Journal of Materials Science and Chemical Engineering, 2013, 1, 1-7
http://dx.doi.org/10.4236/msce.2013.14001 Published Online September 2013 (http://www.scirp.org/journal/msce)
Behaviors of Polypyrrole Soft Actuators in LiTFSI or NaCl
Electrolyte Solutions Containing Methanol
Tetsuya Kadoyama, Jun Yamasaki, Futo Tsumuji, Satoshi Takamiya, Shou Ogihara,
Daiki Hoshino, Yasushiro Nishioka*
Department of Precision Machinery, College of Science & Technology, Nihon University, Chiba, Japan
Received June 18, 2013; revised July 18, 2013; accepted August 18, 2013
Copyright © 2013 Tetsuya Kadoyama et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Organic soft linear actuators were fabricated using galvanostatic electropolymerization of the polypyrrole (PPy) thin
film using a methyl benzoate electrolyte solution of N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis
(trifluoromethanesulfonyl) imide. The electrochemical deformation behaviors of the PPy actuators were investigated in
aqueous solutions of an electrolyte, lithium bis (trifluoromethanesulphonyl) imide (LiTFSI) or sodium chloride (NaCl),
containing different concentrations of methanol. The actuating strain of approximately 9% was achieved when the ac-
tuator was driven by a potential between –1 and 1 V with the potential sweep rate of 10 mV/s corresponding to 0.0025
Hz in the LiTFSI electrolyte containing 40% to 50% of methanol under a load stress of 0.3 MPa. However, the PPy
actuator could not catch up with the higher frequency. On the other hand, the PPy actuator caught up with the potential
sweep up to 0.1 Hz in the NaCl solutions with a methanol concentration between 40% and 60% with the expense of the
actuating strain to approximately 1%.
Keywords: Soft Actuator; Polypyrrole; LiTFSI; NaCl
1. Introduction
Organic soft linear actuators made of conducting poly-
mers such as polypyrrole (PPy) films are of special in-
terest for application in microelectromechanical systems
(MEMS) because they generate large electrochemical
stress between 3 and 5 MPa and large strain [1-8]. Their
electrochemical strains (actuating strain) were between
1% and 3%. Recently, it has been reported that some PPy
actuators exhibited actuating strains of more than 10%
[9-14], and that some of those even achieved actuating
strains of up to 40% [14]. The improved actuating strain
has been mostly achieved using an electrolyte of tetra-n-
butylammonium bis (trifluoromethansulfonyl) imide
(TBATFSI) during PPy electropolymerization. These
actuators generally function under a low potential voltage
range less than 1 V.
In the research of PPy actuators, not only larger elec-
trochemical strain and stress but their operation speed is
another important issue, and a PPy bending actuator was
reported to operate at the frequency up to 90 Hz [15].
However, the force generated by the bending actuator
was much smaller compared to the PPy linear actuators
[16,17]. Similar PPy linear actuators were also reported
to operate with at the frequency of 30 Hz [18,19]. On the
other hand, the performance of PPy actuators were re-
ported to strongly dependent on the different kind of
cations in the electrolyte solutions during actuation
[20,21]. Kaneto et al. made systematic researches using
different kinds of electrolytes such LiCl, NaCl, etc. for
[22-24]. Hara et al. also reported that their TFSI-doped
porous PPy films exhibited increased actuating strains
when their aqueous lithium bis (trifluoromethansulfonyl)-
imide (LiTFSI) electrolyte solutions contained propylene
carbonate [25]. They attributed those effects to the
swelling of the PPy film caused by the penetration of
propylene carbonate. The swelled PPy film could more
easily pass TFSI anions. Hoshino et al. also found that
the PPy films showed notable increase of actuating
strains when they were functioned in LiTFSI solutions
containing 2-propanol [26] or methanol [27]. However,
the PPy actuators in the electrolyte solutions showed
notable electrochemical creep after repeated actuation
processes.
In this paper, we report on increased actuating strain of
PPy actuators but with minimal increase of electro-
chemical creep in a LiTFSI electrolyte solution contain-
*Corresponding author.
C
opyright © 2013 SciRes. MSCE

T. KADOYAMA ET AL. 2
ing optimized amounts of methanol. Moreover, we focus
on the influences of different ions in the electrolyte solu-
tions containing LiTFSI and NaCl during operation.
2. Experiments
The polymerization of PPy films was carried out using a
computer-controlled potentio-galvanostat. A counter elec-
trode (Ti), a reference electrode (Ag/AgCl), and a work-
ing electrode (Ti) were immersed into methyl benzoate
solutions of 0.25 M pyrrole and 0.2 M N,N-diethyl-N-
methyl-N-(2-methoxyethyl) ammonium bis (trifluoro-
methanesulfonyl) imide, and the potential voltage was
controlled to keep a constant current of 0.2 mA/cm2 for 4
h at 20˚C between the counter electrode and the working
electrode. These chemicals were purchased from Sigma-
Aldrich Inc.
The PPy actuator was used as the working electrode in
the 1 M LiTFSI aqueous electrolyte solutions or in the 1
M NaCl aqueous solutions with different methanol con-
centrations of 0% - 100%. The PPy actuator exhibited
expansion and contraction motions under the alternating
potential with the triangular wave shape applied between
the PPy actuator and the counter electrode. The peak
values of the potential voltage were −1 and +1 V, and the
potential sweep rates were 10 - 400 mV/s that correspond
to the frequencies between 0.0025 and 1 Hz. The exten-
sion and contraction of the PPy actuator was measured
by monitoring the displacement of the weight position
using a laser displacement sensor as described in the pre-
vious publications [26-29]. A load stress of 0.3 MPa was
applied on the PPy actuator.
3. Results and Discussion
Figure 1 shows a typical measurement result for the re-
lationship between the measured strain and time during
repeated actuations at 0.0025 Hz. This measurement was
performed in the LiTFSI electrolyte solution containing
40% of methanol. The averaged strain continuously
shifts to the positive strain direction due to electro-
chemical creep. Here, the electrochemical strain is de-
fined as the change of the averaged strain as shown the
dotted line in Figure 1. The difference between the peak
values and the bottom values of the strain is defined as
the actuating strain as indicated by the arrow in Figure 1.
Figures 2(a) and (b) show the comparison of the strain
as a function of time under the repeated potential voltage
change for actuators that were functioned in the LiTFSI
solutions containing various concentrations of methanol.
The actuators in the electrolyte solution with 20% and
80% methanol exhibited increased actuating strain com-
pared to that in the electrolyte solution without methanol,
and the actuating strains stayed at the similar level after
10 cycles of actuations as shown in Figure 2(a).
Figure 1. Relationship between strain and time for PPy
actuator in LiTFSI electrolyte solution containing 40%
methanol during electrochemical actuations at 0.0025 Hz.
(a)
(b)
Figure 2. Relationship between strain and time during elec-
trochemical actuations of PPy actuators in LiTFSI electro-
lyte solutions with different methanol concentrations at
0.0025 Hz.
The electrochemical creep (continuous back ground
change) gradually increased and approached to approxi-
mately 4%. On the other hand, when the methanol con-
centration was increased from 60% to 100%, the actuat-
ing strains of these actuators rapidly decreased after the

T. KADOYAMA ET AL. 3
repeated actuations as shown in Figure 2(b). On the
other hand, the electrochemical creep seemed to be sig-
nificantly smaller.
Figure 3 shows the relationships between the actuat-
ing strain and the methanol concentration after 10 cycles
of actuations. The actuating strain showed the maximum
value of 9% for the actuators functioned in the electrolyte
solutions containing 40% or 50% of methanol. The actu-
ating strain for the actuators functioned in the electrolyte
solutions containing more than 60% of methanol exhib-
ited rapid decreases.
Figure 4 shows the relationship between the electro-
chemical creep and the methanol concentration, and the
electrochemical creep continuously decreased as a func-
tion of the methanol above 40%.
Figures 5(a) and (b) compares the corresponding cy-
clic voltammograms of the PPy actuators functioned in
the electrolyte solutions containing various concentra-
tions of methanol. The current in the positive potential
voltage range corresponds to the motion of large TFSI−
anions, and the current in the negative potential voltage
range corresponds to the motion of small sized Li+
cations. Thus, a large volume change occurs in the posi-
tive potential voltage range. The largest hysteresis of the
PPy actuator driven in the electrolyte solution with
methanol implicates the enhancement of the TFSI ionic
motions into or outwards the PPy actuator. The largest
current with a large hysteresis occurred for the actuator
functioned in the electrolyte solution containing 40%
methanol, and the hysteresis curves continuously de-
creased as the methanol concentration increased above
60%. This may explain the decreased actuating strain
above the methanol concentration of 60%.
However, consistent explanations for these behaviors
of the PPy actuators functioned in the LiTFSI solutions
containing various concentration of methanol have not
been obtained. Table 1 compares the surface tension and
viscosity of pure water and methanol at 20˚C. The data
Figure 3. Change of actuating strain of PPy actuators in
LiTFSI electrolyte solutions with different methanol con-
centrations as measured at 0.0025 Hz.
Figure 4. Change of electrochemical creep of PPy actuators
in LiTFSI electrolyte solutions with different methanol
concentrations as measured at 0.0025 Hz.
(a)
(b)
Figure 5. Cyclic voltammograms of the PPy actuators func-
tioned in LiTFSI electrolyte solutions with different metha-
nol concentrations.
Table 1. Comparison of surface tension and viscocity.
Surface tension (dyn/cm) Viscosity (mPa·s)
Water 72.8 1.01
Methanol22.6 0.59

T. KADOYAMA ET AL. 4
were taken from the web page of the National Institute of
Standards and Technology (NIST).
The surface tensions of methanol are 22.6 dyn/cm,
which are nearly 30% that of water. Therefore, when the
PPy actuator is positively biased, TFSI anions along with
methanol molecules might more easily penetrate into the
porous structure of PPy. Thus, the increased expansion
was observed in the LiTFSI electrolyte solutions with
20% - 50% of methanol. The TFSI ions diffused into the
PPy porous structure in the positive potential region
could be disturbed to escape from the PPy structure due
to viscosity of the electrolyte solution in the negative
potential region. In contrast, the reduced disturbance for
the out diffusion of the TFSI anions from the PPy film
was expected because the viscosity of the methanol was
smaller than that of water. However, the decreased actu-
ating strains of the actuators in the electrolyte solutions
containing more than 60% of methanol after the repeated
actuations can not be fully explained by the discussions
above.
Higashi et al. recently reported the increase of
Young’s modulus after repeated actuation in aqueous
LiTFSI electrolyte solutions containing 0% (water), and
20% of methanol as summarized in Table 2 [29].
Young’s modulus increased from 0.13 to 0.45 GPa after
10 cycles of actuations in the electrolyte solution without
methanol, and it increased to 4.15 GPa after 10 cycles of
actuations in the electrolyte solution containing 20% of
methanol. In addition, notable reduction in the tensile
strength and the strains at break of the PPy films actuated
in the electrolyte solutions containing methanol were also
observed. Although the introduction of methanol in the
LiTFSI electrolyte solutions improves the actuating
strain of the PPy actuators, the hardening of the PPy ac-
tuators after the repeated actuations could be more sig-
nificant when the concentration of methanol was larger
than 60%, which may explain the decreased actuating
strains after the repeated actuations in the electrolyte
solutions containing methanol more than 60%.
Figure 6(a) shows comparisons of the relationships
between the strain and time of the PPy actuators in the
LiTFSI electrolyte solution and in the NaCl electrolyte
solution at the potential sweep rate of 200 mV/s corre-
sponding 0.05 Hz, and Figure 6(b) shows the relation-
ship between the strain and time during electrochemical
actuations of PPy actuators in NaCl electrolyte solutions
with different methanol concentrations at the frequency
of 0.05 Hz. The PPy actuator in the NaCl solution clearly
caught up with the bias change. Based on these results,
the PPy actuators were measured in aqueous solutions of
NaCl with different methanol concentrations of 0%, 20%,
40%, and 60%. The electrochemical strain was measured
with the frequency of 0.0125, 0.025, 0.05, 0.075 and 0.1
Hz. The PPy actuator in the solution containing 0%
(a)
(b)
Figure 6. (a) Comparison of relationships between the
strain and time of PPy actuators in LiTFSI electrolyte solu-
tion and in NaCl electrolyte solution at the potential sweep
rate of 200 mV/s corresponding 0.05 Hz, and (b) Relation-
ship between the strain and time during electrochemical
actuations of PPy actuators in NaCl electrolyte solutions
with different methanol concentrations at 0.05 Hz. The ini-
tial strain was adjusted for better view of the plots.
Table 2. Comparisons of Young’s modulus, tensile strength,
and tensile strain before and after 10 cycles actuations in
the LiTFSI solutions containing 0% of methanol, and the
electrolyte solution containing 20% of methanol.
Initial Young’s
modulus (GPa)
Tensile
strength (MPa)
Tensile strain
at break (%)
Before
actuation 0.13 6.5 19
Aqueous
solution 0.45 6.4 6.5
20%
methanol 4.15 3.8 4.5
methanol did not function at 0.075 Hz. However, the
actuator in the solutions containing 20%, or more
methanol functioned at 0.075 Hz. In addition, the PPy
actuator in the solutions containing 0% and 20% metha-

T. KADOYAMA ET AL. 5
nol did not catch up with the 0.1 Hz actuation. This phe-
nomenon was attributed to the fact that NaCl was not
sufficiently ionized. The actuator caught up with 0.1 Hz
actuation in the solutions containing 40% and 60%
methanol.
The electrochemical strains of the solutions containing
20% methanol became the largest. These results attribute
this phenomenon to the low viscosity and low surface
tension of methanol comparison with the water. There-
fore dopant might diffuse into the PPy film smoothly.
Figure 7 shows 1) relationships between the strain and
time of the PPy actuators in NaCl electrolyte solution at
the potential sweep rate of 300 mV/s corresponding
0.075 Hz, and 2) relationships between the strain and
time of PPy actuators in NaCl electrolyte solution at the
potential sweep rate of 400 mV/s corresponding 0.1 Hz.
Improvement of the strain was observed by mixing
methanol in the NaCl solution. The actuating strain was
increased by mixing 20% or more of methanol. The ac-
tuating strain was the largest by mixing 20% methanol
concentration at 0.075 Hz. On the other hand, the actuat-
ing strain was larger for the NaCl solution containing
40% - 60% at 0.1 Hz. Thus, the most suitable NaCl con-
centration depends on the actuation frequency. The actu-
ating strain in each methanol concentration at each oper-
ating speed was summarized in Table 3.
4. Summary
Soft actuators were fabricated using galvanostatic elec-
tropolymerization of a PPy thin film using a methyl ben-
zoate electrolyte solution of N,N-Diethyl-N-methyl-N-
(2-methoxyethyl) ammonium bis (trifluorome-thanesul-
fonyl) imide. The electrochemical deformation behaviors
of the PPy actuator were investigated in aqueous solu-
tions of an electrolyte, LiTFSI NaCl, containing different
concentrations of methanol. The actuating strain of ap-
proximately 9% was achieved when the actuator was
driven by a potential between –1 and 1 V with the poten-
tial sweep rate of 10 mV/s corresponding to 0.0025 Hz in
the LiTFSI electrolyte containing 40% to 50% of metha-
Table 3. The relationship between the actuation frequency
and associated actuating strain.
Methanol concentration (%)
Frequency (Hz)
0 20 40 60
0.0125 0.7 3.0 1.2 1.0
0.025 0.3 1.8 1.1 1.3
0.05 0.8 1.5 1.0 0.8
0.075 * 1.1 0.5 0.5
0.1 * * 0.5 0.7
*Actuation strains were under detection limit.
(a)
(b)
Figure 7. (a) Relationships between the strain and time of
PPy actuators in NaCl electrolyte solutions at the potential
sweep rate of 300 mV/s corresponding 0.075 Hz, and (b)
relationships between the strain and time of PPy actuators
in NaCl electrolyte solution at the potential sweep rate of
400 mV/s corresponding 0.1 Hz. The initial strain was
adjusted for better view of the plots.
nol under a load stress of 0.3 MPa. However, the PPy
actuator could not catch up with the higher frequency.
On the other hand, the PPy actuator caught up with the
potential sweep up to 0.1 Hz in the NaCl solutions with a
methanol concentration between 40% and 60% with the
expense of the actuating strain to approximately 1%.
5. Acknowledgements
The authors express appreciations to the staffs of the Mi-
cro Functional Device Research Center of Nihon Univer-
sity.
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