Nafion membranes that enable the fabrication of ionic polymer-metal composite actuators with high conductivity and output force. Soft actuation materials move ...
10.2417/spepro.004983
Casting membranes for ionic polymer-metal composite actuators Yanjie Wang, Hualing Chen, and Yongquan Wang
Using additives in the casting process introduces characteristics to Nafion membranes that enable the fabrication of ionic polymer-metal composite actuators with high conductivity and output force. Soft actuation materials move in response to stimuli such as voltage, pressure, and exposure to chemicals. In this class of materials, ionic polymer-metal composites (IPMCs) have the capability of converting electrical energy to mechanical energy. These composites have attracted the attention of numerous researchers due to their potential applications in many fields, including robotics, aerospace, and biomedicine. A typical IPMC has a sandwich structure consisting of a base membrane, usually Nafion (a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer), with two thin metallic electrodes on either side. Currently, most studies on IPMCs are limited to the base membrane of the commercial Nafion series (e.g., N117 and N1110, with thicknesses of 183 and 254m, respectively), which are not suitable for a variety of applications because they are relatively thin and have a consequently low output force. The easiest and most effective way to enhance the output force of an IPMC is to increase the thickness of the base membrane.1, 2 There are currently two methods used to form thick membranes: hot pressing1 and solution casting.2 The former integrates multi-layers of commercial Nafion membranes, but a membrane formed in this way is quickly delaminated (divided into several layers) by repeated actuation.3 The latter process avoids this drawback by enabling a membrane to be fabricated with arbitrary thickness. The formation of a membrane by solution casting is affected by many factors, including temperature, curing time, and the choice of additive. The temperature and curing time have been researched for fuel cell and chlor-alkali applications, and their effects on the properties of membranes have been clarified.4, 5 Recently, researchers focusing on the influence of additives have found that they seriously affect the formation of the morphology of membranes during the solution casting process.6–8 In addition, the key properties of the membranes
Figure 1. Atomic force microscopy topography images. Images (a), (b), (c), (d), and (e) correspond to the topography of ethylene glycol (EG), dimethyl sulfoxide (DMSO), N,N 0 -dimethyl formamide (DMF), N-methyl formamide (NMF), and Nafion 117, respectively.
(e.g., water content, conductivity, and modulus) are altered, which has a significant influence on the performance of IPMCs. To investigate their effect, we prepared Nafion membranes with four different additives—ethylene glycol (EG), dimethyl sulfoxide (DMSO), N,N 0 -dimethyl formamide (DMF), and N-methyl formamide (NMF)—and analyzed the performance of IPMCs based on these membranes. Before solution casting, an additive is added to the precursor solution. This additive fully escapes once the Nafion membrane is cured, but special microstructures are formed within it as a result. We characterized several important properties of these composites, including morphology, water content, ion-exchange capacity (IEC), and conductivity by scanning electron and atomic force microscopy (AFM), hydrostatic weighing, the titration method, and a lowresistivity meter, respectively. We analyzed the thermal properties of the IPMCs by differential scanning calorimetry (DSC) and measured
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Figure 2. Results of an actuation voltage applied to solution-cast membrane-based ionic polymer-metal composites (IPMCs) with a variety of additives, and a Nafion 117-based IPMC as a reference. (a) The calculated strain energy density. (b) The dynamic energy conversion factor, which represents the ratio of the instantaneously generated mechanical energy and the instantaneously consumed electrical energy.
their electromechanical performance using a testing platform composed of a signal generator, current sensor, laser displacement sensor, and load cell.9 AFM topography micrographs of samples with EG, DMSO, DMF, and NMF as additives, and Nafion 117 as a reference, show that the Nafion backbones are looser and more amorphous—see Figure 1(a) and (b)—while the distribution of polymer chains (the brighter regions) and ionic domains (the darker regions) are more compact and uniform: see Figure 1(c–e). These variations are responsible for the differences of compatibility between Nafion chain molecules and additives, and seriously affect the characteristics of the membranes, including water content and conductivity.10
The water content of solution-cast membranes follows the sequence EG > DMSO > Nafion 117 > DMF > NMF; while the sequence of the corresponding conductivities at room temperature is EG > DMSO > Nafion 117 > NMF > DMF. The IECs of IPMCs partly depend on the additives used, and Nafion 117 has a higher IEC than other samples. However, we found that the membrane with higher conductivity does not always have higher IEC, as this property is influenced by many factors. In addition, the data revealed that the conductivities of the samples are more closely related to the water content than to the IEC properties of the membranes. By comparing the DSC data, we found that all of the samples showed similar glass transition temperature values but different melting point values. These differences could be attributed to the changes in the microstructure of the solution-cast membranes due to the additives used. A detailed discussion of these differences can be found in the literature.10 Finally, we used a laser-displacement sensor (Keyence LK-G80) and a microforce sensor (Transducer Techniques GSO-10) to measure actuation displacements and output forces of the IPMCs as a function of time at 2V DC voltage. Furthermore, the strain energy densities and dynamic energy conversion factors (a ratio of the instantaneously generated mechanical energy and the instantaneously consumed electrical energy) were evaluated by measuring the deformation: see Figure 2. The Nafion 117-based IPMC actuator has the largest value of strain energy, reaching 0.0181J/mm3 before relaxation occurs, due to the larger tip deformation and relatively thin membrane. For the solution-cast membrane-based IPMC actuators, the strain energy of the EG-based IPMC actuators reached up to 0.0113J/mm3 , while the NMF-based IPMC obtained a poor value of 0.0012J/mm3 : see Figure 2(a). It should be noted that the EG-based IPMC displays larger values (0.69%) than all-solution-cast membrane-based IPMC actuators and is lower only than the Nafion-based IPMC actuator: see Figure 2(b). In summary, the microstructures of membranes cast from solutions with EG and DMSO as additives are looser and more amorphous, leading to higher water content and thus higher conductivity than those with DMF, NMF, and Nafion 117. Among the solution-cast membranebased IPMC actuators, the EG-based IPMC actuator (whose electromechanical properties are the closest to those based on Nafion 117) has larger deformation and output force, and higher strain energy density and conversion efficiency at 2V DC voltages. Our future work will focus on exploring other additives and other methods, including the doping and irradiation of Nafion, to improve the performance of the solution-cast membrane-based IPMCs. This study was supported by the National Natural Science Foundation of China (51290294) and the Natural Science Basis Research Plan in Shaanxi Province of China (2012JM7002). Continued on next page
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Author Information Yanjie Wang, Hualing Chen , and Yongquan Wang School of Mechanical Engineering Xi’an Jiaotong University Xi’an, China Yanjie Wang is a graduate student currently working on his PhD. He received his undergraduate degree in mechanical engineering from China University of Mining and Technology. His current research explores the performance improvement of ionic polymer-metal composites. Hualing Chen is a professor in the Department of Mechanical Engineering. She received an MSc and a PhD in mechanical manufacturing from Xi’an Jiaotong University and began her teaching career there in 1984. Her research interests focus on smart materials and structures. Yongquan Wang received a PhD in mechanical manufacturing and is now a teaching fellow in the Department of Mechanical Engineering at Xi’an Jiaotong University. His research is focused on the modeling and simulation of electroactive polymers. References 1. S. J. Lee, M. J. Han, S. J. Kim, J. Y. Jho, H. Y. Lee, and Y. H. Kim, A new fabrication method for IPMC actuators and application to artificial fingers, Smart Mater. Struct. 15 (5), p. 1217, 2006. doi:10.1088/0964-1726/15/5/008 2. K. J. Kim and M. Shahinpoor, A novel method of manufacturing three-dimensional ionic polymer-metal composites (IPMCs) biomimetic sensors, actuators, and artificial muscles, Polymer 43 (3), pp. 797–802, 2002. doi:10.1016/S0032-3861(01)00648-6 3. H. L. He, X. H. Zhan, L. Wang, and J. P. Wang, The preparation and electromechanical characteristic of the multilayer artificial muscle, J. Funct. Mater. S3, 2011. 4. C. Li, G. Sun, S. Ren, J. Liu, Q. Wang, Z. Wu, H. Sun, and W. Jin, Casting Nafion– sulfonated organosilica nano-composite membranes used in direct methanol fuel cells, J. Membr. Sci. 272 (1–2), pp. 50–57, 2006. doi:10.1016/j.memsci.2005.07.032 5. F. Mohammadi and A. Rabiee, Solution casting, characterization, and performance evaluation of perfluorosulfonic sodium type membranes for chlor-alkali application, J. Appl. Polym. Sci. 120 (6), pp. 3469–3476, 2011. doi:10.1002/app.33526 6. C. H. Ma, T. L. Yu, H. L. Lin, Y. T. Huang, Y. L. Chen, U. S. Jeng, Y. H. Lai, and Y. S. Sun, Morphology and properties of Nafion membranes prepared by solution casting, Polymer 50 (7), pp. 1764–1777, 2009. doi:10.1016/j.polymer.2009.01.060 7. H. L. Lin, T. L. Yu, C. H. Huang, and T. L. Lin, Morphology study of Nafion membranes prepared by solutions casting, J. Polym. Sci. Part B Polym. Phys. 43 (21), pp. 3044– 3057, 2005. doi:10.1002/polb.20599 8. S. J. Lee, T. L. Yu, H. L. Lin, W. H. Liu, and C. L. Lai, Solution properties of nafion in methanol/water mixture solvent, Polymer 45 (8), pp. 2853–2862, 2004. doi:10.1016/j.polymer.2004.01.076 9. Y. J. Wang, H. L. Chen, B. Luo, and Z. C. Zhu, Design and optimization of small-sized actuators for driving optical lens with different shapes based on IPMCs, Proc. SPIE 8340, p. 83401I, 2012. doi:10.1117/12.917413 10. Y. J. Wang, H. L. Chen, Y. Q. Wang, B. Luo, L. F. Chang, Z. C. Zhu, and B. Li, Influence of additives on the properties of casting Nafion membranes and SO-based ionic polymer-metal composite actuators, Polym. Eng. Sci., 2013. Published online 23 June
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