This paper discusses a new family of ferroelectric polymorphs fluoro-terpolymers comprising vinylidene difluoride (VDF), trifluoroethylene (TrFE), and a chloro-containing third monomer, including vinyl chloride (VC), 1,1-chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE), with narrow molecular weight and composition distributions. The slightly bulky chlorine atom serves as a kink in the polymer chain, which spontaneously alters the chain conformation and crystalline structure. Comparing with the corresponding VDF/TrFE copolymer, the slowly increasing chlorine content (< 8 mol% of ter-monomer) gradually changes the all-trans chain conformation to tttg+tttg- conformation, without significant reduction of overall crystallinity. Curie (F-P) phase transition temperature between the mixed ferroelectric phases and paraelectric phase (tg+tg- conformation) also gradually reduced to near ambient temperature, with very small activation energy. Consequently, the terpolymers show high dielectric constant (>80) and large electrostrictive response (>5%) at ambient temperature, and exhibiting common ferroelectric relaxor behaviors with a broad dielectric peak that shifted toward higher temperatures as the frequency increased, and a slim polarization hysteresis loop at ambient temperature.
This paper discusses a new ferroelectric polymer with high dielectric constant (>50 at 1K-1M Hz) and large electrostrictive response (~5%) at ambient temperature, which is based on a processable semicrystalline terpolymer comprising vinylidene difluoride (VDF), trifluoroethylene (TrFE), and chlorotrifluoroethylene (CTFE). This VDF/TrFE/CTFE terpolymer was prepared by a combination of a borane/oxygen initiator and bulk polymerization process at ambient temperature. The control of monomer addition afforts the terpolymers with high molecular weight and relatively narrow molecular weight and composition distributions. The incorporated bulky CTFE units homogeneously distributed along the polymer chain seem to reduce the thickness of ferroelectric crystalline domains without destroying the overall crystallinity. This nano-size semicrystalline morphology results in the reduction of ferroelectric-paraelectric (F-P) phase transition to near ambient temperature with a very small energy barrier. Some terpolymers exhibited common ferroelectric relaxor behaviors with a broad dielectric peak that shifted toward higher temperatures as the frequency increased, and a slim polarization hysteresis loop at near the dielectric peak (around ambient temperature) that gradually evolved into a normal ferroelectric polarization hysteresis loop with reduced temperature.
In our earlier work, we have demonstrated that the high- energy electron irradiation modifies (VDF-TrFE) copolymers from a normal ferroelectric to a relaxor ferroelectric with high electromechanical response. Here, we present two approaches we are taking recently. One is to explore the non-irradiation approach to modify the PVDF-based material to achieve high electromechanical response. A ter-monomer (HFP and CTFE are used here) with a relative large size is added to the copolymer to act as modifiers. The electromechanical and dielectric properties in the terpolymers seem to be similar to those in irradiated copolymers. The other approach addresses the fundamental issue of the low dielectric constant in the currently available electroactive polymers. By making use of composite approach and ultra-high dielectric constant in CuPc, a polymeric composite with very high dielectric constant but the elastic modulus similar to polymer has been demonstrated. The preliminary results indicate that the polymer composite has the potential to generate high strain under much lower field. In parallel to the material development, we investigated device performance based on the irradiated copolymers. The performance of irradiated copolymer multilayers with a thickness up to 1 mm was characterized. The design and device performance of a flextensional actuator fabricated from the irradiated copolymer multiplayer are presented. The flextensional actuator, whose resonance frequency is at a frequency of a few kHz to more than 10 kHz, exhibits more than 1 mm displacement and high force output, which are attractive for many applications.