Facile Preparation of Highly Alkaline Stable Poly(arylene-imidazolium) Anion Exchange Membranes through an Ionized Monomer Strategy

Article abstract of DOI:10.1021/acs.macromol.0c02612

The preparation of polyelectrolytes with excellent stability in high alkaline conditions still faces several challenges. In this study, we report a novel imidazolium-ionized monomer PhIM [PF6], modified by the propylbenzene group on the position of N1/N3, which can be copolymerized with 1,1,1-trifluoroacetone under superacid-catalyzed conditions. The main-chain poly(arylene-imidazolium) polymers with a controllable degree of functionalization, high molecular weight, and good solubility are successfully prepared without further postfunctionalization. Two copolymers (block-like and random) can also be prepared by introducing biphenyl as a comonomer and adjusting the feed order. Due to their different microstructures, the two copolymers exhibit different ion conduction behaviors, water absorption capacities, and mechanical properties. The obtained polymer membranes exhibit high hydroxide conductivities and high chemical stability even after treatment in a 10 M NaOH solution at 80 °C. In summary, we highlight a simple, effective, and low-cost strategy to prepare highly alkaline stable poly(arylene-imidazolium) polymers.

Full text of DOI:10.1021/acs.macromol.0c02612

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Article
Facile Preparation of Highly Alkaline Stable Poly(arylene−
imidazolium) Anion Exchange Membranes through an Ionized
Monomer Strategy
Boxin Xue, Weidong Cui, Shengyang Zhou, Qifeng Zhang, Jifu Zheng,* Shenghai Li, and Suobo Zhang*
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ABSTRACT: The preparation of polyelectrolytes with excellent stability in high alkaline conditions still faces several challenges. In
this study, we report a novel imidazolium-ionized monomer PhIM [PF6], modified by the propylbenzene group on the position of
N1/N3, which can be copolymerized with 1,1,1-trifluoroacetone under superacid-catalyzed conditions. The main-chain
poly(arylene−imidazolium) polymers with a controllable degree of functionalization, high molecular weight, and good solubility
are successfully prepared without further postfunctionalization. Two copolymers (block-like and random) can also be prepared by
introducing biphenyl as a comonomer and adjusting the feed order. Due to their different microstructures, the two copolymers
exhibit different ion conduction behaviors, water absorption capacities, and mechanical properties. The obtained polymer
membranes exhibit high hydroxide conductivities and high chemical stability even after treatment in a 10 M NaOH solution at 80
°C. In summary, we highlight a simple, effective, and low-cost strategy to prepare highly alkaline stable poly(arylene−imidazolium)
polymers.
of the ring-opening degradation, β-H elimination, or nucleo-
philic substitution of imidazoliums is sufficiently large. As such,
they exhibit higher alkaline stabilities than most reported
quaternary ammonium cations.18−20 Additionally, imidazoliums
have multiple substitution sites that are easily modified to obtain
various structures, making them the ideal candidates to improve
the relationship between the structure of an ionic group and its
alkaline stability. For example, Yan et al.14−16 and Coates et
al.17,21 designed and prepared various imidazoliums with
different substitutions at the N1/C2/N3/C4/C5 position and
demonstrated that substituting patterns at different positions
have significant effects on their overall alkaline stability.
Holdcroft et al.18,22 developed more imidazoliums with excellent
alkaline stability and highlighted the influence of large sterically
INTRODUCTION
■
As a key component of anion exchange membrane fuel cells
(AEMFCs), anion exchange membranes (AEMs) play a vital
role in conducting OH− and isolating the fuel and oxidant
between the anode and the cathode.1−3 Currently, one of the
main factors restricting the application of AEMs is their poor
alkaline stabilities.4−7 Especially in the specific case of fuel cell
applications, the alkaline concentration in the system where the
membrane material is located is very high. Besides, high
temperatures accelerate the degradation of the ionic groups and
the polymer skeleton in the membrane, resulting in the loss of
ion-exchange capacity (IEC) and mechanical properties, which
is not conducive to the long-term stable operation of fuel cells.
Although investigations into suitable AEMs for low-concen-
tration alkaline conditions have made gratifying progress,6−13 to
date, the preparation of stable AEMs in high alkaline
concentrations remains elusive.
Received: November 22, 2020
Revised: January 29, 2021
Published: February 15, 2021
Imidazoliums are very important organic cations that have
increasingly been studied in recent years, especially their
potential role in the preparation of highly alkaline stable
AEMs (see Figure 1a).14−17 Owing to factors such as the
delocalized charge effect and steric hindrance, the energy barrier
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Figure 1. (a) Structure of alkaline stable imidazolium cation, (b) general structure of the superacid-catalyzed polymer precursor, and (c) schematic
diagram of preparation of the poly(arylene−imidazolium) polymer in this work (the black chain represents the polymer backbone; the counterions in
conceived polymers are omitted for clarity).
hindered substituents on the stability of imidazoliums through
theoretical calculations. Based on their report, several
polyarylimidazolium polymers with large steric hindrance that
exhibited high chemical stability at high temperatures and under
strongly alkaline conditions (80 °C, 10 M alkaline solution)
were prepared. Conventionally, to obtain a poly(arylene−
imidazolium) skeleton structure consistent with the designed
imidazolium cation, it is necessary to modify the structure at the
monomer preparation stage, which is mostly focused at the C2,
C4, and C5 positions. Moreover, to prepare the corresponding
imidazolium cations or imidazolium-containing polymers, it is
usually necessary to use precious metal and other oxygen/water-
sensitive catalysts (e.g., Pd and Ni), which increases the
complexity and cost of preparing membrane materials. Besides,
postfunctionalization on the obtained high-molecular-weight
polyarylimidazole precursor polymers usually requires extremely
harsh conditions and it is difficult to ensure that the
postfunctionalization reaction proceeds completely.18 As such,
it is crucial to develop a simple and effective synthetic strategy to
connect the alkaline stable imidazolium cations to the polymer
backbone.
Superacid-catalyzed polymerization reaction has the advan-
tages of simple synthesis, high reaction efficiency, and ease of
obtaining high-molecular-weight polymers, and has become an
important method for the preparation of non-ether polymers. It
has been widely used in alkaline, acidic, flow batteries, sodium
borohydride, and other forms of fuel cell membranes.23−30
Generally, the copolymerization of commercial ketones and
aromatic monomers first yields polymer precursors (see Figure
1b), which are then converted into ionic group-functionalized
AEMs through nucleophilic substitution.1,7 However, there are
still several challenges of incomplete postfunctionalization for
the reported methods, especially when grafting large sterically
hindered imidazole onto the polymer precursors.30
propylbenzene group was designed and prepared. Subsequently,
a main-chain poly(arylene−imidazolium) poly-PhIM [OH]
with precise functionalization positions and 100% functionaliza-
tion degree is successfully prepared by the copolymerization of
the ionized monomer and 1,1,1-trifluoroacetone under super-
acid-catalyzed conditions (see Figure 1c). Moreover, block-like-
type poly-PhIM-b-biPh [OH] and random-type poly-PhIM-co-
biPh [OH] poly(arylene−imidazolium) are also obtained by
introducing biphenyl as a comonomer and adjusting the feeding
sequence. Solvent-cast AEMs were characterized, with the focus
on solubility, thermal stability, mechanical properties, water
uptake, swelling ratio, ion-conducting behavior, micromorphol-
ogy, and alkaline stability.
EXPERIMENTAL SECTION
■
Materials. All reagents and solvents were purchased from Innochem
Co., Aldrich, and TCI Chemical Co., and were used without further
purification.
Synthesis of Monomer PhIM [PF6]. 1,2-Diphenylethanedione
(29.64 g; 0.2 mmol) and 2,4,6-trimethylbenzaldehyde (42.04 g; 0.2
mmol) were combined with ammonium acetate (38.50 g; 0.5 mmol) in
CH3COOH (300 mL) and stirred at 60 °C for 12 h. After cooling to 25
°C, the solution was poured into deionized water, and the residue was
dissolved in chloroform, washed with H2O, dried with magnesium
sulfate, filtered, and concentrated under reduced pressure. The crude
imidazole precursor was further purified via recrystallization. Then, 33.8
g (0.1 mol) of imidazole precursor was dissolved in N,N-
dimethylacetamide, excess potassium carbonate was used as a base,
and 59.7 g (0.3 mol) of 1-bromo-3-phenylpropane was added for
quaternization. The reaction was carried out at 160 °C for 12 h. After
cooling to room temperature, the solvent was distilled under reduced
pressure, and the remaining residue was washed with water and filtered
to obtain a crude imidazolium salt. Then, the crude imidazolium salt
was dissolved in methanol, potassium hexafluorophosphate was added
for ion exchange, and finally, the target product PhIM [PF6] was
obtained and used without further purification. Yield: 70.5%. 1H NMR
(500 MHz, DMSO-d6; ppm) δ 7.50−7.52 (m, 4H), 7.39−7.47 (m,
6H), 7.11 (s, 2H), 7.06−7.09 (m, 6H), 6.72−6.74 (m, 4H), 3.75 (t,
4H), 2.36 (s, 3H), 2.21 (t, 4H), 2.12 (s, 6H), 1.51 (m, 4H); 13C NMR
(125 MHz, DMSO-d6; ppm) δ 143.38, 143.06, 139.99, 139.04, 132.26,
Herein, to introduce the highly alkaline stable imidazolium
cation into the polymer backbone while avoiding problems such
as postfunctionalization, a novel imidazolium-ionized monomer
PhIM [PF6] with its N1/N3 position modified by the
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Figure 2. Synthesis route of monomer PhIM [PF6] and 1H NMR spectra of PhIM [Cl] (0.02 M) in 3 M NaOD/CD3OD/D2O (7:3 wt CD3OD:D2O)
after heating at 80 °C as a function of time.
131.34, 130.69, 129.89, 129.39, 128.72, 128.19, 126.38, 125.87, 118.18,
45.93, 31.73, 30.12, 21.47, 19.87; 31P NMR (202.5 MHz; DMSO-d6;
ppm): δ −144.27; a sevenfold peak was present, with 85% phosphoric
acid as the external standard. 19F NMR (470.93 MHz; DMSO-d6;
ppm): δ −83.43 ppm, d, J = 768 Hz; ESI-MS: 575.0 M+.
Homopolymer number-average molecular weight (Mn), weight-average
molecular weight (Mw), and molecular weight distributions (PDI =
Mw/Mn) were measured by size exclusion chromatography (SEC). The
SEC analyses were performed on a Waters 1515 instrument equipped
with a PLMIXED 7.5 mm × 50 mm guard column and two PLMIXED-
C 7.5 × 300 columns and a differential refractive index detector using
N,N-dimethylformamide as the eluent (with 0.02 M of LiBr to shield
ionic interactions) at 35 °C at a flow rate of 1 mL min−1. The molecular
weight information of the block-like copolymer was obtained by high-
temperature SEC (eluent: 1,2,4-trichlorobenzene (TCB) stabilized
with 0.0125% butylated hydroxytoluene (BHT); temperature: 150 °C;
flow rate: 1 mL min−1). Mn, Mw, and PDI of the random copolymer
cannot be obtained by the above two SEC test systems, which may be
due to its poor solubility in the eluents or strong interaction with the
columns. Differential scanning calorimetry (DSC) thermograms of the
homopolymers are obtained on a PerkinElmer DSC-7 under a N2
atmosphere, and the temperature range is 20−180 °C (scan rate: 5 °C
min−1).
Synthesis of Homopolymers and Ion Exchange. Imidazolium
monomer PhIM [PF6] (7.21 g; 0.1 mmol) and 1,1,1-trifluoroacetone
(1.35 g; 0.12 mmol) were added into a dry pressure bottle at 0 °C, 23
mL of trifluoromethanesulfonic acid was used as the catalyst, and 23 mL
of dichloromethane (DCM) was used as the solvent. Then, the reaction
system was moved to room temperature and stirred vigorously. The
solid content was controlled at 20%. After 24 h of reaction, the reaction
system was poured into methanol. Thereafter, the obtained gumlike
solid was dissolved in dimethyl sulfoxide (DMSO) and the solution was
poured into deionized water. A large amount of a white flocculent resin
fibrous solid was precipitated; then, it was washed with water and
vacuum dried to obtain the poly(arylene−imidazolium) polyelectrolyte
−
poly-PhIM [CF3SO3], whose counterion was CF3SO3 . Poly-PhIM
The ion-exchange capacities (IECs) of poly-PhIM [OH], poly-
PhIM-b-biPh [OH], and poly-PhIM-co-biPh [OH] were determined by
a typical Mohr titration method. First, the quality of the membrane was
tested after vacuum drying. The sample was then immersed in a
standard hydrochloric acid solution for 48 h. Then, the acid solution
was titrated with a standard NaOH solution using phenolphthalein as
an indicator. A pink-colored change indicates the completion of
titration. The mechanical property of membranes was measured by an
Intrson-1211 under ambient conditions at a stretching speed of 5 mm
min−1. Small-angle X-ray scattering (SAXS) was conducted by a Xeuss
system (Xenocs, Sassenage, France) with a Pilatus detector. The atomic
force microscopy (AFM) phase diagrams were observed on a Bruker
AFM instrument using a silicon-based n-type cantilever in the tapping
mode after the 1 wt % polymer solution is dripped on the mica flakes
and dried. Ultra-high-resolution transmission electron microscopy
(TEM) analysis was performed on a JEOLJEM-2010FEF at an
accelerating voltage of 200 kV, which was a bright-field measurement.
The membranes on the copper grid were stained with a 1 wt % H2PtCl6
aqueous solution for 30 seconds and later rinsed with deionized water.
Before the TEM measurements, the copper grids were taken out from
deionized water and put in a vacuum oven at 60 °C for 30 min to
remove the water.31 The thermostability of the polymers was measured
by thermogravimetric analysis (TGA) (instrument: PerkinElmer TGA-
2; condition: N2 flow; heating rate: 5 °C min−1).
[CF3SO3] was then treated in concentrated hydrochloric acid and
heated at 50 °C to exchange CF3SO3− to Cl− and the obtained polymer
was poly-PhIM [Cl]. Then, poly-PhIM [OH] could be obtained by
soaking poly-PhIM [Cl] in 1 M alkaline solution for 48 h. Yield: 86.3%.
Synthesis of Copolymers Poly-PhIM-b-biPh [OH] and Poly-
PhIM-co-biPh [OH]. Poly-PhIM-b-biPh [OH]. Imidazolium salt
monomer PhIM [PF6] (7.21 g; 0.1 mmol) and 1,1,1-trifluoroacetone
(1.344 g; 0.12 mmol) were added into a dry pressure bottle at 0 °C, 25
mL of trifluoromethanesulfonic acid was used as the catalyst, and 25 mL
of DCM was used as the solvent. Then, the reaction system was moved
to room temperature and stirred vigorously. After 12 h, another 0.849 g
(0.05 mmol) of biphenyl and 0.672 g (0.06 mmol) of 1,1,1-
trifluoroacetone were injected into the system, and polymerization
was continued for another 12 h. The process of polymer post-treatment
and ion exchange is exactly the same as that for poly-PhIM [OH]. Yield:
80.5%.
Poly-PhIM-co-biPh [OH]. Imidazolium salt monomer PhIM [PF6]
(7.21 g; 0.1 mmol), biphenyl (0.849 g; 0.05 mmol), and 1,1,1-
trifluoroacetone (2.016 g; 0.18 mmol) were added into a dry pressure
bottle at 0 °C, 25 mL of trifluoromethanesulfonic acid was used as the
catalyst, and 25 mL of DCM was used as the solvent. Then, the reaction
system was moved to room temperature and stirred vigorously for 24 h.
The process of polymer post-treatment and ion exchange is exactly the
same as that of poly-PhIM [OH]. Yield: 90.0%.
1
Characterization. H NMR, 13C NMR, 19F NMR, and 31P NMR
Water uptake (WU) was calculated by formula 1
experiments were conducted on an AV 500 spectrometer. Mass spectra
were obtained using electrospray mass spectrometry (ESMS).
WU (%) = [(mw − md)/md] × 100%
(1)
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Figure 3. Preparation of poly(arylene−imidazolium) polymers.
where md is the quantity of a dried membrane and mw refers to the
quantity of the water-swollen membrane.The swelling ratio (SR) of the
membranes was calculated by formula 2
imidazolium-ionized aromatic monomer was obtained through
the nucleophilic substitution reaction between 1-bromo-3-
phenylpropane and the imidazole precursor. The initial crude
product was purified through ion exchange and PhIM [PF6] (the
content in parentheses represents the counterion for all
monomers) was successfully prepared in good yield. The
chemical structure of PhIM [PF6] was confirmed by 1H NMR
(Figure S1), 13C NMR (Figure S2), 31P NMR (Figure S3a), 19F
NMR (Figure S4a), and mass spectra (Figure S5). Then, as a
model compound, the alkaline stability of the ionized monomer
was further evaluated in 3 M NaOD/CD3OD/D2O (7:3 wt
CD3OD/D2O) at 80 °C. Before the measurements were taken,
the counterion PF6− of PhIM [PF6] was exchanged with Cl− via
treatment in concentrated hydrochloric acid at 50 °C for 6 h
SR (%) = [(Lw − Ld)/Ld] × 100%
(2)
where Ld is the length of a dried membrane and Lw is the length of the
water-swollen membrane.The hydroxide conductivity (σ) was
measured by four-point probe AC impedance spectroscopy on an
electrochemical workstation (BioLogic VSP) with the frequency
ranging from 100 MHz to 100 kHz. Before the test, the membrane
samples were cut into dimensions of 10 mm × 40 mm and immersed
into a fresh alkaline solution for 24 h to ensure that its counterion is in
hydroxide form. Ultrapure water is needed to bubble with N2 to
eliminate CO2. Thereafter, in a glovebox protected by N2, the
membrane sample was fully washed with the prepared ultrapure
water, assembled in the conductivity clamp, and placed in a flask
containing enough ultrapure water (set in a thermo-controlled
chamber). The test temperature range was 30−80 °C. At each test
temperature, the test system should equilibrate for at least 0.5 h. The
−
because PF6 is a hydrophobic ion that could lead to poor
solubility of the imidazolium cation in CD3OD/D2O solution.
−
As shown in Figure S3b, the P signal from the PF6 of PhIM
[PF6] disappeared completely, demonstrating the complete
exchange of PF6− with Cl− and successful preparation of PhIM
[Cl]. The 1H NMR spectra of PhIM [Cl] as a function of time
are collected in Figure 2; after 100 d of the treatment in 3 M
NaOD/CD3OD/D2O (7:3 wt CD3OD/D2O) at 80 °C, it can be
seen that in addition to the normal deuterium signal exchange
(Ha and Hb),22 no new degradation signals appeared, indicating
that the imidazolium cation PhIM [Cl] maintains good stability
in the strongly alkaline solution.
Polymer Synthesis and Characterization. Given the
excellent alkaline stability of the ionized aromatic monomer, the
next step was to synthesize a relevant polymer with this skeleton.
PhIM [PF6] was chosen as the monomer to copolymerize with
1,1,1-trifluoroacetone in trifluoromethanesulfonic acid
(TFSA)/dichloromethane (DCM) at room temperature with
vigorous stirring (the synthesis route is shown in Figure 3). After
24 h, the reaction solution was poured into methanol and a large
gumlike solid was obtained. Thereafter, the solid was dissolved
in DMSO and the solution was poured into deionized water,
forming a fibrous solid that was collected as shown in Figure 3.
The molecular weight information of the polymer was obtained
through the SEC test: Mw = 104 kg mol−1, Mn = 57 kg mol−1, and
PDI = 1.80 (Figure S7). The degree of polymerization of the
polymer calculated from the number-average molecular weight
is about 70, indicating that each polymer chain consists of about
70 repeating units. The obtained polymer can be cast from
dimethyl sulfoxide (DMSO) into a tough and transparent
test was repeated until the resistance value was constant. The OH
conductivity was calculated according to formula 3
̅
(3)
σ = d/twR
where d is the distance between the reference electrodes, R is the
ohmic resistance value, and t and w represent the thickness and width of
the membrane sample, respectively.
To compare the alkaline stability of the new imidazolium monomer
more scientifically and reasonably, the alkaline stability test condition
was set at 3 M NaOD/CD3OD/D2O (7:3 wt CD3OD/D2O) according
to the reported literature.18,22 The imidazolium monomer solution was
injected into an NMR tube and then sealed by a gas lamp as shown in
Figure S6. Then, the NMR tube was placed in an oven at 80 °C for heat
treatment. At regular periods of time, the tube was taken out of the oven
and the 1H NMR test was performed to study the structural changes of
the imidazolium. To assess the alkaline stability of the membranes, the
membrane samples were immersed in 10 mol L−1 NaOH aq. at 80 °C;
at regular periods of time, a portion of the membrane was taken out for
1H NMR and conductivity tests.
RESULTS AND DISCUSSION
■
Preparation and Alkaline Stability of Imidazolium-
Ionized Monomer PhIM [PF6]. First, we designed a new type
of imidazolium-ionized monomer PhIM [PF6] by modifying
terminal aromatic substituents (propylbenzene structure) at the
N1 and N3 positions. As shown in Figure 2, the imidazole
precursor was first prepared by the conventional bis-diketone/
dialdehyde/ammonium reaction,17,18 and then the target
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Figure 4. 1H NMR spectra of (a) PhIM [PF6], (b) poly-PhIM [Cl], (c) poly-PhIM-b-biPh [Cl], and (d) poly-PhIM-co-biPh [Cl].
a
Scheme 1. Preparation of Block-like Polymer Poly-PhIM-b-biPh [OH] and Random Copolymer Poly-PhIM-co-biPh [OH]
a
The ion-exchange process is exactly the same as homopolymer Poly-PhIM [OH].
membrane. Notably, during the aforementioned polymerization
reaction, no cross-linking side reaction occurred, demonstrating
that only the phenyl groups in the long alkyl chain at the N1 and
N3 positions participated in the growth process of the polymer
chain. To confirm that the phenyl groups at the C2, C4, and C5
positions were not involved in the chain-growing process, a
model reaction was carried out as shown in Scheme S1: a
reported imidazolium cation 1,3-di-methyl-2-mesityl-4, 5-
diphenylimidazolium iodide with phenyl groups substituted at
the C2, C4, and C5 positions was used as the monomer to
copolymerize with 1,1,1-trifluoroacetone under the same
conditions. Throughout more than three days of stirring, the
viscosity of the reaction system did not increase and the
precipitated solid was in the form of a powder. As expected, the
polymerization reaction did not occur at all, which means that
the phenyl groups substituted at the C2, C4, and C5 positions
have no nucleophilic activity; therefore, such aromatic
monomers will not be involved in the polymer chain growth
with superelectrophilic activated fluorocarbonyl ketones under
the conditions catalyzed by TFSA. This might be due to the
aromatic substituents on C2, C4, and C5 positions that were
conjugated with the imidazolium cations, resulting in an electron
deficiency of the benzene ring and reduced nucleophilic activity.
These results indicate that the polymerization occurred through
a highly regio-controlled monoprotonation mechanism.32
Since the polymerization reaction is carried out in TFSA,
−
−
CF3SO3 will exchange with PF6 , so that the first obtained
−
membrane is in the form of CF3SO3 . No evident P signal was
observed in the 31P NMR (Figure S3c) of the membrane. For 19F
NMR, most of the F signals from PF6− (at −83.43 ppm) in PhIM
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Table 1. Properties of the Poly(arylene−imidazolium) Membranes
IEC (mmol g−1
)
a
b
c
c
NMR
calculated
η
conductivity
tensile strength
(MPa)
Young’s modulus
elongation at
c
membrane
theoretical titrimetric
(dL g−1
)
(mS cm−1
)
(MPa)
break (%)
d
poly-PhIM [OH]
poly-PhIM-b-biPh
[OH]
1.46
1.23
1.42
1.21
1.47
1.21
1.10
1.28
13.2 (−)
1.43
17.53
92.10
1047.40
1.76
68.87
7.8 (56.8)
poly-PhIM-co-biPh
[OH]
1.23
1.19
1.17
1.47
7.2 (69.4)
22.13
830.77
6.82
a
b
Intrinsic viscosity was tested at a concentration of 0.5 g dL−1 in NMP at 30 °C. Tested at 30 °C, RH = 100%; the data in parentheses were
c
d
obtained at 80 °C, RH = 100%. Mechanical properties are measured at 30 °C and under ambient humidity conditions. The test cannot be
performed due to the excessive swelling of the membrane.
[PF6] disappeared, and another two F signals at −81.45 and
−91.05 ppm could be attributed to the −CF3 from 1,1,1-
[OH] were measured by high-temperature SEC. The SEC trace
of poly-PhIM-b-biPh [OH] is shown in Figure S10 (Mw
=
−
trifluoroacetone and CF3SO3 , respectively (Figure S4b). The
74 780, Mn = 38 633, and PDI = 1.94), indicating that it is a
relatively uniform polymer. The molecular weight of the random
copolymer cannot be obtained because the polymer cannot flow
out from the chromatographic column. This may be due to the
poor solubility of the random polymer in TCB or strong
interaction with the chromatographic column. The inherent
viscosity was measured by an Ubbelohde capillary viscometer at
30 0.1 °C with a polymer concentration of 0.5 g dL−1 in N-
methyl pyrrolidone (NMP). The viscosity of the imidazolium
homopolymer poly-PhIM [OH] is a little lower than the block-
like and random copolymers, though it has a higher Mw value
than the block-like copolymer (Table 1). We suppose this may
be because the higher cation concentration increases the
repulsive force of the polymer chain and thus causes the
decrease of the viscosity value. The chemical structures of poly-
PhIM-b-biPh [OH] and poly-PhIM-co-biPh [OH] were
direct exchange of CF3SO3 with OH− was extremely difficult
−
despite immersing it in an alkaline solution for a long time, so
−
CF3SO3 was first exchanged with Cl− and then Cl− was
exchanged with OH−. The obtained membranes were named
poly-PhIM [CF3SO3], poly-PhIM [Cl], and poly-PhIM [OH],
respectively. In 19F NMR spectra of poly-PhIM [Cl] (Figure
−
S4c), the F signal from CF3SO3 disappeared completely,
−
indicating that CF3SO3 had been completely exchanged with
Cl−. The chemical structure of the poly-PhIM [Cl] was further
analyzed by 1H NMR spectra (Figure 4b), in which all
characteristic signals of the ionized monomer PhIM [PF6] in
the high-field region were completely retained and a new H
signal from −CH3 in 1,1,1-trifluoroacetone appeared at 1.76
ppm. Finally, poly-PhIM [OH] was easily obtained by
immersing poly-PhIM [Cl] in a 1 M alkaline solution for 48 h.
The differential scanning calorimetry (DSC) thermograms of
poly-PhIM [CF3SO3], poly-PhIM [Cl], and poly-PhIM [OH]
are summarized in Figure S8. Poly-PhIM [CF3SO3] and poly-
PhIM [OH] underwent a glass transition process at a glass
transition temperature (Tg) of 137.7 and 125.6 °C, respectively.
In contrast, poly-PhIM [Cl] did not undergo glass transition
throughout the tested process, which might be due to the change
of the polymer chain mobility induced by the incorporation of
different counterions.
1
analyzed by H NMR spectra as shown in Figure 4c,d. The
chemical shift of most of the hydrogen signals was similar for
both the resulting block-like and random copolymers. The most
remarkable difference between them was the distribution of the
H signal at 1.98 ppm (f), 1.89 ppm (f″), and1.79 ppm (f′) of the
−CH3 from 1,1,1-trifluoroacetone. The H signal at 1.79 ppm
originated from the chemical shift of the −CH3 in the pure
imidazolium region, which was consistent with that of poly-
PhIM [OH] in Figure 4b. In contrast, the H signal at 1.98 ppm
originated from the chemical shift of the −CH3 in the pure
biphenyl region.34 As for the block-like copolymer, its biphenyl
segments were longer than the random copolymer; hence, its H
signal at 1.98 ppm is more evident than that of the random
copolymer. Of course, we do not rule out the case that some
biphenyl molecules are directly connected with two imidazolium
monomers or imidazolium segments through CF3−C−CH3
moieties (as shown in Figure S11b) because there is a weak
characteristic peak at 1.89 ppm in Figure 4c, which is assigned to
the chemical signal of methyl as shown in Figure S11b. The
characteristic peak value is not very high, indicating that such
connections are relatively rare; hence, it is a block-like
copolymer. In the random copolymer, the chemical shift of its
H signal that appeared at 1.89 ppm in Figure 4d is very evident,
indicating that there are many connections as shown in Figure
S11b, further confirming that it is a random copolymer. In
addition, the mole fraction of the repeating unit in the
copolymers can be quantified by calculating the integral ratio
of the signals of Hc to Hf, Hf′, and H″ as 1.00:1.17 and 1.00:1.24
by 1H NMR (Figure S12). According to this ratio, the
corresponding IEC values can be calculated to be 1.21 and
1.17, respectively (Table 1). These results are very close to the
theoretical IEC calculated by the experimental titration and the
opolymers containing hydrophobic units usually have
C
lower water absorption and swelling rates.33 This quality is
beneficial for adjusting the solubility of the homopolymer.
According to the kinetic studies, we found that the polymer-
ization reaction activity of biphenyl is very high and its polymer
molecular weight growth rate is significantly faster than that of
the imidazolium monomer PhIM [PF6] (see Figure S9).
Therefore, utilizing different reaction activities between the
aromatic ionized monomer and the biphenyl in polymerization,
a block-like copolymer poly-PhIM-b-biPh [OH] was prepared
by feeding extra biphenyl and 1,1,1-trifluoroacetone into the
reaction system sequentially (Scheme 1). Different from the
traditional method of preparing block copolymers (blending
system of two oligomers), this method of gradually adding
monomers can avoid material transfer loss and is more beneficial
to control the concentration of reactants. For comparison, a
random copolymer poly-PhIM-co-biPh [OH] was also synthe-
sized by adding three raw materials at one time. The amount of
incorporated biphenyl into the block-like and random
copolymer was held constant at 33.3 mol %, as such similar
IEC values were obtained for the resulting block-like and
random copolymer (Table 1). The molecular weight and
distribution of poly-PhIM-b-biPh [OH] and poly-PhIM-co-biPh
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Figure 5. (a) Water uptake and swelling ratio of prepared membranes, (b) photographs of AEMs in dry and wet states, and (c) conductivities under
fully hydrated (immersed) conditions.
Figure 6. (a−c) AFM tapping phase images (1.0 μm × 1.0 μm) of poly-PhIM [OH], poly-PhIM-b-biPh [OH], and poly-PhIM-co-biPh [OH]. (d−f)
AFM tapping phase images (500 nm × 500 nm) of poly-PhIM [OH], poly-PhIM-b-biPh [OH], and poly-PhIM-co-biPh [OH]. (g−i) TEM images
(the scale bar is 20 nm) of poly-PhIM [OH], poly-PhIM-b-biPh [OH], and poly-PhIM-co-biPh [OH].
feeding proportion, indicating that the two copolymers have a
similar molar composition of repeating units. This result further
proves that our feeding method can effectively avoid the loss
caused by material transfer and is beneficial to obtain two
copolymers with very similar IECs.
Solubility and Mechanical Properties. The solubility of
poly-PhIM [OH], poly-PhIM-b-biPh [OH], and poly-PhIM-co-
biPh [OH] in different solvents is summarized in Table S1.
Besides high-boiling solvents such as DMF, DMSO, and NMP,
poly-PhIM [OH] showed good solubility in hot water (80 °C),
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methanol, ethanol, and DCM. The good solubility can be
assigned to two reasons: (1) the main-chain-type structure of the
poly(arylene−imidazolium) and (2) the large steric hindrance
of the imidazolium cations. These two factors make the ionic
groups on the polymer chain have a strong mutual repulsion
effect, which reduces the entanglement of the polymer chain and
increases the solubility. However, poly-PhIM [OH] was
insoluble in alkaline solution even at high temperatures. This
behavior was also exhibited by the polyarylimidazole poly-
electrolyte reported by Fan et al.22 The introduction of
hydrophobic biphenyl segments caused the solubility of poly-
PhIM-b-biPh [OH] and poly-PhIM-co-biPh [OH] to be
somewhat different from that of poly-PhIM [OH]. In pure
water, these two copolymers cannot be dissolved even at 100 °C.
Additionally, their solubility in alcohol was reduced to a certain
extent.
The role of the biphenyl segment was also reflected by the
improvement in mechanical properties (Table 1). The tensile
strength of homopolymer poly-PhIM [OH] was only 1.43 MPa.
Differently, the tensile strength of two copolymers increased 10-
fold owing to the introduction of the biphenyl segment. This can
be assigned to the π−π interaction of the aromatic ring in the
biphenyl segment, which effectively enhanced the chain
entanglement of the polymer. Moreover, the block-like polymer
has a higher elongation at break and Young’s modulus, while the
random copolymer has higher tensile strength. This provided a
new idea for adjusting the mechanical properties of ionomers by
controlling the order of feeding into the reaction system.
Water Uptake, Swelling Ratio, Conductivity, and
Micromorphology. The water uptake and swelling ratio of
poly-PhIM [OH], poly-PhIM-b-biPh [OH], and poly-PhIM-co-
biPh [OH] are summarized in Figure 5a. Compared with poly-
PhIM [OH], the two copolymers showed a much lower water
uptake and swelling ratio. This is because the introduction of
hydrophobic biphenyl fragments reduces the IEC of the
copolymer membranes, making the membranes insoluble in
water and having lower water uptake. In addition, the block-like
copolymer poly-PhIM-b-biPh [OH] simultaneously showed a
higher water uptake and swelling ratio than the random polymer
poly-PhIM-co-biPh [OH]. A comparison of the photographs of
the three samples under dry and wet conditions is shown in
Figure 5b; at 80 °C, in a hydrated state, poly-PhIM [OH] is
basically dissolved, the block-like copolymer swells very
obviously and becomes opaque, and the random copolymer
does not change significantly. This can be attributed to
hydrophilic and hydrophobic segments being randomly
distributed in the random copolymers, thus interrupting the
continuous aggregation of the hydrophilic regions, effectively
limiting the water absorption capacity.
absorption diluting the OH− concentration. The lower water
uptake of the random copolymer at high temperatures endowed
it with a higher OH− concentration, effectively improving its
conductivity. In short, in addition to the chemical structure of
the monomer, this study showed that the length and sequence of
the control block-like polymers are directly related to the ion
transport opening new avenues for future studies on the
structure−activity relationship of AEMs.
The micromorphology of poly-PhIM [OH], poly-PhIM-b-
biPh [OH], and poly-PhIM-co-biPh [OH] was first investigated
in their dry state via small-angle X-ray spectroscopy (SAXS). As
shown in Figure S13, no obvious ionomer peak was detected in
the range q = 0.1−1.9 nm−1, which may be due to the fact that
the main-chain-type ionomer structure makes the difference in
electron cloud density between hydrophilic and hydrophobic
regions insignificant. Then, to further study their micro-
morphology, AFM and TEM measurements were carried out.
In Figure 6a−f, evident light and dark regions were observed in
the AFM phase diagrams of the three polymers at the scales of
1.0 μm × 1.0 μm and 500 nm × 500 nm, suggesting the
difference between the hydrophilicity and hydrophobicity
(softness and hardness) on the surface of the polymers, which
may be a manifestation of phase separation. The TEM images in
Figure 6g−i show the microstructures of poly-PhIM [OH],
poly-PhIM-b-biPh [OH], and poly-PhIM-co-biPh [OH] at the
nanoscale, respectively. The test membrane samples were
stained with H2PtCl6 in advance to achieve an exchange of
OH− to PtCl62−. The dark regions reflect the ionic hydrophilic
domains and the light regions show the hydrophobic domains
(alkyl chain and biphenyl segments), which suggest the
formation of well-defined phase-separated structures in the
membranes. Although they are main-chain-type polymers, the
hydrophilicity and hydrophobicity of the long alkyl substitutes at
the position N1/N3 of the imidazolium ring are quite different
from that of the ion center, so the phase separation structure
described above exists. Moreover, the size and shape of the ion
clusters of the three polymers are different from each other,
which may be due to the difference in the content of the
imidazolium cation and the different distributions of the
imidazolium segments, so that different water absorption
properties and conductivities are finally obtained.
Thermal Stability and Alkaline Stability. The thermal
stability of poly-PhIM [OH], poly-PhIM-b-biPh [OH], and
poly-PhIM-co-biPh [OH] was studied by thermogravimetric
analysis (TGA) and differential thermogravimetry (DTG)
(Figure S14). According to the TGA curve, below 200 °C, a
small amount of weight loss is associated with the evaporation of
residual solvent and water in the membranes. Due to the
excellent thermal stability of imidazolium cations, all three
samples began to decompose at very high temperatures (>350
°C). Based on DTG data, the temperature point of
weightlessness can be judged more accurately. The decom-
position temperatures of poly-PhIM [OH], poly-PhIM-b-biPh
[OH], and poly-PhIM-co-biPh [OH] are 434.9, 429.9, and 435.4
°C, respectively. Here, poly-PhIM [OH] and poly-PhIM-co-
biPh [OH] undergo similar thermal degradation processes.
Differently, the thermal decomposition of poly-PhIM-b-biPh
[OH] includes two stages. In addition to the first stage, there is
another decomposition process at 535.6 °C, which can be
attributed to the decomposition of the biphenyl segments within
the block-like copolymer. These findings indicate that the
difference in the structure of the chain segments will be reflected
in the thermal decomposition process of the polymers.
Ionic conductivity is one of the most important factors in
determining fuel cell performance.35,36 The hydroxide con-
ductivity of the three polymers is summarized in Figure 5c.
Although their IEC values are not high, these poly (arylene−
imidazolium)-based AEMs still have excellent conductivity of up
to 60−70 mS cm−1 at 80 °C under fully hydrated conditions
(Table 1). Additionally, the block-like and random copolymer
showed evidently different OH− conduction behavior. Between
30 and 60 °C, as for the block-like polymer, its conductivity
increased rapidly with the increase in temperature. In contrast,
the conductivity increase rate for the random copolymer was
significantly lower than that of the block-like copolymer. Above
60 °C, the conductivity increase rate for the block-like polymer
slowed down, which might be attributed to the excessive water
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Figure 7. 1H NMR spectra of (a) poly-PhIM [OH], (b) poly-PhIM-b-biPh [OH], and (c) poly-PhIM-co-biPh [OH] in 10 M NaOH at 80 °C for
different times. (d) Conductivity at 60 °C under fully hydrated conditions of poly-PhIM-b-biPh [OH] and poly-PhIM-co-biPh[OH] after alkaline
stability treatment.
Table 2. Some Reported AEMs with Excellent Alkaline Stability in Recent Years
alkaline stability test
conditions
alkaline stability
(h)
membrane
ionic group/skeleton structure
preparation method
ref
poly-PhIM [OH]
poly-PhIM-b-biPh
[OH]
imidazolium/main-chain type
imidazolium/main-chain type
Superacid-Catalyzed Polymerization
Superacid-Catalyzed Polymerization
10 M NaOH at 80 °C
10 M NaOH at 80 °C
>2400
>720
this work
this work
poly-PhIM-co-biPh
imidazolium/main-chain type
imidazolium/main-chain type
imidazolium/main-chain type
imidazolium/main-chain type
Superacid-Catalyzed Polymerization
10 M NaOH at 80 °C
1 M KOH at 80 °C
10M KOH at 80°C
>720
>720
>240
this work
[OH]
AAEM-2
Ring-opening metathesis
copolymerization
Yamamoto-coupling
homopolymerization
21
18
PAImBB(14)
HMT-PMPI
TPN
Microwave polycondensation
10 M KOH at 100 °C
1 M NaOH at 90 °C
>168
>720
22
10
quaternary ammonium/side-chain Superacid-Catalyzed Polymerization
type
PAP HEMs
quaternary ammonium/side-chain Superacid-Catalyzed Polymerization
type
1 M KOH at 100 °C
>2000
11
Good alkaline stability is beneficial for the long-term stable
operation of AEMFCs.1,5 According to the result of this study,
the ionized monomer PhIM [Cl] exhibited excellent alkaline
resistance, which enabled us to investigate the imidazolium-
based AEMs containing the same building blocks with long-term
alkaline stability. The alkaline stability of poly-PhIM [OH] to
caustic solutions was investigated by immersing it in aqueous
hydroxide solutions at elevated temperatures for extended
periods. When poly-PhIM [OH] was immersed in 10 M NaOH
and heated at 80 °C for 100 days (2400 h), no evident
degradation of the polymer was observed by 1H NMR spectra, as
shown in Figure 7a, highlighting the excellent stability of poly-
PhIM [OH]. For comparison, the alkaline stability of poly-
PhIM-b-biPh [OH] and poly-PhIM-co-biPh [OH] was also
studied under the same conditions (Figure 7b,c). The prepared
membrane maintained most of the H signals and no new H
signals appeared throughout the testing process (30 d), which
further confirmed that the membrane had excellent alkaline
stability. Notably, after completion of the alkaline exchange
process (exchanging the counterion Cl− with OH−), in the 1H
NMR spectra (Figure 7a) of the homopolymer, a small H signal
at 7.77 ppm appeared, and it did not increase with the extension
of alkaline treatment. A similar phenomenon was also observed
in the alkaline treatment of the two copolymers (Figure 7b,c).
Moreover, this H signal was not generated by the degradation of
the polymer but might have been caused by some changes in the
aryl substituents at the end of the polymer chain under alkaline
conditions. The pictures of the membrane before and after the
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alkaline stability study are shown in Figure S15. Poly-PhIM
[OH] is very fragile and easy to break in a hot alkaline solution
(but does not dissolve), though no degradation was found in its
chemical structure. Both copolymer membranes can maintain
their shape in alkaline solution, and there is no problem of
membrane rupture after long-term treatment. The conductivity
study of copolymer membranes during the alkaline stability test
is shown in Figure 7d. As for the block-like and random
copolymer membrane, after 30 days of alkaline treatment, most
of their conductivity remained unchanged, which further
demonstrates that these membranes have good alkaline stability
in 10 M NaOH at 80 °C. Some of the previously reported AEMs
in the literature with excellent alkaline stability in recent years
and imidazolium-based AEMs by our group are summarized in
Table 2, including their preparation methods and alkaline
stabilities. The preparation of poly(arylene−imidazolium)
polymers was facile, and their alkaline stabilities were superior
to most of the previously reported AEMs, indicating that these
poly(arylene−imidazolium) polymers can be potentially applied
to AEMs working at high temperatures and under strongly
alkaline conditions.
and Technology of China, Hefei 230026, China; orcid.org/
0000-0002-1442-7801; Email: jfzheng@ciac.ac.cn
Suobo Zhang − Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China; University of Science
and Technology of China, Hefei 230026, China; Jiangsu
National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing 211816, China; orcid.org/0000-0002-
1002-6197; Email: sbzhang@ciac.ac.cn
Authors
Boxin Xue − Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China; University of Science
and Technology of China, Hefei 230026, China
Weidong Cui − Inner Mongolia University of Technology,
Hohhot 010021, China
Shengyang Zhou − Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China; University of Science
and Technology of China, Hefei 230026, China
Qifeng Zhang − Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China; University of Science
and Technology of China, Hefei 230026, China
Shenghai Li − Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China; University of Science
and Technology of China, Hefei 230026, China
CONCLUSIONS
■
In summary, based on a novel imidazolium-ionized monomer,
we have successfully attached alkaline stable imidazolium to a
polymer backbone via a simple superacid-catalyzed polymer-
ization without further postfunctionalization. This ionized
monomer polymerization strategy avoids the use of oxygen/
water-sensitive precious metal catalysts and the potential for
incomplete postfunctionalization issues. The obtained polymers
showed a controllable degree of functionalization, high
molecular weight, and good solubility. By introducing biphenyl
as a comonomer and adjusting the feeding sequence, block-like
and random copolymers are prepared to solve the problem of
excessive swelling of the homopolymer. Remarkably, owing to
different microstructures, the two copolymers exhibit signifi-
cantly different ion conduction behaviors, water absorption
capacities, and mechanical properties. Alkaline stability test
results show that the poly(arylene−imidazolium) poly-PhIM
[OH] showed excellent resistance to alkaline solutions (after
treatment in 10 M NaOH at 80 °C for 100 d, no evident
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.0c02612
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21774123; 21875240; 22075276;
U19A2016), the Development of Scientific and Technological
Project of the Jilin Province (Nos. 20200801051GH;
20190302057GX), and the Jilin Provincial-Chinese Academy
of Sciences High-Tech Industrialization Cooperation Program
(No. 2019SYHZ0007).
1
degradation was observed by H NMR spectra). The alkaline
stability is superior to most of the previously reported AEMs,
indicating that these poly(arylene−imidazolium) polymers can
be potentially applied to AEMs working at high temperatures
and under strongly alkaline conditions. Future research will
focus on improving the mechanical properties of these
poly(arylene−imidazolium) polymers and further exploring
their application in electrochemical energy conversion devices.
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The Supporting Information is available free of charge at
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