3.2.1  Ionic Polymer–Metal Composites

Synthetic composite nanomaterials that act as artificial muscles due to the electric field effect are called ionic polymer–metal composites (IPMCs). The surfaces of IPMCs consist of an ionic polymer. Due to the stress applied across the IPMC strip, ion migration and redistribution cause bending deformation. The applied voltage can cause various deformations, such as bending, rolling, torsion, rotation, rotational, and unsymmetrical bending deformation. When these deformations are applied to IPMC strips, they generate an output voltage signal. Therefore, IPMCs are known EAPs. They work well in air and liquid media. They can generate a peak force of approximately 40 times their weight and have a wide bandwidth (4-11).

3.2.2  Ion Gels

An ion gel is a material that consists of an inorganic or polymer matrix immobilized ionic liquid (12, 13). An ion gel can be obtained by mixing or synthesizing the solid matrix and ionic liquid in situ or by using a block copolymer polymerized in solution with the ionic liquid. The aim is to create a self-assembled nanostructure in which ions can be dissolved selectively. Ion gels can be synthesized using materials such as oxides, non-copolymer polymers, or boron nitride. Ion gels can be polymeric and inorganic. The main purpose of ion gel applications is to electrically insulate the matrix components to provide ionic conductivity (14).

Ion gels have been used as insulators (15), dielectrics (16), and electrolytes (12) in many electrical device systems. Solid-state and flexible ion gels are especially preferred in mobile devices (17). The high viscosity of ion gels makes electrolytes and separators between anodes and cathodes, especially in battery applications. In addition, the viscoelastic flow that occurs in gels under stress creates a quality electrode or electrolyte interface contact, which highlights ion gels.

Ion gels can withstand ≤300°C before they degrade (18). Due to their high-temperature capability, they have high thermal stability (19) and this stability is well above the capacity of existing commercial electrolytes. For example, it has been used in lithium-ion batteries (LIBS) to run cells at 175°C (20).

3.2.3  Carbon Nanotubes

Carbon nanotubes (CNTs) have attracted significant attention since their discovery due to their broad mechanical and electrical properties. They are single-walled carbon nanotubes (SWCNTs) approximately 1 nm in diameter. There are multi-walled carbon nanotubes (MWCNTs) composed of interlocking SWCNTs that are weakly bonded due to Van der Waals interactions (21).

CNTs have excellent tensile strength (22) and thermal conductivity (23, 24) dues to the strength of the bonds in carbon atoms and their nanostructure. Their electrical conductivity is high (25), and chemical modifications are possible (26). Because of these properties they are valuable in many fields, such as optics, electronics, and nanotechnology. A report was recently published on super elastic CNT air gel muscles (27). CNT air gel sheets have 220% anisotropic linear elongation and strain rates. Airgel can be permanently frozen at temperatures between 80 and 1900 K. Unlike conventional CNT electrolytes actuators are not required, and actuation is accomplished by applying a positive voltage to the counter electrode. When CNTs are used as reinforcements in polymers, they are first randomly dispersed in a solvent or polymer fluid/melt by shear mixing or sonication; then further processing is performed to form the composite (28, 29). In addition, poor dispersion is generally specified as a processing limitation (30) and the critical reduction factor (31–33). Improving the CNT dispersion might be possible in the presence of surfactants (34), oxidation, or chemical functionalization of the surface (35, 36)

3.2.4  Polymer Dots

Polymer dots (PDs) were synthesized using the grass hydrothermal route (37). PDs are new nanomaterials composed of conjugated organic polymers with conjugated PDs. Semiconductor PDs have emerged as a fluorescent carbon-based material class.

PDs are from linear polymers or monomers from the clustered or crosslinked polymer. In addition, a carbon core and bonded polymer can form PDs. The synthesis of PDs from polymers is shown in Figure 3.1.

Figure 3.1   PDs synthesis from polymers (38).

PDs show essential potential applications in drug delivery and therapeutics, biological imaging, and detection due to the advantages of their structure, good biocompatibility, optical properties, and accessible surface modification. These nanomaterials have the potential for fluorescent imaging and optical detection (39, 40).

Fluorescent polymer nanoparticles were prepared to determine nitro-explosive picric acid (41). PDs were prepared by polymerizing carbon tetrachloride and ethylenediamine (42). In another study, a path that was not linearly conjugated from polymers to fluorescent PDs was used (43).

3.2.5  Molecularly Imprinted Polymers

Molecularly imprinted polymers (MIPs) are prepared by the copolymerization of monomers (44). First, polymerization of functional monomers occurs in the presence of a template molecule. Then, highly selective gaps are formed by removing the template molecule (45, 46). MIPs have several advantages, such as stability at different pHs and temperatures compared with natural recognition receptors, such as antibodies (47, 48).

MIPs are used in several nanosensor applications, such as antioxidants (49), antibiotics (50, 51), toxic compounds (52, 53), and drugs (54, 55). MIPs can be synthesized by different methods, such as electropolymerization, photopolymerization, and free radical polymerization (56). In electropolymerization, they can control the polymer film. This makes them more advantageous than other methods (57). A thin MIP layer and carbon-based materials formed on the electrode surface can increase the sensor’s conductivity (58–61). In addition, the electrode combined with MIP can help prevent impurities, which are the biggest problem in electrochemical determination.

Based on the properties of the polymers, MIPs have different applications. The suspension (62), two-step swelling (63), bulk (64), precipitation, emulsion, and core-shell polymerization (65) are examples. Scanning electron microscopy (SEM) provides excellent resolution and can be used to study the morphology of MIPs.

In Figure 3.2, the printed polymer surface is shown at different magnifications. The polymer has a homogeneous microsphere structure.

Figure 3.2   SEM images of molecularly imprinted polymers with magnitude of: (a) 25,000 ×; and (b) 100,000 × (66).

3.2.6  Conductive Polymers

Conductive polymers (CPs) were discovered approximately 20 years ago and remain of interest. CPs are an important invention that can replace metallic and semiconductors.

CPs of polyenes polyaromatics have been studied extensively. Polyaniline (PANI) attracts great attention due to its different transmission mechanisms and good environmental stability. It is one of the oldest known CPs. PANI has magnetic, electronic, and optical properties similar to metals. PANI can be used in various fields, such as supercapacitors, electromagnetic shielding devices, battery electrodes, anti-corrosion coatings, light-emitting diodes, non-linear optical devices, molecular sensors, electrochromic displays, and microelectronic devices (67, 68). PANI is an excellent active cathode material for LIBs (69). The polymerization scheme of aniline is shown in Figure 3.3 (70).

Figure 3.3   Aniline polymerization (70).

Pyrrole’s well-known feature is its high primary step velocity after cation radical formation. After a sufficiently long chain is formed, a thin film is formed on the electrode surface with pyrrole oligomers (71–73). Electronic devices and chemical sensors are two main application areas of pyrrole-related CPs (74).

CPs have an operational efficiency of approximately 1% (75). Because of the resistance between the electrolyte and the polymer (76) and their mechanism based on the migration of ions, the actuation speeds are limited (77). Due to the electrolyte requirement, the CP actuator might need to be encapsulated (78).

3.2.7  Bistable Electroactive Polymers

Bistable electroactive polymers (BSEPs) are newly developed EAPs (79). BSEPs exhibit soft elastomer behavior above the glass transition temperature. Depending on the shape of the material, there might be changes in their movements. If it is a thin film compressed between the electrodes, it can act as a rigid capacitor, or a variable capacitor if it is in an elastomeric state. Poly(tert-butyl-acrylate) (PTBA) is a BSEP material that has gained much attention and research (79, 80). It shows a significant glass transition at 50°C, and exhibits outstanding strain stability and stress recovery. Operational features are available with a fault field strength of >250 MV/m.

BSEP has two balanced actuation characteristics based on a high voltage and specific power density. Dielectric materials consume energy when activated due to current leakage through the film. BSEPs can maintain their shape without effort and time. Therefore, BSEPs are important EAPs. (81). Figure 3.4 shows the variable stiffness and activation of BSEPs (82).

Figure 3.4   Rigid-to-rigid actuation mechanism of BSEP (82).

BSEPs contain crystalline clusters of long alkyl side chains in a crosslinked polymer matrix. This crosslinked polymer matrix makes the polymer film translucent (83–85). The abrupt and reversible phase transition of the crystalline aggregates of polymers causes a rapid shift between the solid and rubbery states of the polymers during temperature cycles. Cooling the material can maintain this deformation. The polymer regains its original shape on reheating.

3.2.8  Ferroelectric Polymers

Ferroelectric polymers are especially crystalline polar polymers that can provide permanent electrical polarization (86, 87). These properties are exhibited by poly(vinylidene difluoride) (PVDF), PVDF copolymers, polyamides (88), and mixtures of these (89, 90). It is not sufficient for polymers to have only polar side groups to exhibit ferroelectric behavior. In addition, they need to maintain molecular configurations when there is no polarity.

Polymers, such as poly (vinyl chloride), do not exhibit ferroelectric behavior since the bond must regulate itself in the helical conformation, which results from the relatively large steric influences of Van der Waals forces. Due to internal steric and electrostatic interactions, potential energy is generated in the chains.

PVDF consists of a repeating unit (–CH2CF2–). It is associated with negatively charged fluorine atoms and positively charged hydrogen atoms. (Figure 3.5). The repeating unit’s (–CH2CF2–) dipole moment is µ = 7×10^–30 Cm (2.1 D). Since these dipoles are closely attached to the main chain, their direction varies depending on the conformation and packaging of the molecules. The resulting crystal has a large self-polarization (P_s) responsible for the ferroelectricity of PVDFs. The values of a, b, and c are lattice constants (91).

Figure 3.5   Showing: (a) molecular; (b) chain; and (c) crystal structures of PVDF (91).

Ferroelectric polymers have been included in many applications and they are undergoing further research. For example, new ferroelectric polymer composites with a high dielectric constant are being developed. These are essential for many applications because they exhibit good pyroelectric and piezoelectric responses and have low acoustic impedance.

In addition, ferroelectric materials are used as sensors. High-pressure sensors are examples of these (92). They exhibit piezoluminescence against stress (93). They can be applied in the robotic and biomedical fields.

3.2.9  Dielectric Elastomers

Dielectric elastomers (DEs) are innovative materials that cause significant stress. They are EAPs that can convert electrical energy into mechanical work. They have a fairly good elastic energy density, and have many prototype implementations.

DE actuators are compatible variable capacitors (Figure 3.6). They consist of a thin elastomeric film coated on both sides with electrodes. When an electric field is applied to the electrodes, the electrostatic attraction between opposite charges and the repulsion between similar charges places pressure on the film. Most of the elastomers used are incompressible. If there is any reduction in their thickness, this causes an increase at the same time (81).

Figure 3.6   Sandwich structure of two compatible electrode layers and dielectric elastomer membrane.

Many elastomeric materials were investigated, including silicones, isoprene, polyurethanes, and fluoroelastomers between 1990 and 2000 (94–97). Polyurethanes, acrylics, and silicones have been identified as promising material groups (98).

Grease films loaded with carbon powder or carbon black as electrodes are potential applications for DEAs. However, the reliability of these materials is not very good, and if more advanced properties are desired, graphene sheets, liquid metal, embedded metallic nanoclusters, corrugated or patterned metal films, and carbon nanotube coatings could be used (99, 100). Acrylic elastomers and silicones are other alternatives.

Elastomer materials should have the following properties: (1) low material hardness, (2) high dielectric constant, and (3) high electrical breakage resistance. The mechanically prestressed elastomer film improves the electrical breaking strength. The film thickness is reduced and requires lower voltage to achieve the same pressure. The electrode must be compatible and conductive. This is important so that the elastomer is not restricted mechanically.

Significant research has been carried out on DEs that are based on silicon and natural rubber (101). When comparing them, acrylic elastomer based DEs are more advantageous due to their efficiency and fast response times (102).

3.2.10  Polymer Electrets

Electrets are insulating materials that show piezoelectric effects. Unequal area load distributions cause them to exhibit these effects (103, 104). Current polymer electrets consist of a highly porous polymer. The porous films’ charging voltages (5–10 kV) vary. For electrical discharge, charges accumulate at the polymer–gas interface. There are charges on opposite sides of the pores that form macroscopic dipoles according to the direction of the applied area (105). Pores in the films can be distorted when a compression force is applied. The applied voltage will cause a change in the thickness of the materials.

Polymer electrets can be used as sensors or actuators. Compared with solid PVDF ferroelectric polymers, the converter coefficient (d33) is higher (106).

Due to the increase in performance demands, research into polymer electret mixes is ongoing. Recently, studies based on mixtures with poly(2,6-dimethyl-1,4-phenylene ether) and polystyrene have been reported (107). Good performance with the new electrets has been achieved.

3.2.11  Electrostrictive Polymers

Recently, the development of electrostrictive polymers has created new research areas for high voltage actuators. Research on the use of electrostrictive polymers for the conversion of mechanical to electrical energy or energy harvesting has some potential applications.

Electrostrictive polymers have an inherent electrical polarization. The change in the dipole density of the material causes electrostriction. These polymers contain nanocrystalline or molecular polarizations from the applied electric field effect.

Other materials discussed in this section are electrostrictive graft copolymers, PVDF copolymers with nano-sized crystalline areas, and liquid crystal elastomers (LCEs). LCEs were first used in artificial muscles (108), where the polymer network allowed sufficient movement. LCEs are nematic and smectic and have different mechanisms.

Ferroelectric polymers are based on PVDF copolymers, and they have two main limitations. First, electrically induced paraelectric-ferroelectric transition only allows operation above the Curie temperature. Second, the presence of strong hysteresis makes control difficult (105).