Poly(aryl ether ketone)s with carboxylic acid groups: Synthesis, sulfonation and crosslinking

Article abstract of DOI:10.1039/b806690f

Carboxylic acid-containing poly(aryl ether ether ketone) (PFEEK-COOH) and poly(aryl ether ether ketone ketone) (PFEEKK-COOH) were synthesized from the monomer 9,9-bis(4-hydroxyphenyl)-fluoren-4-carboxylic acid. The latter polymer, PFEEKK-COOH, was post-sulfonated to yield SPFEEKK-COOH whereby the site and the degree of sulfonation (DS from 0 to 3) could be controlled. The reaction progress and structures of the resulting sulfonated polymers were identified by 1D and 2D NMR techniques. The carboxylic acid group was used as to crosslink SPFEEKK-COOH by reaction with poly(vinyl alcohol) (PVA) to prepare crosslinked membranes. The proton conductivity of a crosslinked SPFEEKK-COOH-1.6/PVA membrane was ～0.15 S cm^-1 at 65 °C, while maintaining acceptable dimensional stability in water. Thermal stability, water uptake and proton conductivity of carboxylated, carboxylated/sulfonated, and crosslinked membranes were investigated.

Full text of DOI:10.1039/b806690f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Poly(aryl ether ketone)s with carboxylic acid groups: synthesis, sulfonation
and crosslinking†
b
a
a
Baijun Liu,ab Wei Hu, Gilles P. Robertson and Michael D. Guiver
*
Received 21st April 2008, Accepted 18th July 2008
First published as an Advance Article on the web 28th August 2008
DOI: 10.1039/b806690f
Carboxylic acid-containing poly(aryl ether ether ketone) (PFEEK–COOH) and poly(aryl ether ether
ketone ketone) (PFEEKK–COOH) were synthesized from the monomer 9,9-bis(4-hydroxyphenyl)-
fluoren-4-carboxylic acid. The latter polymer, PFEEKK–COOH, was post-sulfonated to yield
SPFEEKK–COOH whereby the site and the degree of sulfonation (DS from 0 to 3) could be controlled.
The reaction progress and structures of the resulting sulfonated polymers were identified by 1D and 2D
NMR techniques. The carboxylic acid group was used as to crosslink SPFEEKK–COOH by reaction
with poly(vinyl alcohol) (PVA) to prepare crosslinked membranes. The protonꢂconductivity of
a crosslinked SPFEEKK–COOH-1.6/PVA membrane was ꢀ0.15 S cmꢁ1 at 65 C, while maintaining
acceptable dimensional stability in water. Thermal stability, water uptake and proton conductivity of
carboxylated, carboxylated/sulfonated, and crosslinked membranes were investigated.
content by monomer ratio adjustment. However, relatively few
Introduction
sulfonated monomers have been developed so far,3 for reasons
such as control of sulfonation and monomer purification.
Although some post-sulfonated polymers may exhibit poor
controllability over both DS and sulfonation site, there are
a number of post-sulfonated polymer systems with well-defined
structure through rational design of the monomers that
comprise the polymer structure.11
Fuel cells, which are devices for transforming chemical energy
directly into electricity, are considered to be promising clean
power sources. A proton-exchange membrane (PEM) is a key
component in the fuel cell membrane electrode assembly
(MEA) for transferring protons from the anode to the cathode
while providing a barrier to fuel crossover between the elec-
trodes in proton-exchange membrane fuel cells (PEMFCs) and
in direct methanol fuel cells (DMFCs).1,2 There is no doubt that
perfluorinated sulfonic acid (PFSA) copolymers, such as
Nafion, have some excellent properties for fuel cell devices as
well as some negative aspects. The search for non-fluorinated
hydrocarbon polymers having high proton conductivity, low
fuel crossover, excellent chemical and electrochemical stability,
low cost and lower environmental impact as alternatives to
PFSAs is an ongoing area of intensive research.3 To date, many
sulfonated high-temperature hydrocarbon-based polymers,
such as polyimides,4,5 polyphenylene,6,7 polybenzimidazole8 and
poly(phenylquinoxaline)9 have been investigated. Poly(aryl
ether)-type polymers (PAEs), such as poly(aryl ether ketone)s
(PAEKs) and poly(aryl ether sulfone)s, are a class of high
performance engineering thermoplastics known for their excel-
lent combination of chemical stability, physical and mechanical
properties.10 Most of the sulfonated PAE-type polymers were
developed based on post-sulfonated polymers or on copolymers
produced from sulfonated monomers.3 In comparison with
some polymers prepared by the post-sulfonation method,
copolymers obtained by direct copolymerization of sulfonated
monomers have advantages in the controllability of sulfonic
Ionomeric hydrocarbon-based polymers with sulfonic
(–SO3H), phosphonic (–PO(OH)2),12–14 and carboxylic (–COOH)
acid groups15–17 are attracting increasing attention for their
potential applications as proton-exchange membranes in fuel
cells. The number of carboxylated polymers investigated as PEM
materials are rather limited due to their lower acidity compared
with –SO3H. However, PEM materials containing –COOH,
particularly those that have chemical stability, are of potential
interest as they present convenient sites for crosslinking,18,19
which is often otherwise achieved through the –SO3H group.
Maintaining dimensional stability for high proton-conduc-
tivity membrane materials under humidification is one of the key
issues that needs to be addressed in the design of effective PEM
materials. The introduction of sites for crosslinking membranes
by covalent or other interaction may be an effective avenue for
controlling dimensional swelling. In polymers with an optimal
degree of crosslinking, improved properties such as solvent
resistance, thermal stability, mechanical strength and oxidative
stability are often observed.20–22 Lee et al. recently reported
PEMs crosslinked by an esterification reaction between –COOH
and –OH.18,19 In the present study, two types of poly(aryl ether
ketone)s containing carboxyl acid group attached to fluorenyl
were prepared. The polymer PAEEKK was post-sulfonated
under mild reaction conditions for different time periods to yield
sulfonated derivatives (SPFEEKK–COOH) with DS ꢀ0 to 3 per
repeat unit and at three specific sulfonation sites. The resulting
SPFEEKK–COOH could be crosslinked at the –COOH site
with poly(vinyl alcohol) (PVA). Selected properties of these
aInstitute for Chemical Process and Environmental Technology, National
Research Council, Ottawa, Ontario, Canada K1A 0R6. E-mail: michael.
guiver@nrc-cnrc.gc.ca; Fax: +1 613-991-2384
bAlan G. MacDiarmid Institute, Jilin University, Changchun 130012, P.R.
China
† NRCC publication No. 49179.
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carboxylated, carboxlated/sulfonated, and crosslinked polymers
have been investigated, which are expected to be suitable PEMs
in direct methanol fuel cells.
Results and discussion
Synthesis of monomer and polymers containing a –COOH group
A
carboxylic acid-containing monomer, 9,9-bis(4-hydrox-
yphenyl)-fluorenone-4-carboxylic acid, was synthesized in pure
monomer-grade form using an improved two-step reaction, as
shown in Scheme 1. A synthesis and purification procedure to
produce the monomer on a scale of hundreds of grams was
briefly described in a patent by us23 and is described in more
detail here. The structure of the monomer was confirmed by
elemental analysis and NMR spectra. The fully assigned 1H
and 13C NMR spectra of the monomer are shown in Fig. 1.
Polymerization of the carboxylic acid-containing monomer
withactivateddifluoromonomers, suchas1,4-bis(4-fluorobenzoyl)
benzene or 4,40-difluorobenzophenone, was conducted by typical
nucleophilic polymerization in dimethyl sulfoxide (DMSO) in
the presence of K2CO3. No crosslinking occurred, even after
prolonged polymerization time, at temperatures in the range of
160 to 210 ꢂC. High molecular weight polymers containing
pendant carboxylate were readily obtained within several hours,
which suggested that the monomer was of sufficient purity and
high reactivity (Table 1). The obtained polymers in potassium
carboxylate form were transformed to the acid form (PFEEK–
COOH and PFEEKK–COOH), as shown in Scheme 2. Both
PFEEK–COOH and PFEEKK–COOH could form strong,
flexible and transparent films by solution casting from DMSO.
Fig. 1 1H and 13C NMR spectra of 9,9-bis(4-hydroxyphenyl)-fluoren-4-
carboxylic acid.
Table 1 Viscosity and thermal properties of the polymers
Polymer
ha/dL gꢁ1
Tg/ꢂCb
TD/ꢂCc
T5%/ꢂCd
PFEEK–COOH
PFEEKK–COOH
SPFEEKK–COOH
(DSꢀ1.6)
0.9
0.7
1.4
281
273
—
229
217
327
283
277
350
SPFEEKK–COOH
(DSꢀ2.0)
1.5
—
—
—
—
—
—
—
342
242
243
241
356
266
271
284
SPFEEKK–COOH-
2.0/PVA-1
SPFEEKK–COOH-
2.0/PVA-2
SPFEEKK–COOH-
1.6/PVA
Sulfonation reaction of PFEEK–COOH and PFEEKK–COOH
Sulfonation is an electrophilic reaction, and the ultimate
substitution position of –SO3H groups on the benzene rings is
generally determined by the existing substituents on the benzene
rings. Generally, sulfonation reactions occur more readily on
benzene rings with activated substituents (electron donating),
such as methyl (–CH3) and methoxy (–OCH3), than on ones
with deactivated substituents (electron withdrawing), such as
trifluoromethyl (–CF3) and carbonyl (–CO–). By adjustment of
polymer structure, post-sulfonation reactions can produce
polymers with well-controlled sulfonation positions, under mild
reaction conditions and short reaction times.11 An advantage of
polymer post-sulfonation is that it is relatively simple in
comparison with copolymerization of sulfonated monomers
because the latter are based on sulfonated monomers, which may
need multistep synthesis and purification. The presence of
excessive ionic character from salts such as –SO3ꢁM+ and –OꢁM+
may lead to poor reaction mixture solubility, especially at the
a
Inherent viscosity. b Glass transition temperature. c Onset temperature
of decomposition. Temperature at 5% weight loss.
d
initial stages of polymerization of the growing chain, leading to
difficulties in obtaining high-molecular weight polymers. This
situation is less prone to occur in polymerizations based on
unsulfonated monomers.
Several fluorenyl-containing polymers have recently been
reported for PEM applications, which showed some promising
properties, such as high proton conductivity and low methanol
crossover.24–30 In particular, Miyatake and Watanabe recently
reported several sulfonated poly(aryl ether sulfone)s containing
fluorenyl moieties through a post-sulfonation method.27–29 In
their work, chlorosulfonic acid was used and both the amount of
sulfonation reagent and copolymer make-up were employed to
Scheme 1 Synthesis of 9,9-bis(4-hydroxyphenyl)-fluoren-4-carboxylic acid.
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Scheme 2 Synthesis of poly(aryl ether ether ketone)s containing carboxylic groups.
control DS. However, the ionomers were insoluble in CH2Cl2
and precipitated out of the solution soon after the polymer
and chlorosulfonic acid solutions were mixed, which could have
a negative impact on the progress of sulfonation.
In contrast with the sulfonation of fluorenyl-containing
poly(aryl ether sulfone)s,27–29 concentrated H2SO4 was utilized as
both the sulfonation reagent and solvent for the sulfonation of
the present carboxylated polymers (Scheme 3). The carboxyl-
ated/sulfonated poly(aryl ether ether ketone)s were readily
soluble in concentrated H2SO4 and no precipitation occurred
during the sulfonation process. Using mild sulfonation condi-
tions of 95–98% sulfuric acid at room temperature, the resulting
polymers containing both –COOH and –SO3H groups had
well-defined structure. It is of interest to observe that there are
three specific sulfonatable sites per repeat unit, which is quite
different from the structural characterization reported for the
post-sulfonation of fluorenylated PES without –COOH groups
in the ClSO3H–CH2Cl2 system.27–29 In their studies of the post-
sulfonation of fluorenyl-containing poly(arylene ether)s, no
sulfonation on the main chain was reported as occurring.
The good solubility of both carboxylated polymers and
post-sulfonated ones in DMSO made it possible to monitor the
sulfonation reaction by NMR. Fig. 2 shows the reaction kinetics
of PFEEKK–COOH in H2SO4 at a concentration of 3.0 g
polymer per 100mL H2SO4 at room temperature. It is noted
that the reaction rate was fast within the first 10 days and was
especially rapid during the first day, and a DS of 2.0 was achieved
after a five-day reaction. The reaction rate was much slower after
10 days, and DS close to 3.0 was achieved after a 45-day reaction.
Fig. 2 Sulfonation reaction of PFEEKK–COOH in 95–98% H2SO4 at
a concentration of 3.0 g polymer per 100mL H2SO4 at room temperature.
stacked 1H and 13C spectra, respectively, of unsulfonated
PFEEK–COOH and PFEEKK–COOH along with sulfonated
PFEEKK–COOH (DSꢀ3). The 1H NMR spectrum of
SPFEEKK–COOH (DSꢀ2) is also displayed in Fig. 3C but its
corresponding 13C spectrum is not shown in Fig. 3 due to its
complexity resulting from random sulfonation at the equivalent
X and Y sites as described later. The NMR signals of the
fluorenyl monomer (Fig. 1) and of the unsubstituted polymers
(Fig. 3 and 4) were easily assigned. Characterization of the
sulfonated polymers was arduous as up to three –SO3H groups
could be introduced onto the polymer. Nevertheless, we were
successful in identifying the sulfonation sites and the rate of
reaction using 1D and 2D NMR techniques. The distinctive
NMR analysis of the polymers
All the polymer spectra were fully characterized by 1D (1H, 13C)
and 2D (COSY, HSQC, HMBC) NMR. Fig. 3 and 4 show
Scheme 3 Sulfonation of poly(aryl ether ether ketone)s containing carboxylic group.
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Fig. 4 13C NMR spectra of PFEEK–COOH (A), PFEEKK–COOH (B)
and SPFEEKK–COOH (DSꢀ3.0) (C).
Fig. 3 1H NMR spectra of PFEEK–COOH (A), PFEEKK–COOH
(B), SPFEEKK–COOH (DSꢀ2.0) (C) and SPFEEKK–COOH
(DSꢀ3.0) (D).
linkages and exclusively on the fluorenyl monomer residue as
shown by the large chemical shift displacements of C17 (20 ppm
shift to 138.0 ppm) and H16 (0.38 ppm shift to 7.58 ppm)
towards higher frequencies. The reaction rate on the main chain
was monitored by comparing the intensity of the invariable H9
with the intensities of the distinct low frequency H19–H22
resulting from the presence of a –SO3H at the Y position. The X
and Y positions of the polymer in Fig. 4 are equivalent since
a spatial representation of the molecule would show the fluorenyl
group perpendicular to the main chain due to C14. Sulfonation
therefore takes place at both sites randomly. Fig. 3C shows
SPFEEKK–COOH DS 2 where Y and X have been labeled
exclusively as –SO3H and –H for practical reasons only; signals
of Fig. 3C which are labeled with numbers bearing a prime
(e.g. 220) indicate atoms on or near an unsulfonated ring.
Fig. 3D and 4C show the simple 1H and 13C NMR spectra
resulting from symmetric, highly substituted repeat units of
SPFEEKK–COOH (DSꢀ3). The actual DS is slightly lower
than 3 (ꢀ2.9) because residual unsubstituted H170–H190 and
H220 could still be observed in the 7.05–7.15 ppm region even
after a long reaction time. SPFEEK–COOH polymer spectra
are not reported here because the polymer’s physical properties
were less attractive than the SPFEEKK–COOH derivatives.
Nevertheless, thorough NMR analysis of the sulfonation reac-
tions on the fluorenyl and main chain of SPFEEK–COOH
polymer samples revealed the exact same site-specific reactivity
leading to a DSꢀ3 polymer as seen in SPFEEKK–COOH.
The main difference was in the sulfonation reaction rate. The
fluorenyl group underwent rapid sulfonation as seen before and
proton H9, present on all studied derivatives, always appeared
in the high frequency area (8.3 ppm) due to the strong electron
de-shielding effect on H9 of the nearby carboxylic acid’s
anisotropic induced magnetic field. The intensity of the H9 signal
was later used to estimate the degree of sulfonation on the main
polymer chains. 1H and COSY spectra showed very small
changes in chemical shift values for the hydrogen atoms of spin
system H3–H4–H5 present in the fluorenyl monomer and in
the fluorenyl moiety of all the synthetic polymers, including
the sulfonated ones. The electron-withdrawing carboxylic
acid deactivates the benzene ring, hence protecting it against
sulfonation. On the other hand, the spin system H9–H10–H11–
H12 of the other fluorenyl benzene ring is clearly altered by the
1
sulfonation reaction. H NMR spectra of Fig. 3 show a distinct
shift of H10 and H12 to higher frequencies resulting from the
substitution of H11 by an electron-withdrawing –SO3H group.
In addition to that, the carbon signal from C11 shifted nearly
20 ppm towards higher frequencies (147.8 ppm) as a result of its
new linkage with the electronegative sulfur atom; C11 also shows
a heteronuclear 3-bond coupling in the HMBC spectrum with the
1
distinct proton H9 at high frequency. Analysis of the H NMR
spectra of the sulfonation reactions indicated a faster sulfonation
rate on the fluorenyl ring when compared with the reactivity at
the main chain. This was readily observed by the fast transition
of H10–H12 to higher frequencies. Polymer main-chain
sulfonation occurs specifically at the ortho position of the ether
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1
observed once again by H NMR of specific (H10–H12) signal
Properties of the membranes
displacements towards higher frequencies. On the other hand,
the main-chain sulfonation was significantly accelerated. The
sulfonation rate on the main chain could not be monitored by
1H NMR, as it was in the case of SPFEEKK–COOH, due to
the complexity of overlapping signals at low frequencies.
However, a clear and simple spectrum of the symmetric DSꢀ3
repeat units was observed after only 5 days. This interesting
result may be explained by a more reactive PFEEK–COOH
towards nucleophilic sulfonation reaction since electron delo-
calization over a longer ketone–aryl–ketone in PFEEKK–
COOH when compared with a single ketone in PFEEK–COOH
results in decreased nucleophilicity at the ortho-ether sites of the
fluorenyl moieties.
The carboxylated polymers, PFEEK–COOH and PFEEKK–
COOH, had high Tg values of 281 and 273 ꢂC, respectively
(Table 1). As shown in Fig. 5, the thermal stabilities of the
membranes were evaluated in air by TGA. Similar to the many
examples of sulfonated poly(arylene ether ether ketone)s, the
present carboxylated polymers showed obvious two-stage
thermal decomposition. The first stage at around 220 ꢂC may be
associated with the loss of –COOH groups, and the second one
around 500 ꢂC may be caused by the decomposition of the
polymer backbone. It was clear that the first-stage weight loss
increased in magnitude with the incorporation of –SO3H
groups. The crosslinked SPFEEKK/PVA was thermally stable
ꢂ
up to 200 C in air, and its first-stage weight loss may be from
the combination of the loss of carboxylic/sulfonic acid groups
and the degradation of PVA.
Crosslinking of SPFEEKK–COOH and PVA
Water uptake and dimensional stability of PEMs are closely
related to proton conductivity. The water within the membrane
provides a carrier for the proton, and maintains high proton
conductivity. Increasing the –SO3H groups in the polymer
structure enhances the proton conductivity but is often
A number of PEMs that were crosslinked through the sulfonic
acid group were previously reported.2,3 While it is an effective
method, the crosslink utilizes some of the proton-conducting
sulfonic acid groups, thereby reducing conductivity. This can
be partly alleviated by starting with highly sulfonated polymers
that may even be water soluble. The crosslinking reaction to
yield ester linkages between –COOH groups of sulfonated
polymers and –OH groups of PVA is reported in several
articles.18,19 It is well known that a good solvent for both
polymer and crosslinking reagent is especially important in
order to get a compatible system. In this study, DMAc and
PVA (75% hydrolysis and average Mw of 3000) were selected as
solvent and crosslinking reagent, respectively. An improved
procedure including membrane making and then crosslinking
was employed for the thermal crosslinking reaction of this new
blend (Scheme 4). All films before and after curing were clear
and transparent, which suggested the compatibility of the
SPFEEKK–COOH/PVA system. PVAs with high molecular
weight and high percentage hydrolysis were not suitable for this
system because of their poor solubility in DMAc. Low molec-
ular diols, such as 1,4-butanediol and 1,5-pentanediol, showed
good solubility in DMAc and could form transparent films
with SPFEEKK–COOH. However, they did not exhibit good
characteristics as crosslinking agents, since they could not be
well retained in the film when treated at crosslinking tempera-
ture.
Fig.
5 TGA curves of PFEEEK–COOH, PFEEKK–COOH,
SPFEEKK–COOH (DSꢀ1.6 and 2.0) and cross-linked SPFEEKK-2.0/
PVA-1 in air.
Scheme 4 Plausible schematic representation of sulfonated poly(aryl ether ether ketone ketones)s crosslinked by ester linkages.
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accompanied by excessive water uptake and dimensional change
of the membranes. As shown in Table 2, carboxylated polymers,
PFEEK–COOH and PFEEKK–COOH possessed extremely
low water uptakes (# 5%) and swelling ratio (< 5%) at room
temperature. After sulfonation, the water uptake of SPFEEK–
COOH-2.0 increased to 240%. At 65 ꢂC, SPFEEKK-2.0 was
completely dissolved into water. Crosslinking is an effective
method for reducing dimensional change for PEMs that operate
under a humid environment. After crosslinking, the membranes
had significantly lower water uptake and swelling ratios than
the corresponding membranes before crosslinking. For example,
the room-temperature water uptake for the crosslinked
SPFEEKK–COOH-2.0-PVA-1 and -2 was reduced three-fold to
ꢀ80%. Although both SPFEEKK–COOH-2.0 and PVA were
soluble in hot water, the crosslinked membranes were not
soluble in hot water. In addition, the ratio of PVA and
SPFEEKK–COOH-2.0, which are associated with the density of
crosslinking, would impact the water uptake and swelling ratio.
It is noted that the dimensional instability of crosslinked
membranes at elevated temperature (ꢀ100 ꢂC) occurs due to the
high DS values from the amount of sulfonation. The crosslinked
membrane based on SPFEEKK–COOH-1.6, which contained
the lowest amount of –SO3H groups, showed acceptable
dimensional stability, with the swelling ratio having a value of
30% at 65 ꢂC. Although the incorporation of aliphatic PVA
polymer through ester linkages leads to a reduction in ther-
mooxidative stability, the crosslinked SPFEEKK/PVA
membranes maintain dimensional shape after 15 minutes of
treatment with Fenton’s reagent (3% H2O2 containing 2 ppm
FeSO4) at 80 ꢂC. The thermooxidative stability may be suffi-
cient for the crosslinked SPFEEKK/PVA membranes to be
utilized in DMFCs.
Fig. 6 Proton conductivities of PFEEK–COOH, PFEEKK–COOH,
SPFEEKK–COOH (DSꢀ1.6 and 2.0) and crosslinked SPFEEKK/PVA
membranes.
showed acceptable dimensional stability in water was around
0.15 S cmꢁ1 at 65 C.
ꢂ
Experimental
Characterization and measurements
1H and 13C NMR spectra were obtained on a Varian Unity Inova
NMR spectrometer operating at frequencies of 399.95 MHz for
1H and 100.575 MHz for 13C at 35 ꢂC. An indirect detection
probe was used for the acquisition of 1D and 2D spectra.
Deuterated dimethylsulfoxide (DMSO-d6) was selected as the
solvent and the DMSO signals at 2.50 ppm (1H NMR) and
39.51 ppm (13C NMR) were used as the chemical shift references.
Inherent viscosities (hinh) were measured using an Ubbelohde
viscometer at aꢂpolymer concentration of 0.5g dLꢁ1 in DMSO
solutions at 30 C.
Proton conductivity is strongly related to the content of acid
groups and is a crucial parameter for the practical application
of PEMs in fuel cells. Carboxylated polymers exhibit much
lower relative proton conductivity compared with sulfonated
polymers because of their lower relative acidity caused by a low
degree of dissociation (Table 2 and Fig. 6). At room tempera-
ture, proton conductivity of PFEEK–COOH and PFEEKK–
COOH was 6.6 ꢃ 10ꢁ4 and 5.8 ꢃ 10ꢁ4 S cmꢁ1, respectively.
When sulfonated, the proton conductivity of these carboxylated
polymers (SPFEEKK–COOH-2.0) increased to 7.5 ꢃ 10ꢁ2 S cmꢁ1
at room temperature, which was even higher than that of
Nafion 117. However, it was not suitable for fuel cell applica-
tions due to its poor dimensional stability in water. The proton
conductivity of crosslinked SPFEEKK–COOH-1.6/PVA that
A
TA Instruments thermogravimetric analyzer (TGA)
instrument model 2950 was used for evaluating thermal stability
of the polymers. The samples for TGA analysis were preheated at
150 ꢂC for 40 min to remove moisture, and then heated at 10 ꢂC
minꢁ1 from 80 to 800 ꢂC in air. The resulting curves were shifted
to make the starting weight as 100%. A TA Instruments differ-
ential scanning calorimeter (DSC) model 2920 was used for
Table 2 Water uptake, swelling and proton conductivity of the membranes
WU (%)a
18 ꢂC/65 ꢂ
SR (%)b
18 ꢂC/65 ꢂ
sc/S cmꢁ1
18 ꢂC/65 ꢂ
C
Polymer
C
C
PFEEK–COOH
PFEEKK–COOH
4/5
3/4
2/3
1/2
6.6 ꢃ 10ꢁ4/1.3 ꢃ 10ꢁ3
5.8 ꢃ 10ꢁ4/9.2 ꢃ 10ꢁ4
3.2 ꢃ 10ꢁ2/1.3 ꢃ 10ꢁ1
7.5 ꢃ 10ꢁ2/—
SPFEEKK–COOH-1.6
SPFEEKK–COOH-2.0
SPFEEKK–COOH-2.0/PVA-1
SPFEEKK–COOH-2.0/PVA-2
SPFEEKK–COOH-1.6/PVA
64/125
240/—
80/ꢀ500
88/ꢀ2000
50/91
19/36
64/—
17/ꢀ100
30/ꢀ200
16/30
2.8 ꢃ 10ꢁ2/7.7 ꢃ 10ꢁ2
4.8 ꢃ 10ꢁ2/1.4 ꢃ 10ꢁ1
3.0 ꢃ 10ꢁ2/1.5 ꢃ 10ꢁ1
a
Water uptake in weight%. b Swelling ratio in volume%. c Proton conductivity.
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measuring m.p. (monomer) and Tg (polymers). m.p. was taken
from the first run at a heating rate of 10 ꢂC minꢁ1. Tg was
evaluated from the second heating run at a heating rate of 10 ꢂC
minꢁ1 under a nitrogen atmosphere, after an initial heat–quench
cycle.
1H NMR (DMSO-d6) (in ppm): 8.23 (d, 8.0 Hz, 1H), 7.93 (dd,
8.0, 1.6 Hz, 1H), 7.75 (dd, 8.0, 1.6Hz, 1H), 7.62 (d, 8.0 Hz, 1H),
7.60 (td, 8.0, 1.6 Hz, 1H), 7.44 (d, 8.0 Hz, 1H), 7.40 (td, 8.0,
1.6 Hz, 1H).
13C NMR (DMSO-d6) (in ppm): 192.09, 167.90, 142.53,
142.18, 135.98, 135.32, 134.48, 133.56, 129.91, 129.24, 128.17,
126.49, 125.69, 123.72.
The proton conductivity was estimated from AC impedance
spectroscopy data, obtained over a frequency range of 1 to
107 Hz with oscillating voltage of 100 mV, using a Solartron
1260 gain phase analyzer. Specimens in the form of 20 ꢃ 10 mm
strips were soaked in deionized water for at least 24 h prior to the
test. Each specimen was placed in a temperature-controlled cell
open to the air at 100% RH at ambient pressure. Measurements,
in two-point mode, were carried out after sample conditioning in
the closed cell overnight or longer. The conductivity (s) of the
samples in the longitudinal direction was calculated, using the
relationship s ¼ L/(R ꢃ d ꢃ W), where L is the distance between
the electrodes, d and W are the thickness and width of the
sample stripe respectively. R was derived from the low intersect
of the high frequency semi-circle on a complex impedance plane
with the Re (Z) axis.
Synthesis of 9,9-bis(4-hydroxyphenyl)-fluoren-4-carboxylic
acid. 9-Fluorenone-4-carboxylic acid (538 g, 2.4 mol), excess
phenol (1.13 kg, 12 mol) and mercaptopropionic acid (catalytic
amount) were stirred while bubbling in anhydrous HCl gas under
inert atmosphere at 50 ꢂC for 6 h. Following completion of
the reaction, excess phenol was removed by steam distillation.
The filtrate was thoroughly washed with boiling water to give
crude 9,9-bis(4-hydroxyphenyl)-fluoren-4-carboxylic acid (yield
$90%). Crude monomer (400 g) was placed into a 20 L beaker
with 8 L of water and heated to 60 ꢂC. Sodium bicarbonate
(400 g) was added in portions until effervescence ceased. The
total volume was made up to 14 L by addition of water and then
the mixture was heated to boiling. After overnight cooling and
crystallization, the product in the sodium carboxylate form was
filtered using a Buchner funnel. The crystalline monomer was
redissolved in 14 L of hot water, and decolorizing charcoal
was added and stirred at ꢀ80 ꢂC for 1 h. The mixture was filtered
through Celite and the resulting clarified solution was neutralized
by adding concentrated hydrochloric acid. Then the suspension
was heated to boiling to coagulate the precipitate. After filtering,
washing and drying, the purified product was obtained (Scheme
1). This product was further purified using the same purification
procedure (adjusting for bicarbonate quantity), resulting in
Water uptake and swelling ratio measurements
The membranes were dried at 100 ꢂC overnight prior to the
measurements. After measuring the lengths and weights of dry
membranes, the sample films were soaked in deionized water for
24 h at pre-determined temperatures. Before measuring the
lengths and weights of hydrated membranes, the water was
removed from the membrane surface by blotting with a paper
towel.
The water uptake content was calculated by: water uptake
(wt.%) ¼ (Wwet–Wdry)/Wdry ꢃ 100%, where Wdry and Wwet are
the weights of dried and wet samples, respectively.
ꢂ
ꢀ60% yield from crude monomer. m.p. 304 C (DSC).
Elemental analysis: C: 79.17%; H: 4.60%; found: C: 78.90%; H:
4.89%.
The swelling ratio was calculated by: swelling ratio (%) ¼
(Lwet–Ldry)/Ldry ꢃ 100%, where Ldry and Lwet are the lengths of
dry and wet samples, respectively.
1H NMR (DMSO-d6) (in ppm): 13.36 (s, 1H), 9.33 (s, 2H), 8.23
(d, 7.6 Hz, 1H), 7.66 (d, 7.6Hz, 1H), 7.53 (d, 7.6 Hz, 1H), 7.35
(m, 4H), 6.89 (d, 8.8Hz, 4H), 6.61 (d, 8.8Hz, 4H).
13C NMR (DMSO-d6) (in ppm): 169.21, 155.92, 153.29,
152.53, 137.55, 136.90, 135.37, 128.58, 128.01, 127.98, 127.91,
126.93, 126.87, 125.62, 124.04, 114.81, 62.76.
Chemicals and materials
Diphenic acid, phenol and mercaptopropionic acid were
obtained from Sigma-Aldrich Ltd, and used as received. 1,4-
Bis(4-fluorobenzoyl)benzene (Jilin University) and 4,40-difluor-
obenzophenone (Sigma-Aldrich Ltd) were recrystallized from
chlorobenzene before use. Anhydrous potassium carbonate
(Sigma-Aldrich Ltd) was ground into a fine powder using a high-
speed blender and thoroughly dried in a vacuum oven before use.
Poly(vinyl alcohol) (PVA) with an average molecular weight of
3000 g molꢁ1 and hydrolysis of 75% was obtained from Sigma-
Aldrich Ltd. All other chemicals were obtained from commercial
sources, and used without further purification.
Synthesis of polymers containing carboxylic acid (–COOH)
group. A 250 mL three-necked flask equipped with an argon inlet,
a mechanical stirrer, and a Dean–Stark trap with a condenser
was charged with 9,9-bis(4-hydroxyphenyl)-fluoren-4-carboxylic
acid (19.73 g, 0.05 mol), 1,4-bis(4-fluorobenzoyl)benzene
(10.91 g, 0.05 mol), anhydrous potassium carbonate (13.82 g,
0.10 mol, 1.5 mol equivalent), DMSO (120 ml) and toluene
(30 mL). The mixture was heated to 160 ꢂC under an argon
atmosphere, and stirred at reflux temperature for 3 h. After
ꢂ
distilling off toluene, the reaction mixture was heated at 180 C
Synthesis of 9-fluorenone-4-carboxylic acid. A mixture of
for 6 h to effect polymerization. The viscous solution was poured
into ethanol to obtain a fiber-like polymer. The resulting polymer
was washed with water, and then ethanol several times to remove
residual salts and solvents. The polymer in acid form was
obtained after treating the polymer fine powder in 2 N H2SO4 for
several days. After washing with water to remove the excess acid,
the polymer (PFEEKK–COOH) was dried at 100 ꢂC in a vacuum
oven for 24 h.
diphenic acid (500 g, 2.06 mol) and concentrated sulfuric acid
ꢂ
(1.25 L) was stirred at 140 C for 35 min. The red solution was
cooled and poured into 15 L of deionized water to precipitate the
product. The resulting precipitate was boiled in fresh water
several times. The resulting yellow 9-fluorenone-4-carboxylic
acid (463 g, 81% yield) was dried and used for ꢂthe next step
without further purification (Scheme 1). m.p. 226 C (DSC).
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 4675–4682 | 4681

View Article Online
The same synthetic procedure was applied to synthesize
PFEEK–COOH, as shown in Scheme 2.
(ꢀ 0.15 S cmꢁ1) at 65 ꢂC. The crosslinked PEMs are expected to
be applicable in the direct methanol fuel cell.
Sulfonation reaction for the polymers. Dry powdered polymer
(3.0 g, PFEEK–COOH or PFEEKK–COOH) and 95–98%
concentrated sulfuric acid (100 mL) were placed in a 250 mL
single-necked flask with a cover. The mixture was stirred at room
temperature for various reaction times. The resulting homoge-
nous solution was poured into a mixture of ice and water. The
precipitate was thoroughly washed with deionized water until
neutralized, filtered and dried for 24 h in a vacuum oven at
100 ꢂC. The resulting yellowish sulfonated polymers (SPFEEK–
COOH or SPFEEKK–COOH) had different DS values
depending on the sulfonation time (Scheme 3).
Acknowledgements
Financial support for this project, provided by the joint research
cooperation program between the National Science Council of
Taiwan (R.O.C.) and the National Research Council of Canada
(NRC), is gratefully acknowledged.
References
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to as SPFEEKK-1.6/PVA, were also prepared using the same
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Conclusions
New carboxylated poly(aryl ether ketone)s with high Tg values
have been prepared from 9,9-bis(4-hydroxyphenyl)-fluoren-4-
carboxylic acid and difluorinated monomers. They could be cast
into flexible and transparent membranes. The carboxylated
polymers were readily post-sulfonated with 95–98% H2SO4 at
room temperature, and the sulfonation reaction of carboxylated
polymers occurred on three specific sites per repeat unit, reaching
a DS of ꢀ3. A detailed structural analysis using 1D and 2D
NMR techniques revealed the specific sulfonation sites and
enabled the kinetics of the reaction to be determined. In contrast
with previous post-sulfonation structural studies of other
fluorenyl-containing PEMs, detailed NMR measurements in the
present study indicate that sulfonation occurs on the ortho
position of the main-chain ether linkages exclusively on the
fluorenyl monomer residue, as well as on one position on the
fluorenyl in this polymer. An initial investigation of crosslinking
cast carboxylated/sulfonated membranes with PVA through the
carboxylic acid was conducted. The crosslinked membranes
showed an obvious improvement in the dimensional stability,
and water uptake and swelling ratio of crosslinked membranes
(DS ꢀ2) were decreased by several times compared with the
corresponding carboxylated/sulfonated polymer. The cross-
linked SPFEEKK–COOH-1.6/PVA film exhibited both accept-
able dimensional stability and high proton conductivity
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4682 | J. Mater. Chem., 2008, 18, 4675–4682
This journal is ª The Royal Society of Chemistry 2008