**2. Principle**

in early stage. They both claimed that an actuator comprises an ion exchange membrane and a pair of electrodes attached to opposite surfaces of the ionic polymer, which refers to the cation or anion exchange membrane. Adolf et al. even named the actuator electrically controlled polymeric gel actuators, which maybe is the first normal name of ionic polymer actuator. After that, many researchers were attributed themselves to explore the essential properties of this actuator. They give different names to this special actuator based on different understandings, such as ionic polymeric gel actuator [6], electrically controllable artificial muscle [7], ion-exchange membrane metal composites [8], Nafion-Pt composite actuators (ICPF) [9] and ionic polymermetal composites (IPMC) [10], which is the most common names so far. At this stage, it is dominant to clarify the actuating mechanism of this kind of actuator. So several electromechanical and physical models were gradually developed by de Gennes et al. [11], Newbury and Leo [12, 13], Nemat-Nasser et al. [14], Tadokoro et al. [9], Zicai Zhu et al. [15, 16] and so on. Meanwhile, for this ionic polymer actuator, the ionomer layer and conductive layer are critical components. The substitutes of components are an important way to improve the electromechanical performance of the actuator. Generally, perfluorinated polymers, such as Nafion (sulfonated) or Flemion (carboxylated), are employed as ionomer layer. The actuation ability of the ionic polymer actuator seriously is dependent on fixed anions, mobile cations and nanochannels inside Nafion or Flemion. Based on this property, a lot of novel hydrocarbon ion-exchangeable membranes are introduced to replace the ionomer layer [2]. These membranes include commercial products, blending and synthetics, some of which overcome the back-reversal problem and show much larger bending deformation compared to the Nafion- or Flemion-IPMC, such as poly(styrene-alt-maleimide) (PSMI)-incorporated poly(vinylidene fluoride) (PVDF) and chitosan/polyaniline interpenetrating polymer network. Likewise, the electrode layer plays an important role in IPMC actuation. It is considered to be easier to modify the electrode layer to optimize the IPMC property than the ionomer layer. Of all metals, gold and platinum with excellent conductivity and chemical stability are the most widely used electrode materials. Because of high cost, inexpensive electrode materials are still in need to replace gold and platinum. Palladium [17], silver [18] or their complex [19] has been considered as substitute.

**Figure 1.** Universal architecture and mechanism of ionic polymer actuators.

40 Actuators

As we all know that charged particles will have a directional migration effect when put in the electric field. Generally, parallel plate capacitors would create a uniform electric field between the plates. Special dielectric is added into capacitor, which has unique property with solid– liquid two-phase microstructure. Charged particles (such as cations) do not exist alone in solution environment, and they tend to bind to a certain amount of solvent molecules forming solvated cations. Charged particles together with solvent molecules travel through the liquidphase microstructure of dielectric from one side to another side when voltage is applied to the plates. This will result in mass plentiful on one side and exhausted on the other side. At this point, mechanical local strain will occur on both sides. These constitute the basic principle of ion polymer actuators.

#### **2.1. Composition**

As mentioned in Section 1, normally, an IPMC consists of an ionomer membrane plated on both sides with metal electrodes and neutralized with the necessary amount of mobile ions and fixed counterions. Metal electrodes form the outermost layers, followed by the intermediate layer. The intermediate layer comprises of metal particles dispersed inside the polymer matrix, which contains the ionomer, the counter ions and solvent molecules inside the membrane as shown in **Figure 2**. Nafion by DuPont or Flemion by Asahi Glass are most used as ionomer. The differences between them are in the functional groups (sulfonate and carboxylate groups respective) and ion-exchange capacities. The chemical structure of Nafion and Flemion are shown in **Figure 2(a)**. The commonly used cations inside the membrane include the alkali metal cations, such as Li, Na, K, Rb and Cs while the solvent mainly refer to water and ionic liquid [21, 22]. For electrode layer, due to their corrosion resistance and high conductivity, platinum and gold are

**Figure 2.** Pd typed IPMC: (a) electrode interfacial morphology; (b) microstructure of Nafion; (c) schematic diagram of composition.

the cathode with water molecules. The asymmetric distributions of the concentration of cations and water cause the IPMCs to swell near the cathode and generates extensional stress in the polymer, which causes the IPMC to swell near the cathode and shrink beside the anode. Finally, a bending motion is generated toward the anode [11]. Likewise, when an external stimuli was applied to the IPMC, the distributions of ions and water molecules inside IPMCs changes. Potential difference appears on both sides of the IPMC, which could be viewed as sensing signal. The properties of sensing and actuating of IPMC depend on the types of cation and solvent, surface resistant, interface morphology and temperature

**Figure 3.** Components of electrode film and electrolyte film of bucky gel actuator (BGA) (a) Schematic representation of three-layered BGA (b) and the bending motion to the anode side (c) Chemical structure of imidazolium-type IL

Ionic Polymer Actuators: Principle, Fabrication and Applications

http://dx.doi.org/10.5772/intechopen.75085

43

In general, the current is generated by ion transport in BGAs and the three-layered BGAs show a bending motion to the anode side when voltages are applied. The electric charge is stored capacitively in BGAs during applying voltages [25]. This implies that our BGA is a capacitor. Baughman et al. reported SWCNT sheets (bucky papers) show the expansion and contraction (actuation) in aqueous electrolyte solution against a counter electrode [27]. They proposed the actuation mechanism in which C-C bond distance in SWCNT changes by charge injection originated from quantum and double-layer electrostatic effects. On the other hands, we consider the actuation mechanism of BGAs is due to C-C bond distance changing in CNTs [27, 28], volume change of the electrodes by sorption/desorption of ions [29], and electrostatic effect in the electrical double-layer [30]. Kiyohara and Asaka theoretically investigated the actuation mechanism of BGAs by a method of Monte Carlo simulation [31, 32]. We also studied the actuation mechanism of BGAs by a combination of symmetrical analysis, elasticity theory, and experimental results in the bulk scale [33]. As a result, it was found that the cathode expands and the anode contracts resulting in the bending motion of

and humidity, etc.

(EMIBF4) and base polymer (PVDF-HFP) (d).

BGAs to the anode side.

commonly used [1, 23]. In our lab, we developed palladium typed IPMC because of its relative low price and optimized its preparation process [17, 24].

Bucky gel actuators (BGAs) are composed by carbon nanotubes (CNTs), ionic liquid (IL), and base polymer (BP). Single-walled CNT (SWCNT) is one of good nanocarbons as conductive electrode material. Not only imidazolium-type ILs but also ammonium-type ILs can be used as electrolyte source. Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) is used as a BP. BGAs have a three-layered structure as shown in **Figure 3**, that is, one electrolyte layer is laminated by two electrode layers. A gel like self-standing electrolyte film is made with IL and BP. Generally, the electrode films are made from CNTs, IL, and BP. Some additives such as conductive or non-conductive nanoparticles can be added in the electrode layers in order to tune the electrochemical and mechanical properties of electrode [25, 26].

In contrast to IPMC and BGA, we can see that they both have very similar structures, with the exception of the ingredient of the electrode layer and interlayer, separately. This, of course, will finally result in the difference in preparation process and electromechanical performance.

#### **2.2. Bending mechanism**

The working mechanism of IPMC can be explained through electromechanical transduction. When applying an electric field, the cations inside the base membrane move toward

**Figure 3.** Components of electrode film and electrolyte film of bucky gel actuator (BGA) (a) Schematic representation of three-layered BGA (b) and the bending motion to the anode side (c) Chemical structure of imidazolium-type IL (EMIBF4) and base polymer (PVDF-HFP) (d).

the cathode with water molecules. The asymmetric distributions of the concentration of cations and water cause the IPMCs to swell near the cathode and generates extensional stress in the polymer, which causes the IPMC to swell near the cathode and shrink beside the anode. Finally, a bending motion is generated toward the anode [11]. Likewise, when an external stimuli was applied to the IPMC, the distributions of ions and water molecules inside IPMCs changes. Potential difference appears on both sides of the IPMC, which could be viewed as sensing signal. The properties of sensing and actuating of IPMC depend on the types of cation and solvent, surface resistant, interface morphology and temperature and humidity, etc.

commonly used [1, 23]. In our lab, we developed palladium typed IPMC because of its relative

**Figure 2.** Pd typed IPMC: (a) electrode interfacial morphology; (b) microstructure of Nafion; (c) schematic diagram of

Bucky gel actuators (BGAs) are composed by carbon nanotubes (CNTs), ionic liquid (IL), and base polymer (BP). Single-walled CNT (SWCNT) is one of good nanocarbons as conductive electrode material. Not only imidazolium-type ILs but also ammonium-type ILs can be used as electrolyte source. Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) is used as a BP. BGAs have a three-layered structure as shown in **Figure 3**, that is, one electrolyte layer is laminated by two electrode layers. A gel like self-standing electrolyte film is made with IL and BP. Generally, the electrode films are made from CNTs, IL, and BP. Some additives such as conductive or non-conductive nanoparticles can be added in the electrode layers in order

In contrast to IPMC and BGA, we can see that they both have very similar structures, with the exception of the ingredient of the electrode layer and interlayer, separately. This, of course, will finally result in the difference in preparation process and electromechanical performance.

The working mechanism of IPMC can be explained through electromechanical transduction. When applying an electric field, the cations inside the base membrane move toward

to tune the electrochemical and mechanical properties of electrode [25, 26].

low price and optimized its preparation process [17, 24].

**2.2. Bending mechanism**

composition.

42 Actuators

In general, the current is generated by ion transport in BGAs and the three-layered BGAs show a bending motion to the anode side when voltages are applied. The electric charge is stored capacitively in BGAs during applying voltages [25]. This implies that our BGA is a capacitor. Baughman et al. reported SWCNT sheets (bucky papers) show the expansion and contraction (actuation) in aqueous electrolyte solution against a counter electrode [27]. They proposed the actuation mechanism in which C-C bond distance in SWCNT changes by charge injection originated from quantum and double-layer electrostatic effects. On the other hands, we consider the actuation mechanism of BGAs is due to C-C bond distance changing in CNTs [27, 28], volume change of the electrodes by sorption/desorption of ions [29], and electrostatic effect in the electrical double-layer [30]. Kiyohara and Asaka theoretically investigated the actuation mechanism of BGAs by a method of Monte Carlo simulation [31, 32]. We also studied the actuation mechanism of BGAs by a combination of symmetrical analysis, elasticity theory, and experimental results in the bulk scale [33]. As a result, it was found that the cathode expands and the anode contracts resulting in the bending motion of BGAs to the anode side.
