Conducting polymers are simultaneous sensing actuators

Conducting polymers are simultaneous sensing actuators

Fransisco G. Córdovaa, Yahya A. Ismailb, Jose G.Martineza, Ahmad S.Al Harrasic Toribio.F. Otero1

aCentre for Electrochemistry and Intelligent Materials (CEMI), Universidad Politécnica de

Cartagena, ETSII, E- 30203, Cartagena, Spain. bDepartment of Chemistry, College of Applied Sciences, A’Sharqiyah University,

Ibra – 400, Oman cDepartment of Biological Sciences and Chemistry, University of Nizwa, Nizwa-616,

Oman

ABSTRACT

Conducting polymers are soft, wet and reactive gels capable of mimicking biological functions. They are the

electrochemomechanical actuators having the ability to sense the surrounding variables simultaneously. The sensing and actuating signals are sent/received back through the same two connecting wires in these materials. The sensing ability is a general property of all conducting polymers arises from the unique electrochemical reaction taking place in them. This sensing ability is verified for two different conducting polymers here – for an electrochemically generated polypyrrole triple layer bending actuator exchanging cations and for a chemically generated polytoluidine linear actuator exchanging anions. The configuration of the polypyrrole actuator device corresponds to polypyrrole-dodecyl benzene sulfonate (pPy-DBS) film/tape/ pPy-DBS film in which the film on one side of the triple layer is acted as anode and the film on the other side acted as cathode simultaneously, and the films interchanged their role when move in the opposite direction. The polytoluidine linear actuator was fabricated using a hydrgel microfiber through in situ chemical polymerization. The sensing characteristics of these two actuators were studied as a function of their working conditions: applied current, electrolyte concentration and temperature in aqueous electrolytes. The chronopotentiometric responses were studied by applying square electrical currents for a specified time. For the pPy actuator it was set to produce angular movement of ± 45º by the free end of the actuator, consuming constant charges of 60 mC. In both the actuators the evolution of the muscle potential along the electrical current cycle was found to be a function of chemical and physical variables acting on the polymer reaction rates: electrolyte concentration, temperature or driving electrical current. The muscle potential evolved decreases with increasing electrolyte concentrations, increasing temperatures or decreasing driving electrical currents. The electrical energy consumed during reaction was a linear function of the working temperature or of the driving electrical current and a double logarithmic function of the electrolyte concentration. Thus, the conducting polymer based actuators exchanging cations or anions during electrical current flow is a sensor of the working physical and chemical conditions which is a general property of all conducting polymers.

Keywords: Conducting polymer, sensors, sensing actuator, electrochemical actuator, polypyrrole, polytoluidine, polyanilines

1. INTRODUCTION Conducting polymers are soft, wet and reactive gels capable of mimicking biological functions. A unique feature of conducting polymers is that they can be subjected to oxidization and reduction just like inorganic semiconductors. These reactions yield a very complex polymer-ions-solvent material combination. Due to this oxidation / reduction from their neutral state, conducting polymers experience a volume change. This volume change produced as a result of the electrochemical reactions can be exploited to perform a linear movement or an angular movement and can thus produce mechanical energy from chemical energy1,2. Thus they are regarded as potential material for the design

Electroactive Polymer Actuators and Devices (EAPAD) 2013, edited by Yoseph Bar-Cohen, Proc. of SPIE Vol. 8687, 868708 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2009609

Proc. of SPIE Vol. 8687 868708-1

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of artificial muscles3. The first conducting polymer actuators date back to 19924. Since the reactions leading to the volume change are electrochemical in nature, these actuators are also sensors of the environmental conditions surrounding the device5.

In conducting polymers, the electrochemical reactions leading to the volume changes are produced as a result of the flow of an electric current through the materials in presence of electrolytes1, 6 -8. According to the principles of chemical kinetics any physical or chemical variable acting on the reaction rate should promote a change of the electrode potential when the reaction occurs under constant current. Under such conditions the potential evolution of the reactive material during reaction is expected to be a sensor of the surrounding conditions.

Polypyrrole films exchanging anions during reactions can sense temperature and concentration variations9,10. Bilayer and three-layer artificial muscles constructed with those films sense any changes of those variables and the weight of any trailed material11,12. Tactile muscles able to sense the presence of an obstacle, providing information about the mechanical resistance have been developed13,14. In these cases the actuating signal (i.e. current) and the sensing signal (i.e. potential) are carried by the same two connecting wires.

For conducting polymers exchanging cations during reactions, volume changes and movements are in opposite direction to those of the anion exchange materials. These material swells during reduction and shrinks during oxidation. Recently we have demonstrated that both polypyrrole-DBS films and bilayer artificial muscles interchanging cations also sense, while reacting, surrounding physical and chemical variables15-17.

Apart from using electrochemically generated conducting polymer films for the demonstration of actuation and sensing characteristics, chemically generated conducting polymers are also proved to function as actuators and sensors. This is generally achieved by hybridizing the conducting polymers with other polymer (or hydrogel) nanofibers or microfibers 18-20.

We argue that all conducting polymer based actuators have the ability to sense their working conditions and this sensing ability is a general property of all conducting polymer based actuators. Here we verify this aspect by taking two different conducting polymers: an electrochemically generated polypyrrole based triple layer bending actuator exchanging cations and a chemically generated polytoluidine/hydrogel microfiber linear actuator exchanging anions.

2. EXPERIMENTAL 2.1. Chemicals

Pyrrole (Fluka®) and toluidine (Aldrich) were purified by distillation under vacuum and stored under nitrogen atmosphere at −5 ºC. Anhydrous lithium perchlorate (Fluka®) and dodecylbenzenesulfonic acid (DBSA) solution (70 wt. % in 2-propanol; Aldrich), Ammonium per sulphate (Aldrich), , Chitosan with an average molecular weight of 2 x 105 and 76% degree of deacetylation (Jakwang Co., S.Korea), methane sulfonic acid (Aldrich) and HCl (Aldrich) were used as received. Ultrapure water from Millipore Milli-Q equipment was used for preparing solutions.

2.2. Polypyrrole Film preparation and triple layer fabrication

Polypyrrole (pPy) films were prepared at room temperature (20 ± 2 ºC) in dark conditions in a one-compartment electrochemical cell containing 50mL of an aqueous solution of 0.2 M DBSA and 0.2 M pyrrole. An AISI 316 stainless steel sheet, with thickness of 1.24 ± 0.01 mm and having a surface area of 2×5 cm ± 0.05 cm each side was used as the working electrode. Deposition was performed on both sides of the electrode having an area of 2×3.3 cm ± 0.05 cm on each side. Two large electrodes of the same material were used as counter-electrodes. They were symmetrically placed at a distance of 1 ± 0.1 cm of separation from the working electrode to obtain a uniform electric field. A standard Ag/AgCl electrode (Metrohm®) was used as reference electrode.

The pPy films were generated by applying a constant anodic current density of 0.75 mA cm−2 for 1 hr, 40min. The overall charge consumed during the electro-polymerization was 60 C. After the electrogeneration, the pPy was polarized over the stainless steel working electrode at 0.5 V for 4 min in order to obtain an intermediate oxidation level. Two separate films were obtained from the electrode faces, with a mass of 26.42 ± 0.1 mg each, determined by means of a precision balance (Sartorious®, SC2 ). After peeling from the working electrode, the films were



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excess DBSA es of 36 ± 2 μ

mploying doube pPy-DBS fi± 2 μm. One si

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re adopted iugh in situ chersulfate (APtaining 0.01 m

of 0.0125 mol on was carried at a temperathanol, and all

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in the earlierhemical polymPS) as a catalmoles of toluof APS in 50

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The experiments related to the sensing characteristics were performed in a three electrode electrochemical cell assembly. The triple-layer actuator was held by an alligator clip with independent contacts, to the working electrode and to the counter electrode (Fig. 1). For pPy triple layer actuator, the electrochemical measurements were carried out in aqueous solutions of LiClO4. In all experiments, sensing abilities of the triple layer actuator were studied to describe the same amplitude of the angular movement of ± 45º. The angular motion measurement was obtained from images taken by a vision system using EVI-D31 SONY® digital cameras controlled by a Matrox® card and a control system. For experiments with the chemically generated hybrid microfiber, the Cs/pTd hybrid microfiber was connected to a platinum wire using conductive carbon paste and the electrochemical studies were performed using HCl as the electrolyte. An Ag/AgCl was used as reference electrode and a stainless steel plate was used as the counter electrode.

3. RESULTS AND DISCUSSION

3.1. Voltammogram of pPy triple layer actuator The pPy-DBS/tape/pPy-DBS triple layer actuator was characterised by using the cyclic voltammetric technique using the triple layer actuator functioning as the working electrode, stainless steel sheet as the counter electrode and an Ag/AgCl as the reference electrode. The cyclic voltammogram (CV) was recorded in 0.1 m LiClO4 aqueous solution at room temperature, between the potential limits of -0.8 V and 0.3 V at a scan rate of 5 mV/s. The CV shows the presence of one anodic maximum at 0.046V and a cathodic maximum at -0.54V (figure 2)8,12. A large potential separation of about 0.6 V between the peaks is due to the slow diffusion of balancing counter ions through the thick polypyrrole films. By integration of the anodic and cathodic branches of the voltammogram we could obtain the involved charge in the film oxidation, Qox (218.5 mC) reduction, Qred (216.9 mC).

Figure 2: Voltammogram of the pPy-DBS triple layer actuator ( Electrolyte 0.1M LiClO4, scan rate : 5mV s-1)

3.2. Sensing characteristics

Sensing characteristics of the pPy-DBS/tape/pPy-DBS triple layer actuator were studied under galvanostatic conditions. Before subjecting the microfiber to chronopotentiometric (CP) responses, the fiber was stabilized by recording the CV for up to10 cycles. Also, the triple layer was allowed to undergo an initial polarization by applying a constant current of -0.01mA for a period of 250 seconds. Then it was subjected to consecutive square



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gure 12. It hitosan is s at 0.38V raldine to on of the excellent

en -0.2 and

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3.3.1 Cs/pTd microfiber senses current Current sensing abilities of the pTd microfiber was studied using a galvanostatic procedure by recording CP and employing a similar procedures adopted for the demonstration of sensing abilities pPy triple layer actuator. Before studying the CP, the microfiber was stabilized by subjecting it to 10 voltammetric cycles. Then the microfiber was subjected to three consecutive square waves of currents. The microfiber was subjected to varying currents ranging from 5 µA to 75 µA and by passing a constant electrical charge of 1.35mC in 1M aqueous HCl solution. Higher potentials were required at higher currents before the actual electrode process begins. This is due to the different types of resistance associated with the microfiber (the microfiber contains insulating Cs in its matrix). Then the potential gradually increases for increasing anodic

-60 -30 0 30 60

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Elec

trica

l ene

rgy

/ mJ

Current / μA

Anodic Cathodic

Figure 13: Electrical energy consumed by a pTd microfiber as a function of the applied current obtained by the integration of CP for the anodic and cathodic processes.

currents following the oxidation of pTd and then decreases towards higher negative potentials for the increase of cathodic currents following its reduction. Then the electrical energy consumed during these oxidation and reduction process were calculated as Ee = i∫Edt, where I is the constant current density, E the potential and t is the time. Figure 13 shows the variation of the consumed electrical energy as a function of applied current. The linear fit of consumed electrical energy at different applied current for both the anodic and cathodic processes, indicates that the Cs/pTd microfiber actuator can sense current during electrochemical actuation.

3.3.2 Cs/pTd microfiber senses electrolyte concentration

Figure 14 shows the variation of consumed electrical energy during reaction ontained through the integration of CP responses for the anodic and cathodic process when the microfiber is subjected to a constant applied current of 0.02 mA for 30 sec for varying electrolyte concentrations (0.01M to 1M) of HCl. It can be seen that a semi logarithmic dependence of consumed electrical energy on electrolyte concentration in accordance with a gradual decrease of electrode potential with an increase of electrolyte concentration. It means that the Cs/pTd microfiber can act as a concentration sensor.

The electrode process occurring at the Cs/pTdy microfiber is :

(pTd)s + (nA-)solvent ↔ ( pTd)sn+ (A-)n (solvent)m + ne -

The forward process is the anodic oxidation during which the anion insertion occurs and polymer swells.

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-5 -4 -3 -2 -1 0

-0.40-0.35-0.30-0.25-0.20-0.15-0.10-0.050.000.050.100.150.200.250.300.350.400.45

Elec

trica

l ene

rgy

/ mJ

Ln([HCl]/M)

Anodic Cathodic

Figure 14: Electrical energy consumed by a pTd microfiber as a function of electrolyte (HCl) concentration by flow of ± 0.02 mA of currents in aqueous solution at room temperature.

The reverse process is the cathodic reduction during which anions goes out of the polymer chain and the polymer shrinks. The electrode potential evolved in this process is governed by the Nernst equation., i.e, the evolved potential changes as the concentration of the electrolyte changes. Therefore, keeping the current and temperature constant, the rate of anodic and cathodic process increases with increase of electrolyte concentration.

3.3.3 Cs/pTd microfiber senses temperature The reaction rates for the anodic and cathodic process, follow an Arrhenius dependence on temperature. Therefore, keeping current and concentration constant, the elctrode reactions will occur at a lower potential (lower resistance) at higher temperatures. For studying the temperature sensing character the pTd microfiber was subjected to a constant applied current of 0.025 mA for 38 second at different temperatures ranging

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5 10 15 20 25 30 35-0.8

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1.0

Elec

trica

l ene

rgy

/ mJ

Temperature

Anodic Cathodic

Figure 15: Electrical energy consumed by a pTd microfiber as a function of the temperature obtained from CP responses for anodic and cathodic processes for passage of constant electrical charge of 1.35mC in IM HCl.

from 5oC to 30oC. Figure 15 shows the variation of consumed electrical energy during the electrochemical reaction of the pTd hybrid microfiber as a function temperature. It can be seen that the consumed electrical energy during the electrochemical process varies linearly as a function of temperature - decreases (decrease of potential) for anodic process and increases for cathodic process with gradual increase of temperature suggesting that the pTd microfiber can act as a linear temperature sensor.

4. CONCLUSION Conducting polymer based actuators have the ability to sense their working conditions simultaneously while working. This sensing ability is a general property of all conducting polymer based actuators. We have verified this aspect by taking two different conducting polymers: an electrochemically generated polypyrrole triple layer bending actuator exchanging cations and a chemically generated polytoluidine/hydrogel microfiber linear actuator exchanging anions. The sensing characteristics were studied as a function of applied current, electrolyte concentration and temperature in aqueous electrolytes by recording the chronopotentiograms. For both the materials, the muscle potential evolved decreases with increasing electrolyte concentrations, increasing temperatures or decreasing driving electrical currents. The electrical energy consumed during reaction was found to be a linear function of the working temperature or of the driving electrical current and a double logarithmic function of the electrolyte concentration. These simultaneous and self-sensing properties derive from the reactions taking place in dense gels of conducting polymers: polypyrrole and polytoluidine. The devices based on these materials have an advantage that the reference electrode can be short circuited with the counter electrode, they provide simplicity and easiness in designing actuators for practical applications able to sense the ambient while working. We propose that any reactive device based on the same material and reaction (batteries, smart windows, electron-ion transducers, and so on) can sense their surrounding conditions.

REFERENCES [1] Madden, P. G. A., Madden, J. D. W., Anquetil, P. A., Vandesteeg, N. A., Hunter, I. W., "The relation of conducting polymer actuator material properties to performance", IEEE J. Oceanic Eng. 29(3), 696-705 (2004).

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[2] Aldissi, M (Ed.), [Intrinsically Conducting Polymers: An Emerging Technology], Kluwer Academic Publishers. Nato ASI Series E. Dordrecht., Vol. 246, (1993 and 2010).

[3] Hara, S., Zama, T., Takashima, W., Kaneto, K.., "TFSI-doped polypyrrole actuator with 26% strain", J. Mater. Chem. 14(10), 1516-1517 (2004).

[4] Otero,T.F, Angulo,E, Rodrigues,J, Santamaria C., "Electrochemomechanical properties from a bilayer –polypyrrol nonconducting and flexible material artificial muscle", J. Electroanal.chem. 341(1-2), 369-375 (1992).

[5] Otero, T. F., "Soft, wet, and reactive polymers. Sensing artificial muscles and conformational energy", J. Mater. Chem. 19(6), 681-689 (2009).

[6] Smela, E, "Conjugated polymer actuators for biomedical applications", Adv. Mater. 15 (6), 481-494 (2003).

[7] Otero T F, Sansinena J M, "Soft and wet conducting polymers for artificial muscles", Adv. Mater. 10 (6), 491- 94 (1998).

[8] Otero T F, Sansinena J M "Artificial Muscles based on conducting polymers", Bioelectrochem. Bioenerg. 38 411-14 (1995).

[9] Otero, T. F., Cortes, M. T., "A sensing muscle", Sensor. Actuat. B-Chem. 96(1-2), 152-156 (2003).

[10] Valero L, Arias-Pardilla J, Smit M, Cauich-Rodríguez J, and Otero T F., "Polypyrrole Free-Standing electrodes sense temperature or current during reaction" Polym. Int. 59, 337–342 ( 2010).

[11] Otero, TF, Cortes M T., "Artificial muscle: movement and position control", Chem. Commun..3, 284-85, (2004).

[12] Shoa T, Madden J D W, Mirfakhrai T, Alici G, Spinks G M and Wallace, G G, "Electromechanicalcoupling in polypyrrole sensors and actuators", Sensors and Actuators A: Physical 161, 127-33 (2010).

[13] Otero T F, and Cortés M T., "Artificial muscles with tactile sensitivity", Adv. Mat. 15, 279-82 (2010).

[14] Valero L, Arias-Pardilla J, Cauich-Rodríguez J, Smit M and Otero T F, "Sensing and tactile artificial muscles from reactive materials." Sensors, 10, 2638-674 (2010).

[15] Valero L, Arias-Pardilla J, Cauich-Rodríguez J, Smit M., and Otero, T F, "Characterization of the movement of polypyrrole-DBS-ClO4/tape artificial muscles. Faradaic control of reactive artificial molecular motors and muscles", Electrochim. Acta,56, 3721-3726 (2011).

[16] Valero L, Arias-Pardilla J, Smit M, Cauich-Rodríguez J, and Otero T. F, "Polypyrrole Free-Standing electrodes sense temperature or current during reaction", Polym. Int. 59, 337–342, (2010).

[17] Skotheim, T. A., Reynolds, J. R., [Handbook of Conducting Polymers], CRC Press, New York, (2006).

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[18] Ismail, Y. A., Shin, S. R., Shin, K. M., Yoon, S. G., Shon, K., Kim, S. I., Kim, S. J., "Electrochemical actuation in chitosan/polyaniline microfibers for artificial muscles fabricated using an in situ polymerization", Sensor. Actuat. B-Chem. 129(2), 834-840 (2008).

[19] Ismail, Y.A., Shin, M.K.,.Kim, S.J. " A nanofibrous hydrogel templated electrochemical actuator- from single mat to rolled up structures" Sensor. Actuat. B-Chem. 136, 438-443, (2009)

[20] Y.A. Ismail, J.G. Martinez, A.S. Al Harrasi, S.J. Kim, T.F. Otero "Polypyrrol/Chitosan hydrogel hybrid microfiber as sensing artificial muscle" Proced. SPIE 7976, Electroactive Polymer Actuators and devices (EAPAD) 2011, 79761L.

[21] Shimoda S, Smela E, "The effect of pH on polymerization and volume change in PPy(DBS)", Electrochim. Acta, 44 (2-3), 219-38 (1998).

[22] Koncar V,Cochrane C, Lewandowski M, Boussu F, Dufour C, "Electro-conductive sensors and heating elements based on conductive polymer composites", Int J Cloth Sci Tech. 21, 82-92 (2009).

[23] Otero T.F, Cortes M.T, Arenas, G.V, "Biomimetic sensing actuators based on conducting polymers" Electrochim. Acta. 53, 1252-58 (2007).

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Conducting polymers are simultaneous sensing actuators