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Cite this: RSC Adv., 2021, 11, 19012

Acidity eﬀects of medium ﬂuids on anhydrous
proton conductivity of acid-swollen block polymer
electrolyte membranes†
Takato Kajita, a Atsushi Noro,and Naoki Nakamura c*ab Takahiro Seki,a

Yushu Matsushita‡aProton-conductive polymer electrolyte membranes (PEMs) were prepared by inﬁltrating sulfuric acid (Sa) or
phosphoric acid (Pa) into a polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S–P–S) triblock copolymer.
When the molar ratio of acid to pyridyl groups in S–P–S, i.e., the acid doping level (ADL), is below unity, theP-block/acid phase in the PEMs exhibited a moderately high glass transition temperature (Tg) of 140  C
because of consumption of acids for forming the acid–base complexes between the pyridyl groups andthe acids, also resulting in almost no free protons in the PEMs; therefore, the PEMs were totally glassy
and exhibited almost no anhydrous conductivity. In contrast, when ADL is larger than unity, the Tgs ofthe phase composed of acid and P blocks were lower than room temperature, due to the excessive
molar amount of acid serving as a plasticizer. Such swollen PEMs with excessive amounts of acidreleasing free protons were soft and exhibited high conductivities even without humidiﬁcation. In
particular, an S–P–S/Sa membrane with ADL of 4.6 exhibited a very high anhydrous conductivity of 1.4 
101 S cm1 at 95  C, which is comparable to that of humidiﬁed Naﬁon membranes. Furthermore, S–P–
Received 14th February 2021
Accepted 19th May 2021

S/Sa membranes with lower Tgs exhibited higher conductivities than S–P–S/Pa membranes, whereas thetemperature dependence of the conductivities for S–P–S/Pa is stronger than that for S–P–S/Sa,

DOI: 10.1039/d1ra01211h

suggesting Pa with a lower acidity would not be eﬀectively dissociated into a dihydrogen phosphatersc.li/rsc-advances

anion and a free proton in the PEMs at lower temperatures.

1. Introduction
Fuel cells can generate electrical energy together with water
through an electrochemical reaction between hydrogen andoxygen gases; thus, they are highly promising as a clean power
generation system.1–3 In particular, polymer electrolyte fuel cells(PEFCs) with proton-conductive polymer electrolyte membranes(PEMs) have already been used in commercially available fuelcell vehicles (FCVs) and household fuel cell cogeneration
systems.4–8 Peruorosulfonic acid polymers such as Naon® arethe best-known proton-conductive PEMs.9–12 Naon membranes
exhibit a very high proton conductivity of 0.1 S cm1 or higheraDepartment of Molecular & Macromolecular Chemistry, Graduate School ofEngineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: noro@nagoya-u.jpbInstitute of Materials Innovation, Institutes of Innovation for Future Society, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japanc
FC Material Development Dept., Electrication & Environment Material Engineering
Div., Advanced R&D and Engineering Company, Higashifuji Technical Center,
TOYOTA Motor Corporation, 1200 Mishuku, Susono, Shizuoka, Japan† Electronic supplementary
10.1039/d1ra01211h

information(ESI)

available.

See

DOI:

‡ Present address: Toyota Physical and Chemical Research Institute, 41-1,
Yokomichi, Nagakute, Aichi 480-1192, Japan.

19012 | RSC Adv., 2021, 11, 19012–19020

under humidication in the presence of water molecules at 70–
90  C,13–15 where the hydrophobic phase with uorine moiety
and the sulfonic acid group/water mixed phase are separated
from each other at a microscopic scale.16–18 The former plays
a part in keeping the membrane shape, and the latter serves as
the proton-conductive paths. Hydrocarbon-based protonconductive PEMs,19,20 such as sulfonated polyetheretherketone(PEEK),21–23 polysulfone,24–28 polyphenylene,29–31 and poly(styrene sulfonic acid),32,33 have also been widely studied;however, these PEMs exhibit almost no proton conductivity
under dry conditions so that fuel cell reactions never occur.
Thus, PEFCs using these PEMs require humidiers or humidication systems to circulate water vapor, which complicates andupsizes the PEFC systems.
To simplify the PEFC systems, PEMs exhibiting moderate
proton conductivities even without humidication have been
actively studied.34–43 A representative example is a phosphoricacid (Pa)-doped polybenzimidazole (PBI) membrane thatexhibits the low or moderate conductivities of 0.0056 S cm1 at
130  C and 0.04 S cm1 at 190  C.44 Because of the lower
conductivities, the PEM of Pa-doped PBI cannot be an alternative to the humidied Naon membrane. Eﬀorts have also been
made to enhance the anhydrous proton conductivity of PEMs
composed of acid-doped PBI. For example, a membrane© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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exhibiting the anhydrous conductivity of 0.038 S cm1 at 140  C
was prepared by hybridizing PBI and sulfonic-acid-modiedsilica and further doping Pa into the composite
membrane.45–51 However, the conductivity was still lower thanthat of humidied Naon membranes, probably because theability to retain Pa in the membrane was not high; substantially,
merely the surface of the PBI membranes might be dissolved in
Pa due to the rigidity of PBI. To overcome the above mentioned
problems, some Pa-doped composite membranes composed ofsulfonated PEEK52–54 or ether bonds-incorporated PBI55–57 havealso been developed, which exhibit high anhydrous conductivities due to the higher Pa doping level.
Another eﬀective solution to the problem may be to preparea vinyl-polymer-based PEM by inltrating a nearly nonvolatile
low-molecular-weight liquid serving as a proton carrier. Vinyl
polymers have a primary structure composed of a relatively
exible C–C single bond instead of a rigid PBI with aromaticgroups in the backbone chain, although a mixture with a large
amount of low-molecular-weight liquid is typically a solution
and cannot be used as a membrane at the operating temperature of the fuel cell. To make the mixture solid-like material, thepolymer in the mixture should be chemically or physically crosslinked, and it can be swollen with the low-molecular-weightliquid of medium uid serving as a proton carrier. Especially,a self-assembled block polymer58,59 that forms vitried microdomains serving as physical cross-links60–62 is an excellent
candidate material for anhydrous proton-conductive PEMs
because of their thermoplastic property. There have been
several studies of the preparation of anhydrous protonconductive PEMs by inltrating nearly nonvolatile ionicliquids63 into block polymers. For example, Park and coworkers
pioneeringly prepared a proton-conductive PEM exhibiting ananhydrous conductivity of 0.045 S cm1 at 160  C by inltrating
an aprotic ionic liquid into a vinyl-based diblock copolymer
with sulfonic acid groups.64 Segalman and coworkers also
prepared a PEM exhibiting an anhydrous conductivity of
0.01 S cm1 at 140  C by incorporating a protic ionic liquid intoa vinyl-based diblock copolymer,65 whereas Lodge, Hillmyer,
and coworkers reported a PEM exhibiting an anhydrous
conductivity of 0.014 S cm1 at 180  C by inltrating a protic
ionic liquid into a cross-linked diblock copolymer.66 Despite
this progress, the anhydrous conductivities of these protonconductive PEMs are not still suﬃciently higher than thehydrous conductivity of the conventional Naon membrane.
Recently, we developed an anhydrous PEM exhibiting
comparatively high conductivity over 0.1 S cm1 at around100  C, regardless of an anhydrous condition, by inltratinga large amount of nearly nonvolatile sulfuric acid (Sa) intoa chemically cross-linked vinyl-based poly(4-vinylpyridine)
membrane via acid–base complexation and ionic interactions.67
Furthermore, we prepared an anhydrous highly protonconductive PEM by swelling a self-assembled polystyrene-bpoly(4-vinylpyridine)-b-polystyrene (S–P–S) triblock copolymer
with Sa. In terms of tensile properties and processability, theblock polymer-based PEM is superior to the chemically crosslinked PEM, indicating that the S–P–S membrane swollen
with Sa is a promising anhydrous proton-conductive PEM.

© 2021 The Author(s). Published by the Royal Society of Chemistry

RSC AdvancesFig. 1 Schematic illustration of the preparation of self-assembled
block polymer-based proton-conductive PEMs.

Nevertheless, the triblock copolymer-based PEM swollen withacid has been little investigated. Even the eﬀects of diﬀerent
acids on the anhydrous proton conductivities of the PEMs havenot been reported to date; however, anhydrous proton
conductivities of the acid-swollen PEMs must be aﬀected by theacidity of acidic liquids in the PEMs. Thus, investigation on
acidity eﬀects of the block polymer-based PEMs swollen with
diﬀerent acids is crucial to design highly proton-conductive
PEMs under anhydrous conditions for next-generation fuelcells. In this study, by swelling the S–P–S block copolymer
membrane with Sa or Pa, as shown in Fig. 1, we investigated theeﬀects of diﬀerent acidic liquids on the anhydrous proton
conductivity of the triblock copolymer-based PEMs.

2.

Experimental

2.1

Synthesis and characterization of a triblock copolymer

As shown in Scheme 1, S–P–S was synthesized by reversible
addition–fragmentation chain transfer (RAFT) polymerization68,69 (see also Scheme S1, where the exact chemical structureof S–P–S with the RAFT agent residue is shown, ESI†). First,
a styrene monomer (TCI Co., Ltd.) puried by passing throughan alumina column (activated, basic, Brockmann I, SigmaAldrich) was polymerized by mixing a bifunctional RAFT agentof S,S0 -bis(a,a0 -dimethyl-a00 -acetic acid)trithiocarbonate70 anda radical initiator of azobisisobutyronitrile (AIBN, Kishida
Chemical Co., Ltd.), and the mixture was heated at 130  C for six

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Scheme 1

Synthesis of S–P–S (note that the trithiocarbonate unit in the ﬁnal product was omitted in the chemical structure for simpliﬁcation).

hours, where the relatively high reaction temperature of 130  C
was adopted to promote the propagation reaction of styrene
polymerization. Aer the polymerization, polystyrene (S) witha trithiocarbonate unit at the center of the polymer chain, i.e.,a macro-RAFT agent, was puried by reprecipitation. S–P–S wassynthesized by mixing the macro-RAFT agent, AIBN, and a 4vinylpyridine monomer (Sigma-Aldrich) and the mixture washeated at 80  C for 1.5 hours, where the 4-vinylpyridine monomers were inserted next to the sulfur atoms of trithiocarbonate
unit in the precursor S.68,69 See also details of synthesis ofa similar block copolymer by RAFT polymerizations in theprevious literature.67,71–73
1
H NMR spectroscopy by an Ascend 500 MHz (Bruker Corp.)
was used to determine the degree of polymerization of the
precursor S, and composition of the block polymer. Deuterated
chloroform was used as a solvent. Size exclusion chromatography (SEC) was also performed to determine the molecularweight distribution (Đ) of polymers and to conrm successful
synthesis of a block polymer. SEC was conducted using an
HPLC system (HPLC Pump: Shimadzu LC-20AD; column oven:
Shimadzu CTO-20A; RI detector: Shimadzu RID-10A) equippedwith two TSKgel G5000HHR columns (Tosoh Corp.) and N,Ndimethylformamide as the eluent solvent at 40  C.
2.2

Paper

Preparation of acid-swollen membranes

Acid-swollen membranes with a thickness of 0.5 mm wereprepared by swelling neat S–P–S membranes with Sa or Pa,
which are referred to as S–P–S/Sa or S–P–S/Pa, respectively. First,
a solution of 10 wt% was prepared by dissolving an S–P–S
powder in pyridine, and then solution casting was conducted at
50  C for two days followed by vacuum drying at 50  C for one
day. This resulted in the preparation of a neat S–P–S membrane.
A 98 wt% aqueous solution of Sa or an 85 wt% aqueous solutionof Pa was dissolved in methanol that more easily evaporates
than water, and the S–P–S membrane was then immersed in thesolutions, where a 20-fold weight of methanol to the neat S–P–S
was used. Aer evaporating the volatile methanol and waterfrom the solution at 50  C for 12 hours, methanol was added
again to homogeneously inltrate the acid, followed by solution
casting at 50  C for 36 hours and vacuum drying at 50  C for 24hours. The weight content of acid in S–P–S-based PEMs wasvaried from 0 wt% to 80 wt%. Both S–P–S/Sa and S–P–S/Pa withlower than the acid content of 50 wt% were glassy and hard; incontrast, samples with an acid content of more than 60 wt%
were so and gel-like membranes (see also Fig. S1, ESI†). Notethat polystyrene (S) cannot be dissolved in Sa and Pa, whereaspoly(4-vinylpyridine) (P) is soluble in Sa and Pa (Fig. S2, ESI†);

19014 | RSC Adv., 2021, 11, 19012–19020

therefore, Sa and Pa selectively inltrated into the P phase of
S–P–S membranes, where the vitried S phase retained theshape. In other words, the membranes swollen with acid kept
adopting the same morphology as the neat S–P–S membrane
before swelling.74–76
The prepared membranes are coded as S–P–S/X(Y), where X
represents the inltrated acid, Sa or Pa, and Y denotes theweight percentage of acid in the membrane. The molar ratio ofacid to pyridyl group in S–P–S is dened as the acid doping level(ADL),44,51 which is estimated from eqn (1).
ADL ¼

nacid
wacid =Macid
wacid =Macid
¼
¼
nbase
wbase =Mbase
wSPS w0 P =MP monomer(1)

where nacid, wacid, and Macid are the molar amount, weightfraction, and molecular weight of acid, respectively, while nbase,wbase, and Mbase are those of the base monomer unit, respec0
tively. wS–P–S, w P ; and MP monomer are the weight fraction of
S–P–S in the swollen membranes, weight fraction of P in neat S–
P–S, and molecular weight of 4-vinylpyridine monomer (105 gmol1), respectively.
2.3

Measurements

Transmission electron microscopy (TEM) was used to observe
the nanostructure of the neat S–P–S membrane, where themorphology depends on the volume fraction77 of the S block in
the S–P–S, the molecular weight78 and Đ79 of the S–P–S, and soon. The sample specimen of neat S–P–S membrane wasembedded into epoxy resin, and the ultrathin section witha thickness of 80 nm was prepared by a microtome in a wet
condition, followed by staining with iodine (I2) vapor at 60  C
for 40 minutes. The TEM observation was conducted with JEM1400 (JEOL Ltd.) at an acceleration voltage of 120 kV.
To determine the conductivities of the PEMs, alternating
current (AC) impedance spectroscopy was carried out witha potentio/galvanostat SP-300 (BioLogic Sciences Instruments)
in the frequency range of 1  100 to 1  106 Hz at a signal
amplitude of 50 mV by using a two-probe method that is easier
to measure the impedance than a four-probe method. A test cell
was prepared by xing the distance between the platinum
electrodes (l) at 7 mm and using the test specimen withsectional area (A) of 2 mm2.80,81 The temperature wascontrolled in the benchtop-type temperature and humidity
chamber SH-242 (ESPEC Corp.) within a temperature range of
20–95  C below the Tg of polystyrene (100  C) at no humidication, where the humidity in the chamber at all temperatures
was close to 0 %RH which was determined with the thermohygrometer Testo-645 (Testo SE & Co. KGaA). Note that the© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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RSC Advancestrithiocarbonate of the RAFT agent residue in the S–P–S would
not be degraded within the temperature range adopted in thisstudy.82 To estimate the conductivity, sDC, by using eqn (2),a bulk resistance (R) was evaluated from the Nyquist plot of thetest specimen, by reading the extrapolated value of the plot on
the horizontal axis, Z0 , which is the real part of the complex
impedance, Z ¼ Z0  iZ00 , i.e., nonzero Z0 intercept.83,84
sDC ¼

l
AR

(2)

The glass transition temperatures, Tgs, of the membranes
were evaluated by diﬀerential scanning calorimetry (DSC)
measurements using Q2000 (TA Instruments Inc.) with a heating rate of 10  C min1 in the temperature range of 90  C to180  C.

3.

Results and discussion

3.1

Synthesis and characterizations of a block polymer

S–P–S was synthesized through two-step RAFT polymerization.
Fig. 2a shows a 1H NMR spectrum of S–P–S, and Fig. 2b shows
SEC chromatograms of S–P–S and the precursor S. The numberaverage molecular weight of S (Mn,S) was determined to be 31k
according to end-group analysis using the 1H NMR spectrum(see also Fig. S3, ESI†). SEC measurements revealed that Đs of S
and S–P–S were 1.1 and 1.8, respectively, where the relatively

large Đ of S–P–S was probably caused by synthesizing relatively
high molecular weight polymers. From the peak shi to a lower
elution time compared to the peak of S, successful synthesis of
S–P–S was conrmed. The mole fraction of the S block in S–P–S
was determined to be 0.067 with 1H NMR spectroscopy bycomparing the integral of peaks from 8.0 to 8.6 ppm originatingfrom two protons (He) on the pyridyl groups of the P block withthat of peaks from 6.0 to 7.2 ppm originating from ve protons(Ha, Hb and Hc) on the phenyl groups of the S block and two
protons (Hd) on the pyridyl groups of the P block (Fig. 2a). By
using the mole fraction of S in the block polymer and the Mn,S0estimated by the end-group analysis, w P and the total numberaverage molecular weight of S–P–S were estimated to be 0.93
and 461k, respectively. The volume fraction of the S block in
S–P–S (fS) was 0.074, which was calculated from the bulk
densities of S (1.05 g cm3) and P (1.17 g cm3).
A TEM image of a neat S–P–S membrane is shown in Fig. 2c,
where the S phase appears brighter while the P phase does
darker because of I2 vapor staining. A brighter spherical S phase
was observed in the darker P phase as a matrix. The distance
between the domains (D) was estimated to be 40 nm in the
TEM image, which roughly agreed with where spherical
domains are assumed to be packed in a body-centered cubic(bcc) manner and q* (0.13 nm1) is a scattering vector of the
SAXS prole as shown in Fig. S4 (ESI†). Since Sa and Pa selectively inltrated into the P phase of S–P–S membranes (seeFig. S2, ESI†), the membranes swollen with acid probably keep
adopting the spherical morphology as the neat S–P–S
membrane before swelling.74–76

3.2 Anhydrous conductivity of acid-swollen S–P–S
membranes(a) 1H NMR spectrum and the chemical structure of S–P–S
synthesized. (b) SEC chromatograms of S–P–S (solid line) andprecursor S (dashed line). (c) A TEM image of the neat S–P–S
membrane.

Fig. 2© 2021 The Author(s). Published by the Royal Society of Chemistry

AC impedance spectroscopy was performed to determine
anhydrous conductivities of acid-swollen S–P–S membranes.
Fig. 3a and b shows Nyquist plots of S–P–S/Sa(80) and S–P–S/
Pa(80) membranes with almost the same size by varying
temperatures from 50  C to 95  C, respectively. The vertical andhorizontal axes in Fig. 3 are the imaginary part (Z00 ) and thereal part (Z0 ) of the complex impedance (Z), respectively. The
nonzero Z0 intercept in the plot was taken as R of the test
specimen.83,84 In the measurement frequency range, the data of
S–P–S/Sa(80) and S–P–S/Pa(80) did not represent a semicircle
type plot which is oen observed in Nyquist plots for PEMs
probably because of relatively fast proton transport. It is evident
that S–P–S/Sa(80) in Fig. 3a gives a R value of 1.6  102 U, which
is clearly much lower than that of S–P–S/Pa(80) (5.9  102 U) at
95  C. Using these R values, the anhydrous conductivities (sDCs)
of S–P–S/Sa(80) and S–P–S/Pa(80) were estimated to be 1.4 
101 and 6.1  102 S cm1 at 95  C from eqn (2), respectively.
As the temperature decreased, the nonzero Z0 intercept of both
S–P–S/Sa(80) and S–P–S/Pa(80) became higher; in other words,
the R value increased as the temperature decreased.
We further determined sDCs for S–P–S/Sa and S–P–S/Pa with
diﬀerent acid contents using the same procedure and plotted
them against the reciprocal of the absolute temperature (T) in
Fig. 4. Table S1 (ESI†) also summarizes the values of sDC. For the

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Nyquist plots of membranes of (a) S–P–S/Sa(80) (ﬁlled symbols)and (b) S–P–S/Pa(80) (open symbols).

Fig. 3

membranes with an Sa content below 40 wt%, the impedance
spectra could not be correctly acquired due to a resistancehigher than the detection limit. From Fig. 4, S–P–S/Sa(50) withan Sa content of 50 wt% exhibits the relatively low anhydrous
sDC of 2.0  104 S cm1 at 95  C, while S–P–S/Sa(60) gives
moderately high, 1.2  102 S cm1 at the same temperature. It
should be emphasized that the sDC suddenly increased by 2
orders of magnitude by adding merely 10 wt% more Sa. As thePaper
Sa content increased further, S–P–S/Sa(70) and S–P–S/Sa(80)exhibit the higher sDC of around 101 S cm1; particularly, S–
P–S/Sa(80) under anhydrous conditions records the very highconductivity of 1.4  101 S cm1, which is comparable to thatof humidied Naon membranes, regardless of anhydrous
conditions. Furthermore, even though ADL of S–P–S/Sa(80) was
4.6 that is lower than that of previously-reported hightemperature-PEMs (HT-PEMs with ADL higher than 10), theanhydrous sDC of S–P–S/Sa(80) was higher than that of the HTPEMs at around 100  C. For example, a Pa-doped SPEEK/ionicliquid composite membrane with ADL of 10 exhibited theanhydrous sDC of 5.33  102 S cm1 at 110  C.52 Anotherexample is a Pa-doped PBI/sulfonated titania composite
membrane with ADL of 12.1, which showed the anhydrous sDC
of 6.7  102 S cm1 at 100  C.51 These suggests that higher
conductivities of S–P–S/Sa(80) should originate from the higher
acidity of Sa than that of Pa. The results were also comparable tothose from membranes of chemically cross-linked poly(4vinylpyridine) swollen with Sa in our previous study; namely,
the anhydrous conductivities of block polymer-based PEMs alsostrongly depended on the Sa content in the PEMs. It should be
noted that sDC of S–P–S/Sa(80) at 95  C was still around 94% of
the initial sDC at 95  C, even in approximately 330 hours aer
keeping the PEM at 95  C (Fig. S5, ESI†). Furthermore, the S–P–
S/Sa(80) membrane have retained the shape and highconductivities.
The sDC of membranes with a Pa content below 40 wt% was
not correctly estimated, either, due to the very high resistance.
On the other hand, S–P–S/Pa(50) exhibited the anhydrous sDC of
2.4  104 S cm1, almost the same as that of S–P–S/Sa(50) at
95  C. Below 80  C, the sDCs of S–P–S/Pa(50) was lower thanthose of S–P–S/Sa(50). Similar to S–P–S/Sa membranes, theanhydrous sDC of S–P–S/Pa increased as the Pa content
increased, and S–P–S/Pa(80) exhibited the comparatively highsDC of 6.1  102 S cm1 at 95  C. Roughly speaking, the sDCvalues for S–P–S/Pa membranes was lower than those for S–P–S/
Sa with the same acid content.

Fig. 4 Temperature dependence of anhydrous conductivity of a series of membranes of (a) S–P–S/Sa (ﬁlled symbols) and (b) S–P–S/Pa (opensymbols). Solid lines are drawn just as guide for the eyes.

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Fig. 4 also shows that the anhydrous sDC of S–P–S/Pa has
a stronger temperature dependence than that of S–P–S/Sa at thesame acid content. This is probably because highly acidic Sa
would be almost completely dissociated into a hydrogen sulfate
anion and a free proton in the membranes, even at low
temperatures. Namely, the concentration of free protons in the
S–P–S/Sa membrane would be almost constant regardless oftemperatures; therefore, the main factor in the temperaturedependence of anhydrous sDC could be the proton-hopping
mechanism expressed by the Arrhenius equation with theactivation energy.85,86 On the other hand, Pa with lower acidity
would not be eﬀectively dissociated into a dihydrogen phosphate anion and a free proton in the membranes at low
temperatures near room temperature, whereas the dissociationof Pa becomes easier as the temperature increases. Therefore,
the concentration of the free protons in S–P–S/Pa would
increase as the temperature increases, resulting in the stronger
temperature dependence of the anhydrous sDC for S–P–S/Pa,
which is not followed by the Arrhenius equation.

3.3

Thermal analysis of acid-swollen S–P–S membranes

From the macroscopic appearance, it is apparent that both S–P–
S/Sa and S–P–S/Pa membranes with acid content lower than
50 wt% were glassy and hard; in contrast, the samples withmore than 60 wt% acid content were so and gel-like

Fig. 5 DSC thermograms of a series of membranes of (a) S–P–S/Sa
and (b) S–P–S/Pa.

© 2021 The Author(s). Published by the Royal Society of Chemistry

RSC Advances
membranes (see also Fig. S1, ESI†), indicating that the Tgs ofthe membranes were drastically aﬀected by the acid content.
DSC thermograms of the S–P–S/Sa and S–P–S/Pa membranes
are shown in Fig. 5a and b, respectively, and the Tgs estimatedfrom the thermograms are summarized in Table S2 (ESI†). The
Tgs were observed at 96  C and 155  C in the thermogram ofneat S–P–S without acid, which are due to the end S blocks andthe middle P block, respectively. For both S–P–S/Sa and S–P–S/
Pa membranes swollen with acid, the Tg originating from the
S blocks cannot be read from the thermograms. This is because
the volume fraction of S blocks in the acid-swollen membranes
was actually much smaller than that of neat S–P–S upon addition of large amount of the acids. The Tg of the P-blockcontaining phase, i.e., P/acid mixed phase in the S–P–S/Sa
membranes slightly increased up to the Sa content of 30 wt%,
and it decreased when the Sa content increased further; itshould be noted that the Tg suddenly dropped at 50 wt% of Sa
content. The Tg of the P/acid mixed phase in the S–P–S/Pa
membranes also exhibited the same tendency as that for the
S–P–S/Sa membranes.
The Tgs associated with the P block were plotted against the
ADL and the acid weight content in the membranes in Fig. 6a tocompare the acidity eﬀects of two acids on the anhydrous sDC of
the PEMs. At ADL < 1, where the stoichiometry of acid to thepyridyl group is below unity, the P/acid mixed phase in both S–
P–S/Sa and S–P–S/Pa exhibited high Tgs above 140  C. The
results are presumably attributed to restricted segmental
motions of polymer chains due to the formation of rigid acid–base complexes between the pyridyl groups and the acids in themembranes (see also the Fourier transform infrared spectra of
S–P–S/Sa and S–P–S/Pa in Fig. S6 (ESI†), indicating the formation of the acid–base complex). The Tgs of the S–P–S/Sa
membranes with ADL below unity were slightly higher thanthose of the S–P–S/Pa membranes, probably because the highly
acidic Sa had a greater ability to form acid–base complexes. On
the other hand, the Tgs of the P/acid mixed phase in both S–P–S/
Sa and S–P–S/Pa underwent a sudden drop when ADL exceeded
unity, since the excessive acid in the phase served as a plasticizer. It should also be noted that the Tgs of the S–P–S/Sa
membranes were lower than those of the S–P–S/Pa
membranes at ADL larger than unity, where Sa having
a melting point (10  C)87 lower than that of Pa (42  C)88 canplasticize P polymer more eﬀectively. Since the acids inltrated
into S–P–S/Sa or S–P–S/Pa served as both an agent for forming
the acid–base complexes and a plasticizer to the P blocks, thevariation in the Tgs of the P/acid mixed phase was not simple
and did not agree with the Fox equation89 (Fig. S7, ESI†) used toexpress the composition dependence on the Tg of blends
composed of a polymer and a plasticizer.
The anhydrous sDCs of S–P–S/Sa and S–P–S/Pa at 95  C werealso plotted against ADL and the weight content of acid inFig. 6b to compare the acidity eﬀects of medium liquids on thesDCs of the PEMs. With ADL < 1, both S–P–S/Sa and S–P–S/Pa
exhibited quite low sDC (close to 0 S cm1) at 95  C. This isbecause there must be almost no free protons to contribute to
the conductivity due to consumption of acids for forming theacid–base complexes between the pyridyl groups and the acids.

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phase. Second, highly acidic Sa would be almost completely
dissociated into a hydrogen sulfate anion and a free proton in
the membranes, even at a low temperature. Note that Pa would
not be eﬀectively dissociated into a dihydrogen phosphateanion and a free proton in the membranes at low temperaturesdue to lower acidity.

4. Conclusions

Fig. 6 (a) Tg of P/acid mixed phase and (b) anhydrous conductivity at
95  C for a series of S–P–S/Sa and S–P–S/Pa membranes against ADL
and the weight content of acid in the membranes. Solid lines are drawn
as a guide for the eyes.

In addition, the restricted segmental motions of the polymer
chains below the Tgs of the P/acid mixed phase probably lowered conductivity. On the other hand, when ADL exceeded unity,
the segmental motions of the polymer chains became active
owing to the suﬃciently low Tgs of the P/acid mixed phase, andsome free protons were released from an excessive amount ofacid, resulting in enabling proton conduction in anhydrous
conditions. As ADL increased further, these eﬀects wereenhanced and higher anhydrous sDC was attained.
Comparing S–P–S/Sa and S–P–S/Pa in Fig. 6b, S–P–S/Sa
exhibited a higher anhydrous sDC than S–P–S/Pa when ADL
was larger than unity. The tendency is probably due to two main
reasons. First, the segmental motions of the P/acid mixed phasein S–P–S/Sa were more activated than those of S–P–S/Pa, which
originates from the lower Tgs of the P/acid mixed complex

19018 | RSC Adv., 2021, 11, 19012–19020

A vinyl polymer-based S–P–S triblock copolymer was synthesized and swollen with Sa or Pa to investigate the eﬀects ofacidity of these two acidic liquids on anhydrous proton
conductivity of the triblock copolymer-based PEMs. With ADL
below unity, both S–P–S/Sa and S–P–S/Pa exhibited quite low
conductivities under non-humidied conditions, because there
were almost no free protons derived from the consumption ofacids for forming the acid–base complexes between the pyridylgroups and the acids. On the other hand, when ADL exceeded
unity, the Tgs of the P/acid mixed phase in both S–P–S/Sa and S–
P–S/Pa suddenly dropped, and the PEMs exhibited moderate orhigh anhydrous conductivities. When ADL increased further,
the concentration of free protons in the PEMs increased and theTgs of the P/acid mixed phase decreased, revealing the highconductivities of both S–P–S/Sa and S–P–S/Pa under dry conditions, whereas S–P–S/Sa exhibited higher anhydrous conductivities than S–P–S/Pa. Notably, S–P–S/Sa(80) (ADL ¼ 4.6) witha large amount of highly acidic Sa exhibited the very highconductivity of 1.4  101 S cm1 at 95  C, even under dry
conditions, which is comparable to that of humidied Naonmembranes and some non-humidied Pa-doped composite
PEMs. It should be noted that anhydrous sDC had a stronger
temperature dependency for S–P–S/Pa than for S–P–S/Sa withthe same acid content, probably because highly acidic Sa would
be almost completely dissociated into a hydrogen sulfate anion
and a free proton in the PEMs, even at low temperatures nearroom temperature. On the other hand, Pa with a lower acidity
would not be eﬀectively dissociated into a dihydrogen phosphate anion and a free proton in the PEMs at low temperatures,
but the dissociation of Pa would become easier as the temperature increased, resulting in an increase in the concentration offree protons in the S–P–S/Pa. The results in this study will help
to design highly proton-conductive PEMs even under dry
conditions at the molecular level for next-generation fuel cells.
In the future, we will investigate the conductivities of such
PEMs at temperatures lower than 20  C or higher than 100  C
under non-humidied conditions. In addition, we will evaluate
the mechanical properties of the PEMs, nanostructures, the cell
performance, and the leaching-out of the acidic liquids in thePEMs, which will also provide useful insights on designing highperformance anhydrous PEMs.

Author contributions
The manuscript was written with the contributions of all
authors. All authors have given approval to the nal version of
the manuscript.

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Conﬂicts of interest
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There are no conicts to declare.

Acknowledgements
The authors thank Mr Hikage at High Intensity X-ray Diﬀraction
Laboratory in Nagoya University for his kind help in performing
SAXS measurements. This work was partially supported througha project (JPN P20003) subsidized by the New Energy andIndustrial Technology Development Organization (NEDO), and
KAKENHI grant numbers (21K05197 (A. N.) and 20J11033 (T.
K.)) from JSPS, Japan. A. N. is also grateful for Toyota Most
Advanced Collaborative Research Grant.

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