Behaviors of Polypyrrole Soft Actuators in LiTFSI or NaCl Electrolyte Solutions Containing Methanol

doi:10.4236/msce.2013.14001

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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

Email: *[email protected]

Received June 18, 2013; revised July 18, 2013; accepted August 18, 2013

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.

T. KADOYAMA ET AL.

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.

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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