Conducting Polymer Actuators

356 Pages·2014·25.41 MB·English

Conducting Polymer Actuators
by
John David Wyndham Madden
B.Sc. Physics, University of British Columbia (1991)
M.Eng. Biomedical Engineering McGill University (1995)
Submitted to the Department of Mechanical Engineering
in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SEPTEMBER 2000
2000 Massachusetts Institute of Technology. All rights reserved.
Signature of Author..... ..............................
Department of Mechanical Engineering
31 July, 2000
Certified by..........................................................
Ian W. Hunter
Hatsopoulos Professor of Mechanical Engineering
Thesis Supervisor
A ccepted by ............................................................
Ain A. Sonin
Professor of Mechanical Engineering
OF TECHNOLOGY
Chairman, Department Committee on Graduate Students
OCT 0 2 2000
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Conducting Polymer Actuators
by
John David Wyndham Madden
Submitted to the Department of Mechanical Engineering on July 31, 2000
in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in
Mechanical Engineering
Abstract
Conducting polymers are unusual organic materials featuring high electronic
conductivity. Recently it has been observed that some of these polymers change in
dimension when their oxidation state is altered. Dimensional changes induced by
electrochemical activation in the conducting polymer polypyrrole are investigated.
Swept sine and step voltage and current inputs are employed to investigate the nature of
this electro-mechanical coupling. Strains of up to 6 %, strain rates of 4 %/s, power to
mass ratios of 40 W/kg and forces of up to 34 MN/m2 are achieved. Polypyrrole nearly
equals mammalian skeletal muscle in power to mass and exceeds it in force by two orders
of magnitude. A model is developed which predicts the electrochemical impedance, and
relaf electrical input to mechanical output. Observations and modeling indicate that
diffusidh and capacitive charging limit strain rate. The use of thinner films is predicted to
increase strain rate and power to mass ratios by at least two orders of magnitude. Initial
applications are likely to be in microelectromechanical systems.
Thesis Supervisor: Ian W. Hunter
Title: Hatsopoulos Professor of Mechanical Engineering
Acknowledgements
Obtaining a Ph.D. is often regarded as an individual achievement. .However, the process
is one of maturation. The support, insight, criticism, vision and faith of others are an
essential part of the accomplishment.
Ian Hunter had the vision to foresee the importance of conducting polymers as a versatile
class of materials that may enable the low-cost manufacture of highly integrated, and
even life-like devices. As in many other instances, Ian was not daunted by the prospect
of entering a new field of research. His foresight and enthusiasm led me to investigate a
relatively unexplored property of conducting polymers, namely their ability to actuate.
Ian's vision is founded upon a deep and broad understanding of science and engineering.
The volume of specific and often quantitative knowledge he has amassed on topics that
include mathematics, mechanics, electronics, biology, chemistry, electromagnetism,
material science and history never ceases to amaze me.
I am very grateful to my wife Ann Madden for her faith in me, and for the sacrifice of
career and country she made so that we could be together.
My committee, Professors Tim Swager, Donald Sadoway, Michael Rubner and Seth
Lloyd, have made many valuable suggestions that have improved the quality of this
work. It is a pleasure to interact with them and to learn from them.
Peter Madden, Patrick Anquetil and Ann Madden all spent many hours proof-reading this
thesis, for which I am very grateful. Peter has been particularly helpful in discussing and
evaluating ideas, experimental methods, and results.
Tanya Kanigan and Peter Madden greatly improved the tensile strengths, conductivities,
and dimensions of polypyrrole samples. These developments led to the great
improvements in actuator performance and understanding of the underlying mechanisms.
Others also contributed to the synthesis process, including Patrick Anquetil, Kevin Joyce,
Yun-Ju Lee, Ryan Cush, and Gerda Nolan. Ryan Cush assisted greatly in the fabrication
and characterization of bilayer actuators.
Colin Brenan's suggestions and criticisms helped guide and improve this work at an early
stage. Serge Lafontaine's experience in characterizing shape memory alloy actuators,
and his vast expertise in computing and data acquisition were of enormous assistance. I
also benefited greatly from discussions with Dr. Sylvain Martel and James Tangorra.
Thanks to Richard Perilli for being there to help whenever and wherever it was needed.
Many thanks also to Susan Melillo for going beyond the call of duty to keep the lab
running smoothly. Leslie Regan greatly enhanced the MIT experience with her caring,
positive nature and pragmatic approach to bureaucracy.
Finally, I would like to thank my parents. I have been spoiled by their devotion, and am
humbled in striving to follow their examples. To them and to Ann I owe "the debt
immense of endless gratitude"*.
* Satan speaking of his creator in Paradise Lost, Book IV by John Milton, www.literature.org.
Table of Contents
Chapter 1: Introduction ................................................................... 10
1.1 Conducting Polymers............................................................. 10
1.2 Conducting Polymer Actuators ............................................. 13
1.3 Aim s..................................................................................... 16
1.4 Scope of the W ork ................................................................. 17
1.5 Reference List......................................................................... 18
Chapter 2: Conducting Polymer Actuators ..................................... 19
2.1 Conducting Polymers: Background....................................... 20
2.2 History of Conducting Polymer Actuators.............................. 22
2.3 Polyacetylene, Polyaniline and Polypyrrole Actuators...........26
2.3.1 Polacetylene......................................................................... .............. 2 6
2.3.2 Polyaniline ........................................................................... .............. 28
2.3.3 Polypyrrole Synthesis, Conductivity and Actuation.............. .............. 3 3
2.4 Polym er Actuator Performance .............................................. 43
2 .4 .1 S train ................................................................................... .............. 43
2 .4 .2 L o a d ..................................................................................... 43 ..............
2.4.3 Strain Rate ........................................................................... .............. 44
2.4.4 Power to M ass...................................................................... .............. 44
2.4.5 Efficiency....................................................................................... 44
2.4.6 Lifetime ......................................................................................... 45
2.5 Reference List........................................................................ 46
Chapter 3: Electrochemical Theory ................................................ 51
3.1 Electrolyte Impedance .......................................................... 52
3.2 Interfacial Impedance........................................................... 54
3.2.1 Double Layer Capacitance: M etal/Liquid Electrolyte ..................... 54
3.2.2 Electrochemical Reaction Impedance ............................................. 57
3.2.3 Reversible Reactions and the Nernst Equation................................. 58
3.2.4 Non-equilibrium Electrode Processes: Kinetics and Mass transport.... 65
3.3 Reference List...................................................................... 69
Chapter 4: Polypyrrole Synthesis..................................................... 70
4.1 Electrodeposition................................................................. 72
4.2 Film Characterization................................................................ 72
4.3 Reference List...................................................................... 75
Chapter 5: Actuator Characterization: Step Current and Voltage ... 77
5.1 Experimental M ethods ........................................................... 79
5.1.1 The Electromechanical Test Cell.................................................... 84
5.1.2 The Potentiostat .............................................................................. 87
5.1.3 The Current Source .......................................................................... 88
5.1.4 Load Cell and Amplifier................................................................. 89
5.1.5 The Stepper Mo tor, Controller and Linear Stage.............................. 89
5.1.6 Data Acquisition and Output .......................................................... 89
5.1.7 The Computer and Graphical User Interface................................... 90
5.2 Results................................................................................... 93
5.2.1 Passive M echanical Properties......................................................... 93
5.2.2 Electromechanical Response ............................................................ 101
5.2.3 Efficiency......................................................................................... 123
5.3 Discussion and Summary........................................................ 125
5.4 Reference List......................................................................... 126
Chapter 6: Actuator Operation Outside a Liquid Environment ..... 128
Chapter 7: M aximizing Strain Rate and Power ............................... 134
Chapter 8: System Identification and M odeling...............................144
8.1 Polymer Electromechanical Mo deling..................................... 147
8.2 Background ............................................................................ 147
8.3 Theory .................................................................................... 153
8.3.1 Assumptions .................................................................................... 153
8.3.2 M odel derivation: The Diffusive-Elastic 'M etal'. ............................. 156
8.3.3 Discussion and Implications of the M odel Admittance ..................... 168
8.3.4 Electro-M echanical Coupling ........................................................... 172
8.4 Summary: Diffusive-Elastic M etal.......................................... 185
8.5 Other M odels.......................................................................... 186
8.5.1 Admittance/Impedance M odels ........................................................ 187
8.5.2 M odels of Electro-M echanical Coupling .......................................... 197
8.5.3 M odel Uniqueness............................................................................ 199
8.6 M odel Variable Evaluation ..................................................... 199
8.7 Conclusion..............................................................................202
8.8 Reference List......................................................................... 203
Chapter 9: Experimental M ethods....................................................208
9.1 Advantages and Disadvantages of Swept Sine Analysis .......... 208
9.2 Description of the Apparatus................................................... 213
9.2.1 The Electrochemical-M echanical Cell.............................................. 216
9.2.2 The Dynamic Signal Analyzer.......................................................... 222
9.2.3 The Dynamic Signal Analyzer: Settings and Data Retrieval ............. 227
9.2.4 The Potentiostat ............................................................................... 228
9.2.5 Load Cells and Amplifier ................................................................. 241
9.2.6 The Stepping M otor, Controller and Linear Stage............................. 242
9.2.7 Data Acquisition and Output ............................................................ 242
9.2.8 The Computer and Graphical User Interface..................................... 243
9.3 M easurem ent Procedures ........................................................ 245
9.4 Sum ma ry ................................................................................ 249
9.5 Reference List......................................................................... 249
Chapter 10: Experim ental Results......................................................250
10.1 Polypyrrole Sample Properties ................................................ 251
10.2 Polym er Solvent Content ........................................................ 252
10.3 Passive M echanical Properties ................................................ 256
10.3.1 Dry to We t Transition ...................................................................... 256
10.3.2 Stress-Strain Curves ......................................................................... 258
10.4 Adm ittance ............................................................................. 262
10.4.1 Linearity .......................................................................................... 262
10.4.2 Surface/End Currents ....................................................................... 264
10.4.3 Adm ittance During Swelling ............................................................ 266
10.4.4 Estimating Double Layer Capacitance: Swelling and Aging ............. 270
10.4.5 Comparison of Modeled and Measured Admittances........................ 274
10.4.6 Effect of Ion Concentration .............................................................. 281
10.4.7 Effect of Film Thickness .................................................................. 283
10.4.8 Discussion of Admittance M odel...................................................... 288
10.5 Relationship between Strain and Charge ................................. 290
10.5.1 Linearity of the Strain to Charge Ratio ............................................. 292
10.5.2 Strain to Charge and Film Thickness ................................................ 292
10.5.3 Load Dependence of the Strain to Charge Ratio ............................... 295
10.5.4 Strain to Charge Summary ............................................................... 295
10.6 Relationship between Stress and Charge ................................. 297
10.7 Efficiency ............................................................................... 301
10.8 Overview ................................................................................ 306
10.9 Reference List......................................................................... 307
Chapter 11: Discussion and Conclusions ............................................ 309
11.1 Reference List......................................................................... 316
Chapter 12: Contributions to K nowledge...........................................317
12.1 M easurem ent of Actuator Properties ....................................... 317
12.2 M odeling ................................................................................ 321
12.3 Reference List......................................................................... 323
Appendix: Java Code............................................................................ 324