Synthesis and characterization of poly(para-phenylene disulfonic acid), its copolymers and their n-alkylbenzene grafts as proton exchange membranes: High conductivity at low relative humidity

Article abstract of DOI:10.1039/c2jm33066k

Water insoluble poly(para-phenylene disulfonic acid) and its copolymers were synthesized by direct polymerization of 1,4-dibromobenzene-2,5-disulfonic acid and 4,4′-dibromobiphenyl-3,3′-disulfonic acid lithium salts using Ullmann coupling and subsequent g

Full text of DOI:10.1039/c2jm33066k

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PAPER
Synthesis and characterization of poly(para-phenylene disulfonic acid), its
copolymers and their n-alkylbenzene grafts as proton exchange membranes:
high conductivity at low relative humidity
a
b
c
a
Kun Si, Daxuan Dong, Ryszard Wycisk and Morton Litt*
Received 15th May 2012, Accepted 2nd August 2012
DOI: 10.1039/c2jm33066k
Water insoluble poly(para-phenylene disulfonic acid) and its copolymers were synthesized by direct
0
0
polymerization of 1,4-dibromobenzene-2,5-disulfonic acid and 4,4 -dibromobiphenyl-3,3 -disulfonic
acid lithium salts using Ullmann coupling and subsequent grafting of long-tail alkylbenzene groups
ꢀ1
onto the polymer backbones. Copolymers with ion exchange capacities of 4.3 to 7.7 meq. g were
obtained. Polymers and copolymers prepared under optimized polymerization conditions were
characterized by NMR, TGA, DSC and viscometry. The physicochemical characteristics of the
copolymers were tailored by adjusting monomer compositions and by varying the grafting reaction
temperature and time. These polymers could hold eight or more strongly bound water molecules per
acid group, which facilitated their high conductivity. In the dry state, the polymers were very brittle.
Membranes prepared from these polymers exhibited proton conductivity as much as ten times higher
than that of Nafionꢀ 212, at elevated temperature and low relative humidity.
Introduction
confer high thermal stability, mechanical strength and reason-
ably good resistance to oxidation.
Proton exchange membrane fuel cells (PEMFCs) have attracted
the attention of many researchers as one of the next-generation
power technologies for automotive, stationary and portable
applications. The proton exchange membrane (PEM) is one of
the key challenges in the PEMFC development in terms of
performance and production cost. Perfluorosulfonic acid (PFSA)
polymers, such as Nafionꢀ, Aciplexꢀ, and Flemionꢀ, stand as
the state-of-art PEM materials due to their efficient proton
conduction, permselectivity, and long term thermal and chemical
During the past decade our group studied a number of rigid
rod, nematic liquid crystalline poly(aromatic sulfonic acid)
11,14,21–31
polymers and copolymers.
Since their backbones are
packed in parallel, incorporation of bulky grafts forces the chains
apart and generates long nanopores lined with sulfonic groups
that hold water very tightly. Conductivity remains relatively
high at low relative humidity, in contrast with Nafionꢀ and
poly(phenylene ether sulfonic acid) systems. We call this induced
21
inter-chain porosity a ‘‘frozen-in free volume’’. Initial work
11,25,29–31
used poly(naphthalene bis-imide)s
1–3
ꢀ1
stability. Nafion conductivity reaches 100 mS cm in the fully
but those polymers
hydrated protonic form, but decreases at elevated temperature
ꢁ
hydrolyzed slowly in the acid form. Recent efforts have focused
on poly(phenylene sulfonic acid)s, which do not hydrolyze and
show exceptional stability.
(
above 80 C) because of water loss and structural and
morphological changes. Thermally stable polymers with good
water affinity and reliability at higher temperatures are desired
for solid polymer electrolyte fuel cells. Acid-functionalized
aromatic hydrocarbon polymers such as poly(phenylene
A new synthetic method was developed to prepare the rigid rod
polymers, poly(biphenylene disulfonic acid), PBPDSA, and
poly(para-phenylene disulfonic acid), PPDSA, using the Ullmann
14,21–24,26
4
,5
6–8
9–11
sulfone)s,
poly(arylene ether ketone)s,
polyimides,
reaction.
Their conductivity at low relative humidity was
12–14
15–20
poly(phenylene)s,
and polybenzimidazoles
have been
remarkable compared to other PEMs. It met or exceeded US
Department of Energy objectives for 2010 and later. However,
these homopolymers were soluble in water and brittle when dry. In
the present study, we describe a new approach to obtain water-
insoluble materials by grafting long-tail alkylbenzenes on PPDSA
and its biphenyl disulfonic acid copolymer to generate hydro-
phobic alkyl phenyl sulfone substituent groups. The reaction
conditions were optimized and the resulting polymer and copol-
ymer structures were characterized using NMR, TGA, DSC and
viscometry. The structural characteristics of the polymers and
extensively investigated as alternative membrane materials for
PEMFCs. These polymers have rigid aromatic backbones that
a
Department of Macromolecular Science and Engineering, Case Western
Reserve University, Cleveland, OH 44106, USA. E-mail: mhl2@case.
edu; Fax: +1 216 368 4202; Tel: +1 216 368 4174
b
Department of Chemistry, Case Western Reserve University, Cleveland,
OH 44106, USA
c
Department of Chemical and Biomolecular Engineering, Vanderbilt
University, Nashville, TN 37235, USA
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ꢁ
copolymers could be readily tailored by adjusting monomer
compositions andbyvarying the graftingreactiontemperature and
time. The water uptake, dimensional swelling and proton
conductivity were studied. The membranes were relatively brittle
when dry. They exhibited proton conductivity much higher than
that of Nafionꢀ at elevatedtemperature and low relativehumidity.
100 mL 3-neck round bottom flask, heated to 80 C and stirred at
100 rpm. After 60 minutes, a homogeneous solution formed.
n-Octylbenzene, OcB (1.9 mL, 8 mmol), was added to the solution.
P O
2 5
(22.5 g, 160 mmol) was then added in batches of 0.5 g in order
ꢁ
to keep the temperature below 80 C. After the P
2
O
5
addition was
complete, an argon inlet and a condenser were fitted to the flask,
ꢁ
the temperature was raised to 125 C and the reaction mixture was
stirred for 20 hours under argon. After cooling to room temper-
ature, ice water (50 mL) was added to the mixture with vigorous
stirring; concentrated hydrochloric acid (15 mL) was added to
precipitate the polymer. The precipitate was centrifuged, washed
Experimental
Materials
All chemicals were purchased from Sigma-Aldrich Corporation.
Copper powder (99%, 400 mesh) was activated according to a
ꢁ
twice with 6 M hydrochloric acid and dried under vacuum at 90 C
for 24 hours. The product was further purified by dissolving it in
methanol (10 mL) and pouring the solution into hexanes (600 mL)
to precipitate the polymer and remove unreacted OcB. The
3
2
previously reported procedure, vacuum dried and used imme-
diately. N-Methyl-2-pyrrolidone (NMP) was dried by stirring
ꢁ
with calcium hydride at 60 C and distilled under vacuum before
0
ꢁ
precipitate was filtered and dried under vacuum at 90 C for
use. 1,4-Dibromobenzene (98%), 4,4 -dibromobiphenyl (98%),
4
8 hours. 1.70 g of purified grafted polymer was obtained. The
3
fuming sulfuric acid (12–17% SO ), n-octylbenzene (OcB, 98%),
grafting degree was determined by NMR as 11%. The grafted
polymer was labelled PPDSA-g-OcB11%. PPDSA-g-OcB17%,
B20P80-g-OcB07%, B20P80-g-OcB18% and n-dodecylbenzene
n-dodecylbenzene (DDB, 97%) and other reagents and solvents
were used as received.
0
0
4
,4 -Dibromobiphenyl 3,3 -disulfonic acid (DBBPDSA) was
0
(
DDB) grafted polymers were also obtained using similar proce-
synthesized from 4,4 -dibromobiphenyl using fuming sulfuric acid
dures by varying the reaction temperature and time.
26
according to a previously reported procedure. dH(600 MHz;
O; ddioxane ¼ 3.75): 8.05 (2H, d), 7.62 (2H, d), 7.35 (2H, q);
(150 MHz, D H), 138.4 (C–
The grafted polymers and copolymers were dissolved in iso-
propanol to prepare ꢂ20 wt% solutions and cast in 7 cm diameter
Teflon dishes. The dishes were covered with a filter paper and
stored in a closed cabinet for 24 hours at room temperature to let
D
2
d
C
2
O, ddioxane ¼ 67.19): 142.5 (C–SO
3
C), 136.2 (C–H), 131.0 (C–H), 127.5 (C–H), 119.4 (C–Br). 1,4-
Dibromobenzene 2,5-disulfonic acid (DBBDSA) was synthesized
the isopropanol evaporate. The dishes were then heated for
ꢁ
23
from 1,4-dibromobenzene using fuming sulfuric acid.
MHz; D (150 MHz, D
O; ddioxane ¼ 3.75): 8.34 (s, 2H); d
dioxane ¼ 67.19): 145.4 (C–SO H), 135.6 (C–H), 118.7 (C–Br).
d
H
(600
4
8 hours at 90 C under vacuum. The dried membranes could
2
C
2
O,
easily be removed from the dishes. 1 ꢃ 4 cm strips were then cut
d
3
from the membranes.
Polymerization
Characterizations
DBBDSA–Li salt(16.1 g, 0.04 mole) and freshly activated and pre-
dried copper powder (25.4 g, 0.4 mole) were placed in a 1000 mL
three-neck heavy-duty flask with a magnetic stirring bar, and dried
1
13
H and C spectra were recorded on a Varian Inova NMR
spectrometer operating at a proton frequency of 600 MHz and a
carbon frequency of 150 MHz. Deuterium oxide or deuterated
methanol was used to dissolve polymer samples, with added
dioxane or solvent residual peak as chemical shift reference.
Differential Scanning Calorimetry (DSC) was carried out on a
Modulated TA Q2000 Differential Scanning Calorimeter at a
ꢁ
at 135 C under high vacuum (0.03 Torr) for 48 hours. Freshly
distilled NMP (750 mL) was added to the system using a double-
tipped needle under argon pressure. The mixture was degassed by
ꢁ
bubbling argon for 30 minutes and then polymerized at 140 C for
3
6 hours under vigorous stirring. After polymerization, the
ꢁ
ꢀ1
heating rate of 10 C min at a nitrogen flow rate of 40 mL
mixture was filtered. The solid was stirred at room temperature
with 800 mL DI water for 24 hours and the mixture was filtered to
remove insoluble copper powder and CuBr. The water solution
was concentrated to ꢂ50 mL and purified by ultrafiltration using
ꢀ
1
min . The samples were equilibrated in a relative humidity
chamber before being hermetically sealed in an aluminium pan.
Thermal analysis was performed on a TA Instrument Q500
Thermogravimetric Analyzer (TGA) at a nitrogen flow rate of
0
.01 M HClsolution (2000mL). Thepurifiedpolymer solution was
ꢀ
1
ꢁ
ꢀ1
+
passed through an H ion exchange column to remove any
5
0 mL min and a heating rate of 20 C min using a sample
ꢁ
weight of 2–4 mg. The samples were dried at 110 C under
vacuum (10 Torr) for 48 hours before TGA analysis.
strongly bound cations. After water was evaporated, the polymer
ꢁ
was dried under vacuum (10 Torr) at 90 C for 48 hours to give
The viscosities of the polymers and copolymers were measured
as follows: a 50 mg polymer sample was dissolved in 10 mL of
dimethylformamide (DMF) and the solution was filtered using a
poly(para-phenylene, 2,5-disulfonic acid) (PPDSA) (7.8 g, yield ¼
0
8
4%). Poly(biphenylene 3,3 -disulfonic acid) (PBPDSA) and pol-
y(biphenyl disulfonic acid-co-phenylene disulfonic acid) copoly-
mers were made following a similar procedure. Thecopolymersare
denoted as BxP100-x (e.g., B20P80, B40P60), where ‘x’ is the mol
0
.45 mm syringe filter. The polymer solution was diluted six times
ꢁ
and the solution flow time (t) at 35.0 C was measured at each
concentration using a Cannon-Ubbelohde viscometer. The DMF
%
of biphenyl segments in the copolymer.
solvent flow time (t ) was also measured under the same condi-
0
tions. The relative viscosity (h ) is defined as h ¼ t/t and the
rel
rel
0
Grafting
specific viscosity (hsp) is defined as hsp ¼ hrel ꢀ 1. The reduced
viscosity is defined as hsp/c ¼ (hrel ꢀ 1)/c, where c is the
A typical grafting reaction was performed as follows: PPDSA
ꢀ1
(
2.0 g, 8 mmol) and H PO
3
4
(16.8 g, 150 mmol) were placed in a
concentration of polymer in grams per deciliter (g dL ).
2
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ꢁ
The equivalent weights (EWs) and ion-exchange capacities
IECs) of the polymer membranes were determined by titration
membranes were dried under vacuum (10 Torr) at 90 C for 24
hours and their length (Ldry), width (Wdry) and thickness (Tdry
(
)
of the acid forms. The membranes were dried in a vacuum
ꢁ
were measured again. The dimensional changes, DL, DW, DT,
and DV are defined as
(
10 Torr) at 90 C for 24 hours, weighed, and placed in an
aqueous 2 M NaCl solution for at least 24 hours. The solutions
were then titrated with a 0.01 M NaOH solution using bromo-
thymol blue as an indicator. The NaOH solution was calibrated
using potassium hydrogen phthalate immediately before use.
Our NMR and TGA studies showed that the membranes still
contained one water molecule per sulfonic acid group under
L
wet ꢀ Ldry
DL ¼
DW ¼
DT ¼
;
L
dry
W
wet ꢀ Wdry
;
W
dry
21,22
these drying conditions.
When determining EW, this one
ꢀ1
water molecule must be subtracted. So, the EW [g mol ] and
ꢀ
1
IEC [meq. g ] of the membranes were calculated using the
following equations:
T
wet ꢀ Tdry
;
T
dry
Wtdry
EW ¼ V
ꢀ MH O
(1)
(2)
and
2
NaOH ꢃ MNaOH
DV ¼ (1 + DL)(1 + DW)(1 + DT) ꢀ 1
1
000
IEC ¼
EW
where Wtdry is the weight of the ‘‘dry’’ membrane in grams and
NaOH is the molarity of the NaOH solution. MH O is the
The membrane proton conductivity was measured by elec-
trochemical impedance spectroscopy (EIS) using a four-electrode
cell connected to a Solartron 1260 frequency response analyzer
(FRA) and a Solartron 1287 potentiostat. The custom made
PTFE cell with four graphite electrodes was enclosed in a
stainless steel humidity chamber placed in a heating oven with a
built-in temperature controller. The cell temperature was set to
M
2
ꢀ1
molecular weight of water, 18 g mol , and VNaOH is the volume
of NaOH required to reach the end point in liters.
The membrane water uptake was determined by equilibrating
the samples in a series of relative humidity chambers at room
ꢁ
temperature. The membranes were first dried at 90 C for 48
ꢁ
hours and quickly weighed on a digital electronic balance. The
dry membranes in open weighing bottles were then put into each
of nine saturated salt solution chambers employed for humidity
80 C and the relative humidity was controlled by placing a small
container with a saturated salt solution inside the humidity
chamber. The following saturated salt solutions were employed
34
for humidity control at 80 C: LiCl (10.5%), KAc (15.1%),
MgCl (26.1%), K CO (41.1%), NaBr (51.4%), NaNO (57.4%),
33,34
ꢁ
control at room temperature:
MgCl (33.1%), K CO (43.2%), Mg(NO
66.0%), NaCl (75.9%), KCl (85.1%), and K
LiCl (11.3%), KAc (23.1%),
(54.4%), NaNO
SO (97.6%). The
2
2
3
3
)
2
2
2
2
3
2
(
2
4
3 2 4
NaNO (65.4%), KCl (78.9%), and K SO (94.1%). After the
membranes were weighed after reaching equilibrium (ꢂ48 hours)
at that humidity. Wt and Wt are the weights of the wet and
sample was equilibrated at a given humidity (ꢂ12 hours), AC
wet
dry
impedance scans were performed over a frequency range of 1 Hz
0
00
dry membranes and the water uptake (WU) was calculated by
to 20 kHz. The real (Z ) and imaginary (Z ) impedance
components were plotted and the resistance (R) was taken as
Wtwet ꢀ Wtdry
0
00
WU ¼
(3)
the value of Z when the curve was extrapolated to Z ¼ 0.
The humidity was increased by changing the saturated salt
solution, starting with the lowest relative humidity container.
The in-plane proton conductivity of the membrane was calcu-
lated using eqn (5):
Wtdry
Our earlier studies showed that the dry membranes still con-
tained one water molecule per sulfonic acid group under the
21,22
current drying conditions.
The number of water molecules
per sulfonic acid, l, at a given relative humidity can be calculated
L
s ¼
(5)
by the following equation:
À
R ꢃ W ꢃ T
Á
ꢀ
1
Wtwet ꢀ Wtdry ꢃ ðEW þ MH O
Þ
2
where s is the proton conductivity (S cm ), L the distance
between the reference electrodes (cm), R the membrane resis-
tance (U), W the membrane width (cm) and T the membrane
thickness (cm).
l ¼
þ 1
(4)
Wtdry ꢃ MH O
where EW is the equivalent weight of the polymer and MH O the
2
2
ꢀ1
molecular weight of water, 18 g mol . The water contents of the
1
equilibrated and the dry samples were also analyzed by
H
NMR. The samples were dissolved in deuterated methanol and
the water to polymer peak was correlated. In order to correct the
OH content of the deuterated methanol, its NMR spectrum was
Results and discussion
Polymerization
0
measured. The solution CD HOD area was multiplied by the
2
1,4-Dibromobenzene and 4,4 -dibromobiphenyl were sulfonated
OH/CD
2
HOD area ratio in the reference and subtracted from
using fuming sulfuric acid to obtain 1,4-dibromobenzene-2,5-
0
0
the OH area.
disulfonic acid (DBBDSA) and 4,4 -dibromobiphenyl-3,3 -
disulfonic acid (DBBPDSA) monomers. Homopolymerization
of DBBDSA and DBBPDSA, and their copolymerization at a
The lengths (Lwet), widths (Wwet) and thicknesses (Twet) were
also measured for membranes equilibrated using saturated KCl
solution (85.1% RH) at room temperature. Then these
ꢁ
4 : 1 mole ratio were carried out at 140 C in N-methyl
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d(b) ¼ 7.85, and d(c) ¼ 7.64 ppm. The small peaks at 8.22 and
0
8
.37 ppm are from the end group 2 and 2 protons. The ratio of
the peak areas at 8.45 to that at 8.22 ppm gives a measure of the
degree of polymerization of PBPDSA, 42 for this polymer, a
ꢀ
1
.
molecular weight of 13 000 g mol
1
A typical H NMR spectrum of PPDSA is shown in Fig. 1(B).
From the PPDSA structure, one would expect a single peak;
however, there are two main peaks about 0.1 ppm apart in the
region of 8.00–8.20 ppm. The peak area at 8.12 ppm is usually
about 1/3 that at 8.20 ppm. This smaller peak is probably due to
conformational isomerism of the rigid rod backbone. Other
small peaks at 7.86, 7.88, 7.94, 8.05, and 8.15 ppm (referred to as
the solvent CD HOD peak at 3.31 ppm) are attributed to end
2
group protons. If there are no side reactions, we would expect
one or at most, two peaks from the end residues. Since there are
many more small peaks, it is possible that some of the copper
activated C–Br end groups reacted with trace amounts of water
during or after polymerization to form C–H end groups. Using
the 7.94 ppm peak area as reference (one terminal hydrogen), the
degree of polymerization of PPDSA is estimated to be 120, a
Scheme 1 Synthesis of PPDSA, PBPDSA and B20P80.
ꢀ
1
molecular weight of 28 000 g mol
.
1
Fig. 1(C) shows the H NMR spectrum of copolymer B20P80.
pyrrolidinone (NMP) using copper as the coupling reagent
0
(
Ullmann reaction), as outlined in Scheme 1.
The two main peaks in the PPDSA homopolymer, assigned as a ,
The reaction is heterogeneous and the polymer precipitates
become narrower in copolymer B20P80, with one extra peak in
the middle. As shown in Fig. 1, the a proton in the copolymer has
the same chemical shift as in PBPDSA homopolymer, but b and c
protons shift to a higher ppm compared to the homopolymer due
to the changed chemical environment in the copolymer. Since the
end group peaks in the copolymer spectrum overlap with the
main peaks, the degree of polymerization of B20P80 could not be
during polymerization, which limits the molecular weight. The
mechanism of Ullmann coupling has been extensively
35–38
studied.
It was proposed that coordination of the electron-
withdrawing group ortho to the carbon–halogen bond with the
copper surface facilitates the transfer of electron density from the
35,36
copper to the carbon attached to the halogen.
This complex
1
then acts as a nucleophile to attack another molecule of aryl
estimated by H NMR.
halide, resulting in the desired carbon–carbon bond formation.
1
The H NMR spectrum of PBPDSA in CD OD is shown in
Some chemical shift differences between the copolymer and
13
homopolymers can also be seen in their C NMR spectra, Fig. 2.
3
Fig. 1(A). The three main peaks are assigned to three chemically
Since the biphenyl moieties and phenyl moieties have very similar
structures and any interactions between these structures are
distinguishable protons in the PBPDSA backbone: d(a) ¼ 8.45,
1
Fig. 1 600 MHz H NMR spectra of PBPDSA (A), PPDSA (B) and B20P80 (C) in CD
3
OD.
2
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13
3
Fig. 2 150 MHz C NMR spectra of PBPDSA (A), B20P80 (B) and PPDSA (C) in CD OD.
4
–5 bonds away, the changes between homo- and co-polymers
in Scheme 2. The grafting degree, defined as the mol% of sulfonic
acid in the polymer or copolymer reacted with alkylbenzene to
form sulfone groups, was controlled by varying the reaction time
1
are very tiny in C spectra.
3
1
and temperature. It was measured by H NMR.
1
A typical H NMR spectrum of the purified PPDSA-g-
Grafting reaction
n-Octylbenzene (OcB) and n-dodecylbenzne (DDB) were grafted
OcB11% is shown in Fig. 3. The peaks at 0.89, 1.29, 1.67 and
onto PPDSA and B20P80 in polyphosphoric acid, as illustrated
2
.66 ppm are from the n-octyl group protons of the grafted OcB.
The peaks from 7 to 9 ppm are from the PPDSA backbone
protons plus the aromatic protons of the grafted OcB. For an
OcB grafted PPDSA, the grafting degree was calculated as
follows. The aliphatic proton area between 0.5 and 3.0 ppm was
set to 17. An area of 4 (remaining protons on the grafted alkyl-
phenyl sulfone) was subtracted from the total aromatic proton
area between 7.0 and 9.0 ppm. The numerical inverse of the
remaining aromatic proton area is the grafted mole fraction. For
Fig. 3, the grafting degree is calculated as 1/(13.2 ꢀ 4) ꢃ 100% ¼
1
1%. The grafted polymer is denoted as PPDSA-g-OcB11%. It
was soluble in DMF, DMSO, DMAc, methanol, ethanol,
-propanol, 1-butanol, 1-hexanol, etc., but insoluble in hexane,
2
methylene chloride and water. The other OcB and DDB grafted
polymers were obtained using the same procedure, but varying
the reaction temperature and time.
Characterizations
The viscosities of the protonated polymers and grafted polymers
ꢁ
in DMF solution were measured at 35.0 C. The reduced viscosity
as a function of concentration for some representative polymers
ꢀ
1
is shown in Fig. 4. The reduced viscosities at ꢂ0.5 g dL are 0.6
ꢀ
1
and 0.7 dL g for PPDSA and B20P80 respectively. The reduced
viscosities of both PPDSA and B20P80 increase when the poly-
ꢀ
1
mer concentration is below 0.2 g dL . The viscosities of B20P80-
Scheme 2 Synthesis of n-octylbenzene (OcB) and n-dodecylbenzene
(DDB) grafted polymers.
g-OcB18% and PPDSA-g-DDB24% are higher than those of the
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1
3
Fig. 3 600 MHz H NMR spectrum of PPDSA-g-OcB11% in CD OD (water and solvent peaks were suppressed for illustration purpose).
ꢀ
1
ꢀ1
1
base polymers, 1.2 and 1.9 dL g at ꢂ0.5 g dL , rising when the
macroions rather than due to conformational changes. A self-
consistent mode-coupling theory was proposed by Miyazaki
ꢀ
polymer concentration is below 0.3 g dL . All the grafted
polymers behave similarly. When viscosities were measured in
dilute LiCl solutions (not shown here), they were slightly lower
but followed the same trend as in salt-free solution, rising when
44
ꢀ1
et al. The reduced viscosities at ꢂ0.5 g dL concentration, plus
other characterization, for some polymers and grafted polymers
are listed in Table 1. The reduced viscosities of all grafted poly-
mers are higher than those of base polymers; the higher the OcB
or DDB content, the higher the reduced viscosity.
ꢀ1
polymer concentration dropped below 0.2–0.3 g dL . This is not
a typical polyelectrolyte effect. A similar phenomenon was also
39
40
41
found in nitrated, sulfonated and carboxylated poly(p-
phenylene) polymers. These polymer backbones are linear and
rigid, very different from flexible polyelectrolytes in dilute solu-
tion. Several theories have been proposed to describe these
The equivalent weights (EWs) and ion exchange capacities
(IECs) of PPDSA, B20P80 and their grafted polymers were
determined by titration; the results are given in Table 1. Our
ꢁ
earlier studies showed that vacuum dried (10 Torr, 90 C)
42–44
43
unusual characteristics.
Ballauff et al. suggested that this
PPDSA membranes still contained about one water molecule per
21,22
‘
very pronounced polyelectrolyte effect’ was caused by a long-
sulfonic acid group.
The EW values listed in Table 1 have
range intermolecular electrostatic repulsion of the dissolved
already been corrected for that water content and are in good
agreement with the theoretical ones within experimental error.
As a comparison, data for Nafion NR-212 are also listed.
The thermal stability of the polymer and grafted polymer
membranes was investigated by thermogravimetric analysis
(
TGA). Fig. 5 shows representative TGA curves for some poly-
mers. Most of them show a 3-stage degradation behaviour.
ꢁ
Although the polymers were dried under vacuum at 110 C for
4
8 hours immediately prior to the analysis, they still showed an
initial weight loss due to the evaporation of tightly bound water,
ꢁ
and plateaued at or above 210 C. All tightly bound water was lost
ꢁ
ꢁ
below 210 C. PPDSA remains stable to about 310 C; the other
polymers start losingweight ata somewhatlower temperature. The
weight fraction lost in the second weight loss stage for the base
polymer corresponds to a loss of about one acid plus one water
molecule for every two acid groups. A third stage of weight loss
ꢁ
starts at about 510 C; this might be associated with the degrada-
tion of polymer aromatic backbones or further acid loss. The TGA
curve of B20P80 is similar to that of PPDSA. The initial weight loss
is due to the evaporation of tightly bound water. B20P80 has two
further stages of weight loss, like PPDSA. However, B20P80 starts
Fig. 4 The reduced viscosity as a function of concentration for proton-
ated PPDSA, B20P80, B20P80-g-OcB18% and PPDSA-g-DDB24% in
ꢁ
ꢁ
to lose sulfonic acid groups at 280 C, 30 C lower than PPDSA;
ꢁ
DMF at 35.0 C. (Cannon-Ubbelohde viscometer, type: 0C C453).
because it has 33% p-phenylene mono-sulfonic acid moieties, it is
2
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Table 1 Reduced viscosity (hsp/c), equivalent weight (EW), ion exchange capacity (IEC), proton conductivity (s) and solubility of PPDSA, B20P80 and
their grafted polymers, together with Nafion NR-212 as comparison
a
sp/c
b
c
b
d
s
h
EW
EWcal
IEC
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
S cm
Sample
dL g
g mol
g mol
meq. g
Solubility (H-form)
PPDSA
B20P80
0.6
0.7
1.0
1.1
0.9
1.4
1.2
1.2
1.9
n/a
120
125
130
157
159
160
175
193
230
1050
118
126
128
148
154
157
173
191
227
1100
8.3
8.0
7.7
6.4
6.3
6.3
5.7
5.2
4.3
0.95
0.39
0.44
0.28
0.26
0.15
0.18
0.13
0.09
0.05
0.03
H
H
H
2
2
2
O, alcohol, DMF, DMAc, DMSO
O, alcohol, DMF, DMAc, DMSO
O, alcohol, DMF, DMAc, DMSO
PPDSA-g-DDB03%
B20P80-g-OcB07%
PPDSA-g-OcB11%
PPDSA-g-DDB10%
PPDSA-g-OcB16%
B20P80-g-OcB18%
PPDSA-g-DDB24%
Nafion NR-212
alcohol, DMF, DMAc, DMSO
alcohol, DMF, DMAc, DMSO
alcohol, DMF, DMAc, DMSO
alcohol, DMF, DMAc, DMSO
alcohol, DMF, DMAc, DMSO
alcohol, DMF, DMAc, DMSO
n/a
a
ꢀ1
ꢁ
b
c
d
Measured at 0.5 g dL in DMF, 35.0 C. Determined by titration. Calculated EW, based on structure. Measured at 80 C, 51% RH.
ꢁ
less stable than PPDSA with 100% p-phenylene di-sulfonic acid
repeat groups (two strong electron-withdrawing groups). After the
initial water loss, grafted polymers PPDSA-g-DDB24% and
B20P80-g-OcB18% start to lose sulfonic acid groups at about
Water uptake and dimensional changes
The water uptake of a PEM plays an important role in its proton
conductivity and mechanical properties. Generally, the absorbed
water facilitates proton transport, but excessive water uptake
decreases polymer modulus and strength as well as proton
conductivity. Therefore, water uptake needs to be controlled in
the appropriate range. The water uptake and l at room
ꢁ
ꢁ
2
55 C. The large fractional weight loss between 255 and 500 C
implies that grafted long-tail alkylbenzene groups probably are
being lost, as well as acid groups.
The water content of the dried polymer samples can be esti-
mated from the TGA curves. For PPDSA, the drop due to water
ꢁ
loss up to 210 C is 13.1 weight percent; the remaining polymer
weight percent is 86.9. l, the mole ratio of water (molecular weight,
ꢀ
1
ꢀ1
ꢀ1
1
8 g mol ) to polymer PPDSA (EW ¼ 118 g mol ), is 13.1 ꢃ 118/
(
18 ꢃ 86.9) z 1.0. For B20P80 (EW ¼ 126 g mol ), the weight
ꢁ
loss up to 210 C is 11.4 percent; the remaining polymer is 88.6
weight percent; l is 11.4 ꢃ 126/(18 ꢃ 88.6) z 0.9. For PPDSA-g-
ꢀ
1
DDB24% (EW ¼ 227 g mol ), l is 8.3 ꢃ 227/(18 ꢃ 91.7) z 1.1;
ꢀ1
while for B20P80-g-OcB18% (EW ¼ 191 g mol ), l is 9.7 ꢃ 191/
18 ꢃ 90.3) z 1.1. The dry polymer water content determined by
TGA is in good agreement with the titration results, Table 1, and
(
21,22
consistent with our previous findings for PPDSA.
Fig. 5 TGA curves for PPDSA, B20P80, PPDSA-g-DDB24% and
Fig. 6 Water uptake (top) and lambda (bottom) as a function of relative
humidity at room temperature for PPDSA, PPDSA-g-DDB10% and
PPDSA-g-DDB24% membranes.
ꢀ1
B20P80-g-OcB18% at a nitrogen flow rate of 40 mL min and at a
ꢀ1
ꢁ
heating rate of 20 C min
.
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respectively. B20P80-g-OcB07% and B20P80-g-OcB18%
membranes have a Young’s modulus of ꢂ6 GPa and an elon-
gation at break of ꢂ1% at 20% RH (stress–strain curves not
shown), which was indicative of inherent brittleness of this
material. At high relative humidity, the membranes became very
soft and lost their mechanical strength. In boiling water, the
membranes broke into small pieces but did not dissolve.
The water uptake and dimensional changes of some
membranes from 0 to 85% RH were also investigated and the
results are summarized in Table 2. At 85% RH, both PPDSA and
B20P80 absorbed more water than their own weights and became
very soft, making it impossible to measure their dimensional
changes. Within experimental error, all the grafted polymer films
swelled equally in lateral dimensions (in-plane direction) but
much more in thickness, indicating that these membranes are
anisotropic. It may be speculated that almost all polymer mole-
cules are in domains with their backbone parallel to the
membrane surface. As the membrane absorbs water, chain
separation increases. Swelling in the plane is hindered by other
domains with chains oriented perpendicular to the swelling
direction, but the through-plane swelling has no such hindrance.
It can be seen from Table 2 that through-plane swelling
decreased as long-tail alkyl benzene grafting increased. This can
be rationalized by postulating that the long-tails act as physical
crosslinks due to hydrophobic bonding and limit the swelling in
this direction.
State of water
Since the polymers and their grafts absorbed a large amount of
water when equilibrated at high RH, the question arises – what
fraction was strongly bound? Therefore, low-temperature DSC
was performed on samples equilibrated at different RHs; typical
examples are shown in Fig. 8.
Fig. 7 Water uptake (top) and lambda (bottom) as a function of relative
humidity at room temperature for B20P80, B20P80-g-OcB07% and
B20P80-g-OcB18% membranes.
temperature of some selected polymer membranes as a function
of relative humidity are shown in Figs. 6 and 7.
B20P80-g-OcB18% equilibrated at 98% RH shows an
ꢁ
exothermic freezing peak at about ꢀ25 C in the cooling cycle
PPDSA fractional water uptake increases with increasing
relative humidity, reaching 2.13 at 98% RH which corresponds to
absorption of 17.1 water molecules per sulfonic acid group. At
this high humidity the PPDSA membrane loses its mechanical
stability, deforms and sticks to the glass weighing vial. The water
uptake at high relative humidity decreases dramatically with
increasing grafted long-tail alkylbenzene content. At 98% RH, a
dried 10% DDB graft’s weight increased by a factor of 1.40 while
that of a 24% DDB graft increased only 0.77, corresponding to ls
of 14.6 and 11.6, respectively. The decreasing water uptake and l
is probably due to the hydrophobic bonding of the grafted
alkylbenzene groups that act as physical crosslinks preventing
the polymer gel from expanding indefinitely at high relative
humidity. Below 75% RH, l at a given relative humidity is
independent of the graft content; the curves overlap that of
PPDSA. DDB grafted PPDSA membranes are brittle at low
relative humidity. At high relative humidity, the materials
become very soft and lose their strength very quickly.
ꢁ
and an endothermic melting peak at about ꢀ5 C in the heating
cycle, which indicates the presence of loosely bound, freezable
water in the polymer membrane. The areas of these two peaks are
ꢀ1
very close; the average is about 68 J g . Since the heat of fusion
ꢀ1 45
of water is about 333.6 J g
, the freezable water in this grafted
copolymer was calculated as 0.20 g per gram of sample. The
water uptake was about 0.98 for B20P80-g-OcB18% equilibrated
Table 2 Water uptakes and dimensional changes of some membranes
from 0 to 85% RH at room temperature
Dimensional change
a
a
a
a
DT
a
DV
Sample
WU
DL
DW
PPDSA
B20P80
B20P80-g-OcB07%
PPDSA-g-OcB11%
B20P80-g-OcB18%
PPDSA-g-DDB24%
1.32
1.19
0.95
0.94
0.74
0.59
—
—
0.14
0.13
0.13
0.12
—
—
0.14
0.13
0.14
0.13
—
—
0.49
0.48
0.42
0.29
—
—
0.94
0.89
0.83
0.63
The fractional water uptake of B20P80 reached 1.97 at 98%
RH, corresponding to 16.7 water molecules per sulfonic acid
group. Grafting with n-octylbenzene, OcB, reduced the frac-
tional water uptake to 1.38 for B20P80-g-OcB07%, and 0.98 for
B20P80-g-OcB18%, corresponding to ls of 13.8 and 12.3,
a
WU, DL, DW, DT and DV are the water uptake, length, width,
thickness and fractional volume changes of the membranes from 0 to
8
5% RH.
2
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Fig.
9
Proton conductivity of PPDSA and its grafted polymer
ꢁ
Fig. 8 DSC thermograms of the B20P80-g-OcB18% membrane (heating
ꢀ1
membranes as a function of relative humidity at 80 C.
ꢁ
rate: 10 C min ). Samples were equilibrated at 98% or 54% RH prior to
measurements.
temperature and 100% RH, the numbers of non-freezable bound
water molecules per sulfonic group in Nafion 117 were reported
47
as 9 and 10 by Lue and Shieh, and Corti et al., respectively at
at 98% RH, corresponding to ꢂ0.54 g water per gram of sample,
46
3
including one water molecule per SO H in the dried sample. The
4
8
1
00% RH using FTIR, but about 5–7 using DSC by Pineri et al.
freezable water fraction, determined by DSC, is 0.20/0.54 ¼ 0.37
of the total water uptake. The rest, 0.63, is non-freezable
water, strongly bound to the sulfonic acid groups. When the
water content is calculated as lambda, out of the total 12.3 water
molecules per sulfonic acid group, there were 4.6 freezable water
molecules and 7.7 non-freezable water molecules. When the
polymer was equilibrated at 54% RH, l ¼ 5.5, there were no
exotherms or endotherms within the scanned temperature range
ꢁ
48
At 0 C and 100% RH, Pineri et al. found a l of 6.0. Table 3
shows that the effect of grafting OcB or DDB is to decrease the
loosely bound, freezable water in the polymers.
Proton conductivity
Proton conductivity is very important for fuel cell performance.
High proton conductivity is needed to achieve high power
densities in PEMFCs. The proton conductivity of PPDSA,
B20P80 and their grafted polymers, as well as Nafion NR-212 for
(
Fig. 8, bottom), suggesting that all the absorbed water was
strongly bound. The total l and its freezable and non-freezable
components for all samples equilibrated at 98% RH, determined
by DSC, are listed in Table 3. Within the experimental error, the
non-freezable water content for all membranes is ꢂ8 water
molecules per sulfonic acid. As a comparison, at room
comparison, as a function of relative humidity was measured at
ꢁ
8
0 C, Fig. 9 and 10.
As can be seen, all the new polyelectrolytes have higher proton
ꢁ
conductivity than Nafionꢀ NR-212. At 80 C, PPDSA and
B20P80 conductivities are about 11-14 times higher than that of
Table 3 The nature of absorbed water determined by DSC for PPDSA,
B20P80 and their grafted polymers equilibrated at 98% RH at room
temperature compared with that of Nafion 117 (at 100% RH)
l
a
b
frz
c
Sample
WU
Total
nfrz
PPDSA
B20P80
2.13
1.97
1.89
1.38
1.44
1.40
1.20
0.98
0.77
0.24
0.22
0.18
17.1
16.7
16.3
13.8
14.8
14.6
13.7
12.3
11.6
14.7
13.4
11.0
8.1
7.9
7.0
6.7
7.0
6.1
6.1
4.6
3.5
5.6
3.1
4.2
9.0
8.8
9.3
7.1
7.8
8.5
7.6
7.7
8.1
9.1
10.3
6.8
PPDSA-g-DDB03%
B20P80-g-OcB07%
PPDSA-g-OcB11%
PPDSA-g-DDB10%
PPDSA-g-OcB16%
B20P80-g-OcB18%
PPDSA-g-DDB24%
Nafion 117
Nafion 117
Nafion 117
d
e
f
a
b
c
Water uptakes. Freezable water. Non-freezable water. Data
d
e
f
calculated from ref. 46. Data calculated from ref. 47. Data
calculated from ref. 48.
Fig. 10 Proton conductivity of B20P80, B20P80-g-OcB07%, B20P80-g-
ꢁ
OcB18% and Nafion NR-212 as a function of relative humidity at 80 C.
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Nafionꢀ NR-212 over the measured relative humidity range. It
should be noted that the conductivity of B20P80 exceeds that of
PPDSA. We believe that the presence of the biphenyl segments
might contribute to an increase in the frozen-in free volume with
a resultant increase of proton conductivity. No structural char-
acterization was performed, however, to quantify the confor-
mational or configurational differences. Since PPDSA and
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2
ꢀ1
B20P80 have a very high IEC ($8.0 meq. g ) and are water
soluble, at high relative humidity the membranes absorb large
amounts of water, swell significantly and eventually deform; their
conductivity could not be measured above 65% RH. Among all
the grafted polymer membranes, the conductivity of PPDSA-g-
DDB03% is very close to that of PPDSA, but the resultant
membrane is still water-soluble, losing its mechanical strength
and dimensional stability at higher relative humidity. The proton
conductivity of the polymer membranes decreases with the
increase of alkylbenzene graft fraction. However, PPDSA-g-
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PPDSA-g-OcB11%
and
B20P80-g-OcB07%
membranes are water insoluble with proton conductivity about
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ꢀ
1
ꢁ
0
.14 S cm at 30% RH and 80 C reaches or exceeds the US
6
715.
ꢀ1
Department of Energy 2015 target of 0.10 S cm . The observed
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21
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Conclusions
Poly(para-phenylene disulfonic acid), PPDSA, and poly-
(
biphenyl disulfonic acid-co-phenylene disulfonic acid), B20P80,
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4
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were measured. These polymers had a very high IEC and
absorbed water strongly at low humidity, which facilitated fast
proton transport. Membranes cast from these polymers exhibi-
ted excellent proton conductivity at elevated temperature and
low relative humidity, tenfold higher than that of Nafionꢀ 212.
This exceptional conductivity could make these materials great
candidates for membranes in proton exchange fuel cells oper-
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2
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