Ionic Electroactive Polymer Composite Actuators

Open Access
Author:
Liu, Yang
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 25, 2012
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Srinivas A Tadigadapa, Committee Member
  • Shizhuo Yin, Committee Member
  • Ralph H Colby, Committee Member
Keywords:
  • Ionic Electroactive Polymer
  • Ionic Liquids
  • Actuator
Abstract:
In the past decades, ionic electroactive polymers (i-EAPs) have become attractive transduction materials due to their relatively large electromechanical actuations that can be generated under low voltages (~ a few volts) and many other advantages, such as flexibility and light weight. These i-EAP actuators hold promises for applications including artificial muscles, robots, micro-electromechanical systems (MEMS), nano-electromechanical systems (NEMS), and energy harvesting. On the other hand, one critical issue in utilizing i-EAPs for those applications is how to significantly improve the electromechanical performance, including the actuation speed, actuation strain level and efficiency. Particularly, extremely low efficiency is the major drawback of current i-EAP actuators. This dissertation discusses the physics in ionic polymer composite actuators and develops solutions to improve the actuation strain, speed, and efficiency. One general approach through the entire dissertation is to electrically and electromechanically analyze ionic polymer composite actuators to identify the limiting factors, and design a strategy to guide material selection or device fabrication for better performance. Two types of ionic polymer actuators will be discussed: (i) ionic polymer conductive network composite (IPCNC) actuators that consist of an ionic liquid (IL) containing polymer membrane and large surface area composite electrodes, referred to as the “IPCNC actuator”; and (ii) IL containing ionic polymer membranes with planar Au electrodes, referred to as the “ionic polymer membrane actuator”. While the composite electrodes in the IPCNCs provide large specific capacitance for ion storage which enhances the bending actuation, the ionic polymer membrane actuators offer a simple structure for device modeling and material analysis. IPCNC actuators are fabricated and discussed first. IPCNCs with disordered CNC nanomorphology (nanoscale morphology), mostly made with conducting nanoparticles, are discussed in Chapter 3, while IPCNCs with ordered CNC nanomorphology, in which vertically aligned carbon nanotubes (VA-CNTs) are implemented, are discussed in Chapter 4. In these chapters, electrical and electromechanical models are established to analyze the performance of IPCNC actuators. Firstly, electronic equivalent circuits are developed to model the complex frequency-dependent impedance and explain the initial response (t<10s) of ionic polymer composite actuators. It is found that the low frequency responses of these actuators indicate Warburg diffusion (semi-finite diffusion). When there is no CNC or when CNC is very thin (tens of nm), a simple equivalent circuit, which only consists of Warburg impedance AW, bulk resistance R, and electric double layer capacitance Cdl can be used, where Cdl and AW dominate the fast and slow responses of the actuator, respectively. When the CNC is thick (several µm), the aforementioned equivalent circuit is no longer adequate; therefore, the de Levi transmission line is introduced to model the CNC layer, which shows that the device transport time constant increases with CNC thickness. Secondly, by combining the time domain electric and electromechanical responses, a two-carrier model is established to explain the long time actuation response (t>10s), and also provide quantitative information on transport behavior of the two mobile ions (i.e., cation and anion) in IPCNCs. By employing this model, the total excess ions stored and strains generated by the cations and anions, and their transport times in the nanocomposites can be determined, which all depend critically on the morphologies of the conductor network nanocomposites. The model further reveals that, for EMI-Tf or EMI-BF4, anions have a larger effective ion size than cations, and thus dominate the long time response (t>10s) of the ionic polymer actuators, while cations account for the initial response (t<10s) of the actuators. Finally, IPCNCs with high volume fraction VA-CNTs are fabricated to improve the electromechanical performance of IPCNC actuators. The results demonstrate that the VA-CNTs create non-isotropic elastic modulus in the composite electrodes which markedly enhance the actuation strain (8.2%) compared to that of IPCNCs with RuO2 nanoparticles (2.1%), and provide inter-VA-CNT ion channels that enable faster actuation (τ=0.82 s) than that of IPCNCs with RuO2 nanoparticles (τ=3 s). Besides studies on IPCNC actuators, two other studies are performed on ionic polymer membrane actuators to understand the physics of ions transport in ionic liquid electrolytes and ion-polymer interaction, which are difficult to be tackled by directly studying the complex IPCNC actuators. Electrically, the ion distribution of the electrical double layer and the diffuse layer, which has been extensively analyzed with equivalent circuits, is of great interest. Hence, direct observations of the ion distribution in ionic polymer membrane actuator with 1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl) electrolyte are performed by employing Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The results reveal that Cl- and BMMI+ accumulate near both cathode and anode due to a strong Coulomb force between the ions and the ion size difference (17 times) causes the BMMI+ space charge layers (~6 µm) to be much thicker than those of Cl- (~0.5 µm). These findings emphasize the importance of engineering the electrolyte for IPCNC actuators, as well as show the dominance of large ions on the long time actuation response. At last, progress in addressing the electromechanical coupling between the ions and the electroactive polymers, is made by applying P(VDF-CTFE), crosslinked P(VDF-CTFE)/PMMA, and crosslinked P(VDF-TrFE-CFE) for ionic polymer membrane actuators. Compared to the conventional ionic polymers, such as Nafion and Aquivion ionomers that were popularly utilized in previous IPCNC actuator studies, these PVDF based polymers remarkably enhance the stress generation and significantly depress the charge consumption of the ionic polymer membrane actuators, both of which leads to higher energy conversion efficiency.