Synthesis and alkaline stability of novel cardo poly(aryl ether sulfone)s with pendent quaternary ammonium aliphatic side chains for anion exchange membranes

Article abstract of DOI:10.1016/j.polymer.2010.09.049

Novel cardo poly(aryl ether sulfone)s containing pendent -[CH₂CH₂NMe₃]⁺OH^-, or -[CH₂CH₂CH₂NMe₃]⁺OH^- groups, and partially fluorinated

Full text of DOI:10.1016/j.polymer.2010.09.049

Polymer 51 (2010) 5407e5416
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and alkaline stability of novel cardo poly(aryl ether sulfone)s with
pendent quaternary ammonium aliphatic side chains for anion exchange
membranes
,
,
,
,
a
Qiang Zhang a b, Qifeng Zhang a b, Junhua Wang a b, Suobo Zhang a , Shenghai Li
*
a Key Laboratory of Advanced Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Novel cardo poly(aryl ether sulfone)s containing pendent e[CH2CH2NMe3]þOHꢀ, or
e[CH2CH2CH2NMe3]þOHꢀ groups, and partially fluorinated cardo poly(aryl ether sulfone) containing
pendent e[CH2CH2CH2NMe3]þOHꢀ groups were synthesized and examined with the focus of under-
standing how the polymer chemical structure affects their morphology, ion conductivity and alkaline
stability. The resulting quaternized polymers exhibited outstanding solubility in polar aprotic solvents.
The partially fluorinated cardo poly(aryl ether sulfone)s (PES-PF-OH) with ion-exchange capacity of 1.44
meq gꢀ1 displayed the highest ion conductivities, varied from 3.0 ꢁ 10ꢀ2 to 4.1 ꢁ 10ꢀ2 S cmꢀ1 over the
Article history:
Received 30 May 2010
Received in revised form
14 September 2010
Accepted 16 September 2010
Keywords:
range 20e60 ꢂC. TEM revealed that PES-PF-OH membrane exhibited
a distinct phase-separated
Anion exchange membrane
Hydroxide ion conductivity
Dimensional stability
morphology comprised of interconnected ionic clusters in size of 1e2 nm. The studies on alkaline
stability of the membranes revealed that the PES-PF-OH polymer was not stable under strong basic
conditions.
The
quarternized
polymers
containing
pendent
e[CH2CH2NMe3]þOHꢀ,
or
e[CH2CH2CH2NMe3]þOHꢀ groups mainly underwent Hofmann elimination and SN2 substitution
reaction.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
electrocatalysts, thus drastically reducing the cost of the fuel cell
[5]. Furthermore, ion transport within the membrane is from the
Ion-conducting polymers are of interest for a variety of appli-
cations, such as sensors, actuators, batteries, and ion-exchange
membranes or resins. In recent years, studies on ion-conducting
polymers have been strongly promoted as a result of a considerable
interest in the development of high performance polymer elec-
trolyte membrane fuel cells for transportation, stationary and
portable power applications [1e3]. Polymer electrolyte membrane
fuel cells (PEMFCs) based on proton exchange membrane (PEM),
especially Nafion membrane, have been intensively studied [4]. It
has been recognized that some specific limitations exist for
PEMFCs, including slow electrode-kinetics, CO poisoning of Pt and
Pt-based electrocatalysts at low temperatures, and high cost of the
membrane and catalyst. In view of this, alkaline anion exchange
membrane fuel cells (AEMFCs) have attracted much attention since
it has many attractive advantages over PEM fuel cell, including
performance and cost. The faster kinetics of oxygen reduction
reactions in alkaline media allows the use of nonprecious metal
cathode to the anode, opposing the direction of methanol crossover
from anode to cathode, which will reduce the methanol perme-
ability. Challenges in the development of AEMFCs include the
development of stable anion exchange membranes (AEMs) at high
pH. The commercial membranes containing substituted poly-
styrene are not stable in basic medium. For the design of novel
AEMs, aromatic ionomers are the preferred candidates for fuel cell
applications due to their excellent thermal and mechanical prop-
erties as well as their resistance to oxidation and stability in acidic
and alkaline conditions. Chloromethyl groups can be introduced
onto the phenyl ring of the aromatic backbone via the chlor-
omethylation reaction. Subsequently, the AEMs are prepared by
amination of chloromethlated polymers with various amines. Using
this general method, polysulfones containing quaternary ammo-
nium [6e8], guanidinium [9] and quaternary phosphonium groups
[10] have been prepared and evaluated for the application in
AEMFCs. However, chloromethylation reaction is usually restricted
due to their lack of precise control over the degree and location of
functionalization [6,11] and the use of a toxic regent, chloromethyl
methyl ether [12,13].
* Corresponding author. Tel.: þ86 431 5605139.
E-mail address: sbzhang@ciac.jl.cn (S. Zhang).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.09.049

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Q. Zhang et al. / Polymer 51 (2010) 5407e5416
HO
OH
HO
OH
N
HCl
N
+
H2N
O
reflux 24 h
N
O
O
Scheme 1. The synthetic route of monomer PPH-DMEA.
A strategy to improve the stability and conductivity of aromatic
ionomers is to distinctly separate the hydrophilic functional groups
from the hydrophobic polymer main chain by, e.g., locating the
sulfonic acid groups on side chains grafted onto the polymer main
chain [14e16]. Most of the sulfonated proton exchange membranes
were obtained by post-sulfonation process and direct polymerization
from sulfonated monomers to introduce sulfonic acid group to poly-
mer side chain, while aromatic ionomer containing quaternary
ammoniumaliphaticsidechainswasseldomachievedprobablydueto
the lack of appropriate and commercially available monomers and
convenient approach to convert them into anion-conductive
materials.
In this study, a series of novel cardo poly(aryl ether sulfone)s
containing pendent e[CH2CH2NMe3]þOHꢀ, or e[CH2CH2CH2
NMe3]þOHꢀ groups, and partially fluorinated cardo poly(aryl ether
sulfone) containing pendent e[CH2CH2CH2NMe3]þOHꢀ groups
were synthesized. The tertiary monomers were easily prepared by
the reaction of phenolphthalein with commercially available N,N-
dimethyl-1,3-propanediamine (DMPA) or N,N-dimethylethane-1,2-
diamine (DMEA). With these monomers, cardo poly(aryl ether
sulfone)s with pendent tertiary amine groups were then prepared.
Alkaline anion exchange membranes were made by the reaction of
the tertiary amine groups with iodomethane to form quaternary
ammonium groups which acted as the counterion for hydroxide
anion. An advantage of this method is avoiding the use of toxic
chloromethylation reagents. Their properties, such as the solubility,
thermal stability, mechanical strength, water uptake behavior,
hydroxide ion conductivity, area resistance, static transport number,
morphology, and alkaline stability have been investigated in detail.
(DCDPS), N,N-dimethyl-1,3-propanediamine (DMPA), N,N-dime-
thylethane-1,2-diamine (DMEA) and iodomethane were purchased
from Aldrich and used as received. Dimethylsulfoxide (DMSO) and
N-Methyl-2-pyrrolidinone (NMP) were firstly dried over CaH2 and
then distilled under reduced pressure before use. All other reagents
were obtained from commercial sources and used as received.
2.1.1. Synthesis of 2-(3-(dimethylamino)propyl)-3,30-bis(4-
hydroxyphenyl)isoindolin-1-one (PPH-DMPA)
The monomer PPH-DMPA was prepared according to the
previously reported method [17]. Yield: 85%. mp: 236.0e237.0 ꢂC.
9.55 (2H, s), 7.65e7.68 (1H, d), 7.49e7.54 (1H, t), 7.43e7.45 (1H, t),
7.39e7.41 (1H, d), 6.91e6.94 (4H, d), 6.70e6.73 (2H, d), 1.91 (6H, s),
0.84e0.91(2H, m), FT-IR (KBr): 3201 cmꢀ1 (OeH), 2963 cmꢀ1
(
yCeH), 1685 cmꢀ1 (C]O of phthalimidine).
2.1.2. Synthesis of 2-(2-(dimethylamino)ethyl)-3,30-bis(4-
hydroxyphenyl)isoindolin-1-one (PPH-DMEA)
The monomer PPH-DMEA was prepared according to the
previously reported method [18]. Mp: 264.5e265.5 ꢂC, the yield
was 82%. 1H NMR (DMSO-d6-ppm): 9.58 (2H, s), 7.67e7.68 (1H, d),
7.52e7.55 (1H, t), 7.44e7.46 (1H, t), 7.39e7.41 (1H, d), 3.37e3.40
(2H, t), 6.94e6.96 (4H, d), 6.73e6.74 (2H, d), 1.91 (6H, s), 1.55e1.58
(2H, t). IR (KBr): 2963 cmꢀ1
dine), 3280 cmꢀ1
yOeH
( (yC]O of phthalimi-
yCeH), 1683 cmꢀ1
(
)
2.1.3. Synthesis of 3,30,4,40-tetrafluorodiphenylsulfone (TFDPS)
The monomer TFDPS was prepared by the same method
reported by our group in previous paper [19].
2.1.4. Synthesis of cardo poly(aryl ether sulfone)s containing
pendent tertiary amine groups
2. Experimental part
Cardo poly(aryl ether sulfone)s containing e(CH2CH2CH2NMe2)
(PES-P-N) or e(CH2CH2NMe2) (PES-E-N) groups were prepared
according to the previously reported method [17,18]. A typical
synthetic procedure, illustrated by the preparation of PES-P-N poly-
mer, is described as follows. Polymerizations of PPH-DMEA (3.8846 g,
2.1. Materials and chemicals
Phenolphthalein (PPH) was purchased from Beijing Chemical
Reagent Company, and purified by recrystallization from mixed
solvent of ethanol and water. 4,40-dichlorodiphenylsulfone
O
HO
OH
O
O
S
n
O
O
S
K2CO3 / Toluene / NMP
140 oC, 4 h 180 oC, 10 h
+
Cl
Cl
N
N
N
N
O
O
O
O
S
O
S
O
O
O
O
n
n
O
O
CH3I / 24 h / 20 o
C
(1) cast film
(2) 1 N KOH
N
N
I
N
N
OH
O
O
Scheme 2. The synthetic route of the PES-E-OH.

Q. Zhang et al. / Polymer 51 (2010) 5407e5416
5409
O
PES-E-OH
O
O
S
n
O
N
N
OH
O
O
S
PES-P-OH
O
O
n
O
N
N
OH
O
F
F
O
S
O
O
PES-PF-OH
n
O
N
N
OH
O
OH
N
N
OH
O
S
O
S
PES-B-OH
O
O
O
O
0.65
0.35
O
O
Fig. 1. The chemical structures of the ionomers synthesized in this study.
10 mmol) and DCDPS (2.8716 g,10 mmol) were conducted in a flame
dried three-necked flask. The flask was fitted with a nitrogen inlet,
over-head stirrer and Dean Stark trap fitted with a condenser. After
charging all the monomers and solvent, the reaction flask was heated
in an oil bath to 140 ꢂC while the toluene was allowed to reflux for 4 h
to remove any water present from the hydrated atmosphere or
hydrated monomers. The toluene was then removed over a 60 min
time interval. The Dean Stark trap was emptied and the reaction
temperature was slowly increased to 180 ꢂC for 10 h. The resulting
solution media became very viscous. The viscous solution was cooled
to room temperature and then diluted with NMP. The polymer solu-
tion was filtered to remove the salt byproduct and then isolated by
precipitation in deionized (DI) water. The resulting fibrous polymer
was washed thoroughly with water several times and dried in
a vacuum oven at 120 ꢂC for 24 h. Yield: 98%.
at room temperature for a few minutes and then slowly heated at
90 ꢂC for 10 h. After cooling to room temperature, an additional
30 mL of NMP was added so as to dilute the highly viscous solution,
after which the solution was filtered and dropped into stirred
deionized water. The fiber-like precipitate was filtered off and
washed with hot water three times prior to being dried under
vacuum to produce the final product. Yield: 98%.
2.1.6. Synthesis of cardo poly(aryl ether sulfone)s containing alkyl
ammonium groups (PES-E-I, PES-P-I and PES-PF-I)
A typical synthetic procedure, illustrated by the preparation of
PES-E-I polymer, is described as follows. Iodomethane was added at
a molar ratio of 2 compared to amino groups to a solution of PES-E-N
in DMSO and the mixture was stirred for 24 h. The polymer solution
was isolated by precipitation in deionized (DI) water, separated by
filtration and dried in a vacuum oven at 120 ꢂC for 24 h.
2.1.5. Synthesis of partially fluorinated cardo poly(aryl ether
sulfone)s containing pendent tertiary amine groups
A flame dried 100 mL three-necked flask equipped with
a nitrogen inlet and over-head stirrer was charged with PPH-DMPA
(2.0125 g, 5 mmol), TFDPS (1.4512 g, 5 mmol), K2CO3 (1.38 g,
10 mmol), and dry NMP (34 mL, 10% solids). The mixture was kept
2.2. Membrane preparation
The membranes were prepared by the dissolution of the qua-
ternized polymers in NMP (5% w/v). The solution was then filtered
Table 1
Viscosity values, molecular weights and mechanical properties of membranes in dry state.
Polymer
h
(dLꢀ1 g)
Mn ꢁ 103 (g/mol)
Mw ꢁ 103 (g/mol)
PDI
Tensile strength (MPa)
Young Modulus (GPa)
Elongation break (%)
PES-E-N
PES-E-OH
PES-P-N
PES-P-OH
PES-PF-N
PES-PF-OH
0.50
1.20
0.44
1.13
0.52
1.15
41.5
54.0
40.7
52.9
50.3
70.4
89.7
125.6
91.0
127.4
92.0
2.16
2.33
2.24
2.42
1.83
1.96
52.9
34.8
50.2
31.9
60.0
48.3
1.09
0.59
1.08
0.54
1.94
0.86
10.6
33.9
12.5
31.7
17.9
23.8
138.0
Samples were dried at ambient conditions for 1 day and tested at 30 ꢂC, 30% RH.

5410
Q. Zhang et al. / Polymer 51 (2010) 5407e5416
Fig. 2. 1H NMR spectra of PES-P-N and PES-P-OH (a), PES-PF-N and PES-PF-OH (b) in DMSO.
through a 0.45
m
m Teflon syringe filter and cast onto a clean glass
a solvent at a flow rate of 1.0 mL/min. The thermogravimetric anal-
yses (TGA) were obtained in nitrogen with a Perkin Elmer TGA-2
thermogravimetric analyzer (Inspiratech 2000 Ltd., UK) at a heating
rate of 10 ꢂC/min. Tensile measurement was performed with
a mechanical tester Instron-1211 instrument (Instron Co., USA) at
aspeed of2 mm/min at ambienthumidity(w30%relativehumidity).
The ion-exchange capacity, defined in this study as mmol (OHꢀ) gꢀ1
(dry AEM), was determined using the standard back-titration tech-
nique reported previously [20]. Hydroxide anion conductivity of the
membranes was measured by two-probe electrochemical imped-
ance spectroscopy (EIS) using a Solartron 1260 frequency response
analyzer coupled to a Solartron 1287 potentiostat.
substrate. The membranes were slowly dried in an oven for 12 h at
80 ꢂC and then in vacuo at 100 ꢂC for 24 h, forming thin films about
30
m
thick after complete evaporation of the organic solvent.
The above quaternized membranes were soaked in 1.0 M
potassium hydroxide aqueous solution at room temperature for
48 h, followed by washing with deionized water several times and
soaking in deionized water with many times exchanges within 48 h
prior to evaluation.
2.3. Characterization
Nuclear magnetic resonance (NMR) spectra were recorded using
a Varian Unity Inova spectrometer at a resonance frequency of
400 MHz for 1H NMR and 376 MHz for 19F NMR in DMSO-d6. The
chemical shifts relative to tetramethylsilane for 1H NMR and fluo-
robenzene for 19F NMR are reported on the ppm scale. Attenuated
total reflectance infrared (ATR-IR) characterization of the
membranes surface was made with a Bio-Rad digilab Division FST-
80 spectrometer. Inherent viscosities of all polymers were deter-
mined at 30 ꢃ 0.1 ꢂC using Ubbelodhe viscometer with 0.5 dL/g
concentration in NMP. Molecular weight measurement was per-
formed via gel permeation chromatography with JASCO PU-2080
Plus with two polystyrene gel column (TSK GELs; GMHHR-M). N,N-
dimethylformamide (DMF) containing 0.01 M LiBr was used as
2.3.1. Water uptake and swelling ratio measurements
The membrane sample (30e40 mg per sheet) was dried at 50 ꢂC
under vacuum for 48 h until constant weight as dry material was
obtained. It was immersed into deionized water at given temper-
ature for 8 h and then quickly taken out, wiped with tissue paper,
and quickly weighted on a microbalance. Use Eq. (1) to calculate the
water uptake (wu):
hꢀ
ꢁ.
i
Water uptakeð%Þ ¼
Wwet ꢀ Wdry Wdry ꢁ 100%
(1)
where Wdry and Wwet are the weight of the dry and the corre-
sponding water-swollen membranes, respectively. The water
swelling ratio of the membranes was investigated by immersing
the round-shaped samples into water at room temperature for
a given time, and the swelling ratio (sr) was calculated from:
1.0
PES-P-I
PES-P-N
hꢀ
ꢁ.
i
PES-P-OH
0.9
Swelling ratioð%Þ ¼
lwet ꢀ ldry ldry ꢁ 100%
(2)
0.8
0.7
0.6
0.5
0.4
0.3
Here, ldry and lwet are the length of the dry and wet samples,
respectively.
Table 2
IEC, water uptake and swelling ratio of membranes.
Polymer
IEC (meq gꢀ1
)
Water uptake
(%)
Swelling ratio
(%)
Theoretical
Experimental
20 ꢂC
60 ꢂC
20 ꢂC
60 ꢂC
PES-E-N
PES-E-OH
PES-P-N
PES-P-OH
PES-PF-N
PES-PF-OH
PES-B-OH
5.7
25.8
5.5
25.1
3.0
7.2
27.5
6.9
28.3
4.4
3.0
4.2
3.1
4.2
0
3.9
5.5
3.8
5.4
1
1.57
1.54
1.52
1.50
100
200
300
400
500
600
700
800
o
temperature ( C (
1.46
1.50
1.44
1.51
15.1
20.0
18.4
22.7
2.7
4.0
3.0
4.2
Fig. 3. TGA curves for PES-P-N, PES-P-I and PES-P-OH membranes in N2.

Q. Zhang et al. / Polymer 51 (2010) 5407e5416
5411
Fig. 4. TEM micrographs of polymer membranes: (a) PES-B-OH, (b); (c) PES-P-OH and (d) PES-PF-OH.
2.3.2. Transmission electron microscopy
In order to obtain TEM images, the membrane was first dyed
with PdCl24ꢀ by immersing in a PdCl24ꢀ/HCl solution for 2 days, and
then rinsed with excessive water and finally dried at room
temperature for 12 h. The dyed sample was embedded in epoxy
resin and sectioned using a microtome to yield a 50 nm thick
sample which was placed on copper grids. Images were taken on an
ultrahigh-resolution transmission electron microscope (JEOLJEM-
2010FEF) using an accelerating voltage of 200 kV.
0.05
0.04
0.03
0.02
0.01
PES-P-OH
PES-E-OH
PES-PF-OH
PES-B-OH
2.3.3. Membrane area resistance and the transport number
The measurement of membrane area resistance and the trans-
port number was based on the procedure in the literature [21] with
a
few modifications. The area resistance was measured in
Table 3
Area resistance and static transport number of membranes.
Polymer
Thickness IEC
m) (meg/g dry) (%)
PES-PF-OH 23
Water content Rm
t )
(OHꢀ
ꢀ
(m
(U )
cmꢀ2
20
30
40
50
60
1.44
1.50
1.52
1.51
15.1
17.7
29.9
28.5
35.2
0.91
0.92
0.92
0.93
o
)
C
(
Temperature
PES-P-OH
PES-E-N
PES-B-OH
22
22
24
25.1
25.8
20.0
Fig. 5. Temperature dependence of the hydroxide ion conductivities of PES-E-OH, PES-
P-OH, PES-PF-OH and PES-B-OH membranes.

5412
Q. Zhang et al. / Polymer 51 (2010) 5407e5416
Table 4
The mole fraction of degradation and viscosity values change.
Polymer
Mole fraction of degradation
Viscosity values change
x
y
x þ y
Viscosity before
testing (dL/g)
Viscosity after
testing (dL/g)
Percentage of viscosity
change (%)
PES-P-OH
PES-E-OH
PES-PF-OHa
PES-B-OH
0.14
0.15
e
0.08
0.42
e
0.22
0.57
e
1.13
1.20
1.15
1.15
1.02
0.99
0.25
1.08
9.7
17.5
78.3
6.1
0.16
e
0.16
x: the mole fraction of degradation caused by substitution reaction. y: the mole fraction of degradation caused by Hofmann elimination reaction.
a
The percent of degradation could not calculate from 1H NMR due to overlap of the peaks.
conductivity cells. The measurement was carried out in 0.1 mol Lꢀ1
NaOH solution. The resistance was calculated by the resistance of
conductivity cell subtracting the resistance of electrolyte solutions.
The electromotive force (E) between both sides of a membrane,
which caused by different concentrations, was measured by two
reverse Ag/AgCl electrodes connected to an auto voltmeter. The
membrane potential is obtained from the difference of E and the
standard solution potential E0:
KOH solution, respectively. MðꢀNðMeÞ3Þ is the mole of ꢀNðMeÞ3
groups in original membranes. The integral values in 1H NMR
spectra were used in the calculation of x and y values.
3. Results and discussion
3.1. Synthesis and characterization of monomers and polymers
The typical synthetic routes for the biphenol monomer (PPH-
DMEA) and quternized polymer containing e[CH2CH2NMe3]þOHꢀ
groups, PES-E-OH, were shown in Scheme 1 and 2, respectively. The
monomer was synthesized by condensation of phenolphthalein with
N,N-dimethylethane-1,2-diamine (DMEA) in the yield of 82%
according to the literature method for the synthesis of monomer
PPHeDMEA reported by our groups [18]. Polymer containing
e[CH2CH2CH2NMe3]þOHꢀ groups, PES-P-OH, and partially fluori-
nated polymer, PES-PF-OH, were prepared from PPH-DMPA and
DCDPS, PPH-DMPA and 3,30,4,40-tetrafluorodiphenylsulfone, respec-
tively. For the sake of comparison, polymer containing
e[CH2NMe3]þOHꢀ groups, PES-B-OH, was prepared according to our
previously reported method [19]. All structures of the quaternized
polymers in this study were shown in Fig.1. In a typical procedure for
RT C2
E ¼ E ꢀ E0 and E0 ¼ ꢀ ln
(3)
F
C1
where R is gas constant, T is absolute temperature, F is faraday
constant,C1 and C2 are the concentration of aqueous solution in the
two sides. The static transport number t of the hydroxyl in the
ꢀ
membrane is given by Eq. (4):
2E
E0
t
ꢀ
¼
(4)
2.3.4. Alkaline stability
The alkaline stability of the membranes was evaluated in 10 M
KOH solution at 30 ꢂC. The percentage of degradation (x, y) was
expressed as:
O
ꢂ
ꢃ
ꢂ
ꢃ
O
O
S
SN2 substitution reaction
M ꢀ NðMeÞ2
ꢂ
M ꢀCH ¼ CH2
n
O
ꢃ
ꢂ
ꢃ
x ¼
; y ¼
(5)
M ꢀ NðMeÞ3
M ꢀ NðMeÞ3
N
N
OH
where MðꢀNðMeÞ2Þ and MðꢀCH ¼ CH2Þ are the molar of ꢀNðMeÞ2
and eCH]CH2 groups generated by degradation after test in 10 M
O
10 M KOH 30 o
C
O
S
O
O
n
O
N
N
O
O
S
O
O
Hofmann degradation
n
O
N
N
H
O
OH
10 M KOH 30 o
C
O
S
O
O
n
O
N
O
Fig. 6. 1H NMR spectra of PES-E-OH (a) and PES-E-OH treating with KOH for 24 h (b) in
DMSO.
Scheme 3. Degradation mechanism of PES-E-OH in KOH solution (10 M) at 30 ꢂC.

Q. Zhang et al. / Polymer 51 (2010) 5407e5416
5413
the hydrogen signal assignment in the 1H NMR spectra of PES-P-OH
and PES-PF-OH. Two typical peaks of a tertiary amine group were
observed at
N. After quaternization, proton signals of methylene and methyl
were shifted to higher frequencies ( 3.14 (eCH2e) and 2.86 (eCH3))
d 1.95 (eCH2e), and 2.19 (eCH3) for PES-P-Nand PES-PF-
d
as a result of the deshielding fromthe quaternary nitrogen atom. The
integration ratio of Hi to Hk was close to 9:2, suggesting a quantita-
tive conversion of tertiary amine to ammonium groups. The peak at
d
7.89e7.92 ppm was attributed to the hydrogen atom (Ha) in the
ortho position tothe sulfone group. The integration ratio of Ha and Hi
was very close to 4:9, as expected for the composition of PES-P-OH.
FT-IR spectra of PES-PF-OH revealed the vibration bands at 1683 and
724 cmꢀ1, which corresponded to the stretch vibration of carbonyl
and AreF bonds groups, respectively.
3.2. Thermal and mechanical properties
The thermal stabilities of the membranes were analyzed by
recording their TGA curves under flowing nitrogen. The TGA traces
of the resulting polymers were presented in Fig. 3 for PES-P-N, PES-
P-I and PES-P-OH membranes as representative cases. The tertiary
amine polymer PES-P-N was a thermally stable polymer and
exhibited the starting decomposition temperature around 260 ꢂC. A
two-step degradation profile was observed for the polymer con-
taining e(CH2CH2CH2NMe3)þIꢀ groups (PES-P-I). The first weight
loss observed for PES-P-I at 188 ꢂC was assigned to the degradation
of the side chain of PES-P-I. The second weight loss region (at
temperature >373 ꢂC), the polymer residues were further
degraded, corresponded to the decomposition of the main chains of
PES-P-I. The polymer containing e(CH2CH2CH2NMe3)þOHꢀ groups
(PES-P-OH) showed three-step weight loss. The first weight loss
from 128 to 190 ꢂC was due to Hofmann elimination of quaternary
ammonium hydroxides. The result indicated that PES-P-OH is less
stable than PES-P-I and PES-P-N.
Fig. 7. 1H NMR spectra of PES-P-OH (a) and PES-P-OH treating with KOH for 24 h (b) in
DMSO.
the synthesis of PES-E-N, a stoichiometric ratio of the PPH-DMPA and
DCDPS, an excess of potassium carbonate, and NMP with a 20% solid
content were used. The reaction was first held at 140 ꢂC for 4 h under
nitrogen to azeotrope off water with toluene and then was heated up
to 180 ꢂC for 10 h to afford a high-molecular-weight polymer. All
polymers containing tertiary amine groups were converted to the
quaternized poly(aryl ether sulfone)s by reaction with iodomethane
at 20 ꢂC for 24 h. The mole ratio of iodomethane to tertiary amine
groups was set to be 2:1 to ensure complete conversion of the tertiary
amine units into the quaternary ammonium moieties.
Table 1 lists the mechanical properties of membranes as
measured at room temperature and 30% RH. The tertiary amine
polymer membranes had tensile strength at maximum load in the
range of 50e60 MPa, Young’s modulus of 1.08e1.94 GPa, and
elongation at break of 10.6e17.9%. After the quaternization, the
tensile strength and Young’s modulus decreased, and elongation at
break improved due to the introduction of strong polar quaternary
ammonium moieties in side chains. The quaternized polymer
membranes still showed good mechanical properties with tensile
strength, elongation at break and Young’s modulus in the ranges of
32e48 MPa, 23.8e33.9%, and 0.54e0.86 GPa in dry state, respec-
tively. The tension results showed that they were strong and flex-
ible membrane materials.
The high-molecular-weight polymers can be successfully
synthesized by nucleophilic aromatic substitution polymerization,
each of which had high-molecular-weight (Mn > 40.7 ꢁ 103 g/mol,
M
w > 89.7 ꢁ103 g/mol), as evidenced by GPC analyses and their high
viscosity values (Table 1). These polymers showed good solubility in
polar aprotic solvents, such as dimethylacetamide (DMAc), dime-
thylsulfoxide (DMSO), and NMP. The chemical structures of the
polymers were confirmed by FT-IR and 1H NMR spectra. Fig. 2 shows
3.3. IEC, water uptake, and swelling ratio
IEC, water uptake and swelling ratio of membranes were
summarized in Table 2. The ion-exchange capacity was determined
using the standard back-titration technique reported previously
[20]. In all cases, the experimental value of IEC agreed with the
theoretical one, demonstrating that Iꢀ ions were completely
replaced by OHꢀ ions.
The water within membranes provides a carrier for the OHꢀ ions
and maintains high OHꢀ ions conductivity. However, excessive
water uptake in AEMs leads to unacceptable dimensional change
and loss of dimensional shape. Therefore, the preparation of qua-
ternizated membranes with appropriate water uptake and excel-
lent dimensional stability is one of the critical requirements for
their application as AEMs.
Fig. 8. 19F NMR spectra of PES-PF-OH (a) and PES-PF-OH treating with KOH for 24 h (b)
in DMSO.
The water uptake and swelling ratio of the membranes are
reported in Table 2. It could be seen that PES-PF-OH (wu: 15.1%, sr:

5414
Q. Zhang et al. / Polymer 51 (2010) 5407e5416
Fig. 9. 1H NMR spectra of PES-PF-OH (a) and PES-PF-OH treating with KOH for 24 h (b) in DMSO.
2.7% at 20 ꢂC) exhibited a lower water uptake and swelling ratio
than those of PES-P-OH (wu: 25.1%, sr: 4.2% at 20 ꢂC) and PES-E-OH
(wu: 25.8%, sr: 4.2% at 20 ꢂC), because of more hydrophobic poly-
mer backbone. All of polymers displayed proper water uptake and
excellent dimensional stability.
(PES-B-OH) exhibited a hydroxide conductivity lower than those of
others containing pendent quaternary ammonium groups on
aliphatic side chains. For instance, as temperature increases from 20
to 60 ꢂC, the conductivity increases from 0.021 to 0.031 S/cm for
membrane PES-P-OH (IEC ¼ 1.50 meq gꢀ1), from 0.031 to 0.042 S/cm
for PES-PF-OH, whereas the conductivity increases steadily from
0.018 to 0.026 S/cm for membrane PES-B-OH (IEC ¼ 1.51 meq gꢀ1).
The main chain of PES-PF-OH is composed of a partly fluorinated
poly(aryl ether sulfone), while the side chain segments containing
the ionic groups are composed of flexible, aliphatic chains. The
introduction of F atom in the polymer main chain increases the
hydrophobicity of the polymer backbone of PES-PF-OH, which
facilitates to form hydrophilic/hydrophobic microphase separation
3.4. Membrane morphology and hydroxide ion conductivity
Transmission electron micrographs (TEMs) of membranes were
shown in Fig. 4. To investigate phase separation and ionic aggre-
gation, membranes were stained with PdCl24ꢀ anion; thus, dark
areas correspond to regions of high ionicity and brighter areas to
hydrophobic regions. It was observed that all membranes
possessed a phase-separated morphology characterized by ionic
clusters. The polymer’s chemical structure plays an important role
in phase separation and ionic aggregation. Ionic clusters of PES-B-
OH (ca. 6e8 nm in diameter) and PES-P-OH (ca. 20e30 nm in
diameter) were observed. This morphology is very similar to that of
quaternized poly(aryl ether sulfone) reported by Zhuang’s group
[7], which possesses tens of nanometers ionic clusters. However,
after introduction of fluorine into the main chain, the morphology
remarkably changed. The ionic cluster size of PES-PF-OH
membrane (1e2 nm) shows a substantially decrease and the
number density of the ionic clusters demonstrates great increase
compared with those of PES-P-OH membrane (Fig. 4d).
The hydroxide ion conductivities of the PES-B-OH, PES-E-OH,
PES-P-OH and PES-PF-OH membranes were measured in water in
the temperature range of 20e60 ꢂC, and the results were illustrated
in Fig. 5. As expected, the hydroxide ion conductivity increased with
increasing temperature. All the samples exhibited room tempera-
ture conductivities higher than 1 ꢁ 10ꢀ2 S/cm. Hydroxide ion
conductivity values for all membranes with similar IEC value ranged
between 0.018 and 0.030 S/cm at 20 ꢂC. It can be seen that the order
of conductivity is: PES-PF-OH > PES-E-OH w PES-P-OH > PES-B-OH.
In the entire temperature range studied, PES-PF-OH with the lowest
IEC value showed the highest hydroxide ion conductivity compared
with other samples. The main chain quaternary ammonia ionomer
Fig. 10. 1H NMR spectra of PES-B-OH (a) and PES-B-OH treating with KOH for 24 h (b)
in DMSO.

Q. Zhang et al. / Polymer 51 (2010) 5407e5416
5415
structure. This ultimately results in better-connected ionic domains
and high density of ionic clusters leading to the excellent conduc-
tivities (Fig. 4d). By combining the microscopic observation and
hydroxide conducting behavior, it is concludedthatboththe size and
density of ionic clusters have a great influence on hydroxide
conductivity in AEMs.
from the attachment of electron-withdrawing groups. The electron
density at the -carbon of PES-E-OH is small in comparison to PES-
P-OH because of the strong electron-withdrawing effect of
nitrogen. The decrease in the electron density at the -carbon of
PES-E-OH makes it easier for the hydroxyl ion to abstract proton.
The ease of the -hydrogen abstraction facilitates decomposition of
b
b
b
quaternary ammonium groups to alkene.
3.5. Area resistance and static transport number
The PES-PF-OH membrane breaks into pieces after being treated
with 10 M KOH for 24 h. The decomposition of partially fluorinated
PES-PF-OH membrane was analyzed by the 19F NMR measurements
and the 19F NMR spectra were shown in Fig. 8. In the 19F NMR
spectra of the initial membrane, only a single peak attributed to the
meta fluorine atoms (relative to the sulfone group) on the main
chain was observed at ꢀ15.8 ppm. Four new peaks are clearly
evident in the 19F NMR spectra of the PES-PF-OH after being treated
with 10 M KOH for 24 h at ꢀ25.6, ꢀ16.3, ꢀ16.0 and ꢀ15.9 ppm. It is
concluded that F in the polymer backbone was substituted by
hydroxide ion and the main chain of polymer split off. However, 1H
NMR spectra (Fig. 9) indicated that SN2 substitution reaction and
Hofmann elimination of quaternary ammonium hydroxides did not
occur in PES-PF-OH membrane.
In order to further evaluate the practicability for AEMs, area
resistance and static transport number were determined and the
results were presented in Table 3. Because the thickness of
membrane has great effect on area resistance and static transport
number, the membranes with the similar thickness were used to
test in the experiment. As far as the transport number was con-
cerned, all of them had a value above 0.91. Area resistance of the
membranes ranged from 35.2 to 17.7
U
cm2 and they could also
satisfy the requirement for desalination membranes [22]. From
Table 3, it can be concluded that the order of area resistance is: PES-
B-OH (35.2
U U
cmꢀ2) > PES-P-OH (29.9 cmꢀ2) w PES-E-OH
(28.5
U
cmꢀ2) > PES-PF-OH (17.7
U
cmꢀ2). The sequence is opposite
to that of hydroxide ion conductivity. Compared with the PES-B-OH
membrane, the PES-P-OH exhibited lower area resistance. This was
attributed to more easily form the ion conductive paths through the
membrane resulting from ionic groups on the flexible aliphatic side
chain of the polymer, which led to the ion migrate through the
membrane very easily.
Meanwhile, 1H NMR spectra of PES-B-OH (Fig. 10) showed that
the degradation caused by SN2 substitution reaction is more
significantly than others, demonstrated by the characteristic peak
at 2.1 ppm. This was attributed to that the benzyl group was easily
substituted by hydroxide ions. However, it can be seen from the
data in Table 4 that the PES-B-OH exhibited the lowest mole frac-
tion of degradation (x þ y), which indicated that PES-B-OH was
more stable against hydroxide ions attack compared with other
membranes, owing to the lack of Hofmann elimination of quater-
nary ammonium hydroxides. And PES-PF-OH main chain was the
mostly unstable. It was inferred from the above that their alkaline
stability in 10 M KOH solution follows the order: PES-B-OH > PES-
P-OH > PES-E-OH > PES-PF-OH.
3.6. Alkaline stability of the membranes
A key property in developing AEMs for fuel cells is the chemical
stability of the cationic groups attached to the membrane. Even in
an electrochemical cell without any added electrolyte, the localized
pH within the ion-conducting channels of membranes will be quite
high. High pH can lead to chemical attack on the quaternary
ammonium groups and the polymer chain, most commonly by
either an E2 (Hofmann degradation) mechanism or an SN2 substi-
tution reaction [23]. In addition, the partially fluorinated copolymer
materials were used as AEMs recently, which provided higher ion
conductivity [19,24]. But the alkaline stability of these materials has
not been investigated. The present section reports the influence of
the chemical structures on their alkaline stability.
The alkaline stability of the membranes was evaluated in 10 M
KOH solution at 30 ꢂC. The mole fraction of degradation caused by
SN2 substitution or Hofmann elimination reactions was denoted as
x or y, respectively. The values of x or y were calculated by
comparing the integral of the signals originating specifically from
tertiary amine (2.1 ppm) or alkene groups (4.9 ppm) with the
integral of the quaternary ammonium at 2.9 ppm and the results
were listed in Table 4.
4. Conclusion
A series of novel poly(aryl ether sulfone)s that contain pendent
quaternary ammonium groups on aliphatic side chains have been
prepared as hydroxide ion conductive materials. We demonstrated
that using functionalized monomer it is possible to directly obtain
different quaternized poly(aryl ether sulfone)s for anion exchange
membranes. Compared with traditional approach, this process
could precisely control the amount of quaternary ammonium
groups and their location along the polymer backbone. Additional,
this new route avoided the use of chloromethyl methyl ether-a toxic
chemical, and thus it was more environmentally friendly. The
obtained quaternized ionomers exhibited excellent solubility and
formed flexible and tough membranes by casting from NMP solu-
tion. The introduction of hydrophobic fluorine groups in the poly-
mer main chainresulted in the lowwater uptake, area resistance and
the excellent dimensional stability of the PES-PF-OH membranes. It
showed high hydroxide conductivity up to 3.0 ꢁ 10ꢀ2 S cmꢀ1 in
water at 20 ꢂC. All membranes possessed a phase-separated
morphology characterized by ionic clusters. The alkaline stability of
the membranes was evaluated in 10 M KOH solution at 30 ꢂC,
investigated by 1H and 19F NMR, and viscosity change. The result
shows that PES-B-OH membrane displays better alkaline stability
than others. It is observed that the presence of fluorine atoms in the
aromatic ring leads to a poor stability in KOH solution.
The 1H NMR spectra of PES-E-OH before and after treatment with
10 M KOH solution at 30 ꢂC for 24 h was given in Fig. 6. Three typical
alkene peaks were observed at 4.54e4.57 ppm (d), 4.92e4.97 ppm
(d), 6.61e6.70 ppm (q) after test with KOH solution for 24 h and the
integration ratio of the three peaks is 1:1:1. The alkene resulted from
Hofmann elimination of quaternary ammonium hydroxides, as
shown in Scheme 3. Moreover, the quaternary nitrogen atoms are
transformed to tertiary nitrogen by SN2 substitution reaction. A
proof of this assertion is the presence of the characteristic peak for
the tertiary amines at 2.1 ppm in 1H NMR spectra.
The PES-P-OH membrane exhibited a lower x value than that of
PES-E-OH (Fig. 7 and Table 4). Compared with PES-P-OH, the poor
alkaline stability of PES-E-OH resulted from chemical decomposi-
Acknowledgements
tion by
b
elimination (Scheme 3). The ease of the abstraction of the
-carbon that results
We thank the National Basic Research Program of China (No.
2009CB623401), the National Science Foundation of China
proton depends on the electron density at the
b

5416
Q. Zhang et al. / Polymer 51 (2010) 5407e5416
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(Nos.50825302, 50973106), and the Development of Scientific
Technological Projects of the Jilin Province (No. 20080620) for
financial support.
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