Synthesis of 4,4′-diacryloylphenyl ether to conveniently crosslink sulfonated poly(ether ether ketone) membranes via electron beam irradiation

Full text of DOI:10.1002/bkcs.10586

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DOI: 10.1002/bkcs.10586
BULLETIN OF THE
E.-B. Lee et al.
KOREAN CHEMICAL SOCIETY
Synthesis of 4,40-Diacryloylphenyl Ether to Conveniently
Crosslink Sulfonated Poly(ether ether ketone) Membranes via
Electron Beam Irradiation
Eun-Byul Lee,† Naeun Kang,‡ Junhwa Shin,§ and Youn-Sik Lee†,‡,
*
†Department of Energy Storage and Conversion Engineering, Chonbuk National University, Jeonju 561-756,
Republic of Korea. *E-mail: yosklear@jbnu.ac.kr
‡Division of Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National
University, Jeonju 561-756, Republic of Korea
§Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongup-si,
Republic of Korea
Received July 13, 2015, Accepted August 24, 2015, Published online November 16, 2015
Keywords: Nonhydrolyzable crosslinking agent, 4,40-Diacryloylphenyl ether, Crosslinked polymer,
Electron beam, Proton conductivity
Nafion membranes have been widely used in the fabrication of
polymer electrolyte membrane fuel cells (PEMFCs) owing to
their good thermal and chemical stabilities and high proton
conductivities.1,2 However, these polymer membranes have
some disadvantages such as high cost and decreased proton
conductivity at elevated temperatures. Sulfonated aromatic
hydrocarbon polymer membranes including poly(ether ether
ketone) (PEEK), poly(ether sulfone), poly(ether ketone),
poly(phosphazene), and polybenzimidazole have been actively
investigated as alternatives to the commercial membranes
owing to their good stability and high proton conductivity.3–6
These highly sulfonated membranes, however, usually have
serious problems such as low-dimensional stability and
mechanical strength, because of excess water uptake.
synthesized in a large quantity; and (3) it is a polar compound
and readily dissolves in solutions of sulfonated polymers in
highly polar organic solvents such as dimethylacetamide
(DMAc). Nonpolar crosslinking agents such as divinylben-
zene are not immediately dissolved in DMAc.
DAPE was reported to be synthesized in two steps.16 How-
ever, we synthesized the compound via one-step reaction with
an isolation yield of 75%, and employed it for the preparation
of crosslinked sulfonated PEEK (c-SPEEK) via electron beam
irradiation. The properties of the resulting membranes were
characterized with respect to the swelling ratio, water uptake,
thermal stability, mechanical property, proton conductivity,
oxidative stability, and methanol permeability.
The problems of the highly sulfonated polymer membranes
associated with water uptake can be overcome by crosslinking
of the membranes. Among various crosslinking methods,7–14
the electron beam-induced crosslinking appears to be a very
convenient method, because it does not require any additive
and heating. Furthermore, not only the membrane surface,
but also the membrane interior can be crosslinked. For the
crosslinking of the sulfonated aromatic polymer membranes,
the crosslinking agent should be acid-resistant, because pro-
tons move across the membranes during the operation of
PEMFCs. Divinylbenzene, a nonhydrolyzable crosslinking
agent, is commercially available; however, it is so highly reac-
tive that it can undergo inhomogeneous and inefficient cross-
linking with other comonomers.15 One approach to overcome
this problem is to synthesize a new agent such as 1,6-bis(4-
vinylphenyl)hexane (BVPH), where two styrene molecules
are separated by a hexane chain, but the synthesis of such com-
pound was reported to be difficult.7,15
Experimental
Materials. Sulfuric acid (95–98%), DMAc, diphenyl ether,
acryloyl chloride, AlCl3, and MgSO4 were purchased from
Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) NaCl,
NaOH, CH2Cl2, and ethyl acetate were purchased from
Samchun (Gyeonggi-do, Korea). Aqueous phenolphthalein
(1%), and 1.0 N HCl, H2O2 (35%) solutions were purchased
from Daejung (Gyeonggi-do, Korea). PEEK was purchased
from Victrex® USA Inc. (West Conchohocken, PA, USA).
All the chemicals were used as received without purification.
Sulfonated PEEK. PEEK was sulfonated following the
reported procedure.8 PEEK powder (35 g) was dissolved in
concentrated sulfuric acid (500 mL) at 40 ꢀC for 1 h. The tem-
perature was increased to 60 ꢀC and maintained for 2 h. The
reaction mixture wascooled to5 ꢀCandthen pouredintowater
with stirring. The coagulated solid was washed several times
with water and dried in a vacuum oven at 50 ꢀC for 12 h and
then at 70 ꢀC for 24 h. The degree of sulfonation of the SPEEK
was estimated to be 60% based on its 1H NMR spectrum.9
Onepossiblealternative maybe4,40-diacryloylphenylether
(DAPE) for the crosslinking of such aromatic polymers
because of the following characteristics: (1) it cannot be
hydrolyzed under acidic conditions; (2) it can be easily
4,40-Diacryloylphenyl ether.
2.94 mmol), acryloyl chloride (0.8 g, 8.81 mmol), and CH2Cl2
Diphenyl ether (0.5 g,
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(18 mL) were added in a two-neck flask under a nitrogen
atmosphere and stirred for 30 min, followed by the addition
of AlCl3 (1.12 g, 8.41 mmol) at 0 ꢀC. The mixture was allowed
to react for 4 h, quenched by the addition of water (1 mL), and
then extracted thrice with CH2Cl2. The combined organic
phase was dried over anhydrous magnesium sulfate and con-
centrated using a rotary evaporator. The crude product was pur-
ified by column chromatography (hexane/ethyl acetate 10/1) to
solventssuchasDMAcanddimethylformamide.Asmentioned
earlier, the solubility characteristics are very important for
the crosslinking of SPPEK, because nonpolar hydrocarbon
crosslinking agents are not readily dissolved in highly polar
solvents that are required to dissolve highly polar SPEEK.
A commercial PEEK was sulfonated to obtain the SPEEK,
following the reported procedure.8 The degree of sulfonation
(DS) of the SPEEK, defined as the average number of
sulfonic acid groups per repeat unit of SPEEK, was estimated
tobe60%basedonits1HNMRspectrum.9 Thegelfractions of
the membranes were determined from the difference in the
weights before and after dissolving in DMAc. The c-SPEEK
membranes prepared by irradiation at 400 kGy were
brittle, and thereafter all the SPEEK membranes were irra-
diated at a constant dose of 350 kGy. When PEEK membranes
without DAPE were irradiated with electron beam, the result-
ing membranes were completely dissolved in DMAc, indicat-
ing that the SPEEK chains are not effectively crosslinked in
the absence of the crosslinking agent. As expected, the gel
fraction of the c-SPEEK membranes gradually increased from
96 to 98% with increasing DAPE loading from 5 to 20 wt %,
becauseofahigherdegreeofcrosslinkingathigherloadingsof
DAPE. The gel fractions of the c-SPEEK membranes
(96–98%) are similar to those (93–98%) of the crosslinked
membranes (DS: 55%, BVPH loading: 5–15 wt %) prepared
using BVPH under similar conditions.7 DAPE is expected to
crosslink polymer chains via radical process. We proposed
one of possible mechanisms for crosslinking SPEEK with
4-vinylbenzyl chloride on electron beam irradiation.17 A sim-
ilar mechanism may also occur in this system.
The IEC of the membranes decreased with increasing
DAPE loading, because the addition of DAPE decreased the
weight fraction of the sulfonic acid groups in the membranes.
The experimentally measured IEC value of the SPEEK mem-
brane (1.69 mmol/g) is very close to the theoretical value
(1.71 mmol/g), whereas the experimental IEC values of all
the c-SPEEK membranes were smaller than their
corresponding calculated values, as listed in Table 1. Perhaps,
this discrepancy resulted from incomplete ion exchange
between SO3H groups in the c-SPEEK membranes and NaCl
dissolved in the aqueous solution, probably because it may be
moredifficultforNa+ ions todiffuseinto SO3Hgroupsforion
exchange within the more highly crosslinked membranes.17
As expected, the dimensional change and water uptake of
the c-SPEEK membranes decreased with increasing DAPE
1
afford a white solid (0.61 g, 75%). H-NMR (400 MHz,
CDCl3): δ 7.99–8.02 (d, 2H), 7.16–7.2 (t, 1H), 7.1–7.13 (d,
2H), 6.44–6.48 (d, 1H), 5.92–5.95 (d, 1H). Anal. Calcd. for
C16H14O3: C, 77.68; H, 5.07. Found: C, 77.65; H, 5.14.
c-SPEEK Membranes. The dried SPEEK was dissolved in
DMAc (10 wt %), followed by the addition of DAPE. The
solutions were cast on glass plates and then dried at 70 ꢀC
for 1 h. The casting membranes were irradiated with electron
beams at a dose of 350 kGy with a dose rate of 6 kGy/min at
room temperature (Korea Atomic Energy Research Institute,
Advanced Radiation Technology Institute, Jeongup, Korea).
The irradiated membranes were dried at 120 ꢀC for 4 h. The
c-SPEEK membranes were designated as 5c-SPEEK, 10c-
SPEEK, 15c-SPEEK, and 20c-SPEEK, where the numbers
indicate the wt % of DAPE with respect to the SPEEK.
Measurements. Gel fraction, ion-exchange capacity (IEC),
dimensional stability, water uptake, proton conductivity,
and methanol permeability of polymer membranes were deter-
mined following the reported procedures.17 The thermal and
mechanical properties were also measured following the
reported procedures, usingaQ-50thermogravimetric analyzer
(TA instruments, New Castle, DE, USA) and an INSTRON
Series IX Universal Testing System Model 4400 (Instron
Co. Norwood, MA, USA), respectively.
Results and Discussion
DAPE was synthesized from diphenyl ether and acryloyl chlo-
ride via a simple one-step Friedel-Craft acylation in 75% yield.
Its proton NMR spectrum (Figure 1) along with the elemental
analysis data confirmed that DAPE was successfully synthe-
sized with high purity. DAPE is well soluble in polar aprotic
Table 1. IEC values of SPEEK and c-SPEEK membranes.
IEC (mmol/g)
Sample
Calculated
Measured
SPEEK
1.71
1.63
1.56
1.48
1.43
1.69
1.43
1.41
1.32
1.17
5c-SPEEK
10c-SPEEK
15c-SPEEK
20c-SPEEK
Figure 1. 1H NMR spectrum of DAPE in CDCl3.
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loading, owing to a higher degree of crosslinking (Table 2).
For example, the crosslinking with 15 wt % DAPE reduced
the increment in length of the membranes from 18 to 2%
and water uptake from 28 to 11%. It was reported that the
crosslinking SPEEK membranes (DS: 55%) with 15 wt %
BVPH reduced the increment in length from 18 to 4% and
water uptake from 28 to 13%.7 This comparison indicates that
DAPE improves the dimensional stability ofthe membranes to
the almost same degree as BVPH.
As shown in Figure 2, the SPEEK membrane exhibited a
two-step thermal degradation, a typical behavior of most
SPEEK membranes.18,19 The first weight loss in the range
of 270–380 ꢀC and the second weight loss above 430 ꢀC were
attributed to the elimination of the sulfonic acid groups and the
decomposition of the polymer main chains, respectively. All
the c-SPEEK membranes exhibited similar degradation pat-
terns as the SPEEK membrane; however, their degradation
onset temperatures were slightly lower than that of the original
SPEEK membrane. The lowered degradation temperatures
may be because of the unreacted or partially reacted DAPE.
If some polymer chains had been cleaved into shorter chains
during the electron beam irradiation process, the resulting
small polymer molecules would also contribute to the slightly
decreased thermal stability of the c-SPEEK membranes.17
As shown in Table 3, the tensile strength and Young’s mod-
ulus of the c-SPEEK membranes gradually increased with
increasing DAPE loading up to 15 wt % and decreased there-
after (increment: 62% in tensile strength, 77% in Young’s
modulus). In contrast, the elongation at break decreased sig-
nificantly from 130 to 15% with increasing DAPE loading
up to 15 wt % (reduction: 88%). The increased tensile
strengths and Young’s moduli at high loadings of DAPE are
because of the increased compactness of the membranes.
However, thedecreasedtensile strength andYoung’smodulus
of the 20c-SPEEK membrane probably resulted from its brit-
tleness, becauseofahighdegreeofcrosslinking. However, the
crosslinking of SPEEK membranes with 15 wt % BVPH was
reported to slightly increase the tensile strength and modulus
of the membranes (increment: 13% in tensile strength, 10% in
Young’s modulus) but reduce the elongation at break from
47 to 18% (reduction: 62%).7 This comparison indicates that
DAPE increases the tensile strength and Young’s modulus
more effectively with more reduction in the elongation than
BVPH, probably due to the difference in the chemical struc-
tures of BVPH and DAPE where DAPE is shorter and more
rigid than BVPH.
The proton conductivity of the membranes increased with
increasing temperature as shown in Figure 3. Even though
the SPEEK membrane exhibited higher proton conductivity
than Nafion 212 at various temperatures up to 75 ꢀC, its proton
conductivity could not be measured at 90 ꢀC owing to its
excessive water uptake and swelling. In contrast, the proton
conductivities of the c-SPEEK membranes were lower than
that of SPEEK and decreased with increasing DAPE loading,
because of the decrease in the water uptake and IEC value as a
result of the crosslinking. The decreased proton conductivities
of the c-SPEEK membranes may also result from the decrease
in the free volume, and subsequently narrowed water channels
within the polymer membranes.14 However, notably, the pro-
ton conductivities of the c-SPEEK membranes did not
decrease too much from that of the original SPEEK membrane
and were comparable to that of Nafion 212. A similar result
was also reported for the BVPH-derived membranes.7
The membranes were immersed in Fenton’s reagent (2 ppm
FeSO4 + 2% H2O2) at 80 ꢀC, and the breaking times of the
Table 2. Dimensional stability and water uptake of membranes.
Water
uptake (%)
Δla (%)
Δtb (%)
Sample
SPEEK
5c-SPEEK
10c-SPEEK
15c-SPEEK
20c-SPEEK
Nafion212
25 ꢀC 80 ꢀC 25 ꢀC 80 ꢀC 25 ꢀC
80 ꢀC
18
4
25
12
10
9
25
13
10
9
36
28
26
17
13
16
28
14
12
11
10
16
65
40
35
21
15
20
3
2
1
5
8
5
20
7
a Swelling ratio in length = (l – ldry)/ldry × 100 where ldry and l represent
the length of the membranes before and after the swelling experiment,
respectively.
b Swelling ratio in thickness = (t – tdry)/tdry × 100 where to and t represent
the thickness of the membranes before and after the swelling
experiment, respectively.
Table 3. Mechanical properties of the membranes.
Tensile
strength
(MPa)
Young’s
modulus
(GPa)
Elongation at
break (%)
Sample
SPEEK
22.0
33.5
35.5
35.7
26.5
33.3
0.66
1.09
1.15
1.17
1.05
0.19
130
21
18
15
14
5c-SPEEK
10c-SPEEK
15c-SPEEK
20c-SPEEK
Nafion212
240
Figure 2. TGA curves of SPEEK and c-SPEEK membranes.
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Table 4. Breaking time and methanol permeability of membranes.
Breaking
time (min)
Methanol permeability
(cm2/s)
Sample
SPEEK
7
2.8 × 10−6
1.5 × 10−7
5.3 × 10−8
3.8 × 10−8
1.0 × 10−8
2.2 × 10−6
5c-SPEEK
10c-SPEEK
15c-SPEEK
20c-SPEEK
Nafion 212
208
233
272
310
—
Research Foundation of Korea funded by the Ministry of Sci-
ence, ICT & Future Planning. This study was also supported
by the Human Resources Development program
(No. 1501001882) of the Korea Institute of Energy Technol-
ogy Evaluation and Planning (KETEP) grant funded by the
Korea government Ministry of Trade, Industry, and Energy.
Figure3. ProtonconductivitiesofSPEEKandc-SPEEKmembranes.
resulting membranes were assessed by the naked eye. The
original SPEEK membrane was broken apart into pieces
within 7 min, but the c-SPEEK membranes became so highly
oxidation-resistant that the breaking times of the c-SPEEK
membranes increased from 208 to 272 min as the DAPE load-
ing increased from 5 to 15 wt % (Table 4). This experimental
data indicate that the oxidative stability of the SPEEK mem-
brane increases with increasing DAPE loading owing to a
higher degree of crosslinking of the membrane. However,
the oxidative stabilities of the c-SPEEK membranes are lower
than those of the BVPH-derived membranes (breaking time:
33–38 h), probably due to the radical-sensitive carbonyl
groups of DAPE moiety.20
The methanol permeabilities of the membranes were meas-
ured in an aqueous 3.0 M methanol solution at 35 ꢀC, follow-
ing the reported procedure.17 The methanol permeability of
the original SPEEK membrane (2.8 × 10−6 cm2/s) was close
to that of Nafion 212, because of the high content of the sul-
fonic acid group. However, as the DAPE loading increased
from 5 to 15%, the methanol permeability decreased progres-
sively from 1.5 × 10−7 to 3.8 × 10−8 cm2/s, because of more
compact membrane structures. The degree of reduction in
themethanolpermeabilityisclosetothatoftheBVPH-derived
membranes (1.7 × 10−8 cm2/s).7
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Acknowledgment. This study was supported by the Radia-
tion Technology R&D program through the National
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