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Title

Synthesis and Proton transport property ofPoly(aspartic acid) thin film on SiO̲2 substrate

Author(s)Nagao, Yuki; Iguchi, Fumitada; Sata, Noriko;
Yugami, Hiroo

Citation

Solid State Ionics, 181(3‑4): 206‑209

Issue Date

2009‑04‑05

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/10630

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Yuki Nagao,
Fumitada Iguchi, Noriko Sata, Hiroo Yugami, Solid
State Ionics, 181(3‑4), 2009, 206‑209,
http://dx.doi.org/10.1016/j.ssi.2009.03.011

Description

Japan Advanced Institute of Science and Technology

1Synthesis and Proton transport property ofPoly(aspartic acid) thin film on SiO2 substrate

Yuki Nagaoa,b, Fumitada Iguchib, Noriko Satab, and Hiroo YugamibaCenter Research Strategy and Support, Tohoku University, 2-1-1 Katahira, Aoba-ku,

Sendai 980-8577, Japan and bGraduate School of Engineering, Tohoku University, 6-6-01
Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japanemail: y_nagao@energy.mech.tohoku.ac.jp

AbstractPoly(aspartic acid) and its thin film were synthesized to study proton transport properties.
The average degree of polymerization and molecular weight are 50 and 7000, respectively.
The proton conductivities of the Poly(aspartic acid) and its thin film were 4.3  10-4 and 5.1 
10-3 S cm-1, respectively under the relative humidity of 70 % and 298 K. The proton
conductivity of the thin film is one order of magnitude higher than that of the pelletizedPoly(aspartic acid). The activation energies for the proton conduction of the pelletizedPoly(aspartic acid) and thin film are 0.65 and 0.53 eV, respectively. The open circuit voltageof the pelletized sample exhibited ~0.9 V under the relative humidity of 70 % and 298 K.

Keywords: proton conduction, thin films

2

1. Introduction
The design of highly proton-conductive solid electrolyte is essential to many applicationsin the research field of solid state ionics. One of the fundamental methods to make highly
proton-conductive material is chemical modification. The properties of chemical stability andmechanical strength are often unfavorable by using chemical modification like sulfonation,
however the proton conductivity exhibit excellent value. Due to such a trade-off relation
between the proton conductivity, mechanical strength, and chemical stability, a new method
to make highly proton-conductive material has been expected for a long time. The ionic
conductivity enhancement in the 2-D system has been reported by many researchers [1 - 12].
These anomalies are supposed to be attributed to interfacial effects like structural effects
and/or effects of a space charge region. The interfacial effects for the proton conduction in the
2-D system also attract considerable attention. But the enhancement of the proton
conductivity has not been clearly reported in the thin films of the organic conductors.
Amino acid homopolymers and their derivatives, which have amide linkages as well as a
peptide, have been known to exhibit interesting biochemical properties. Poly(aspartic acid)has free carboxylic acid groups at the side chains, and its proton at the free carboxylic acidgroups is candidate for the mobile proton in the proton conduction. In this paper, we
synthesized Poly(aspartic acid), and investigated the proton transport properties of thepelletized sample and its thin film.

2. Experimental
Poly(aspartic acid) was synthesized as shown in Scheme. D, L-Aspartic acid and ophosphoric acid of reagent grade were commercially available and used without further
purification. D, L-Aspartic acid of 1 g and 85 % o-phosphoric acid of 600 l were added to3

100 ml round-bottom flask and mixed for 10 minutes under Ar atmosphere. The mixture washeated at 170 oC for 1 hour. The product was washed with methanol and water. The residue
was washed with methanol and dried under reduced pressure. The dried sample of 0.7 g and asolution of sodium hydroxide of 0.3 g with a deionized water of 20 ml were added to a 200 ml
beaker with stirring for 20 minutes. After the reaction, the pH of the solution was adjusted to~6 by the addition of 35 % aqueous HCl into the solution. The white precipitate was obtainedby adding methanol of 100 ml. The sample was washed with methanol, and was separated
from the supernatant, in which the centrifuge technique was used in the washing andseparation procedures. The obtained sample was dried under reduced pressure.
The quality of the sample was checked by the 1H NMR and Fourier-transform infrared(FT-IR) spectra measurements. The 1H NMR spectra in deuterium oxide, using Sodium 3(trimethylsilyl)propionate-2,2,3,3-d4 (TSP) as internal reference, were obtained on a ECA600 (JEOL) operated at 14 T. For the FT-IR absorption measurements, the sample was diluted
with KBr and the mixture was pressed into a pellet. The FT-IR spectra were measured with a
PerkinElmer Spectrum GX in the frequency range of 400 – 4000 cm-1. The average degree ofpolymerization was determined by comparing the ratio of the peak area derived from theproton of –CH2 at the amino end groups to that of the intrachain –CH2 and –CH2 at the
carboxyl end groups in the 1H NMR spectrum. The degree of protonation was determined by
the titrimetric method.
The thin films were synthesized by using a Active ACT-200 Spincoater on amorphous
SiO2 substrates. The thickness of the thin film was determined by using a KLA-Tencor P-10
contact stylus profiler. The obtained thin film is flat, and the thickness is 60 nm.
Impedance measurements of the pelletized sample and thin films were carried out in therelative humidity (RH) range of 40 – 70 % with a Solartron 1260 Impedance/Gain-Phase4analyzer and a 1296 Dielectric Interface system. The RH and temperature were controlledwith an Espec Corp. SH-221 humidity- and temperature-controlled chamber. In the case of
the impedance measurements of the pelletized sample, the sample was processed into pellets
of 2.5 mm under pressure of ~1 GPa and porous gold paint (Tokuriki Chemical Research,
SILBEST No. 8560) was used as electrodes. In the case of the impedance measurements ofthe thin films, the electrode configuration was selected to obtain measurements of the current
flow in the plane parallel to the substrate surface. The size of the membrane on SiO2 substrateis ~4.5 x 8.4 mm. The electrode by using Au paste is located at the edge of the membrane,
and electrode configuration is parallel. The distance between electrodes is 4.0 mm. The Au
electrode is also covered at the side of the membrane. The conductivity of the amorphous
SiO2 substrate is negligible because its resistance is quite higher than that of the thin film.
For the measurement of the open circuit voltage, the sample was processed into pellets of
13 mm. Pt-loaded carbon paper sheets (Pt: 1 mg cm-2) were used as electrodes. The
pelletized sample was sandwiched between anode and cathode. Humidified hydrogen (100 ml/ min) and oxygen (100 ml / min) were fed to anode and cathode, respectively. The celltemperature and relative humidity were controlled to be 298 K and 70 %, respectively.

3. Results and Discussion
3.1 CHARACTERIZATION
Figure 1 shows the NMR spectra of D, L-Aspartic acid and synthesized sample. The peakwidth in the synthesized sample becomes broad. This broad peak width suggests that thesynthesized sample is polymerized. The similar spectrum of sodium polyaspartate wasobserved by Tabata et al. [13]. They determined that sodium polyaspartate was composed of
30 % of -amide and 70 % of -amide units from the NMR spectrum as shown in Figure 2.
Our sample is found to be composed of 24 % of -amide and 76 % of -amide units. The

5inset Figure 1 shows magnified figure for the chemical shift from 2.0 to 3.4 ppm in the NMR
spectrum of synthesized polymer. These broad bands at 2.95 – 3.25 ppm are due to the protonof the –CH2 at the amino end groups. The average degree of polymerization was determinedby comparing the ratio of the peak area derived from the proton of –CH2 at the amino end
groups to that of the intrachain –CH2 and –CH2 at the carboxyl end groups. The average
degree of polymerization is found to be ~50. The degree of protonation in the carboxylic acidgroups was also determined by the titration method. The degree of protonation in the
carboxylic acid groups is found to be 6 %. The average molecular weight is calculated by the
degree of polymerization and the degree of protonation, and found to be ~7000. Thissynthesized polymer is determined to be called P-Asp in this paper.
Figure 3 shows FT-IR spectra of the D, L-Aspartic acid, Polysuccinimide, and P-Asp. In
the spectrum of Polysuccinimide, the broad peak at 1670 – 1770 cm-1 can be assigned as the
CO bond of the –CO-N-CO- (imide) groups. In the spectrum of P-Asp, two broad peaks areobserved at 1600 and 1650 cm-1. These peaks are due to the CO bond of -amide (-CH-CONH-) and -amide mode (-CH2-CO-NH-). These attributions are in accord with those of the
NMR spectra. The spectrum of P-Asp exhibits the broad absorption band over 3000 cm-1,
which is due to the OH and NH bonds. This result is suggested that P-Asp can adsorb watermolecules.

3.2 PROTON TRANSPORT PROPERTIES OF THE PELLETIZED P-ASP AND P-ASP THIN FILM
Figure 4 shows the RH dependence of the proton conductivity () for the pelletized P-Asp
and P-Asp thin film. A resistance is directly given by the intersection of a depressed
semicircle with the real axis in the complex plane impedance plots. In the proton conduction

6of the pelletized P-Asp, the proton conductivity increases by over three orders of magnitudewith increasing the RH from 40 to 70 %. Such a large dependence of the proton conductivity
on the RH could arise from the proton conduction paths developed by the water uptake. The
open circuit voltage of the pelletized sample exhibited ~0.9 V, even if the tablet was found tobe cracked after the measurement of open circuit voltage. This result indicates that the protontransport number is high and the carrier is proton in the sample, however we do not know yet
whether Na+ ions affect AC conductivity or not. The proton conductivity of the pelletized PAsp is 4.3  10-4 S cm-1 under the RH of 70 % and 298 K. In the proton conduction of P-Asp
thin film, the proton conductivity of the 60 nm thick thin film is found to be 5.1  10-3 S cm-1
under the RH of 70 % and 298 K. This value is found to be over one order of magnitude
higher than that of the pelletized P-Asp under the same condition, and is half of that of Nafion
117 membrane (1.3  10-2 S cm-1) [14].
Figure 5 shows the temperature dependence of the proton conductivity (Arrhenius plots)
for the pelletized P-Asp and P-Asp thin film at the RH of 70 %. The proton conductivity ofthe thin film exhibits higher values than that of the pelletized P-Asp. The activation energy ofthe pelletized P-Asp and P-Asp thin film is 0.65 and 0.53 eV, respectively. The activationenergy of P-Asp thin film is found to be 0.12 eV smaller than that of the pelletized P-Asp.
One of the origins for increasing the proton conductivity of P-Asp thin film is due to thedecrease of the activation energy for the proton conduction.
We observed the proton conductivity was enhanced by the synthesis of the thin film. In
many kinds of the inorganic ionic conductors, some theoretical models including the spacecharge models for understanding the enhancement phenomena of the ionic conductivities
have been discussed [15]. But the obvious thickness dependence of the proton conductivity
for P-Asp thin films has not been observed yet. Therefore this enhancement phenomenon

7

could be originated from other effects like a molecular orientation by the synthesis of the thinfilm on the substrate rather than the interface effects like the space charge models in thissystem. The amide bond causes interchain hydrogen bonds between the polymers. The amidegroups also exhibit the proton exchange property because of the amide/imidic acidtautomerism. The proton conduction paths could be expected to be formed by using theseproperties and water molecules including in the sample considering the relatively highactivation energy. One of the possible reasons for this enhancement of the proton conductivity
is due to the molecular orientation because it causes that the proton can easily move in thesample along these paths. This supposition can explain the reason why the activation energy
for the proton conduciton of the thin film decreases as compared to that of the pelletizedsample. Further investigation is required to understand this phenomenon.

4. Conclusions
Proton-conductive polyamide, Poly(aspartic acid), was synthesized to investigate theproton transport properties of the pelletized sample and its thin film. The average degree ofpolymerization and molecular weight are 50 and 7000, respectively. The degree ofprotonation in the carboxylic acid groups is 6 %. The open circuit voltage of the pelletizedsample exhibited ~0.9 V under the relative humidity of 70 % and 298 K. The proton
conductivity of the pelletized sample and its thin film is 4.3  10-4 and 5.1  10-3 S cm-1,
respectively under the RH of 70 % and 298 K. The activation energy of the pelletized P-Asp
and P-Asp thin film is 0.65 and 0.53 eV, respectively. The activation energy of P-Asp thinfilm is found to be 0.12 eV smaller than that of the pelletized P-Asp. We observed the proton
conductivity was enhanced by the synthesis of the thin films.

8

Acknowledgements
The authors thank M. Sasaki for helping the synthesis of the proton-conductive amides andthank Dr. H. Maekawa and M. Ando for giving me a chance to measure the NMR spectra.
This study was supported by Program for Improvement of Research Environment for Young
Researchers from Special Coordination Funds for Promoting Science and Technology (SCF)
commissioned by the Ministry of Education, Culture, Sports, Science and Technology(MEXT) of Japan.

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Scheme Caption
Scheme : Chemical structure and reaction scheme of Poly(aspartic acid) from D, L-aspartic
acid.

11

Scheme12

Figure Captions
Fig. 1:. 1H NMR spectra of D, L-Aspartic acid and synthesized sample. Inset figure ismagnified figure for the chemical shift from 2.0 to 3.4 ppm in the NMR spectrum ofsynthesized sample.
Fig. 2: Structure of sodium polyaspartate composed of 30 % of -amide and 70 % of -amide
units.
Fig. 3: IR spectra of the D, L-Aspartic acid, Polysuccinimide, and P-Asp.
Fig. 4: RH dependence of the proton conductivity with the pelletized P-Asp and its thin film.
Fig. 5: Temperature dependence of the proton conductivity with the pelletized P-Asp and its
thin film.

13Fig. 1

14

Fig. 2

15

Fig. 3

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Fig. 4

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Fig. 5