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Title

Influence of Confined Polymer Structure on Proton
Transport Property in Sulfonated Polyimide Thin
Films

Author(s)Krishnan, Karthik; Yamada, Tomoaki; Iwatsuki,
Hiroko; Hara, Mitsuo; Nagao, Shusaku; Otsubo,
Kazuya; Sakata, Osami; Fujiwara, Akihiko;Kitagawa, Hiroshi; Nagao, Yuki

Citation

Electrochemistry, 82(10): 865‑869

Issue Date

2014‑10‑05

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/14065

RightsCopyright (C) 2014 The Electrochemical Society ofJapan. Karthik KRISHNAN, Tomoaki YAMADA, Hiroko
IWATSUKI, Mitsuo HARA, Shusaku NAGANO, Kazuya
OTSUBO, Osami SAKATA, Akihiko FUJIWARA, Hiroshi
KITAGAWA, Yuki NAGAO, Electrochemistry, 82(10),2014, 865‑869.
http://dx.doi.org/10.5796/electrochemistry.82.865

Description

Japan Advanced Institute of Science and Technology

Influence of Confined Polymer Structure on Proton Transport Property in SulfonatedPolyimide Thin Films

Karthik KRISHNAN,a Tomoaki YAMADA,b Hiroko IWATSUKI,c Mitsuo HARA,c Shusaku
NAGANO,d Kazuya OTSUBO,e Osami SAKATA,f Akihiko FUJIWARA,g Hiroshi
KITAGAWA,e Yuki NAGAOa,*a

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1

Asahidai, Nomi, Ishikawa 923-1292, Japan.
b

Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho,

Chikusa, Nagoya 464-8603, Japanc

Department of Molecular Design & Engineering, Graduate School of Engineering, Nagoya

University, Furo-cho, Chikusa, Nagoya 464-8603, Japand

Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa,

Nagoya 464-8603, Japane

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japanf

Synchrotron X-ray Station at SPring-8, National Institute for Materials Science (NIMS), 1-1-1

Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japang

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho,

Sayo-gun, Hyogo 679-5198, Japan*Corresponding author: ynagao@jaist.ac.jp1Abstract
The organized structure and proton transport property in sulfonated polyimide (SPI) thin film
have been studied, much less is known at the interfacial charge transport during confinement ofsuch materials. The proton conductivity of SPI thin film is correlated well with the highlyoriented polymer structures as determined by infrared (IR) p-polarized multiple-angle incidence
resolution spectrometry (p-MAIRS) and in situ RH dependent synchrotron grazing incidence Xray diffraction (GI-XRD). The high proton conductivity of 2.8 × 10−1 S cm−1 at 80 ºC and 90%
RH was achieved in the highly oriented SPI thin film. In contrast, the bulk SPI showed the dropof conductivity value at high relative humidity region, which is due to the significant structural
disorder accompanied by the strong interaction between the sulfonic acid side chains and watermolecules.

Keywords
Conductivity enhancement, Confined polymer thin film, Organized structure2

1. Introduction
Molecular nano-structure organizations have grown in importance for efficient ionictransportation.1-4 Especially, molecular orientation and interfacial structure in the nanostructured
materials have been an active area of research for proton transportation in fuel cellapplications.5-7 In polymer ultrathin films, the interfacial confinement at the polymer-substrateboundary and free surface effects causes the specific interaction between the adjacent polymer
chains, which results the distinct polymer properties relative to that of bulk materials.8 For
instance, in our previous studies, we observed the enhancement of proton conductivity inpyromellitic dianhydride based sulfonated polyimide (PMDA/SPI) thin film when the films
confined into the nanoscale regime. Owing to the interfacial polymer chain confinement in thein-plane direction favors the formation of long-range proton conduction path and thus exhibitedthe high proton conductivity.9 Consequently, the preferred chain packing along the in-plane/outof-plane direction in the confined film could be an additional impact for forming themacroscopic proton transport path as like proton wires to improve the charge transportcharacteristics in proton exchange membrane fuel cells (PEMFCs).10,11 Therefore, achieving thedirected assembly of polymer chains in thin films has attracted intense interest in recent
years.12,13
Many investigations concerning the relationship between chemical structures and application
performances for polyimide (PI) systems are devoted because the chemical structure of PI is oneof the essential factors associated with the secondary and tertiary structures and the physicalproperties.14-16 For PEMFC applications, the sulfonated polyimides have received much attention
due to its excellent thermal, chemical, mechanical stability and resistive to swelling.17-19 In
particular, the PI based systems have been prepared using the sulfonic acid groups as side chains3directly bound to the polymer backbone.20,21 This has led to the hydrolytic stability and it greatly
depends on the basicity of the amido nitrogen. Also, the enhancement of hydrolytic stability canbe possible when the spacer group is positioned in between the –SO3H group and phenyl ring.22
In general, the sulfonated groups in the polymer electrolyte exhibit excessive swelling underfully hydrated conditions, which deteriorates the performance of fuel cell operation. Therefore, anumber of structural investigations have been done on SPI to improve the proton conductivity
and stability profiles of SPIs.23,24
This work elucidates the relationship between polymer orientation and proton transportcharacteristics in SPI systems. The p-MAIRS25,26 and in situ RH dependent synchrotron GI-XRD
were taken into account to investigate the polymer orientation and internal structure when
confined into thin film. The diffraction patterns of SPI film can be attributed to the lamellar
structure falls parallel to the substrate; in addition, the ordering of lamellar phase significantly
varies with the humidity change. The impedance studies under both RH and temperaturedependent were used to characterize the proton transport property in SPI. This insight has thepotential to understand more clearly on the structure-property relation in such systems.

2. Experimental
2.1. Materials
3,3'-Dihydroxybenzidine, propanesultone, and 1,4,5,8-Naphthalenetetracarboxylic dianhydride(NTDA) were used as received from TCI, Japan and Sigma–Aldrich, respectively. Acetic acid,acetic anhydride, nitric acid, methanol, ethanol and acetone were purchased from Wako
Chemicals, Japan. Hydrochloric acid (Nacalai Tesque, Japan), sodium hydroxide (Kishida4Chemical, Japan), m-cresol, and triethylamine (TEA) (Kanto Chemicals, Japan) were used asreceived.
2.2. Synthesis of sulfonated diamine monomers
Scheme 1 represents the steps involved in preparing of the sulfonated naphthalenic
polyimides. The reaction starts with the mixtures of 3,3'-dihydroxybenzidine (5 g) acetic acid(1.5 ml), acetic anhydride (79 ml) and distilled water (200 ml). Subsequently, this reaction
mixture was stirred at 70 °C for 1 hr. The obtained precipitates were washed several times withcooled acetone and separated using centrifugation. Presence of modified acetyl functional group
was confirmed by using 1H NMR. Then, the sulfonation was carried out using acetylated 3,3'dihydroxybenzidine, propanesultone (7.3 g), sodium hydroxide (2.4 g) and methanol (150 ml).
The mixed products were refluxed constantly at 80 °C under an inert atmosphere for 6 hr.
Obtained final precipitates were washed with cold methanol to remove the unreacted species.
Then, the sulfonated monomers were dissolved in water to precede the ion-exchange process(using Amberlyst). For removing the acetate group, we used the concentrated hydrochloric acid(HCl) to the ion-exchanged sulfonated product. Refluxing was conducted at 120 °C for 1 hrunder an inert atmosphere. After the mixture was refluxed, HCl was evaporated at 150 °C; inaddition, the HCl vapor was carefully neutralized with a concentrated base. Prior to each step, allof the prepared monomers were characterized by 1H NMR. The degree of sulfonation for the
final product was estimated from 1H NMR data and it is more than 96%. The calculated ion
exchange capacity is ~2.8 mequiv/g.
2.3. Synthesis of Sulfonated naphthalenic polyimides
The sulfonated monomers (1 mol) were polymerized with the required amount of NTDA (1mol), m-cresol (5 ml), and TEA (0.3 ml). m-Cresol was used as solvent, and the polymerization5

reaction was carried out under an inert atmosphere at 150 °C for 5 hr. The polymerized productwas collected after being washed with cooled acetone. The final product was obtained by an ionexchange process at least 8 times using a mixture solution of concentrated nitric acid and ethanol(1/12 by volume). The obtained product was washed with ethanol and dried in vacuumed
desiccator. GPC measurement was used to estimate the molecular weight of the preparedpolymer, which was found to be 1.5 × 105.
2.4. Preparation of thin films
SPI thin film on quartz substrate was prepared using an Active ACT−200 spin-coater. Prior
to film deposition, the substrate was subjected to several cleaning processes, and plasma
treatment was performed using a vacuum plasma system (Cute-MP; Femto Science, Korea) toimprove surface hydrophilicity. The thickness of the thin film was measured using a surface
profiler (KLA-Tencor P-15 profiler).
2.5. Conductivity measurements of the thin films
Proton conductivity was estimated through impedance spectroscopy measurements using a
Solartron 1260 frequency response analyzer equipped with a high-frequency Solartron 1296
dielectric interface. The relative humidity (RH) and temperature were established using acomputer-controlled environmental test chamber (SH-221; Espec Corp). The conductivity
measurements were performed at 298 K and at various RH values. For the thin film
measurements, the electrode configuration was selected to obtain measurements of the current
flow in the plane parallel to the substrate surface and two-probe method was employed to obtain
the impedance data. Gold contacts were used as the thin film electrodes for the conductivity
measurements. The impedance data were collected between the frequency range of 1 Hz and 10

6

MHz, with an applied alternating potential of 50 mV. Thin-film conductivity (σ) was estimatedby the following relation,

σ=

d
Rlt

(1)

where d is the distance between the gold electrodes, R is the resistance value obtained directlyfrom the impedance measurement, l is the length of the contact electrodes, and t is the thicknessof the film (Figure 1).
2.6. p-Polarized multiple angle incidence resolution spectrometry (p-MAIRS)
The polymer orientation in thin films has been studied using p-MAIRS. The Si wafers wasused as substrate and the surface of the original Si wafer was oxidized before the measurement.
The p-MAIRS measurements were obtained with a Fourier-transform infrared (FTIR)
spectrometer (Nicolet 6700; Thermo−Fisher Scientific) equipped with a mercury cadmium
telluride (MCT) detector. The single-beam spectra were collected from 38° to 8° in 6° steps
between the angle of incidence.
2.7. Fourier-transform infrared attenuated total reflectance (FTIR-ATR)
The bulk SPI has been studied using FTIR-ATR. The spectrum was obtained with a Fouriertransform infrared (FTIR) spectrometer (Nicolet 6700; Thermo−Fisher Scientific) equipped witha deuterated triglycine sulfate (DTGS) detector and an optional SMART iTR. Sample waspelletized, and then the flat surface of the pellet was used for the ATR measurement.
2.8. The in situ synchrotron grazing incidence X-ray diffraction (GI-XRD)
The XRD measurements at a fixed grazing incident angle (α) for thin films were carried out
with various relative-humidity (RH) at room temperature using a six-axis diffractometer installed
at a BL13XU beamline in SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo,
7Japan). The incident angle (α) was set to 0.4º. RH was controlled by BEL-Flow apparatus (BEL
Japan, Japan). The wavelength (λ) of the radiation used in this experiment was equal to 0.1550
nm. The data were collected as α-2θ mode in 1 s as step time and the 2θ step angle was 0.2º witha NaI(Tl) scintillation detector.

3. Results and discussion
The difference in proton conductivity between the pelletized bulk SPI (0.40 mm) and thinfilm (1.0 µm) have been studied using both RH and temperature dependent measurements.
Figure 2 represents the RH dependent proton conductivity plots for both bulk SPI and thin filmcases. It can be clearly seen that the Log conductivity increases linearly up to 95% RH in SPI
thin film, unlike the bulk SPI. The conductivity drops appeared at after 80% RH in the bulk SPI
sample. The strong interaction between the water molecules and sulfonated groups in SPI
polymer chains under high humidified condition causes the structural instability, which may be
due to the origin of excessive swelling. This structural instability exhibits the drop of
conductivity at high humidity range. The maximum proton conductivity is achieved in the SPI
thin film (2.0 × 10−1 S cm−1, at 95% RH and 298 K), which is nearly one order of magnitude
higher than the conductivity of bulk SPI (1.2 × 10−2 S cm−1, at 80% RH and 298 K). It is
anticipated that the specific interaction between the polymer chain at interfaces in thin film
regime influence the variation of proton conductivity than bulk sample.
The temperature dependent proton conductivity plots with a constant RH of 90% for bothbulk SPI and thin film are shown in Figure 3. It is observed that the proton conductivity increases
with the increase in temperature and reaches its maximum value (2.8 × 10−1 S cm−1 at 80 ºC) in
the SPI thin film. On the other hand, the proton conductivity of bulk SPI shows the value of 9.88

× 10−3 S cm−1 at 50 ºC, which is almost one order of magnitude lower than the SPI thin film (2.2
× 10−1 S cm−1 at 50 ºC). Similar observation has already been reported to be the value of proton
conductivity of SPIs in water were around 2.0 × 10−1 S cm−1 at 50 ºC and RH above 80%, whichis due to the high ion exchange capacity and high water uptake capability accompanied with thewater stability.20 But, the results presented in this work shows the enhanced proton conductivity
value than the previous report. Because, the thin film confinement at nanoscale regime can be
responsible for obtaining both the enhanced structural stability and proton conductivity. It is also
expected that the large scale interrupted domains in the randomly oriented bulk SPI cannot leads
to the long-range proton transport path, which possibly disrupt the fast ion conduction (Figure 4).
According to the Arrhenius equation (2), the activation energy (Ea) was estimated and the values
for the bulk SPI and thin film are 0.11 eV and 0.10 eV, respectively.
−𝐸𝐸𝑎𝑎𝜎𝜎𝜎𝜎 = 𝜎𝜎0 𝑒𝑒𝑒𝑒𝑒𝑒 �

𝑅𝑅𝑅𝑅

�

(2)

where Ea, R, T and σ0 represents the activation energy, gas constant, temperature and preexponential factor, respectively. The conductivity patterns are quite similar with our previous
observation in the PMDA/SPI samples.9
The infrared p-MAIR spectroscopy used in this investigation is an important tool for theidentification of molecular orientation in thin films. Thereby, the in-plane (IP) and out-of-plane(OP) SPI orientation in thin film has been investigated using p-MAIRS (Figure 5). The typicalnaphthalimide absorption bands were observed at 1716, 1676 and 1349 cm−1, which corresponds
to the symmetric, asymmetric vibrations of C=O groups and vibration of C−N group of the sixmembered imide rings, respectively. The observed broad band at 1248 and 1195 cm−1
corresponding to the asymmetric and symmetric vibration of O=S=O in the sulfonic acid9

groups.27,28 The polymer orientation in SPI thin film has been distinguished according to thechanges in the intensity of p-MAIR spectra. In principle, the maximum intensity of the p-MAIR
signal arise when the electric field vector of IR rays and the orientation of the molecularvibration are parallel.29 Consequently, the IP vibrational modes of C=O and C−N groups areapproximately two times stronger intensity than the OP component, revealing that the SPI thinfilm makes the highly oriented structure parallel to the substrate plane. Furthermore, there isnoticeable peak shift to the higher wavenumber region (1680 cm−1) is found in the C=O
asymmetric mode27,28 of OP component relative to that of the IP component (1676 cm−1). Theband shift in the p-MAIR spectra is regarded due to the Berreman effect.30,31 Also, this highwavenumber shift suggests that the longitudinal mode may contributes a slight surface
perpendicular component.32
In the bulk SPI, the FTIR-ATR measurement was done and the characteristic vibrationalmodes are shown in Figure 6. Owing to the random orientation of the bulk SPI, consideration of
the intensity of vibration mode is insignificant for investigation. It is also essential to mention
that the frequency downshift is observed for both the asymmetric C=O (1711 cm−1)27 andsymmetric C=O (1666 cm−1) stretching vibrations of bulk SPI than that of the stretching modes
in oriented film.
The change in internal polymer structure and crystalline order of the SPI thin film wasdetermined by in situ RH dependent synchrotron GI-XRD and thereby correlating with theproton transport property. Figures 7(a) and 7(b) shows the diffraction pattern and d-spacing
values for SPI thin film with various RH conditions. The measurement has been taken only in theout-of-plane direction and it plays a vital role for understanding the variation in the molecular
structure with the change in RH. Essentially, the 1D GI-XRD pattern shows the shift in peak10

position towards the lower angle region with the increase in RH. Initially (at 0% RH), the peak islocated at 7.1º (2θ) and it tends toward the lower angle region with the value of 4.6º at fully
hydrated region (at 95% RH). The corresponding d-spacing values for these two peaks are 1.25
nm and 1.93 nm, respectively. This large deviation in the polymer structure is due to theperturbed interlamellar stacking in the out-of-plane direction by the introduction of watermolecules. Also, it suggests that the spacing between the interlamellar stacking increases with
the increase in RH, which reasons the shift in peak position. To gain further insight into theeffect of RH in the SPI thin film, the relative intensity of the diffraction pattern must be
considered. The relative intensity of the diffraction increases gradually with an increase in the
RH. This implies that the degree of crystallinity improved by RH and causes the aggregatedstructures such as liquid-crystal-like and ordered crystalline structures.33 The increase inhumidity results the remarkable interaction between the aggregated lamellar structures and watermolecules, which possibly rearranged the crystalline order. Understanding of this structuralproperty well correlates with our earlier finding of the expansion of lamellar structure in the outof-plane direction when the PMDA/SPI system hydrated fully.9 The aggregation and oriented
molecular structure in SPI thin films have been investigated; in addition, the strong
intermolecular interaction reasons the higher degree of spontaneous chain orientation, which
could improve the performance properties of polymer electrolytes. For instance, some reports
were evidenced that the oriented PI structures parallel to substrate plane could an additional
impact for the improving the physical properties, such as birefringence and thermal expansion
coefficient.34,35 Mostly, the rigid and planar PI molecular structures can be responsible for thehigher degrees of in-plane orientation than those having bent and three dimensional molecular
structures.33
11This study reveals that the organized structure in SPI thin film significantly exhibit the highproton conduction. The chemical stability also plays an essential role in obtaining the highproton conduction, which derived from the spacing between the sulfonic acid group andpolyimide main chain. Furthermore, the lyotropic lamella structure parallel to the substrate plane
formed the long-range proton transport path when the SPI confined into film. As mentioned in
the previous studies,14,33,36 the planar PI molecular structure mostly orients the polymer chain in
the in-plane direction, which possibly enhances the proton conductivity in such thin film system.
4. Conclusions
The significance of molecular orientation on the proton transport property in sulfonatednaphthalenic polyimide thin film was studied using p-MAIRS and in situ RH dependent
synchrotron GI-XRD. The p-MAIR spectra disclose the highly oriented polymer structure in the
IP direction when the SPI polymer confined into thin film, whereas the bulk SPI showed
randomly oriented polymer structure, confirmed by FTIR-ATR. Hydration effect on theinterlamellar stacking influenced the variation of diffraction patterns in GI-XRD. It is speculated
that the specific interaction between the water molecules and side chain sulfonic acid groups in
SPI rearranged the molecular structure in the out-of-plane direction, can be termed as a lyotropic
liquid-crystal-like phase. Furthermore, the enhancement of crystalline order with an increase in
RH can be due to the ch-packing between the continuous polyimide chains aligns more readilyon the out-of-plane direction. As consequence, the highly oriented polymer structure in SPI thinfilm influenced the enhancement of proton conductivity, which exhibits one order of magnitude
more conductivity value than the randomly oriented bulk SPI system. These structure-property
relationships promise to assist in the development of fuel cell performance with rationallydesigned highly oriented polymer materials and enhanced proton transport properties.
12

Acknowledgments
Synchrotron XRD measurements were supported by the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal No.: 2012A1072). This work was financially supported by
the Japan Society for the Promotion of Science (JSPS) through the Funding Program (GR060)
for Next Generation World-Leading Researchers (NEXT Program), initiated by the Council for
Science and Technology Policy (CSTP). This work was partially supported by the Ogasawara
Foundation for the Promotion of Science and Engineering.

13

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16

Scheme caption
Scheme 1. Synthesis of sulfonated naphthalenic polyimides

Figure caption
Figure 1. Schematic representation of highly oreinted thin film used to measure in-plane proton
conductivity.
Figure 2. RH dependent proton conductivity plots for both the pelletized bulk SPI and thin film.
Figure 3. Arrhenius plots of conductivity for both the pelletized bulk SPI and thin film.
Figure 4. (a) Proposed structure of the long range proton transfer path in the highly oriented SPI
interfacial structure and (b) interrupted domains in the randomly oriented bulk SPI
Figure 5. Infrared p-MAIR spectra for the SPI thin film, measured from 38° to 8° in steps of 6°.
Figure 6. FTIR-ATR spectrum for the bulk SPI.
Figure 7. (a) diffraction patterns and (b) the estimated d-spacing values of SPI thin film withvarious RH using in situ synchrotron GI-XRD.

17

O

HO
H2N

NH2

AcOH/Ac2O

OH

H
N
Ac
HO

H
N
Ac
OH

S

O
O

NaOH
MeOH

H
N
Ac

H
N
Ac
O

Ion ExchangeO

3 3'-Dihydroxybenzidine

1

2

NaO3S

SO3Na

HO3S
H
N
Ac

H
N
Ac
O

O

H2N
HCl

NH2
O

O

O

O

O

O ,TEA

O

120oC
HO3S

SO3H

3

HO3S

O
m-cresol
150oC

O

Ion Exchange

O

O

N

N

O O

O

SO3H

4

5

Scheme 1. K. Krishnan et al.

Figure 1. K. Krishnan et al.
18

SO3H

n

Figure 2. K. Krishnan et al.

Figure 3. K. Krishnan et al.
19

Figure 4. K. Krishnan et al.

Figure 5. K. Krishnan et al.

20

Figure 6. K. Krishnan et al.

(b)

(a)

RH95%
RH70%
RH40%
RH0%

Log (Intensity)

Log (Intensity)

RH95%
RH70%
RH40%
RH0%

3

4

5

6

7

8

2θ / degree

9

10

0.5

1

1.5

2

2.5

d / nmFigure 7. K. Krishnan et al.
21

3

3.5

4