The field of Electroactive Polymers has experienced a considerable amount of expansion over the last decade. Much of this work has been concentrated on developing polymeric materials that mimic biological systems or that exhibit electronic and optical properties similar to inorganic materials. This paper briefly reviews some of the nearer term applications that electroactive polymers might impact: image processing and sonar. In addition, a review of compliant actuators based on the unique properties inherent in electroactive polymers is provided. Emphasis will be placed on the mechanisms responsible for actuation and on the limited mechanical, electrical and chemical data current available. A comparison between mammalian muscle properties and electroactive polymer actuator properties is provided.

In recent years, electroactive polymers (EAP) materials have gained recognition as potential actuators with unique capabilities having the closest performance resemblance to biological muscles. Ion-exchange membrane metallic composites (IPMC) are one of the EAP materials with such a potential. The strong bending that is induced by IPMC offers attractive actuation for the construction of various mechanisms. Examples of applications that were conceived and investigated for planetary tasks include a gripper and wiper. The development of the wiper for dust removal from the window of a miniature rover, planned for launch to an asteroid, is the subject of this reported study. The application of EAP in space conditions is posing great challenge due to the harsh operating conditions that are involved and the critical need for robustness and durability. The various issues that can affect the application of IPMC were examined including operation in vacuum, low temperatures, and the effect of the electromechanical and ionic characteristics of IPMC on its actuation capability. The authors introduced highly efficient IPMC materials, mechanical modeling, unique elements and protective coatings in an effort to enhance the applicability of IPMC as an actuator of a planetary dust-wiper. Results showed that the IPMC technology is not ready yet for practical implementation due to residual deformation that is introduced under DC activation and the difficulty to protect the material ionic content over the needed 3-years durability. Further studies are under way to overcome these obstacles and other EAP materials are also being considered as alternative bending actuators.

The single crystalline (SC) film of copolymer of vinylidene fluoride and trifluoroethylene is a highly crystallized; highly double-oriented, and optically transparent ferroelectric film, which can be prepared by annealing a uniaxially drawn film in the paraelectric phase with its ends being clamped and its surfaces free. The SC film is composed of endlessly extended chain crystals whose c-axis aligns along the stretching direction almost perfectly and the polar b-axis is oriented ±π/6 off the film normal. It does not contain lamellar crystals nor amorphous phase. Owing to such unique structure, the SC film exhibits the ferroelectricity and related properties inherent to the crystal more clearly than the films composed of lamellar crystals. In this paper, we present the results of our recent studies on the SC films: (1) the structure and morphology studied by POM and X-ray diffiractions; (2) the ferroelectric, piezoelectric, and mechanical properties in the ferroelectric phase; (3) molecular chain motions in the paraelectric phase as revealed by dielectric, X-ray diffraction and shear mechanical studies; and (4) applications to shear ultrasonic transducers. The followings are the main results to be emphasized: (A) the Young's modulus in the direction along the stretching axis is much larger (120 GPa at 10K) than that of LC films (15 GPa); (B) the SC film is the most effective piezoelectric polymer film among piezoelectric polymers; (C) the SC film in the paraelectric phase is a liquid crystal of ID-liquid and 2D-solid, and each chain molecule of TGTG' conformation undergoes rotational and flip-flop motions independently of neighboring chains in a highly regular hexagonal crystal lattice, which leads to anomalous dielectric behaviors in the paraelectric phase; (D) SC film transducers for shear ultrasonic waves are effectively usable for NDE.

Structure changes in the ferroelectric phase transitions of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers were investigated on the basis of a set of experimental data of WAXS, SAXS, IR and Raman measurements. An intimate relation has been clarified between the structural changes in the crystal lattice and the morphological changes. These structural changes were found to originate ultimately from the remarkable change in the molecular conformation between trans and gauche forms. In order to extract the essentially important factors governing this trans-gauche conformational change in the phase transition, the molecular dynamics calculation was carried out, giving relatively good reproduction of the observed structural changes.

Recently it has been demonstrated that using a high electron energy irradiation, the electromechanical properties of polyvinylidene fluoride - trifluoroethylene, P(VDF-TrFE) copolymers can be improved significantly for example; high electric field induced strain (∼ 4.5%) with high elastic energy density, high dielectric constant (∼ 65 with loss less than at 1kHz), high piezoelectric coefficient (∼ 350 pm/V ) and high electro-mechanical coupling coefficient (∼0.45). It was found, depending on sample processing and electron irradiation conditions these properties can be controlled. In this talk we will present the experimental data on the effect of processing and irradiation conditions on structural and transitional behavior of copolymers. These structural information coupled with the electro-mechanical properties will be presented in order to show the possibilities for further improvement.

This paper focuses on the molecular structure-electric property relationships of VDF/TrFE/HFP terpolymers, containing vinylidene difluoride (VDF), trifluoroethylene (TrFE) and hexafluoropropene (HFP) units. Several terpolymers were synthesized and evaluated with the corresponding VDF/TrFE copolymers. In general, a small amount of bulky HFP units in the polymer chain prevents the long sequence of crystallization and results in smaller ferroelectric (polar) micro-domains, which show the improved electric properties. The resulting terpolymer possesses a interesting combined properties with good processibility, low Curie transition temperature and high dielectric constant, narrow polarization hysteresis loop, and high strain response (2.5%) at relatively low electric field (50 MV/m).

In the present study, the strain response of a new class of copolymers of PVF2 is investigated. Electrostrictive strains were measured in poly(vinylidene-fluoride hexafluoropropylene), P(VF2-HFP), using a capacitance method (air-gap capacitor), as a function of electric field. Three different thermal treatments (ice water quenched, air quenched and slow cooled) were given to samples of composition 5% and 15% HFP. Strains greater than 4 % were observed in the 5% HFP ice water quenched P(VF2-HFP) copolymer. Values of elastic modulus were lower for the quenched 5 % films than for the slow cooled ones, and in both cases they were higher than previously studied polyurethane elastomers and poly(vinylidene-fluoride trifluoroethylene) copolymers. Key words: Electrostrictive Polymer, Poly(vinylidene-fluoride hexafluoropropylene) copolymer, Ferroelectic, X-ray, DSC.

The electro-mechanical properties of high energy electron irradiated poly(vineylidene fluoride-trifluorethylene) (P(VF-TrFE)) copolymers under different mechanical stress conditions are reported. In stress free condition, the electric field induced longitudinal and transverse strains of the irradiated P(VDF-TrFE) copolymer films at room temperature (RT) can reach about 5% and more than 3% respectively. The longitudinal strain response of the material under hydrostatic a pressure up to 83 atmospheses was studied at RT. The transverse strain response of the material at RT was studied under uniaxial tensile stress. It was found that the material has a high load capability and for stretched films along the stretching direction the induced strain remains high, even at about 45 MPa. The temperature dependence of the strain response is also characterized. Both the temperature and stress dependence of the strain response indicate that the electric field induced strain response originates from the electric field induced local phase transition.

Recently, it was discovered that the transduction / strain capability of p(VDF-TrFE) copolymers can be enhanced by more than an order of magnitude by irradiating the copolymer with large doses (40–300 Mrads) of β-particles (1–3 MeV electrons). The goal of this project is to understand how irradiation improves the electromechanical properties of p(VDF-TrFE) copolymers while simultaneously identifying the secondary effects of the irradiation process on the material (not critical to the electromechanical properties) and attempting to separate the two contributions. It has been found that β-irradiation affects the material profoundly in several different manners. Reduction in the melt temperature, degree of crystallinity, and the resulting crystal quality have been observed for increasing doses of β-particles. Similar results have been observed for the Curie transition, especially in the energy associated with, and the breadth otf the transition. In addition, thermogravimetric analysis indicates that irradiation causes both chain scission and network polymer formation. Solid-state NMR results are discussed in reference to postulated dehalogenation, dehydrohalogenation, and olefinic bond formation activities.

The pyroelectric response of a copolymer of vinylidene fluoride with trifluoroethylene was measured as a function of time by applying a laser pulse to give an abrupt increase in temperature. The response curve shows several processes: the thickness extensional mode TEM, the length extensional mode LEM and the heat transfer to the environment. By fitting a theoretical curve for the TEM mode, the pyroelectric primary and secondary effects are separated. The primary effect is found to be small at room temperature. Additional gradual response shows that there exists a surface layer which has smaller polarization than in the bulk.

The temperature dependence of the pyroelectric response curve shows that the primary effect increases with temperature. It dominates the response at just below the ferroelectric-paraelectric transition temperature.

The ferroelectric switching curve of vinylidene fluoride / trifluoroethylene copolymer exhibits a characteristic time evolution consisting of two processes; an initial gradual increase in proportion to t0 5 followed by a rapid increase according to an exponential function with particularly large exponent 6. Such a switching curve was analyzed by means of computer simulation based on a modified nucleation-growth mechanism. It was found that the initial gradual increase is attributed to generation of considerably large nuclei that grow according to a random-walk scheme. Once such nuclei gain a critical size, they start to grow automatically either one or two dimensionally. The time required to generate critical nuclei serves as an incubation time for the later growth process to result in the large exponent.

A piezoelectric polymeric blend system has been developed. The system contains two components: ferroelectric poly(vinylidene fluoride-trifluoroethylene) and graft elastomer. The remanent polarization, Pr, and the piezoelectric strain coefficient, d31, of the blends have been studied as a function of relative composition of the two components, temperature and frequency. Both blended copolymer and graft unit in the elastomer contribute to the total crystallinity of the blend-system, and hence to the remanent polarization and piezoelectricity. The piezoelectric strain coefficient, d31, of the blend systems shows dependence on both the remanent polarization and the mechanical stiffness, which in turn are determined by the fraction of the two components in the blends. This mechanism makes it possible for the piezoelectric strain response of the blend to be tailored by adjusting the relative composition. Although Pr of the copolymer is higher than that of the blends, the blend films containing 75 wt.% copolymer exhibit a higher d31 at room temperature, possibly due to their lower modulus. The blend films containing 50 wt.% copolymer exhibit a constant value of d31, from room temperature to 70°C.

The structural and electric properties of newly synthesized vinylidene fluoride (VDF) oligomer thin films evaporated on various substrate were investigated. The structural behavior of the VDF oligomer thin films strongly depended on the kinds of substrate and substrate temperature during film preparation. In particular, the VDF oligomers epitaxially grew on KBr(001), aligned their molecular chain along the <110> direction of substrate surface, and were similar to polar Form Itype crystals of poly(VDF). While, the thin films evaporated on NaCl(001), SiO2/Si were non-polar Form II-type or mixture of both Form I- and Form II-type crystals with their molecular chain normal to the surface. These facts indicated that crystal field of substrate, based on van der Waals and electrostatic interactions, strongly influenced to ferroelectric phase transition of the VDF oligomer. In addition, we demonstrate the formation of the local polarized domains in the epitaxial crystals by applying electric pulses from a conducting AFM probe used as a positionable topelectrode, and confirm their piezoelectricity of the VDF oligomers for the first time.

Non-electrical poling was proposed in novel ferroelectric polymers including such as polythioureas, polycyanophenylenesulfides, and polyvinyfluorides. This poling method utilizing the cooperativity of molecular dipoles can be called “surface energy poling” and takes agvantage of the energy difference inthe top and bottom surface of apolar aggregate (dipole glass) to form a remanent polarization. A ferroelectric polymer film sandwiched between a metal with higher surface energy and PTFE film with lower surface energy were heated to Tp=Tc (glass tramsition temperature or ferroelectric transition) × 1.2 and cooled slowly to room temperature. In the thin films less than 10μm, we observed the remanent polarization which gives a large pyroand piezo-electric constant as well as that obtained by electrical poling. This poling is effective for homogenious structures such as amophous ferroelectric polymers.

The strain response of dielectric elastomers sandwiched between compliant electrodes was studied. These electroactive polymer artificial muscle (EPAM) materials show excellent overall performance and appear more attractive than many competing actuator technologies. Based on the available data, the actuation mechanism is due to the free charge interaction of the compliant electrodes, enhanced by the dielectric properties of the elastomer (Maxwell stress). Strains over 200%, actuation pressures up to 8 MPa, and energy densities up to 3.4 J/cm3 have been demonstrated with silicone rubber and acrylic elastomers. Response time is rapid, and the potential efficiency is high. The fabrication of EPAM actuators can be simple and low cost. A wide range of small devices have been made, to demonstrate the potential of the technology and reveal more about performance and fabrication issues. These devices include bending beam actuators for scanners and clamps, diaphragm actuators for pumps and valves, stretched-film actuators for electro-optics, and bow actuators for muscle-like actuators for small robots and other micro machines.

Efficient actuators that are lightweight, high performance and compact are needed to support telerobotic requirements for future NASA missions. In this work, we present a new class of electromechanically active polymers that can potentially be used as actuators to meet many NASA needs. The materials are graft elastomers that offer high strain under an applied electric field. Due to its higher mechanical modulus, this elastomer also has a higher strain energy density as compared to previously reported electrostrictive polyurethane elastomers. The dielectric, mechanical and electromechanical properties of this new electrostrictive elastomer have been studied as a function of temperature and frequency. Combined with structural analysis using x-ray diffraction and differential scanning calorimetry on the new elastomer, structure-property interrelationship and mechanisms of the electric field induced strain in the graft elastomer have also been investigated. This electroactive polymer (EAP) has demonstrated high actuation strain and high mechanical energy density. The combination of these properties with its tailorable molecular composition and excellent processability makes it attractive for a variety of actuation tasks. The experimental results and applications will be presented.

The Young's Modulus (E) and piezoelectric coefficient (d31) have been investigated as a function of dehydration time for bovine cornea at room temperature. The piezoelectric and mechanical responses observed were anisotropic for bovine cornea and d31 decreased, while E increased with dehydration. In addition, water molecules appear to increase the crystallinity (of collagen) in the cornea. With dehydration of the cornea, reduction of crystallinity and changes in hydrogen bonding were observed by Fourier Transform Infra Red (FTIR) and Wide Angle X-ray Diffracion (WAXD) measurements.

Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high transducer constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene. In particular, measurements of the direct and inverse transducer constants, the thermal stability of the charge, and Young's modulus are discussed. Assuming reasonable charge distributions and charge densities, the calculated transducer constants are in good agreement with the measured values. Both the theoretical model and the measurements show the reciprocity of the transducer constants.

Amorphous polyimides containing polar functional groups have been synthesized and investigated for potential use as high temperature piezoelectric sensors. The thermal stability of the piezoelectric effect of one polyimide was evaluated as a function of various curing and poling conditions under dynamic and static thermal stimuli. First, the polymer samples were thermally cycled under strain by systematically increasing the maximum temperature from 50°C to 200°C while the piezoelectric strain coefficient was being measured. Second, the samples were isothermally aged at an elevated temperature in air, and the isothermal decay of the remanent polarization was measured at room temperature as a function of time. Both conventional and corona poling methods were evaluated. This material exhibited good thermal stability of the piezoelectric properties up to 100°C.

Ferroelectric liquid crystalline polymer films exhibit a number of unique physical properties and have been extensively studied during the last few years. Ferroelectric polymer films exhibiting a high level of piezoelectric activity would provide a new route to piezo- and pyro-electric sensors.

We report the fabrication of free-standing ferroelectric liquid crystal films which exhibit piezoelectric coefficients up to 4.4pC/N. We also report the investigation of the physical properties of these films by dynamic mechanical analysis.