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Title

Cross-linkable Sulfonated Polyimide Thin Film With
High Proton Conductivity

Author(s)Yao, Yuze; Hara, Mitsuo; Nagano, Shusaku; Aoki,
Kentaro; Nagao, Yuki

Citation

Conference Proceedings of STEPI-12, 12: 49-59

Issue Date

2024

Type

Conference Paper

Text version

author

URL

http://hdl.handle.net/10119/19046

RightsThis paper is posted here with permission of
University of Montpellier. Copyright (C) 2024
University of Montpellier. Conference Proceedingsof STEPI-12, 2023, 12, pp. 49-59.

Description

STEPI-12 (12th Conference on POLYIMIDES) ,Montpellier, France, June 4-7, 2023

Japan Advanced Institute of Science and Technology

Cross-linkable Sulfonated Polyimide Thin Film With High Proton
Conductivity
Yuze YAO,a Mitsuo HARA,b Shusaku NAGANO,c Kentaro AOKI,a Yuki NAGAOaa School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,

Ishikawa 923-1292, Japan, ynagao@jaist.ac.jp

b Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japanc Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-ikebukuro, Toshima,

Tokyo 171-8501, Japan

Sulfonated semi-alicyclic polyimides have issues with poor mechanical and chemical stability inwater. As described herein, we synthesized a new sulfonated semi-alicyclic polyimide
composed of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BOEDA) and 3,3'bis-(sulfopropoxy)-4,4'-diaminobiphenyl (BSPA), of which the contained dienophile structure
can undergo Diels–Alder (D-A) reaction. Facile cross-linking reaction was conducted via Fe3+catalyzed D-A reaction between BSPA-BOEDA and silica nanoparticles (NPs) modified by (3cyclopentadienylpropyl) triethoxyslane (CPTS). Compared with BSPA-BOEDA, the cross-linked
BSPA-BOEDA-NP membrane and its thin film showed better stability in water. Humiditycontrolled grazing incidence X-ray scattering (GIXRS) revealed that an isotropic phasesegregation of 2 nm-1 formed in both BSPA-BOEDA and BSPA-BOEDA-NP thin films. The BSPABOEDA and BSPA-BOEDA-NP thin films exhibited high proton conductivity of 4 × 10-2 S cm-1
and 1 × 10-2 S cm-1, respectively, under 25 °C and 95% relative humidity (RH).
1. Introduction
Alkyl sulfonated polyimides (ASPIs) exhibit high proton conductivity and well-definedlamellar-organized structures under humidified conditions because of a lyotropic liquid
crystalline (LC) property [1-10]. In practical use for polymer electrolyte fuel cells (PEFCs), thepolymer electrolyte ionomer in electrochemical reaction parts is always exposed under highhumidity conditions. Therefore both mechanical and chemical stabilities are required in theswelling state. However, ASPIs are adversely affected by poor mechanical and chemical
stability in water.
Sulfonic acid groups on the side chains of ASPIs can readily adsorb water molecules
through hydrogen bond interaction under high-humidity conditions. The water adsorption leads
to excessive swelling or even dissolution, resulting in a decrease in mechanical properties.
Generally, cross-linking can inhibit the plasticization of polymers and can enhance theirchemical and mechanical stabilities [11-13]. By introducing functional groups that are usefulfor cross-linking in the main or side chains of SPI and by selecting appropriate cross-linking
agents, cross-linking effects on SPI properties can be explored systematically.
Some reports have described cross-linked SPI through the chemical cross-linking
method [14-16]. Okamoto et al. obtained membranes with markedly improved oxidative
stability by covalently cross-linked SPIs containing benzimidazole groups in polyphosphoricacid.[17] Guan et al. obtained SPI membranes with improved solvent resistance, oxidative
stability and mechanical stability by introducing pendant tetrafluorostyrol groups in the side
chains and thermally cross-linking at 260 °C [18]. Han et al. investigated the details of theeffects of cross-linking on SPI membranes based on the reaction between acridine groups anddibromoalkanes with different lengths. The hydrolytic stability of all cross-linked SPI films was
better than that of the uncross-linked SPI membranes [19]. Generally, these crosslinked
membranes exhibit lower water uptake and proton conductivity than uncross-linked
membranes. These outcomes are also caused by the introduction of SPI components that areincapable of acting as proton-conducting carriers.
This study was conducted to develop a new design for cross-linked ASPI by Diels–Alder(D–A) reaction to improve its mechanical properties and stability in solvents without reducing
its proton conductivity. Simultaneously, we explored cross-linking effects on water uptake
using an in situ quartz crystal microbalance (QCM) and the ordered structure using RH-

controlled grazing-incidence X-ray scattering (GI–XRS) of ionomer thin films. Findings revealed
that the cross-linked structure increased the stability of ionomer thin film in water while it did
not affect water uptake or proton conductivity markedly. The cross-linked thin film still
exhibited comparable proton conductivity to those of the reported materials.
2. Experimental
2.1 Materials
Triethylamine (TEA) and NaNO3 were used as received from Kanto Chemical Co. Inc.,
Japan. Hydrochloric acid, m-cresol, acetic acid, acetic anhydride, methanol, acetone, N,N’dimethylformamide (DMF), and tetrahydrofuran (THF) were obtained from Fujifilm Wako Pure
Chemical Corp., Japan. 1,2,3,4-Cyclopentanetetracarboxylic dianhydride was purchased from
Tokyo Chemical Industry Co. Ltd., Japan. Cross-linking silica nanoparticles modified by 3(cyclopentadienylpropyl)triethoxysilane (CPTS) were obtained from Admatechs Co., Ltd.
2.2 Synthesis of monomer and cross-linkable polymer
3,3'-Bis(sulfopropoxy)-4,4'-diaminobiphenyl (BSPA) was synthesized according
methods described in earlier reports, as shown in Figure 1 [1]. The final yield was 47%.

to

Scheme 1: Synthetic route of BSPA monomer
The synthetic route of BSPA-BOEDA (polyimide) is shown in Scheme 2. First, 0.46 g (1mmol) of BSPA and 0.25 g (1 mmol) of BOEDA were added to a three-necked flask withmagnetic stirring. Then 10 ml of m-cresol was added to the three-necked flask as a solvent. It
was then filled with 600 µl of triethylamine. Under the protection of argon atmosphere, thereaction was first performed at 80 C for 2 h. Then the temperature was raised to 180 C. The
reaction was continued for 20 h. After the reaction, the resulting solution was cooled to room
temperature. It was further cooled in an ice bath for 30 min. The cooled solution was added to
100 ml of cold fresh acetone to obtain a white flocculent precipitate. After the solid productwas obtained by centrifugation, it was washed several times with acetone, and then dried
under vacuum overnight. The dried product was then dissolved in water. Ion exchange wasconducted using a glass column packed with ion exchange resin (Amberlyst 31WET; Organo
Corp.). This process was repeated several times to ensure that sulfonate was exchanged tosulfonic acid to the greatest extent possible. The solvent was removed by rotary evaporation.
The final product BSPA-BOEDA was obtained after complete drying.

Scheme 2: Synthetic scheme of BSPA-BOEDA as a new ASPI.
2.3 Membrane preparation
Cyclopenta-1,3-diene and its substituents are commonly used dienes in Diels–Alder (DA) reactions. The route used to synthesize the cross-linked BSPA-BOEDA by the crosslinker ofsilica nanoparticles is shown in Scheme 3. 90 mg of BSPA-BOEDA and 10 mg of nanoparticles(NPs) were dissolved in 2 ml of mixed solution of water and THF (1:1 in w/w). When the BSPABOEDA and NPs were completely dissolved, ferric chloride hexahydrate (FeCl3·6H2O) was
added, followed by continued stirring until completely dissolved. Then, the solution wastransferred into a mold and heated at 50 °C to dry until the solvent was evaporated completely.
The prepared membrane was immersed into 100 ml of 1 M hydrochloric acid (HCl) to remove
Fe3+. After rinsing the membrane with H2O, it was vacuum-dried at 60 °C overnight. Scanningelectron microscopy and energy dispersive X-ray analysis (SEM-EDX) confirmed that Fe3+ was
ion-exchanged completely. The obtained membrane is denoted as BSPA-BOEDA-NPs.

Scheme 3: Preparation of cross-linked BSPA-BOEDA-NPs.
2.5 Thin film preparation
Spin coating (ACT-200 spin-coater; Active Co. Ltd.) method was used to prepare thinfilms. BSPA-BOEDA was dissolved in the mixed solution of water and THF (1:1 in w/w). The
thin film thickness was controlled at ca. 500 nm. The Si (E&M Co. Ltd.), SiO2 substrates(Sendai Sekiei Glass Seisakusho, Japan), and SiO2-coated 9 MHz quartz crystals (Seiko EG&G
Co. Ltd.) were washed in advance with 2-propanol. Before thin film deposition, plasma
treatment (Cute-MP; Femto Science, Korea) was conducted to improve the hydrophilic
properties of the substrate surface.
To obtain cross-linked BSPA-BOEDA-NP thin films with good surface roughness, the NPs
were first dissolved in 300 µl of mixed solution of water and THF (3:4 w/w). Then they were
flowed through a filter film with 1 μm pore size to remove aggregated particles. To the filteredsolution of nanoparticles, 9 mg of BSPA-BOEDA was added and sonicated until it was dissolved
completely. Then the solution was cooled in an ice-water bath for 30 min before the addition of
1 mg of FeCl3·6H2O. The spin coating method was used to prepare thin films, with the
thickness controlled at ca. 500 nm. After the prepared BSPA-BOEDA-NP thin films were
immersed into 100 ml of 1 M hydrochloric acid (HCl) to remove Fe3+, they were washed withwater and vacuum dried at 60 °C overnight. Scanning electron microscopy and energy
dispersive X-ray analysis (SEM-EDX) confirmed that Fe3+ was completely ion exchanged.
2.6 Characterization
2.6.1 NMR spectroscopy
1
H NMR spectra (Bruker Avance III 400; Bruker Analytik GmbH) were collected toascertain whether the monomer and polymer were synthesized successfully. Deuterated
dimethyl sulfoxide (DMSO-d6) containing 0.03% of trimethylsilane (TMS) was used.
2.6.2 Infrared (IR) spectroscopy
Fourier transform infrared spectroscopy of monomer and polymer was applied byattenuated total reflection (FT–IR ATR, Nicolet 6700; Thermo Fisher Scientific Inc.) with a
ZnSe crystal. The analysis software used was OMNIC (Thermo Fisher Scientific Inc.). The
background spectrum and the sample spectrum were scanned 64 times. The obtained
spectrum was corrected using Advanced ATR Correction in the software. The measurement
range was 900–2100 cm-1.
2.6.3 Gel permeation chromatography (GPC)
Molecular weight of BSPA-BOEDA was evaluated by gel permeation chromatography(GPC). An LC-2000plus (Jasco Corp.) was used with Shodex GF-1G 7B and GF-7M HG columns.

The eluent was obtained by mixing 10.26 g of NaNO3, 600 ml of water, 400 ml of DMF, and 18
mL of acetic acid. The sample molecular weight was calibrated using standard polystyrene.
2.6.4 Water uptake
The water uptake was measured using a humidity-controlled in situ quartz crystal
microbalance (in situ QCM) system. Various degrees of relative humidity (RH) were generatedusing a humidity generator (Bel Flow; MicrotracBEL Corp.) at 298 K. The QCM substrate wasplaced in a homemade humidity control chamber equipped with a high-resolution RH sensor
and was connected to the oscillation circuit and a frequency counter (5313A; Agilent
Technologies) through a circuit. The weight change of the film is calculable using the
Sauerbrey equation as
Δ

=

×

×

× (−Δ ),

(2-1)

where S represents the electrode surface area. Also, ρ and μ respectively stand for the quartz
density and quartz shear modulus. F stands for the fundamental frequency of QCM substrate.
The number of water molecules adsorbed per sulfonic acid group (denoted as λ) was calculatedusing the formula shown below.
=
−1 ×
,
(2-2)

Therein, m represents the mass of the film under each humidity condition, m0 represents thefilm mass at the dry condition, M(H2O) is the molecular weight of water molecules, and EW
expresses the equivalent weight of each polymer.
2.6.5 Density functional theory (DFT) calculation
The optimized molecular structures were calculated (Material Studio 2020; Dassault
Systèmes). The calculations were done based on DFT using a DMol3 module. The generalized
gradient approximation (GGA) functional with the Perdew–Burke–Ernzerhof (PBE) type wasused to model the exchange and correlation interactions. The convergence threshold for the
maximum force and maximum displacement for normal geometry optimization were set,
respectively, as 0.002 Ha Å-1 and 0.005 Å.
2.6.6 Proton conductivity
The proton conductivity of the BSPA-BOEDA and BSPA-BOEDA-NP thin films in the
direction parallel to a substrate surface was measured using alternating current (AC)electrochemical impedance spectroscopy (EIS). The frequency response analyzer and highfrequency dielectric interface (SI1260 and SI1296; Solartron Analytical) were used to measure
the impedance. A humidity-controlled and temperature-controlled chamber (SH-221; Espec
Corp.) was used to control the humidity and temperature during the experiment. The
impedance was measured when AC voltage of 50 mV was applied. The frequency was scannedin the range of 1 Hz to 10 MHz. Also, the RH was controlled to 40% – 95%. The temperature
was fixed at 298 K. The collected impedance values (R) were converted to the conductivity of athin film directly using
= ,
(2-3)
where t and l respectively stand for the film thickness and contact electrode length, and whered represents the distance between the two gold electrodes.

3. Results and discussion
3.1 1H NMR spectra
The expected structure and 1H NMR spectra of the BSPA-BOEDA before and after ion
exchange are shown in Figure 1. Peaks from the possible by-products of amide or carboxylgroup were not observed, which proved that the imidization reaction was complete. The
integration of proton peaks in 1H NMR spectra showed good agreement with the number ofprotons in both the expected backbone and side chain of BSPA-BOEDA structure. Before ion
exchange, the prepared polymer contained the triethylammonium as a counter cation of SO3unit, denoted as red arrows. After ion exchange to proton, the amount of triethylammonium
had decreased considerably, to less than 1% of that before ion exchange.

Figure 1: 1H NMR (DMSO-d6) spectra for BSPA-BOEDA before and after ion exchange. Red
arrows indicate the positions of triethylammonium.
3.2 FT-IR ATR spectra
The FTIR-ATR spectra of BSPA and BSPA-BOEDA are shown in Figure 2. Absorption
peaks of νsym(C=O), νasym (C=O) and ν(C-N), which stood for the imide group, were observed
respectively at 1776 cm-1,1710 cm-1, and 1382 cm-1 [20-23]. Because δ(N-H) observed at
1629 cm-1 of BSPA monomer disappeared in BSPA-BOEDA, the imidization reaction was
indicated as completed. In addition, because δ(C-N-H) (1530 cm-1) of amic acid, which is anintermediate of imidization reaction, was also not observed, we inferred that the imidization
reaction was completed. Absorption peaks at 1502 cm-1 and 1573 cm-1 derived from thevibration modes of the phenylenediamine benzene ring skeleton ν(C-C) were observed [24,25]. Asymmetric stretching vibration bands of the sulfonated group were also observed
respectively at 1230 cm-1 and 1189 cm-1 [26]. The symmetric stretching vibration peak ofsulfonated groups was observed at 1031 cm-1 [27]. Results of the FTIR-ATR measurements
demonstrated that imidization reaction proceeded sufficiently because amide and amic acid
were not observed. Furthermore, no by-product or unreacted product peak was observed.

Figure 2: FTIR ATR spectra for 3,3'-BSPA monomer and BSPA-BOEDA.
3.3 Molecular weight by GPC
Molecular weight of BSPA-BOEDA was confirmed by GPC chromatogram. The calculation
results of molecular weight are shown in Table 1. The weight average molecular weight (Mw) of
BSPA-BOEDA was ca. 7.5 × 104. Because no other prominent peak in the monomer region wasobserved, the polymer was regarded as having polymerized sufficiently.

Table 1: Molecular weight of BSPA-BOEDA by GPC

BSPA-BOEDA

Mw
7.5 × 104Mn
2.5 × 104

Mw/ Mn3.0

3.4 Solubility test
To demonstrate that the D-A reaction can take place between BSPA-BOEDA and crosslinker, we carried out a solubility test by water. Solubilities were compared by immersing
BSPA-BOEDA (before crosslinking) and BSPA-BOEDA-NPs (after crosslinking) as membrane
forms in deionized water (Figure 3). The uncross-linked BSPA-BOEDA membrane dissolved
quickly, even without stirring, but the cross-linked BSPA-BOEDA-NP membrane was difficult todissolve. The marked change in solubility proves successful cross-linking by the D-A reaction.

Figure 3: Solubility test of BSPA-BOEDA membrane and BSPA-BOEDA-NP membrane in water.
3.5 Water uptake
Because the hydrogen bonding networks produced by water promote the transport ofprotons, water uptake strongly affects the proton conductivity. Figure 4 shows the RHdependent water uptake of BSPA-BOEDA and BSPA-BOEDA-NP thin film, as measured using
QCM measurement. The adsorption isotherm of water molecules showed a similar tendency tothe adsorption isotherm of non-porous multimolecular adsorption. This uptake manner was
regarded as a change in the type of adsorbed water around the sulfonic acid groups [25, 26].
The BSPA-BOEDA and BSPA-BOEDA-NP thin film adsorption behaviors were similar withrespect to RH. The water uptake value increased gradually, concomitantly with RH. The water
uptake values of the two thin films were equal under low humidity (< 60% RH), but the BSPABOEDA thin film showed a higher water uptake value (λ = 13.8) than BSPA-BOEDA-NPs (λ =10.5) at 95% RH.

Figure 4: Water uptake of BSPA-BOEDA and BSPA-BOEDA-NP thin films.
3.6 RH in situ GIXRSGIXRS is a powerful tool for detecting molecular packing and ordering in molecularorganized thin films [28, 29]. We took RH-dependent in situ GIXRS measurements of the

BSPA-BOEDA and BSPA-BOEDA-NP thin films to assess semi-alicyclic main chain effects on thelyotropic ordered structure. In Figures 5(a–d) and Figures 5(e–h), 1D and 2D scattering
images are shown respectively for the BSPA-BOEDA and BSPA-BOEDA-NP thin films. One
isotropic broad scattering peak around 2 nm-1 in both out-of-plane and in-plane directions wasobserved in both BSPA-BOEDA and BSPA-BOEDA-NP thin films under 95% RH, indicating thatthe phase segregation was formed. According to our earlier reports, ASPIs can form lamellar
structures because of lyotropic LC property [5, 8, 10]. However, for this study, BSPA-BOEDA
and BSPA-BOEDA-NP thin films only formed a phase segregation structure: not a long-rangeordered lamellar structure along the thin film surface.

Figure 5: 1D GIXRS profiles and 2D GIXRS images obtained BSPA-BOEDA and BSPA-BOEDA NPs thin films at 0% and 95% RH. (a) 1D GIXRS profile in the out-of-plane direction for the
BSPA-BOEDA thin film. (b) 1D GIXRS profile in the in-plane direction for the BSPA-BOEDA thinfilm. (c) 2D GIXRS profile in the out-of-plane direction for the BSPA-BOEDA thin film. (d) 1D
GIXRS profile in the in-plane direction for the BSPA-BOEDA thin film. (e) 1D GIXRS profile in
the out-of-plane direction for the BSPA-BOEDA-NP thin film. (f) 1D GIXRS profile in the inplane direction for the BSPA-BOEDA-NP thin film. (g) 2D GIXRS profile in the out-of-plane
direction for the BSPA-BOEDA-NP thin film. (h) 1D GIXRS profile in the in-plane direction for
the BSPA-BOEDA-NP thin film.
To elucidate reasons why only phase segregation was observed in BSPA-BOEDA and
BSPA-BOEDA-NP thin films, we strove to investigate the planarity of the main chain which
related with the lyotropic LC property. Figure 6 depicts the optimized oligomeric structures of 5
repeating units for BSPA-BOEDA and BSPA-CPDA (Figure 6b [10]) by DFT calculation. The
main chain of BSPA-BOEDA units showed lower planar rigidity. Therefore, the BSPA-BOEDA
and BSPA-BOEDA-NP thin films remained in an isotropic phase-segregated structure rather
than an anisotropic lamellar structure because the main chain rigidity necessary for the
expression of lyotropic LC properties was much lower.

Figure 6: Optimized structures of five repeating units of BSPA-BOEDA and BSPA-CPDA. The
chemical structure of BSPA-CPDA is shown in Figure 6b [10].

3.7 Proton conductivity
The proton conductivity for the BSPA-BOEDA and BSPA-BOEDA-NP thin film is shown in
Figure 7 as a function of humidity at 25 °C. The proton conductivity increased concomitantlywith increasing RH, which was observed in typical proton-conducting polymers. The BSPABOEDA thin film exhibited higher proton conductivity than BSPA-BOEDA-NP thin film under all
humidity conditions. The maximum proton conductivity reached 4 × 10-2 S cm-1 and 1 × 10-2 S
cm-1 under 25 °C and 95% RH, respectively, for BSPA-BOEDA and BSPA-BOEDA-NP thin films.
Compared with ASPIs reported earlier, the proton conductivities of BSPA-BOEDA and BSPABOEDA-NP were lower. This is true because BSPA-BOEDA and BSPA-BOEDA-NP only exhibit anisotropic phase segregated structure, which is unable to promote proton transport effectively
along the in-plane direction. Table 2 shows the summary of thickness, proton conductivity andits measurement conditions of reported cross-linked polyimide membranes [14-16, 19, 30-37].
Because our research assumes a material for ionomers, the thickness is reduced tosubmicrometer order. Therefore, it is markedly thinner than those of other reported
membranes. Reportedly, the proton conductivity of Nafion membranes decreases considerably
as the thickness decreases [38]. This report is the first to describe proton conductivity
measured using a thin film. The results revealed that the proton conductivity obtained iscomparable to that of other thick membranes.
-1log (σ / S·cm-1)-2

BSPA-BOEDA
BSPA-BOEDA-NPs

-3
-4
-5
-6
30

40

50

60

70

80

90

100

RH / %

Figure 7: Proton conductivity of BSPA-BOEDA and BSPA-BOEDA-NP thin films at 25 °C.
Table 2: Comparison of form, thickness, and proton conductivity of cross-linked materialsσ
T
RH
Material
Form, Thickness/ S cm/ °C / % 1
BSPA-BOEDA-NPs
Thin film, approx. 500 nm(this work)
Ref. 15
Membrane, approx. 50 μm
Ref. 16
Membrane, approx. 40 μm
Spin coating, no thickness
Ref. 19
information
Ref. 25
Membrane, approx. 30 μm
Ref. 30
Membrane, approx. 50 μm
Ref. 31
Membrane, 40–60 μm
Ref. 32
Membrane, 20–60 μm
Ref. 33
Membrane, approx. 30 μm
Ref. 34
Membrane, no thickness information
Ref. 35
Membrane, 35–45 μm
Ref. 36
Membrane, no thickness information
Ref. 37
Membrane, no thickness information

25

95

0.01

80
80

100
100

0.11
0.11

90

100

0.07

30
90
40
60
120
80
60
30
60

90
90
100
100
100
90
100
95
100

0.093
0.1
0.0014
0.16
0.2
0.15
0.14
0.061
0.17

4. Conclusion
As described herein, a semi-cycloaliphatic sulfonated polyimide (BSPA-BOEDA) with adienophile in its backbone was newly synthesized. A simple and readily available cross-linked
thin film (BSPA-BOEDA-NPs) was obtained through the Fe3+-catalyzed D-A reaction. The
stability in water, water uptake, morphology, and proton conductivity were investigated forboth BSPA-BOEDA and BSPA-BOEDA-NP thin films. Compared with BSPA-BOEDA, the crosslinked BSPA-BOEDA-NPs showed better stability in water. The water uptake of the two thinfilms was similar under low-humidity conditions, but the water uptake of BSPA-BOEDA-NP thinfilm was less than that of BSPA-BOEDA under high-humidity conditions. Neither BSPA-BOEDA
nor BSPA-BOEDA-NPs formed well-defined ordered structures: they only formed phaseseparation structures because of their less lyotropic LC property. The proton conductivity of
BSPA-BOEDA-NP thin film was slightly lower than that of BSPA-BOEDA thin film because ofcross-linking, respectively reaching maximum values of 0.01 and 0.04 S cm-1 at 25 °C and
95% RH.
Acknowledgments
The authors thank Admatechs Co., Ltd. for the development and supply of silica
nanoparticles with surface treatment with cross-linker. This work was supported in part by
JSPS KAKENHI Grant Number JP21H00020 and JST CREST Grant Number JPMJCR21B3, Japan.
References[1] K. Krishnan, H. Iwatsuki, M. Hara, S. Nagano, Y. Nagao, Proton conductivity enhancement inoriented, sulfonated polyimide thin films, J. Mater. Chem. A, 2014; 2(19): 6895-6903.
[2] K. Krishnan, T. Yamada, H. Iwatsuki, M. Hara, S. Nagano, K. Otsubo, O. Sakata, A. Fujiwara, H.
Kitagawa, Y. Nagao, Influence of Confined Polymer Structure on Proton Transport Property in
Sulfonated Polyimide Thin Films, Electrochemistry, 2014; 82(10): 865-869.
[3] K. Krishnan, H. Iwatsuki, M. Hara, S. Nagano, Y. Nagao, Influence of Molecular Weight on
Molecular Ordering and Proton Transport in Organized Sulfonated Polyimide Thin Films, J. Phys.
Chem. C, 2015; 119(38): 21767-21774.
[4] Y. Nagao, K. Krishnan, R. Goto, M. Hara, S. Nagano, Effect of Casting Solvent on Interfacial
Molecular Structure and Proton Transport Characteristics of Sulfonated Polyimide Thin Films,
Anal. Sci., 2017; 33(1): 35-39.
[5] Y. Ono, R. Goto, M. Hara, S. Nagano, T. Abe, Y. Nagao, High Proton Conduction of OrganizedSulfonated Polyimide Thin Films with Planar and Bent Backbones, Macromolecules, 2018; 51(9):3351-3359.
[6] Y. Nagao, K. Ohno, S. Tsuyuki, K. Suetsugu, M. Hara, S. Nagano, Effect of Molecular Orientation
to Proton Conductivity in Sulfonated Polyimides with bent backbones, Mol. Cryst. Liq. Cryst.,
2019; 686(1): 84-91.
[7] Y. Nagao, T. Tanaka, Y. Ono, K. Suetsugu, M. Hara, G. Wang, S. Nagano, T. Abe, Introducing
planar hydrophobic groups into an alkyl-sulfonated rigid polyimide and how this affects
morphology and proton conductivity, Electrochim. Acta, 2019; 300: 333-340.
[8] K. Takakura, Y. Ono, K. Suetsugu, M. Hara, S. Nagano, T. Abe, Y. Nagao, Lyotropic ordering forhigh proton conductivity in sulfonated semialiphatic polyimide thin films, Polym. J., 2019; 51
31–39.
[9] R. Goto, Y. Ono, M. Hara, T. Seki, Y. Nagao, S. Nagano, Preparation and structuralcharacterization of shear-aligned films in a high proton conductive alkyl-sulfonated polyimide
with lyotropic liquid crystallinity, Mol. Cryst. Liq. Cryst., 2021; 727(1): 23-32.
[10] Y. Yao, H. Watanabe, M. Hara, S. Nagano, Y. Nagao, Lyotropic Liquid Crystalline Property andOrganized Structure in High Proton-Conductive Sulfonated Semialicyclic Oligoimide Thin Films,
ACS Omega, 2023; 8(8): 7470-7478.
[11] B. B. Narzary, B. C. Baker, N. Yadav, V. D'Elia, C. F. J. Faul, Crosslinked porous polyimides:
structure, properties and applications, Polym. Chem., 2021; 12(45): 6494-6514.
[12] A. Bos, I. G. M. Pünt, M. Wessling, H. Strathmann, Plasticization-resistant glassy polyimide
membranes for CO2/CO4 separations, Sep. Purif. Technol., 1998; 14(1): 27-39.
[13] Y. Xiao, B. T. Low, S. S. Hosseini, T. S. Chung, D. R. Paul, The strategies of moleculararchitecture and modification of polyimide-based membranes for CO2 removal from naturalgas—A review, Prog. Polym. Sci., 2009; 34(6): 561-580.
[14] H. B. Park, C. H. Lee, J. Y. Sohn, Y. M. Lee, B. D. Freeman, H. J. Kim, Effect of crosslinked chain
length in sulfonated polyimide membranes on water sorption, proton conduction, andmethanol permeation properties, J. Membr. Sci., 2006; 285(1): 432-443.
[15] M. Hu, B. Zhang, J. Chen, M. Xu, D. Liu, L. Wang, Cross-linked polymer electrolyte membrane
based on a highly branched sulfonated polyimide with improved electrochemical properties forfuel cell applications, Int. J. Energy Res., 2019; 43(14): 8753-8764.

[16] S.-J. Yang, W. Jang, C. Lee, Y. G. Shul, H. Han, The effect of crosslinked networks withpoly(ethylene glycol) on sulfonated polyimide for polymer electrolyte membrane fuel cell, J.
Polym. Sci. B Polym. Phys., 2005; 43(12): 1455-1464.
[17] G. Zhang, X. Guo, J. Fang, K. Chen, K.-i. Okamoto, Preparation and properties of covalently
cross-linked sulfonated copolyimide membranes containing benzimidazole groups, J. Membr.
Sci., 2009; 326(2): 708-713.
[18] H. Yao, Y. Zhang, Y. Liu, K. You, N. Song, B. Liu, S. Guan, Pendant-group cross-linked highly
sulfonated co-polyimides for proton exchange membranes, J. Membr. Sci., 2015; 480: 83-92.
[19] S. Sundar, W. Jang, C. Lee, Y. Shul, H. Han, Crosslinked sulfonated polyimide networks aspolymer electrolyte membranes in fuel cells, J. Polym. Sci. Part B Polym. Phys., 2005; 43(17):2370-2379.
[20] Y. Yin, Y. Suto, T. Sakabe, S. W. Chen, S. Hayashi, T. Mishima, O. Yamada, K. Tanaka, H. Kita, K.
Okamoto, Water stability of sulfonated polyimide membranes, Macromolecules, 2006; 39(3): 11891198.
[21] J. Sung, D. Kim, C. N. Whang, M. Oh-e, H. Yokoyama, Sum-Frequency Vibrational
Spectroscopic Study of Polyimide Surfaces Made by Spin Coating and Ionized Cluster Beam
Deposition, J. Phys. Chem. B, 2004; 108(30): 10991-10996.
[22] K. Sakamoto, R. Arafune, N. Ito, S. Ushioda, Y. Suzuki, S. Morokawa, Determination of molecularorientation of very thin rubbed and unrubbed polyimide films, J. Appl. Phys., 1996; 80(1): 431439.
[23] T. J. Shin, B. Lee, H. S. Youn, K.-B. Lee, M. Ree, Time-Resolved Synchrotron X-ray Diffraction
and Infrared Spectroscopic Studies of Imidization and Structural Evolution in a Microscaled
Film of PMDA-3,4‘-ODA Poly(amic acid), Langmuir, 2001; 17(25): 7842-7850.
[24] K. Miyatake, T. Yasuda, M. Hirai, M. Nanasawa, M. Watanabe, Synthesis and properties of apolyimide containing pendant sulfophenoxypropoxy groups, J. Polym. Sci., Part A: Polym. Chem.,
2007; 45(1): 157-163.
[25] G. D. Hietpas, J. M. Sands, D. L. Allara, Formation of A Molecularly Reconstructed Surface
Layer during Unidirectional Rubbing of Polyimide Films, Macromolecules, 1998; 31(10): 3374-3378.
[26] J. H. Fang, X. X. Guo, S. Harada, T. Watari, K. Tanaka, H. Kita, K. Okamoto, Novel sulfonatedpolyimides as polyelectrolytes for fuel cell application. 1. Synthesis, proton conductivity, andwater stability of polyimides from 4,4 '-diaminodiphenyl ether-2,2 '-disulfonic acid,
Macromolecules, 2002; 35(24): 9022-9028.
[27] R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto, A. Zecchina, Interaction of H2O, CH3OH,(CH3)2O, CH3CN, and Pyridine with the Superacid Perfluorosulfonic Membrane Nafion: An IR
and Raman Study, The Journal of Physical Chemistry, 1995; 99(31): 11937-11951.
[28] S. Nagano, Inducing Planar Orientation in Side‐Chain Liquid‐Crystalline Polymer Systems
via Interfacial Control, Chem. Rec., 2016; 16(1): 378-392.
[29] S. Nagano, S. Kodama, T. Seki, Ideal Spread Monolayer and Multilayer Formation of Fully
Hydrophobic Polythiophenes via Liquid Crystal Hybridization on Water, Langmuir, 2008; 24(18):10498-10504.
[30] C. H. Lee, H. B. Park, Y. S. Chung, Y. M. Lee, B. D. Freeman, Water Sorption, Proton Conduction,and Methanol Permeation Properties of Sulfonated Polyimide Membranes Cross-Linked with
N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic Acid (BES), Macromolecules, 2006; 39(2): 755764.
[31] H. Pan, S. Chen, Y. Zhang, M. Jin, Z. Chang, H. Pu, Preparation and properties of the crosslinked sulfonated polyimide containing benzimidazole as electrolyte membranes in fuel cells, J.
Membr. Sci., 2015; 476: 87-94.
[32] J. Fang, F. Zhai, X. Guo, H. Xu, K. Okamoto, A facile approach for the preparation of crosslinked sulfonated polyimide membranes for fuel cell application, J. Mater. Chem., 2007; 17(11):1102-1108.
[33] Y. Yin, S. Hayashi, O. Yamada, H. Kita, K. Okamoto, Branched/crosslinked sulfonatedpolyimide membranes for polymer electrolyte fuel cells, Macromol. Rapid Commun., 2005; 26(9):696-700.
[34] Y.-S. Ye, Y.-J. Huang, C.-C. Cheng, F.-C. Chang, A new supramolecular sulfonated polyimide
for use in proton exchange membranes for fuel cells, Chem. Commun.,, 2010; 46(40): 7554-7556.
[35] N. Endo, K. Matsuda, K. Yaguchi, Z. Hu, K. Chen, M. Higa, K. Okamoto, Cross-Linked SulfonatedPolyimide Membranes for Polymer Electrolyte Fuel Cells, J. Electrochem. Soc., 2009; 156(5): B628.
[36] C. H. Lee, S. Y. Hwang, J. Y. Sohn, H. B. Park, J. Y. Kim, Y. M. Lee, Water-stable crosslinked
sulfonated polyimide–silica nanocomposite containing interpenetrating polymer network, J.
Power Sources, 2006; 163(1): 339-348.
[37] M. Kido, Z. Hu, T. Ogo, Y. Suto, K. Okamoto, J. Fang, Novel Preparation Method of Cross-linked
Sulfonated Polyimide Membranes for Fuel Cell Application, Chem. Lett., 2007; 36(2): 272-273.
[38] Y. Nagao, Progress on highly proton-conductive polymer thin films with organizedstructure and molecularly oriented structure, Sci. Tech. Adv. Mater, 2020; 21(1): 79-91.