Effect of nucleation time on bending response of ionic polymer–metal composite actuators
Introduction
Electro-active polymers (EAPs), which show a change in size or shape when stimulated by an electric field are very promising transduction materials [1], [2]. EAPs are used for applications including energy harvesting, biomimetic robots, artificial muscles and micro- and nano-electromechanical systems (MEMS and NEMS) due to their flexibility, low weight, fracture tolerance and ability to be molded into a desirable configuration [2], [3], [4], [5], [6]. There are many EAPs materials such as dielectric elastomer, polymer gel, conductive polymers, ionic polymer–metal composites (IPMC) and ferroelectric polymers [3], [4], [5], [6], [7], [8].
Among EAPs, IPMC is one of the promising materials as actuators due to relatively large displacement under low voltage (<5 V), relative insensitivity to damage and actuation capability in both dry and wet conditions [4], [5]. IPMC is composed of an ionic polymer membrane plated with metal electrodes on both surfaces. Up until now the electrodes of choice for IPMC have generally been noble metals such as Pt and Au due to their good chemical stability and electrical conductivity. The metal is mostly plated by electroless impregnation-reduction (IR) method in which the metal cation is incorporated inside the membrane and later reduced by the aid of reducing agent [9].
Although IPMC prepared by electroless IR method of noble metal exhibit good actuation performance, the main drawback is the time consuming and expensive processing. For example, it has been reported that Pt electroless IR method requires overall processing time of more than 48 h [10].
As such, there have been great efforts to replace the noble metals into non-precious metals [11], [12]. Among many non-precious metals, Ni is a good substitute for noble metal as the electrodes because of its similar electrical conductivity to that of noble metals and its lower cost. Park et al. fabricated Ni-IPMC through IR method, which took more than 30 h [13]. We thought that this problem could be solved if we use auto-catalytic electroless plating of nickel, which is expected to significantly reduce the overall processing time for fabricating Ni-IPMC.
The schematic of the autocatalytic electroless plating of nickel is shown in Fig. 1. The overall process is similar to electro-less IR method except the formation of Pd nuclei that is used as catalysts in initial reduction reaction. With Pd catalysts and resulting Ni self-catalysis, Ni particles are formed much faster than IR method. Thus, we can dramatically reduce the processing time using the autocatalytic electro-less plating. As the nucleation time of Pd–Sn colloids is the major determining factor of overall processing time in the auto-catalytic electro-less plating, we focused on the nucleation time as the control parameter. The surface concentration of Pd nuclei increases as the nucleation time increases. At the same time, the penetration depth of Pd nuclei increases as a function of nucleation time. However, the amount of Ni ions and reducing agent ions was stationary. Therefore, as the nucleation time increased, the density of Ni particles in the intermediate layer increased initially (Fig. 2(a) and (b)) and then remained constant (Fig. 2(b) and (c)). These tendencies of the intermediate layer affect the performance of Ni-IPMC such as the bending response. Therefore, in order to optimize the performance of Ni-IPMC, we systematically studied the relationship between the nucleation time, surface/interface morphology, and various electrical properties. Furthermore, in order to replace the impregnation-reduction method by the auto-catalytic electro-less plating, we report that it shows comparable performance of Ni-IPMC, as well as reduces the processing time when compared to the noble metal counterpart.
Section snippets
Fabrication of IPMC
In this study, Nafion 117 membrane from Dupont was used as an ionic polymer. To fabricate IPMC, nickel electrode was electroless plated on both side of Nafion as described in Fig. 1.
Electrical properties
We measured the sheet resistance of the nickel electrode as a function of nucleation time as shown in Fig. 4. The sheet resistance decreases from 2.85 to 1.65 Ω/sq as nucleation time increased from 5 min to 10 h and remained almost constant as nucleation time increased from 10 to 36 h. In order to understand the observed trend of the sheet resistance as a function of nucleation time, we analyzed the potential factors that contribute to the sheet resistance including electrode thickness, Ni particle
Conclusions
In conclusion, we investigated the influence of the nucleation time of Ni electroless plating on the electrical properties of the electrode, interfacial morphology between electrode and ionic polymer, and bending response of Nafion-Ni composite, which is known as ionic polymer–metal composite. We found that as nucleation time increased from 5 min to 36 h, displacement of IPMC increased initially up to 10 h, then remained almost constant. Based on the morphology and composition analysis using SEM
Acknowledgements
This research was supported by the Mid-career Researcher Program (No. 2010-0015063) and Conversion Research Center Program (No. 2011K000674) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20103020060010) funded by the Ministry of Knowledge Economy, Korea. Work at Argonne National Laboratory (S.H., data
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