Fluorene-based poly(arylene ether sulfone)s containing clustered flexible pendant sulfonic acids as proton exchange membranes

Article abstract of DOI:10.1021/ma2015968

A new bisphenol monomer, 9,9-bis(3,5-dimethoxy-4-hydroxyphenyl) fluorene, was synthesized and polymerized to form fluorene-based poly(arylene ether sulfone) copolymers containing tetra-methoxy groups (MPAES). After converting the methoxy group to the reac

Full text of DOI:10.1021/ma2015968

ARTICLE
pubs.acs.org/Macromolecules
Fluorene-Based Poly(arylene ether sulfone)s Containing Clustered
Flexible Pendant Sulfonic Acids as Proton Exchange Membranes†
||
Chenyi Wang,‡, Nanwen Li,‡ Dong Won Shin,§ So Young Lee,§ Na Rae Kang,‡ Young Moo Lee,*,‡,§ and
Michael D. Guiver*,‡,^
‡WCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea
§School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea
^Institute for Chemical Process and Environmental Technology, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
School of Materials Science and Engineering, Changzhou University, Changzhou, 213164, China
ABSTRACT: A new bisphenol monomer, 9,9-bis(3,5-dimeth-
oxy-4-hydroxyphenyl) fluorene, was synthesized and polymer-
ized to form fluorene-based poly(arylene ether sulfone) copoly-
mers containing tetra-methoxy groups (MPAES). After converting
the methoxy group to the reactive hydroxyl group, the res-
pective side-chain type sulfonated copolymers (SPAES) were
obtained by sulfobutylation. The polymers were characterized
by 1H NMR, thermogravimetric analysis (TGA), water uptake,
and proton and methanol transport for fuel cell applications.
These SPAES copolymers had good overall properties as
polymer electrolyte membrane (PEM) materials, having high proton conductivity in the range of 0.061ꢀ0.209 and 0.146ꢀ
0.365 S/cm at 30 and 80 ꢀC (under hydrated conditions), respectively. SPAES-39 (IEC = 1.93 mequiv/g) showed higher or
comparable proton conductivity than that of Nafion 117 at 50ꢀ95% RH (relative humidity). The methanol permeabilities of
these membranes were in the range of 3.22 to 13.1 ꢁ 10ꢀ7 cm2/s, which is lower than Nafion (15.5 ꢁ 10ꢀ7 cm2/s). In comparison
with some reported sulfonated poly(arylene ether sulfone)s containing pendent sulfophenyl groups, the present fluorene-based
SPAES containing clustered flexible pendent aliphatic sulfonic acid groups displayed better properties, such as lower water uptake
and higher proton conductivities. A combination of high proton conductivities, low water uptake, and low methanol permeabilities
for selected SPAES indicates that they are good candidate proton exchange membrane materials for evaluation in fuel cell
applications.
’ INTRODUCTION
ion exchange capacity (IEC) and high water content, which
consequently lead to large dimensional variations and poor
mechanical properties. The dimensional stability and proton
conductivity of aromatic PEMs are crucial issues that require
improvement through careful structural design.
Polymer electrolyte membrane fuel cells (PEMFC) have
attracted considerable attention as candidates for alternative
power sources due to their high power density, good energy
conversion efficiency, and zero emissions levels.1ꢀ4 Perfluoro-
sulfonic acid (PFSA) polymers such as Nafion (DuPont) are the
most established and state-of-the-art polymer electrolyte mem-
branes (PEMs) for PEMFC because of their excellent chemical
stability and high proton conductivity, but their limited operating
temperature range up to 80 ꢀC, high methanol/gas diffusion,
environmental recyclability, and high cost are perceived as signi-
ficant disadvantages. These challenges have driven the inves-
tigation of aromatic hydrocarbon polymers as alternative PEM
materials.5ꢀ7 The most widely reported aromatic PEMs include
sulfonated derivatives of poly(phenylene)s,8,9 poly(arylene ether
ketone)s,10ꢀ12 poly(arylene ether sulfone)s,13,14 poly(arylene
sulfide sulfone)s,15ꢀ17 poly(arylene ether)s,18,19 and poly-
imides.20ꢀ25 Generally, the sulfonic acid groups in these poly-
mers are located on the main chain, and the rigid polyaromatic
backbone prevents continuous ionic clustering from occurring
to form distinct phase-separated structures.26 As a result, these
sulfonated polymers only attain suitable conductivities at high
A viable strategy to improve the properties and performance of
aromatic PEMs is to design polymer architecture that distinctly
separates the hydrophilic sulfonic acid groups from the hydro-
phobic polymer main chain by locating the sulfonic acid groups
on flexible side chains, which may induce improved nanophase
separation between hydrophilic and hydrophobic domains.27
Nafion is a statistical copolymer comprising a highly hydropho-
bic perfluorinated backbone that contains a number of short,
flexible pendant side chains with single strongly hydrophilic
sulfonic acid groups. The nanophase-separated structure of
Nafion may be responsible for its excellent thermal, mechanical,
and electrochemical properties. Currently, several research
groups are focusing their work on the development of side-chain
Received: July 12, 2011
Revised:
August 9, 2011
Published: August 25, 2011
r
2011 American Chemical Society
7296
dx.doi.org/10.1021/ma2015968 Macromolecules 2011, 44, 7296–7306
|

Macromolecules
ARTICLE
type sulfonated hydrocarbon-based polymers. Jannasch and
co-workers reported a sulfophenoxybenzoyl polysulfone and sul-
fonaphthaoxybenzoyl polysulfone that was prepared by attaching
pendant sulfonated aromatic side chains to polysulfone, showing
proton conductivities of 0.11ꢀ0.32 S/cm at 120 ꢀC.28 Jiang
and co-workers,29 Na and co-workers,30,31 and Zhang and co-
workers32 reported sulfonated aromatic polymers containing one
or two pendant sulfoalkyl groups in each hydrophilic unit. The
highest conductivity of 0.179 S/cm was obtained for these
copolymers (IEC = 1.82 mequiv/g) under a hydrated state at
80 ꢀC, which is higher than that of Nafion 117 (0.146 S/cm).31
Okamoto and co-workers33 and Watanabe and co-workers34
reported two types of SPIs bearing a pendant sulfonic acid group
on an aliphatic side chain. Compared with main-chain type SPIs,
these side-chain type SPIs exhibited much improved hydrolytic
stability and high conductivities. In our previous work, we
reported a series of pendant or comb type poly(arylene ether
sulfone)s by attaching pendant sulfonated aromatic side chains to
the backbone.35ꢀ37 The results also indicated that these side-
chain type sulfonated polymers displayed advantageous proton
conductivities with relatively low water content.
Another approach to enhance PEM performance can be
realized by introducing locally and densely sulfonated structures
into the aromatic polymer backbone.38ꢀ41 The large difference in
polarity between the locally and densely sulfonated units and
hydrophobic units of the polymers results in the formation of
well-defined nanophase-separated structures and can induce
efficient proton conduction. Hay and co-workers reported the
synthesis of linear poly(sulfide ketone)s and branched poly-
(ether ketone)s bearing clusters of six sulfonic acid moieties
situated on the chain ends, by regioselective postsulfonation.38,39
These ionomers possessed good microphase-separated morphol-
ogy and displayed relatively high proton conductivity (0.091 S/cm
at room temperature with 100% RH (relative humidity)) at very
low IEC (1.09 mequiv/g). However, it is difficult to further
increase their IEC values and proton conductivity based on this
methodology because the sulfonated groups are located only on
the chain ends. Ueda and co-workers reported locally sulfonated
poly(ether sulfone)s with 10 sulfonic acid groups in each repeat-
ing unit by postsulfonation with sulfuric acid.40,41 The sulfonated
polymers (IEC = 1.96ꢀ2.38 mequiv/g) exhibited good proton
conductivity under low humidity conditions, which is com-
parable to the proton conductivity of Nafion 117 over a wide
range of RH. Densely sulfonated segmented poly(arylene ether
sulfone)s showed nanophase separation of hydrophilic and
hydrophobic blocks.42
’ EXPERIMENTAL SECTION
Materials. 2,6-Dimethoxyphenol, 9-fluorenone, mercaptopropionic
acid (MPA), 4,40-(hexafluoroisopropylidene)diphenol (6F-BPA), boron
tribromide (BBr3), 1,4-butanesultone, and bis(4-fluorophenyl)sulfone
were purchased from Sigma-Aldrich. N-Methyl-2-pyrrolidone (NMP),
dimethyl sulfoxide (DMSO), and all other solvents and reagents were
reagent grade and were used as received.
9,9-Bis(3,5-dimethoxy-4-hydroxyphenyl)fluorene (DMHF).
To a 250 mL four-necked round-bottom flask, equipped with a reflux
condenser, magnetic stirrer, nitrogen inlet, thermometer, and a drop
funnel, 7.20 g (0.04 mol) of 9-fluorenone, 15.42 g (0.1 mol) of 2,6-
dimethoxyphenol, 0.1 mL of MPA, and 10 mL of toluene were
introduced. The reaction mixture was stirred under nitrogen atmosphere
at 30 ꢀC for 30 min. A portion of 1.5 mL of 98 wt % H2SO4 was then
added dropwise in 10 min, and the temperature was thereafter raised to
55 ꢀC for 4ꢀ6 h with stirring. When the reaction mixture became solid, it
was cooled down and transferred into cold water. Light yellow solid
product was obtained by filtration. Crude product was recrystallized
from toluene twice to afford 10.54 g (yield: 56%) of pure white crystal-
line9,9-bis(3,5-dimethoxy-4-hydroxyphenyl)fluorene, mp: 186ꢀ187 ꢀC.
1H NMR (300 MHz, DMSO-d6; ppm): 8.34 (s, 2H, ꢀOH), 7.90 (d, J =
7.2, 2H, Ha), 7.50 (d, J = 7.2, 2H, Hd), 7.40 (t, J = 7.5, 2H, Hb), 7.30 (t, J =
7.5, 2H, Hc), 6.31 (s, 4H, He), 3.53 (s, 12H, ꢀOCH3).
Synthesis of Poly(arylene ether sulfone)s Containing
Methoxy Groups (MPAES-xx). A typical synthetic procedure,
illustrated by the preparation of MPAES-39 copolymer (xx: DMHF/
6F-BPA = 39:61), is described as follows. Samples of 1.101 g (2.34
mmol) of DMHF, 1.231 g (3.66 mmol) of 6F-BPA, 1.526 g (6 mmol) of
bis(4-fluorophenyl)sulfone, 1.076 g (7.8 mmol) of K2CO3, 16 mL of
NMP, and 8 mL of toluene were added into a 100 mL three-neck flask
equipped with a mechanical stirrer, a DeanꢀStark trap, and a nitrogen
inlet. The solution was allowed to reflux at 140 ꢀC, while the water was
azeotropically removed from the reaction mixture. After 4 h, the toluene
was removed from the reaction by slowly increasing the temperature
to 155 ꢀC, and then the reaction was allowed to continue for another
5ꢀ8 h. After the reaction, 5 mL of NMP was added to the mixture to
reduce the solution viscosity. The solution was poured into 500 mL of
deionized water with vigorous stirring. The resulting fibrous copolymer
was washed with deionized water and hot methanol several times and
dried at 100 ꢀC under vacuum for 24 h.
Conversion of a Methoxy Group (MPAES-xx) to a Hydroxyl
Group (HPAES-xx). A sample of 2.0 g of MPAES-39 was dissolved
into 50 mL of CH3Cl in a 100 mL three-neck flask equipped with a
mechanical stirrer and a nitrogen inlet. BBr3 (2 mL) was mixed with
CH3Cl (20 mL), and the resulting solution was added dropwise to the
MPAES-39 solution at 0 ꢀC (ice bath). After 6 h, the resulting copolymer
(HPAES-39) was filtered, washed with boiling water, recovered, and
then dried under vacuum at 100 ꢀC for 24 h.
Over the past few years, much research focus has been on
fluorene-based sulfonated poly(arylene ether)s.43ꢀ49 It has been
demonstrated that fluorene-based PEMs show some attractive
properties, not only for chemical, thermal, and mechanical
stability, but also good proton conductivities and fuel cell per-
formance over several thousand hours.46 Herein, we introduce a
new fluorene-based bisphenol monomer and derived sulfonated
poly(arylene ether sulfone) copolymers containing clusters of
four butylsulfonic acid units per repeat unit along the main
chain. To the best of our knowledge, this is the first reported
example of fluorene-based PEMs containing locally and densely
populated flexible butylsulfonic acid pendant units. The syn-
thesis and properties of this new series of PEMs are investi-
gated and compared with a previously reported side-chain type
poly(arylene ether sulfone)s containing pendant sulfophenyl
groups.
Preparation of Sulfonated Copolymer (SPAES-xx). Samples
of 2.0 g of HPAES-39 and 0.60 g of NaOH were dissolved into 30 mL of
DMSO at room temperature and stirred for 10 min under nitrogen
atmosphere. Then 2 mL of 1,4-butanesultone was added, and the reac-
tion was heated to 100 ꢀC for another 6 h. The viscous solution was
precipitated into 500 mL of isopropanol and washed with boiling water
several times before being dried under vacuum at 100 ꢀC for 24 h.
Membrane Preparation. The dried sulfonated copolymers were
readily dissolved as 10ꢀ15 wt % solutions in DMSO at 60 ꢀC. The
solutions were filtered and cast onto glass plates and left to dry and then
treated in vacuum at 60 ꢀC for 20 h. The membranes were immersed
into 2.0 M H2SO4 solution for 24 h and then washed in deionized water
for another 24 h to obtain the H+ form membranes.
Measurements. 1H NMR spectra were measured on a 300 MHz
Bruker AV 300 spectrometer using DMSO-d6 or CDCl3 as solvent. The
7297
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
inherent viscosities were determined on 0.5 g dLꢀ1 concentration of
polymer in NMP with an Ubbelohde capillary viscometer at 30 ( 0.1 ꢀC.
The thermogravimetric analyses (TGA) were obtained in nitrogen with
a Perkin-Elmer TGA-2 thermogravimetric analyzer at a heating rate
of 10 ꢀC/min. The gel permeation chromatographic (GPC) analysis
was carried out with Tosoh HLC-8320 instrument (tetrahydrofuran as
eluent and polystyrene as standard).
Scheme 1. Synthesis of the Monomer DMHF
The oxidative stability of the membranes was tested by immersing the
films into Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at
80 ꢀC. The retained weights (RW) of membranes after treatment in
Fenton's reagent for 1 h, and the dissolution time (t) of polymer mem-
branes into the reagent, were used to evaluate the comparative oxidative
resistance.
The proton conductivity (σ, S/cm) of each membrane coupon (size:
1 cm ꢁ 4 cm) was obtained using σ = d/LsWsR (d: distance between
reference electrodes, and Ls and Ws are the thickness and width of the
membrane, respectively). The resistance value (R) was measured over
the frequency range from 100 mHz to 100 kHz by four-point probe
alternating current (ac) impedance spectroscopy using an electrode
system connected with an impedance/gain-phase analyzer (Solartron
1260) and an electrochemical interface (Solartron 1287, Farnborough,
Hampshire, UK). The membranes were sandwiched between two pairs
of gold-plate electrodes. The membranes and the electrodes were set in a
Teflon cell, and the distance between the reference electrodes was
1.0 cm. Conductivity measurements under fully hydrated conditions
were carried out in a thermo-controlled chamber with the cell immersed
in liquid water.
The methanol permeability was determined by using a cell consisting
of two-half-cells separated by the membrane, which was fixed between
two rubber rings. Methanol (2 M) was placed on one side of the diffusion
cell, and water was placed on the other side. Magnetic stirrers were used
on each compartment to ensure uniformity. The concentration of the
methanol was measured by using a Shimadzu GC-1020A series gas
chromatograph. Peak areas were converted into methanol concentration
with a calibration curve. The methanol permeability was calculated by
the following equation:
Figure 1. 1H NMR spectrum of bisphenol monomer DMHF.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of the Monomers and
Polymers. The monomer DMHF was synthesized by reacting
9-fluorenone with 2,6-dimethoxyphenol, by a phenol condensa-
tion reaction catalyzed by MPA and sulfuric acid. Scheme 1
shows the synthesis of monomer DMHF; the structure was
confirmed by 1H NMR spectroscopy in DMSO-d6. As shown in
Figure 1, the ꢀOH signal appeared at 8.34 ppm, and the signals
at 3.53 ppm correspond to ꢀOCH3. The clean spectrum of
aromatic protons Ha to He confirms the structure of the new
monomer DMHF.
Poly(arylene ether sulfone) copolymers containing methoxy
groups (MPAES-xx, xx: mole ratio (%) of DMHF) were syn-
thesized by polycondensation using various feed ratios of
DMHF/HFDP, resulting in copolymers with different molar
percentages of pendant groups, as shown in Scheme 2. The
polymerization reactions proceeded smoothly to high molecular
weight (Mn > 50 000 g/mol), and no cross-linking was evident
when the temperature was controlled by an oil bath (less than
165 ꢀC) and the reaction time was less than 10 h, because the
methoxy groups are not reactive under these conditions. Table 1
shows the molecular weight, polydispersity index, and inherent
viscosity of MPAES-xx copolymers.
The conversion of the ꢀOCH3 to reactive ꢀOH for grafting
was conducted using BBr3 in chloroform. Although the MPAES-
xx copolymers were readily soluble in chloroform, the resulting
HPAES-xx copolymers (containing ꢀOH) were insoluble due to
the polar nature of the ꢀOH group. The conversion of MPAES-
xx to HPAES-xx copolymer (Scheme 2) resulted in precipitation
from chloroform. Comparative 1H NMR spectra of the MPAES-
xx (ꢀOCH3) and HPAES-xx copolymers (ꢀOH) confirmed
that complete demethylation occurred; ꢀOCH3 proton signals
A DK
CBðtÞ ¼
CA ðt ꢀ t0Þ
3
L
ð1Þ
3
3
VB
where CA and CB are the methanol concentrations of feed side and
permeated through the membrane, respectively. A, L, and VB are the
effective area, membrane thickness, and the liquid volume of permeate
compartment, respectively. DK is defined as the methanol permeability.
t0 is the time lag.
Characterization Methods. The membrane density was mea-
sured from a known membrane dimension and weight after drying at
100 ꢀC for 24 h. Water uptake was measured after drying the membrane
in acid form at 100 ꢀC under vacuum overnight. The dried membrane
was immersed in water at 30 ꢀC and periodically weighed on an
analytical balance until a constant water uptake weight was obtained.
Then, the volume-based water uptake (WU) was obtained. A volume-
based IEC (IECv) was obtained by multiplying the membrane density by
the IECw values which were estimated from the copolymer structure.
This calculation resulted in IECv (dry) based on the dry membrane
density. An IECv (wet) was then calculated based on membrane water
uptake.35,50 For transmission electron microscopy (TEM) observations,
the membranes were stained with lead ions by ion exchange of the
sulfonic acid groups in 0.5 M lead acetate aqueous solution, rinsed with
deionized water, and dried in vacuum oven for 12 h. The stained
membranes were embedded in epoxy resin, sectioned to 70 nm thick-
ness with a RMCMTX Ultra microtome, and placed on copper grids.
Electron micrographs were taken with a Carl Zeiss LIBRA 120 energy-
filtering transmission electron microscope using an accelerating voltage
of 120 kV.
7298
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
Scheme 2. Synthesis of Copolymers MPAES-xx, HPAES-xx, and SPAES-xx
Table 1. Molecular Weights and Inherent Viscosities of
MPAES-xx Copolymers
structures into the aromatic polymer backbone. Hvilsted and co-
workers recently demonstrated that the parent polymers contain-
ing hydroxyl groups in the para-position of the aromatic ring could
be sulfopropylated through a nucleophilic ring-opening reaction
with 1,3-propanesultone using NaOH.51 Na and co-workers also
prepared a series of sulfonated poly(arylene ether ketone)s by the
same method through hydroxyl groups in the ortho-positon of the
aromatic ring grafting and with 1,4-butanesultone using NaOH.31
In the present work, we applied this synthetic method. Scheme 2
shows the grafting reaction of sulfobutyl groups onto HPAES-xx
by a nucleophilic ring-opening reaction with 1,4-butanesultone
using NaOH. A series of polymers with different butylsulfonate
contents were obtained by adjusting the ꢀOH content in HPAES-
xx copolymers, which were reacted with excess 1,4-butanesultone
and NaOH. The sulfonated polymer series were readily soluble in
DMSO and NMP and afforded flexible and transparent films by
polymer solution casting. The chemical structures of SPAES-xx
were confirmed by 1H NMR spectra (Figure 3). As expected, the
ꢀOH proton signal at 9.43 ppm disappeared, while the signals for
the four sulfobutyl methylene groups (Hj, Hk, Hl, and Hm)
appeared at lower frequencies. Interestingly, the proton of He
appeared as a broad multiplet, because the steric hindrance of
the bulky sulfobutyl groups prevents rotational freedom of the
benzene ring, which further supports the formation of SPAES-xx
bearing sulfobutyl groups.
copolymers Mna (104 g/mol) Mw (104 g/mol) PDI ηinhb (dL/g)
MPAES-25
MPAES-30
MPAES-33
MPAES-36
MPAES-39
6.12
7.01
7.28
5.43
6.83
15.97
19.49
19.65
11.08
15.09
2.61
2.78
2.70
2.04
2.21
0.64
0.71
0.73
0.58
0.66
a Measured at 30 ꢀC using THF as a solvent and polystyrene as a
standard. b 0.5 g/dL in NMP at 30 ꢀC.
at 3.48 ppm disappeared, and ꢀOH proton signals appeared at
9.43 ppm, as shown in Figure 2.
A variety of side-chain type PEMs containing one or two
pendant sulfoalkyl groups in each hydrophilic unit have been
prepared by the direct copolymerization method29,33,34 or by
chemically grafting the pendants onto polymers.30ꢀ32 Compared
to chemical grafting, the direct copolymerization method allows
close control of sulfonation content and the polymer structure.
However, the synthesis and purification of sulfonated monomers
may be complicated and difficult, especially for those containing a
high content of sulfonic groups. The chemical grafting method
avoids the need to prepare sulfonated monomers and offers an
easier route to introduce locally and densely populated sulfonated
7299
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
Table 2. Thermal and Mechanical Properties
Td5% tensile strength Young's modulus elongation at
copolymers (ꢀC)a at break (MPa)
(GPa)
break (%)
SPAES-25
SPAES-30
SPAES-33
SPAES-36
SPAES-39
292
250
250
242
237
52.8 (58.8)b
49.5 (61.3)
48.7 (53.4)
49.4 (50.1)
36.9 (43.6)
1.23 (1.92)
1.22 (1.88)
1.12 (1.64)
0.99 (1.48)
0.83 (1.24)
34.1 (9.3)
24.3 (9.7)
27.4 (12.0)
18.2 (14.3)
16.7 (12.2)
a 5% weight loss temperature in N2 gas (acid form membrane). b Dry
sample values in parentheses.
Figure 2. Comparative 1H NMR spectra of the MPAES-39 (ꢀOCH3)
and HPAES-39 (ꢀOH) copolymers.
Figure 4. TGA curves of SPAES-xx (acid form) membranes from a
measurement run at 10 ꢀC/min in N2.
properties with tensile moduli of 1.24ꢀ1.92 GPa, elongations
at break of 9.3ꢀ14.3%, and tensile strength at break of 43.6ꢀ
61.3 MPa. These results indicate that the side-chain type SPAES
membranes were sufficiently tough and ductile for potential use
as PEM materials in a fuel cell.
IEC, Water Uptake, Swelling Ratio, and Oxidative Stability.
IEC represents the amount of exchangeable protons in ionomer
membranes. The IEC values from titration tests were in the range
1.36ꢀ1.93 mequiv/g, which were close to the theoretical values,
indicating close to quantitative reaction yields for methoxy con-
version and sulfobutylation.
Water uptake (weight- and volume-based) of PEMs is an
important parameter for IEC, proton conductivity, dimensional
stability, mechanical strength, and membrane-electrode com-
patibility. Although weight-based IECs (IECw) are most often
reported, a more realistic comparison of water uptake among
different membranes is achieved by using volumetric IECs (IECv,
mequiv/cm3), which is defined as the molar concentration of
sulfonic acid groups per unit volume containing absorbed water.
Table 3 compares the density, IEC, and water uptake (weight-
and volume-based) of the copolymer membranes and Nafion.
The water uptake of SPAES-xx increased with IECw and IECv
(dry), due to the increased hydrophilicity, as shown in Figure 5a,
b. The volume-based IECv data in Figure 5b shows a very similar
trend as the weight-based IECw data in Figure 5a. The highest
water uptake (wt %) and water uptake (vol %) was 70%
and 111% at 80 ꢀC for the highest IECw membrane SPAES-39
(IECw = 1.93), respectively, which was nearly twice that of
Figure 3. 1H NMR spectrum of SPAES-39.
Thermal and Mechanical Properties. The thermal stabilities
of the polymers were investigated by TGA, and the 5% weight
loss temperatures are listed in Table 2. Figure 4 shows the TGA
curves of SPAES-xx in the sulfonic acid form, which appear to
have three distinct degradation steps. The initial degradation step
observed at around 220ꢀ330 ꢀC is likely associated with the
thermal degradation of the (sulfo)butoxy groups, while the
second and third steps starting at about 370 ꢀC correspond to
the remainder of the polymer.
The mechanical properties of the membranes are listed in
Table 2. The SPAES membranes in 40% RH and 80 ꢀC had
tensile moduli in the range of 0.83ꢀ1.23 GPa, elongations at
break of 16.7ꢀ34.1%, and tensile strength at break of 36.9ꢀ52.8
MPa. In the dry state, the samples also showed good mechanical
7300
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
Table 3. IEC, Density, and Water Uptake of the SPAES-xx and Nafion Membranes
water uptake
IECw (mequiv/g)
IECvb (mequiv/cm3)
wt %c
vol %d
expta
dry
wet
30 ꢀC
80 ꢀC
30 ꢀC
80 ꢀC
copolymers
density (g/cm3)
calcd
SPAES-25
SPAES-30
SPAES-33
SPAES-36
SPAES-39
Nafion 117
1.42
1.63
1.75
1.86
1.97
1.36
1.61
1.70
1.86
1.93
0.90
1.46
1.54
1.56
1.57
1.58
1.98
1.99
2.48
2.65
2.92
3.05
1.78
1.81
2.00
2.02
2.08
2.06
1.30
6.7
15.4
20.2
25.6
30.5
18.5
16.5
30.4
35.8
46.8
70.3
30.6
9.8
23.7
31.5
40.2
48.2
24.1
46.8
55.8
73.5
111.1
60.6
36.6
c
a Determined by acidꢀbase titration. b Based on volume of dry and/or wet membranes (IECv(wet) = IECv(dry)/(1 + 0.01WU)). WU (wt %) =
(Wwet ꢀ Wdry)/Wdry ꢁ 100. d WU (vol %) = ((Wwet ꢀ Wdry)/δw)/(Wdry/δm) ꢁ 100. (Wwet and Wdry are the weights of the wet and dry membranes,
respectively; δw is the density of water (L g/cm3), and δm is the membrane density in the dry state.)
Figure 5. Water uptake dependence of IEC (IECw), IECv (dry), and IECv (wet) values of SPAES-xx membranes.
Nafion membrane. The water uptake of the SPAES-xx series
increased quasilinearly up to SPAES-36 (IECw = 1.86) and then
dramatically increased at SPAES-39, similar to that reported for
comb-shaped sulfonated PEMs in the literature and related to a
percolation threshold.37
The IECv (wet) reflects the concentration of ions within the
polymer matrix under hydrated conditions, without distinguish-
ing between those protons that are mostly associated with the
sulfonic acid groups and those that are fully dissociated. The IECv
(wet) of SPAES-xx increased from 1.81 to 2.08 mequiv/cm3 with
7301
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
the corresponding increase of IECw from 1.36 to 1.86 mequiv/g
at 30 ꢀC. However, the IECv (wet) of SPAES-39 was lower than
that of SPAES-36, because the hydration of SPAES-39 resulted in
excessive swelling and dilution of the ion concentration after
equilibration with water. This is observed in Figure 5c whereby
the slope of the 30 and 80 ꢀC curves assumes a reverse direction
due to high water uptake (vol %) and reduced IECv (wet).
The IECv is plotted as a function of temperature in Figure 6.
Similar to Nafion, the IECv values decrease with increasing
temperature due to increased water volume within the polymer
matrix. These side-chain type membranes displayed higher IECv
than that of Nafion membranes over the temperature range
investigated. The SPAES-25 membrane showed approximately
the same trend in IECv values as Nafion, since the differences
in gravimetric IEC were counterbalanced by the differences in
density (1.98 g/cm3 for Nafion and 1.46 g/cm3 for SPAES-25).
The IECv (wet) of SPAES-39 membrane with the highest IECw
decreased sharply at elevated temperatures, to values lower than
others at 80 ꢀC, which indicated that this membrane underwent
excessive swelling in water.
Figure 7 compares the dimensional swelling ratio of SPAES-xx
and Nafion in the through-plane and in-plane direction at 80 ꢀC.
Besides SPAES-39, all of the SPAES PEMs exhibited a lower
dimensional swelling ratio than that of Nafion (Table 4). The
side-chain type SPAES-xx membranes showed generally a
lower swelling ratio than many main-chain type sulfonated
poly(arylene ether)s with similar IEC values.10,11,49 Compared
with main-chain type SPAES, the side-chain type SPAES having
sulfonic acid units on long flexible side chains may be more
effective in suppressing the swelling behavior of the polymer
films by inducing hydrophobic and hydrophilic domain phase
separation. In addition, the fluorene-based SPAES-xx displayed
mildly anisotropic membrane swelling, in which the dimensional
change was larger in thickness direction than in plane; all of the
PEMs except that with the highest IEC had lower dimensional
swelling than Nafion 117.
Figure 6. Volumetric IECv of SPAES-xx and Nafion 117 in water as a
function of temperature.
The oxidative stability of the polymers was evaluated in
Fenton's reagent at 80 ꢀC. This method is regarded as one of
the standard tests to gauge relative oxidative stability and to
simulate accelerated fuel cell operating conditions. All of the
polymer films exhibited good oxidative stability, as shown in
Table 4. Weight retention for all samples was above 93% after
treatment in Fenton's reagent at 80 ꢀC for 1 h, and most samples
remained undissolved in Fenton's reagent within 2.0 h of treat-
ment at 80 ꢀC, comparable to reported side-chain type PEMs
with pendant sulfophenyl groups.52
Proton Conductivity and Methanol Permeability. Proton
conductivities of the SPAES-xx PEMs measured in the RH range
of 40ꢀ100% at different temperatures are listed in Table 4.
Figure 8 shows the proton conductivities as a function of tem-
perature at 100% RH. The proton conductivities of SPAES-xx
Figure 7. Dimensional swelling of SPAES-xx membranes and Nafion
117 at 80 ꢀC.
Table 4. Dimensional Swelling, Proton Conductivity, and Methanol Permeability of the SPAES-xx Membranes
dimensional swelling (%)
proton conductivity
in water (S/cm)
proton conductivity
Δt (thickness)
Δl (in plane)
oxidative stability
(S/cm) (at 80 ꢀC)
methanol permeability
(10ꢀ7 cm2/s)
relative
30 ꢀC 80 ꢀC 30 ꢀC 80 ꢀC RWa (%) tb (h)
30 ꢀC
80 ꢀC
RH 95%
RH 50%
copolymers
selectivity
SPAES-25
SPAES-30
SPAES-33
SPAES-36
SPAES-39
Nafion 117
2.7
5.1
7.5
13.3
17.1
22.9
28.1
25.3
1.9
3.6
4.8
11.4
13.1
16.8
22.6
17.7
97
96
96
94
93
98
4
0.061
0.110
0.135
0.176
0.209
0.077
0.146
0.187
0.250
0.318
0.365
0.165
0.061
0.107
0.130
0.206
0.223
0.131
0.0031
0.0051
0.0055
0.0131
0.0160
0.0160
3.22
6.64
3.81
3.33
3.85
4.40
3.21
1.00
3.5
3.5
2.5
2
7.3
5.4
7.05
9.1
7.6
8.06
13.9
12.7
13.10
15.50
12.7
11.6
>6
a Retained weights of membranes after treating in Fenton's reagent for 1 h. b Dissolution time of polymer membranes.
7302
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
Figure 8. Proton conductivity of SPAES-xx membranes and Nafion 117
at different temperatures under fully hydrated conditions.
Figure 10. (a) Relative water uptake as a function of relative proton
conductivity. (b) Relative proton conductivity as a function of relative
methanol permeability.
obvious hydrophilic/hydrophobic separation and improve pro-
ton conductivity.
Figure 9. Proton conductivity of SPAES-xx membranes and Nafion 117
at 80 ꢀC as a function of RH.
Figure 10a shows the relative water uptake (vol %) as a func-
tion of relative proton conductivity. For comparison, the proper-
ties of some other side-chain type poly(arylene ether sulfone)s
(S2-PAES-70 and S4-PAES-35) containing pendant sulfophenyl
groups35 are also shown in Figure 10a. It should be noted that
high relative water uptake and dimensional swelling can lead to
difficulties in MEA fabrication, membrane-electrode interfacial
resistance, and membrane creep and deformation. On the other
hand, conductivity values below 0.05 S/cm can lead to significant
ohmic losses under fuel cell operation, since a minimum mem-
brane thickness is often practically limited due to membrane
fabrication or mechanical properties. In comparison with S2-
PAES-70 (IECw = 1.84) and S4-PAES-35 (IECw = 1.82) in our
previous study,35 SPAES-36 exhibited a much higher relative
proton conductivity and lower relative water uptake. This further
indicates that the present approach of introducing multiple
flexible sulfonic acid groups onto the fluorine-cardo structure is
effective in improving the properties of PEMs.
(IECw > 1.61) are higher than that of Nafion and increased with
temperature. In comparison with similar IEC value side-chain-
type sulfonated PEMs containing only two aliphatic sulfonic acid
groups per repeat unit (IECw = 1.72, σ = 0.114), the present
SPAES-33 show much higher proton conductivities (IECw
=
1.70, σ = 0.250), which indicates that more densely populated
aliphatic sulfonic acid groups along the polymer chain appear to
be more effective in proton conduction.31 Figure 9 shows the RH
dependence of proton conductivities at 80 ꢀC. As expected, all of
the films exhibited increased proton conductivities with increas-
ing RH. SPAES-39 and SPAES-36 showed the highest proton
conductivity values, which were comparable to or even higher
than that of Nafion at g50% RH.
Nonfluorinated randomly sulfonated PEMs generally require
high IEC values to attain high proton conductivity because of
lower acidity and side-chain flexibility of the sulfonic acid groups
and smaller hydrophilic/hydrophobic differences compared to
Nafion. In the SPAES-xx copolymers, the incorporation of clus-
ters of flexible side-chain sulfonic acid groups could be benefi-
cial to aggregate the ionic clusters, which would lead to more
The membrane properties of SPAES were also investigated for
their potential use in direct methanol fuel cells (DMFC). Metha-
nol permeability through the PEM often follows a similar rela-
tionship to proton conductivity; that is, the proton conductivity
7303
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
Figure 11. TEM photographs of (a and b) SPAES-33 membrane and (c and d) SPAES-39 membrane.
has a strong trade-off relationship with the methanol perme-
ability. Target membranes suitable for DMFC are ideally located
in the upper left-hand corner, as shown by the dotted circle, that
is, high conductivity and low methanol permeability. As shown in
Figure 10b, SPAES-27, SPAES-30, SPAES-33, and SPAES-36 are
located in this target area. Membranes intended for DMFC must
have both high proton conductivity and be effective barriers for
methanol crossover from the anode to the cathode compartment.
Nafion has good proton conductivity due to a strongly intercon-
nected ionic domain structure but it also has high methanol
permeability. The present SPAES exhibited low methanol per-
meability, with values for 10% methanol concentration at room
temperature in the range of 3.22 ꢁ 10ꢀ7 to 13.1 ꢁ 10ꢀ7 cm2/s,
which is lower than the value for Nafion of 15.5 ꢁ 10ꢀ7 cm2/s.
Although SPAES-25 has a lower proton conductivity and higher
IECv than Nafion, it has the potential to achieve improved
DMFC performance through its low methanol permeability
(i.e., better selectivity) (target area in Figure 10b). The selec-
tivity, which is the ratio of proton conductivity to methanol
permeability, is often used to evaluate the potential performance
of DMFC membranes. The relative selectivities of copolymers
are higher than that of Nafion as listed in Table 4.
Proton conductivity and dimensional stability of the membranes
are closely related to their morphology. Wide proton-conducting
channels formed by hydrophilic domains are helpful to the
movement of protons but are possibly detrimental for mechan-
ical properties and dimensional stability in hot water.37 The
typical microstructure of membranes SPAES-33 and SPAES-39
studied by TEM is shown in Figure 11. The bright and dark
regions are derived from the hard segments corresponding to the
hydrophobic units and the soft segments corresponding to the
hydrophilic units containing water, respectively. Both SPAES-33
and SPAES-39 showed well-defined nanophase-separated struc-
tures and hydrophilic domains, which are indicated as the darker
regions, were well-interconnected. The narrow and intercon-
nected ionic channels (∼2ꢀ5 nm) are believed to assist the
proton conduction and be beneficial in improving the dimen-
sional stability of membranes SPAES-33 and SPAES-39.
’ CONCLUSIONS
A novel fluorene-based bisphenol monomer containing four
methoxy groups has been successfully synthesized in high
yield by a facile phenol condensation reaction. Sulfonated poly-
(arylene ether sulfone)s (SPAES) with four pendant aliphatic
sulfonic acid groups were obtained by sequential polyconden-
sation, methoxy conversion, and sulfobutylation. In comparison
with other representative SPAES structures and with Nafion
Microstructure of the Membranes. Two-phase separated
morphology has been observed in the microstructure of Nafion
as well as some multiblock sulfonated copolymers and some
PEMs containing densely and locally sulfonated acid sites.6,7,42
7304
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
membranes, these membranes exhibited very low water uptake
and dimensional change, even at elevated temperatures (80 ꢀC).
Anisotropic swelling of the present aliphatic side-chain fluorene-
based SPAES was observed, which was greater in the through-
plane direction than in the in-plane direction. The SPAES-36
and SPAES-39 membranes displayed good proton conductivity,
which were comparable to or even higher than that of Nafion
at g50% RH. It appears that the unique nanophase separa-
tion architecture and interconnected narrow ionic channels
(∼2ꢀ5 nm) of both SPAES-33 and SPAES-39 are responsible
for the high proton conductivities and low dimensional swelling.
Moreover, the methanol permeability values of these SPAES
membranes are much lower than Nafion. The combination of
facile synthetic routes for monomer and polymers, good relative
proton and methanol transport, and relatively low water uptake
and swelling ratio makes these membranes attractive as PEM
materials for further investigation in fuel cell applications.
(17) Phu, D. S.; Lee, C. H.; Park, C. H.; Lee, S. Y.; Lee, Y. M.
Macromol. Rapid Commun. 2009, 30, 64–68.
(18) Gao, Y.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li,
X.; Kaliaguine, S. Macromolecules 2005, 38, 3237–3245.
(19) Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.;
Zawodzinski, T. A.; McGrath, J. E. Macromol. Symp. 2001, 175, 387–395.
(20) Genies, C.; Mercier, R.; Sillion, B.; Cornet, N.; Gebel, G.;
Pineri, M. Polymer 2001, 42, 359–373.
(21) Miyatake, K.; Zhou, H.; Matsuo, T.; Uchida, H.; Watanabe, M.
Macromolecules 2004, 37, 4961–4966.
(22) Yin, Y.; Suto, Y.; Sakabe, T.; Chen, S.; Hayashi, S.; Mishima, T.;
Yamada, O.; Tanaka, K.; Kita, H.; Okamoto, K.-I. Macromolecules 2006,
39, 1189–1198.
(23) Li, N.; Cui, Z.; Zhang, S.; Li, S. J. Polym. Sci., Part A: Polym.
Chem. 2008, 46, 2820–2832.
(24) Park, C. H.; Lee, C. H.; Sohn, J. Y.; Park, H. B.; Guiver, M. D.;
Lee, Y. M. J. Phys. Chem. B 2010, 114, 12036–12045.
(25) Li, N.; Zhang, S.; Liu, J.; Zhang, F. Macromolecules 2008,
41, 4165–4172.
(26) Kreuer, K. D. J. Membr. Sci. 2001, 185, 29–39.
(27) Norsten, T. B.; Guiver, M. D.; Murphy, J.; Astill, T.; Navessin,
T.; Holdcroft, S. Adv. Funct. Mater. 2006, 16, 1814–22.
(28) Lafitte, B.; Puchner, M.; Jannasch, P. Macromol. Rapid Commun.
2005, 26, 1464–1469.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ymlee@hanyang.ac.kr; michael.guiver@nrc-cnrc.gc.ca.
(29) Pang, J. H.; Zhang, H. B.; Li, X. F.; Ren, D. F.; Jiang, Z. H.
Macromol. Rapid Commun. 2007, 28, 2332–2338.
(30) Zhang, Y.; Wan, Y.; Zhao, C. J.; Shao, K.; Zhang, G.; Li, H. T.;
Lin, H. D.; Na, H. Polymer 2009, 50, 4471–4478.
(31) Shao, K.; Zhu, J.; Zhao, G. J.; Li, X. F.; Cui, Z. M.; Zhang, Y.; Li,
H. T.; Xu, D.; Zhang, G.; Fu, T. Z.; Wu, J.; Na, H.; Xing, W. J. Polym. Sci.,
Part A: Polym. Chem. 2009, 47, 5772–5783.
Notes
†NRCC No. 53026.
’ ACKNOWLEDGMENT
Funding for the project by the WCU program, National
Research Foundation (NRF) of the Korean Ministry of Science
and Technology (No. R31-2008-000-10092-0) is gratefully
acknowledged.
(32) Gao, L.; Zhang, F.; Zhang, S. B.; Liu, J. J. Membr. Sci. 2011,
372, 49–56.
(33) Yin, Y.; Yamada, O.; Suto, Y.; Mishima, T.; Tanaka, K.; Kita, H.;
Okamoto, K. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1545–1553.
(34) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.;
Watanabe, M. J. Am. Chem. Soc. 2006, 128, 1762–1769.
(35) Li, N. W.; Shin, D. W.; Hwang, D. S.; Lee, Y. M.; Guiver, M. D.
Macromolecules 2010, 43, 9810–9820.
(36) Li, Z.; Ding, J. F.; Robertson, G. P.; Guiver, M. D. Macro-
molecules 2006, 39, 6990–6996.
(37) Kim, D. S.; Robertson, G. P.; Guiver, M. D. Macromolecules
2008, 41, 2126–2134.
’ REFERENCES
(1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1,
5–39.
(2) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102,
253–269.
(3) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath,
J. E. Chem. Rev. 2004, 104, 4587–4611.
(4) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463–1502.
(5) Peckham, T. J.; Holdcroft, S. Adv. Mater. 2010, 22, 4667–4690.
(6) Higashihara, T.; Matsumoto, K.; Ueda, M. Polymer 2009,
50, 5341–5357.
(7) Park, C. H.; Lee, C. H.; Guiver, M. D.; Lee, Y. M. Prog. Polym. Sci.
2011, 36, 1443–1498.
(8) Fujimoto, C. H.; Hickner, M. A.; Cornelius, C. J.; Loy, D. A.
Macromolecules 2005, 38, 5010–5016.
(38) Matsumura, S.; Hlil, A. R.; Lepiller, C.; Gaudet, J.; Guay, D.;
Hay, A. S. Macromolecules 2008, 41, 277–280.
(39) Matsumura, S.; Hlil, A. R.; Lepiller, C.; Gaudet, J.; Guay, D.; Shi,
Z. Q.; Holdcroft, S; Hay, A. S. Macromolecules 2008, 41, 281–284.
(40) Matsumoto, K.; Higashihara, T.; Ueda, M. Macromolecules
2009, 42, 1161–1166.
(41) Matsumoto, K.; Higashihara, T.; Ueda, M. J. Polym. Sci., Part A:
Polym. Chem. 2009, 47, 3444–3453.
(42) Li, N. W.; Hwang, D. S.; Lee, S. Y.; Liu, Y. L.; Lee, Y. M.; Guiver,
M. D. Macromolecules 2011, 44, 4901–4910.
(43) Miyatake, K.; Chikashige, Y.; Higuchi, E.; Watanabe, M. J. Am.
Chem. Soc. 2007, 129, 3879–3887.
(44) Bae, B.; Miyatake, K.; Watanabe, M. Macromolecules 2010,
43, 2684–2691.
(45) Bae, B.; Yoda, T.; Miyatake, K.; Uchida, H.; Watanabe, M.
Angew. Chem., Int. Ed. 2010, 49, 317–319.
(46) Aoki, M.; Chikashige, Y.; Miyatake, K.; Uchida, H.; Watanabe,
M. Electrochem. Commun. 2006, 8, 1412–1416.
(47) Shang, X. Y.; Li, X. H.; Xiao, M.; Meng, Y. Z. Polymer 2006,
47, 3807–3813.
(48) Liu, B. J.; Hu, W.; Robertson, G. P.; Guiver, M. D. J. Mater.
Chem. 2008, 18, 4675–4682.
(9) Wu, S. Q.; Qiu, Z. M.; Zhang, S. B.; Li, Z. Y. Polymer 2006,
47, 6993–7000.
(10) Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.;
Wang, K.; Kaliaguine, S. J. Membr. Sci. 2004, 229, 95–106.
(11) Gao, Y.; Robertson, G. P.; Guiver, M. D.; Wang, G. Q.; Jian,
X. G.; Mikhailenko, S. D.; Li, X.; Kaliaguine, S. J. Membr. Sci. 2006,
278, 26–34.
(12) Shang, X.; Tian, S.; Kong, L.; Meng, Y. J. Membr. Sci. 2005, 266,
94–101.
(13) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.;
McGrath, J. E. J. Membr. Sci. 2002, 197, 231–242.
(14) Chikashige, Y.; Chikyu, Y.; Miyatake, K.; Watanabe, M. Macro-
molecules 2005, 38, 7121–7126.
(15) Schuster, M.; Kreuer, K. D.; Andersen, H. T.; Maier, J. Macro-
molecules 2007, 40, 598–607.
(16) Bai, Z.; Dang, T. D. Macromol. Rapid Commun. 2006, 27,
1271–1277.
(49) Liu, B. J.; Kim, D. S.; Murphy, J.; Robertson, G. P.; Guiver,
M. D.; Mikhailenko, S.; Kaliaguine, S.; Sun, Y. M.; Liu, Y. L.; Lai, J. Y.
J. Membr. Sci. 2006, 280, 54–64.
7305
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306

Macromolecules
ARTICLE
(50) Kim, Y. S.; Einsla, B.; Sankir, M.; Harrison, W.; Pivovar, B. S.
Polymer 2006, 47, 4026–4035.
(51) Dimitrov, I.; Jankova, K.; Hvilsted, S. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 7827–7834.
(52) Liu, B. J.; Robertson, G. P.; Kim, D. S.; Guiver, M. D.; Hu, W.;
Jiang, Z. H. Macromolecules 2007, 40, 1934–1944.
7306
dx.doi.org/10.1021/ma2015968 |Macromolecules 2011, 44, 7296–7306