Constitutive model of aligned carbon nanotube/nafion nanocomposite ionic electroactive polymer actuators

Abstract

Research is progressing rapidly on composite nano-structured materials, including aligned carbon nanotube (A-CNT) polymer nanocomposites (PNC), and with device construction utilizing these novel materials. However, the material constitutive relations of the PNCs and tailoring of these is lagging. In this work, A-CNT and Nafion PNC electrodes are manufactured and investigated as the key active element in ionic-electroactive polymer (i- EAP) devices. i-EAP actuators are known to create relatively large strains with low input voltages, and could therefore be used for future high-strain actuation mechanisms such as synthetic muscles, microfluidic drug delivery, or low-frequency energy harvesting. Tailoring the PNC is accomplished by process control and moving to higher volume fraction (Vf) A-CNTs, in order to increase both the rate and magnitude of strain through increased nonisotropy and specific surface area for ionic transfer efficiency. This work shows methods to increase the A-CNT synthesis yield from below 50% to ~90%, while keeping quality and morphology stable. Porous A-CNT based i-EAP electrodes are synthesized with CNT and Nafion volume fractions of 10-40% and 10-25%, respectively. Key elements of the non-isotropic mechanical, electrical and electroactive constitutive law are discussed and calculated for different morphologies of the electrodes. Tailored high Vf i-EAP devices at ±3 V show ±4,130 microstrain in the transverse direction and an order of magnitude smaller strain (±420 microstrain) in the CNT axis direction, with A-CNT and Nafion Vfs of ~20% and ~15%, respectively. These large non-isotropic strains correspond to electroactive coupling coefficients of 7.1 x 10-12 m=V in the transverse direction and 7.2 x 10-13 m=V along the CNT axis. In addition, the i-EAP material's moduli are measured to be ~85 MPa in the CNT axis direction and ~50 MPa in the transverse direction, with a relative dielectric permittivity of 79, for this polymeric based i-EAP device. These coefficients constitute the first assembly of mechanical, electrical and electroactive properties into a constitutive relation for such an active nano-structured material. More extensive data sets for completing the full constitutive model and reducing measurement uncertainty are recommended for future work.