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Ionic Polymer Metal Composites (IPMCs)
Smart Multi-Functional Materials and Artificial Muscles Volume 1
By Mohsen Shahinpoor The Royal Society of Chemistry
Copyright © 2016 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-78262-258-1
CHAPTER 1
Fundamentals of Ionic Polymer Metal Composites (IPMCs)
MOHSEN SHAHINPOOR
Department of Mechanical Engineering, University of Maine, Orono, Maine 04469, USA
Email: shah@maine.edu
1.1 Introduction
Ionic polymers such as polyelectrolytes in a nano-composite form with a conductive phase such as a metal, a synthetic metal or a conductive polymer of carbon, graphite or graphene are active actuators, sensors and energy harvesters that show large deformation in the presence of a low applied voltage and yet generate a transient voltage signal if subjected to mechanical deformations, as sensors and energy harvesters. In particular, ionic polymer metal composites (IPMCs) have been shown to be excellent candidates for low-voltage biomimetic robotic soft actuation and self-powered biomimetic robotic sensing and energy harvesting. They have been modeled as both capacitive and resistive element actuators that behave like biological muscles and provide an attractive means of actuation as artificial muscles for biomechanics and biomimetics applications. Grodzinsky, Grodzinsky and Melcher and Yannas and Grodzinsky were the first to present a plausible continuum model for the electrochemistry of deformation of charged poly-electrolyte membranes such as collagen or fibrous protein and were among the first to perform the same type of experiments on animal collagen fibers essentially made of charged natural ionic polymers, and were able to describe the results through the electro-osmosis phenomenon. Kuhn, Katchalsky, Kuhn, Kunzle and Katchalsky, Kuhn, Hargitay and Katchalsky and Kuhn and Hargitay, however, should be credited as the first investigators to report the ionic chemomechanical deformation of polyelectrolytes such as polyacrylic acid (PAA)–polyvinyl chloride (PVA) systems. Kent, Hamlen and Shafer were also the first to report the electrochemical transduction of the PVA–PAA polyelectrolyte system. Recently, revived interest in this area concentrates on biomimetic artificial muscles, which can be traced to Shahinpoor and coworkers, Adolf et al., Oguro, Takenaka and Kawami, Oguro et al., Asaka et al., Guo et al., De Rossi et al. and Osada et al. and Brock, et al. Essentially, polyelectrolytes possess ionizable groups on their molecular backbone. These ionizable groups have the property of dissociating and attaining a net charge in a variety of solvent media. According to Alexanderowicz and Katchalsky, these net charge groups that are attached to networks of macromolecules are called polyions and give rise to intense electric fields of the order of 10 V m-1. Thus, the essence of electro-mechanical deformation of such polyelectrolyte systems is their susceptibility to interactions with externally applied fields as well as their own internal field structure. In particular, if the interstitial space of a polyelectrolyte network is filled with liquid containing ions, then the electrophoretic migration of such ions inside the structure due to an imposed electric field can also cause the macromolecular network to deform accordingly. IPMC researchers have recently presented a number of plausible models for the micro-electromechanics of ionic polymeric gels as electrically controllable artificial muscles in different dynamic environments. The reader is referred to the references of this chapter for the theoretical and experimental results on dynamics of ion-exchange membrane–platinum composite artificial muscles.
1.1.1 History of IPMCs
IPMCs as multi-functional smart materials with actuation, energy harvesting and sensing capabilities were first introduced in 1997–1998 by Shahinpoor-Bar-Cohen–and co-workers as a member of the electroactive polymer (EAP) family based on research work supported by NASA–Jet Propulsion Laboratory (JPL) and under the leadership of Dr Yousef Bar-Cohen at JPL and Mohsen Shahinpoor, director of the University of New Mexico's Artificial Muscles Research Institute. However, the original idea of ionic polymer and polymer gel actuators goes back to the 1991–1993 time period of Osada et al., as depicted in the references at the end of the chapter. The two original patents on IPMCs were awarded in 1993 to Adolf et al. and Oguro, Takenaka and Kawami. These patents were followed by additional related patents on both the sensing and actuation of IPMCs (Shahinpoor and Mojarrad, Shahinpoor and Shahinpoor and Kim). It should also be mentioned that Tanaka, Nishio and Sun introduced the phenomenon of ionic gel collapse or phase transition in an electric field, which led to a large number of publication by Tanaka and co-workers out of MIT. It should also be mentioned that Hamlen, Kent and Shafer introduced the electrochemical contraction of ionic polymer fibers. Credit should also be extended to Caldwell and Taylor for their early work on chemically stimulated gels as artificial muscles. Research on polyacrylonitrile gels in the form of contractile muscles that are either pH activated or electrochemically activated has also been reported by Shahinpoor and co-workers.
1.2 Chemistry of Manufacturing IPMCs
The IPMC actuators, sensors and artificial muscles used in our investigation are composed of a perfluorinated ion-exchange membrane, which is chemically composited with a noble metal such as gold, palladium, platinum and silver. A typical chemical structure of one of the ionic polymers (Nafions®) used in our research is shown below in Figure 1.1.
Note that in Figure 1.1, n is such that 5<n<11 and m is ~1, and M+ is the counter ion (H+, Li+, Na+, etc.). One of the interesting properties of this material is its ability to absorb large amounts of polar solvents (i.e. water). A typical perfluorinated ionic polymer is the well-known Nafions®, discovered in late 1960s and patented in early 1970s (US patent 3 784 399) by Dr Walther G. Grot of IBM with a chemical formula of C7HF13O5S·C3F7 per pendant group. Figure 1.2 depicts the Nafions® basic chemical formula.
Nafions® is essentially a perfluorosulfonated proton conductor (H+) and incorporates perfluorovinyl ether groups attached to pendant sulfonate SO3-H+ groups over a tetrafluoroethylene (Teflon) backbone. Nafions® is heavily used as a proton conductor for proton exchange membranes in fuel cells, water filtration and caustic soda production, among others. Protons on the sulfonic acid groups are capable of "hopping" from one acid site to another. Nafions® pores allow movement of cations but do not allow movement of anions or electrons. Polymeric actuation and sensing technology has advanced in the past decade primarily due to the unique properties of EAPs' large strain, soft actuation, easy manufacturing and built-in sensing capabilities.
Another similar ion exchange material by the name of Flemions® has also been studied by a number of authors (Nemat-Nasser and Wang et al.). Flemions® is a carboxylic acidic ionomer with a chemical backbone similar to Nafions® except for the carboxylic versus sulfonic charged pendant groups, as shown in Figure 1.3.
Flemions®-based IPMC performance has been observed to be inferior by not being capable to work in air and dynamically slower than Nafions®-based IPMCs and thus have not enjoyed as much attention as perfluorosulfonated IPMCs.
Based on Nafions®-based IPMCs, a number of materials that could provide new applications for industrial, biomedical, defense and space applications have emerged. Obviously, there is a great potential for IPMCs to be adopted as soft biomimetic robotic actuators, artificial muscles, dynamic sensors and energy harvesters in nano-to-micro-to-macro size ranges. The base polymeric materials are typically ion-exchange materials that are designed to selectively pass ions of a single charge (either cation or anion). They are often manufactured from polymers that consist of fixed covalent ionic groups — perfluorinated alkenes with short side chains terminated by ionic groups or styrene/divinylbenzene-based polymers in which the ionic groups are substituted from the phenyl rings where the nitrogen atom fixes an ionic group. These polymers are highly cross-linked. Under an imposed electric potential across the material, ions are usually transported through the material, termed "migration", and the direction of ions migration is determined by the polarity of the electrodes and the vectorial direction of the imposed electric field. The ion migration rate is determined by the applied potential and the properties of the materials. In practice there are two types of ion-exchange materials: homogeneous and heterogeneous. Homogenous materials are coherent ion-exchange materials having the form of thin films or sheets. Heterogeneous materials are typically fabricated by embedding fine resin particles in inert thermoplastic binders, thereby forming thin sheets or films. Improving the mechanical properties of resulting membranes is of interest. However, they have some disadvantages, showing high electric resistance and reduced long-term integrity due to repeated swelling and de-swelling.
1.3 Introduction to Manufacturing IPMCs
Manufacturing an IPMC begins with selection of an appropriate ionic polymeric material. Often, ionic polymeric materials are manufactured from polymers that consist of fixed covalent ionic groups. The currently available ionic polymeric materials that are convenient to be used as IPMCs are:
(1) Perfluorinated alkenes with short side chains terminated by ionic groups (typically sulfonate or carboxylate [SO3- or COO- for cation exchange or ammonium cations for anion exchange [see Figures 1.1 and 1.3]). Large polymer backbones determine their mechanical strength. Short side chains provide ionic groups that interact with polar liquids such as water and the passage of appropriate ions.
(2) Styrene/divinylbenzene-based polymers in which the ionic groups have been substituted from the phenyl rings where the nitrogen atom is fixed to an ionic group. These polymers are highly cross-linked and are rigid.
The current state-of-the-art IPMC manufacturing technique incorporates two distinct preparation processes: an initial redox operation to embed a conductive medium within the material and an eventual surface electroding process. Due to different preparation processes, morphologies of precipitated metals are significantly different. The initial compositing process requires an appropriate metallic salt such as Pd (NH3)4HCl or other salts such as AuCl2(phenonthroline)Cl in the context of chemical oxidation and reduction processes similar to the processes evaluated by a number of investigators including Takenaka et al. and Millet et al. Noble metals such as gold (Au) or platinum (Pt), in the form of charged (oxidized) metal ions, which are dispersed throughout the hydrophilic regions of the polymer, are subsequently reduced to the corresponding metal atoms. This results in the formation of dendritic type electrodes within the molecular network of the polymer. The principle of the electroplating process is to metalize the inner surface of the material by a chemical reduction means such as LiBH4 or NaBH4. The ion-exchange polymer is soaked in a salt solution to allow metal-containing cations to diffuse through via the ion-exchange process. Later, a proper reducing agent such as LiBH4 or NaBH4 is introduced to metalize the polymeric materials by molecular plating. The metallic particles are not homogeneously formed across the material but concentrate predominantly near the interface boundaries. It has been experimentally observed that the metallic particulate layer is buried few microns deep within the IPMC boundary surface and is highly dispersed. The range of average particle sizes has been found to be around 40–60 nm due to reduction around micellar nanoclusters, as shown in Figure 1.4.
These micellar-type nanoclusters generate fractal formations of reduced metallic particles, as shown in Figure 1.5.
An effective recipe for the manufacturing of IPMCs is:
(i) Surface roughening and bead blasting to enhance molecular diffusion of a metallic salt during oxidation;
(ii) Ion-exchange processes by oxidation caused by exchanging the H+ cations with positively charged metallic cations such as Pt+ (oxidation);
(iii) Metallic molecular deposition by a reduction process, using a strong reducer such as sodium borohydride (NaBH4) or lithium borohyride (LiBH4), which converts the oxidized Pt+ to Pt and deposits them on macromolecules around the nanoclusters and exchanges the H+ cations with Na+ or Li+; (iv) Surface plating and placement of electrodes.
1.4 Mechanisms of Actuation and Sensing in IPMCs
A typical 200 micron ionic polymeric membrane after the above chemical plating will look like what is depicted in Figure 1.6. In the presence of chemically plated electrodes shown in Figure 1.6, an imposed electric field enables the cations such as Na1 or Li1 to migrate towards the cathode, causing the cathode side to expand due to injection of cations and thus create a pressure gradient across the thickness of the membrane to cause it to bend towards the anode electrode, as depicted in Figure 1.7.
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Excerpted from Ionic Polymer Metal Composites (IPMCs) by Mohsen Shahinpoor. Copyright © 2016 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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