HIGH ELECTROMECHANICAL RESPONSE ELECTROACTIVE POLYMERS AND THEIR APPLICATIONS FOR SOLID STATE ACTUATORS

Open Access
Author:
Liu, Sheng
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 22, 2010
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Craig Grimes, Committee Member
  • Srinivas A Tadigadapa, Committee Member
  • Ralph H Colby, Committee Member
Keywords:
  • electroactive polymers
  • actuators
  • electromechanical
  • ionic polymer
  • electrostrictive polymers
Abstract:
The emerging large integration micro- and macro-electromechanical systems based on soft materials such as microfluidics, biomedical devices, flexible optical display, energy harvesting, and robotics demand actuators with high performance, flexibility, fracture tolerance, light weight, and on-chip integration availability, which cannot be met by traditional inorganic actuator materials such as piezoceramics and shape memory alloys. Electroactive polymers (EAP) with large strain, high elastic energy density, easy processing, and tailored properties hold the promise to meet these challenges. The ionic polymer conductor network composite (IPCNC) actuators which are operated at very low operation voltage (<5 V) offer additional benefits in micro- and macro-scale applications because they can be directly integrated with advanced microelectronics. Compared with other ionic EAPs which can also be operated with low operation voltage, IPCNC actuators which provide broader improvement space with their flexible structure and material selectivity are chosen in this dissertation study. This dissertation discusses the limiting factors in IPCNC actuators and develops solutions to overcome the barriers of low actuation speed and electromechanical efficiency. Two important components, conductor network composites (CNCs) and electrolytes, which determine performance of IPCNCs are investigated and analyzed for developing improvement solutions. CNCs where the insertion of ions generates actuation strain, have been found to play a key role on IPCNC performance. This is the first time to clarify the importance of CNC thickness in determining the IPCNC actuator speed. In IPCNC actuators, thin CNC layers will lead to fast actuation speed. The possibility of realizing fast speed in IPCNC actuators with thick CNCs is also investigated by controlling CNC microstructure morphology. In IPCNC actuators, it is CNC layers that control actuator speed. Ion transport in traditional porous composite electrodes is slow because of long transport paths caused by both large thickness and random morphology. CNCs with thickness more than 20 &#956;m are usually fabricated due to the limitation brought by the fabrication methods. To solve the problem, layer-by-layer (LbL) self assembly with fine thickness control ability is used to fabricate CNC layers, with ultrathin thickness (sub-micrometer). Increases in actuator speed (response time &#8776; 0.18 s) is demonstrated with LbL CNCs. An additional improvement is also shown where strain has been increased to 6.8%, compared with traditional IPCNC or ionic polymer metal composite (IPMC) (maximum strain &#8776; 3.3%). Also, thin CNCs have led to both high electrical and electromechanical efficiency. The importance of CNC morphology is demonstrated in IPCNC actuators, for the very first time thanks to recent advances in fabricating controlled-morphology vertically aligned carbon nanotubes (VA-CNTs) with ultrahigh volume fraction. Compared with traditional CNCs with random distributed nanoparticles, VA-CNTs offer three advantages: creating continuous paths through inter-VA-CNT channels for fast ion conduction, minimizing electrical conduction resistance due to the continuous CNTs, and by tailoring modulus anisotropically to enhance actuation strain. A large strain level of 8.2% is achieved in IPCNC actuators with VA-CNT/Nafion CNCs. Faster actuation combined with higher ionic conductivity has also been demonstrated. Electrolytes providing for and transporting mobile ions are another component in IPCNC actuators. Traditional electrolytes, i.e. water and organic solvents have several issues including solvent evaporation and short lifetime. Ionic liquids with several unique features including nonvolatility, nonflammability, large electrochemical window, and high ionic conductivity, are introduced into IPCNCs, bring lots of opportunities for optimizing performance. The effect of ion size on actuator speed and efficiency has been investigated in IPCNCs with four imidazolium ionic liquids with different ion sizes. Besides single ions with different ion volume, the effect of ion clusters is identified in this dissertation by both experimental and theoretical approaches using ab initio method. The results could be utilized as guide in future IPCNC performance improvement. Traditionally, only bending actuators are fabricated for IPMCs, however, linear actuators which can make full use of the large strain generation of the IPCNCs are highly desirable. The possibility of linear IPMC has been realized by approaches demonstrated in this study. By examining the strain response in linear IPCNCs, additional information of individual ion transport of cation and anion is supplied. Another class of EAP, the electrostrictive polymers (poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) P(VDF-TrFE-CFE)) are more mature for commercialized actuators and they are also studied in this dissertation. P(VDF-TrFE-CFE)) have large elastic energy density (1 J•cm-3) and are usually quite thin (thickness ~ 1 µm) for operation voltage reduction. In this study several approaches for further improving their characteristics are discussed and demonstrated. The polymer blend approach is adopted to increase modulus by blending P(VDF-TrFE-CFE) with P(VDF-CTFE) (CTFE: chlorotrifluoroethylene). This approach results in alleviating of electrode clamping effect, i.e. strain reduction caused by electrodes. Electrical breakdown under high electrical field reduces device lifetime. Conductive polymers with a breakdown self-healing feature are employed as electrodes. The traditional deposition methods including in-situ polymerization and screen printing have several problems, such as taking a long time and damaging the thin polymer films. A mist spray method is developed for depositing conducting polymers with much increase deposition speed, improved coverage, less damage, and better thickness control (± 0.1 &#956;m). The low modulus of the conductive polymer further reduces the electrode clamping effect. An actuator with compact size (2mm diameter and 30mm length) and high performance (large displacement (1mm) and force output (1.5 N)) has been developed for refreshable full page Braille display and graphic display. These devices provide the capability for fulfilling so many dreams of the blind and visually-impaired.