Fully aromatic naphthalene-based sulfonated poly(arylene ether ketone)s with flexible sulfoalkyl groups as polymer electrolyte membranes

Article abstract of DOI:10.1039/c4ra12651c

A series of sulfonated naphthalene-based poly (arylene ether ketone)s (SNPAEK-xx) with pendant sulfoalkyl groups were prepared by polycondensation of 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene and o-methylhydroquinone, followed by a demethylation a

Full text of DOI:10.1039/c4ra12651c

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Fully aromatic naphthalene-based sulfonated
poly(arylene ether ketone)s with flexible sulfoalkyl
groups as polymer electrolyte membranes†
Cite this: RSC Adv., 2015, 5, 536
a
b
a
b
a
a
*
Baolong Wang, Zhenzhen Cai, Na Zhang, Bin Zhang, Duo Qi, Chengji Zhao
a
*
and Hui Na
A series of sulfonated naphthalene-based poly (arylene ether ketone)s (SNPAEK-xx) with pendant sulfoalkyl
groups were prepared by polycondensation of 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene and o-
methylhydroquinone, followed by a demethylation and sulfobutylation reaction. The sulfonate degree of
SNPAEK-xx could be controlled easily by adjusting the ratio of 1,4-butane sultone to the hydroxyl
content in the demethylated polymers. Flexible and tough membranes with reasonably high mechanical
strength were prepared. SNPAEK-xx membranes showed a high ionic exchange capacity (IEC) in the
range of 1.13 to 2.27 mequiv. gꢀ1, and the highest proton conductivity of 0.191 S cmꢀ1 at 80 ꢁC. They
exhibited low methanol permeability in the range of 1.25–10.22 ꢂ 10ꢀ7 cm2
s
ꢀ1, which was much lower
than that of Nafion 117. Transmission electron microscopy analysis of SNPAEK-xx revealed that they had
a more obvious phase separated structure between the hydrophilic side chain and hydrophobic fully
aromatic domains at a higher IEC. Combining their high thermal and mechanical stability, high selectivity,
lower water swelling ratio, SNPAEK-xx membranes could be promising materials as alternative to Nafion
membranes for direct methanol fuel cell applications.
Received 18th October 2014
Accepted 24th November 2014
DOI: 10.1039/c4ra12651c
www.rsc.org/advances
lower production cost, high-temperature stability and good
mechanical strength. These materials developed by many
Introduction
research groups include sulfonated poly(arylene ether
ketone),6–8 sulfonated poly(arylene ether sulfone),9–14 sulfonated
polyimides,15–17 sulfonated poly(arylene ether nitrile),18,19 and
sulfonated polybenzimidazole.20,21 Generally, the sulfonated
groups in these polymers are located on the main chain and
directly attached to the rigid backbone of polymers; therefore,
these polymers are unable to form distinct phase-separated
structures and have more dead-end channels compared to
Naon. As a result, these main-chain-type sulfonated polymers
only achieve sufficient conductivities comparable with Naon at
much higher ion exchange capacity (IEC). However, high IECs
usually lead to excess water swelling at elevated temperatures,
thereby causing large dimensional variations and failing to
afford enough mechanical strength. To balance the tradeoff
between dimensional stability and proton conductivity of
aromatic polymers is crucial for improving PEM performance by
careful structural design. One of the most promising
approaches to solve these shortages of main-chain-type
aromatic polymers is to simulate the chemical structure of
Naon and to separate the hydrophilic sulfonic acid domain
from the hydrophobic main-chain domain by locating these
ionic groups far away on the pendent side chains to restrict the
Polymer electrolyte fuel cells (PEFCs) are gaining more and
more attention due to their high durability and potentially
lower cost for stationary and portable power applications.1–3 The
polymer electrolyte membrane (PEM), which separates two
electrodes and transports protons, plays a dominant role in a
PEFC system. PEMs usually require a high proton conductivity,
low methanol permeability and high mechanical strength.
Peruorosulfonic acid polymers, such as Naon produced by
DuPont, which meet the above crucial requirements, are
considered the state-of-the-art PEM materials. However, their
drawbacks, such as high cost, high methanol permeability, and
relatively low operation temperature (<80 ꢁC), restricted their
practical applications in PEFCs.4,5 Thus, the development of
well-balanced alternatives to peruorosulfonic acid polymers is
a major challenge.
Hydrocarbon-based aromatic polymers have been exten-
sively investigated as candidate PEM materials because of their
aAlan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun
130012, PR China. E-mail: zhaochengji@jlu.edu.cn; huina@jlu.edu.cn; Fax: +86-
431-85168870; Tel: +86-431-85168870
bState Key Laboratory of Supramolecular Structure and Materials, College of
water swelling of the sulfonated hydrophilic domain.22–25
A
Chemistry, Jilin University, Changchun 130012, China
variety of side-chain-type sulfonated polymers have been
prepared by the direct copolymerization method or by chemical
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra12230e
536 | RSC Adv., 2015, 5, 536–544
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graing the pendants onto polymers.26,27 In general, they mechanical properties, were also reported here. Moreover, we
demonstrated a comparatively better balance between proton designed a new precise and fast measurement method to
1
conductivity and water dimensional swelling than main-chain- determine the methanol permeability of PEMs, using H NMR
type sulfonated polymers. For the direct copolymerization to calculate the methanol concentration in the permeation cell
method, Pang et al. synthesized a new sulfonated monomer as a function of time, which is different from the conventional
with sulfonic acid groups on exible aliphatic chains and calculated method by using meteorological chromatography or
prepared the sulfonated poly(arylene ether)s with pendant sul- by using refractive index detector. Due to the fully aromatic
foalkyl groups by a direct copolymerization method.28 The structure and naphthalene moieties in the main chain,
sulfonated monomer was prepared by an anhydrous aluminum SNPAEKs with high IEC values still have low methanol perme-
chloride-catalyzed Friedel–Cras acylation of 1-bromo-3- ability, low swelling ratio and high selectivity, which meet the
phenylpropane with 2,6-diuorobenzoyl chloride and subse- fundamental requirements for applications in direct methanol
quently sulfonated with Na2SO3. The generated membranes fuel cells (DMFCs).
exhibited high proton conductivities and a very low swelling
ratio at high temperature. In addition, some side-chain-type
sulfonted polyimides had been prepared from sulfonated
diamine monomers by direct copolymerization.29,30 These
Experimental
Materials
polymers also displayed advantageous conductivity and
membrane hydrodynamic properties compared to most of
main-chain-type sulfonated polymers. Besides the direct
copolymerization method, the graing of sulfonic acid groups
onto polymer side chains is another efficient method to prepare
side-chain-type sulfonated aromatic polymers. Jannasch and
coworkers prepared branched sulfonated polysulfones via lith-
iation and sulfoalkylation of polysulfone.31 Ding and coworkers
synthesized comb-shaped polymers that had rigid aromatic
main chains and exible side chains.32 In our previous work, we
reported a series of novel side-chain-type naphthalene-based
sulfonated poly(arylene ether ketone) copolymers (SNPAEKs)
by a demethylation and sulfobutylation method. The highest
conductivity of 0.179 S cmꢀ1 was obtained for SNPAEKs (IEC ¼
1.82 mequiv. gꢀ1) at hydrated state at 80 ꢁC, which is higher
than that of Naon 117 (0.149 S cmꢀ1) at the same condition.
Furthermore, the introduction of naphthalene moiety into
SNPAEKs increased the free volume and the stiffness of polymer
chain and restricted the water swelling, thereby improving the
in-water dimensional stability and mechanical properties as
PEM materials. However, they had relatively low IEC values in
the range of 0.99–1.82 mequiv. gꢀ1, which hindered the further
improvement in the proton conductivity or related properties.
Because of the highly hydrophobic structure of naphthalene
moieties in the main chain, achieving PEMs with higher IEC
values, higher proton conductivity, but acceptable dimensional
stability is also possible. Moreover, increasing IEC enhances the
difference between hydrophilic sulfonic acid domains and
hydrophobic domains, thus inducing more distinct phase-
separated morphology.
1,5-Bis(4-uorobenzoyl)-2,6-dimethoxynaphthalene
(DMNF)
was synthesized in our lab according to our previous report.3
2,6-Dimethoxynaphthalene was obtained from Dalian Jinzhou
Chemical plant. 4-Fluorobenzoyl chloride and 1,4-butane sul-
tone were purchased from Sigma-Aldrich. Boron tribromide
(BBr3), o-methylhydroquinone and NaH (60%) were purchased
from Aladdin Scientic Co. Ltd. Shanghai. The solvents, N,N-
dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), and N-
methyl-2-pyrrolidone (NMP) were vacuum-distilled before used.
Other reagents were commercially available grade and used
without further purication.
Synthesis of naphthalene-based poly(arylene ether ketone)
containing methoxyl groups
A typical synthetic procedure for the preparation of MM-PAEK is
described as follows. In a 250 mL three-necked ask equipped
with a mechanical stirrer, a Dean–Stark trap and a nitrogen
inlet, DMNF (8.64 g, 0.02 mol), o-methylhydroquinone (2.48 g,
0.02 mol), K2CO3 (2.76 g, 0.02 mol), NMP (50 mL) and toluene
ꢁ
(15 mL) were placed. The mixture was slowly heated to 120 C
and kept at this temperature for about 3 h to remove the water
produced by reaction. The temperature was slowly raised to
180 ꢁC and maintained for another 12 h. The resulting viscous
solution was cooled down to room temperature, precipitated
into a large excess of deionized water with vigorous stirring. The
resulting brous polymer was ltered and washed thoroughly
withꢁdeionized water several times and dried under vacuum at
120 C for 24 h. The polymer was denoted as MM-PAEK.
Synthesis of naphthalene-based poly(arylene ether ketone)
containing phenolic hydroxyl groups
In this study, the fully aromatic naphthalene-based
sulfonated poly(arylene ether ketone)s with high density of
alkylsulfonic acid groups were successfully prepared. The IEC The methoxy groups in MM-PAEK polymer were converted into
values for this novel series of side-chain-type SNPAEKs can hydroxyl functionalities according to a modied procedure of
reach up to 2.27 mequiv. gꢀ1, accompanying with high proton demethylation reaction as McOmie et al. reported.33 The typical
conductivities of 0.094 S cmꢀ1 at 25 ꢁC and 0.191 S cmꢀ1 at demethylation reaction is described as follow: MM-PAEK (1.0 g)
80 ꢁC, respectively. Fortunately, the fully aromatic structures in was dissolved in 20 mL rened CH2Cl2. The solution was cooled
the polymer main chain suppressed the water swelling and down using liquid N2, and 1 M BBr3 solution of CH2Cl2 12 mL
maintained dimensional stability in spite of high IEC values. was added dropwise. Then the reaction was increased to room
The morphology and the detailed properties of PEMs, such as temperature and stirred for another 12 h. Then the mixture was
water uptake, swelling ratio, proton conductivity, thermal and stripped out with 100 mL ice-water, and then washed with
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ethanol and deionized water several times. The ꢁresulted poly-
mer (HO-PAEK) was dried under vacuum at 120 C for 24 h to
remove the remaining solvents.
Ion exchange capacity, water uptake and swelling ratio
The IEC and water uptake of membranes were determined by
the method similar to that reported previously.8 The dimen-
sional changes of these membranes were measured in the
vertical and plane direction, which were characterized by
Synthesis of naphthalene-based poly(arylene ether ketone)
with pendent sulfoalkyl groups
Twet ꢀ Tdry
DT ¼
(1)
Tdry
The sulfobutylation reaction is described as follows: HO-PAEK
(1.0 g)polymer was dissolved in 20 mL anhydrous DMSO at
room temperature for 24 h in oil bath, and then heated to 60 ꢁC
under N2 atmosphere. Aer HO-PAEK was completely dissolved
in DMSO, NaH (0.30 g) solid was added quickly and reacted for
another 30 min. 0.80 mL 1,4-butane sultone was subsequently
added to the mixture, and kept stirring at 60 ꢁC for 30 min. Then
the reaction was heated at 120 ꢁC for 24 h in oil bath, and
precipitated into anhydrous acetone. The obtained pale yellow
solid powder was obtained by ltered, washed with acetone and
deionized water for several times, and dried in vacuum oven at
Lwet ꢀ Ldry
DL ¼
(2)
Ldry
where Tdry, Twet, Ldry and Lwet are the thicknesses and lengths of
dry and wet samples, respectively.
Proton conductivity
The in-plane proton conductivity (s) was measured by a four-
electrode AC impedance method from 0.1 Hz to 100 KHz,
using Princeton Applied Research Model 2273 potentiostat/
galvanostat/FRA. All the membranes were cut into 1 cm ꢂ 4
cm pieces and the thicknesses were measured. The membranes
were dipped in deionized water for at least 24 h before analysis.
A sheet of membrane was placed in the test cell, which was
immersed in water with 100% relative humidity at desired
temperature. The proton conductivity (s) was calculated from
the following relationship.
ꢁ
120 C for 24 h to remove the remained solvents. The resulted
polymers were nominated as SNPAEK-xx, where xx stood for the
contents of exible sulfoalkyl side chain of every repeat unit.
Membrane formation and proton exchange
Membranes were prepared by casting SNPAEK-xx (in their
sodium form) solution in DMSO onto the glass plates and dried
L
s ¼
(3)
ꢁ
RS
at 100 C for 24 h. The membranes were then peeled off from
the substrates, and immersed in 1 M HCl at room temperature
for 24 h to thoroughly convert into acid form. In order to remove
any remaining solvent, the membranes were dried under
where L is the distance between the electrodes (cm), R is the
membrane resistance, and S is the cross-sectional area of
membrane (cm2).
ꢁ
vacuum at 120 C for another 48 h.
Methanol permeability
Methanol permeability through the membranes was measured
using a homemade two-chamber diffusion cell, which consists
of two compartments separated by a vertical membrane. One
compartment was lled with 10 M methanol aqueous solution
(donor) and the other with deionized water (receptor). Each
liquid compartment was stirred by a magnetic stirrer to ensure
uniformity. Methanol concentration (measured in mol Lꢀ1) in
the donor compartment was held at a constant value, and the
methanol concentration in the receptor compartment as a
function of time was measured by 1H NMR. For this purpose, 1
mL methanol solution was collected from the receptor
compartment at a desired time by using 1 mL pipette and
transferred into a standard 5 mm NMR tube with DMSO-d6 as
solvent. Then 10 mL HPLC THF was also added to the NMR tube
by using a 10 mL microinjector as standard substance. Aer
Measurements
1H NMR spectra were recorded on a Bruker 510 instrument
using CDCl3 and/or DMSO-d6 as solvent and tetramethyl silane
(TMS) as an internal standard. The inherent viscosities of the
obtained polymers were determined with a concentration of 0.5
g dLꢀ1 polymer in DMSO and/or NMP at 30 C by using Ubbe-
ꢁ
lohde capillary viscometer. The glass transition temperatures
(Tg) of polymers were determined by using differential scanning
calorimeter (DSC) measurement performed on a TA DSC Q20
instrument at heating and cooling rate of 10 ꢁC minꢀ1 in a
ꢁ
temperature range of 100–300 C under nitrogen. The thermo
gravimetric analyses (TGA) were performed on a Pyris 1 thermo
gravimetric analyzer (Perkꢀin1-Elmer) from 100 to 700 ꢁC at a
heating rate of 10 ꢁC min under nitrogen atmosphere. The
mechanical properties of thin membranes were evaluated at
room temperature with a universal testing instrument (SHI-
MADZU AG-I 1KN) at a speed of 2 mm minꢀ1. TEM images were
obtained with JEM-2100F electron microscope operating at 200
kV. Before test, the polymers were rst converted into Ag+ form
by immersing the polymers in AgNO3 solutions for 24 h. The
SNPAEK-Ag in DMSO solutions was then casted onto copper
grids for TEM use.
1
mixing them by ultrasonic, H NMR measurements were per-
formed on a Bruker 510 instrument. The methanol concentra-
tion (CB) was then calculated from integral area ratio of the
1
signals of methanol and HPLC THF in the obtained H NMR
spectra. Last, the methanol permeability of SNPAEK
membranes was calculated as follows:
P ¼ DK
(4)
538 | RSC Adv., 2015, 5, 536–544
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À Á
A DK
CB t ¼
CAðt ꢀ t0Þ
(5)
VB
L
where A (cm2) and L (cm) are the effective area and the thickness
of the membrane, respectively; VB (cm3) is the volume of diffu-
sion compartment; CA and CB (mol Lꢀ1) are the methanol
concentration in the donor and receptor compartment,
respectively; D and K are the membrane diffusivity and partition
coefficient between the membrane and the adjacent solution,
respectively; P denotes the methanol permeability (cm2 sꢀ1).
Results and discussion
Synthesis and characterization of polymers
According to our previous work, a naphthalene-based monomer
DMNF was prepared by an anhydrous ferric chloride catalyzed
Friedel–Cras acylation of 2,6-dimethoxynaphthalene with 4-
uorobenzoylchloride. Then, as shown in Scheme 1, the
naphthalene-based poly(arylene ether ketone) polymer con-
taining methoxy groups (MM-PAEK) was synthesized by a
standard K2CO3-catalyzed aromatic nucleophilic substitution
polycondensation of DMNF and o-methylhydroquinone. The
polymerization reaction proceeded smoothly to high molecular
weight as no evident cross-linking was found even at the
temperature higher than 180 ꢁC. MM-PAEK exhibited high
intrinsic viscosity (1.0 g dLꢀ1), which indicated the resulted
polymer had high molecular weight.
Scheme 2 Synthesis of HO-PAEK and SNPAEK-xx.
sulfobutyl groups were graed onto HO-PAEK by a nucleophilic
ring opening reaction with 1,4-butane sultone in the presence of
NaH. SNPAEK-xx with different contents of sulfobutyl groups
were obtained by adjusting the ratio of 1,4-butane sultone to the
hydroxyl content in HO-PAEK.
Fig. 1 also shows a representative 1H NMR spectrum of
SNPAEK-xx, which has the signal assignments consistent with
the designed molecular structure. As expected, the hydroxyl
protons at around 9.83 ppm in HO-PAEK almost disappeared,
while new signals for the methylene protons were observed at
3.97 ppm, 2.33 ppm, 1.52 ppm, 1.45 ppm, which demonstrated
that the sulfonic acid groups were introduced to polymers by
sulfobutylation reaction. By calculating integral area ratio of
3.97 ppm (Hm) and 7.78 ppm (Ha), the degree of sulfonation
(DS) of SNPAEK ranging from 0.65 to 1.60 was obtained
according to the following equation.
As shown in Scheme 2, the demethylation of MM-PAEK
polymer to the reactive hydroxyl-containing HO-PAEK for
graing was conducted at room temperature in dichloro-
methane using BBr3. HO-PAEK was precipitated during the
reaction due to the polar nature of phenolic hydroxyl groups.
1
Fig. 1 shows the H NMR spectra of MM-PAEK and HO-PAEK.
The characteristic peak at 3.78 ppm attributing to the
methoxy protons in the initial MM-PAEK disappeared in
HO-PAEK, while the new peak at around 9.83 ppm corre-
sponding to the proton of –OH group was observed in the
spectrum of HO-PAEK. The results indicated that the deme-
thylation reaction had completely proceeded. Finally, the
AH
m
DS ¼ 2
(6)
AH
a
where AH is the peak area of Hm, and AH corresponds to the
m
a
peak area of Ha, respectively.
Mechanical and thermal properties
As shown in Table 1, SNPAEK-xx membranes showed much
higher inherent viscosities than that of the corresponding
MM-PAEK because of the graing of sulfobutyl groups, which
enhanced intermolecular associations and gave a stronger
interaction between polymer chains. The high viscosities also
indicated that there was no obvious polymer decomposition or
crosslinking occurred under basic conditions during the sul-
fobutylation reaction.
Typical tensile stress–strain curves of all the SNPAEK-xx
membranes are shown in Fig. 2 under ambient condition, and
the results are also listed in Table 1. For the MM-PAEK
membranes containing naphthalene moieties, it exhibited a
Scheme 1 Synthesis of the monomer DMNF and MM-PAEK.
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reasonably high mechanical strength with a tensile strength at
maximum load 63.7 MPa, Young's modulus of 1.08 GPa, and
elongation at break of 7.4%. Aer sulfobultylation, SNPAEK-xx
membranes still showed better mechanical properties with
high tensile strength ranging from 32.0 to 67.0 MPa, and
Young's modulus ranging from 0.84 to 1.76 GPa than Naon
117 which has a tensile strength of 29.4 MPa and a Young's
modulus of 0.357 GPa, respectively. Furthermore, it can be seen
from Fig. 2 that SNPAEK-xx membranes exhibited a decreasing
tendency in tensile strength and Young's modulus with the
sulfobutyl content increasing. It's worth to mention that the
elongation at break of SNPAEK-xx signicantly increased from
14.8% to 77.7% as the sulfobutyl content increased from 0.65 to
1.60. This could be attributed to the introduction of exible
aliphatic side-chain segments, which acted as a plasticizer to
the high chain rigidity of aromatic polymer backbone. As shown
in Table 1, compared with ambient condition, fully hydrated
SNPAEK-xx membranes also showed good mechanical proper-
ties with tensile strength ranging from 35.1 to 62.3 MPa, and
Young's modulus ranging from 0.16 to 0.53 GPa, respectively. As
the water molecule in the hydrated membranes acted as the
plasticizer, all these SNPAEK-xx membranes exhibited high
elongation at break ranging from 31.1 to 40.2%. All the results
Fig. 1 1H NMR spectra of MM-PAEK (a), HO-PAEK (b) and SNPAEK-xx (c).
Fig. 2 The curve of stress–strain of different membranes at room
temperature ambient condition.
Table 1 Mechanical properties and inherent viscosities of polymers and Nafion 117
Ambient condition
Fully hydrated state
hinh
(dL gꢀ1
Tensile strength
(MPa)
Young's
modulus (GPa)
Elongation at
break (%)
Tensile strength
(MPa)
Young's
modulus (GPa)
Elongation at
break (%)
Samples
)
MM-PAEKa
1.00
1.49
1.06
1.67
1.70
2.03
—
63.7 ꢃ 4.0
67.0 ꢃ 4.0
52.0 ꢃ 2.0
42.6 ꢃ 1.6
32.0 ꢃ 2.2
35.6 ꢃ 2.1
29.4c
1.08 ꢃ 0.09
1.76 ꢃ 0.07
1.39 ꢃ 0.04
1.09 ꢃ 0.07
0.84 ꢃ 0.04
0.88 ꢃ 0.02
0.357c
7.4 ꢃ 0.7
14.8 ꢃ 3.8
17.3 ꢃ 7.9
40.3 ꢃ 4.5
55.0 ꢃ 5.3
77.7 ꢃ 7.5
270c
—
—
—
SNPAEKb-0.65
SNPAEKb-1.00
SNPAEKb-1.30
SNPAEKb-1.47
SNPAEKb-1.60
Naon 117c
62.3 ꢃ 3.5
45.5 ꢃ 1.0
45.0 ꢃ 3.4
44.0 ꢃ 2.5
35.1 ꢃ 1.7
—
0.29 ꢃ 0.03
0.26 ꢃ 0.02
0.35 ꢃ 0.03
0.53 ꢃ 0.04
0.16 ꢃ 0.02
—
33.8 ꢃ 4.6
31.1 ꢃ 2.7
35.6 ꢃ 6.4
33.2 ꢃ 3.7
40.2 ꢃ 1.7
—
a
The polymer was dissolved in NMP. b The polymer was dissolved in DMSO. c The mechanical properties of Naon 117 are adapted from ref. 34.
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IEC, water uptake and swelling ratio
Table 2 shows the IEC values of SNPAEK-xx membranes as
determined through NMR method. IEC was a constant repre-
senting the amount of the exchangeable protons in the
membrane. The calculated IEC values of SNPAEK-xx in acid-form
membranes were in the range of 1.13 to 2.27 mequiv. gꢀ1, which
were close to the theoretical values derived from the molar ratio
of 1,4-butane sultone to the hydroxyl content in HO-PAEK,
indicating that the sulfonic acid groups were quantitatively
graed by sulfobutylation reaction.
The water uptake and swelling ratio of PEMs are signicantly
related to the membrane properties, such as proton conduc-
tivity, dimensional stability and mechanical properties. On one
hand, the protons within PEMs could not be conducted unless
they are hydrated by water;35 On the other hand, excessive water
swelling would deteriorate the dimensional stability, thus
resulting in the loss of mechanical properties. Therefore, the
preparation of sulfonated polymers with appropriate water
uptake is one of the critical requirements for their application
as PEMs. The water uptake and swelling ratio of SNPAEK-xx
membranes were evaluated by comparing the mass and
dimensions of their hydrated state membranes with dry state
membranes, respectively. Fig. 5 and Table 2 showed a clear
Fig.
polymers.
3 The Tg curve of MM-PAEK, HO-PAEK and SNPAEK-0.65
indicated that these membranes were strong and tough for
potential use as PEM materials in fuel cells.
Due to the high chain rigidity of fully aromatic structures,
MM-PAEK containing methoxyl groups and HO-PAEK contain-
ing hydroxyl groups exhibited the high glass transition
temperatures (Tg) of 239 ꢁC and 227 ꢁC, respectively (Fig. 3).
However, no obvious glass transition could be observed below
decomposition temperature for SNPAEK polymers containing
sulfobutyl groups. To further investigate the thermal stabilities
of SNPAEK polymers, TGA analysis was carried out under
nitrogen. Fig. 4 shows the TGA and their derivative curves (DTG)
of SNPAEK polymers with different sulfobutyl groups. All the
polymers displayed a two-stage weight loss behavior. The rst
ꢁ
decomposition stage around 230–350 C was attributed to the
decomposition of the sulfonic acid groups in theꢁside chains,
and the second decomposition stage around 450 C was likely
related to the degradation of main and side chains. The 5 wt%
weight loss temperatures of the SNPAEK polymers were all
higher than 230 ꢁC, indicating that they had sufficient thermal
stability for the PEM applications in medium temperature fuel
cells.
Fig. 4 The TGA and DTG curve of SNPAEK-xx.
Table 2 IEC, water uptake, swelling ratio, proton conductivity (s), methanol permeability (P), and selectivity of SNPAEK-xx acid and Nafion 117
membranes
Swelling ratio (%)
Water uptake
(%)
s (S cmꢀ1
)
DT
DL
P
Selectivity
Polymer membranes IECa (mequiv gꢀ1
)
25 ꢁ
C
80 ꢁ
C
25 ꢁ
C
80 ꢁ
C
25 ꢁC 80 ꢁ
C
25 ꢁ 80 ꢁ
C
C
(ꢂ10ꢀ7 cm2 sꢀ1
)
(ꢂ104 S s cmꢀ3
)
SNPAEK-0.65
SNPAEK-1.00
SNPAEK-1.30
SNPAEK-1.47
SNPAEK-1.60
Naon 117
1.13
1.60
1.90
2.14
2.27
0.92
9
20
30
34
37
18
14
31
52
79
101
28
2
2
4
5
6
5
3
7
10
16
17
14
2
3
5
7
7
3
10
13
17
20
17
0.010 0.027
0.035 0.092
0.061 0.145
0.076 0.181
0.094 0.191 10.22
0.075 0.149 23.80
1.25
4.42
7.32
9.28
8.00
7.92
8.33
8.19
9.20
3.15
11
a
IEC was calculated by 1H NMR.
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Fig. 6 Proton conductivity of SNPAEK-xx membranes.
Fig. 5 Water uptake of SNPAEK-xx acid form membranes.
cmꢀ1 at 25 C, and from 0.027 to 0.191 S cm at 80 C, which
were suitable for practical applications as PEMs in fuel cells.
Especially for SNPAEK-1.47 and SNPAEK-1.60, the proton
conductivity was much higher than those of Naon 117 (0.149 S
cmꢀ1 at 80 ꢁC) and those of other side-chain-type SNPAEKs
reported previously.3 It is well known that the proton conduc-
tivity of sulfonated aromatic polymers is mainly dependent on
the IEC values. Especially for the randomly non-uorinated
sulfonated polymers, it required high IEC values to achieve
high proton conductivity because of lower acidity of aliphatic
sulfonic acid group.36 The morphology of the PEM is another
crucial factor to determine the proton conductivity. For the side-
chain-type SNPAEK-xx membranes, the sulfonic acid groups on
the exible aliphatic side chains would be benecial to aggre-
gate the ionic clusters, the highly rigid naphthalene groups on
the polymer backbone could increased the free volume and
hydrophobic of segments. Considering the hydrophilic sulfonic
acid groups on the exible side chains and the hydrophobic
naphthalene moieties on the main chains, SNPAEK membranes
are favor to form more continuous and larger transport chan-
nels at higher sulfobutyl content, and thus improving the
proton conductivity.
ꢀ1
ꢁ
ꢁ
trend of increasing water uptake with sulfobutyl content and
IEC values at a given temperature. However, at lower sulfobutyl
content, the water uptake of side-chain-type sulfonated poly-
mers increased much slowly as the temperature increasing from
ꢁ
ꢁ
25 C to 80 C than those of main-chain-type sulfonated poly-
mers. At a high sulfobutyl content, SNPAEK-1.60 membrane
with a high IEC (2.27 mequiv. gꢀ1) exhibited a sharply increase
in water sorption. This indicated that the aggregation of the
sulfonic acids at higher sulfobutyl content could induce the
formation of the obvious phase separation morphology, which
could lead to a rapid increase of water uptake.
As shown in Table 2, the swelling ratio also increased with
the IEC and temperature increasing. But interestingly, all the
SNPAEK-xx membranes showed a much lower swelling ratio,
which were lower than those of main-chain-type sulfonated
poly(arylene ether ketone)s.12 The side-chain-type sulfonated
polymers locating the sulfonic acid groups on the exible side
chains were more effective in separating hydrophilic sulfonic
acid groups from the hydrophobic polymer main chain. Thus,
the molecular water would be restricted to hydrophilic domains
and separated from hydrophobic domains, which suppressed
the swelling behavior of the polymer membranes. Additionally,
the rigidity naphthalene moieties in the polymer backbone
remarkably increased the rigidity and hydrophobic of polymer
chain, which can further restricted the water swelling at high
IEC values. Although SNPAEK-1.60 has the highest IEC up to
2.27 mequiv. gꢀ1, the swelling ratio is as low as 7% at 25 ꢁC and
20% at 80 ꢁC. This result indicates that the increasing intro-
duction content of the naphthalene group and long alkylsulfo-
nated side chains can effectively enhance the dimensional
stability of the membranes.
Methanol permeability and selectivity
Membranes for the practical application in DMFC must possess
both high proton conductivity and low methanol permeability.
Fig. 7 shows the 1H NMR spectra of methanol aqueous solution
collected from the receptor compartment at a pre-set time in a
diffusion cell, which was separated by a SNPAEK-xx membrane
with different DS values. The methanol concentration for
calculating the methanol permeability was measured by the
deconvoluted peak area of the methyl groups of methanol (or
hydroxyl proton of methanol) relative to the methylene groups
of THF (internal standard) in the spectrum. The signal corre-
sponding to the methyl groups (H1) and hydroxyl proton (H2) of
Proton conductivity
The proton conductivity of SNPAEK-xx membranes at 100% RH methanol increased apparently as the IEC of SNPAEK-xx
was measured as a function of temperature and is displayed in increased from 1.13 to 2.27 mequiv. gꢀ1. As shown in Table 2,
Fig. 6. As the IEC values increased from 1.13 to 2.27 mequiv. the calculated methanol permeability increased from 1.25 to
gꢀ1, the proton conductivity increased from 0.010 to 0.094 S 10.22 ꢂ 10ꢀ7 cm2 sꢀ1, which was much lower than the value
542 | RSC Adv., 2015, 5, 536–544
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still necessary to induce the exible long alkyl side chains to
aggregate into larger ionic domains and form better phase
separation microstructure. SNPAEK-1.60 with the highest IEC
values was observed to have more continuous and larger
transport channels than that of SNPAEK-1.30 with lower IEC.
The larger and connected hydrophilic domain was responsible
to the efficient proton transportation and excellent proton
conductivity. It was also contributed to the increase of water
uptake and methanol permeability with IEC increasing.
Conclusions
A series of novel side-chain-type fully aromatic naphthalene-
based sulfonated poly(arylene ether ketone) polymers were
successfully prepared from HO-PAEK derived from the original
monomer. The IEC (1.13–2.27 mequiv. gꢀ1) of SNPAEK-xx could
be readily controlled by using different amount of 1,4-butane
sultone and NaH. The polymers possessed high molecular
weights, revealed by their high viscosities and formation of
tough and exible membranes. The SNPAEK-xx membranes
showed anisotropic membrane swelling in water with lower
swelling in thickness than in plane, which was helpful to
improve the dimensional stability. From SNPAEK-1.30 to
SNPAEK-1.60, they exhibited high proton ꢁconductivity, ranging
from 0.145 S cmꢀ1 to 0.191 S cmꢀ1 at 80 C. They also showed
lower methanol permeability than that of Naon 117. SNPAEK-
1.30 and SNPAEK-1.60 showed the methanol permeability of
7.32 ꢂ 10ꢀ7 cm2 sꢀ1 and 10.22 ꢂ 10ꢀ7 cm2 sꢀ1, respectively.
TEM images revealed that long alkylsulfonated side chains
aggregated into hydrophilic clusters to form a continuous
network at a sulfobutyl content of 1.60. The SNPAEK-xx
membranes exhibited low methanol permeability, lower water
swelling, and high proton conductivity and could be the
promising materials for DMFC applications.
Fig. 7 1H NMR spectra of methanol water solution and HPLC THF as
standard substance.
2.38 ꢂ 10ꢀ6 cm2 sꢀ1 of Naon 117. As mentioned earlier, the
transportation of methanol also required channels with good
connectivity to form by the aggregation of hydrophilic clusters.
Thus, the increasing trend of methanol permeability from
SNPAEK-0.65 to SNPAEK-1.60 is easy to understand because of
the increased water uptakes and IECs. The selectivity, which is
dened as the ratio of proton conductivity to methanol
permeability, is used to estimate the membrane potential
performance in DMFC. As shown in Table 2, all the SNPAEK-xx
membranes had the relatively higher selectivity than that of
Naon 117, indicating SNPAEK membranes have great potential
for DMFC applications.
Microstructure of the SNPAEK membranes
The morphology of SNPAEK-xx polymers was investigated by
TEM. Fig. 8 shows the TEM micrographs corresponding to the
morphologies of SNPAEK-1.30 and SNPAEK-1.60. In the TEM
images, the dark areas corresponding to the ionic clusters
stained by silver ions represent the hydrophilic domains and
bright areas represent the domains formed by hydrophobic
polymer backbones. The silver stained hydrophilic clusters are
dispersed throughout the polymer matrix. With the increase of
IEC, these silver clusters became more obvious and their
density as well as average size of clusters increased. However,
compared to Naon membrane, the rigid fully aromatic main
chains of SNPAEK hindered the formation of pronounced
phase-separated morphology.37 It indicated that a higher IEC is
Acknowledgements
This work was supported by the National Nature Science
Foundation of China (Grant no. 21074044, 21374034, 21474036)
and Science and Technology Development Plan of Jilin Province
(Grant no. 20130522138JH).
Notes and references
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