Electroactive polymers (EAPs) are capable of converting energy in the form of electric charge and voltage to mechanical force and movement and vice versa. Several electroactive polymer actuator materials whose responses are controlled by external electric fields, e.g. poly(vinylidene fluoride-trifluoroethylene) based fluoroterpolymers, have generated considerable interest for use in applications such as artificial muscles, sensors, parasitic energy capture, and integrated bio-microelectromechanical systems (BioMEMS) due to their high electric-field induced strain, high elastic modulus, high electromechanical coupling and high frequency operation, etc. The combination of micro-optics and MEMS, referred to as micro-opto-electromechanical systems (MOEMS), makes a new opportunity for innovation in the EAP field. There is a lot of pioneering work on optical beam deflection by electromechanically driven digital micromirrors. In this paper we describe a flexible polymer deformable micromirror (PDM) light-valve technology based on high-performance electroactive polymer materials and microactuators for high-quality electronic projection display and imaging systems. The excellent electromechanical properties of these electroactive polymer microactuators greatly improve the electro-optical properties of the deformable micromirrors and light valves, e.g., optical switching behavior, deformation amplitude and contrast, and low-voltage and high-frequency operation. The material selection, device fabrication, characterization, and a theoretical analysis using the finite element analysis code will be investigated. This technology is compatible with CMOS technology for an active matrix addressing on a chip. High-resolution phase-modulating polymer light valves may permit a lot of future applications, and electroactive polymer micromachining lends flexibility to displays application.
The development of high dielectric constant polymers as active materials in high-performance devices is one of the challenges in polymeric electronics and opto-electronics such as flexible thin-film capacitors, memory devices and microactuators for deformable micromirror technology. A group of poly(vinylidene fluoridetrifluoroethylene) P(VDF-TrFE) based high-dielectric-constant fluoroterpolymers have been developed, which have high room-temperature dielectric constant (K>60) and very high strain level and high energy density. The longitudinal and transverse strain of these materials can reach about -7% and 4.5%, respectively, and the elastic energy density is around 1.1 J/cm^3 under a high electric field of 150 MV/m. The influence on the electromechanical properties of copolymerizing poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) with a third monomer, chlorofluoroethylene (CFE), was investigated. It was found that increasing the CFE content from 0 to 8.5% slowly converts the ferroelectric structure of the copolymer to a relaxor ferroelectric system. This allows for a greatly decreased polarization and dielectric hysteresis and a much higher strain. Above 8.5%, increased CFE content substantially degrades the bulk crystallinity and the Young's modulus. These terpolymers have the potential to achieve above 10 J/cm^3 whole capacity energy density, which makes them good candidates for applications in pulse power capacitors. An all-polymer percolative composite by the combination of conductive polyaniline particles (K>10^5) within a fluoroterpolymer matrix, is introduced which exhibits very high dielectric constant (>7,000). The experimental results show that the dielectric behavior of this new class of percolative composites follows the prediction of the percolation theory and the analysis of the conductive percolation phenomena. The very high dielectric constant of the all-polymer composites which are also very flexible and possess elastic modulus not very much different from that of the insulation polymer matrix makes it possible to induce a high electromechanical response under a much reduced electric field (a strain of 2.65% with an elastic energy density of 0.18 J/cm^3 can be achieved under a low field of 16 MV/m). Data analysis also suggests that in these composites, the non-uniform local field distribution as well as interface effects can significantly enhance the strain responses. Furthermore, the experimental data as well as the data analysis indicate that the conduction loss in these composites will not affect the strain hysteresis. Flexible high dielectric constant electroactive polymers provide potential applications in high-energy-density (HED) energy storage and conversion systems such as lightweight field effect actuators and capacitors.
This paper reports two classes of electroactive polymers developed recently which exhibit very high strain and elastic energy density. In the first class of the electroactive polymer, i.e., the defects modified poly(vinylidene fluoridetrifluoroethylene)(P(VDF-TrFE)) polymers, an electrostrictive strain of more than 7% and an elastic energy density above 1 J/cm3 can be induced under a field of 150 MV/m. The large electrostrictive strain in this class of polymers originates from the local molecular conformation change between the trans-gauche bonds and all trans bonds, which
accompanies the field induced transformation from the non-polar phase to the polar phase. The second class of the polymer is an all organic composite, which shows a very high dielectric constant (>400) and high strain induced with a low applied field (2% strain under 13 MV/m). The strain is proportional to the applied field and the composite has an elastic modulus near 1 GPa.
The uniqueness of liquid crystals (LCs) lies in the large anisotropies in their properties, which can be utilized to generate high electromechanical responses. In a properly oriented liquid crystal polymer system, an external electric field can induce re-orientation of the mesogenic units possessing a dielectric anisotropy, which, when coupled with the shape anisotrophy of the mesogenic units, can in turn produce large mechanical strain. Anisotropic liquid crystal gels, which can be obtained by in situ photopolymerization of the reactive LC molecules in the presence of non-reactive LC molecules in an oriented state, are an example of such liquid crystal polymer systems. It has been shown that a homeotropically aligned liquid crystal gel in its nematic phase exhibits high electrically induced strain (>2%) with an elastic modulus of 100MPa and a high electromechanical conversion efficiency (75%) under an electric field of 25 MV/m. These anisotropic LC polymeric materials could provide a technologically compatible system for such applications as artificial muscles and as micro-electromechanical devices.
The recent discovery of high electromechanical performance in high-energy electron irradiated P(VDF-TrFE) copolymers opens a new avenue for developing high performance electroactive polymers. From basic materials consideration, it is expected that one can achieve high electromechanical performance by means of nonirradiation approach, such as introducing ter-monomer to form PVDF based terpolymer. The basic requirement for the ter-monomer is discussed in order to achieve a high electromechanical performance in P(VDF-TrFE) based terpolymer. Based on the conclusion, P(VDF-TrFE-CFE) terpolymer has been synthesized and the experimental results indicate that the terpolymer exhibits better electromechanical performance compared with irradiated copolymers. For example, both the electric induced strain and Young's modulus in P(VDF-TrFE-CFE) terpolymer could be higher than that in irradiated copolymers. X-ray diffraction, DSC and FTIR were employed to determine the structure and molecule conformation. Furthermore, a serious theoretical simulation was carried out for P(VDF-TrFE) based terpolymers with different ter-monomers. The results show that indeed the terpolymer with CFE favors gauche conformation, consistent with the experimental results.