NANOSTRUCTURED ELECTROACTIVE POLYMER ACTUATOR MATERIALS

Open Access
Author:
Lin, Junhong
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 22, 2011
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Ralph H Colby, Committee Member
  • Qing Wang, Committee Member
  • James Patrick Runt, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • actuator
  • polymer
  • charge dynamics
  • ionic liquids
  • ionomer
Abstract:
This dissertation investigates nanostructured materials, including the nanorods of Field Activated Electro Active Polymer (FEAP) and Ionic Electro Active Polymer (IEAP) systems. As one of the most important FEAPs, the large electromechanical responses in the ferroelectric relaxor poly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) P(VDF-TrFE-CFE) terpolymers make them attractive to nanoelectromechanical systems (NEMS), as well as nano-actuator and nanosensor applications. This dissertation develops the fabrication process for the nanorod array of P(VDF-TrFE-CFE) relaxor ferroelectric terpolymer using an anodic aluminum oxide (AAO) template. Nanorod arrays in the rod diameter have been fabricated down to 25 nm. In the relaxor ferroelectric terpolymers, the bulky CFE monomers act as the random defects that break the long range polar-ordering in the ferroelectric P(VDF-TrFE), and the freezing of the random dipoles leads to the relaxor behavior. Making use of the nanorod arrays, the evolution of the relaxor ferroelectric behavior of the P(VDF-TrFE-CFE) terpolymers was investigated for nanorods with diameters reduced from 200 nm to 25 nm. It was observed that all the nanorods exhibited relaxor ferroelectric behavior, as characterized by the dielectric peak shifting toward high temperatures with frequency. The frequency-permittivity peak temperature characteristics fit well with the Vogel-Fulcher-Tammann (VFT) relation. Moreover, the freezing temperature in the VFT relation decreases with the reduction of the nanorod diameter, indicating that the reduction of the nanorod’s diameter influences the relaxor ferroelectric behavior of the terpolymer. The existence of ferroelectric relaxor properties in terpolymer nanorods as small as 25 nm suggests the possibility of terpolymers for NEMS and nanoactuator applications. It also provides an interesting ferroelectric material system with which to study the finite size effect in ferroelectric relaxor. In the IEAPs, the ions transport through the ionic systems under an applied field and the subsequent accumulation and depletion of excess ions at the electrodes determine the response behavior of the electroactive devices, such as IEAP actuators and supercapacitors. Moreover, recent experimental results reveal the potential of ionic liquids (ILs) in enhancing the IEAP device performance. For instance, the vapor pressure of ILs is negligibly low and as a result they will not evaporate out of the IEAP devices when operated in ambient conditions. Their wide electro-chemical window (~4 V) allows the IEAP to utilize higher applied voltages than the dilute water solution electrolyte. ILs also offer the possibility of achieving high mobile ion concentration and high ion mobility. This dissertation investigates the charge dynamics of ILs in two kinds of nanostructured IEAPs, which possess distinctively different polymer nano-morphologies, and it is of great interest to know how these morphologies affect the charge dynamics of ILs: (i) Aquivion ionomer, which forms percolating nanostructured ionic clusters above a critical uptake of ILs for fast ion transport. In addition, its shorter side chain (in comparison to Nafion) could also lead to better IEAP actuator performance, owing to superior coupling to the polymer backbone. (ii) Poly(dimethylaminoethyl methacrylate-co-diethylene glycol methylether methacrylate) P(DMAEMA-co-DiglymeMA) copolymer, which was synthesized by Prof. Tim Long’s group at Virginia Tech as a new class of IEAP. A time domain electrical characterization method was developed and employed to systematically study the charge dynamics of ILs in these IEAPs. Compared with the frequency domain method, this method offers the possibility of probing the charge dynamics over a broad voltage range. In the Aquivion membrane swelled with EMI-Tf, the ionic conductivity and mobility show strong uptake dependent behaviors and undergo abrupt enhancement transitions close to the critical uptake, which suggests that the minimum uptake for the IEAP application is above its critical uptake. It was found that the ionic conduction of ILs is coupled with the segmental motion of the ionic phase of the Aquivion membrane implying that the enhancement of the ionic conduction is mainly due to the reduction of the glass transition temperature of the ionomer matrix with an increased uptake of EMI-Tf. The activation energies for ions to dissociate do not show substantial uptake dependence. With the same uptake of EMI-Tf, both Aquivion and Nafion show almost the same charge dynamics, while the short side chain Aquivion shows a better electromechanical coupling per charge than that of the longer side chain Nafion. In the P(DMAEMA-DiglymeMA) copolymers with 10, 30, 50 and 70 mole% of DMAEMA ionic chains blended with 40 wt% uptake of EMI-Tf systems, a rapid drop of the conductivity, mobility and mobile ion dissociation ratio with increasing ionic chain density is observed. The conductivity follows Debye-Stokes-Einstein relation closely implying that the ionic conduction is coupled with the copolymer’s segmental motion in this system. The electrostatic interactions among charges and ionic chains may slow down the chain mobility and increase the energy barrier for ion dissociation. These results suggest that the suppression of ionic conductivity, mobility and mobile ion dissociation ratio may be mainly caused by the increase in the ionic charged density of the local environment where the ions transport. This dissertation shows that at voltages <1 V the charge dynamics in these IEAPs in a short time are dominated by the electric double layer (EDL), in which the charging time &#61556;DL follows the classic model &#61556;DL =&#61472;&#61472;&#61548;Dd/(2D) (<< 0.1 seconds at room temperature for the IEAPs studied here) very closely where &#61472;&#61548;D is the Debye length, d the membrane thickness, and D the diffusion constant. However, my study reveals that the EDL charges do not make much of a contribution to IEAP device responses, which are instead dominated by a much longer time charging process (> 0.1 seconds). Several recent theoretical models suggest that the charge dynamics at the later stage should follow the random diffusion time &#61556;diff = d2/(4D), indicating that by reducing the membrane thickness d the IEAP device response speed can be increased significantly. After examining the charge dynamics in the Aquivion membrane with 40 wt% uptake of EMI-Tf and pure BMI-PF6 over a broad thickness range, my results show that in the longer time regimes (t >> &#61556;DL) the charge dynamics become dependent on the applied voltage. At a low applied voltage (0.1 V) the charge response seems to follow the d2 dependence, whereas at a high applied voltage (> 0.5 V) where substantial actuation occurs, the charging responses do not show significant thickness dependence. These results imply that the movement of charges is mainly from the near electrode region, which might be due to the very high mobile ion concentration of the ionomer/IL systems studied here. The finding that actuation is mainly driven by the slow nonlinear charges instead of the double layer charges explains why the substantial actuation only occurs with voltage > 1 V and longer times.