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Title

Proton conductivity enhancement in oriented,
sulfonated polyimide thin films

Author(s)Krishnan, Karthik; Iwatsuki, Hiroko; Hara,
Mitsuo; Nagano, Shusaku; Nagao, Yuki

Citation

Journal of Materials Chemistry A, 2014(19): 6895‑
6903

Issue Date

2014‑03‑27

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/12610

RightsCopyright (C) 2014 Royal Society of Chemistry.
Karthik Krishnan, Hiroko Iwatsuki, Mitsuo Hara,
Shusaku Nagano and Yuki Nagao, Journal of
Materials Chemistry A, 2014(19), 2014, 6895‑6903.
http://dx.doi.org/10.1039/C4TA00579A ‑ Reproducedby permission of The Royal Society of Chemistry

Description

Japan Advanced Institute of Science and Technology

Journal of Materials Chemistry A

RSCPublishing

ARTICLE

Cite this: DOI: 10.1039/x0xx00000x

Proton Conductivity Enhancement in Oriented,
Sulfonated Polyimide Thin Films
Karthik Krishnan,† Hiroko Iwatsuki,‡ Mitsuo Hara,‡ Shusaku Nagano,‡ and Yuki
Nagao†,*

Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/

Studies of proton transport in confined thin polymer electrolytes are essential for providing
additional information regarding the structure–property relationships of such materials. Usinga combination of proton transport measurements and structural characterization, we exploredthe effect of proton conductivity in sulfonated polyimide (SPI) under both bulk andnanostructured thin film systems. SPI film confined to a thickness of approximately 530 nm
shows significant proton conductivity enhancement to a value of 2.6 × 10−1 S/cm (95% RH at
298 K), which is almost one order of magnitude more proton conductive than the bulk system(3.0 × 10−2 S/cm at 90% RH and 298 K). In thin film, the preferred chain packing along the inplane direction can have considerable influence on the charge transport characteristics, whichleads to the enhanced proton conductivity. The infrared (IR) p-polarized multiple-angle
incidence resolution spectrometry (p-MAIRS) and in situ grazing-incidence small-angle X-ray
scattering (GISAXS) were used to investigate the direction of polymer orientation and thechanges in the internal polymeric structure under various humidity conditions, respectively.
Under high-humidity conditions, the strong interaction between the side-chain sulfonic acidgroups and water molecules causes an abrupt change in the internal structure in bulk SPI. Suchstructural rearrangement results in a liquid-crystal-like ordered polymer structure. The RH
dependent FTIR−ATR studies reveal that, the accumulation of large fraction of watermolecules with stronger hydrogen bonding at high humidity region causes the huge deviationin the internal morphology of the bulk samples.

1. Introduction
Aromatic polyimides have been widely investigated because of
their high thermal stability, excellent chemical resistance, andfavorable dielectric and gas permeability properties.1 A
polyimide (PI) functionalized with fluorinated and/or
semialiphatic groups exhibits unique physical properties, which
can be applicable to novel electronic and optical materials.2–4
Recently, acid-functionalized or sulfonated polyimides (SPIs)
have attracted interest as essential polymer electrolytes for fuelcells.5,6 Because of their high molecular weights, the ease withwhich they can be formed into films, their enhanced proton
conductivity, and high durability, SPIs have been used as
possible substitutes for perfluorinated ionomer membranes.7,8However, proton conductivity, stability, and durability are still
issues for SPIs because the sulfonated groups in the polymer
electrolyte exhibit excessive swelling under fully hydrated
conditions, which deteriorates their performance during fuelcell operation. Therefore, a number of structural investigations
have been conducted to improve the proton conductivity andstability profiles of SPIs.9,10 For example, Miyatake et al.11
reported that the high proton conductivity of 1.67 Scm−1 (at120 °C and 100% RH) is due to the high water-holding
capability of the sulfonated polyimide containing fluorenyl
groups. The strong interchain interaction among the polymer
molecules allows for the dense packing of polymer chains,which reasons the possibility of stability under hydrated
conditions, even at high temperatures. Notably, the properties

This journal is © The Royal Society of Chemistry 2013of polymeric materials are not only influenced by theirchemical modifications but also by the intermolecularaggregated structures that they form, such as the degree ofchain packing and molecular ordering.
Conjugated polymer structures and the orientation of PI
chains in thin films have been investigated because theysignificantly influence the resulting physical and chemical
characteristics. For example, Terui et al.12 have reported that, in
the in-plane direction, highly oriented PI films exhibit asignificant in-plane/out-of-plane birefringence property. In
addition, the oriented PI membrane influenced the diffusion ofgas molecules through the oriented skin layer, and the gas
diffusive selectivity increased with the decrease in thickness ofthe skin layer.13 In particular, polymer films confined tothicknesses that approach the characteristic length scale of thepolymer can cause anisotropy in the orientation of polymer
chains. This anisotropic polymer orientation is of particularimportance because the majority of the electrical, mechanical,
and optical properties of polymers depend on the degree ofpolymer chain orientation and the direction of orientation.14,15
In general, the preparation method, solvent effect, annealing
temperature, type of substrate, etc. are important parameters for
obtaining the desired polymer orientation in thin films.14−18 In
recent years, numerous researchers have investigated thetransport characteristics of polymer films in oriented molecular
structures, because, during confinement, the self-assembled
polymer structure exhibits a significant change in the protontransport characteristics relative to that of the bulk scale.19−23

J. Name., 2013, 00, 1‐3 | 1

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In our previous studies,24 we observed an enhancement ofproton conductivity in highly oriented polypeptide structures on
MgO(100) substrates, where the conductivity was nearly one
order of magnitude greater than that of the bulk sample. In
addition, the molecular orientation of the sulfonic acid groups
significantly affects the proton transport property because themicrophase separation, as well as the orientation of the protonconductive groups, is an essential parameter for enhancing
proton conductivity.25 Typically, both the surface interactionthat results from thin-film confinement and the wettinginteractions between the substrate and the interface play animportant role in the self-assembly of polymers. Proton
conductivity and water uptake in confined systems largely
depend on the surface properties of the substrate, themorphology of the thin films, the origin of interfacial effects,
the alignment of domains, the domain spacing, etc.26−31 The
orientation of ionomer domains has already been reported to be
influenced by the respective substrate wetting interactions.32
For example, films cast on hydrophobic substrates display aparallel orientation of the ionomer channels, whereas films cast
onto hydrophilic substrates exhibit an isotropic orientation.
Consequently, the orientation of ionomer domains in aparticular direction provides unique domain spacing incomparison with that of the bulk system, and this uniquedomain spacing causes the observed changes in the water
uptake, swelling, and proton transport properties. However, alimited number of studies on the degree of the interfacial effecton proton transport characteristics have been reported. Both thechange in polymer structure during confinement and the originof the interfacial effect on polymer orientation are offundamental interest in understanding the structure–property
relationships of these materials.
In the present study, we describe the effect of proton
conductivity on highly oriented SPI films prepared using thespin-coating technique. We employed a combination of pMAIRS,33,34 GISAXS, and impedance studies to investigate theproton transfer characteristics in the oriented polymer structures.
We obtained clear evidence that the SPI films exhibit highlyoriented polymer chains in the in-plane direction, with a slightpolymer tilt in the backbone.

2. Experimental section
2.1. Materials
3,3'-Dihydroxybenzidine, propanesultone, and pyromellitic
dianhydride (PMDA) were used as received from TCI, Japan.
Acetic acid, acetic anhydride, methanol, and acetone werepurchased from Wako Chemicals, Japan. Hydrochloric acid
(Nacalai Tesque, Japan), sodium hydroxide (Kishida Chemical,
Japan), m-cresol, and triethylamine (TEA) (Kanto Chemicals,Japan) were used as received.
2.2. Synthesis of SPI
Acetylation
was
initially
performed
using
3,3'dihydroxybenzidine as the starting material because the aminogroups in 3,3'-dihydroxybenzidine were protected byacetylation. In a typical reaction, 5 g 3,3'-dihydroxybenzidine
was mixed with 1.5 ml acetic acid, 79 ml acetic anhydride, and
200 ml distilled water. This mixture was stirred at 70 °C for 1
hr. After the mixture cooled to room temperature, theprecipitate was collected and separated using centrifugation.
The products were washed several times with acetone toremove unreacted species, and the obtained precipitate was

2 | J. Name., 2012, 00, 1‐3

Journal Namedried under vacuum. The incorporation of the acetyl functionalgroup was confirmed by NMR analysis. Prior to each step, allof the prepared monomers were characterized by 1H NMR.
Sulfonation of the acetylated 3,3'-dihydroxybenzidine wasaccomplished using 7.3 g propanesultone as the reagent. Thus,
2.4 g sodium hydroxide was dissolved in methanol (150 ml)
and was added to the propanesultone mixture of acetylated 3,3'dihydroxybenzidine. The reaction mixture was refluxed at
80 °C under an argon atmosphere for 6 hr. A pale-white
precipitate was collected and washed several times with cold
methanol. The obtained monomer was completely dissolved in
distilled water, and an ion exchange process was performed
using Amberlyst. Using a rotary vacuum evaporator, weseparated the ion-exchanged monomers and dried them undervacuum. To remove the acetate groups, we added concentratedhydrochloric acid (HCl) to the final product obtained from theaforementioned step, and we subsequently refluxed the mixture
at 120 °C for 1 hr under an inert atmosphere. After the mixture
was refluxed, the cooled product was heated at 150 °C toevaporate the concentrated HCl. The HCl vapor was carefully
neutralized with a concentrated base.
The sulfonated monomers (0.43 g) were polymerized withthe required amount of PMDA (0.22 g), m-cresol (5 ml), andTEA (0.3 ml). m-Cresol was used as the solvent, and thepolymerization reaction was conducted under an argon
atmosphere at a constant temperature of 150 °C for 5 hr. The
polymerized product was collected after being washed withcooled acetone. The final product was again subjected to an
ion-exchange process using Amberlyst. GPC measurements
were used to estimate the molecular weight of the preparedpolymer, which was found to be 2.6 × 105.
2.3. Preparation of thin films
Nanostructured SPI thin films cast onto quartz substrates wereprepared using an Active ACT−200 spin-coater. Prior to film
deposition, each substrate was subjected to several cleaning
processes, and plasma treatment was performed using a vacuum
plasma system (Cute-MP; Femto Science, Korea) to improvesurface hydrophilicity. The thicknesses of the thin films were
measured using a surface profiler (KLA-Tencor P-15 profiler).
2.4. Conductivity measurements of the thin films
Proton conductivity was examined through impedance
spectroscopy measurements using a Solartron 1260 frequency
response analyzer equipped with a high-frequency Solartron
1296 dielectric interface. The relative humidity (RH) andtemperature were established using a computer-controlledenvironmental test chamber (SH-221; Espec Corp). The
conductivity measurements were performed at 298 K and at
various RH values. For the thin film measurements, a twoprobe method was employed to obtain the conductivity 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 MHz, with anapplied alternating potential of 50 mV. Thin-film conductivity(σ) was estimated by the following relation,



d
Rlt

(1)

where d is the distance between the gold electrodes, R is theresistance value obtained directly from the impedance
measurement, l is the length of the contact electrodes, and t is
the thickness of the film.

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2.5. p-Polarized multiple
spectrometry (p-MAIRS)ARTICLE
angle

incidence

resolution

p-MAIRS was used to investigate the polymer orientations of
the nanostructured SPI thin films on Si wafers in both the inplane and out-of-plane directions. The surface of the original Si
wafer was oxidized before the measurement. The p-MAIRS
measurements were collected with a Fourier-transform infrared(FTIR) spectrometer (Nicolet 6700; Thermo−Fisher Scientific)
equipped with a mercury cadmium telluride (MCT) detector.
Single-beam spectra were collected from 38° to 8° in 6° steps
between the angle of incidence.

proton conductivity of the thin film is 2.6 × 10−1 S/cm (at 95%
RH and 298 K). The conductivity of the pelletized bulk SPI(0.39 mm) reaches its maximum value of 3.0 × 10−2 S/cm at
90% RH and 298 K. Under high-humidity conditions, excessive
swelling may accelerate the structural heterogeneity of the bulk
sample, which causes a significant decrease in the conductivity.
Hence, the proton conductivity of the thin film shows one orderof magnitude higher than that of the pelletized bulk SPI.

2.6. Grazing incidence small angle X-ray scattering(GISAXS)
GISAXS measurements were performed on a Rigaku FR-E Xray diffractometer with an R-AXIS IV two-dimensional (2D)detector. The sample stage was composed of the goniometerand a vertical stage (CHUO Precision Industria ATS-C316EM/ALV-300-HM) with a humidity-controlled cell. The cell
had polyimide film (Kapton) windows and the humiditycontrolled cell was obtained from BEL Japan. Nitrogen carriergas was used as-received (without further dehumidification)from the gas cylinder to control the humidity. We used Cu Kα
radiation (λ = 1.542 Å) with a beam size of approximately 300
μm × 300 μm, and the camera length was 300 mm. The
incidence angle was chosen in the range from 0.18° to 0.22°.
The diffraction patterns of the out-of-plane and in-plane
directions that were obtained from thin film samples were used
to extract information regarding the presence of liquid
crystalline lamellar structures and the repeating unit of thepolyimide main chain.
Small-angle X-ray scattering (SAXS) measurements in thebulk state were performed using the same X-ray diffractometer.
A dry sample was placed in a capillary tube and was irradiatedwith the X-ray beam without further adjustments. In the case of
the high-humidity condition, we placed both a portion of dry
sample and a water droplet in the same capillary tube and thensealed it. After 2 days, we used the sealed capillary sample as
the humid bulk state for SAXS measurements.
2.7. DFT calculations
Density functional theory (DFT) calculations were performed
using the DMol3 package in Materials Studio v6.0.0 (Accelrys
Software). The Perdew−Burke−Ernzerhof (PBE) function waschosen. The convergence threshold for the maximum force andmaximum displacement for normal geometry optimization wereset, respectively, to 0.02 Ha Å−1 and 0.05 Å.

Fig. 1 Relative-humidity-dependent proton conductivity plots
for the pelletized bulk SPI and thin film.
The Arrhenius plot of conductivity for the bulk SPI and thinfilm in the temperature range of 20-80 ºC is shown in Figure 2.
The conductivity data were measured with a constant RH valueof 90%. It is clearly seen that, the large deviation in the
conductivity is obtained between the bulk SPI and thin film.
The maximum proton conductivity of 2.8 × 10−1 S/cm isreached at 80 ºC in the thin film. This value is significantly
higher than that of the bulk SPI (5.8 × 10−2 S/cm). The
estimated activation energy values (Ea) for both the bulk SPI
and thin film are 0.13 eV and 0.10 eV, respectively. Thissignificant deviation in conductivities between the bulk SPI andnanostructured film has attracted and triggered further
investigations into the internal structural characteristics of thethin film. The observation that the thin film confinement affectstrongly enhances the proton conductivity of this material isparticularly important.

3. Results and discussion
3.1. Proton conductivity of SPI
This study illustrates the effect of proton conductivity on SPI at
the bulk scale and in nanostructured thin film. Figure 1 shows
the RH-dependent conductivity plots for both the bulk and thinfilm materials. The proton conductivity of the thin film
increases gradually and reaches a maximum value at 95% RH
and 298 K; in addition, unlike the bulk SPI, the thin film does
not exhibit a decrease in conductivity under this high-humidity
condition. In the thin film, confinement effects are expected toplay a vital role in the polymer orientation and transport
properties. In RH dependent conductivity data, the maximumThis journal is © The Royal Society of Chemistry 2012

Fig. 2 Temperature-dependent proton conductivity plots for thebulk and thin SPI film.

J. Name., 2012, 00, 1‐3 | 3

ARTICLE
We speculate that an ordered molecular orientation andconfinement effects in thin films are most likely able to induce
proton transport properties. However, not much report to date isavailable for the relationship between polymer orientation andproton transport behavior in a confined thin film. Hence, wehave performed a more detailed structural analysis of the SPI
thin film to obtain a more complete understanding of theassociated structure–property relationships.
3.2. Evaluation of SPI orientation by p-MAIRSInfrared p-MAIR spectroscopy is an important tool for theinvestigation of molecular orientation in thin films. The
measured in-plane (IP) and out-of-plane (OP) p-MAIR spectraof the SPI thin film are shown in Figure 3. In the IR spectra,
three strong vibrational modes are observed at 1381, 1504, and
1726 cm−1. The band at 1381 cm−1 is due to the C−N−C bondof the imide groups, and the mode at 1504 cm−1 is attributed tothe phenyl C−C stretching vibration. The two vibrational modes
at 1726 and 1781 cm−1 correspond to the C=O asymmetric andsymmetric stretching vibrations of imide groups, respectively.35
The orientation dynamics in confined systems are determinedaccording to the change in intensity profiles of p-MAIR spectra.
Typically, the strongest absorbance arises when the electricfield vector of the IR rays and the orientation of the molecularvibration are parallel.36

Fig. 3 Infrared p-MAIR spectra for the SPI thin film, measured
from 38° to 8° in steps of 6°.
Accordingly, the absorbances of the IP vibrational modes of
C−N−C and phenyl C−C are two times more intense than theabsorbance of the OP component, suggesting that the SPI thinfilm is highly oriented parallel to the substrate plane. Moreover,a notable finding is obtained from the C=O asymmetric andsymmetric stretching modes in the IP spectrum. The peakstrength of the asymmetric vibrational mode is significantly
reduced relative to that of the OP component. Simultaneously,
the symmetric stretching of the IP component is more
prominent than that of the OP component. On the basis of these
two stretching modes, the asymmetric stretch has a dipole
moment that is directed normal to the backbone, whereas thesymmetric stretch has a dipole moment that falls along the
backbone direction. This result indirectly indicates a small
degree of polymer tilting or that the molecule protrudes fromthe surface toward the normal direction, thereby causing the
reduction of the asymmetric peak intensity in the IP spectra.35

4 | J. Name., 2012, 00, 1‐3

Journal Name
To gain further insight into the effect of the C=O asymmetricmode in the OP direction, the presence of band shifts in theabsorption spectrum must be considered. The appearance ofpeak shifts in a vibrational spectrum can be attributed to the
influence of physical and chemical contributions. In the present
OP spectra, the asymmetric vibrational mode is significantly
shifted to higher positions (1735 cm−1) relative to that of the IP
component (1726 cm−1). The observation of this shift in peak
position and intensity profile correlates with the occurrence ofthe tilted polymer symmetry or the surface-perpendicular C=O
component.37 This p-MAIRS study provides quantitative
support for the earlier assumption of the relationship between
SPI orientation and proton transport behavior in conductivity
analysis.
For the pelletized bulk SPI, the FTIR-ATR measurement wasused along the three orthogonal directions, and thecharacteristic vibrational modes are shown in Figure 4. As aresult of the random orientation of the bulk material, themaximum absorbance in the vibrational mode is limited ascompared with the oriented thin film; hence, the bulk SPI
exhibits the same observance strengths in all three directions. In
addition, a significant frequency downshift is observed for both
the C−N−C (1376 cm−1) and C=O asymmetric (1720 cm−1)
stretching vibrational modes.

Fig. 4 FTIR-ATR spectra for the pelletized bulk SPI, measured in
the three orthogonal directions.
3.3. Structural analysis using GISAXS
To explain the change in internal structure and/or morphologicalcharacteristics of the thin film, in situ GISAXS measurements wereconducted (Figure 5) for the SPI thin film under various humidified
conditions. Figure 5 (a, b) represents the 2D GISAXS profiles for the
SPI thin film, taken from 0% RH to 95% RH. Under conditions ofincreasing humidity, the 2D scattering pattern clearly demonstrates
the origin of the new scattering arc (the white arrow in Figure 5b).
The 1D GISAXS profiles of both the in-plane and out-of-plane
directions for SPI thin film are shown in Figure 5 (c, d). Under the
95% RH condition, two scattering intensity at 0.37 and 0.28 Å−1 (qy)are observed in the in-plane direction (Figure 5c). The corresponding
d-spacing values for these two peaks are 1.6 and 2.2 nm, respectively.
The 1D GISAXS patterns for the out-of-plane direction show thepeak shift toward the smaller-angle direction (qz from 0.39 to 0.34
Å−1, Figure 5d), suggesting that the expansion of the out-of-planestructure is the result of water uptake under the high RH range.

This journal is © The Royal Society of Chemistry 2012

Journal Name

ARTICLE

(a)

(c)

(b)

(d)

Fig. 5 (a, b) The 2D GISAXS patterns at 0% RH and 95% RH, respectively, and the humidity-dependent 1D GISAXS profiles in the (c) inplane and (d) out-of-plane directions of the SPI thin film.
The appearance of scattering intensity at approximately 0.75 and0.84 Å−1 (qy) corresponds to the diffraction pattern of the reference
film (Kapton) used during the GISAXS measurements.
PIs possess rigid and planar molecular structures that are oriented
parallel to the substrate plane and exhibit mixed aggregatedstructures, such as liquid-crystal-like and ordered crystalline
structures.38 Therefore, the in-plane scattering peaks correspond to
the periodic length of the repeating units in the ordered structure. To
estimate the repeating unit of the primary chain, a DFT calculation
was performed. The rod-like molecular structure of SPI wasinvestigated, yielding the periodic distance value that is shown in
Figure 6. The periodic unit almost matches the experimental dspacing values of 1.6 and 2.2 nm (qy = 0.37 and 0.28 Å−1,
respectively). We propose that the ordered structure of SPI films isthe driving state, as described below. SPI backbones aggregate in thein-plane direction with bridging of the sulfonated alkyl side chain tothe out-of-plane direction, forming a lamellar structure parallel to thesubstrate. With increasing humidity, the sulfonated interlamellar
spacing is selectively hydrated. Consequently, the in-plane highconductivity behavior is attained by the expansion of theinterlamellar proton conduction path into the out-of-plane direction.
These behaviors are essentially identical to that of the lamellar phaseof the lyotropic liquid crystal (LC) system.39
Figure 7 shows the SAXS pattern for bulk SPI, measured
under both dry and high-relative-humidity conditions. The
obtained results give more information regarding the significant
change in internal structure of SPI under the high-relativehumidity condition. The scattering intensity of q= 0.41 Å−1 (d =ca. 1.4 nm) under the dry condition is an overlapping peak that
occurs due to the repeating unit of the main chain and the driedlamella phase in the partially ordered domains. Under the highhumidity condition, the emergence of a stronger intensity at
approximately 0.21 Å−1 (q) is evident in the diffraction pattern,
confirming the highly ordered lamellar phase as the majordiffraction peak following the water uptake expansion behavior.
The corresponding d-spacing value (2.9 nm) for this peak isvery large. The expanded ordered lamella structure can beinterpreted as a resulting lyotropic LC-like phase, which arises

This journal is © The Royal Society of Chemistry 2012as a result of the strong interaction between the sulfonic acidside chain units and water molecules, as well as thehydrophobic packing of the PI main chains.

Fig. 6 The DFT geometrical analysis performed for a rod-like SPI
backbone and the obtained mean inter-distance values.
In comparison with the case of the high-humidity region, theobserved new small-angle peak originates from the lyotropic ordereddomains, in which the ch-packing38 of the neighboring SPI chains ishighly ordered in the out-of-plane direction. The strong structural
rearrangement that occurs at the bulk scale is due to the vigoroustranslation motion of sulfonic side groups during the high-humidity
exposure. Similarly, other authors have proposed that the vigorous
motion of SPI chains can induce molecular rearrangement during
thermal imidization as a result of the bent molecular structures.38
Possibly, the bent (or twisted) molecular structure is also essential
for the enhancement of the packing order of SPI chains under thehigh-relative-humidity condition, which supports the interpretationof a highly orientated liquid-crystalline-like order in the bulk case.
From these observations, the molecular rearrangement due tostructural instability clearly increases when the system approaches
the bulk scale.

J. Name., 2012, 00, 1‐3 | 5

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Journal Name

(a)

(b)

Fig. 7(a) The 2D SAXS scattering profiles and (b) 1D SAXS patterns for the bulk SPI, measured under dry and humid conditions.
As indicated by the preceding GISAXS results, the inherent
anisotropic lyotropic lamella structure parallel to the substrate plane
is formed in the thin film. Occurrence of this inherent in-plane
polymer orientation might be due to the spin coating technique as aresult of the centrifugal force applied during spinning.40 The surface
energy of the substrate may also influence the interfacial interaction
between the polymer chains and the polymer orientation along thesurface parallel in a film.41−43 Consequently, the oriented structure
can have a significant advantage in in-plane conduction behavior
because the in-plane-oriented lamellar structure constructs the longrange continuous proton conduction paths with the sulfonic layers.
On the other hand, the bulk state does not exhibit this anisotropic
structure. The isotropic multi-oriented domain cannot lead to longrange pathways. On the basis of the previous discussion, the highlyoriented lyotropic lamellar structure enhances the proton
conductivity in comparison with that in the bulk by expanding theproton conduction pathways.

bonding results in a prominent deviation in the internal morphologyof the bulk samples, which causes more intense absorption peaks inthe entire spectrum. The appearance of the bending frequency ofwater molecules in the FTIR-ATR spectra further supports theinterpretation presented in the previous discussion of the strong
structural reconstruction in the randomly ordered pelletized bulk SPI,
following the decrease in conductivity at higher humidity region.

3.4. Humidity-dependent FTIR−ATR studies
We attempted to analyze the change in the internal structure of thepolymer under various RH conditions using FTIR-ATR
measurements.44 This approach is more quantitative and provides
additional support to our previous discussion. Figure 8 shows the
RH-dependent FTIR-ATR spectra of the pelletized bulk SPI system.
In the pelletized bulk SPI, the observed broad band between 3000
and 3720 cm−1 corresponds to the OH stretching vibration of watermolecules. The absorbance of the OH stretching mode increases
gradually up to 70% RH. When the system reaches RH values of
80% and greater, the absorbance of OH traces increases significantly.
More importantly, the new peak at 1637 cm−1 is observed in thehigh-humidity region, corresponding to the bending frequency ofwater molecules. According to the unique physical state of watermolecules, the bending frequency can vary from 1630 to 1670
cm−1.45 The appearance of a band between 1630 and 1645 cm−1
corresponds to a pure state of liquid water. In association with thechange in absorbance and frequency values, a frequency downshift
can be observed from 95% RH (1637 cm−1) to 70% RH (1633 cm−1).
The difference in the vibrational mode can be interpreted as a resultof the change in hydrogen bonding strength within the system.
Notably, the bending frequency of water molecules in the higher
wavenumber region is assigned to the characteristics of stronghydrogen bonding, whereas the frequency downshift in this same
region is ascribed to the weaker hydrogen bonding of watermolecules.45−47 The result obtained at RH greater than 90% is
unexpected and interesting with respect to understanding the changes
in internal morphology of the pelletized bulk SPI. The accumulationof a significant fraction of water molecules with stronger hydrogen

6 | J. Name., 2012, 00, 1‐3

Fig. 8 An observation of the OH traces using FTIR-ATR spectraof the pelletized bulk SPI under various humidity conditions.
RH-dependent FTIR-ATR measurements were also conducted on
SPI thin film (Figure 9). In the present spectra there are three distinct
characteristic bands have been observed: the first in the region
between 3600 and 3400 cm−1 is due to the contemporary presence ofwater molecules stretching ν3 and ν1. The interpretation of the band
between 3300 and 3200 cm−1 includes the overtone of water bending.
The third band at approximately 3060 cm−1 is possibly a combinationand overtone of the matrix.45 All these bands demonstrate the linear
relationship between the OH absorbance value and the relative
humidity. Because of the nanoscale dimensionality of the thin film,
the intensity of this band is not similar to that of the pelletized bulk
SPI. The structural alteration is minimally apparent in the fully
hydrated region; in addition, a limited increase in the intensity of the1637 cm−1 mode corresponding to the OH bending vibration isevident. This result provides indirect evidence of the fact that theabrupt change in internal structure is restricted when the system isconfined as a thin film. The primary polymer orientation in the inplane direction also limits further changes in internal structure underThis journal is © The Royal Society of Chemistry 2012

Journal Name
the higher humidity condition. The peak observed between 2250 and
2400 cm−1 corresponds to the stretching of O=C=O molecularvibrations present in atmospheric air. These results also suggest thatthe higher proton conductivity observed in the SPI thin film is due
primarily to the greater ordering of the polymer chain orientations,which is accompanied by high structural stability.

ARTICLE
demonstrates that the interfacial confinement of polymer structures
significantly influences the proton transport characteristics.

Acknowledgments
This work was supported in part by the Nanotechnology Platform
Program (Molecule and Material Synthesis) of the Ministry of
Education, Culture, Sports, Science and Technology (MEXT), Japan.
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).

Author information
Corresponding author*ynagao@jaist.ac.jp Phone: +81(Japan)-761-51-1541

Notes and references
Fig. 9 An observation of the OH traces in the SPI thin film using
FTIR-ATR measurements.

4. Conclusions
The effect of polymer orientation on the transport properties of SPI
was investigated using a wide range of structural analysis methods.
We have demonstrated that the thin film confinement effectsignificantly affects the polymer orientation and proton-conducting
properties of SPI. In SPI thin film, the inherent anisotropic lyotropic
lamellar structure falls parallel to the substrate plane and develops
long-range continuous proton-conducting paths, which is in contrast
to observations for the bulk case. Thus, the observed proton
conductivity value of SPI thin film exhibits one order of magnitude
greater than that of the bulk sample. In the bulk case, an abruptchange in internal structure as a result of the increased hydrogenbonding behavior of the water molecules results in a sudden decreasein the conductivity value at RH values greater than 90%. The pMAIRS measurements used in this study represent an important toolfor determining the changes in internal structure of polymer films.
Based on the p−MAIR spectra, the polymer orientation of SPI ishighly ordered in the in-plane direction. The existence of a small
degree of polymer tilting or the surface-perpendicular C=O
component is evidenced by the combination of a change in the peakintensity and a peak shift in the out-of-plane component. The in situ
GISAXS diffraction profile reveals a significant deviation in theinternal symmetry of polymer chains under high-humidity conditions.
Because of the strong interaction between the sulfonic acid side
chain units and water molecules, the structures of PI chain packing,including the more ordered domains, can be interpreted as alyotropic LC-like phase. On the basis of the diffraction profiles, thedegree of lyotropic LC-like order increases in the bulky molecular
structure because the ch-packing between the contiguous PI chainsorders more rapidly in the out-of-plane direction due to excessive
water absorption. Humidity-dependent FTIR-ATR studies of boththe bulk and thin SPI samples also support the observation ofchanges in internal polymer structure. Under high-humidity
conditions, the emergence of a new peak at 1637 cm−1 and theincrease in peak intensities indicate that strong structural alterations
dominate within the bulk sample. Consequently, this work

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