Single-Walled Carbon Nanotubes — Ionic Polymer Electroactive Hybrid Transducers

Abstract

Ionic electroactive polymers, sometimes referred to as artificial muscles, have the ability to generate large bending strain and moderate stress at low applied voltages. Typical types of ionic electroactive polymer transducers include ionic polymers, conducting polymers, and carbon nanotubes. Preliminary research combining multiple types of materials proved to enhance certain transduction properties. Bennett and Leo (Materials Research Society Symposium Proceedings, vol. 785, 2003b) showed that the speed of response, maximum strain, and quasi-static actuation are improved by adding a layer of conducting polymer on an ionic polymer transducer. In this work, a recently developed fabrication method called the direct assembly process (DAP) plating is used to build SWNT/RuO2 hybrid transducers. The DAP consists of mixing a conducting powder with an ionomer solution. This technique has demonstrated improved response time and strain output as compared to previous methods. Electrodes applied using this new technique of mixing RuO2 (surface area 45—65 m2/g) particles and NafionTMdispersion provided 5× the displacement and 10× the force compared to a transducer made with conventional methods. Furthermore, previous studies demonstrated that the response speed of the transducer is optimized by varying the composition of metal in the electrode (Akle, B.J., Bennett, M.D., Leo, D.J., Wiles, K.B. and McGrath, J.E. 2007. "Direct Assembly Process: A Novel Fabrication Technique for Large Strain Ionic Polymer Transducers,'' Journal of Mat. Sci., 42:7031—7041). For RuO2, the optimal loading was approximately 45vol%, while carbon nanotubes electrodes have an optimal performance at 30vol%. Due to low percolation threshold, carbon nanotubes actuators perform better at a lower loading compared to other conducting powders. The addition of single-walled carbon nanotubes (SWNT) to the electrode increases both the strain rate and the maximum strain of the hybrid actuator. The strain rate of the transducer increased proportional to the ratio of SWNT to RuO2 in the electrode. A maximum peak-to-peak strain of 10.6% (±2V) is attained in a 15vol% SWNT/30 vol% RuO2 hybrid transducer. The maximum strain rate of 2.7%/s is generated by a 20vol% SWNT/25 vol% RuO2 hybrid transducer.