**4.2.1 The experimental procedures were already described in the previous section. 4.3 Results and discussion**

Figure 4.1. shows the time dependences of the strain of the actuators, as measured by the displacement of the weight as a function of time under the load stress of 0.3 MPa. The strain in Fig. 4.1. was defined as the length change of the PPy actuators divided by the length prior to deformation. No potential voltage was given for the first 30 s, and after that repeated voltage sweeps with a period of 400 seconds at the potential sweep rate of 10 mVs-1 were applied to the PPy actuators.

Water 72.8 1.01

2-propanol 21.7 2.37

Table 2. Comparison of surface tension and viscocity for water and 2-propanol at 20 oC (data taken from the web page of the National Institute of Standards and Technology

Soft actuators were fabricated by 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 the aqueous solutions of an electrolyte, lithium bis(trifluoromethanesulphonyl)imide (LiTFSI), mixed with various concentrations of 2-propanol. The potential voltage range given to the actuators was between –1 and 1 V with a sweeping rate of 10 mVs-1, and a stress of 0.3 MPa was applied. The actuator exhibited nearly 3% of the electrochemical strain in the electrolyte solution without 2-propanol, and exhibited an electrochemical strain of up to 12% in the electrolyte solutions with the 2-propanol concentration between 20 and 40%. However, the reduction of the electrochemical strain and the acceleration of the creeping effect were observed when the actuators were deformed in the electrolyte solution with 2-propanol concentration of

**4. Dynamic behaviors of polypyrrole actuators in electrolyte solution mixed** 

Hoshino et al. tried to immerse TFSI-doped PPy films into several organic chemicals, and found that the PPy films showed notable swelling in 2-propanol. They reported that the PPy actuators showed the increased electrochemical strains in aqueous LiTFSI solutions containing 20 to 40% of 2-propanol as described in the previous section (Hoshino et al.,

In this section, the dynamic electrochemical deformation characteristics of TFSI-doped PPy soft actuators under potential sweep rates between 10 and 25 mVs-1 in aqueous LiTFSI

**4.2.1 The experimental procedures were already described in the previous section. 4.3** 

Figure 4.1. shows the time dependences of the strain of the actuators, as measured by the displacement of the weight as a function of time under the load stress of 0.3 MPa. The strain in Fig. 4.1. was defined as the length change of the PPy actuators divided by the length prior to deformation. No potential voltage was given for the first 30 s, and after that repeated voltage sweeps with a period of 400 seconds at the potential sweep rate of 10 mVs-1 were

solutions with different 2-propanol concentrations are reported.

(http://webbook.nist.gov/chemistry/fluid/).

**3.4 Conclusions** 

more than 60%.

**with 2-propanol 4.1 Introduction** 

**4.2 Experimental procedure** 

applied to the PPy actuators.

**Results and discussion** 

2011).

Surface tension (dyn.cm-1) Viscosity (mPa.s)

Fig. 4.1. Relationships between the strain of the actuator and time under the repeated operation of the actuator for 10 times with the potential sweep rate of 10 mVs-1. The actuators were electrochemically deformed in aqueous solutions of 1 M LiTFSI mixed with 2-propanol concentrations of 0 (water) and 20%. The load stress during the deformation was 0.3 MPa.

The characteristics of the actuators were measured in aqueous solutions of LiTFSI with the different 2-propanol concentrations of 0 (water) and 20%. During the initial 130 s of the first cycle, the PPy actuators immersed in the electrolyte solutions with the 2-propanol concentrations of 0 and 20% showed slight reduction of the PPy length. Here, it should be noted that the strain of the measured PPy actuator consists of the electrochemical strain caused by anion motions and the swelling (creep) strain. Therefore, the electrochemical strain was defined by the difference of the peak value minus the lowest values of the strain for each potential cycle. The electrochemical strain of the PPy actuator driven in the 0% 2 propanol electrolyte for the second cycle exhibited nearly 7%. In contrast, the electrochemical strain of the PPy actuator driven in the aqueous solution of LiTFSI containing 20% of 2-propanol was 12%. In this case, the electrochemical strain of the PPy actuator gradually decreased after the repeated potential cycles but it stayed around 10%, while the creep strain of the actuators continuously increased. This creep strain of the PPy actuator in the 20% 2-propanol electrolyte was larger than that in the 0% 2-propanol (water) electrolyte. This seems to suggest that the creep strain is larger in the former case due to the swelling of the PPy film in 2-propanol. These creep behaviors look similar to creep effects in metal deformation processes. Sendai et al. recently reported in their detailed study on creep effects of PPy actuators, and concluded that this elongation could be recovered by releasing the stress during the deformation (Chida et al., 2010). Hence, they called this phenomenon the memory effect. The dotted straight lines correspond to the tangents of the second peaks, and the tangential slopes correspond to the electrochemical strain rate of 0.25%s-1 for the actuator in the 20% 2-propanol electrolyte and 0.08%s-1 for the actuator in the 0% 2-propanol (water) electrolyte, respectively.

Polypyrrole Soft Actuators 177

Figure 4.3. shows the relationships between the creep strains and time of the PPy actuators driven in the electrolyte solutions with the different potential sweep rates. The time dependences look similar, which possibly suggests that the creep strain of the PPy actuators mostly depends on time but not on the potential sweep rate. This may mean that the PPy films swell both in the water and 2-propanol water solutions, and that the swelling of the PPy is larger in the 2-propanol electrolyte solution. This situation is more clearly described in Fig. 4.4. Figure 4.4. shows the potential sweep rate dependence of the creep strain measured at the time of 1000 s. The creep strains are mostly determined by the time under

Fig. 4.4. Sweep rate dependences of creep strains for PPy actuators in LiTFSI solutions with

Figure 4.5. shows the potential sweep rate dependences of electrochemical strain rate of the PPy actuator in the electrolyte solution containing 0 and 20% of 2-propanol. The electrochemical strain evidently is independent of the potential sweep rate, and those for the electrolyte solution with 20% 2-propanol are always larger than those for the electrolyte solution with 0% 2-propanol. If the ionic flow rate in the electrolyte solution influences the electrochemical strain, the electrochemical strain rate should be dependent on the potential sweep rate. This is because the potential sweep rate should modify the ionic flow rate in the electrolyte solution. Since this is not the case, another mechanism should be considered in the PPy actuator functions. Hara et al. reported that their TFSI-doped PPy actuators exhibited the maximum deformation when their aqueous LiTFSI electrolyte solution contains 20 to 40% of propylene carbonate (Hara et al., 2005). They pointed out that these phenomena were due to the swelling of the PPy film with propylene carbonate. The PPy film was reported to have porous and sponge like structures, and the swelling of the film

2-propanol concentrations of 0 (water) and 20%.

the 0.3 MPa stress.

It is interesting to investigate whether the response time of the PPy actuator improves when the potential sweep rate is increased. Figure 4.2. shows the potential sweep rate dependences of the electrochemical strain of the PPy actuators in the electrolyte solutions with 0 and 20% 2-propanol. The electrochemical strains continuously decreased with the sweep rate. The electrochemical strain of the PPy actuator in the electrolyte solution with 20% 2-propanol is larger than those in the electrolyte solution with 0% 2-propanol in the sweep rate range between 10 and 25 mVs-1.

Fig. 4.2. Sweep rate dependences of electrochemical strains for PPy actuators in LiTFSI solutions with 2-propanol concentrations of 0 (water) and 20%.

Fig. 4.3. Comparison of creep strains for PPy actuators driven in LiTFSI solutions of 0 (water) and 20% of 2-propanol with different potential sweep rates.

It is interesting to investigate whether the response time of the PPy actuator improves when the potential sweep rate is increased. Figure 4.2. shows the potential sweep rate dependences of the electrochemical strain of the PPy actuators in the electrolyte solutions with 0 and 20% 2-propanol. The electrochemical strains continuously decreased with the sweep rate. The electrochemical strain of the PPy actuator in the electrolyte solution with 20% 2-propanol is larger than those in the electrolyte solution with 0% 2-propanol in the

Fig. 4.2. Sweep rate dependences of electrochemical strains for PPy actuators in LiTFSI

Fig. 4.3. Comparison of creep strains for PPy actuators driven in LiTFSI solutions of 0

(water) and 20% of 2-propanol with different potential sweep rates.

solutions with 2-propanol concentrations of 0 (water) and 20%.

sweep rate range between 10 and 25 mVs-1.

Figure 4.3. shows the relationships between the creep strains and time of the PPy actuators driven in the electrolyte solutions with the different potential sweep rates. The time dependences look similar, which possibly suggests that the creep strain of the PPy actuators mostly depends on time but not on the potential sweep rate. This may mean that the PPy films swell both in the water and 2-propanol water solutions, and that the swelling of the PPy is larger in the 2-propanol electrolyte solution. This situation is more clearly described in Fig. 4.4. Figure 4.4. shows the potential sweep rate dependence of the creep strain measured at the time of 1000 s. The creep strains are mostly determined by the time under the 0.3 MPa stress.

Fig. 4.4. Sweep rate dependences of creep strains for PPy actuators in LiTFSI solutions with 2-propanol concentrations of 0 (water) and 20%.

Figure 4.5. shows the potential sweep rate dependences of electrochemical strain rate of the PPy actuator in the electrolyte solution containing 0 and 20% of 2-propanol. The electrochemical strain evidently is independent of the potential sweep rate, and those for the electrolyte solution with 20% 2-propanol are always larger than those for the electrolyte solution with 0% 2-propanol. If the ionic flow rate in the electrolyte solution influences the electrochemical strain, the electrochemical strain rate should be dependent on the potential sweep rate. This is because the potential sweep rate should modify the ionic flow rate in the electrolyte solution. Since this is not the case, another mechanism should be considered in the PPy actuator functions. Hara et al. reported that their TFSI-doped PPy actuators exhibited the maximum deformation when their aqueous LiTFSI electrolyte solution contains 20 to 40% of propylene carbonate (Hara et al., 2005). They pointed out that these phenomena were due to the swelling of the PPy film with propylene carbonate. The PPy film was reported to have porous and sponge like structures, and the swelling of the film

Polypyrrole Soft Actuators 179

film on a platinum wire (125 m in diameter) that was wrapped around with thinner (25 m in diameter) platinum wire. A maximum strain of 5% and the response of 10%s-1 were achieved. A similar structure to minimize the response time of PPy actuators was also reported by Hara et al. (Hara et al., 2003, 2004a, 2004b). They deposited a PPy film on a tungsten helical coil (250 m in diameter) made of a tungsten wire with a diameter of 30 m. This fibrous PPy actuator exhibited a strain of 11.6 % under the load of 0.2 N. The tungsten wire also helped to reduce the potential drop within the PPy for the improved performance. Another trial to increase the extension and contraction ratios employing a corrugated PPy structure has been reported (Morita et al., 2010), and a bimorph structure of a PPy actuator

In contrast, we considered applying these PPy soft actuator techniques to the actuation of the very small mechanisms used in silicon microelectromechanical systems (MEMS). To the best of our knowledge, there have been very few applications of these techniques to the actuation of small MEMS mechanisms (Chida et al., 2010, Guo et al., 1996, Hutchison et al., 2000). In this section, two kinds of PPy thin film actuators with or without a silicon MEMS

Figure 1 describes the designed actuator. The silicon microspring has a width of 0.5 mm, length of 15 mm, and thickness of 60 m. The microspring consists of silicon wires having a cross-section of 10 x 60 m2. The surface of this microspring is covered by a 91 m-thick PPy

Fig. 5.1. Design of the PPy-driven silicon linear actuator; (a) corresponds to designed silicon micro spring, and (b) corresponds to the designed linear actuator driven by PPy expansion

for more uniform bending has also been reported (Chida et al., 2010).

microspring were fabricated and compared.

**5.2 Linear actuator design** 

and contraction motions.

increased the porous spacing. This will make the anions more easily penetrate into the PPy film, which resulted in the increase of the deformation of the PPy actuators.

Fig. 4.5. Sweep rate dependences of electrochemical strain rates for PPy actuators in LiTFSI solutions with 2-propanol concentrations of 0 (water) and 20%.

#### **4.3 Conclusion**

The influences of the potential sweep rate on electrochemical and creep strains PPy actuators in aqueous LiTFSI electrolyte solutions with 2-propanol concentrations of 0 and 20% were compared under the load stress of 0.3 MPa. The electrochemical strain rates in the 0 and 20% 2-propanol solution were found to be approximately 0.1%s-1 and 0.25%s-1, respectively, and they are nearly independent of the sweep rate between 10 and 25 mVs-1. These results suggest that the TFSI anion penetration into the PPy films might be limited by the interactions between the electrolyte and the PPy surface, and the reduced viscosity in the LiTFSI electrolyte solution containing 2-propanol possibly enhances the doping and dedoping of TFST anions along with the swelling effect of the PPy film by 2-propanol. The creep rate was more rapidly increased as time elapsed in the electrolyte solution containing 20% 2-propanol, which was due to the swelling of PPy film in the electrolyte.
