Synthesis and characterization of sulfonated poly(ether sulfone)s containing mesonaphthobifluorene for polymer electrolyte membrane fuel cell

Article abstract of DOI:10.1166/jnn.2014.9464

The novel sulfonated poly(ether sulfone)s containing mesonaphthobifluorene (MNF) moiety were synthesized and characterized their properties. The prepared polymers have highly conjugated aromatic structure due to the MNF group which is an allotrope of carb

Full text of DOI:10.1166/jnn.2014.9464

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Nanoscience and Nanotechnology
Vol. 14, 7948–7953, 2014
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Synthesis and Characterization of Sulfonated Poly(ether
sulfone)s Containing Mesonaphthobifluorene for
Polymer Electrolyte Membrane Fuel Cell
Youngdon Lim, Dongwan Seo, Soonho Lee, Md. Awlad Hossain, Jinseong Lim,
Sangyoung Lee, Taehoon Hong, and Whangi Kim∗
Department of Applied Chemistry, Konkuk University, Chungju, 380-701, Korea
The novel sulfonated poly(ether sulfone)s containing mesonaphthobifluorene (MNF) moiety were
synthesized and characterized their properties. The prepared polymers have highly conjugated
aromatic structure due to the MNF group which is an allotrope of carbon and one atom thick
planar sheets of sp2-bonded carbon atoms. Poly(ether sulfone)s bearing tetraphenylethylene
on polymer backbone were synthesized by polycondensation and followed intra-cyclization from
tetraphenylethylene to form MNF by Friedel-craft reaction with Lewis acid (FeCl3). The sulfonation
was performed selectively on MNF units with conc. sulfuric acid. The structural properties of the
1
sulfonated polymers were investigated by H-NMR spectroscopy. The membranes were studied by
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ion exchange capacity (IEC), water uptake, and proton conductivity. The synthesized polymer elec-
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trolyte membranes showed better thermal and dimensional stabilities owing to the inducted highly
Copyright: American Scientific Publishers
conjugated aromatic structure in the polymer backbone. The water uptake of the synthesized mem-
branes ranged from 23∼52%, compared with 32.13% for Nafion 211® at 80 C. The synthesized
ꢀ
ꢀ
membranes exhibited proton conductivities (80 C, RH 90%) of 74ꢀ6∼100ꢀ4 mS/cm, compared with
102.7 mS/cm for Nafion 211®.
Keywords: PEMFC, Polymer Electrolyte Membrane, Mesonaphthobifluorene, Intra-Cyclization.
1. INTRODUCTION
polymer structures are susceptible to nucleophilic attack by
hydrogen peroxide or peroxide radicals made by PEMFC
operating system. The chemical modification of sulfonic
acid groups attached on aromatic rings of polymer side
chain provides better stability due to less reactivity of
nucleophilic substitution reaction.7 Furthermore, IEC val-
ues of polymers are quite an important factor because
proton conductivity is strongly affected by IEC. Although
sulfonated aromatic membranes with high IEC values can
achieve high proton conductivity, such membranes show
excess water swelling or even dissolve in water.3ꢀ8 Consid-
ering this critical issue, we made a plan to introduce highly
conjugated aromatic structure highly conjugated aromatic
structure into polymer backbone. This structure is a pla-
nar sheet of sp2-bonded carbon atoms that are consisted of
6 benzenes with 2 cyclopentadienes. This structure is pre-
dicted to have remarkable properties, such as high thermal
conductivity, superior mechanical properties, and excellent
Polymer electrolyte membranes (PEMs) based on
sulfonated perfluoropolymers, Nafion and Flemion have
been widely used for polymer electrolyte membrane
fuel cells (PEMFCs) because of their high proton
conductivity and outstanding stability.1ꢀ2 However, they
have some drawbacks, suꢀch as high cost, limited oper-
ating temperature (0∼80 C), and insufficient durability
etc. To overcome these impediments, extensive efforts
have been made to develop alternative acid-functionalized
aromatic hydrocarbon-based polymers.3–5 And there have
been significant developments in the synthesis of sul-
fonated aromatic polymers as alternative PEMs.6 Most of
the chemical structure of polymer membranes had ether
linkage and sulfonic acid groups directly attached on
aromatic rings of polymer main chain. These kinds of
∗Author to whom correspondence should be addressed.
7948
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 10
1533-4880/2014/14/7948/006
doi:10.1166/jnn.2014.9464

Lim et al.
Synthesis and Characterization of Sulfonated Poly(ether sulfone)s Containing Mesonaphthobifluorene for PEMFC
ꢀ
electronic transport properties.9–13 This work is an attempt
to synthesize sulfonated poly(ether sulfone)s containing
MNF moiety, a highly conjugated aromatic structure. The
designed polymer membrane including a MNF structure,
a conjugated system between six phenyl rings and two
cyclopentadiene groups, is a promising unit to maintain
excellent dimensional stability owing to superior mechan-
ical property of highly conjugated aromatic structure. The
membranes were studied by ion exchange capacity (IEC),
water uptake, and proton conductivity.
at 80 C for 24 h. PDHTPE 40 and 50 were synthesized
by same procedure with different mol% of DHTPE units
(40 and 50 mol%).
2.4. Synthesis and Sulfonation of Poly(ether sulfone)s
Containing of MNF, (PMNFs)
To
a flask, PDHTPEs (1.0 g) was dissolved in
dichloromethane (100 mL) at N2 atmosphere. FeCl3
(0.02 mol) in nitromethane (0.4012 mol) was added slowly
to the reaction mixture. The resultant mixture was stirred
at room temperature for 10 h. Then, the reaction mixture
was poured into methanol and collected by filtration, and
washed with hot water and methanol. Filtrated polymer
2. EXPERIMENTAL DETAILS
2.1. Materials
ꢀ
was dried in a vacuum oven at 80 C for 24 h. The sul-
Diphenylmethane, 2.5 M solution of n-butyl lithium
in hexane, 4, 4-dimethoxybenzophenone, ammonium
chloride, p-toluenesulfonic acid, boron tribromide, bis(4-
hydroxyphenyl) sulfone, bis(4-fluorophenyl) sulfone,
FeCl3, nitromethane, sodium chloride, sodium hydrox-
ide, hydrochloric acid, anhydrous K2CO3, and conc.
sulfuric acid were purchased from Aldrich and used as
received. Commercial grade dimethylacetamide (DMAc)
and toluene were dried over calcium hydride and distilled
prior to use. Other common solvents were also used
without any further purification.
fonation was carried out in a flask with PMNFs (0.5 g)
and 6 mL conꢀc. sulfuric acid and the reaction mixture was
stirred at 45 C for 24 h. The mixture was then poured
into cold water. Then the sulfonated PMNFs (SPMNFs)
were thoroughly washed 3 times with water, and dried in
ꢀ
a vacuum oven at 80 C for 24 h. SPMNF 30, 40, and 50
were synthesized same procedure.
2.5. Membrane Preparation and Characterization
Membranes preparation and characterization were fol-
lowed by previous report.14ꢀ15
2.2. Synthesis of 4, 4ꢁ-(2, 2-Diphenylethenylidene)
Diphenol, (DHTPE)
The synthesis of DHTPE was successCfuollpyycriagrhrite:dAomuet rviciaan Scientific Publishers
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2.6. Accelerated Chemical Degradation
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Membrane degradation was studied by immersion in Fen-
ton reagent (4 ppm Fe2+, 3% H2O2ꢁ at 75 C, following
general method.16ꢀ17 The investigated samples were linear
sulfonated poly(ether sulfone)s (S-PES 40), SPMNFs, and
Nafion 211®.
ꢀ
McMurry coupling and demethylation by following our
previous report14 as shown in Scheme 1.
2.3. Synthesis of Poly(ether sulfone)s, (PDHTPE 30)
To a flask fitted with dean-stark trap, condenser, nitro-
gen inlet/outlet, and a magnetic stirrer, monomer DHTPE
(1.18 mmol), bis(4-fluorophenyl) sulfone (3.93 mmol),
bis(4-hydroxyphenyl) sulfone (2.75 mmol), K2CO3
(4.72 mmol), DMAc (10 mL), and toluene (10 mL) were
charged. The mixture was refluxed for 3 h at 130 ꢀC. Upon
dehydration and slow removal of toluene, the temperature
3. RESULTS AND DISCUSSION
3.1. Monomer and Polymer Synthesis
The DHTPE monomer was synthesized by McMurry
coupling reaction18 and followed demethylation using
BBr3 with 62% yield. This monomer synthesis was
conducted according to our previous report.14 A series of
PDHTPE (30, 40, and 50) were successfully synthesized
via a K2CO3 catalyzed aromatic nucleophilic substitution
reaction of DHTPE with bis(4-fluorophenyl)sulfone and
bis(4-hydroxyphenyl) sulfone in DMAc/toluene solvent
system. The polymers, PDHTPE 30, 40, and 50, were
designed to different mol% of DHTPE unit, respectively.
A series of PMNF (30, 40, and 50) with a highly con-
jugated aromatic structure were successfully synthesized
via an intra-cyclization by Friedel-craft reaction with lewis
acid (FeCl3ꢁ. The sulfonation was carried out selectively
on MNF units with conc. sulfuric acid (Scheme 2). The
chemical structures of the PDHTPE, PMNF and SPMNF
ꢀ
was gradually increased to 160–180 C and the reaction
mixture was stirred for about 2 h until a highly viscous
solution was obtained. The resulting mixture was cooled
to room temperature and then poured into a mixture of
methanol (100 mL)/water (100 mL)/HCl (10 mL) to be
precipitated as a white fibrous polymer and collected by
filtration and washed with water several times. The poly-
mer collected by filtration was dried in a vacuum oven
BuLi/THF
4,4-dimethoxybenzophenone
PTSA/toluene
BBr3
HO
OH
1
were confirmed by H NMR. In Figure 1 (PDHTPE), Ha
peaks of sulfones’ ortho protons and Hb peaks of sulfones’
meta protons appeared at 7.95–8.09 and 7.25–7.30 ppm,
Scheme 1. Synthesis of DHTPE.
J. Nanosci. Nanotechnol. 14, 7948–7953, 2014
7949

Synthesis and Characterization of Sulfonated Poly(ether sulfone)s Containing Mesonaphthobifluorene for PEMFC
Lim et al.
O
S
O
S
HO
OH
F
F
O
O
HO
OH
Polymerization
O
O
S
O
S
O
S
O
O
O
O
O
O
m
n
PDHTPE
Friedel-craft
O
S
O
S
O
O
O
n
O
S
O
O
O
O
m
PMNF
HO3S
SO3H
Sulfonation
O
S
O
S
O
S
O
O
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O
O
O
O
m
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Copyright: American Scientific Publishers
SPMNF
SPMNF 30 (n=0.3, m=0.7), SPMNF 40 (n=0.4, m=0.6),
SPMNF 50 (n=0.5, m=0.5)
Scheme 2. Synthesis of SPMNFs.
respectively. The Hc peaks of tetraphenylethylene groups’
protons emerged at 6.86–7.18 ppm. The integration ratios
of proton peaks were confirmed by molar ratios of each
component of copolymer. In PMNF, Hd and He peaks of
MNF groups appeared at 8.38 and 8.53 ppm each. Hf and
degradation of the polymer main chain. The thermoox-
idative stability of polymers decreased as the content of
sulfonic acid increased. The IECs of SPMNF 30, 40, and
50 were 1.13, 1.42 and 1.71 meq./g, respectively, whereas
the IEC of the Nafion 211® was 0.90 meq./g as shown
in Table I. By increasing the percentage of sulfonic acid
group, there was an increase in the IEC of SPMNFs. The
theoretical IECs and the titrated IECs were in reasonably
good agreement. Table I exhibits the water uptake of SPM-
1
Hg peaks appeared at 7.79–7.84 ppm. H-NMR spectra
showed the decrease of tetraphenylethylene rings’ protons
ratio as new peaks were formed. That because of ortho
positional C–H bonds of multiphenyl rings converted to
C–C bond to form MNF. After sulfonation (SPMNF), Hf
peak was disappeared and He peak was shifted to 8.71 ppm
of Heꢁ .
ꢀ
NFs as a function of the sulfonation level at 80 C. The
water uptake of SPMNF 30, 40 and 50 were 23.24%,
34.17%, and 51.72% compared to 32.13% of Nafion 211®
ꢀ
at 80 C. The SPMNFs provided very low water uptake in
3.2. Properties of SPMNFs
spite of high IEC. The dimensional changes of SPMNFs
were evaluated by through-plane (ꢂt) and in-plane (ꢂl)
of membrane by comparing their hydrated state with dry
Thermooxidative stabilities of SPMNFs were studied by
thermogravimetric analysis (TGA) with results shown in
Figure 2.
ꢀ
state. The results of SPMNFs’ swelling in water at 80 C
The initial weight loss around 100–200 ꢀC was
attributed to residual solvent (DMSO) and water ꢀrelease
from SPMNFs. The second weight loss over 250 C was
assigned to the decomposition ofꢀ sulfonic acid groups.
The third weight loss around 500 C was assigned to the
were presented in Table I.
These membranes containing highly conjugated aro-
matic structure showed exceptional dimensional stability
compared to the reported hydrocarbon membrane.19–22 The
water uptakes of hydrocarbon membranes with similar IEC
7950
J. Nanosci. Nanotechnol. 14, 7948–7953, 2014

Lim et al.
Synthesis and Characterization of Sulfonated Poly(ether sulfone)s Containing Mesonaphthobifluorene for PEMFC
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Figure 1. 1H-NMR spectra of polymers.
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ꢀ
(1.7 meq./g) were reported from 70–108%, which are rela-
tively higher than that of SPMNF 50. The small change of
through-plane with high proton conductivity is desirable
behavior for maintaining dimensional stability during oper-
ation of PEMFC stack. Proton conductivities of the series
of SPMNFs were measured as a function of the mole frac-
tion of sulfonic acid groups. The proton conductivities of
SPMNFs were measured at 80 C in the range of 30–90%
ꢀ
RH, and at 40–80 C under 80% RH.
The results were estimated from AC impedance spec-
troscopy data and the results at different conditions were
presented in Figure 3, in comparison with Nafion 211®.
The proton conductivities of the resulting of SPMNFs were
in the range of 74.66 to 100.42 mS/cm at 80 ꢀC with
90% humidity compared to 102.75 mS/cm of Nafion 211®
ꢀ
at the same conditions. At 80 C and under conditions
of varied humidity, the proton conductivity of SPMNFs
showed increasing as humidity increases. The conductivi-
ties of all the samples increased in the range of evaluated
Table I. Properties of membranes.
Polymer
SPMNF 30 SPMNF 40 SPMNF 50 Nafion 211
IEC1 (meq./g)
IEC2 (meq./g)
WU at 30 ꢀC
WU at 80 ꢀC
ꢂt
1ꢃ10
1ꢃ13
1ꢃ40
1ꢃ42
1ꢃ67
1ꢃ71
0ꢃ91
0ꢃ90
–
32ꢃ13%
14ꢃ10%
13ꢃ84%
102ꢃ75
20ꢃ15%
23ꢃ24%
13ꢃ64%
7ꢃ53%
74ꢃ66
28ꢃ93%
34ꢃ17%
13ꢃ88%
11ꢃ35%
93ꢃ17
37ꢃ22%
51ꢃ52%
18ꢃ32%
17ꢃ99%
100ꢃ42
ꢂl
ꢄ (mS/cm)
Notes: IEC1: Theoretical IEC; IEC2: Titrated IEC; WU: Water uptake; ꢄ: proton
conductivity at 80 ꢀC under 90% RH.
Figure 2. TGA of polymers.
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7951

Synthesis and Characterization of Sulfonated Poly(ether sulfone)s Containing Mesonaphthobifluorene for PEMFC
Lim et al.
(A)
(B)
Figure 3. Proton conductivity of membranes A) at 80 ꢀC under different RH, B) at different temperature under 80% RH.
sulfuric acid. Compared with Nafion 211®, these SPM-
NFs showed reasonable IECs from 1.13 to 1.71 meq./g,
water uptake from 23.24 to 51.72%, proton conductiv-
ity from 74.66 to 100.42 mS/cm and high thermal sta-
bility. The proton conductivity of SPMNF 50 was almost
identical to Nafion 211®. These results showed that the
morphology of polymer matrix greatly affected the mem-
branes properties, and membrane included highly con-
jugated aromatic structure have excellent dimensional
stability in spite of high IEC. The properties rendered
by SPMNFs are very remarkable for fuel cell applica-
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tion. This research demonstrated the possibility of promis-
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ing SPMNFs for excellent proton conductivity and cell
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performance.
Figure 4. Accelerated chemical degradation test.
Acknowledgment: This work was supported by the
National Research Foundation of Korea Grant funded by
the Korean Government (MEST) (NRF-2009-C1AAA001-
0093168).
temperatures, and the membrane with higher IEC value
had higher proton conductivity. The proton conductivity of
SPMNF 50 was almost same as that of Nafion 211® at
80∼90% RH.
References and Notes
Ex situ hydrogen peroxide tests were carried out for
all membranes [SPMNFs, Nafion 211®, and SPES 40
(SPES 40: sulfonated linear poly(ether sulfone) containing
40 mol% of 4, 4-biphenol)] at the same temperature. The
tests were performed in hot Fenton reagent for 9 h to inves-
tigate accelerated chemical degradation of the membranes.
Weight loss of the membranes after the tests, were pre-
sented as the remaining weight of membranes as a function
of time, in Figure 4. The curves showed that all SPM-
NFs maintained 82 wt% after 9 h, whereas Nafion 211®
upholded its high stability. However, SPES 40 was almost
degraded after 9 h.
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A series of PMNFs were synthesized by polyconden-
sation and following intra-cyclization by Friedel-craft
reaction with lewis acid (FeCl3ꢁ. And the sulfonation
was carried out selectively on MNF units with conc.
7952
J. Nanosci. Nanotechnol. 14, 7948–7953, 2014

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Received: 5 October 2012. Accepted: 10 July 2013.
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7953