Phosphonated poly(arylene ether)s as potential high temperature proton conducting materials

Article abstract of DOI:10.1016/j.polymer.2011.08.020

The preparation and characterization of new phosphonated polymeric ionomers based on a fully aromatic poly(arylene ether) backbone with applications as proton exchange membranes for fuel cell is reported. The high-molecular-weight polymers were obtained b

Full text of DOI:10.1016/j.polymer.2011.08.020

Polymer 52 (2011) 4709e4717
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Phosphonated poly(arylene ether)s as potential high temperature proton
conducting materials
,
c
d
Ebrahim Abouzari-Lotf a, Hossein Ghassemi b , Abbas Shockravi , Thomas Zawodzinski ,
*
David Schiraldi b
a Membrane Research Group, Iranian Academic Center For Education, Culture & Research (ACECR), Tarbiat Moallem University Branch, Mofatteh Ave. Postal Code 15614, Tehran, Iran
b Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, OH 44106, USA
c Faculty of Chemistry, Tarbiat Moallem University, Mofatteh Ave. No.49, Postal Code 15614, Tehran, Iran
d Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation and characterization of new phosphonated polymeric ionomers based on a fully
aromatic poly(arylene ether) backbone with applications as proton exchange membranes for fuel cell is
reported. The high-molecular-weight polymers were obtained by the polycondensation of the phos-
Received 16 June 2011
Received in revised form
12 August 2011
Accepted 13 August 2011
Available online 22 August 2011
phonated monomers with decafluorobiphenyl in high yields with inherent viscosities up to 0.58 dL gꢀ1
.
The hydrolysis of the phosphonated ester into phosphonic acid groups was carried out quantitatively
under acidic conditions. The polymers were studied by TGA after hydrolysis and showed10% weight loss
above 430 ꢁC. Membranes with total ion-exchange capacities above 6 meq/g showed proton conduc-
tivities of approximately92 mS/cm at 25 ꢁC and 100% relative humidity increasing to ca.150 mS/cm at
140 ꢁC. Their conductivity under dry condition showed values over 2 mS/cm at 120 ꢁC which upon
doping with phosphoric acid jumped to nearly 100 mS/cm.
Keywords:
Phosphonated polymer
Proton-exchange membrane
Conductivity
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Studies on model compounds have shown higher thermal and
electrochemical stability for phosphonic acid based compounds
Polymer electrolyte membrane fuel cells (PEMFCs) are under
consideration as future power sources for transport, stationary and
portable applications [1e3]. While there are continuing improve-
ments in advancing the PEMFC technology, one of the present
challenges is the development of new durable membranes that
allow operation at higher temperatures and low humidity [3,4].
Because of their excellent chemical and physical properties and
well-studied synthesis, membrane based on sulfonic acid con-
taining polymers, have drawn much interest and are often regarded
as the material of choice for use as solid electrolyte membranes in
PEMFCs [5,6]. However, their shortcomings at temperatures above
90 ꢁC or at low relative humidity motivate the search for ionomers
with alternative acidic moieties, such as phosphonic acid, which
have higher chemical and thermal stabilities than sulfonic acid [7].
Aromatic phosphonic acids have lower acidity but better chemical
and thermal stability and greater ability to retain water compared
with their sulfonic acids analog [8e12].
compared with their sulfonic acid analogue. Thehigh degreesof self-
dissociation and hydrogen bonding in phosphonated compounds
lead to high proton conductivities in moderate temperature and low
humidity [8,13,14]. It has been suggested that the significant proton
donor and acceptor properties combined with the high dielectric
constant of phosphonic acid result in high degrees of self-
dissociation [13]. Formation of water due to the self-condensation
equilibrium between phosphonic acid groups especially at higher
temperatures may help their conductivity under very dry condi-
tions. It has been proposed that the conductivity of phosphonate
dionomers under nearly zero humidity may mainly be due to the
presence of this small amount of water [15].
Although phosphonic acid containing polymers show some
advantages relative to sulfonic acid containing polymers for higher
temperature application in PEMFC, they are not well studied due to
relatively limited synthetic procedures and the required high
degree of phosphonation (DP) in order to achieve sufficient proton
conductivity [7,16,41e44,47]. Lower acidity and a higher degree of
hydrogen bonding of the phosphonated polymers compared with
the sulfonated systems lead to reductions in the number of absor-
* Corresponding author. Tel.: þ1 216 368 1549; fax: þ1 216 3684202.
E-mail addresses: e_abouzari@tmu.ac.ir (E. Abouzari-Lotf), hossein.ghassemi@
case.edu (H. Ghassemi), shockravi@tmu.ac.ir (A. Shockravi).
bed water molecules per acid group (
l) in a phosphonated
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.08.020

4710
E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
membrane. Therefore, synthesizing phosphonic ionomers with
increased acidity and ion exchange capacity (IEC) may lead to high
proton conductivities required for the successful application of
such materials as electrolyte in PEM fuel cell at high temperature
and low humidity.
The majority of the phosphonic acid containing ionomers
reported to date have low IEC value and proton conductivity. Some
examples of these materials include: poly(arylene ether)s [17e20],
poly(ethersulfone)s [21e24], polyphosphazenes [25,26], poly(4-
phenoxybenzoyl-1,4-phenylene)s [27] and polymers with per-
fluorinated backbones [7,12,28]. Proton conduction in these
systems is water dependent, similar to observed behavior with
sulfonated polymers. Since the preparation of phosphonated ion-
omers with very high IEC value is achievable compared to sulfonic
ionomers, it is desirable to design phosphonic ionomers with
increased acidity and IEC.
IECcal ¼ equivalents of
ꢀ OH per unit=repeat unit molecular weight
To obtain water uptake values, membranes were dried for 24 h
at 100 ꢁC, weighed (Wdry), and immersed in deionized water at
room temperature until reading a constant mass (2 days). The wet
membranes were blotted dry and immediately weighed (Wwet). The
water uptake is reported as a percentage and determined by taking
the equilibrium weight difference between the wet film (Wwet) and
the dry film (Wdry) and dividing by the dry film (Wdry) weight.
The acid doping level of membranes was determined by
immersing samples in 85% phosphoric acid for various doping
times (1e14 days) at room temperature. The samples were weighed
before and after the immersion and the weight gain was obtained,
and the acid doping level of the membrane was calculated as the
moles of phosphoric acid per phosphonic acid group.
The proton conductivity of the phosphonated polymeric
membranes was measured by placing the membranes between two
platinum electrodes held by two polytetrafluoroethylene (PTFE)
frames. A homemade chamber was used to equilibrate the samples
for conductivity measurement. An impedance spectrum was
recorded from 10 MHz to 10 Hz using an Autolab PGSTAT 30in fully
hydration and water-free conditions at various temperatures. The
In this study, we report new poly(arylene ether)-containing
phosphonic acid substituents on the main chains of ionomers. We
initially prepared diphosphonated bisphenol and biscarbamate
monomers and subsequently polymerized them with decafluor-
obiphenyl to produce the corresponding phosphonatedpoly(ar-
ylene ether)s. The presence of highly concentrated acid groups next
to perfluorinated biphenyl rings should increase acidity and
aggregation of phosphonic acids which may provide a better
condition for higher proton conductivity. The synthesis, structure,
and some properties of the phosphonated ionomers as well as
phosphoric acid doped membrane are reported.
proton conductivity (s) is the reciprocal of the resistance (R) and
reported in units of milli-Siemens per centimeter (mS/cm) from:
s
¼ L/RS where L and S are the thickness and area of the membrane,
respectively.
2.2. Materials
2. Experimental
Diisopropylamine (Aldrich) was dried over KOH and distilled
just before use under nitrogen atmosphere. Triethylamine (Aldrich)
was dried by refluxing over sodium hydroxide pellets for several
hours, distilled and then stored over potassium hydroxide pellets.
Diethyl phosphite distilled before use under argon.
Decafluorobiphenyl (Sigma Aldrich) was used as received. N,N-
Dimethylacetamide (DMAc) (Sigma Aldrich) was purified by stir-
ring over CaH2 for several hours and distilled under reduced
pressure. Anhydrous potassium carbonate (K2CO3) (Aldrich) was
ground into fine powder and stored in a vacuum oven at 120 ꢁC
before use.
2.1. Instrumentation
1H, 13C, 19F, and 31P NMR spectra were measured on a Bruker
model DRX 500 AVANCE apparatus at the following frequencies: 1H
500 MHz, 13C 125 MHz, 19F 470 MHz, and 31P 202 MHz. The signal of
H3PO4 85% in water was used as the external reference (0 ppm) for
31P NMR spectroscopy.Elemental analysis for C, H and N performed
by a Perkin Elmer 2004 (II)analyzer. Melting points (uncorrected)
were measured with an Electrothermal Engineering LTD 9200
apparatus. Inherent viscosities (hinh ¼ ln hr/c at concentration of
0.5 g dLꢀ1) were measured with an Ubbelohde suspended-level
viscometer at 30 ꢁC using N,N-dimethylacetamide (DMAc) as
solvent. Thermogravimetric analyses (TGA) were conducted with
a Stanton Redcraft STA-780 (London, UK) thermal analysis under
nitrogen atmosphere at a heating rate of 10 ꢁC minꢀ1. The experi-
mental ion exchange capacity (IECexp, meq PO3H2/gof dried poly-
mer) of the membranes determined via titration using the
following procedure: Initially membrane samples were vacuum
dried at 80 ꢁC until constant weight, and then the samples were
individually immersed in 30 mL of 0.1 N NaOH/0.1 M NaCl mixed
aqueous solution (1/3 by volume) and stirred for 24 h, to convert
the membrane from the Hþ to the Naþ form completely. Subse-
quently, 15 mL of 0.10 N HCl solutions was added, and the excess
amount of HCl was titrated using 0.025 N NaOH solution with
phenolphthalein as a pH indicator. IECexp was calculated according
to the following equation.
2.3. Monomer synthesis
2.3.1. Tetraethyl 1,4-phenylene diphosphate(O-Ph)
A mixture of hydroquinone (30 mmol, 3.3 g), diethyl phosphite
(62 mmol, 8.56 g, 8 mL), and18 mL of carbon tetrachloride was
stirred and cooled to 0 ꢁC. 8.7 mL (62 mmol, 6.3 g) of triethylamine
was then added drop-wise to the reaction mixture cooled in ice
bath over a period of 1 h so that the temperature was maintained
below 5 ꢁC. After addition, the reaction mixture was stirred at 0 ꢁC
for 1 h, then the ice bath was removed and the mixture was stirred
overnight at room temperature. Water (20 mL) was added, the
reaction mixture was filtered, and the organic layer was dried over
anhydrous sodium sulfate. By removal of solvent on a rotary
evaporator 9.75 g (85%) of colorless liquid was obtained: 1H NMR
(500 MHz, CDCl3);
d
1.33e1.36 (t, 12H, 3J (HeH) ¼ 7 Hz), 4.18e4.23
(m, 8H), 7.18 (s, 4H) ppm; 13C NMR (125 MHz, CDCl3);
d
16.45 (d, 3J
(CeP) ¼ 6.9 Hz), 65.07 (d, 2J (CeP) ¼ 6.2 Hz), 121.48 (d, 3J
IECexp ¼ 0:025ðVNaOH ꢀ VBÞ=Wdry
(CeP) ¼ 4.7 Hz), 147.95 (d, 2J (CeP) ¼ 7.5 Hz) ppm; 31P NMR
where VNaOH is the volume of 0.025 N NaOH aqueous solutions
for the volumetric titration, VB is the volume of NaOH aqueous
solution for blank titration, and Wdry is dry weight of the
membrane. Also the theoretical IEC value for membrane (IECcal
was also calculated as follows:
(202 MHz, D2O):
d
ꢀ5.83 to ꢀ5.68 (m) ppm.
2.3.2. Tetraethyl (2,5-dihydroxy-1,4-phenylene) diphosphate (C-Ph)
A solution of lithium diisopropylamide (LDA) was prepared as
follows: To a solution of diisopropylamine (25.4 mmol, 2.57 g,
)

E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
4711
3.58 mL) in THF (15 mL) at ꢀ78 ꢁC under argon was added n-
butyllithium (25 mmol, 10 mL) and the mixture was stirred for
30 min A solution of O-Ph (6 mmol, 2.3 g) in dry THF (10 ml) was
slowly added with a syringe to a solution of LDA. After stirring for
1 h at ꢀ78 ꢁC, the solid carbon dioxideeacetone bath was removed
and the mixture was allowed to warm up to room temperature and
stir for an additional 1 h. A mixture of saturated solution of
ammonium chloride (15 mL) and diethyl ether (20 mL) was then
added, and the organic layer was separated and the aqueous layer
was extracted with diethyl ether and methylene chloride for several
times. The combined organic layers were washed with water and
dried over MgSO4.The solvent was removed in vacuo to give
a yellowish solid as crude product. Recrystallization from acetone/
water yields 2.42 g (68%) white crystalline solid. Mp 219e220 ꢁC;
1H NMR (500 MHz, CDC13) 1.37e1.40 (t, 12H, 3J (HeH) ¼ 7 Hz),
4.08e4.16 (m, 4H), 4.18e4.25 (m, 4H), 7.01e7.05 (dd, 2H, 2J
(HeP) ¼ 15.7 Hz, 3J (HeP) ¼ 7.4 Hz), 9.75 (s, 2H, (b, OH,
1J (CeP) ¼ 185.5 Hz), 129.71 (m), 149.60 (d, 2J (CeP) ¼ 18.5 Hz),
154.89 ppm; 31P NMR (202 MHz, DMSO-d6);
d 12.77 ppm (at 350 K).
2.4. Polymer synthesis
2.4.1. Synthesis of the phosphonated poly(aryl ether) with
bisphenol method (PAE-1)
A three-necked round bottom flask equipped with a magnetic
stirrer, an argon inlet, and a dean-stark trap was charged with
decafluorobiphenyl (0.6682 g, 2 mmol), anhydrous potassium
carbonate (0.83 g, 6 mmol), diphosphonate diester(C-Ph)(0.7644 g,
2 mmol), NMP 10 mL, and 4 mL of toluene. The reaction mixture was
heated at 115 ꢁC for 20 h (during which the toluene was removed
and the solution became viscous). The temperature was raised
slowly to 140 ꢁC and stirred for 2 h to obtain a light brown viscous
solution. The solution was cooled to room temperature and poured
into 200 mL deionized water containing 2 mL hydrochloric acid. The
precipitated polymer was filtered, washed several times with
deionized water to remove salts, and finally vacuum dried at 100 ꢁC
exchangeable with D2O)), ppm; 13C NMR (125 MHz, CDC13);
d 16.58
(d, 3J (CeP) ¼ 7.0 Hz), 63.60 (d, 2J (CeP) ¼ 5.4 Hz), 116.84 (d, 1J
(CeP) ¼ 173.0 Hz), 119.82 (dd, 2J (CeP) ¼ 14.1 Hz, 3J (CeP) ¼ 6.1 Hz),
153.02 (dd, 2J (CeP) ¼ 17.8 Hz, 3J (CeP) ¼ 6.3 Hz), ppm; 31P NMR
for 24 h yield 90%.1H NMR (500 MHz, DMSO-d6);
d1.15e1.24 (t,12H),
4.11 (b, 8H), 7.63 (b, 2H) ppm; 19F NMR (470 MHz, DMSO-d6);
(202 MHz, CDCl3);
d 19.79e19.98 (m) ppm, Anal. Calcd. for
d
ꢀ139.1 and ꢀ154.5 (major peaks), ꢀ140.7, ꢀ156.7, ꢀ161.4 ppm; 31
P
Cl4H24O8P2: C, 43.97; H, 6.28. Found: C, 43.90; H, 6.20.
NMR (202 MHz, DMSO-d6); d6.98, 10.32, 10.73 ppm. Anal. Calcd.-
2.3.3. Bis-propylcarbomate-masked derivative: (2,5-bis(diethoxy-
phosphoryl)-1,4-phenylene bis(propylcarbamate)) (C-Ph-BC)
forC26H22F8O8P2: C, 46.17; H, 3.28; Found: C, 46.01%; H, 3.22%.
To a solution of C-Ph (2 mmol, 0.764 g) in 25 mL of methylene
chloride was added 1 mL of n-propylisocyanate and 0.7 mL of
triethylamine. The mixture was stirred at room temperature for 3
days to obtain a white precipitate. The solvent was removed and
a crude product was washed with methylene chloride to obtain C-
Ph-BC in a 70% yield. Mp: 170e172 ꢁC, 1H NMR (500 MHz, MeOH-
2.4.2. Synthesis of the phosphonated poly(aryl ether) with
bicarbamate method (PAE-2)
In a procedure similar toPAE-1, a mixture of C-Ph-BC (1.105 g,
2 mmol) and decafluorobiphenyl (0.6682 g, 2 mmol) were dis-
solved in 10 mL of DMAc. Potassium carbonate (0.55 g, 4 mmol) was
added and the reaction temperature was raised slowly to 130 ꢁC
and stirred for 5 h under an argon atmosphere. The brown viscous
solution was cooled to room temperature and poured into 200 mL
deionized water containing 2 mL hydrochloric acid. The precipi-
tated polymer was filtered, washed several times with deionized
water to remove salts, and finally vacuum dried at 100 ꢁC for 24 h
yield 95%. 31P NMR (202 MHz, DMSO-d6); 12.20, 12.80 ppm. Anal.
d4);
d
0.99e1.02 (t, 6H, 3J (HeH) ¼ 7.4 Hz), 1.34e1.37 (t, 12H, 3J
(HeH) ¼ 7 Hz), 1.60e1.65 (h, 4H, 3J (HeH) ¼ 7 Hz), 3.17e3.20 (t, 3H,
J (HeH) ¼ 7 Hz), 4.14e4.20 (m, 8H), 4.34 (s, (amide NH and water in
methanol)), 7.63e7.68 (dd, 2H, 3J (HeP) ¼ 15 Hz, 4J (HeP) ¼ 6.7 Hz,)
ppm; 13C NMR (125 MHz, MeOH-d4);
d
10.58, 15.57 (d, 3J
(CeP) ¼ 6.5 Hz), 22.91, 42.99, 63.58 (d, 3J (CeP) ¼ 6.1 Hz), 127.16 (d,
Scheme 1. Preparation of phosphonate ester functionalized bisphenol (C-Ph) and bis-propylcarbamate derivative (C-Ph-BC).

4712
E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
Calcd. for C26H22F8O8P2: C, 46.17; H, 3.28; Found: C, 46.09%; H,
3.29%. (H, and F NMR data was similar toPAE-1).
3. Results and discussion
3.1. Monomer synthesis
2.5. Dealkylation
As shown in Scheme 1 phosphonated monomer (C-Ph) was
prepared by a two-stage synthetic procedure starting from di-
phosphorylation of hydroquinone by diethylphosphate using
simplified AthertoneTodd method [30], followed by phospha-
teephosphonate rearrangement in the presence of strong base
[31]. Finally the prepared diphosphonated compound was reac-
ted with n-propylisocyanate to give the corresponding
bispropylcarbamate-masked compound (C-Ph-BC).
The membranes in their acid forms were obtained by soaking
the PAE-1 or PAE-2 ester films in concentrated HCl solution. In
a typical reaction, 1 g of PAE-2 was placed in 50 mL of hydrochloric
acid (38%). After 10 days of reflux, the membrane was filtered off
and water repeatedly added and removed in vacuum to remove
excess acid, and then dried at vacuum for 12 h yield more than 98%
(based on 1H NMR spectrum). 1H NMR (500 MHz, DMSO-d6);
The structure of these compounds was confirmed by 1H NMR,
13C NMR, 31P NMR and elemental analysis. The preparation of the
(C-Ph) was confirmed using NMR spectroscopy. Proton NMR
d
7.23e7.48 (2H, aromatic H), 9.40 (b, 4H, phosphonic acid); 31P
NMR (202 MHz, DMSO-d6); 5.58e5.63 (d) ppm. Anal. Calcd.-
forC18H6F8O8P2: C, 38.32; H, 1.07; Found: C, 39.14%; H, 1.35%.
d
Fig. 1. 1H NMR spectrum of C-Ph-BC in MeOH-d4.

E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
4713
showed shift of the protons on the aromatic ring from
to 7.03 ppm (dd), and the appearance of a new peak at
corresponding to the phenol OH groups. The appearance of a new
peak at
116.84 ppm (d) with 1J (173 Hz) corresponding to the
d
7.18 ppm (s)
3.2. Polymer synthesis
d
d
9.75 ppm
Nucleophilic aromatic substitution has been employed
successfully in the synthesis of various high-molecular-weight
poly(arylene ether)s [32,33].It is well known that aromatic
bisphenol with electron withdrawing groups at the ortho position
are not highly reactive in nucleophilic substitution poly-
condensation reaction with bis(halides).We first investigated the
polymerization of the phosphonated bisphenol and masked bicar-
bamate with 4,40-difluorodiphenyl sulfone under different nucle-
ophilic substitution conditions by changing solvent, temperature,
base, etc. However, these attempts were not successful and low
molecular weight oligomers formed presumably due to the low
reactivity of the bisphenol monomer. Therefore, we have examined
the more reactive decafluorobiphenyl monomer (Scheme 2) which
is known to form linear poly(arylene ether)s under relatively mild
condition [34].Careful dehydration during polymerization is
required to obtain high-molecular-weight polymers and it can be
achieved through azeotropic distillation. Another option is using
masked-bisphenol and run the reaction under completely anhy-
drous condition. In both cases, the reaction temperature has to be
controlled carefully to suppress possible side reactions such as
branching or crosslinking. Two parafluorines in decafluorobiphenyl
are more reactive and preferentially react to form linear chain.
The new fluorinated poly(arylene ether)s containing phos-
phonic ester groups were synthesized by polycondensation reac-
d
C
AromaticeP bond in the 13C NMR spectrum, and shift of the phos-
phorus from
observed.
d
ꢀ5.7 ppm to
d
þ 19.85 ppm in the 31P NMR were
Masked bisphenols often give much better results in nucleo-
philic substitution polymerization reactions compared with their
parent bisphenols [29], therefore phosphonated bisphenol (C-Ph)
was converted to its corresponding biscarbamate (C-Ph-BC) as
shown in Scheme 1.
Preparation of bis-propylcarbamate derivative (C-Ph-BC) was
confirmed by the disappearance of signals at
sponding to the protons of OH group and the appearance of new
peaks at 0.99e1.02 (triplet, 6 protons), 1.60e1.65 (multiplet, 4
d 9.75 ppm, corre-
d
protons), and 3.17e3.20 (triplet, 6 protons) corresponding to the
protons of n-propyl group in the 1H NMR spectroscopy (Fig. 1). Also
the appearance of 3 new peaks in the aliphatic region in the 13C
NMR corresponds to the propyl group confirms the formation of
C-Ph-BC compound (Fig. 2). The 31P NMR spectrum of C-Ph-BC at
ambient temperature consists of two multiplet signals at 12.20 and
12.80 ppm with the relative intensities of 7:1. The variable
temperature spectrum at 202 MHz showed line shape changes over
a temperature range of 50 ꢁC. At 321 K and below, the 31P spectrum
for C-Ph-BC consist of two octets (Fig. 3). At higher temperatures,
the two signals broaden and eventually merge to a singlet at around
340 K. As shown in Fig. 3, the minor peak at 12.80 ppm appears to
merges into the high frequency signal (at 12.20) as the temperature
is raised.
tion
between
the
substituted
hydroquinones
and
decafluorobiphenyl (Scheme 2).
The reaction was carried out in DMAc under argon atmosphere
and temperatures below 140 ꢁC. The polycondensations proceeded
Fig. 2. 13C NMR spectrum of C-Ph-BC in MeOH-d4.

4714
E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
readily in a homogeneous solution and tough and stringy materials
formed when the viscous polymer solutions were trickled into the
stirring methanol/water (1/1) mixture. As shown in Table 1, the
inherent viscosities of polymers in the ester form were 0.45 and
0.58 dL gꢀ1 PAE-1 and PAE-2, respectively. The elemental analysis
values were in good agreement with the calculated values for the
proposed structures of polymers. Structural features of these
polymers were verified by H, F, and P NMR spectroscopies.
The 19F NMR spectrum of the PAE-1 polymer shows two major
peaks at ꢀ139.1 and ꢀ154.5 ppm, which were assigned to the meta-
and ortho-fluorine atoms on the polymer main chain. It also revealed
three smaller other peaks at ꢀ140.7, ꢀ156.7 and ꢀ161.4 ppm for the
end-groups. By comparing the integral intensity of the peaks of the
main chain fluorines with those on the end units, the degree of
polymerization (n) and the number average molecular weight
(MNnMR) was calculated (Table 2). Based on this comparison, values of
42 and 56 have been estimated for the degree of polymerization (n)
corresponding to a number average molecular weight (MNnMR) of
28,400 and 37,900 Da for the bisphenol and biscarbamate methods,
respectively. Advantages of the carbamate method over the
conventional bisphenol method are shorter reaction time under
mild conditions, no water generation during the polymerization,
and formation of higher molecular weight polymers.
Polymers in the ester form (PAE-1 and PAE-2) were hydrolyzed
to their corresponding acid form (PAA-1 and PAA-2) after treating
with hydrochloric acid (HCl) for 10 days and dealkylated products
were characterized by 1H, 31P, and 19F NMR spectroscopies. 1H
NMR spectrum revealed only small residue of ethyl groups in the
aliphatic region. Phosphorus peaks at 6.98, 10.32, 10.73 ppm
correspond to the phosphonate esters in the 31P NMR spectrum
were disappeared, and a new peak appeared at 5.61 ppm which
was assigned to phosphonic acid. These changes along with the
CHN analysis indicated the completion of the dealkylation
reaction.
Fig. 3. 31P variable temperature NMR of spectrum of C-Ph-BC in DMSO-d6.
Scheme 2. Synthesis of the polymers in the ester form and hydrolysis to the corresponding acid form.

E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
4715
Table 1
Thermal stability, solubility, and viscosity of phosphonated poly (arylene ether)s in ester, acid and salt forms.
Polymer
Yield
(%)
Thermal
stability
Inherent viscosityb
Solubilitya
d
T10
Cy
hinh
DMAc
DMSO
THF
MeOH
H2O
CHCl3
(ꢁC)c
(%)
(dl/g)
Ester Derivatives
Acid Derivatives
Salt Derivatives
PAE-1
PAE-2
PAA-1
PAA-2
PAS-1
PAS-2
90
95
97f
98f
e
298
299
426
430
430
428
53e
58e
33
32
63
0.45
0.58
e
þ þ
þ þ
þ
þ þ
þ þ
þ
þ
þ
e
e
e
e
þ
ꢃ
e
e
e
e
e
e
e
e
S
ꢃ
S
e
e
e
e
e
þ
þ
e
e
e
e
65
e
e
e
S
(þþ) Soluble at room temperature, (þ) soluble after heating, (ꢃ) partially soluble after heating, (S) swell, (ꢀ) insoluble.
a
Solubility: measured at a polymer concentration of 0.05 g/ml.
b
Measured at a polymer concentration of 0.5 g/dl in DMAc at 30 ꢁC.
c
Temperature at which 10% weight loss occurred under nitrogen atmosphere.
d
Char yield calculated as the percentage of solid residue after heating from room temperature to 700 ꢁC under nitrogen.
e
In 500 ꢁC.
f
Obtained from 1H NMR (Hydrolysis yield).
3.3. Solubility properties
As seen in Fig. 4, the decomposition occurred for PAS and PAA
samples in at least three distinctive temperature ranges. The very
slow mass loss in the salt and proton form occurred in the
temperature range of 100e300 ꢁC is due to the loss water. Water
may be absorbed or involved in condensation even in nominally
dry materials (two phosphonic acid groups form a PeOeP bond
producing water). The next weight loss step is observed from 435 to
480 ꢁC and is most probably connected with the cleavage of the
CeP bond. The final step of the degradation which starts around
480 ꢁC, and should be due to the degradation of the main chain.
For the acid derivatives the 10% weight loss under nitrogen
atmosphere occurred around 430 ꢁC and the polymer produced
more than 30 wt% char residue at 750 ꢁC. PAS polymers showed
T10value around 430 ꢁC similar to PAA samples. However, the char
yields at 750 ꢁC were higher (around65%) under similar conditions.
Solubility of the polymers was tested qualitatively in various
solvents and the results are presented in Table 1. Polymers PAE-1
and PAE-2 with phosphonic ester groups showed very good solu-
bility in a variety of solvents such as NMP, DMF, THF and methanol.
However, after hydrolysis solubility of the polymers decreased and
PAS-1 and 2 with disodium phosphonate groups became insoluble
in all the above solvents and PAA-1 and 2 with phosphonic acid
groups were only soluble in high boiling solvents such as DMSO and
DMAc. PAS polymers in the salt form were prepared by neutralizing
PAA polymers with sodium hydroxide. Both PAA-1 and 2 formed
tough films after casting from DMSO or DMAc solution. Strong
interaction between the phosphonic acid groups is believed to be
responsible for low solubility of phosphonated polymers. It has
been reported that phosphonated polysulfone having more than
0.75 phosphonic acid group per repeating unit were not completely
soluble in DMAc or DMSO [21], and poly(arylene ether)s having two
phosphonic acid groups per repeating unit were found to only swell
in DMSO even at 100 ꢁC [19].
3.5. Degree of phosphonation and IEC
1H NMR was used to determine the degree of hydrolysis or
phosphonation and subsequently amount of repeat units with
residual phosphonate ester groups in the membranes. The degree
of hydrolysis was determined from residual ethyl groups of phos-
phonate ester, resonants at 1.2 ppm (eCH3) and 4.1 ppm (eCH2)
after hydrolysis of PAE polymers. The following equation was used
to calculate degree of hydrolysis by using integration values of ethyl
and aromatic region. As shown in Table 1, degree of hydrolysis was
97% and 98% for PAA-1 and PAA-2, respectively.
3.4. Thermal properties
High thermal stability is a key property for the membranes in
fuel cell application. Thermal stability of the phosphonated poly-
mers in the ester, salt and proton forms was investigated by TGA
measurements. The temperatures which 10% weight loss (T10
)
nh
i.
occur and char yields at 750 ꢁC under nitrogen were determined
from the original thermograms and tabulated in Table 1. As
expected and is clearly shown in Fig. 4, the polymers with ester
groups show lower thermal stability compared with the polymers
in their acid and salt form. The temperature for 10% weight loss
recorded around 290 ꢁC for PAE samples which was due to the loss
of four ethyl groups. Similar TGA results have been reported for
polymers containing phosphonic esters [19,35].
Degree of Hydrolysis ¼
ð10 ꢂ HaromaticÞ ꢀ Hethyl
o
ð10 ꢂ Haromatic
Þ
ꢂ 100
Experimental ion exchange capacity (IEC) was determined by
titration of the membranes in acid form with sodium hydroxide.
The results show IEC values around 6.2 mmol/g compared to the
theoretical value of 7.0 which is calculated by considering both
Table 2
Physical properties of PAA polymers.
Polymer
Polymerization degree
(n)a
MNnMR
Water uptake (%)
IEC
IEC
experimental
theoreticalb
25 (ꢁC)
40 (ꢁC)
60 (ꢁC)
80 (ꢁC)
100 (ꢁC)
PAA-1
PAA-2
42
56
23,700
31,600
36.5
33.8
39.1
35.4
44.6
39.3
53.1
46.3
61.0
55.6
6.23
6.05
7.09
7.09
a
Based on NMR data.
Phosphonic acid is considered a diprotic acid.
b

4716
E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
Fig. 4. TGA comparison of PAE-2 (ester form), PAA-2 (acid form), and PAS-2 (salt form).
protons of each phosphonic acid group in the repeat unit (Table 2).
The small deviation between experimental and theoretical IEC
values may come from the fact that the second proton of phos-
phonic acid is less acidic and may not be completely neutralized
during titration.
Fig. 5. Time dependence doping level of PAA-2 membrane.
conductivity value for PAA-2 membrane reached 92 mS/cm when
fully hydrated at ambient temperature which is comparable with
standard perfluorosulfonic acid membranes [40] and substantially
3.6. Water and phosphoric acid uptake
higher
among
the
reported
phosphonated
polymers
Proton conductivity in polymeric electrolytesrelies strongly on
the concentration of water as mobile phase and sharply decreases
as the water content decreases. On the other hand, high water
uptake causes excessive swelling and dimensional changes and
therefore decreases in the mechanical strength. The diphospho-
nated PAA samples were tested for their water uptake by
comparing the weight of the membranes in dry state and after
submerging in deionized water. The PAA-1 membrane exhibited
slightly higher water uptake (36.5 wt %) than that of the PAA-2
membrane (33.8 wt %) which is in agreement with the higher IEC
value of PAA-1.This moderate level of water uptake at 100% RH
[14,24,39,41e44]. For example, proton conductivity of highly
phosphonated polysulfone has been reported around 12 mS/cm at
100 ꢁC under fully hydrated conditions [44] and for phosphonated
poly(N-phenylacrylamide) with very high IEC of 6.7 was 88 mS/cm
under 95% relative humidity and 80 ꢁC [41]. High proton conduc-
tivity of PAA clearly demonstrates that the high and localized
concentration of phosphonic acids was effective for water-assisted
proton transport.
It was also found that the level of conductivity of the PAA-2
membrane under fully hydrated condition increased by tempera-
ture and reached150 mS/cm around 100 ꢁC and stays almost
constant up to 140 ꢁC (Fig. 6). This behavior might partially be due
to the different amounts of water absorbed by the membrane at
various temperatures (see Table 2). At high temperatures, the
conductivity was seemingly constant with the temperature
possibly due to the large water uptake of the membrane under
fully immersing conditions. Water uptake measurement above
100 ꢁC did not result reproducible data because of technical
difficulties.
corresponds to l (the number of water molecules per acid group) of
about 5e6. Sulfonated polymers with similar structure and number
of acid groups per repeat unit have much higher water uptake and
may even be water soluble [36e38]. Similar observation and
comparison between phosphonated and sulfonated ionomers are
reported in the literature [39]. Because of low number of water
molecules per phosphonic acid groups it is very important to reach
high degree of phosphonation in order to obtain ionomers with
properties useful for fuel cell application. This usually becomes
a limiting factor for post-phosphonation reactions which only one
phosphonic per repeat unit may form even at nearly 100% yield.
It is highly desirable to run PEM fuel cells at temperatures
exceeding 100 ꢁC for better efficiency and less technical complexity.
The system should therefore be able to operate under dryer
condition that is a critical limitation for almost all PEMs which their
proton conductivity depends heavily on minimum level of hydra-
tion. Polybenzimidazole (PBI) doped with phosphoric acid as
mobile phase is one of the best choice for high temperature (up to
200 ꢁC) PEM fuel cells. To investigate proton conductivity of the PAA
membranes at high temperature and water-free conditions, they
were soaked in 85% phosphoric acid at room temperature
Proton conductivity of PAA membrane under dry (no external
humidification) condition increased exponentially by temperature
from nearly 2 ꢂ 10ꢀ3 mS/cm at 25 ꢁC to 2.2 mS/cm at 120 ꢁC (Fig. 7)
which is remarkably high for a non-fluorinated hydrocarbon
membrane. Because of their amphoteric character phosphonic
(membrane thickness was about 40 mm). The level of phosphoric
acid uptake increase over time and reach high level of around
3.5 mol of phosphoric acid per phosphonic group after 14 days
(Fig. 5).
3.7. Proton conductivity
Fig. 6. Temperature dependence of proton conductivity of phosphoric acid doped PAA-
2 membrane ([PO4H3]/[ePO3H2] w3.5) (-) and PAA-2 in the fully hydrated conditions
(:).
Proton conductivity of the phosphonated membranes was
measured under both fully hydrated and dry conditions. The

E. Abouzari-Lotf et al. / Polymer 52 (2011) 4709e4717
4717
References
[1] Roziere J, Jones DJ. Ann Rev Mater Res 2003;33:503e55.
[2] Jannasch P. Fuel Cells 2005;5:248e60.
[3] Hickner MA, Ghassemi H, Kim YS, Einsla BR, McGrath JE. Chem Rev 2004;104:
4587e612.
[4] Ghassemi H, Schiraldi DA, Zawodzinski TA, Hamrock S. Macromol Chem Phys
2011;212:673e8.
[5] Savadogo O. J New Mater Electrochem Syst 1998;1:47e66.
[6] Kordesch KV, Simader GR. Chem Rev 1995;95:191e208.
[7] Kotov SV, Pederson SD, Qiu W, Qiu ZM, Burton DJ. J Fluorine Chem 1997;82:
13e9.
[8] Kreuer KD. Chem Mater 1996;8:610e41.
[9] Kanamura K, Tanaka A, Kennedy V, Adzic R, Yeager EB, Burton D, et al.
J Electrochem Soc 1996;143:2765e70.
[10] Yamada M, Honma I. Polymer 2005;46:2986e92.
[11] Souzy R, Ameduri B, Boutevin B, Gebel G, Capron P. Solid State Ionics 2005;
176:2839e48.
Fig. 7. Proton conductivity vs temperature for PAA-2 membrane under dry condition.
[12] Souzy R, Ameduri B. Prog Polym.Sci 2005;30:644e87.
[13] Schuster M, Rager T, Noda A, Kreuer KD, Maier J. Fuel Cells 2005;5:355e65.
[14] Bock T, Mohwald H, Mulhaupt R. Macromol Chem Phys 2007;208:1324e40.
[15] Steininger H, Schuster M, Kreuer KD, Kaltbeitzel A, Bingol B, Meyer WH, et al.
Phys Chem Chem Phys 2007;9:1764e73.
[16] Allcock HR, Hofmann MA, Wood RM. Macromolecules 2001;34:6915e21.
[17] Meng YZ, Tjong SC, Hay AS, Wang SJ. J Polym Sci. Part A Polym Chem 2001;39:
3218e26.
acids can form strong hydrogen bonding which allows structure
diffusion of protons through formation of a hydrogen bond network
with fast hydrogen bond breaking and forming processes. This
phenomena results in high water retention for phosphonated
membranes at elevated temperatures and high proton conductivity
in a low humidity condition [48].
As expected the proton conductivity of diphosphonated
membranes after doping with phosphoric acid under dry condition
increased significantly and jumped almost three orders of magni-
tude to 1 mS/cm at 30 ꢁC and 100 mS/cm at 140 ꢁC (Fig. 6). Similar
behavior has been reported for PBI/Phosphoric acid system which
shows strong relationship between conductivity and temperature
and level of doping [45,46]. The values of proton conductivity
reported for phosphoric acid doped PBI membranes (6.5H3PO4
molecules per PBI repeat unit) at 140 ꢁC and 5% and 30% RH are 22
and 50 mS/cm, respectively [45]. High proton conductivity of PAA
membrane with relatively low level of doping indicates that
phosphonic acid groups and phosphoric acid molecules effectively
participate in the conductivity process by formation of a hydrogen
bonding network.
[18] Liu B, Robertson GP, Guiver MD, Shi Z, Navessin T, Holdcroft S. Macromol
Rapid Commun 2006;27:1411e7.
[19] Miyatake K, Hay AS. J Polym Sci. Part A Polym Chem 2001;39:3770e9.
[20] Meng YZ, Tjong SC, Hay AS, Wang SJ. Eur Polym J 2003;39:627e31.
[21] Jakoby K, Peinemann KV, Nunes SP. Macromol Chem Phys 2003;204:61e7.
[22] Parvole J, Jannasch P. J Mater Chem 2008;18:5547e56.
[23] Lafitte B, Jannasch P. J Polym Sci. Part A Polym Chem 2005;43:273e86.
[24] Parcero E, Herrera R, Nunes SP. J Membr Sci 2006;285:206e13.
[25] Allcock HR, Hofmann MA, Ambler CM, Lovov SN, Zhou XY, Chalkova E, et al.
J Membr Sci 2002;201:47e54.
[26] Zhou X, Weston J, Chalkova E, Hofmann MA, Ambler CM, Allcock HR, et al.
Electrochim Acta 2003;48:2173e80.
[27] Yanagimachi S, Kaneko K, Takeoka Y, Rikukawa M. Synth Met 2003;135e136:
69e70.
[28] Stone C, Daynard TS, Hu LQ, Mah C, Steck AE. J New Mater Electrochem Syst
2000;3:43e50.
[29] Wang ZY, Carvalho HN, Hay AS. J Chem Soc Chem Commun; 1991:1221e2.
[30] Atherton FR, Openshaw HT, Todd AR. J.Chem Soc; 1945:660e3.
[31] Dhawan B, Redmore D. J Org Chem 1984;49:4018e21.
[32] Kerres J, Zhang W, Cui W. J Polym Sci. Part A Polym Chem 1998;36:1441e8.
[33] Rogers ME, Long TE, Wang S, McGrath JE. Synthesis of poly(arylene ether)s. In:
Rogers ME, Long TE, editors. Synthetic methods in step-growth polymers.
Hoboken, NJ, USA: John Wiley & Sons, Inc.; 2003. p. 327e74.
[34] Ghassemi H, McGrath JE, Zawodzinski TA. Polymer 2006;47:4132e9.
[35] Cabasso I, Jagur-Grodzinski J. J Appl Polym Sci 1974;18:1969e86.
[36] Schuster M, Kreuer KD, Andersen HT, Maier J. Macromolecules 2007;40:
598e607.
4. Conclusions
In the present study, we report successful synthesis of highly
phosphonated poly(arylene ether)s from diphosphonated hydro-
quinone. The phosphonation took place on the hydroquinone ring
at two positions and polymerization with the decafluorobiphenyl
following with acidic hydrolysis lead to polymers with inherent
viscosities up to 0.58 dL gꢀ1. The resulting polymers were soluble in
polar aprotic solvents and could be readily cast into tough and clear
films. The thermal stability of these membranes based on TGA
study showed T10% at about 430 ꢁC. The high phosphonic acid
content membrane reached a conductivity of around 150 mS/cm at
about 100 ꢁC under fully hydrated and 2.2 mS/cm at 120 ꢁC under
dry condition. Once impregnated with phosphoric acid proton
conductivity jumped to near 100 mS/cm under no external
humidification. The combination of all these properties makes this
phosphonated polymeric electrolyte potentially useful in fuel cell
technology.
[37] Liu B, Robertson GP, Guiver MD, Sun YM, Liu YL, Lai JY, et al. J Polym Sci. Part A
Polym Phys 2006;44:2299e310.
[38] Peron J, Ruiz E, Jones DJ, Roziere J. J Membr Sci 2008;314:247e56.
[39] Tayouo R, David G, Ameduri B, Roziere J, Roualdes S. Macromolecules 2010;
43:5269e76.
[40] Slade S, Campbell SA, Ralph TR, Walsh FC. J Electrochem Soc 2002;149:
A1556e64.
[41] Fukuzaki N, Nakabayasi K, Nakazawa S, Murata S, Higashihara T, Ueda M.
J Polym Sci. Part A Polym Chem 2011;49:93e100.
[42] Lafitte B, Jannasch P. J Polym Sci. Part A Polym Chem 2007;45:269e83.
[43] Shaplov AS, Lozinskaya EI, Odinets LL, Lyssenko KA, Kurtova SA, Timofeeva GI,
et al. React Funct Polym 2008;68:208e24.
[44] Abu-Thabit NY, Ali SA, Zaidi SMJ. J Membr Sci 2010;360:26e33.
[45] Ma YL, Wainright JS, Litt MH, Savinell RF. J Electrochem Soc 2004;151:A8e16.
[46] He R, Li Q, Jensen JO, Bjerrum NJ. J Polym Sci. Part A Polym Chem 2007;45:
2989e97.
[47] Johnson FE, Cabasso I. Macromolcules 2010;43:3634e51.
[48] Lafitte B, Jannasch P. On the prospects for phosphonated polymers as proton
exchange fuel cell membranes. In: Zhao TS, Kreuer KD, Nguyen TV, editors.
Advances in fuel cells, vol. 1. Oxford, UK: Elsevier; 2007. p. 119e45.