Andrew M. 5e. TASK NUMBER
Herring, E. Bryan Coughlin
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Colorado School of Mines
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14. ABSTRACT
Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS-b-[PVBTMA]
[BF4]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes
for Alkaline Anion Exchange Membrane Fuel Cells
of the block copolymers were prepared by drop casting from dimethylformamide.
block-Poly(vinyl benzyl trimethylammonium)
Initial evaluation of the microphase separation in these PS-b-[PVBTMA][BF4] materials via SAXS revealed the
formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene-b-poly(vinyl
15. SUBJECT TERMS
amphiphilic block copolymers; anion exchangemembrane fuel cell; atom transfer radical polymerization(ATRP); phase separation;
polymeric electrolyte membranes;polystyrene; poly(vinyl benzyl trimethylammonium); structure;conductivity relationship
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 15. NUMBER 19a. NAME OF RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF PAGES Andrew Herring
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Report Title
Synthesis and structure-conductivity relationship of polystyrene-
ABSTRACT
Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS-b-[PVBTMA]
[BF4]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes of
the block copolymers were prepared by drop casting from dimethylformamide.
Initial evaluation of the microphase separation in these PS-b-[PVBTMA][BF4] materials via SAXS revealed the
formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene-b-poly(vinyl benzyl
trimethylammonium hydroxide) (PS-b-[PVBTMA][OH]) were prepared as polymeric alkaline anion exchange
membranes materials by ion exchange from PS-b-[PVBTMA][BF4] with hydroxide in order to investigate the
relationship between morphology and ionic conductivity. Studies of humidity [relative humidity (RH)]-dependent
conductivity at 80 C showed that the conductivity increases with increasing humidity. Moreover, the investigation
of the temperature-dependent conductivity at RH ¼ 50, 70, and
90% showed a significant effect of grain boundaries in the membranes against the formation of continuous
conductive channels, which is an important requirement for achieving high ion conductivity.
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ARO Report Number 58161.18-CH-MUR
Synthesis and structure-conductivity relationship ...
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© 2012 . Published in Journal of Polymer Science Part B: Polymer Physics, Vol. Ed. 0 (2012), (Ed. ). DoD Components reserve
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Approved for public release; distribution is unlimited.
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Synthesis and Structure–Conductivity Relationship of
Polystyrene-block-Poly(vinyl benzyl trimethylammonium)
for Alkaline Anion Exchange Membrane Fuel Cells
Tsung-Han Tsai,1 Ashley M. Maes,2 Melissa A. Vandiver,2 Craig Versek,3 So€nke Seifert,4
Mark Tuominen,3 Matthew W. Liberatore,2 Andrew M. Herring,2 E. Bryan Coughlin1
1DepartmentofPolymerScienceandEngineering,UniversityofMassachusettsAmherst,120GovernorsDrive,
Amherst,Massachusetts01003
2DepartmentofChemicalandBiologicalEngineering,ColoradoSchoolofMines,Golden,Colorado80401
3DepartmentofPhysics,UniversityofMassachusettsAmherst,411HasbrouckLaboratory,Amherst,Massachusetts01003
4X-rayScienceDivision,ArgonneNationalLaboratory,Argonne,Illinois60439
Correspondenceto:E.B.Coughlin(E-mail:[email protected])
Received22June2012;accepted17August2012;publishedonline
DOI:10.1002/polb.23170
ABSTRACT: Blockcopolymersofpolystyrene-b-poly(vinylbenzyl dependent conductivity at 80 (cid:2)C showed that the conductivity
trimethylammonium tetrafluoroborate) (PS-b-[PVBTMA][BF ]) increaseswithincreasinghumidity.Moreover,theinvestigation
4
were synthesized by sequential monomer addition using atom ofthetemperature-dependentconductivityatRH¼50,70,and
transfer radical polymerization. Membranes of the block 90% showed a significant effect of grain boundaries in the
copolymers were prepared by drop casting from dimethylfor- membranes against the formation of continuous conductive
mamide. Initial evaluation of the microphase separation in channels, which is an important requirement for achieving
thesePS-b-[PVBTMA][BF4]materialsviaSAXSrevealedthefor- high ion conductivity. VC 2012 Wiley Periodicals, Inc. J Polym
mation of spherical, cylindrical, and lamellar morphologies. SciPartB:PolymPhys000:000–000,2012
Block copolymers of polystyrene-b-poly(vinyl benzyl trimethy-
lammonium hydroxide) (PS-b-[PVBTMA][OH]) were prepared KEYWORDS: amphiphilic block copolymers; anion exchange
aspolymericalkalineanionexchangemembranesmaterialsby membrane fuel cell; atom transfer radical polymerization
ion exchange from PS-b-[PVBTMA][BF ] with hydroxide in (ATRP); phase separation; polymeric electrolyte membranes;
4
order to investigate the relationship between morphology and polystyrene; poly(vinyl benzyl trimethylammonium); structure;
ionicconductivity.Studiesofhumidity[relativehumidity(RH)]- conductivityrelationship
INTRODUCTION Proton exchange membrane fuel cells Analkalinefuelcell(AFC)usespotassiumhydroxideasaliq-
(PEMFCs), which convert chemical energy to electrical uid electrolyte.5–9 When compared with PEMFCs, AFC con-
energy through redox reactions, have been developed as ducts hydroxide ion from cathode to anode. The oxygen
renewable and portable energy devices because of their high reduction10 and fuel oxidation11,12 are faster in alkaline con-
efficiency, high energy density, and low formation of pollu- dition than in acidic condition, allowing for the use of non-
tants.1,2 Commercially available NafionVR,3 a perfluorosulfonic noble metal catalysts (i.e., nickel13 and silver) and longer
acid ionomer, has been widely investigated as a proton carbon chain alcohol fuels. Moreover, the corrosion of metal
exchange membrane because of its good chemical stability, catalysts by carbon monoxide is reduced under the alkaline
suitable mechanical properties, and high proton conductivity. conditions of cells.14 However, the use of potassium hydrox-
However, because of the high cost of the membranes and ide solution results in leakage problems. The presence of
their need for noble metal (i.e., platinum)-based electrocata- carbondioxideinhydrogenandairwillreactwithpotassium
lysts, the commercialization of PEMFCs is still limited. Addi- hydroxide to form potassium carbonate, which precipitates
tionally, oxygen reduction andfuel(hydrogen oralcohol) oxi- on the electrodes blocking the surface and reducing the effi-
dation are sluggish under the acidic condition of cells, and ciency of the cells. The use of alkaline anion exchange mem-
thenoblemetalcatalystsareeasilypoisonedbycarbonmon- brane (AAEM) with a suitable stable polymeric electrolyte
oxide at low temperature. These are serious obstacles to the can overcome the issues discussed above.14–16 The require-
extensiveadoptionofPEMFCs asenergydevices.4 ments for a AAEM polymeric electrolyte are a robust
VC 2012WileyPeriodicals,Inc.
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polymer backbone as well as an alkaline stable cationic gyroids.34,35 Polymeric conductive membranes made from
charge to facilitate anion transport. An all-carbon polymer block copolymers can provide well-oriented and continuous
backbone will be hydrolytically stable. Benzyl trimethyl am- conductive hydrophilic channels to enhance ion conductivity.
moniumcationshaveproventobestableunderalkalinecon- Because of the presence of the hydrophobic domain in the
ditions because of the presence of steric hindrance and the membranes, the mechanical property of the membranes can
absenceofb-hydrogenspreventingHofmannelimination.17 also be enhanced. Therefore, more ion conductive groups
can be incorporated into the polyelectrolyte leading to
Recently, Varcoe and coworkers18–20 demonstrated radiation-
higher conductivity [higher ion exchange capacity (IEC) > 1.5
grafted polyvinylidene fluoride, poly(ethylene-co-tetrafluoro-
mequiv/g]. In contrast to random copolymers, it is difficult to
ethylene), and fluorinated ethylene propylene-containing
achievehigherIECbecauseoftheswellingencounteredathigh
polymeric benzyl trimethylammonium hydroxide ions for
statesofhydrationleadingtodisintegrationofthemembranes.
AAEMFCs. Additionally, chloromethylated polysulfones quater-
Severalstudiesaboutstructure–morphology–propertyrelation-
nized by treatment with trimethylamine are another class of
ships of block copolymers for PEM have shown that the mor-
AAEMFCs because of their good mechanical, thermal, and
phologyoftheconductivemembranesstronglyinfluencestheir
chemical stability.21–23 Brominated benzylmethyl-containing
proton conductivity on the aspect of type and orientation of
polysulfones for AAEMFCs, as investigated by Yan and Hick-
structure.36–42 Till now, fundamental investigations about the
ner,24 avoid the chloromethylationstep, which isknown tobe
relationship between morphology and conductivity in AAEM
a toxic and carcinogenic process. Cross-linked tetraalkylam-
madefromwell-definedblockcopolymersaresparse.Theblock
monium-functionalized polyethylenes have been synthesized
copolymers poly(arylene ether)s containing ammonium-func-
by ring-opening metathesis copolymerization of tetraalkylam-
tionalizedfluorenegroupsexploredbyWatanabeetal.43show
monium-functionalized cyclooctenes with unfunctionalized
highIECupto1.93mequiv/gandhighconductivity(144mS/
cyclooctenes.25Thesecrosslinkedstructuresprovidegoodme-
cm) at 80 (cid:2)C. The membrane is mechanically and chemically
chanical properties and allow incorporation of higher propor-
stable, similar to the membranes from random copolymers.
tion of ion conductive groups. Other crosslinked copolymers
Therefore, using block copolymers with hydrophilic blocks
based on imidazolium and bis-imidazolium groups exhibited
could improve the ionic conductivityof AEM by incorporating
high hydroxide ion conductivity and good mechanical proper-
moreconductivegroupwithoutlosingmechanicalproperties.
ties.26,27 The incorporation of novel phosphonium cations
with methoxyphenyl group into polysulfones was studied.28 In the current investigations, we have synthesized block
The electron-donating nature of oxygen in the phosphonium copolymers of polystyrene-b-poly(vinyl benzyl trimethy-
moieties stabilizes the cations, leading to good chemical dura- lammonium tetrafluoroborate) (PS-b-[PVBTMA][BF ]) via
4
bility under basic conditions. Guanidinium-functionalized pol- sequential monomer addition using atom transfer radical
y(arylene ether sulfone) shows higher ion conductivity than polymerization (ATRP)without the needtoperform any post-
trimethylammonium-functionalized polysulfone because of the polymerization modification. The membranes of PS-b-
higher basicity of guanidinium cation.29 There is some ques- [PVBTMA][BF ] diblockcopolymers were readily preparedvia
4
tionaboutthelong-termstabilityofguanidiniumcationunder solvent-casting. Polystyrene-b-poly(vinyl benzyl trimethylam-
highly caustic conditions. Recently, base-stable benzimidazo- monium hydroxide) (PS-b-[PVBTMA][OH]) was subsequently
lium polymers designed with steric protection around the C2 prepared by ion exchange with potassium hydroxide to pro-
position have been developed.30 Solvent-processable poly- duce the AAEMFC materials. The morphology of the mem-
fluorene ionomers containing pendant imidazolium moieties branes of PS-b-[PVBTMA][BF ] and PS-b-[PVBTMA][OH] block
4
were investigated. These polyfluorene ionomer membranes copolymers was determined by small-angle X-ray scattering
show long-term stability under basic condition at elevated (SAXS) at different humidity and temperature conditions. The
temperature, and hydroxide conductivity is above 10(cid:3)2 S/cm effects of the morphologies on the ionic conductivity, meas-
at room temperature.31 A random copolymer of poly(methyl uredbyimpedancespectroscopy,werealsoinvestigated.
methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) was
EXPERIMENTAL
prepared and revealed the feasible preparation for the
AAEM.32 Different types of random copolymers with methyl Materials
methacrylate, vinylbenzyl chloride, and ethyl acrylate were Styrene (>99%; Aldrich) was passed through a column of
fabricated as potential AAEMs for direct methanol AFC.33 Till basic alumina. Anhydrous N, N-dimethylformamide (DMF,
now, most of the studies on AAEM have been based on ran- 99.8%; Alfa Aesar), vinyl benzyl trimethylammonium chlo-
domcopolymerscontainingcationconductivemoieties. ride ([VBTMA][Cl], 99%; Aldrich), sodium tetrafluoroborate
(NaBF , 97%; Alfa Aesar), copper(I) bromide (CuBr,
The use of well-defined block copolymers with polycation as 4
99.999%; Aldrich), (1-bromoethyl)benzene (97%; Alfa
ionic conductive pathway can benefit the investigation of the
Aesar), and 1,1,4,7,10,10-hexamethyltriethylenetetramine
relationship between structure and ionic conductivity of the
(HMTETA, 97%; Aldrich) were used as received. All solvents
membranes. Microphase separation in block copolymers can
wereofACSgrade.
provide a versatile platform for the fabrication of nanostruc-
tured materials withawiderangeofmorphologiesdepending Characterizations
on the segregation strength (v) and the degree ofpolymeriza- 1H NMR spectroscopy was performed on a Bruker DPX-300
tion (N)-accessible structures include cylinders, lamellas, and FT-NMR. Gel permeation chromatography (GPC) was
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performed in tetrahydrofuran (THF) on a Polymer Laborato- NaBF (1.255 g, 11.43 mmol) were dissolved in 200 mL of
4
ries PL-GPC 50IntegratedGPCsystem.Infraredspectroscopy acetonitrile and stirred at ambient temperature overnight.
was performed on a Perkin-Elmer 100 FTIR spectrometer The solution was filtered, and the filtrate was concentrated.
with universal ATR sampling accessory. SAXS experiments White crystals were obtained by precipitation in anhydrous
were performed on beamline 12 ID-B at The Basic Energy diethyletherandthendriedinvacuumat40(cid:2)C.
Sciences Synchrotron Radiation Center at the Advanced Pho-
Synthesis of[PVBTMA][BF ]byATRP
ton Source at Argonne National Laboratory. A Pliatus 2M 4
Nitrogen-purged DMF (2 mL), (1-bromoethyl)benzene (7.03
SAXS detector was usedto collect scattering datawith an ex-
mg, 0.038 mmol), and HMTETA (8.75 mg, 0.038 mmol) were
posure time of 1 s. The X-ray beam had a wavelength of 1 Å
added to a Schlenk tube containing a mixture of
and power of 12 keV. The intensity (I) is a radial integration
[VBTMA][BF ] (1 g, 3.8 mmol) and CuBr (5.45 mg, 0.038
of the 2D scattering pattern with respect to the scattering 4
mmol). The mixture was put under vacuum and refilled with
vector(q).
nitrogen three times. The mixture was degassed by three
Temperature and humidity were controlled within a custom freeze-pump-thaw cycles followed by stirring at 90 (cid:2)C. Ali-
sample oven. Typical experiments studied three membrane quot samples were taken and analyzed to determine conver-
samples and one empty window so that a background spec- sionofthereaction by1HNMR.
trum of the scattering through just the Kapton windows and
Synthesis ofPolystyrene(PS-Br) by ATRP
nitrogen environment could be obtained foreach experimen-
The polymerization of styrene was performed as reported in
tal condition. The humidity of the sample environment was
the literature.45 Styrene (36.36 g, 348.8 mmol) was added to
controlled by mixing heated streams of saturated and dry
a Schlenk tube containing a mixture of CuBr (74.5 mg, 0.519
nitrogen. Sample holders were inserted into an oven envi-
mmol), (1-bromoethyl)benzene (99.0 mg, 0.519 mmol), and
ronment of 40 (cid:2)C and 50% relative humidity (RH). The sam-
HMTETA(119.6mg,0.519mmol).Themixturewasdegassed
ples were allowed to take up water for 20 min before X-ray
by three freeze-pump-thaw cycles followed by stirring at
spectra were taken. A 50% RH was maintained as the tem- 110 (cid:2)C for 5 h. After polymerization, the reaction solution
peraturewas then increased to 50 (cid:2)C and then to 60 (cid:2)C. The
was quenched in an ice bath, then passed through a pad of
spectra were taken 20 min after the temperature set points
basic alumina to remove the copper catalyst, and precipi-
were changed. Temperature was then maintained at 60 (cid:2)C
tated in methanol three times to obtain polystyrene as a
while the RH was stepped up to 75% and then to 95% with
whitepowder.
15-minsteps.
Synthesis ofPS-b-[PVBTMA][BF ] by ATRP
4
ConductivityMeasurement Nitrogen-purged DMF (4 mL) and HMTETA (4.6 mg, 0.05
Ionic conductivity measurements were made by using im- mmol) were added to a Schlenk tube containing a mixture of
pedance spectroscopy using custom electrode assemblies PS-Br (700 mg, M ¼ 35 kg/mol), [VBTMA][BF ] (700 mg,
n 4
and automation software within the humidity- and tempera- 2.67mmol),andCuBr(2.86mg,0.02mmol).Themixturewas
ture-controlled environment of an ESPEC SH-241 test cham- degassed by three freeze-pump-thaw cycles followed by stir-
ber. The free-standing membrane samples of irregular areas ringat90(cid:2)C.Thepolymerwasprecipitatedintomethanol.
were lightly compressed between two gold-plated stainless
PreparationofPS-b-[PVBTMA][BF ]Membranes
steel electrodes, the top having an area of A ¼ 0.07917 cm2 4
PS-b-[PVBTMA][BF ] membranes were drop cast from DMF
(1/8 inch diameter) and bottom having a 1/2 inch diameter, 4
(10 wt % solution) onto a Teflon sheet. The membranes
such that there was exposed material on the top surface.
were first dried at ambient temperature for 7 days and then
The impedance spectra were sampled at regular intervals of
undervacuumat40(cid:2)C.
20 min using a Solartron 1260 Impedance/Gain Phase Ana-
lyzer over a range of 10 MHz to 0.1 Hz in logarithmic steps IonExchange ofPS-b-[PVBTMA][OH]
of 10 points per decade; the portion of each spectrum form- PS-b-[PVBTMA][BF ] membranes were soaked in 1 M KOH
4
ing a‘‘plateau’’ in the impedance magnitude and correspond- aqueous solution for 3 days. The solution was changed sev-
ing to the first local phase minimum nearest the low-fre- eraltimes,andthenthemembraneswereimmersedinwater
quency range was fitted to a constant magnitude function for1day.
and interpreted as the bulk resistance R to ion transport
within the membrane. The thickness t of each membrane WaterUptake
was measured with a micrometer and the effective area for The PS-b-[PVBTMA][OH] membranes were soaked into the
the conductance measurement was estimated as the area of deionized water at room temperature for 24 h. The weight
the smaller top electrode A so that conductivity was com- of hydrated membranes (Wwet) was measured after wiping
puted as conductivity (r) ¼ t/(A (cid:4) R). The thickness of the the excess water on the surface. The weight of dry mem-
membranes is 189, 242, and 310 mm for PS-b- brane (Wdry) was obtained by drying the wet membranes
[PVBTMA][BF ]-1,-2,and-3,respectively. under vacuum at 60 (cid:2)C. The water uptake was calculated
4
usingthefollowingequation:
IonExchange of[VBTMA][BF ]
4 W (cid:3)W
The ion exchange of [VBTMA][Cl] was performed as previ- Wateruptake¼ wet dry(cid:4)100% (1)
W
ously reported.44 [VBTMA][Cl] (2.2 g, 10.39 mmol) and dry
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SCHEME 1 Synthesisofpoly(vinylbenzyltrimethylammoniumtetrafluoroborate)[PVBTMA][BF ]homopolymersandpolystyrene-
4
b-poly(vinylbenzyltrimethylammoniumtetrafluoroborate)PS-b-[PVBTMA][BF ]blockcopolymers.
4
RESULTSANDDISCUSSION 1H NMR. The Ð and M of the resulting block copolymers
n
cannot bemeasureddirectly by GPC due to lackofa suitable
Synthesis ofPS-b-[PVBTMA][BF ] and
4 solvent system to serve as the eluent. The M of the
PS-b-[PVBTMA][OH] n
Amphiphilic block copolymers of polystyrene-b-poly(vinyl [PVBTMA][BF4] was calculated based on the conversion
benzyl trimethylammonium) (PS-b-[PVBTMA]) have been syn- measurements. The conversion analyzed by yield was typi-
cally lower than that measured by 1H NMR because of the
thesized by quaternization with trimethylamine to the block
copolymers polystyrene-b-polyvinyl benzyl chloride that have lossofproductduringprecipitationandcollection.Therefore,
thecomposition andIECoftheblockcopolymerswerebased
already been synthesized by sequential stable free-radical po-
lymerization46 or reversible addition fragmentation transfer.47 ontheconversioncalculatedby1HNMR.
In this study, block copolymers PS-b-[PVBTMA][BF4] were Membranes of the PS-b-[PVBTMA][BF4] were made by drop
synthesized directly by sequential ATRP without the need casting from DMF. To determine the conversion of anion
for any postchemical modification. First, the ATRP of exchange, these membranes were characterized by FTIR. Fig-
[PVBTMA][BF4] was investigated to confirm its living charac- ure 2 shows the FTIR spectrum of PS-b-[PVBTMA][BF4]
ter. As shown in Scheme 1, [VBTMA][BF4] was prepared by before and after ion exchange in 1 M KOH aqueous solution
ion exchange from commercially available [VBTMA][Cl] with for 3 days in a sealed vial. The disappearance of the charac-
NaBF4.44 Because of the ion exchange, this monomer became teristics band at 1048 cm(cid:3)1 corresponding to the
slightly less hydrophilic and was soluble in DMF. The poly-
merization of [VBTMA][BF ] was achievedvia ATRP catalyzed
4
by a CuBr/HMTETA complex at 90 (cid:2)C in DMF. The linear
increase of conversion versus time (shown in Fig. 1) was
observed when the conversion was kept below 50%. Conver-
sion reaches a plateau at 50% probably due to the poor solu-
bility of the polymer chains in the reaction mixture. DMF is
also a solvent for polystyrene. Therefore, block copolymer PS-
b-[PVBTMA][BF ] can be directly synthesized by ATRP in
4
DMFwithoutanypostchemicalmodification.
The synthetic route for PS-b-[PVBTMA][BF ] is shown in
4
Scheme 1. The macroinitiator PS-Br with M ¼ 35 kg/mol
n
and dispersity (Ð) ¼ 1.13 was synthesized via ATRP cata-
lyzed by a CuBr/HMTETA complex. PS-b-[PVBTMA][BF ]
4
materials were then synthesized by ATRP of [VBTMA][BF ]
4
using the PS-Br as macroinitiator in DMF at 90 (cid:2)C. After the
copolymerization, aliquots of solution were analyzed by 1H
NMR in DMSO-d to measure the conversion. The conversion
6
of the copolymerization was also analyzed by yield measure-
ments to confirm the progress of copolymerization. PS-b- FIGURE 1 Time dependence of conversion for polymerization
[PVBTMA][BF4]-x are summarized in Table 1, where x refers of [VBTMA][BF4] by ATRP. [[VBTMA][BF4]]0/[initiator]0/[CuBr]0/
to the theoretical IEC based on the conversion calculated by [HMTETA] ¼100:1:1:1and[VBTMA][BF ]¼1.9M.
0 4
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]R
M
N
g)
v/
ui
q
gCme 36 19 58
IE[( 1. 1. 0.
%)
(
fe
WaterUptak 38.6 25.0 9.2
FIGURE 2 FTIR spectra of the PS-b-[PVBTMA][BF ]-1.19
]MR membrane (upper trace) and the PS-b-[PVBTMA][OH4]-1.19
N
%) membrane(lowertrace).
e(
e ol
Sm 4 8 4 tetrafluoroborate anion and the presence of the characteris-
P[ 6 6 8 tic signal of hydroxide anion (OAH stretching at around
3500cm(cid:3)1)indicatecompleteionexchange.
d Morphology Studiesonthe PS-b-[PVBTMA][BF ]
]BF4 F].4 andPS-b-[PVBTMA][OH]bySAXS 4
TABLE1SamplesofPS-b-[PVBTMA][BF]4 ofofMMn,NMRn,YieldabcConv.Conv.][PVBTMA][BF[PVBTMA][4PS-b-[PVBTMA][BF]-x[(%)][(%)](g/mol)(g/mol)4NMRYield PS-b-[PVBTMA][BF]-1.3656.244.119,60015,5004 ]-1.1935.728.216,10012,700PS-b-[PVBTMA][BF4 PS-b-[PVBTMA][BF]-0.5815.715.16,3006,1004 a1TheconversionwascalculatedbyHNMRspectraofreactionmixtureaftercopolymerizationinDMSO-d.6bTheconversionwascalculatedbytheyieldofcopolymerization.c1Mof[PVBTMA][BF]wasdeterminedbytheconversionfromHNMRaftercopolymerization.n4dMof[PVBTMA][BF]wascalculatedbytheconversionobtainedfromtheyieldofthecopolymerization.n4e1ThemoleratioofPSto[PVBTMA][BF]wascalculatedfromHNMR.4fWateruptakewasmeasuredwithhydroxideanionatroomtemperature.gIonexchangecapacity(IEC)ofthesamplesarecalculatedfromthecompositionratiobetweenPSand[PVBTMA][B aoFvOsfhtvmfrtdrohhohIserriroGgareeo.odongimmhdgspUmewotqemieero,nR]fc.rrdDteaaEotbTohOfemlosfCyolHmamtny3rsMrbtPdF,ofdprrS,Sw,eltttrtoavehh-hArhoohanmbereeeepXrani-epc[tSmstDecmPhucmDecaoaMVoralfeMktpneeseorBnomFsmtcrtprFTior..ttnhdb.baiboMfTvbgiorrrerlSehaooyeAladAof,npetson]rehgXsre[boSetoByceSsotesAmaflbaFfrvooXsoil4mecerntfoSfPd]xaenecitmSips,nttkhmdcdth-eewhesebarefe,mlbfot-eiommrt[amyrlsPPhroblbhoe-VeeSmrfcdtmromaon-BhhaebwnsrtTeobenf-steeis[MohtrenltsPhafPshesw.e-AVania.SeandHne]Be,sdS[goresslBmTiAmsoefewFMhafcXw(emeo4aeDoSpm]ArressvbwCeepert]ebldmnrM[helyrrfBefat,ddoroea)ctFoablenm,ratso4tmmgiedeoctvg]brh[ieshleieslevraemwDoldateeovscigsMnenpnraio(oeroooacsttIFlesssr)ttt--ff,
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(a) --40"C,RH50% (b) --40"C,RH50"/o
100 1 --50"C,RH50"/o --50"C,RH50"/o
--60"C,RH50"/o --60"C,RH50"/o
10
10
::i ::i
~ ~ 1
1
:§1 ~
0.1
0.1
<F39.27nm
d=57.12rm
0.01 0.01
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
q(A'') q(A'')
10 (c) --40"C,RH50"/o (d) --RH50"!.,60"C
100
--50"C,RH50"/o 1 --RH75"!.,60"C
--60"C,RH50"/o --RH95"!.,60"C
10
::i ::i
~ ~
! ! 1
0.1
0.1
d=31.42rm <F57.12rm
0.01 0.01
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
q (A'') q(A'')
(e) - RH50"/o,60"C (f) - RH50"/o,60"C
--RH75"/o,60"C 10 - RH75"/o,60"C
--RH95"/o,60"C - RH95"/o,60"C
10
-
::i ::i
~ ~
1
~ ~
0.1
0.1
<F39.27nm <F31.42rm
0.01 0.01
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
q (A'') q(A'')
FIGURE 4 SAXSprofilesofPS-b-[PVBTMA][OH]membranescastfromDMF.(a)PS-b-[PVBTMA][OH]-1.36;(b)PS-b-[PVBTMA][OH]-
1.19; and (c) PS-b-[PVBTMA][OH]-0.58 at 50% RH with increasing temperature from 40 to 60 (cid:2)C. (d) PS-b-[PVBTMA][OH]-1.36; (e)
PS-b-[PVBTMA][OH]-1.19;and(f)PS-b-[PVBTMA][OH]-0.58at60(cid:2)Cwith50,75,and95%RH.
to perform ion exchange to hydroxide anion for the mem- structures with PS as corona and [PVBTMA][BF ] as core in
4
branes drop cast from DCM were not successful, as judged DCM, disturbing the formation of continuous ion-conductive
by FTIR analysis, likelydue to the formation of a micelle-like channels when drop casting the membranes from DCM.
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Morphology–ConductivityRelationshipsatFixed
Temperaturewith IncreasingRH
The humidity-dependent conductivity of PS-b-[PVBTMA][OH]
membranes at 80 (cid:2)C is shown in Figure 5. From the log con-
ductivity versus humidity plot, the conductivity systemati-
callyincreaseswithincreasinghumidity forallthreesamples
because the water uptake in the membrane facilitates ion
conduction. Additionally, the conductivity increases with
increasing IEC of the materials; however, it does not increase
proportionally. The conductivity increases from 0.36–5.53
mS/cm to 12.55 mS/cm at RH ¼ 90% and 80 (cid:2)C when IEC
changes from 0.58–1.19 mequiv/g to 1.36 mequiv/g. This
unexpected relationship between conductivity and IEC may
result from the inherent nature of the microstructures in
these materials. Balsara and coworkers48 proposed a mor-
phology factor for block copolymers containing one ionic
blockswithanisotropic-oriented structures,f:
FIGURE 5 Humidity (RH)-dependent conductivity for PS-b- r¼f/ r (2)
[PVBTMA][OH]membranesat80(cid:2)C. c c
where r is the measured conductivity, and / and r are the
c c
Membranes drop cast from DMF, while less ordered as volume fraction and intrinsic conductivity of the conducting
shown by SAXS analysis, undergo successful conversion to block,respectively.
the hydroxide counter ion. SAXS data of different PS-b-
Sax and Ottino suggested that a factor of two-thirds be
[PVBTMA][BF ] samples drop cast from DMF are shown in
Figure 3. For4PS-b-[PVBTMA][BF ]-1.36, the SAXS data show applied to ion transport in a lamellar structure (f ¼ 2/3).49
4 Following similar arguments, f is 1/3 for a cylindrical mor-
two scattering peaks at q* and 2q*, and therefore, it likely
forms a lamellar structure. For PS-b-[PVBTMA][BF ]-1.19, phology. The conductivity of PS-b-[PVBTMA][OH]-1.36 with
the appearance of two scattering peaks at q* and 34(1/2)q* lamellar structure and PS-b-[PVBTMA][OH]-1.19 is 12.55
indicates a cylindrical morphology. Only one peak for PS-b- S/cm (rlam) and 5.53 S/cm (rcyl), respectively, at fully
hydrated state (90% RH and 80 (cid:2)C). Derived from eq 1, the
[PVBTMA][BF ]-0.58 is observed by SAXS, and thus, these
4 ratio of r to r is equal to two times of the ratio of vol-
data are not sufficient to distinguish the morphology. From lam cyl
ume fraction of conducting block in these two samples. In
the decreasing volume fraction of [PVBTMA][BF ], it likely
4 this case, the ratio of r to r (2.269) corresponds to
formsasphericalmorphology. lam cyl
twice the ratio of the mole fraction of the [PVBTMA][OH]
The morphology of PS-b-[PVBTMA][OH] membranes was (2.25) in PS-b-[PVBTMA][OH] samples with the assumption
determined by SAXS at a fixed RH at 50% with increasing that the mole fraction is similar to the volume fraction of
temperature from 40 to 60 (cid:2)C and at a fixed temperature of the conducting block because of undefined density of
60 (cid:2)C with different RHs at 50, 75, and 95% (shown in Fig. [PVBTMA][OH] at different RH levels and temperatures.
4). For PS-b-[PVBTMA][OH]-1.36, the SAXS data [Fig. 4(a, d)] With the same derivative, the f morphology factor for spher-
showing scattering peaks at q* and 2q*, 3q*, 4q*, and 5q* ical morphology without well-defined orientation is found
indicate that its microphase separates into a lamellar struc- to be1/3.
ture with long-range order. The SAXS data of PS-b-
[PVBTMA][OH]-1.19 show reflections at q*, 3(1/2)q*, and 7(1/
DifferentRelationship Between IEC andConductivityat
2)q*, exhibiting a cylindrical morphology [Fig. 4(b, e)]. When
comparedwith the SAXS data with BF (cid:3), the increase of peak LowandHigh RHwithElevated Temperature
4 Conductivity data as a function of temperature of PS-b-
intensity and the appearance of high-order peaks are due to
[PVBTMA][OH] membranes at different humidity conditions
the swellingby waterenhancing the scattering contrast. From
the SAXS data, only one peak isseen for PS-b-[PVBTMA][OH]- are shown in Figure 6. The ionic conductivity of these sam-
ples increases with elevated temperature at humidity levels
0.58 due to the lack of long-range order [Fig. 4(c, f)]. From
of 50, 70, and 90%. As shown in Figure 6(a), the conductiv-
the decreasing of [PVBTMA][OH] content, it presumably is a
ity at lower RH, that is, 50%, of PS-b-[PVBTMA][OH]
spherical morphology. The SAXS data for these three mem-
increases with increasing IEC at temperature above 45 (cid:2)C,
branes at RH at 50% with increasing temperature from 40 to
60 (cid:2)C and at 60 (cid:2)C with different RH at 50, 75, and 95% which is the so-called refraction temperature, because there
are more conductive groups in the membranes. However, the
demonstrate that no significant phase transition happens dur-
conductivityamong these samples follows a reverse order at
ing changing the environmental condition. The polystyrene
(T (cid:5) 100 (cid:2)C) as the hydrophobic domain can confine any temperaturebelow45(cid:2)Candat50%RH.
g
structural expansion and contraction of PS-b-[PVBTMA][OH] This unexpected behavior may result from the swelling and
membranesasthehumidityisraisedorlowered. shrinkage of the [PVBTMA][OH] domain and the presence of
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