Multi-fields responsive ionic polymer–metal composite

Abstract

The multi-fields responsive ionic polymer–metal composites, which have wide applications such as actuators, sensors, and dampers, are synthesized through a standard ion-exchange method using Ni particles doped on a Nafion™ film. SEM, EDS, and XRD were utilized to reveal their crystal shape and type. Dynamic mechanical analysis, vibrating sample magnetometry, and cyclic voltammetry were used to investigate the mechanical, magnetic, and electrical properties of the Ni-doped ionic polymer–metal composites. The nano-sized Ni particles (ca. 300 nm) were successfully synthesized on the Nafion™ film with a 2.5 μm layer. The Ni-doped ionic polymer–metal composites demonstrated good magnetic, electrical, and electro-mechanical responses.

Introduction

In 1992, an ionic polymer–metal composite (IPMC) was first reported as an active polymeric material by Oguro et al. [1]. Since then, much attention has been given to IPMCs with the hope that they can be used as soft actuators and as a sensor/transducer material for opportunities in future engineering. Some of attractive characteristics of IPMCs are its large bending capability and its easy processability when compared to electro-active ceramics or shape memory alloys [2], [3], [4], [5]. Additional features are its low driving voltage and easy miniaturization; therefore, IPMCs have been considered as a promising actuator material, in particular for bio-robotic applications [6]. It is now well accepted that IPMCs, in a deposited form with a metal such as gold or platinum, are actuated by the molecular transport of the hydrated ions under an electric field. In order to achieve a bending motion, the IPMCs need to be immersed and/or encased in a polar solvent. A newly adopted technique of using ionic liquid as an electrolyte medium, proposed by Leo and his co-worker [7] is an effective way to widen the operating voltage window up to as much as approximately ±4 V.

In this study, we attempted to investigate the responsive characteristics of IPMCs that are electroded with magnetically responsive materials, such as Ni. The properties of such IPMCs are responsive to multi-fields-not only electrical but also magnetic fields. Such multi-field responsive (MFR) IPMCs are expected to have attractive features including active damping, the replacement of precious metal (Au, Pt, etc.), and the possibility of responding to multiple driving forces—both electrically and magnetically, respectively. In order to make IPMCs be responsive to magnetic fields, transition metals such as Ni, Fe, and Co are initially considered as electrode materials for this project. Generally, transition metals go through a simple oxidation–reoxidation reaction, possess electron storage ability, and produce a magnetic field. In a previous study, Fe was used to synthesize magnetically responsive membranes [8], but it is challenging to keep Fe in a stable state due to the fact it oxidizes readily and has a high electrical resistance. In order to adequately overcome a Fe-precipitated IPMC, Ni was doped on the Nafion™ film. Ni and Fe have similar properties such as enthalpy, atomic radius, and melting point, but Ni is more stable than Fe in an oxidated state.

The MFR IPMCs were synthesized by an in situ standard ion-exchange method in which the Ni particles are doped onto the Nafion™ film using a nickel sulfate hexahydrated solution. XRD, SEM, and EDS were used to investigate the characteristics of the Ni-doped MFR IPMCs, and dynamic mechanical analysis (DMA), vibration sample magnetometry (VSM), and cyclic voltammometry (CV) were used to probe the mechanical, magnetic, and electrical properties of the Ni-doped MFR IPMCs.