Nanoscale characterization of electro-active polymer using atomic force microscopy (AFM)

Abstract

Electro-active polymers (EAPs) are materials capable of mechanical movement due to expansion and contraction of volume by an external electrical signal. Electroactive polymers can be divided into two types according to their operation mechanism. Electronic electroactive polymers (eEAPs) are materials that are deformed by polarization of electric dipoles due to external electric field, ionic electroactive polymers (iEAPs) are materials that are deformed by contraction and expansion due to ion transport. Ionic polymer-metal composite (IPMC), a representative ionic electroactive polymer, is being studied in various actuator fields based on its relatively high operating displacement, low driving voltage, and chemical stability. IPMC is composed of ion conductive polymer and an electrode. Nafion is widely used as a representative ion conductive polymer. The ion conduction mechanism of Nafion involves hydrated ion clusters that are moved by an external electric signal. In this study, using atomic force microscopy (AFM), we analyzed the actuation properties of IPMC actuators and the mechanical and ion conduction properties. First, we fabricated IPMC actuators of different thicknesses and analyzed actuation properties under external voltage application. Based on the results, we used electrochemical and geometric calculation to predict ion movement inside Nafion under external voltage application and analyzed the correlation between the measured actuation properties and the thickness. Second, we analyzed the mechanical properties of Nafion hot-pressed at bulk scale and micro/nano scale. The elastic modulus of the hot-pressed Nafion was measured by uniaxial tensile test and the mechanical properties at the surface were measured by atomic force microscopy indentation method. Based on the properties of Nafion measured by these two methods and crystallinity calculation through X-ray analysis, we proposed a new structure of Nafion fabricated by hot-pressing method. Third, we analyzed the behavior of ions on the Nafion surface using electrochemical strain microscopy (ESM), one of the techniques of atomic force microscopy. With an AFM tip, an external voltage was applied to the surface of the Nafion and induced movement of the ions, allowing us to measure the change in surface ion concentration and electrochemical strain. By acquiring the change in surface topography over time, the movement of ions induced by external voltage could be confirmed.