Aromatic ionomers with highly acidic sulfonate groups: Acidity, hydration, and proton conductivity

Article abstract of DOI:10.1021/ma201759z

A novel sulfonation method that involves iridium-catalyzed aromatic C-H activation/borylation and subsequent Suzuki-Miyaura coupling with sulfonated phenyl bromides was developed for the preparation of aromatic ionomers. Superacidic fluoroalkyl sulfonic a

Full text of DOI:10.1021/ma201759z

ARTICLE
pubs.acs.org/Macromolecules
Aromatic Ionomers with Highly Acidic Sulfonate Groups: Acidity,
Hydration, and Proton Conductivity
Ying Chang,† Giuseppe F. Brunello,‡ Jeffrey Fuller,‡ Marilyn Hawley,§ Yu Seung Kim,^
Melanie Disabb-Miller,|| Michael A. Hickner,|| Seung Soon Jang,‡,* and Chulsung Bae†,*
†Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas,
Nevada 89154-4003, United States
‡School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Dr., Atlanta, Georgia 30332-0245,
United States
§MST-8: Structure/Property Relationship Group and ^MPA-11: Materials Physics and Application, Sensors and Electrochemical Device
Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802,
United States
S Supporting Information
b
ABSTRACT: A novel sulfonation method that involves iridium-
catalyzed aromatic CꢀH activation/borylation and subsequent Suzukiꢀ
Miyaura coupling with sulfonated phenyl bromides was developed for
the preparation of aromatic ionomers. Superacidic fluoroalkyl sulfonic
acid and less acidic aryl and alkyl sulfonic acids were efficiently
incorporated into the aromatic ring of model polystyrene, and the
resulting sulfonated ionomers were characterized for their properties
as proton-conducting membranes. The membrane properties of iono-
mers containing sulfonic acid groups with different acidity strengths were
compared to study the effect of acidity on the water properties, proton
conductivity, and morphology. The superacidic fluoroalkyl sulfonated
ionomer (sPS-S1) exhibited a significantly higher proton conductivity
than that of the less acidic aryl and alkyl sulfonated ionomers (sPS-S2 and
sPS-S3, respectively) at low relative humidity, despite a lower ion exchange capacity and lower water uptake. Hydration behaviors of the
ionomers as a function of relative humidity were studied to correlate the acid strength of the sulfonates and water uptake properties.
Morphology studies of the sulfonated ionomers show that sPS-S1 has a larger hydrophilic domain than that of sPS-S3. Molecular
dynamic simulations were performed to understand the origin of the improved proton conductivity of the superacidic ionomer at the
molecular level. These simulations suggest that the enhanced proton conductivity of sPS-S1 is due to the cumulative effect of higher
acidity of the sulfonate, which leads to increased dissociation to hydronium ions and a higher degree of ionic character in the sulfonate,
and better solvation of the sulfonate with water molecules.
’ INTRODUCTION
Although Nafion has been the most commonly used PEM during
recent decades, itsproperties need to be improved for broader fuel
cell applications. The drawbacks of Nafion include its high cost,
the rare availability of fluorine-containing precursors, high metha-
nol permeability in direct methanol fuel cells, and poor mechan-
ical stability at high temperatures (>100 ꢀC). Furthermore, the
lack of reactive sites in the perfluorinated structure of Nafion
makes modifications to overcome these shortcomings difficult.
Compared with perfluorinated polymers, aromatic-based
polymers are less expensive, more readily available, and easier
to modify.1 Considerable efforts have therefore been devoted to
Proton exchange membrane fuel cells (PEMFCs) are con-
sidered one of the most promising clean energy conversion
technologies for alleviating environmental problems associated
with burning fossil fuels.1ꢀ5 The proton exchange membrane
(PEM), an important component of PEMFCs, functions as a
proton conductor and separates hydrogen fuel from the oxidant
in PEMFCs. State-of-the-art PEMFC technology currently relies
on perfluorosulfonic acid (PFSA) membranes, such as Nafion.6,7
It is generally believed that the high proton conductivity of
PFSAs in PEMFCs, especially at low relative humidity (RH), is
related to the strong acidity of their perfluorosulfonic acid groups
and a distinct nanoscale phase-separated morphology derived
from extremely hydrophobic perfluorinated polymer backbone
and flexible, hydrophilic fluoroalkyl sulfonic acid groups.2,3,7,8
Received: August 1, 2011
Revised:
September 14, 2011
Published: October 11, 2011
r
2011 American Chemical Society
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the development of aromatic PEMs as Nafion alternatives.
Numerous examples of randomly sulfonated aromatic copoly-
mers based on polybenzimidazoles,9 poly(ether ether ketone)s,10
poly(arylene ether sulfone)s,11ꢀ13 polyimides,14ꢀ16 and poly-
phenylenes17 have been prepared by either electrophilic post-
sulfonation of aromatic polymers or direct copolymerization
of sulfonated monomers and investigated as alternative PEM
materials. Some examples of aromatic PEMs show promising
performance in fuel cell operations. However, these types of
PEMs generally have achieved high proton conductivity only at
fully hydrated conditions and their conductivities drop sharply
when RH is decreased. Because PEMFC operation under
reduced RH gives increased system efficiency, the development
of PEMs that are highly conductive at reduced RH is critical for
successful adoption of fuel cell technology in automobile trans-
portation applications.4
Recently, studies of the relationship between morphology and
proton conductivity have drawn considerable attention to in-
crease proton conductivity at low RH in the design of new
PEMs.18ꢀ25 Accordingly, sulfonated multiblock copolymers
composed of hydrophilic and hydrophobic aromatic units have
been investigated to provide continuous proton conduction
pathways through self-assembled domain structures at low RH.
Although multiblock copolymer ionomers can achieve enhanced
proton conductivity at low RH in comparison to that of the
corresponding randomly sulfonated ionomers, the conductivity
at low RH has not reached the level needed for practical low RH
PEMFC operations. Molecular weight control and sequence
length of multiblock copolymer synthesis is also complicated,
which hinders their widespread adoption.
In addition to unique nanoscale phase separation, another
characteristic feature of PFSA ionomer is its strong acidity. Owing
to the strong electron-withdrawing effect of fluorine, the fluori-
nated sulfonic acid in PFSA is an extremely acidic group (also
called a superacid). Although accurate measurement of the
strength of this group is difficult because reliable methods for
assessing solid acids are lacking, the acidity range is expected to be
similar to that of perfluoroalkyl sulfonic acid (approximate pKa =
ꢀ14.1).26 Except for a few recent examples of superacidic
PEMs,27ꢀ34 most aromatic ionomers contain much less acidic
aryl sulfonic acid or alkyl sulfonic acid groups as proton-
conducting moieties. Although those perfluoroalkyl-sulfonated
(superacidic) aromatic ionomers have exhibited better proton
conductivity at reduced RH compared with typical sulfonated
aromatic PEMs, most superacidic ionomers are still restricted in
their synthetic generality; when less efficient Ullmann coupling
was used, low ion-exchange capacity (IEC; <1.5 mequiv/g) was
obtained.27ꢀ31 Importantly, there have been no comprehensive
studies exploring the effect of acidity on fuel cell membrane
properties, such as how increased acidity affects proton conduc-
tivity, morphology, and water transport behavior within a PEM.
Herein, we report a highly efficient synthetic method that
allows the incorporation of fluoroalkyl, aryl, and alkyl-tethered
sulfonate groups with various acidities into a model aromatic
polymer and a detailed study of the effects of acidity on PEM
properties, particularly its hydration and proton conductivity at
low RH. Although polystyrene is not considered an ideal material
for PEM application owing to its poor oxidative stability of the
benzylic CꢀH bond,1 we selected syndiotactic polystyrene (sPS)
as the model PEM material for our study because of its simple
chemical structure, which allows expeditious computational mod-
eling and convenient spectroscopic characterization (see the
Results and Discussion for details). By comparing the properties
of aromatic PEMs with different acidity strengths, we sought to
understand the relationship between the chemical structures of
ionic groups and the property of PEMs at the molecular level. We
also conducted molecular dynamics studies of the synthesized
PEMs to more fully understand the mechanism underlying acid
group interactions with water molecules within the membrane.
’ RESULTS AND DISCUSSION
A. Experimental Studies of sPS Ionomers. Synthetic Strat-
egy for Sulfonated Ionomers. The synthesis of sulfonated sPS
ionomers functionalized with different sulfonic acid groups is
summarized in Scheme 1. The synthetic strategy is based on
sequential reactions of (i) iridium-catalyzed borylation of the
aromatic CꢀH bonds of sPS, (ii) palladium-catalyzed Suzukiꢀ
Miyaura cross-coupling reactions of borylated sPS (sPS-Bpin)
with phenyl bromides that contain different sulfonate pre-
cursor groups (S1, S2, S3 in Scheme 1), and (iii) removal of the
protecting group from the sulfonates and subsequent acidifica-
tion. Three examples of sulfonated sPS ionomers containing
fluoroalkyl sulfonic acid (sPS-S1), aryl sulfonic acid (sPS-S2), and
alkyl sulfonic acid (sPS-S3) were prepared using this synthetic
method. Detailed synthetic procedures for ionomers and sulfo-
nated phenyl bromides are described in the Experimental Section
and the Supporting Information.
Synthesis of Sulfonated Phenyl Bromides. To introduce
sulfonate groups with different acidity strengths into sPS via
the SuzukiꢀMiyaura coupling reaction, we prepared three types
of phenyl bromides functionalized with sulfonate groups: fluor-
oalkyl sulfonated S1, aryl sulfonated S2, and alkyl sulfonated S3
(Scheme 2). All three sulfonated phenyl bromides were capped
with a 3,5-dimethylphenol protecting group at the end and
conveniently prepared from readily available chemicals in high
yields using traditional organic synthetic methods.
Nucleophilic substitution reaction of 4-bromophenol with 1,2-
dibromotetrafluoroethane under basic conditions produces an
ether covalent bond between oxygen and CF2CF2Br.35 The
terminal bromide was converted first to its sodium sulfinate
form (ꢀSO2Na) via reaction with sodium dithionite and sodium
bicarbonate and then to its sulfonyl chloride form (ꢀSO2Cl) by
bubbling chlorine. The sulfonyl chloride was transformed to 3,5-
dimethylphenol-protected sulfonate S1 via reaction with 3,5-
dimethylphenol in the presence of N,N-dimethylaminopyridine.
Ring-opening of 1,3-propane sultone with 4-bromophenol under
basic conditions, subsequent conversion of sodium sulfonate to
sulfonyl chloride, and protection with 3,5-dimethylphenol af-
forded S3 in good yield. S2 was prepared from a condensation
reaction of 4-bromobenzenesulfonyl chloride with 3,5-dimethyl-
phenol in one step. All three protected sulfonates were air- and
moisture-stable and could be stored under ambient conditions.
Synthesis and Characterization of Sulfonated Polymers. We
recently reported a highly effective borylation of aromatic poly-
mers using bis(pinacolato)diboron (B2pin2) as a borylation
reagent in the iridium-catalyzed activation of aromatic CꢀH
bonds.36,37 Because sPS gives a 1H NMR spectrum that is more
finely resolved than that of atactic polystyrene, owing to the high
stereoregularity of the phenyl rings along the polymer main chain
(see Figure 1a), we chose sPS for postfunctionalization in this
study. The iridium-catalyzed borylation substitutes aromatic
CꢀH bonds to CꢀB bonds under mild conditions while tolerat-
ing various functional groups.38,39 As reported elsewhere,36 the
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Scheme 1. Synthesis of Syndiotactic Polystyrene Ionomers Functionalized with Different Sulfonic Acid Groupsa
a Reagents and conditions: (i) B2pin2, [IrCl(COD)]2 (1.5 mol %), dtbpy (3 mol %), cyclooctane, 150 ꢀC, 6 h; (ii) S1, S2, or S3 (2 equiv), Pd(PPh3)4
(4 mol %), K3PO4 (4.5 equiv), THF/H2O (10/1), 80 ꢀC, 12 h; (iii) NaOH (8 equiv), dioxane/H2O, 100 ꢀC, 4 h; and (iv) 1 M H2SO4.
Scheme 2. Synthesis of Sulfonated Phenyl Bromides
CꢀH borylation of sPS [weight-average molecular weight (Mw) =
140.9 kg/mol; polydispersity index (Mw/Mn) = 2.90] was con-
ducted in cyclooctane solvent at high temperature (150 ꢀC).
After borylation of the polymer, a new proton signal corre-
sponding to the four methyl groups of the pinacolboronic ester
(Bpin) appeared at 1.16 ppm (Figure 1b). Integrals of the
resonances of methylene at 1.49 ppm and methine at 2.09
amount of B2pin2 relative to the polymer repeating unit. In this
work, we focused on borylated sPS with 40 mol % Bpin
concentration.
The palladium-catalyzed SuzukiꢀMiyaura cross-coupling re-
action of aryl boron compounds and aryl halides40 has been a
powerful method for biaryl CꢀC bond formation because of its
high efficiency and good tolerance of functional groups. Thus, we
conducted SuzukiꢀMiyaura coupling reactions of the borylated
polymer with sulfonated phenyl bromides (S1, S2, and S3 in
Scheme 1) to incorporate the corresponding sulfonate groups
into sPS. After the coupling reactions, the Bpin resonance of sPS-
1
ppm in the sPS main chain maintained a ratio of 2:1 in the H
NMR spectrum, suggesting that the main-chain structure of sPS
remained intact during the borylation. Thus, the molar concen-
tration of the attached Bpin group in the borylated polymer was
estimated by comparing the resonance integrals of the methyl
group at 1.16 ppm and the methine proton of the polymer main
chain at 2.09 ppm. The incorporated Bpin concentration was well
controlled up to ∼50 mol % simply by changing the loading
1
Bpin at 1.16 ppm disappeared completely in the H NMR
spectra, and a new signal at 1.97 ppm appeared and was assigned
to the two methyl groups of the 3,5-dimethylphenol structure of
the sulfonated sPS (Figure 1c).
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Figure 2. 19F NMR spectra of sulfonated sPS-S1 before (a) and after
(b) deprotection of the 3,5-dimethylphenol group.
In addition to the expected resonances of ꢀCF2CF2SO3Na at
ꢀ80.4 and ꢀ116.2 ppm, two small resonances were observed at
ꢀ86.9 ppm and ꢀ137.2 ppm in the 19F NMR spectrum of
deprotected sPS-S1, and they were identified as ꢀCF2CF2H and
ꢀCF2CF2H, respectively (Figure 2b). Confirmation of the
desulfonated side chain structure was made from its anticipated
chemical shift range and the splitting pattern of CF2CF2H in the
19F NMR spectrum (doublet; J = 59 Hz). The 19F NMR
resonance integral of the desulfonated structure was less than
5% of that of ꢀCF2CF2SO3Na. In a related experiment of basic
deprotection of S1, a small amount of BrꢀC6H4ꢀOCF2CF2H
was also detected in GC-MS and 19F NMR. In contrast, no
Figure 1. 1H NMR spectra of (a) sPS, (b) sPS-Bpin, (c) sPS-S3 in the
3,5-dimethylphenol protected form, and (d) sPS-S3 in the ꢀSO3Na form.
Similar to that in the sPS-Bpin characterization, the concen-
tration of 3,5-dimethylphenol-protected sulfonate groups in the
polymers was calculated based on the integral ratio of the
methine proton of the sPS main chain (at 2.09 ppm) and the
terminal methyl groups in the side chain of sulfonated sPS (at
1.97 ppm). The molar concentration of the sulfonate group
matched well with that of the borylated group in the precursor
polymer (i.e., 40 mol %). In the case of the coupling reaction
with S3, the resulting polymer also showed the characteristic propyl-
ene group of S3 at 3.52, 3.08, and 2.12 ppm with the expected
molar concentrations. Successful incorporation of the fluoroalkyl
sulfonate group after the coupling reaction with S1 was further
confirmed by two clean resonances at ꢀ80.3 ppm (ꢀOCF2ꢀ)
and ꢀ111.9 ppm (ꢀCF2SO3ꢀ) in the 19F NMR spectrum
(Figure 2a). Unlike electrophilic sulfonation of aromatic poly-
mers using chlorosulfonic acid or sulfuric acid, in which polymer
gelation frequently occurs because of undesired side reactions
(cross-linking) in the polymer chains,41 the borylation and the
subsequent coupling reaction caused no gelation.
1
similar desulfonation was observed when we examined the H
and 13C NMR spectra of sPS-S3 and the reaction mixture of
deprotected S3 with GC-MS. These results indicate the desulfo-
nation that occurred during the deprotection step of the sPS-S1
synthesis might be related to the presence of the strong electron-
withdrawing fluoroalkyl group. Although further study is needed
to confirm this hypothesis, it was encouraging to observe that
only a small fraction of the sulfonate group in sPS-S1 underwent
the undesired side reaction.
Membrane Properties. All sPS ionomers (ꢀSO3Na forms)
prepared using this method had a good solubility in polar aprotic
solvents, such as dimethyl sulfoxide (DMSO), and high molec-
ular weights, as reflected in their intrinsic viscosities (Table 1).
Although all sulfonated sPSs have different (weight-based) IECs,
they have the same parent polymer backbone and the same
sulfonation level (i.e., 40 mol %). Thus, the higher viscosity of
sPS-S1 compared to two other polymers in Table 1 is believed to
be derived from properties of superacidic sulfonate group, such
as degree of dissociation. Superacidic sPS-S1 would have the
highest dissociation degree. Thus, in spite of having the lowest
IEC, it showed the highest viscosity. A similar conclusion was
drawn by Lundberg and Makowski that due to stronger associa-
tion and greater polarization present in sulfonate groups than
those of carboxylate groups, melt viscosities of sodium sulfonated
polystyrene ionomers at a given functional level are significantly
higher than their corresponding carboxylated ionomers.42 All
sPS ionomer membranes were readily prepared as transparent
films using a solution casting method and further acidified with
1 M H2SO4 to yield their acid forms. The IECs of the sPS ionomers
were measured by titration and compared to the calculated values
determined from 1H NMR. At a given degree of sulfonation, the
Removal of 3,5-dimethylphenol from the protected sulfonate
groups of sPS using basic hydrolysis and subsequent acidification
with sulfuric acid yielded the final sPS ionomers functionalized
with fluoroalkyl, aryl, or alkyl sulfonic acid groups (sPS-S1, sPS-
S2, and sPS-S3, respectively, in Scheme 1). Basic hydrolysis of the
protected sulfonated sPS was conducted with sodium hydroxide
in a mixture of dioxane and water. Upon completion of the
reaction, the sPS ionomers in the sodium salt form precipitated
owing to a dramatic solubility change in the medium. The ionic
polymers were stirred in a refluxing solution of water and
methanol to remove salt and organic impurities and filtered.
1
After the deprotection step, the H NMR resonance of the
methyl side groups at 1.97 ppm disappeared, and the 19F NMR
resonance of ꢀCF2SO3ꢀ in sPS-S1 shifted to ꢀ116.2 ppm
(Figure 2b). A small amount of desulfonation was also detected
during the deprotection of the 3,5-dimethylphenol group of sPS-S1.
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Table 1. Properties of sPS Ionomers and Nafion
ionomera
IECb
IVc
water uptaked
λe
sPS-S1
1.64 (1.87)
2.29 (2.39)
2.01 (2.10)
0.86 (0.90)
1.41
1.01
0.85
22
32
21
16
7.5
7.8
5.8
9.6
sPS-S2
sPS-S3
Nafion 112
—
a All sPS ionomers contain 40 mol % sulfonate in the repeating unit
based on the 1H NMR spectra of 3,5-dimethylphenol-protected
sulfonate group. b IEC (mequiv/g) measured by titration. Calculated
IEC values are shown in parentheses. c Intrinsic viscosity of sodium salt
form ionomer in 0.1 M NaI/DMSO at 30 ꢀC. d Water uptake (%) =
(Wwet ꢀ Wdry)/Wdry measured at 98% RH and 30 ꢀC during dynamic
RH scans used for conductivity. e Number of water molecules per
sulfonic acid moiety (hydration number).
IEC values decreased in the order of sPS-S2 > sPS-S3 > sPS-S1
because of the increasing sizes of the pendant sulfonate tethers.
Proton conductivity (σH+) is the primary property that
determines PEM performance, and high proton conductivity at
low RH is highly desired for the successful adoption of PEMFC
technology in vehicle applications. The proton conductivity of
sPS-S2 followed the typical proton conductivity pattern of
aromatic PEMs randomly functionalized with aryl sulfonic acid
groups. Although it exhibited high proton conductivity when
fully hydrated, its conductivity dropped sharply at lower RH. In
contrast, sPS-S1 demonstrated proton conductivity that was
higher than that of sPS-S2 below 90% RH despite its lower water
uptake, lower IEC, and similar hydration number (λ). Notably,
the relative slope of the conductivity (on a log scale) vs RH of the
superacidic sPS-S1 was less steep than those of sPS-S2 and sPS-S3
and was almost parallel to that of Nafion: 0.020 for Nation, 0.022
for sPS-S1, 0.029 for sPS-S2, and 0.030 for sPS-S3 (see Figure 3a).
This conductivity behavior is unique when compared with typical
aromatic PEMs. The order of the proton conductivities at
reduced RH follows the same order of the acidities of the pen-
dant sulfonic acid groups: ꢀCF2CF2SO3H (estimated pKa =
ꢀ14)25,43 > ꢀC6H4ꢀSO3H (estimated pKa = ꢀ2.5)27,44
>
ꢀ(CH2)3SO3H (estimated pKa = ꢀ0.6).43,45 Among the sPS
ionomers studied, sPS-S3 had the lowest conductivity across the
entire RH range, possibly resulting from the combined effect of
the weaker acidity of the sulfonic acid and lower WU.
The water uptake (WU) properties of PEMs have a strong
influence on their proton conductivity. The WU of sPS-S2 is the
greatest among the samples studied due to its high IEC
(Figure 3b). Interestingly, the superacid-containing sPS-S1 has a
similar bulk WU to that of sPS-S3, even though the IEC of sPS-S3
is 20% greater than that of sPS-S1. This difference in the WU of
the fluoroalkyl sulfonate sample compared to the alkyl sample
can be attributed to the increased hydration of the superacid as
discussed later. The λ of the alkyl sulfonate sample is lower than
those of two other ionomers, likely due to its lower acidity and a
hydrophobicity effect of the alkyl tether (Figure 3c). Interest-
ingly, sPS-S1 and sPS-S2 had similar hydration numbers, but
clearly the sulfonate moiety of the former is more effective in
promoting conductivity, even at equivalent hydration. Nafion
which has the same fluoroalkyl sulfonate group as sPS-S1 shows
the highest λ across all RHs, which suggests that other factors
such as backbone structure may impact the hydration of the ionic
groups.
Figure 3. (a) Proton conductivity (σH+), (b) water uptake (WU), and
(c) hydration number (λ) of Nafion and sPS ionomers under different
relative humidity conditions.
AFM after exposure of the PEMs to ambient conditions for >6 h.
In the AFM images (Figure 4), domains having higher phase lag
angle appear as dark areas (hydrophilic domains) and domains
having lower phase lag angle appear as bright areas (hydrophobic
domains). All three sPS ionomers exhibited a phase separated
morphology with slightly different hydrophilic and hydrophobic
domain sizes, but no obvious connectivity among the hydrophilic
domains was observed. The hydrophilic domain size of sPSꢀS1
Ionic Domain Morphology. The morphological characteristics
of sulfonated sPS ionomers were investigated using tapping-mode
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sulfonic acid group at the terminal position of flexible pendant
side chains (i.e., sPS-S1 and sPS-S3), improved phase separation
was achieved.
The TEM images of the sPS ionomers showed morphological
features similar to those observed with AFM (Figure 5): indis-
tinct phase separation with poor connectivity among hydrophilic
domains. Because the TEM images were obtained under vacuum
and there are no tip sharpness artifacts as with AFM, the
hydrophilic domains of the PEMs were observed to be much
smaller in TEM than what was obtained using AFM. While sPS
ionomers demonstrated a low level of phase separation with
significantly smaller hydrophilic domains, Nafion showed dis-
tinctive nanoscale phase-separated hydrophilic domains (2ꢀ4 nm
in TEM). Considering that Nafion and sPS-S1 have a terminal
sulfonate group with the same acidity strength, the larger size and
better continuity of the hydrophilic domains in Nafion strongly
suggest that other structural differences (i.e., more hydrophobic
polymer backbone, longer and more flexible side chain of
Nafion) might play critical roles in enhancing the aggregation
of ionic groups and formation of water channels within the PEM.
Although Nafion has a lower IEC than sPS-S1, the former still
achieved a comparable proton conductivity of the latter at the
entire RH possibly due to the higher λ created by the favorable
morphology effect.
Figure 4. Morphology of Nafion and sPS ionomers studied by AFM.
The morphological study by AFM and TEM confirmed that
the hydrophilic and hydrophobic domain structures depend on
several other factors besides acidity, such as polymer architecture,
concentration and distribution of sulfonated groups, difference in
hydrophobicity/hydrophilicity of backbone and sulfonated side
chain, and backbone/side chain stiffness. It is difficult to com-
pletely decouple the acidity effect from other factors in the model
polymer systems we used. However, it is interesting to note that
sPS-S1 exhibited high proton conductivity at low RH even with
somewhat undesirable domain structure (i.e, little observed
domain connectivity), which is in stark contrast with previous
multiblock copolymer approach that endeavor to form highly
connected hydrophilic domains to improve low RH conductivity.
B. Computational Simulation Studies of sPS Ionomers.
The experimental data above demonstrate differences in mem-
brane properties but provide no mechanistic insight into their
cause. Thus, we investigated the effect of acidity strength on
proton conductivity and water properties using molecular dy-
namic simulations. We selected two sulfonated sPS ionomers
containing short ponytail side chains, sPS-S1 and sPS-S3, as model
polymers. Because these ionomers have an identical polymer
backbone structure, an identical concentration of sulfonic acid
groups, and a similar flexibility along their side chains, we believe
that a comparison of their properties by computational studies
provides a theoretical understanding of the acidity effect on PEM
properties. All calculations were conducted for hydrated mem-
branes with 10 and 20 wt % water contents at 80 ꢀC. The
molecular parameters of the computational simulation are listed
in Table 2.
Figure 5. Morphology of Nafion and sPS ionomers studied by TEM.
was ∼10 nm which was larger than that of sPS-S2 (∼8 nm) and
sPS-S3 (∼6 nm). The hydrophilic domain size does not seem to
be strongly related to the acid strength of sulfonated group as
superacidic fluoroalkyl sPS-S1 has a similar domain size as the aryl
sulfonated sPS-S2. These data on the hydrophilic domain sizes of
our ionomers by AFM are supported by measurements of the
hydrophilic domains in other random sulfonated poly(arylene
ether sulfone)s with a similar IEC (1.5ꢀ2.2 mequiv/g).19,46
Compared to Nafion, all three sPS ionomers demonstrated a
low degree of phase separation with smaller isolated hydrophilic
domains. Of the sPS ionomers, the AFM image of sPS-S2
exhibited a somewhat higher degree of phase mixing between
the hydrophobic and hydrophilic domains. By introducing a
Dissociation and Solvation of Sulfonate Groups. Previous
theoretical PEM studies on Nafion,47 dendrion,48,49 and sulfo-
nated poly(ether ether ketone)50 suggest that nanoscale phase-
segregation of hydrated PEMs can form well-connected water
phases within a PEM and that this process requires the solvation
of the hydrophilic sulfonate groups with water molecules in the
first step.
To investigate the relationship between acidity and interac-
tions of sulfonate with water molecules, we analyzed the pair
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Table 2. Composition of Hydrated sPS Membranes and
Simulation Conditions
ionomers
sPS-S1
sPS-S3
molecular weight per chain (Da)
equivalent weight (g/mmol)
degree of polymerization
7756
554
35
6624
473
35
degree of sulfonation
40
40
number of polymer chains
total number of sulfonate groups
4
4
56
56
correlations of sulfonateꢀhydronium pair, FgSꢀO (hydronium), and
sulfonateꢀwater pair, FgSꢀO (water), for sPS-S1 and sPS-S3 with
10 and 20 wt % water contents. The definition of pair correlation
function, gAꢀB(r), is the probability density of finding B atoms at
a distance r from A atoms averaged over the equilibrium
trajectory as shown in eq 1:
ꢀ
ꢁ ꢀ
ꢁ
nB
4πr2Δr
NB
V
gAꢀBðrÞ ¼
=
ð1Þ
where nB is the number of B particles located at a distance r in a
shell of thickness Δr from particle A, NB is the number of B
particles in the system, and V is the total volume of the system.
Therefore, gAꢀB(r) is equivalent to the ratio of the number
density of particle B in a thin shell to the global number density at
a distance r from the position of particle A.
The pair correlation function for the sulfonateꢀhydronium
pair clearly shows an acidity effect (Figure 6a). The stronger acid
group of sPS-S1 can more readily dissociate into the ionized form
and has a lower pair correlation intensity. This is because sPS-S1
can stabilize the ionized sulfonate form with the adjacent
electron-withdrawing CF2 group, whereas sPS-S3 does not have
these strong electron-withdrawing groups for stabilizing the
negative charge on the sulfonate.
Figure 6. Calculated pair correlation functions of (a) sulfonateꢀ
hydronium ion and (b) sulfonateꢀwater in hydrated sPS ionomers.
Transport Properties of Water and Protons. We also investi-
gated the transport properties of water molecules and protons
within the sPS ionomer membranes. The water diffusion coeffi-
cient (Dwater) was calculated from the mean squared displace-
ment of water using the following equation:
As shown in Figure 6b, the intensity of [FgSꢀO(water)(r)]
increased with increasing water content for both ionomers,
indicating that more water molecules gathered around the
hydrophilic sulfonate groups. Between the two sPS ionomers,
sPS-S1 had a higher pair correlation intensity than that of sPS-S3
at both water contents. To quantitatively assess the difference in
pair correlation, we calculated the average number of water
molecules that surrounded each sulfonate group, or the hydra-
tion number (λ), by integrating the first peak in Figure 6b. The
hydration numbers for sPS-S1 were 1.78 and 3.91 at 10 and 20 wt %
water content, respectively, and the corresponding hydration
numbers for sPS-S3 were 1.35 and 3.61. These results suggest
that the sulfonate groups in sPS-S1 attracted more water mol-
ecules than did those of sPS-S3, and this can be explained by that
a stronger acid group has a greater tendency to exist in an ionized
form and requires solvation with more water molecules. Overall,
the data of Figure 6 suggest that more hydronium ions are
dissociated from the sulfonate groups in sPS-S1 because of its
stronger acidity, and the resulting higher degree of ionic char-
acter of the sulfonate group attracts more water molecules for
solvation. The superior solvation of the sulfonate groups in sPS-S1
compared with that of sPS-S3 may account for the larger
hydrophilic domains of the former observed on the AFM images
and contribute to its enhanced proton conductivity at low RH.
This phenomenon is also observed in the hydration data where
sPS-S1 absorbs more water molecules per sulfonic acid group
than sPS-S3.
1
2
ð2Þ
Dwater ¼ lim
ÆðrðtÞ ꢀ rð0ÞÞ æ
t
s
f∞
6t
where r(t) and r(0) are the positions of water at a certain time (t)
and at the beginning (t = 0), respectively. The results shown in
Table 3 suggest that water diffusion increases with increasing
water content, and this relationship is attributed to the better
development of a water phase in hydrophilic domains. As the
sulfonate groups become surrounded by more water molecules,
the local water concentration increases and eventually these
water domains coalesce and enhance water transport. Compar-
ison of sPS-S1 and sPS-S3 showed that the former achieved
greater water diffusion. This result can be understood by con-
sidering the aforementioned discussion of FgSꢀO(water)(r): more
water molecules that are attracted to the acid groups of sPS-S1
enhance the possibility of better solvated sulfonate groups and
facilitate water transport in the hydrophilic phase.
We also investigated the proton diffusion of sPS-S1 and sPS-S3
under 10 and 20 wt % water contents. Proton diffusion can occur
via vehicular diffusion and hopping mechanisms. Through the
vehicular diffusion mechanism, protons are transported as water
molecules carrying protons diffuse through the membrane,
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Table 3. Calculated Diffusion Coefficients of Water and
Protons of sPS Ionomers at 80 ꢀC
Table 4. Computed Proton Conductivity of sPS Ionomers at
80 ꢀC
proton conductivity
proton diffusion
ionomer
sPS-S1
water content (wt %)
(σ) (mS/cm)
water diffusion
vehicular
hopping
10
20
10
20
14.4
24.6
4.7
water content
(wt %)
Dwater
Dvehicular
Dhopping
ionomer
sPS-S1
( ꢁ 10ꢀ5 cm2/s) ( ꢁ 10ꢀ5 cm2/s) ( ꢁ 10ꢀ5 cm2/s)
sPS-S3
10
20
10
20
0.0908
0.4441
0.0584
0.2626
0.0029
0.0221
0.0012
0.0214
0.2511
0.4272
0.0820
0.3303
19.5
sPS-S3
that the effect of strong acidity on proton conduction is more
significant at lower water content.
Lastly, we calculated proton conductivity (σ) based on the
proton diffusion coefficient (Dproton,total = Dvehicular + Dhopping
using the following NernstꢀEinstein equation:
)
whereas in the hopping mechanism, protons are transported by
transfer from one water molecule (proton donor) to another
(proton acceptor). The proton diffusion coefficient under the
vehicular mechanism (Dvehicular) can be described by the classical
molecular dynamics simulation in eq 2. However, calculating
Dproton, totalcz2F2
σ ¼
ð5Þ
RT
proton diffusion coefficient of the hopping mechanism (Dhopping
)
where c and z denote the proton concentration and the charge
carried by the proton, respectively, and F and R denote the
Faraday constant and the gas constant, respectively. T is tem-
perature in Kelvin. The computed proton conductivity results are
summarized in Table 4. The difference in computed proton
conductivities between sPS-S1 and sPS-S3 becomes larger as the
water content decreases, which is consistent with the results of
proton diffusion coefficient calculations: the ratio of the calculated
proton conductivities of sPS-S1 and sPS-S3 increased from 1.3 to
3.1 as the water content decreased from 20 wt % to 10 wt %.
Therefore, the more robust proton conductivity of sPS-S1 com-
pared to the other sPS ionomers at low RH is due primarily to the
strongeracidity ofthe superacidic ionomer anditsresultinghigher
diffusions of water and protons within the membrane.
requires a quantum mechanical treatment because intermolecu-
lar distances between water molecules will change dynamically
during the proton conduction. Therefore, we applied quantum
mechanical transition state theory using eq 3 as described in
previous studies:49,51,52
!
EijðrÞ ꢀ 1=2hωðrÞ
kBT
h
kijðrÞ ¼ kðT, rÞ
exp
ꢀ
ð3Þ
RT
where k(T,r) and ω(r) are the tunneling factor and the frequency
for zero point energy correction (adopted from literature50,51),
respectively, and E(r) is the energy barrier for a proton to be
transferred from donor to acceptor in water when they are at a
distance of r. We first calculated the proton hopping energy
barrier, E(r), for fixed distances between donor and acceptor
oxygen atoms using quantum mechanics (B3LYP with the
6-311G** basis) to see how energy varies as a function of the
distance between the proton and the donor oxygen. We then used
the PoissonꢀBoltzmann self-consistent reaction field model53,54
to correct the solvent effect along the reaction path, and we
recalculated the energy barrier. From the results obtained from
eq 3 and the distances between all pairs of donors and acceptors
determined with the equilibrium molecular dynamics trajectory,
we calculated the proton hopping diffusion coefficient as follows:
’ CONCLUSIONS
We have developed a novel, efficient sulfonation method for
aromatic polymers using a combination of transition metal-
catalyzed borylation of aromatic CꢀH bonds and Suzukiꢀ
Miyaura coupling reactions. The new polymer functionalization
method allows for convenient attachment of a variety of sulfonate
groups with different acidities onto the aromatic polymer. We
prepared polystyrene-based sulfonate ionomers by attachment of
superacidic fluoroalkyl sulfonic acid and less acidic aryl sulfonic
and alkyl sulfonic acids to the backbone, and evaluated their fuel
cell membrane properties. With these systematic series of poly-
mers, we performed a comprehensive study on the effect of acid
strength on the proton conductivity of the sulfonated ionomers.
Despite its lower water uptake and IEC, the fluoroalkyl sulfo-
nated superacidic ionomer (sPS-S1) maintained higher proton
conductivity at low RH compared with the less acidic aryl and
alkyl sulfonated ionomers (sPS-S2 and sPS-S3), and this differ-
ence in proton conductivity gradually increased as the RH
decreased. The water uptake behavior as a function of RH and
the morphology studies show that compared to less acidic iono-
mers sPS-S3, the superacidic sulfonate groups of sPS-S1 attract
more water molecules and create enlarged hydrophilic domains
that could provide more facile transports of hydronium ions and
water and afford higher proton conductivity. Comparison of
properties of two superacidic sulfonated ionomers with different
polymer backbones, Nafion and sPS-S1, suggests that creation of
favorable morphology with larger-sized and better-connected
Z
t f ∞
N
M
1
2
Dhopping
¼
kijrij Pij dt
ð4Þ
∑∑
6Nt
0
i
j
where N is the number of protons and Pij is the probability that a
proton will jump from hydronium i to water j defined as Pij =
kij/∑jkij. Here rij is the distance between all pairs of donors and
acceptors measured from the equilibrium molecular dynamics
trajectory.
Table 3 summarizes the calculated proton diffusion coeffi-
cients of hydrated sPS ionomers. Similar to water diffusion,
proton diffusion became greater as water content increased, and
sPS-S1 has a proton diffusion coefficient higher than that of sPS-S3
under both mechanisms. Especially at 10 wt % water content, the
proton diffusion coefficient of sPS-S1 is almost three times larger
than that of sPS-S3. At 20 wt % water content, however, the
proton diffusion coefficient of sPS-S1 is only 1.3 times larger than
that of sPS-S3. Thus, the computational results strongly suggest
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hydrophilic domains also plays an important role in enhancing
proton conduction.
Here MNaOH and VNaOH are the molar concentration and volume (mL)
of the aqueous NaOH solution used in titration, Wdry (g) is the weight of
dry membrane.
We conducted comparative computational studies of sPS-S1
and sPS-S3 under hydrated conditions to investigate acidity effects
at a molecular level. By analyzing the solvation of sulfonate groups
with water in each system, we confirmed that the sulfonate groups
in sPS-S1 are better solvated than those in sPS-S3. Thus, the
acidity effect not only induces more effective dissociation of
protons (in the form of hydronium ion) from the sulfonate but
also develops better solvated sulfonate groups by surrounding
them with more water molecules. Because of the superacid
solvation effect, sPS-S1 has higher calculated water diffusion
coefficient and proton diffusion coefficient (under both vehicular
and hopping mechanisms) than sPS-S3, and their difference in
proton diffusion coefficient was greater at lower water content.
Overall, our studies on the effect of acidity strength on fuel cell
membrane properties suggest a clear relationship between acidity
strength and proton conductivity: the enhanced proton con-
ductivity of the more acidic ionomer is due to the cumulative
effects of better dissociation of the superacidic sulfonic acid to
hydronium ion and better solvation of the sulfonates through
increased water concentration. We believe that the results
reported herein are a significant step toward the development
of highly conductive hydrocarbon-based ionomers and offer an
alternative approach to morphology-driven high-performance
multiblock copolymer ionomers.
Water Uptake and Hydration Number (λ). Water uptake was
measured as a function of relative humidity at 30 ꢀC using a TA
Instruments Q5000SA dynamic vapor sorption analyzer. The relative
humidity steps and equilibration times were the same those used in the
conductivity experiments. Hydration number (λ) was calculated from
!
ꢀ
ꢁ
WRH ꢀ Wdry
18:01
1000
λ ¼
Wdry ꢁ IEC
where WRH is the sample mass at a given RH, Wdry is the dry mass of the
sample, and IEC is the ion exchange capacity of the sample in
milliequivalents of sulfonate group per gram of polymer.
Proton Conductivity. To measure the proton conductivity of sPS
ionomers, the membrane in sulfonic acid form was immersed in
deionized water for at least 24 h. The proton conductivity of the
membrane was measured using a four-electrode method with a BT-
512 membrane conductivity test system (BekkTech LLC). The proton
conductivity was measured by changing the relative humidity from 20 to
100% at 80 ꢀC. RH control started from 70%, stabilized for 2 h,
decreased to 20% at a rate of 10% RH/20 min, then increase to 100%
at a rate of 10% RH/20 min. The proton conductivity data was collected
from the cycle of 20% RH to 100% RH. The proton conductivity was
calculated according to the following equation:
L
σ ðmS=cmÞ ¼
R ꢁ W ꢁ T
’ EXPERIMENTAL SECTION
where L is the distance between the two inner platinum wires
(0.425 cm), R is the resistance of the membrane, and W and T are the
width and the thickness of the membrane in centimeters, respectively.
Membrane Morphology. Membranes in acid form were stained
by soaking in 0.5 M lead acetate solution at room temperature for 1 day,
then rinsed with deionized water and dried under vacuum at room
temperature overnight. The stained membranes were cut into small
pieces and embedded in Spurr’s epoxy resin and cured overnight at
70 ꢀC. The samples were sectioned to yield slices of 100 nm thickness
using Leica EM UC6 ultramicrotome and placed on copper grids.
Transmission electron microscopy images were taken by TECNAI-
F30 Supertwin TEM using an accelerating voltage of 300 kV. The phase
segregation of membranes was characterized by AFM. Acid form of
membranes were soaked in water for 12 h, dried at 60 ꢀC for 30 min, and
then exposed to ambient conditions for >6 h before the characterization.
A Veeco Metrology D3000 microscope with a Nanoscepe IIIa controller
with standard commercially available SFM 125 μm long silicon canti-
levers with a spring constant of about 40 N/m was used to obtain all
images. Identical operating conditions, i.e., cantilever drive amplitude
and set point, were employed for all aromatic membranes.
Materials and Methods. 4,40-Di-tert-butyl-2,20-dipyridyl (dtbpy),
chloro-1,5-cyclooctadiene iridium(I) dimer ([IrCl(COD)]2), tetrakis-
(triphenylphosphine)palladium(0) (Pd(PPh3)4), 4-dimethylaminopyr-
idine (DMAP), Na2S2O4, 3,5-dimethylphenol, and 4-bromobenzene-
sulfonyl chloride were reagent grade and used without further purification.
Bis(pinacolato)diboron (B2pin2) from Frontier Scientific Co., ICF2CF2I
from Oakwood Products, Inc., BrCF2CF2Br from SynQuest Laboratories,
Inc., Chlorine gas from Praxair Inc., CFC-113fromChemNetwere used as
received. Cyclooctane was dried using sodium and benzophenone,
distilled under vacuum, and stored in a nitrogen-filled glovebox. sPS
(Mw = 140.9 kg/mol with Mw/Mn = 2.90) was obtained from LG
Chemical Ltd., Daejeon, S. Korea and used as received. Anhydrous
tetrahydrofuran (THF) was obtained from EMD Chemicals and collected
from the container using a positive pressure of nitrogen.
1H, 19F, and 13C NMR spectra were obtained using a Varian NMR
spectrometer (400 MHz for 1H, 376 MHz for 19F, and 100 MHz for 13C)
at room temperature and chemical shifts were referenced to TMS (1H
and 13C) and CFCl3 (19F). GC/MS analysis was conducted using a
Shimadzu QP2010S equipped with a 30 m ꢁ 0.25 mm SHR-XLB GC
column and an EI ionization MS detector. FT-IR spectra were recorded
on a Shimadzu IR Prestige-21.
Synthesis of 3,5-Dimethylphenyl 2-(4-Bromophenoxy)tetrafluo-
roethanesulfonate Ester (S1). 3,5-Dimethylphenol (3.10 g, 25.4 mmol,
1.1 equiv) and CH2Cl2 (70 mL) were added to a 250 mL two-neck flask
filled with nitrogen and was cooled to 0 ꢀC. 2-(4-Bromophenoxy)-
tetrafluoroethanesulfonyl chloride (8.57 g, 23.1 mmol), which was
prepared from 4-bromophenol in 61% overall yield using a literature
method,35 and DMAP (3.38 g, 27.7 mmol, 1.2 equiv) were added in
sequence and the mixture was stirred at 0 ꢀC for 2 h and room
temperature for 12 h. The reaction mixture was diluted with CH2Cl2
(80 mL), washed with 2 M HCl (40 mL ꢁ 3), saturated NaHCO3
(40 mL) and brine (30 mL), and combined organic layer was dried over
MgSO4. After evaporation of solvent, resulting crude product was
purified by column chromatography (hexane/ethyl acetate=10:1) to
Ion Exchange Capacity (IEC). The calculated IECs of sPS
ionomers were estimated from the mol % of 3,5-dimethylphenol
protected sulfonated sPS (i.e., sPS-S1ꢀPh, sPS-S2ꢀPh, and sPS-S3
1
ꢀPh in Scheme S3 of the Supporting Information) in the H NMR
spectra. The experimental IECs of sPS ionomers were determined using
a titration method. Membranes were equilibrated in 2 M NaCl solution
at room temperature for 3 days before titration. The protons released
into the aqueous solution were titrated with 0.025 M NaOH solution
using phenolphthalein as an indicator. The experimental IEC values of
the sPS ionomer membranes were calculated according to the equation
below:
1
give 9.66 g of S1 as an yellowish oil (92% yield). H NMR (CDCl3):
IEC ðmequiv=gÞ ¼ MNaOH ꢁ VNaOH=Wdry
δ 7.52 (d, J = 8.8 Hz, 2H), 7.13 (d, J = 8.8 Hz, 2H), 6.99 (s, 1H), 6.92
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(s, 2H), 2.25 (s, 6H). 19F NMR (CDCl3): δ ꢀ81.4 (t, J = 4.3 Hz, OCF2),
ꢀ112.5 (t, J = 4.3 Hz, CF2SO3). 13C NMR (CDCl3): δ 150.0, 147.5,
28.6 Hz), 113.9 (tt, 1JCF = 298 Hz, 2JCF = 39.1 Hz), 21.4. GC/MS: 458,
456, 392, 192, 143, 121 (100%), 91, 77. HRMS (m/z) (CI, NH3): calcd
for C16H13O4BrF4S (M + NH4)+, 473.9992; found, 473.9996.
stirring bar were placed into a 30 mL vial, which was capped with a
Teflon-lined septum. The vial was removed from the glovebox and
placed in an oil bath at 150 ꢀC for 6 h. After cooling to room
temperature, the solution was diluted with chloroform (60 mL) and
filtered through a short plug of silica gel to remove the catalyst. The
filtrate was concentrated by a rotary evaporator to about 10 mL, and cold
methanol (100 mL) was added to precipitate polymer. The dissolution
and precipitation process was repeated one more time. The borylated
polymer was isolated as a white solid and dried under vacuum at 80 ꢀC
(1.04 g, 148% yield based on polymer weight). 1H NMR (benzene-d6):
δ 8.00 (Harom from C6H4ꢀBpin), 7.73 (Harom from C6H4ꢀBpin), 7.08
(Harom), 6.71 (Harom), 2.09 (CH of sPS backbone), 1.49 (CH2 of sPS
140.4, 133.1, 129.9, 123.8, 120.7, 119.2, 115.8 (tt, 1JCF = 277 Hz, 2JCF
=
Synthesis of 3,5-Dimethylphenyl 4-Bromobenzenesulfonate Ester
(S2). 4-Bromobenzenesulfonyl chloride (20.0 g, 78.3 mmol), 3,5-di-
methylphenol (9.56 g, 78.3 mmol), and DMAP (10.5 g, 86.1 mmol, 1.1
equiv) were placed in a 500 mL two-neck flask under nitrogen. CH2Cl2
(250 mL) was added, and the resulting mixture was stirred overnight at
room temperature. The reaction solution was washed with 2 M HCl
solution (50 mL ꢁ 2), saturated NaHCO3 solution (50 mL) and brine
(50 mL), dried over Na2SO4, and evaporated. Crude product was
purified by column chromatography (ethyl acetate/hexane: 1/10 to 3/
1
backbone), 1.16 (CH3 of Bpin). On the basis of analysis of H NMR
spectrum, an average of 40% of polymer repeating unit contains
Bpin group.
1
10) to give 26.4 g of S2 as an off-white solid product (94% yield). H
Preparation of 3,5-Dimethylphenyl-Protected Sulfonate of sPS-S1,
sPS-S2, and sPS-S3. sPS-Bpin (100 mg of 40 mol % Bpin-functionalized
sPS, 0.250 mmol Bpin) and K3PO4 (0.240 g, 1.13 mmol, 4.5 equiv) were
placed in a 25 mL vial. which was capped with a Teflon-lined septum.
Tetrakis(triphenylphosphine)palladium (11.6 mg, 0.01 mmol, 4 mol %)
and THF (4 mL) were added to the vial in a nitrogen-filled glovebox and
the vial was removed from the glovebox. Compound S1 (230 mg, 0.50
mmol, 2 equiv) and water (0.4 mL) were added using syringes. The
solution was stirred at 80 ꢀC for 12 h, cooled to room temperature,
diluted with chloroform (40 mL), and filtered through a short pad of
silica gel. The filtrate was concentrated to about 3 mL and cold methanol
(10 mL) was added to precipitate the polymer. Another cycle of
dissolution in chloroform and precipitation with cold methanol pro-
vided 140 mg of 3,5-dimethylphenol protected sulfonate form of sPS-S1
as a white solid. 1H NMR (benzene-d6): δ 6.62ꢀ7.20 (multiple Harom),
2.18 (CH of sPS backbone), 1.97 (CH3), 1.59 (CH2 of sPS backbone).
19F NMR (benzene-d6): δ ꢀ80.3 (s, 2F, ꢀOCF2), ꢀ111.9 (s, 2F,
CF2SO3ꢀ)
NMR (CDCl3): δ 7.69 (distorted doublet, J = 8.0 Hz, 2H), 7.58
(distorted doublet, J = 8.0 Hz, 2H), 6.89 (s, 2H), 6.61 (s, 1 H), 2.25
(s, 6H). 13C NMR (CDCl3): δ 149.3, 139.7, 134.7, 132.3, 129.9, 128.3,
128.9, 119.6, 21.1. GC/MS: 342, 340, 248, 221, 155, 121 (100%), 109,
91, 77, 65, 50, 41. HRMS (m/z) (CI, NH3): calcd for C14H13O3BrS
(M + NH4)+, 358.0107; found, 358.0115.
Preparation of 3,5-Dimethylphenyl 3-(4-Bromophenoxy)propane-
sulfonate Ester (S3). 4-Bromophenol (5.0 g, 28.9 mmol) and K2CO3
(12.0 g, 86.7 mmol, 3 equiv) were placed in a 100 mL two-neck
flask under nitrogen, and DMF (50 mL) and 1,3-propane sultone
(4.59 g, 37.6 mmol, 1.3 equiv) were added. The mixture was stirred at
110 ꢀC overnight and cooled to room temperature, and the resulting
precipitate was filtered. The filtered solid was stirred with water
(200 mL) for 1 h at room temperature, filtered and dried in air to give
5.83 g of potassium 3-(4-bromophenoxy)propanesulfonate as a white
solid (61% yield). 1H NMR (DMSO-d6): δ 7.42 (d, J = 8.8 Hz, 2H), 6.89
(d, J = 8.8 Hz, 2H), 4.04 (t, J = 6.6 Hz, 2H), 2.53 (t, J = 7.2 Hz, 2H), 1.98
(m, 2H). 13C NMR (DMSO-d6): δ 157.9, 132.1, 116.8, 111.7, 67.0, 47.8,
25.1. Potassium 3-(4-bromophenoxy)propanesulfonate (1.73 g, 5.46
mmol) was added to a 50 mL two-neck flask under nitrogen, and
acetonitrile (17 mL) and phosphorus oxychloride (4.18 g, 27.3 mmol, 5
equiv) were added subsequently. The reaction mixture was stirred at
85 ꢀC overnight, cooled to room temperature, poured into ice water
(∼60 g), and extracted with CH2Cl2 (30 mL ꢁ 3). Combined organic
layer was dried over Na2SO4, and evaporation of solvent gave 1.45 g of
3-(4-bromophenoxy)propanesulfonyl chloride as an off-white solid
The same procedure above with compound S2 produced sPS-S2 in
3,5-dimethylphenol protected sulfonate form. 1H NMR (benzene-d6):
δ 6.56ꢀ8.00 (multiple Harom), 2.03 (CH of sPS backbone), 1.93 (CH3),
1.55 (CH2 of sPS backbone).
The same procedure above with compound S3 produced sPS-S3 in
3,5-dimethylphenol protected sulfonate form. 1H NMR (benzene-d6):
δ 6.56ꢀ7.50 (multiple Harom), 3.53 (OCH2CH2CH2SO3), 3.09 (OCH2-
CH2CH2SO3), 2.12 (CH of sPS backbone and OCH2CH2CH2SO3),
1.97 (CH3), 1.52 (CH2 of sPS backbone).
1
(85% yield). H NMR (DMSO-d6): δ 7.42 (d, J = 9.2 Hz, 2H), 6.89
Preparation of the Sodium Sulfonate Form of sPS-S1, sPS-S2, and
sPS-S3. Above 3,5-dimethylphenol protected sulfonate form of sPS-S1
(100 mg of 40 mol % sulfonated sPS; 0.158 mmol of sulfonate) was
dissolved in dioxane (4 mL) with gentle heating and NaOH (50.6 mg,
1.26 mmol, 8 equiv) and H2O (40 μL) were added. The resulting
solution was stirred at 100 ꢀC for 4 h. After cooling to room temperature,
solvent was evaporated and residue was dissolved in methanol, filtered
through a short plug of silica gel. After concentration of the filtrate,
addition of H2O (20 mL) caused precipitation of polymer which was
filtered and washedwitha refluxing solution ofwater/methanol (3/1, v/v)
for 2 h. Drying under vacuum at 80 ꢀC for 12 h gave 87 mg of polymer
(d, J = 8.8 Hz, 2H), 4.04 (t, J = 6.4 Hz, 2H), 2.64 (t, J = 7.6 Hz, 2H), 2.00
(m, 2H). 13C NMR (DMSO-d6): δ 158.0, 132.2, 116.9, 111.9, 66.7, 48.0,
24.9. 3-(4-Bromophenoxy)propanesulfonyl chloride (1.44 g, 4.59
mmol) and 3,5-dimethylphenol (0.51 g, 4.17 mmol, 0.91 equiv) were
added to a 50 mL two-neck flask under nitrogen and CH2Cl2 (15 mL)
and DMAP (0.56 g, 4.59 mmol, 1 equiv) were added in sequence. The
resulting solution was stirred overnight at room temperature, diluted
with CH2Cl2 (50 mL), washed with 2 M HCl (20 mL ꢁ 2), and dried
over Na2SO4. After evaporation of solvent, crude product was purified
by column chromatography (ethyl acetate/hexane: 1/10 to 2/10) to
give 1.65 g of S3 as a white solid (99% yield). 1H NMR (CDCl3): δ 7.39
(d, J = 8.8 Hz, 2H), 6.94 (s, 1H), 6.88 (s, 2H), 6.77 (d, J = 8.8 Hz, 2H),
4.11 (t, J = 5.6 Hz, 2H), 3.47 (t, J = 7.6 Hz, 2H), 2.45 (m, 2H), 2.32 (s,
6H). 13C NMR (CDCl3): δ 157.6, 149.2, 140.2, 132.6, 129.2, 119.6,
116.4, 113.6, 65.5, 47.4, 24.1, 21.4. GC/MS: 400, 398, 227, 185, 174,
157, 134, 121 (100%), 105, 91, 77, 65, 41.
1
product. H NMR (DMSO-d6) δ: 6.80ꢀ8.10 (multiple Harom), 1.61
(CH of sPS backbone), 1.23 (CH2 of sPS backbone). 19F NMR
(DMSO-d6) δ: ꢀ80.4 (s, OCF2), ꢀ116.2(s, CF2SO3).
The above procedure was used for preparation of sodium salt form of
sPS-S2. The obtained polymer product was purified by stirring in hot
1
methanol for 2 h. Yield: 87% based on recovered polymer weight. H
Preparation of sPS-Bpin (40 mol %). In a nitrogen-filled glovebox,
sPS (700 mg, 6.73 mmol polystyrene repeating unit), B2pin2 (1.37 g,
5.38 mmol, 0.8 equiv), [IrCl(COD)]2 (54.2 mg, 3 mol % iridium based
on the amount of B2pin2), dtbpy (43.3 mg, 3 mol % based on the amount
of B2pin2), cyclooctane (4.30 g, 0.40 mol, 60 equiv), and a magnetic
NMR (DMSO-d6): δ 6.43ꢀ7.64 (multiple Harom), 1.63 (CH of sPS
backbone), 1.28 (CH2 of sPS backbone).
The above procedure was used for preparation of sodium salt form of
sPS-S3. The obtained polymer product was purified by stirring in hot
methanol for 2 h. Yield: 86% based on recovered polymer weight.
8467
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Macromolecules
ARTICLE
1H NMR (DMSO-d6): δ 6.30ꢀ7.50 (multiple Harom), 4.00 (OCH2-
CH2CH2SO3), 2.54 (OCH2CH2CH2SO3), 1.98 (OCH2CH2CH2SO3),
1.62 (CH of sPS backbone), 1.22 (CH2 of sPS backbone).
annealing (between 300 and 600 K) and volume annealing (between
densities of 0.5 to 1.1 times the expected density). This procedure aims to
help the system escape from various local minima and promote the
migration of species required for phase-segregation in heterogeneous
systems, whose detailed steps are described in the previous
publications.47ꢀ50 After finishing the annealing cycles, a 100 ps NVT
MD simulation and a subsequent 5 ns NPT MD simulation were
performed at 353.15 K to finalize the annealing procedure. Then, another
15 ns NPT simulation was performed at 353.15 K for data collection.
Acid Form Membrane Preparation of sPS-S1, sPS-S2, and sPS-S3.
The sodium sulfonate form of sPS ionomers (300 mg) was dissolved in
DMSO (3 mL) and cast on a glass plate. The film was dried at 40 ꢀC
under a positive air flow for 24 h and then at 80 ꢀC under vacuum for
12 h. The acid form membrane was obtained by immersing the
membrane in 1 M H2SO4 for 3 days (during which time the solution
was changed every day) at room temperature, followed by immersing in
deionized water for 1 day (water was changed several times). FT-IR
(cmꢀ1, film) (Figure S1 in Supporting Information) 3450 (OꢀH), 3025
(aromatic CꢀH), 2920 (alkyl CꢀH), 1600 and 1490 (aromatic CꢀH),
1198, 1172 (SdO), 962 (CꢀF).
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, synth-
b
esis schemes, and FT-IR, GC/MS and NMR spectra (1H, 19
F
and 13C). This material is available free of charge via the Internet
at http://pubs.acs.org.
’ COMPUTATIONAL MODELS AND METHODS
Using a full atomistic simulation method, we investigate the nano-
phase-segregated structure and transport properties of the hydrated
sulfonated syndiotactic polystyrene membrane with 10 and 20 wt %
water contents at 353.15 K.
’ AUTHOR INFORMATION
Corresponding Author
*(S.S.J.) Telephone: (404) 385-3356. Fax: (404) 894-9140. E-mail:
seungsoon.jang@mse.gatech.edu. (C.B.) Telephone: (702) 895-
3465. Fax: (702) 895-4072. E-mail: chulsung.bae@unlv.edu.
Force Field and Simulation Parameters. To perform molec-
ular dynamics (MD) simulations, we used the DREIDING55 force field,
which has been used for other fuel cell studies such as Nafion,47
Dendrion,48,49 and sulfonated poly(ether ether ketone)50 as well as for
various molecular systems such as hydrogels,56,57 liquidꢀliquid and
liquidꢀair interfaces,58,59 and molecular self-assembly.60,61 The F3C
force field was employed to describe the water molecules.62 These force
field parameters are described in the original paper and our previous
study on hydrated Nafion.46 The form of the potential energy used,
therefore, is
’ ACKNOWLEDGMENT
C.B. thanks the NSF (CAREER DMR-0747667) and Nevada
Renewable Energy Consortium for their generous support,
Frontier Scientific Co. for a gift of B2pin2, Dr. Longzhou Ma of
UNLV HRC for his help on running TEM, and Tae Soo Jo and
Lacie V. Brownell for conducting the preliminary work for this
study. The Los Alamos authors thank US DOE Fuel Cell
Technologies Program, Technology Development Manager Dr.
Nancy Garland, for financial support. Computational work in this
study was supported as a start-up from School of Materials
Science and Engineering of Georgia Institute of Technology.
Etotal¼ EvdW þ EQ þ Ebond þ Eangle þ Etorsion þ Einversion
ð6Þ
where Etotal, EvdW, EQ, Ebond, Eangle, Etorsion, and Einversion are total energies,
van der Waals, electrostatic, bond stretching, angle bending, torsion, and
inversion components, respectively. The individual atomic charges of
the copolymer were assigned by Mulliken population analysis with
B3LYP and 6-31G** basis set. The atomic charges of the water molecule
were from the F3C water model.61 The particleꢀparticle particleꢀmesh
(PPPM) method63 was used to calculate the electrostatic interactions.
The annealing MD and equilibrium MD simulations were performed
using the MD code LAMMPS (large-scale atomic/molecular massively
parallel simulator) from Plimpton at Sandia64 with modifications to
handle our force fields.47 The equations of motion were integrated using
the velocity Verlet algorithm65 with a time step of 1.0 fs. The Noseꢀ
Hoover temperature thermostat66,67 for the NVT and NPT MD
simulations used a damping relaxation time of 0.1 ps and a dimensionless
cell mass factor of 1.0.
Construction and Equilibration of Amorphous Membrane.
The simulated hydrated membrane systems consist of four chains of sPS
ionomers and water molecules with 10 and 20 wt % as summarized in
Table 2. The degree of polymerization and the degree of sulfonation were
set to 35 and 40, respectively. Thus, the number of sulfonate group per
chain is 14. The sulfonated units were selected randomly from 35
repeating units in the backbone and all of the sulfonic acid groups are
assumed to be ionized as assumed in the previous studies.47ꢀ49
The initial amorphous structures of hydrated sPS ionomers were
constructed using the Amorphous Builder of Cerius2.68 Since such initial
structures of polymeric materials may include unstable conformations,
they were equilibrated using the annealing procedure as used in the
previous studies of Nafion,47 Dendrion,48,49 and sulfonated poly(ether
ether ketone)50 membranes, which accelerates the attainment of equi-
librium by driving the system repeatedly through 5 cycles of thermal
’ REFERENCES
(1) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath,
J. E. Chem. Rev. 2004, 104, 4587–4612.
(2) Harrison, W. L.; Hickner, M. A.; Kim, Y. S.; McGrath, J. E. Fuel
Cells 2005, 5, 201–212.
(3) Alberti, G.; Casciola, M.; Massinelli, L.; Bauer, B. J. Membr. Sci.
2001, 185, 73–81.
(4) Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Chem. Mater. 2003,
15, 4896–4915.
(5) Kreuer, K.-D. Chem. Mater. 1996, 8, 610–641.
(6) Zawodzinski, T. A., Jr.; Neeman, M.; Sillerud, L. O.; Gottesfeld,
S. J. Phys. Chem. 1991, 95, 6040–6044.
(7) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104, 4535–4585.
(8) Kopitzke, R. W.; Linkous, C. A.; Anderson, H. R.; Nelson, G. L.
J. Electrochem. Soc. 2000, 147, 1677–1681.
(9) Ng., F.; Pꢀeron, J.; Jones, D. J.; Roziꢀere, J. J. Polym. Sci., Part A:
Polym. Chem. 2011, 49, 2107–2117.
(10) Liu, B.; Robertson, G. P.; Kim, D. ꢀS.; Guiver, M. D.; Hu, W.;
Jiang, Z. Macromolecules 2007, 40, 1934–1944.
(11) Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.;
Zawodzinski, T. A.; McGrath, J. E. Macromol. Symp. 2001, 175, 387–395.
(12) Li, N.; Shin, D. W.; Hwang, D. S.; Lee, Y. M.; Guiver, M. D.
Macromolecules 2010, 43, 9810–9820.
(13) Katzfuß, A.; Krajinovic, K.; Chromik, A.; Kerres, J. J. Polym. Sci.,
Part A: Polym. Chem. 2011, 49, 1919–1927.
(14) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.;
Watanabe, M. J. Am. Chem. Soc. 2006, 128, 1762–1769.
8468
dx.doi.org/10.1021/ma201759z |Macromolecules 2011, 44, 8458–8469

Macromolecules
ARTICLE
(15) Genies, C.; Mercier, R.; Sillion, B.; Cornet, N.; Gebel, G.;
Pineri, M. Polymer 2001, 42, 359–373.
(49) Jang, S. S.; Goddard, W. A., III J. Phys. Chem. C 2007, 111,
2759–2769.
(16) Chen, X.; Chen, K.; Chen, P.; Higa, M.; Okamoto, K. ꢀI.;
Hirano, T. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 905–915.
(17) Fujimoto, C. H.; Hickner, M. A.; Cornelius, C. J.; Loy, D. A.
Macromolecules 2005, 38, 5010–5016.
(18) Hickner, M. A.; Pivovar, B. S. Fuel Cells 2005, 5, 213–229.
(19) Einsla, M. L.; Kim, Y. S.; Hawley, M.; Lee, H.-S.; McGrath, J. E.;
Liu, B.; Guiver, M. D.; Pivovar, B. S. Chem. Mater. 2008, 20, 5636–5642.
(20) Elabd, Y. A.; Hickner, M. A. Macromolecules 2011, 44, 1–11.
(21) Peckham, T. J.; Holdcroft, S. Adv. Mater. 2010, 22, 4667–4690.
(22) Bae, B.; Yoda, T.; Miyatake, K.; Uchida, H.; Watanabe, M.
Angew. Chem., Int. Ed. 2010, 492, 317–320.
(50) Brunello, G.; Lee, S. G.; Jang, S. S.; Qi, Y. J. Renewable
Sustainable Energy 2009, 1, 033101–1ꢀ033010ꢀ14.
(51) Lill, M. A.; Helms, V. J. Chem. Phys. 2001, 115, 7985–7992.
(52) Lill, M. A.; Helms, V. J. Chem. Phys. 2001, 114, 1125–1132.
(53) Greeley, B. H.; Russo, T. V.; Mainz, D. T.; Friesner, R. A.;
Langlois, J. M.; Goddard, W. A., III; Donnelly, R. E., Jr.; Ringnalda, M. N.
J. Chem. Phys. 1994, 101, 4028–4041.
(54) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.;
Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100,
11775–11788.
(55) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III J. Phys. Chem.
(23) Tian, S.; Meng, Y.; Hay, A. S. J. Polym. Sci., Part A: Polym. Chem.
2009, 47, 4762–4773.
1990, 94, 8897–8909.
(56) Jang, S. S.; Goddard, W. A., III; Kalani, M. Y. S.; Myung, D.;
Frank, C. W. J. Phys. Chem. B 2007, 111, 14440.
(57) Jang, S. S.; Goddard, W. A., III; Kalani, Y. J. Phys. Chem. B 2007,
111, 1729–1737.
(58) Jang, S. S.; Goddard, W. A., III J. Phys. Chem. B 2006,
110, 7992–8001.
(59) Jang, S. S.; Lin, S.-T.; Maiti, P. K.; Blanco, M.; Goddard, W. A.,
III; Shuler, P.; Tang, Y. J. Phys. Chem. B 2004, 108, 12130–12140.
(60) Jang, S. S.; Jang, Y. H.; Kim, Y.-H.; Goddard, W. A., III; Choi,
J. W.; Heath, J. R.; Laursen, B. W.; Flood, A. H.; Stoddart, J. F.; Nørgaard,
K.; Bjørnholm, T. J. Am. Chem. Soc. 2005, 127, 14804–14816.
(61) Jang, S. S.; Jang, Y. H.; Kim, Y.-H.; Goddard, W. A., III; Flood,
A. H.; Laursen, B. W.; Tseng, H.-R.; Stoddart, J. F.; Jeppesen, J. O.; Choi,
J. W.; Steuerman, D. W.; DeIonno, E.; Heath, J. R. J. Am. Chem. Soc.
2005, 127, 1563–1575.
(24) Li, N.; Hwang, D. S.; Lee, S. Y.; Liu, Y. ꢀL.; Lee, Y. M.; Guiver,
M. D. Macromolecules 2011, 44, 4901–4910.
(25) Kim, Y. S.; Pivovar, B. S. Annu. Rev. Chem. Biomol. Eng. 2010, 1,
123–148.
(26) Olah, G. A.; Prakash, G. K. S.; Sommer, J. In Superacids: Wiley-
Interscience: New York, 1985; Chapter 1.
(27) Mikami, T.; Miyatake, K.; Watanabe, M. Appl. Mater. Interfaces
2010, 2, 1714–1721.
(28) Miyatake, K.; Shimura, T.; Mikami, T.; Watanabe, M. Chem.
Commun. 2009, 6403–6405.
(29) Yoshimura, K.; Iwasaki, K. Macromolecules 2009, 42, 9302–9306.
(30) Ghassemi, H.; Shiraldi, D. A.; Zawodzinski, T. A.; Hamrock, S.
Macromol. Chem. Phys. 2011, 212, 673–678.
(31) Li, H.; Jackson, A. B.; Kirk, N. J.; Mauritz, K. A.; Storey, R. F.
Macromolecules 2011, 44, 694–702.
(62) Levitt, M.; Hirshberg, M.; Sharon, R.; Laidig, K. E.; Daggett, V.
(32) Mikami, T.; Miyatake, K.; Watanabe, M. J. Polym. Sci., Part A:
J. Phys. Chem. B 1997, 101, 5051–5061.
Polym. Chem. 2011, 49, 452–464.
(33) Nakabayashi, K.; Higashihara, T.; Ueda, M. Macromolecules
2011, 44, 1603–1609.
(34) Xu, K.; Oh, H.; Hickner, M. A.; Wang, Q. Macromolecules 2011,
44, 4605–4609.
(63) Hockney, R. W.; Eastwood, J. W. In Computer Simulation using
Particles; McGraw-Hill International Book Co.: New York, 1981.
(64) Plimpton, S. J. Comput. Phys. 1995, 117, 1–19.
(65) Swope, W. C.; Andersen, H. C.; Berens, P. H.; Wilson, K. R.
J. Chem. Phys. 1982, 76, 637–649.
(35) Feiring, A. E.; Wonchoba, E. R. J. Fluorine Chem. 2000,
(66) Nose, S. J. Chem. Phys. 1984, 81, 511–519.
105, 129–135.
(67) Nose, S. Mol. Phys. 1984, 52, 255–268.
(36) Shin, J.; Jensen, S. M.; Ju, J.; Lee, S.; Xue, Z.; Noh, S. K.; Bae, C.
Macromolecules 2007, 40, 8600–8608.
(68) Accelrys_Inc. Cerius2Modeling Environment, Release 4.0; Accelrys
Inc.: San Diego, CA, 1999.
(37) Jo, T. S.; Kim, S. H.; Shin, J.; Bae, C. J. Am. Chem. Soc. 2009,
131, 1656–1657.
(38) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura,
N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263–14278.
(39) Chotana, G. A.; Rak, M. A.; Simth, M. R., III J. Am. Chem. Soc.
2005, 127, 10539–10544.
(40) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
(41) Iojoiu, C.; Marꢀechal, M.; Chabert, F.; Sanchez, J.-Y. Fuel Cells
2005, 5, 344–354.
(42) Lundberg, R. D.; Makowski, H. S. In Ions in Polymers; Eisenberg,
A., Ed.; Advances in Chemistry Series No. 187; American Chemical Society:
Washington DC, 1980; Chapter 2.
(43) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463.
(44) Guthrie, J. P. Can. J. Chem. 1978, 56, 2342–2354.
(45) The pKa values of these sulfonic acid groups are estimates
because accurate measurement of these strongly acidic groups in water is
experimentally difficult, but the strong electron-withdrawing effect of
fluoroalkyl groups can be unambiguously found in more weakly acidic
acids, such as substituted carboxylic acids: CF3CO2H (pKa = 0.5) >
C6H5ꢀCO2H (pKa = 4.2) > CH3CO2H (pKa = 4.8) from: McMurry, J.
In Organic Chemistry, 7th ed.; Thomson Brooks/Cole: Belmont, CA,
2008; Appendix B.
(46) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinki, T. A.; McGrath,
J. E. J. Membr. Sci. 2002, 197, 231–242.
(47) Jang, S. S.; Molinero, V.; Cagin, T.; Goddard, W. A., III J. Phys.
Chem. B 2004, 108, 3149–3157.
(48) Jang, S. S.; Lin, S.-T.; Cagin, T.; Molinero, V.; Goddard, W. A.,
III J. Phys. Chem. B 2005, 109, 10154–10167.
8469
dx.doi.org/10.1021/ma201759z |Macromolecules 2011, 44, 8458–8469