JAIST Repository
https://dspace.jaist.ac.jp/

Title

Influence of Molecular Weight on Molecular
Ordering and Proton Transport in OrganizedSulfonated Polyimide Thin Films

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

Citation

The Journal of Physical Chemistry C, 119(38):21767‑21774

Issue Date

2015‑09‑02

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/14271

RightsKarthik Krishnan, Hiroko Iwatsuki, Mitsuo Hara,
Shusaku Nagano, and Yuki Nagao, The Journal of
Physical Chemistry C, 2015, 119(38), pp.21767‑
21774. This document is the unedited author's
version of a Submitted Work that was subsequently
accepted for publication in The Journal of
Physical Chemistry C, copyright (c) American
Chemical Society after peer review. To access the
final edited and published work, see
http://dx.doi.org/10.1021/acs.jpcc.5b03292

Description

Japan Advanced Institute of Science and Technology

Influence of Molecular Weight on Molecular Ordering and Proton
Transport in Organized Sulfonated Polyimide Thin Films
Karthik Krishnan,† Hiroko Iwatsuki,‡ Mitsuo Hara,‡ Shusaku Nagano,§ and Yuki Nagao†,*†

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa
923-1292, Japan.

‡

Department of Molecular Design & Engineering Graduate School of Engineering, Nagoya University, Furo-cho,
Chikusa, Nagoya 464-8603, Japan§

Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

Abstract
The effect of molecular ordering and ordered domain size on molecular weight dependent proton
conductivity in well-organized sulfonated polyimide (SPI) thin films has been investigated using
both low molecular weight (LM w , M w = 1.3 × 104) and high molecular weight samples (HM w ,M w = 2.6 × 105). Analyses of the films by grazing incidence small angle X-ray scattering(GISAXS) revealed that the HM w SPI film exhibits the higher degree of inherent liquid
crystalline-like lamellar ordering, and that it is consistent with a preferential in-plane main chain
packing. Such molecular ordering and the in-plane oriented structure can have considerable
influence on the proton transport characteristics in thin films. The larger ordered domains in the
HM w SPI thin film with a thickness of approximately 500 nm shows the significant proton
conductivity enhancement to a value of 2.6 × 10−1 S/cm (at 25 ºC and 95% relative humidity),
which is more than an order of magnitude higher value than that of the smaller ordered domains
in the LM w SPI thin film with less molecular ordering. The larger ordered domains in the HM w
SPI thin film influence the conducting characteristics attributable to the fewer liquid crystallinelike domain boundaries relative to LM w SPI films because the smaller ordered domains in the1

LM w SPI thin film more obviously exhibit large-scaled domain boundaries, which can disrupt
the fast ion transportation.

Keywords: lyotropic liquid property, molecular ordering, organized structure, confined property,
molecular weight

1. Introduction
Sulfonated polyimides (SPI) containing alkylene segments both in main and side chains arefound to be the essential polymer electrolytes in fuel cells owing to the substantial stability andhigh proton conductivity.1,2 The typical polymer electrolyte for fuel cell is well known to require
the high proton conductivity, mechanical and chemical integrity, low permeation to gases andlimited swelling.3‒5 The hydrophilic sulfonic acid group contained in SPI usually forms ionicchannels upon sorption of water, which promotes the proton transport.6,7 In general, the degree of
sulfonation and interaction strength of water molecules facilitates proton transport characteristics
in polymer electrolytes during fuel cell operation. However, excessive swelling may arise when
the interaction is stronger between water molecules and sulfonic acid group, which significantly
reduces the stability and continuous proton transport paths.8,9 Therefore, confinement of thepolymer structure for enhancing property characteristics constitutes an important issue in thedevelopment of solid H 2 /O 2 fuel cells.10‒14 In fuel cell infrastructure, the development ofpolymer electrolytes for higher proton transportation along with substantial stability has been
done mostly through chemical modification.15‒18 Instead of chemical modification, a rational
design of polymer electrolytes requires fundamental understanding of the role of confinedpolymer structure in influencing proton transport characteristics. Recent investigations have been2

conducted to elucidate the well-organized polymer structure and their interfacial confinement forenhancing the proton transport and stability characteristics.19 How the chain alignment and longrange periodicity of polymer chains control the transport property and how stability leads to thedevelopment of solid polymer electrolytes are two important questions that must be answered.
To date, various reports describe studies of the macroscopic structural ordering to enhance theproton transport, but they are insufficient to explain the interfacial charge transportcharacteristics in confined thin film polymer structures.20-23
In conjugated polymer films, the anisotropic charge transport occurs most efficiently either inthe direction of chain stacking, or along the polymer backbone, which is the fastest transportation
direction inside an aggregate. During film confinement, a higher degree of polymer packing anddomain ordering is also favorable for higher charge transport. Most commonly, the strong
polymer chain interactions promote the favorable polymer packing that often results in largescaled domain order.24,25 Interchain interaction and domain ordering likely are governed by many
parameters such as processing conditions, solubility limit, surface state, and molecularweights.26-28 Amongst the molecular weight and solubility parameters emerged to control thepolymer packing, where polymers of higher solubility promote face-on packing and less-soluble
polymers favors edge-on packing.26 To estimate the SPI polymer packing on proton transport
property, we specifically examine the molecular weight that controls the ordered domains during
SPI film formation. Generally, aromatic polyimide (API) possesses a rigid planar molecular
structure that exhibits higher in-plane orientation than the non-planar molecular structures.29 The
interchain packing and π-stacking of imide and phenyl groups in APIs vary considerablyaccording to differences in aggregation structures, e.g., crystalline, liquid-crystalline (LC) likeand amorphous. In addition, the absence of large-scale three-dimensional ordered domains in the3

API films suggesting mesomorphic ordering between the amorphous and crystalline phases canbe interpreted as reflecting the existence of lyotropic LC like ordered domains.29
The main objective of this report is to ascertain the relation between lyotropic LC domain
ordering and proton conductivity on SPI thin films. The discussion also provides new insights
relates to the correlation between the lyotropic LC domain structure and molecular weight of SPI
thin films. The larger ordered domains in a high molecular weight (HM w ) SPI thin film exhibits
higher proton conductivity than the smaller ordered domains in low molecular weight (LM w ) SPI
thin film. The increase in LC domain structure in the highly oriented HM w SPI thin film strongly
contributes the proton conductivity enhancement.

2. Experimental
2.1. Materials
3,3'-Dihydroxybenzidine, propanesultone, and pyromellitic dianhydride (PMDA) were used asreceived from Tokyo Chemical Industry Co. Ltd., Japan. Acetic anhydride, acetic acid, methanol,
and acetone were purchased from Wako Pure Chemical Inds. Ltd., Japan. Hydrochloric acid
(Nacalai Tesque Inc., Japan), sodium hydroxide (Kishida Chemical Co. Ltd., Japan), m-cresol,and triethylamine (TEA, Kanto Chemicals Co. Inc., Japan) were used as received.
2.2. Synthesis of SPISPIs with various molecular weights were prepared using the same polymerization scheme(Scheme 1). The reaction was initiated by acetylation process using 3,3'-dihydroxybenzidine as
the starting material because the amino groups 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.

4After the mixture cooled to room temperature, the precipitate was collected and separated using
centrifugation. The products were washed several times with acetone to remove unreacted
species, and the obtained precipitate was dried under vacuum. NMR study was used to confirm
the incorporation of the acetyl functional group. Prior to each step, all prepared monomers werecharacterized by 1H NMR.
Acetylated 3,3'-dihydroxybenzidine (6 g) was sulfonated using 7.3 g propanesultone as thereagent. Consequently, 2.4 g sodium hydroxide was dissolved in methanol (150 ml), and then it
was poured into the propanesultone and the acetylated 3,3'-dihydroxybenzidine containing
mixture. The overall reaction mixture was refluxed at 80 °C under an argon atmosphere for 6 hr.
A pale-white precipitate was received and washed several times with cold methanol. For ion
exchange process, the obtained monomer was dissolved completely in distilled water. An ion
exchange step was performed using Amberlyst. Using a rotary vacuum evaporator, the ionexchanged monomers were separated and dried under vacuum. To remove the acetate groups, we
added concentrated hydrochloric acid (HCl) to the final product obtained from the step described
previously. Then we 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 to evaporate the concentrated
HCl. The HCl vapor was neutralized carefully with a concentrated base.

5

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

Ion ExchangeOO

3 3'-Dihydroxybenzidine

1

2

NaO3S

SO3Na

HO3S
H
N
Ac

H
N
Ac
O

O

SO3H

HO3S

3

NH2

H2N
HCl
120oC

O

O

O

O

O

O ,TEAOO
m-cresol
150oC

O

Ion Exchange

O

O

N

N

OO

O

n

SO3H

HO3S

4

5

SO3H

Scheme 1. Steps used for synthesizing sulfonated polyimide

Polymerization was conducted using the prepared sulfonated monomers (0.43 g), PMDA (0.22g), m-cresol (5 ml), and TEA (0.3 ml). m-Cresol was used as the solvent. The polymerization
reaction was processed under an argon atmosphere at a constant temperature of 150 °C for 5 hr.
The polymerized product was collected after washing with cooled acetone. The final product was
subjected again to an ion-exchange process using Amberlyst. In our preparation, we obtained the
SPI polymers with various molecular weights. Among them, we selected the LM w (1.3 × 104)
and HM w (2.6 × 105) SPI samples for comparative study. Both polydispersity indexes are 11-12
for Mw/Mn. The degree of sulfonation for the final product was estimated from 1H NMR data: it
was more than 98%. The calculated ion exchange capacity (IEC) was 3.1 mequiv/g.
2.3. Gel permeation chromatography (GPC)
Gel permeation chromatography (GPC, LC-2000plus; Jasco Inc.) was used. Response to the
HPLC system control of up to four through the (controller) LC-Net II / ADC enables
simultaneous recording of four chromatograms per system. A DMF-, H 2 O-, CH 3 COOH- andNaNO 3 -containing mixture was used as the eluent at a flow rate of 1.0 mL min−1. The polymer
6

solutions were filtered through a 0.50 μm PTFE hydrophobic filter before being injected into thecolumn. The molecular weight was calibrated with a polystyrene standard.
2.4. Preparation of thin films
Active ACT−200 spin-coater was used to prepare nanostructured SPI thin films cast on quartz
substrates. The substrates were subjected to several cleaning processes prior to film deposition.
Plasma treatment was performed using a vacuum plasma system (Cute-MP; Femto Science Inc.,
Korea) to improve surface hydrophilicity. Thicknesses of the thin films were measured using asurface profiler (P-15 profiler; KLA-Tencor Corp.).
2.5. Polarized optical microscopy (POM)
POM was conducted with crossed polarizers to observe the liquid crystalline-like ordereddomains and to ascertain their size in the polymer films (BX51-P; Olympus Corp. / DP28
camera; Olympus Corp.) Data were recorded with Cellscan controller software.
2.6. Grazing incidence small angle X-ray scattering (GISAXS)GISAXS measurements were performed using an X-ray diffractometer (FR-E; Rigaku Corp.)with an R-AXIS IV two-dimensional (2D) detector. The sample stage was composed of thegoniometer and a vertical stage (ATS-C316-EM/ALV-300-HM; Chuo Precision Industrial Co.
Ltd.) with a humidity-controlled cell. The typical cell holds polyimide film (Kapton) windows
and the humidity-controlled cell. Nitrogen carrier gas was used as received, without further
dehumidification, from the gas cylinder to control the humidity. We used Cu Kα radiation (λ =0.1542 nm) with a beam size of approximately 300 μm × 300 μm. The camera length was 300
mm. The incidence angle was chosen in the range from 0.20° to 0.22°. Diffraction patterns of theout-of-plane and in-plane directions obtained from thin film samples were used to extract
information related to the presence of liquid crystalline lamellar structures and the repeating unit

7

of the polyimide main chain. For 1-D out-of-plane and in-plane patterns, the integrated regions
were taken between -0.5° – 0.5° as 2θ from the center (0°) and the width of 1° as 2θ,respectively.
Small-angle X-ray scattering (SAXS) measurements in the bulk state were performed using thesame X-ray diffractometer. A dry sample was placed in a capillary tube and was irradiated with
the X-ray beam without further adjustments.
2.7. p-Polarized multiple angle incidence resolution spectrometry (p-MAIRS)
p-MAIRS technique has been used to investigate the polymer orientations of the nanostructured
SPI thin films on Si wafers in both the in-plane and out-of-plane directions.30,31 The surface ofthe original Si wafer was oxidized before measurements. Then p-MAIRS measurements werecollected with a Fourier-transform infrared (FTIR) spectrometer (Nicolet 6700; Thermo Fisher
Scientific Inc.) 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. Thicknesses of thefilms prepared on the oxidized Si substrate used for this measurement were approximately 500
nm.
2.8. Conductivity measurements of thin films
The proton conductivity of SPI thin films were examined through impedance spectroscopy
measurements using a frequency response analyzer (SI1260; Solartron Analytical) equipped witha high-frequency dielectric interface (SI1296; Solartron Analytical). The relative humidity (RH)
and temperature were controlled using a computer-controlled environmental test chamber (SH221; Espec Corp.). For thin film conductivity measurements, two-probe method was used toobtain proton conductivity on the surface parallel to the film. Gold contacts were used as the thinfilm electrodes for the conductivity measurements. Impedance data were collected for8frequencies of 1 Hz and 10 MHz, with an applied alternating potential of 50 mV. Thin-film
conductivity (σ) was calculated as,

σ=

d
Rlt

(1)

where d signifies the distance between the gold electrodes, R is the resistance value obtained
directly from the impedance measurement, l denotes the contact electrode length, and t
represents the thickness of the film.
2.9. X-ray reflectivity
The X-ray reflectivity measurements were carried out by a high-resolution X-ray diffractometer(ATX-G; Rigaku Corp.) with X-ray wavelength λ= 0.1542 nm (Cu Kα) generated from a Cu
rotating anode (50 kV, 300 mA). X-rays were collimated by a parabolic multilayer mirror andslit collimators.

3. Results and discussion
3.1. LC ordered domain and size
In the previous study, the oriented organized structure has been suggested by the GISAXS andIR measurements. To elucidate whether the lyotropic LC domain morphologies on SPI thin films
are observed or not, polarized optical microscopy was used. Figure 1 shows the polarized optical
microscope images for both LM w and HM w SPI thin films. The LC morphology on the thin films
was identified under cross polarized light when the sample was rotated from 0º (Figure 1a and1c) to 45º (Figure 1b and 1d) with respect to the direction of polarized light. These images showthat the HM w SPI thin film engenders the increase in domain size and macroscopic alignment
over a large area, which suggests that the LC region within the domains is well oriented. Smooth
proton transport occurs with low activation energy in the highly conductive domain, but harder
9transport might occur in between the inter-domain with higher activation energy. This domain
size difference between the LM w and HM w SPI thin films could be related with the internal
structural difference. Therefore in situ GISAXS analysis was carried out.

Figure 1. Polarized optical micrograph images of both (a and b) LM w and (c and d) HM w SPI
thin films at 0º and 45º, respectively.
3.2. Structural interpretation using in situ GISAXS
Differences in the molecular packing and crystalline order obtained by varying the molecularweight in SPI thin films were investigated using in situ GISAXS analysis with humidity control.
The 2D scattering images in the RH-dependent in situ GISAXS measurements were shown,respectively, in Figures S4a–S4d and Figures 2a–2d for HM w and LM w SPI thin films. Figures
S4e, S4f and Figures 2e, 2f respectively show a series of 1D scattering profiles for HM w and
LM w SPI thin films with various RH measured both in in-plane and out-of-plane directions. The
10scattering arcs at the positions of 0.75 and 0.84 Å−1 were artifacts caused by diffraction of the
Kapton film used as the windows for the humidity-controlled GISAXS cell.
In the wide angle regions, the 2D GISAXS patterns of both LMw and HM w SPI thin films at
0% RH condition (dry condition) exhibit strong semicircle scattering at q z = 1.4 Å−1 of d = 0.4
nm (Figures 2a, 2f, S4a and S4f 19). The scattering intensity of the semicircles is anisotropic
unlike the isotropic amorphous halo scattering observed by measuring only a quartz substrate(Figure S5 and S6). The strong intensity of the broad intense peak is observed toward the out-ofplane azimuthal direction, indicating that the repeating ordered structure is formed
perpendicularly to the substrate plane in the SPI film. Ando et al. reported the different types ofordered polyimide structures in thin films using detailed GISAXS analysis.32 Backbones andconjugated planes of aromatic polyimides in thin films in ordered domains parallel to thesubstrate plane, where the scatterings corresponding to the main chain packing (ch-pack) andconjugated plane stacking (π stack) were observed in the out-of-plane direction with d-spacing
ranging from 0.36 to 0.57 nm.32 The broad out-of-plane peak can be assigned to the interchain
packing in the thin film. In the small angle region, the in-plane peak profiles of both LM w andHM w exhibit sharp scattering peak at q y = 0.37 Å−1 of d = 1.7 nm at 0% RH condition (Figure 2eand S4e). The in-plane scattering is attributed to the periodic monomer unit length for thepolyimide chain units for the appropriate positions of the corresponding polyimides.32

11Figure 2(a, b, c and d). The 2D GISAXS patterns at 0% RH, 40% RH, 70% RH and 95% RH,
respectively, and the humidity-dependent 1D GISAXS profiles in the (e) in-plane and (f) out-ofplane directions of the LM w SPI thin film. The peak indicated by arrows in Fig. 2f is discussed in
the manuscript. The mark of * is Kapton window.

The present out-of-plane and in-plane scatterings can be assigned respectively to the interchain
packing and the periodic monomer unit length for the SPI main chains.19 Furthermore, in dry
condition, the HM w SPI thin film exhibits another small angle scattering at 0.39 Å−1 (d = 1.6 nm)
is observed in out-of-plane direction. Our earlier work has shown that the out-of-plane scatteringpeak is attributable to the lamellar structure in the in-plane stacked hydrophobic polyimide
12

backbone with the out-of-plane bridging of hydrophilic sulfonated side chains as shown in
Figure 3a.19 The previous birefringence observation of the polarized optical microscope is
explainable by this internal oriented structure. At high RH conditions, the small angle out-ofplane peak enhanced and shifted to the smaller angle region (Figure S4f). At 95% RH, the outof-plane peak reached 0.34 Å−1 (d = 1.8 nm); it was markedly enhanced relative to the X-rayspecular peak. The in-plane peak profile is almost unchanged by humidity. These results indicatethat the inter-lamellar hydrophilic sulfonated layer during the high humidity region is hydrated
selectively, thereby enhancing of molecular ordering and the expansion of lamellar spacing to theout-of-plane direction as shown in Figure 3b.19

Figure 3. Schematic views of the degree of the molecular ordering and ordered domain size. (a)HM w SPI thin film in dry condition with relatively larger domain size. (b) HM w SPI thin film inhigh humidified condition with improved molecular ordering in the out-of-plane direction. (c)LM w SPI thin film in dry condition with less ordering structure and relatively smaller domain
size. This is an expected structure because of absence of out-of-plane scattering at 0.39 Å−1 (d =1.6 nm). (d) LM w SPI thin film in high humidified condition with improved molecular orderingin the out-of-plane direction and relatively smaller domain size.

13

For the LM w SPI thin film under a dry condition (Figures 2a, 2e, 2f), we were able to obtainfundamentally identical 2D scattering image to that of the HM w SPI thin film, except in theabsence of out-of-plane scattering at 0.39 Å−1 (d = 1.6 nm). In the humidified condition, however,new scatterings are recognized around the small angle region in the out-of-plane peak profile
with overlapping of direct X-ray specular peak. The arc scatterings are observed in the out-ofplane small angle region from 2D profiles of Figure 2c and 2d. As humidity increases, the arc
scattering enhanced and shifted to smaller angle. This change by the humidity can be attributed
to the amphiphilic lamellar expansion.
To clarify the difference of the scattering aspects between the LM w and HM w SPI thin films,
Figure 4 summarize 1D GISAXS profiles of the LM w and HM w SPI thin films in the out-ofplane (a) and in-plane (b) directions under dry and humid conditions. The absence of the small
angle scattering due to the inter-lamellar structure in the LM w SPI thin film indicates that thelower molecular weight leads to higher thermal fluctuation of LC structure by thermodynamic
driving force. As evidence, one additional scattering peak is apparent in in-plane pattern around0.34 Å−1 (d = 1.8 nm) in the LM w SPI thin film of the 95% RH condition (Figure 4a). This peakcorresponds to the same scattering peak around 0.34 Å−1 (d = 1.8 nm) in the out-of-plane pattern(Figure 4b) and is attributable to the nature of lamellar structure. The peak intensity
corresponding to the amphiphilic lamellar structure is weak, even in highly humidified
conditions (95% RH) in the LMw SPI thin film. This result suggests the LM w SPI thin film has aless structural ordering comparing with the HM w SPI thin film. The smaller domain size shownby polarized optical microscope is explainable by this less-structured ordering in the LM w SPI
thin film as shown in Figures 3c and 3d.

14Figure 4. The humidity-dependent 1D GISAXS profiles in the (a) out-of-plane and (b) in-plane
directions of the SPI thin films. The marks of * indicate the scattering from the Kapton windows.

The intensity enhancement (half order magnitude) and scattering peak shift (from 0.42 to 0.34
Å-1) with humidity involve the lamellar expansion (Figure S7a and S7b) and ordering of theamphiphilic lamellar structure to the out-of-plane direction similar to a lyotropic liquid
crystalline phase. Because of the strong interaction between the sulfonic acid side chain units andwater as well as the hydrophobic packing of the SPI main chains, the formation of the lyotropic
lamellar structure is explainable for water uptake. This structural change is completely reversible
versus relative humidity. This SPI film exhibits a humidity dependent self-assembly structure.
Higher in-plane proton conductivity along the polymer chains can be attained from in-planeordering and expansion of the amphiphilic lamellar structure under high-humidity conditions.

3.3. Polymer orientation using p-MAIRS
The p-MAIRS technique coupled with IR spectroscopy used in this investigation offers asignificant molecular information resolved by the in-plane and out-of-plane spectra, dependingon the molecular orientation.30,31 Figure 5 shows p-MAIR spectra of LM w and HM w SPI thin
15

films, measured in both in-plane and out-of-plane modes. The observed vibrational modes at1381 and 1504 cm−1 are attributed to the C−N bonds of the imide groups and to phenyl C−C
stretching vibration, respectively. Two vibrational modes at approximately 1726 and 1781 cm−1
correspond to the C=O asymmetric and symmetric stretching vibrations of imide groups,
respectively.19,33,34 No peak corresponds to the presence of polyamic acid, which is normallylocated at 1530 and 1690 cm−1. The peak of polyamic acid at 1530 cm−1 is ascribable to thestretching of C−N bonds of amide. The peak at 1690 cm−1 results from the carboxylic acid or
amide group of C=O bonds.
According to the band positions and relative intensity between in-plane and out-of-planevibrational modes of p-MAIR spectra, the molecular orientation can be ascertained.35‒37 In
general, the strongest absorbance occurs when the electric field vector of the IR rays and theorientation of the molecular vibration become parallel. However, the randomly oriented polymer
structure in thin films would exhibit identical in-plane and out-of-plane vibrational modes.
Occurrence of greatly deviated in-plane and out-of-plane vibrational modes of SPI obviously
differentiate the anisotropic molecular orientation in thin films. The absorbance strength of thein-plane component is higher than the out-of-plane component, indicating that the highlyoriented polymer chains are parallel to the substrate surface. Spectra of HM w and LM w SPI thinfilms show very similar shapes. The main chains of both HM w SPI and LM w SPI thin films areoriented parallel to the surface. Usually, the higher frequency carbonyl group (C=O) vibrations
are the most expedient for use in characterizing the orientation distribution. Because of the highin-plane orientation of polymer chains, the C=O asymmetric stretching possesses higher intensity
than the phenyl stretching.38 To check the difference of the orientation of the C=O asymmetric16stretching between the HM w SPI and LM w SPI thin films, the degree of the orientation was
estimated according to the following equation,31

ϕ = tan −1

2 I IP
I OP(2)Where I IP and I OP are the in-plane and out-of-plane band intensity of C=O group and ϕ is theorientation angle from the surface normal. The distribution of the orientation cannot be analyzed
from this analysis. The obtained orientation angles of the C=O asymmetric stretching are 44° and
40°, respectively, in the HM w SPI and LM w SPI thin films. These values are not so different.
Other orientation angles of the C−C and C−N groups are also same values. So the large
difference of the molecular orientation between the HM w SPI and LM w SPI thin films has notbeen obtained.

Figure 5. In-plane (IP) and out-of-plane (OP) profiles of p-MAIR spectra for both LM w
and HM w SPI thin films. Reasons for the double IP absorbance are explained in theliterature.14,31
17

3.4. Structure-dependent proton transport property
Figure 6 depicts proton conductivity plots for SPI thin films as a function of molecular weight.
The proton conductivity for HM w SPI thin film showed a remarkably higher value of 2.6 × 10−1
S/cm (at 25 ºC and 95% RH).19 A linear increase in conductivity is observed by the increase in
RH, which is similar for both LM w and HM w SPI thin films. However, the proton conductivityof HM w SPI thin film manifests more than an order of magnitude higher value than that of the
LM w SPI thin film (1.0 × 10−2 S/cm at 25 ºC and 95% RH). From the proposed lamellar structure,
the side chain sulfonic acid functional groups aggregate to form continuous hydrophilic domains
parallel to the surface at the high humidity region that selectively hydrated upon the adsorptionof water molecules. Such continuously formed hydrophilic domains in the highly ordered systempromote the marked enhancement of conductivity. It is also expected that variation of structuralordering, or the change in the degree of liquid crystalline-like polymer packing by changing thepolymer molecular weights, causes a huge difference in proton conductivity between these two
SPI thin films. Elucidation of this structural effect by varying the polymer molecular weight iscrucially important to correlate the structure-property relation by which one can readily tune thepolymer property characteristics without the need for general chemical modifications.

Figure 6. RH-dependent proton conductivity plots for both LM w and HM w SPI thin films.
18

Figure 7 shows temperature-dependent proton conductivity plots for both LM w and HM w SPI
thin films. The conductivity data were measured using various temperatures and constant RH
value of 90%. Substantial deviation in the conductivity is obtained between the LM w and HM w
SPI thin films. Varying the temperature from 20 ºC to 80 ºC elevates the conductivity from 4.8 ×
10−3 to 1.8 × 10−2 S/cm for LM w SPI thin film, and from 1.4 × 10−1 to 2.8 × 10−1 S/cm for HM w
SPI thin film. Differences in the conductivity between LM w and HM w SPI thin films are
explainable by the activation energy (E a ) of proton hopping. According to the Arrhenius
equation (3) for the ionic conduction, the estimated E a for both the LM w and HM w SPI thin films
are 0.20 eV and 0.10 eV, respectively.
−𝐸𝐸𝜎𝜎𝜎𝜎 = 𝜎𝜎0 𝑒𝑒𝑒𝑒𝑒𝑒 � 𝑅𝑅𝑅𝑅𝑎𝑎�

(3)

Therein, E a , R, T, and σ 0 respectively represent the activation energy, gas constant, temperatureand pre-exponential factor. This difference in proton conductivity cannot be explained only by
the chemical formula as a primary structure. The higher-order architectures with the polymer
packing and liquid crystalline-like ordering in SPI thin films can strongly engender protontransport characteristics.

Figure 7. Temperature-dependent proton conductivity plots for LM w and HM w SPI thinfilms.
19

The ordering and orientation of the liquid crystalline-like nanostructure play important roles in
improving the proton conductivity. In the present case, the highly ordered HM w SPI thin film
exhibits one order of magnitude higher proton conductivity than that of the less-ordered LM w
thin film, mainly because of the difference of degree of ordered domain size by the change inmolecular weight.

3.5. Confirmation of molecular weight contribution to the proton transport property
Further investigations were conducted to elucidate why the proton conductivity differs greatly
between LM w and HM w SPI thin films. Structural differences in powder samples as bulk
materials were examined using SAXS study under dried conditions (Figure S8, Supporting
Information). It is necessary to ascertain and to correlate how molecular ordering in the bulk caseaffects the scattering profile. It is particularly interesting that both samples show similar 2D
SAXS and 1D SAXS patterns. The scattering peak in the lower angle region at approximately
0.40 Å−1 (q) corresponds to the repeating units of the main chain polymer and the dried lamellar
phase. The corresponding d-spacing value for this peak is 1.5 nm. Both are similar structure
between HM w bulk SPI and LM w bulk SPI. Therefore the original structure from the bulk
sample is found to be very similar.
It is important that the ordered domain size from POM observations varied considerably withmolecular weight when the SPI is confined into a thin film. Therefore, we strove to ascertain
how the proton concentration and density could change while the SPI was confined into thin film.
X-ray reflectivity study was conducted to estimate the proton concentration by variation of thedensity as function of molecular weight as shown in Figure S9. Table 1 presents data analyzed
from X-ray reflectivity, which are quite similar density values between the LM w and HM w SPI

20

thin films. Because of their same chemical structure, the densities of both LM w and HM w SPI
thin films are almost equal. The in situ IR analysis was also conducted as shown in Figure S10.
Both SPI thin films show almost identical absorbance, which suggests that numbers of adsorbed
water molecules of HMw and LM w SPI thin films are not so different from each other. Therefore,
the proton concentrations might also be similar or equal.

Table 1. Density of both the LM w and HM w SPI thin films by X-ray reflectivity.
ρ (g/cm3)
LM w SPI thin film

1.54

HM w SPI thin film

1.57

Therefore, we propose that the lyotropic lamellar structure of the larger liquid crystalline-likepacking in the HM w SPI thin film can provide long-range proton conducting in-plane channels,
which is more beneficial for obtaining the enhanced proton transport in highly ordered systems.
In the LM w SPI film at 95% RH, the scattering peak corresponding to the sudden change oflyotropic lamellar structure in in-plane direction (Figure 3b), suggesting that the randomly
oriented proton channels arose from the unstable less-ordered domains.
In summary, an organized structure with high proton conduction was achieved using thelyotropic LC property. This macroscopic proton transport is strongly dependent on the degree ofthe molecular ordering and LC domain size in highly oriented SPI thin films. We demonstrated
how organization of the structure as a higher-order structure can contribute to the protontransport property. This finding provides a new strategy for the development of the highly21

proton-conductive materials from the viewpoints of the protonic devices in future. This protonconductive organized structure can open the door to structure-driven protonics.

4. Conclusion
Influence of molecular weight on molecular ordering and proton transport in well-organized SPI
thin films has been demonstrated. Results of this study indicate that the ordering and orientationof the LC domains in a thin film nanostructure with different molecular weights play importantroles for enhancing proton conductivity. The most obvious explanation for the observed gain inproton conductivity derives from improvement of the degree of the molecular ordering and LC
domain size. Both HM w and LM w SPI thin films exhibit the highly oriented polymer chains in
the in-plane direction and organized lyotropic lamellar structure. However, the smaller LC
domains with the less molecular ordering of the LM w SPI thin films reduce the proton transport.
On the basis of diffraction profiles and POM observations, the degree of lyotropic LC likeordering and domain size increases in the HM w SPI thin film.

22

Acknowledgments
This work was supported in part by the Nanotechnology Platform Program (Molecule andMaterial 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). This work was partially supported by the research funds of The Kyoto Technoscience
Center and the Research Foundation for the Electrotechnology of Chubu (REFEC).

Supporting information
Synthesis and characterization of sulfonated polyimide, GISAXS data, bulk SAXS data, X-ray
reflectivity (XRR) data, relative humidity dependent FTIR-ATR, AFM, and GISAXS data for
the quartz substrate. This material is available free of charge via the internet at
http://pubs.acs.org.
Author information
Corresponding author*ynagao@jaist.ac.jp Phone: +81(Japan)-761-51-1541, Fax: +81(Japan)-761-51-1149, Address:
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
Notes
The authors declare no competing financial interest.
References

23

(1) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.; Watanabe, M.
Aliphatic/aromatic Polyimide Ionomers as a Proton Conductive Membrane for Fuel Cell
Applications. J. Am. Chem. Soc. 2006, 128, 1762–1769.
(2) Saito, J.; Miyatake, K.; Watanabe, M. Synthesis and Properties of Polyimide Ionomers
Containing 1H-1,2,4-triazole Groups. Macromolecules 2008, 41, 2415–2420.
(3) Yang, J. S.; Cleemann, L. N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler,
H. A.; Bjerrum, N. J.; He, R. H. High Molecular Weight Polybenzimidazole Membranes forHigh Temperature PEMFC. Fuel Cells 2014, 14, 7–15.
(4) Jang, S. Y.; Han, S. H. Characterization of Sulfonated Polystyrene-block-poly(ethyl-ranpropylene)-block-polystyrene Copolymer for Proton Exchange Membranes (PEMs). J.
Membr. Sci. 2013, 444, 1–8.
(5) Yan, X.; Gu, S.; He, G.; Wu, X.; Benziger, J. Imidazolium-functionalized Poly(ether ether
ketone) as Membrane and Electrode Ionomer for Low-temperature Alkaline Membrane
Direct Methanol Fuel Cell. J. Power. Sources 2014, 250, 90–97.
(6) Fujita, S.; Kamazawa, K.; Yamamoto, S.; Tyagi, M.; Araki, T.; Sugiyama, J.; Hasegawa, N.;
Kawasumi, M. Proton Conductivity under Dry Conditions for Mesoporous Silica with
Highly Dense Sulfonic Acid Groups. J. Phys. Chem. C 2013, 117, 8727−8736.
(7) Li, G.; Xie, J.; Cai, H.; Qiao, J. New Highly Proton-conducting Membrane Based on
Sulfonated Poly(arylene ether sulfone)s Containing Fluorophenyl Pendant Groups, for Lowtemperature Polymer Electrolyte Membrane Fuel Cells. Int. J. Hydrogen Energy 2014, 39,
2639−2648.

24

(8) Fu, L. C.; Xiao, G.Y.; Yan, D. Y. High Performance Sulfonated Poly(arylene ether
phosphine oxide) Membranes by Self-protected Cross-linking for Fuel Cells. J. Mater.
Chem. 2012, 22, 13714–13722.
(9) Peckham, T. J.; Holdcroft, S. Structure-Morphology-Property Relationships of NonPerfluorinated Proton-Conducting Membranes. Adv. Mater. 2010, 22, 4667–4690.
(10) Hoarfrost, M. L.; Tyagi, M. S.; Segalman, R. A.; Reimer, J. A. Effect of Confinement on
Proton Transport Mechanisms in Block Copolymer/Ionic Liquid Membranes.
Macromolecules 2012, 45, 3112−3120.
(11) Dishari, S. K.; Hickner, M. A. Confinement and Proton Transfer in NAFION Thin Films.
Macromolecules 2013, 46, 413−421.
(12) Kim, O.; Jo, G.; Park, Y. J.; Kim, S.; Park, M. J. Ion Transport Properties of SelfAssembled Polymer Electrolytes: The Role of Confinement and Interface. J. Phys. Chem.
Lett. 2013, 4, 2111−2117.
(13) Paul, D. K.; Fraser, A.; Pearce, J.; Karan, K. Understanding the Ionomer Structure and theProton Conduction Mechanism in PEFC Catalyst Layer: Adsorbed Nafion on Model
Substrate. ECS Trans. 2011, 41, 1393–1406.
(14) Nagao, Y.; Matsui, J.; Abe, T.; Hiramatsu, H.; Yamamoto, H.; Miyashita, T.; Sata, N.;
Yugami, H. Enhancement of Proton Transport in an Oriented Polypeptide Thin Film.
Langmuir, 2013, 29, 6798–6804.
(15) Miyatake, K.; Chikashige, Y.; Higuchi, E.; Watanabe, M. Tuned Polymer Electrolyte
Membranes Based on Aromatic Polyethers for Fuel Cell Applications. J. Am. Chem. Soc.
2007, 129, 3879–3887.

25

(16) Tan, S.; Belanger, D. Characterization and Transport Properties of Nafion/polyaniline
Composite Membranes. J. Phys. Chem. B 2005, 109, 23480–23490.
(17) Nakamoto, H.; Noda, A.; Hayamizu, K.; Hayashi, S.; Hamaguchi, H.; Watanabe, M. Protonconducting Properties of a Brønsted Acid-base Ionic Liquid and Ionic Melts Consisting ofBis(trifluoromethanesulfonyl)imide and Benzimidazole for Fuel Cell Electrolytes. J. Phys.
Chem. C 2007, 111, 1541–1548.
(18) Robertson, N. J.; Kostalik, H. A.; Clark, T. J.; Mutolo, P. F.; Abruna, H. D.; Coates, G. W.
Tunable High Performance Cross-Linked Alkaline Anion Exchange Membranes for Fuel
Cell Applications. J. Am. Chem. Soc. 2010, 132, 3400–3404.
(19) Krishnan, K.; Iwatsuki, H.; Hara, M.; Nagano, S.; Nagao, Y. Proton Conductivity
Enhancement in Oriented, Sulfonated Polyimide Thin Films. J. Mater. Chem. A 2014, 2,
6895– 6903.
(20) Qiao, J.; Okada, T.; Ono, H. High Molecular Weight PVA-modified PVA/PAMPS ProtonConducting Membranes with Increased Stability and their Application in DMFCs. Solid
State Ionics 2009, 180, 1318–1323.
(21) Tonozuka, I.; Yoshida, M.; Kaneko, K.; Takeoka, Y.; Rikukawa, M. Considerations ofPolymerization Method and Molecular Weight for Proton-conducting Poly(p-phenylene)
Derivatives. Polymer 2011, 52, 6020–6028.
(22) Zhang, J.; Zhou, T.; Qiao, J.; Liu, Y.; Zhang, J. Hydroxyl Anion Conducting Membranes
Poly(vinyl alcohol)/poly(diallyldimethylammonium chloride) for Alkaline Fuel Cell
Applications: Effect of Molecular Weight. Electrochimica Acta 2013, 111 351– 358.

26

(23) Yang, J. S.; Cleemann, L.N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler,
H. A.; Bjerrum, N. J.; He, R. H. High Molecular Weight Polybenzimidazole Membranes forHigh Temperature PEMFC. Fuel Cells 2014, 14, 7–15.
(24) Salleo, A.; Kline, R. J.; DeLongchamp, D. M.; Chabinyc, M. L. Microstructural
Characterization and Charge Transport in Thin Films of Conjugated Polymers. Adv. Mater.
2010, 22, 3812–3838.
(25) Sirringhaus, H.; Bird, M.; Zhao, N. Charge Transport Physics of Conjugated Polymer FieldEffect Transistors. Adv. Mater. 2010, 22, 3893–3898.
(26) Chen, M. S.; Niskala, J. R.; Unruh, D. A.; Chu, C. K.; Lee, O. P.; Frechet, J. M. J. Control
of Polymer-Packing Orientation in Thin Films through Synthetic Tailoring of Backbone
Coplanarity. Chem. Mater. 2013, 25, 4088−4096.
(27) Bass, M.; Beman, A.; Singh, A.; Konovalov, O.; Freger, V. Surface Structure of Nafion in
Vapor and Liquid. J. Phys. Chem. B 2010, 114,3784−3790.
(28) Bass, M.; Beman, A.; Singh, A.; Konovalov, O.; Freger, V. Surface-Induced Micelle
Orientation in Nafion Films. Macromolecules 2011, 44, 2893−2899.
(29) Wakita, J.; Sekino, H.; Sakai, K.; Urano, Y.; Ando, S. Molecular Design, Synthesis, andProperties of Highly Fluorescent Polyimides. J. Phys. Chem. B 2009, 113, 15212–15224.
(30) Hasegawa, T. Advanced Multiple-angle Incidence Resolution Spectrometry for Thin-layer
Analysis on a Low-refractive-index Substrate. Anal. Chem. 2007, 79, 4385–4389.
(31) Hasegawa, T. A Novel Measurement Technique of Pure Out-of-plane Vibrational Modes inThin Films on a Nonmetallic Material with No Polarizer. J. Phys. Chem. B 2002, 106,
4112–4115.

27

(32) Wakita, J.; Jin, S.; Shin, T. J.; Ree, M.; Ando, S. Analysis of Molecular Aggregation
Structures of Fully Aromatic and Semialiphatic Polyimide Films with Synchrotron Grazing
Incidence Wide-Angle X-ray Scattering. Macromolecules 2010, 43, 1930–1941.
(33) Miyamae, T.; Tsukagoshi, K.; Matsuoka, O.; Yamamoto, S.; Nozoye, H. Surface
Characterization of Polyamic Acid and Polyimide Films Prepared by Vapor Deposition
Polymerization by Using Sum-Frequency Generation. Langmuir 2001, 17, 8125–8130.
(34) Sung, J.; Kim, D.; Whang, C. N.; Oh-e, M.; Yokoyama, H. Sum-Frequency Vibrational
Spectroscopic Study of Polyimide Surfaces Made by Spin Coating and Ionized Cluster
Beam Deposition. J. Phys. Chem. B 2004, 108, 10991–10996.
(35) Kakuda, H.; Okada, T.; Hasegawa, T. Anisotropic Molecular Structure in Dip-Coated Filmsof Linear Poly(ethylene imine) Studied by Infrared Multiple-Angle Incidence Resolution
Spectrometry. J. Phys. Chem. B 2008, 112, 12940–12945.
(36) Matsunaga, M.; Suzuki, T.; Yamamoto, K.; Hasegawa, T. Molecular Structure Analysis in a
Dip-Coated Thin Film of Poly(2-perfluorooctylethyl acrylate) by Infrared Multiple-Angle
Incidence Resolution Spectrometry. Macromolecules 2008, 41, 5780–5784.
(37) Hasegawa, T. A New Spectroscopic Tool for Surface Layer Analysis: Multiple-angle
Incidence Resolution Spectrometry. Anal. Bioanal. Chem. 2007, 388, 7–15.
(38) Perez, M. A.; Ren, Y.; Farris, R. J.; Hsu, S. L. Characterization of PMDA-ODA Polyimide
Films by External Reflectance Infrared Spectroscopy. Macromolecules 1994, 27, 6740–
6745.

28

Table of Contents Graphic

29