Electrochemomechanical characterization of the trilayer structure

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Electro-chemical properties

Before beginning the full characterization, different sample configurations were fabricated to adapt for different measurements (refer to section 2.6.1 for list of fabricated samples). It is worth noting that trilayers of varying thickness were fabricated and tested in actuation, to find a dimension range where significant bending actuation is achieved, for easy characterization. Ten trilayer membranes were fabricated on ten separate silicon wafers, in which the thicknesses of PEDOT electrodes were adjusted from 0.8 µm to 2.3 µm to investigate the effect of the PEDOT’s thickness on the strain output of the trilayer actuator. Bending actuation experiments were observed on 4 samples, where the thicknesses of PEDOT electrodes vary from 1.2 µm to 2.3 µm. These exhibit bending under a voltage excitation. The biggest displacement was seen in a sample having a thickness of PEDOT:NBR/PEO:PEDOT = 2.2 µm:7 µm: 2.2 µm (pre-swelling dimensions). After swelling in ionic liquid, the thicknesses of the PEDOT electrodes and of the NBR/PEO layer are approximately 3.5 µm and 10 µm, respectively. Therefore, this thickness ratio (PEDOT:NBR/PEO:PEDOT = 3.5 µm:10 µm:3.5 µm after swelling in ionic liquid) is chosen for further investigation on the volumetric capacitance, the possible short circuit between two PEDOT electrodes, the Young’s moduli, the damping ratio, and the linear strain as well as the bending strain.

Ionic conductivity of the SPE and PEDOT layers

Ionic conductivity is one of a number of properties that limit current, and, as a result, charging speed. [8]. Therefore, it is a factor that can limit strain rate and bending speed of the trilayer actuator. PEDOT ionic conductivity was determined using an Electrochemical Impedance Spectroscopy (EIS) measurement according to a procedure described previously [8, 9] and summarized in Fig. 2a. A Solartron 1287A Potentiostat/Galvanostat combined with a Solartron 1260A frequency response analyzer was used to obtain the frequency responses. A 4-point measurement across the ionic liquid and the membrane (a bilayer or a trilayer) is used. We impose current between the working electrode (W.E) and the counter electrode (C.E) and detect a local potential drop between two reference electrodes (R.Es) (Fig. 2a). The W.E and the C.E are made of two pieces of glassy carbon, and the R.Es are the classic Ag/AgCl wire filled 4M NaCl. It has been shown that there is an asymmetry between the top and the bottom PEDOT electrodes in a trilayer actuator fabricated by layer-by-layer method (chap. 2, section 2.7) [10]. As demonstrated, the roughness of the top PEDOT electrode is ten times higher than that of the bottom PEDOT electrode. This fact may lead to asymmetric properties between these two electrodes. Therefore, there is a need to characterize separately each electrode. A top bilayer (PEDOT electrode is polymerized on the top of NBR/PEO) and a bottom bilayer (PEDOT electrode is polymerized on a silicon wafer followed by a NBR/PEO layer) were investigated to find out the variation in PEDOT ionic conductivity, shown in Fig. 2b. Before the measurement, an NBR/PEO membrane, as well as the top bilayer and the bottom bilayer samples were immersed into EMITFSI for one week [11], to ensure a full uptake of ions. Because the resistance of each layer depends on their thickness, to separate the PEDOT ionic resistance from other sources of resistance such as NBR/PEO resistance and ionic liquid resistance, the thickness of specimens including the thickness of NBR/PEO and PEDOT (Table 1) was increased during the fabrication process. It is worth of noticing that the thickness of swollen PEDOT layers were not measured directly but it was determined by SEM in its dry state and then multiplied by 1.66, which is the swelling factor in EMITFSI deriving from chapter 2 (section 2.7).

Electrical conductivity of the PEDOT electrodes

Researchers have demonstrated that the electronic conductivity of PEDOT depends on the potential applied [2-4]. Feldman et al. [2] shown that a conductivity of a 13.9 µm thick PPy film in 0.1 M Et4NClO4/CH3CN quite constant at approximately 3 x 10-1 S/cm as the potential of the film is decreased from 0.4 V to 0 V. When the potential continues to decrease to -0.6 V, the conductivity enormously drops to 10-6 S/cm. Warren et al. [3] and Farajollahi et al. [4, 14] found a similar trend where the conductivity of a 25 µm thick PPy film increases from 0.64 S/cm to 270 S/cm when the applied potential raises from -0.8 V to 0.4 V. This drop with decreasing voltage can be explained: the PPy electrode is indeed reduced and the doping level is decreased which reduces the charge carrier concentration, the mobility, and in turn the conductivity. In my work, the electrical conductivities of the thin sIPN PEDOT/PEO electrodes will be characterized to explore their dependences on the potential applied.
Fig. 4 presents a three-electrode system (Solartron potentiostat, ModuLab XM ECS) for cyclic voltammetry analysis. The counter electrode was a platinum foil with a surface area greater than that of the trilayer actuator electrode (at the working electrode). The reference electrode was a standard wire Ag/AgCl filled with 4M NaCl solution. The potentiostat was connected to both trilayer electrodes using platinum clamps and then fully immersed in EMITFSI. This scheme is used to reduce or oxidize the specimens to a certain potential (compared to Ag/AgCl reference electrode). To measure the electrical conductivity as a function of oxidation state, a potential (versus Ag/AgCl reference electrode) was applied to both sides of a trilayer structure in solution (Fig. 5). The dimensions of the specimen are length (L) х width (b) х total thickness (h) = 10 mm х 3 mm х 0.017 mm in the swollen state, the thickness of the PEDOT layers is hP = 3.5 µm and the thickness of NBR/PEO is hS = 10 µm. The length and width of the sample were increased to adapt them to the 4-line measurement setup in Fig. 5.

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Volumetric capacitance of the PEDOT electrodes

The volumetric capacitance describes the amount of charge per volume of electrode that is stored in response to a change of applied voltage. It also describes the ability to accommodate ions of the conducting polymer film during its actuation, since all electronic charge is balanced by ionic charge. Researchers have recently tried to determine the volumetric capacitance of pure PEDOT or PEDOT:PSS thin films [15, 16]. They found that this capacitance not only depends on the thickness and the density of PEDOT – determined by the VPP time and the coating speed – but also on the type of dopant, as smaller dopants increase the volumetric capacitance to a certain extent. In addition, Madden et al. [17, 18] have demonstrated a variation in the volumetric capacitance value of the polypyrrole as a function of applied voltage. Since the trilayer structure works as an actuator, a variation in the applied voltage is expected. For our measurement, the working electrode in Fig. 5 was connected to both electrodes of the trilayer actuator using platinum clamps and then fully immersed in EMImTFSI. The dimensions of the trilayer are length (L) х width (b) х total thickness (h) = 5 mm х 1 mm х 0.017 mm in the swollen state, and the thickness of each layer is hP = 3.5 µm and hS = 10 µm. In the first experiment, the volumetric capacitance of the PEDOT electrodes was investigated as a function of the scanning rate while the potential window was held constant. The potential was swept from U1 = -0.6 V to U2 = 0.7 V vs. Ag/AgCl reference electrode at scan rates, , varying from 0.005 V/s to 1 V/s (Fig. 7a). This potential window is carefully chosen to cover the oxidation and reduction peaks of the PEDOT electrodes during the redox process, but not too high to accelerate other reactions due to the presence of impurity substances inside ionic liquid.

Table of contents :

Abstract
Résumé
Résumé Substantiel
Lay Summary
Preface
Table of Contents
List of Figures
List of Tables
Abbreviations
Acknowledgements
Dedication
Chapter 1: Introduction
1.1 Mammalian muscles
1.2 Artificial muscles
1.3 Motivation and problem statement
1.4 Thesis structure
Chapter 2: PEDOT-based trilayer fabrication process
2.1 Introduction
2.2 The selection of materials for CP-based trilayer actuators
2.2.1 Electrodes of the microactuators
2.2.2 Solid polymer electrolyte layer
2.2.3 The electrolyte
2.2.4 Microactuator fabrication technique
2.3 Materials
2.4 PEDOT synthesis route
2.5 PEDOT-based trilayer fabrication process
2.5.1 Trilayer fabrication process
2.6 PEDOT-based trilayer patterning
2.6.1 Fabrication of samples for the characterization process
2.7 Analysis of the texture of the trilayer structure
2.8 Conclusion
Chapter 3: Electrochemomechanical characterization of the trilayer structure
3.1 Introduction
3.2 Electro-chemical properties
3.2.1 Ionic conductivity of the SPE and PEDOT layers
3.2.2 Electrical conductivity of the PEDOT electrodes
3.2.3 Volumetric capacitance of the PEDOT electrodes
3.2.4 Possible short circuit between two PEDOT layers
3.3 Mechanical properties
3.3.1 Youngs moduli of the SPE layer and of the trilayer actuator
3.3.2 Damping ratio
3.3.3 Blocking force characterization
3.4 Empirical strain-to-charge ratio
3.4.1 Strain to charge ratio
3.4.2 Linear strain
3.5 Conclusion
Chapter 4: Linear dynamic and nonlinear dynamic model to predict PEDOT-based trilayer actuation behavior
4.1 Motivation
4.1.1 Objectives
4.1.2 Proposed methodology
4.2 State of art
4.2.1 A summary of Black box, white box, grey-box models for CP actuators
4.2.1.1 Black-box model
4.2.1.2 Grey-box model
4.2.1.3 White-box model
4.2.2 Why the choice of the Bond Graph language?
4.3 Dynamic Bond Graph modeling
4.3.1 Actuation description
4.3.2 Word Bond Graph model
4.3.3 BG submodels
4.3.3.1 Electrical model
4.3.3.2 Electromechanical coupling
4.3.3.3 Mechanical model
4.3.4 BG global models
4.4 Simulation tests
4.4.1 Software implementation
4.4.2 Comparison between the linear and nonlinear simulations
4.5 Comparison to experimental tests
4.5.1 Time domain responses
4.5.2 Frequency responses
4.6 Parameter sensitivity and power performance analysis
4.7 Conclusion
Chapter 5: Sensing ability and sensing model of the PEDOT-based trilayer actuators 
5.1 Introduction
5.2 Theories on the mechanoelectrical effects
5.3 Sensing modeling
5.3.1 Mechanoelectrical coupling
5.3.2 Global model
5.4 Experimental setup
5.5 Results
5.5.1 Comparison between the model simulation and experimental results
5.6 Discussion
5.6.1 Force in response to a step displacement
5.7 Conclusion
Chapter 6: Conclusion and outlook
Appendix
A.2 Chapter 2: PEDOT-based trilayer fabrication process
A.2.1 Optimization of electrochemical properties of PEDOT electrodes
A.2.2 Surface measurement method
A.3 Chapter 3: Electrochemomechanical characterization of the trilayer structure
A.3.1 Qualitative explanation the apparent capacitance of the PEDOT electrodes at extreme low scan rate
A.4 Chapter 4: Linear dynamic and non-linear dynamic model to predict PEDOT-based trilayer actuation behavior
A.4.1 The coupling matrix derivation method
A.5 Chapter 5: Sensing ability and sensing model of the PEDOT-based trilayer actuators
A.5.1 A possible qualitative sensing model ….

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