Electrothermal Polymer Nanocomposite Actuators

Adaptive structures are desirable for a broad array of technologies, ranging from medical stents to deployable telescopes and morphing air vehicles. [ 1 ] Due to their processability and relatively high strain, [ 2 ] electroactive polymers (EAPs) have become a viable option for low-frequency actuating devices that reversibly drive a mechanically articulated substructure in a manner analogous to muscles controlling the position of an organism’s skeleton. Unfortunately, electric-fi eld driven EAPs generally require high operating voltages and exhibit low force output, challenging broader utilization. Recently, enhancements in electromechanical performance have been realized in a host of polymers through nanoparticle inclusion. [ 3– 7 ] Carbon nanotube (CNT) additives in particular, both below and above electrical percolation, have been shown to reduce the electric-fi elds required to induce actuation by up to two orders of magnitude in various electrostrictive and piezoelectric polymers. For example, Zhang et al. observed nearly a two-fold increase in electrostrictive strain (2.1%) relative to the pristine polymer at an applied fi eld of 54 MVm −1 through the addition of 0.35 vol% MWNTs to P(VDF-TrFE-CFE). [4] Likewise, Park et al. observed an electrostrictive strain of 2.6% in a LaRC-EAP fi lm containing 0.05 vol% SWNTs under an applied fi eld of 0.8 MVm −1 . [ 6 ] These enhancements have been attributed to modifi cation of the local electric-fi eld distribution within the polymer due to fi eld exclusion from the conductive particles and charge accumulation at the polymer–CNT interfaces. In general, peak electromechanical performance was reported around the electrical percolation threshold for CNT addition. The increased conductivity at these higher nanotube loadings, however, can result in a leakage current that decreases the external power effi ciency for capacitive operation of the EAP. Alternatively, if this electrical dissipation is maximized and coupled to a thermodynamically reversible dimensional transformation, effective electromechanical actuation can be envisioned via a materialderived electrothermal effect. For some materials, such as shape memory polymers [ 1 , 8,9 ] and liquid-crystal elastomers, [ 10,11 ] this can be used to replace external heat sources. Generally, with external heating, the response time is limited by the rate of heat transfer from the heater to the sample surface, and then the sample surface into the bulk. Although this can be mitigated to some extent by minimizing the sample thickness, direct incorpo-