Yoshiro Tajitsu

Ferroelectric phase transition in a copolymer of vinylidene fluoride and trifluoroethylene

Abstract The phase transition in a copolymer of 55% vinylidene fluoride and 45% trifluoroethylene near 70°C has been investigated by measuring the temperature dependence of fundamental properties. The remanent polarization disappeared near 70°C. The dielectric relaxation strength showed a peak indicating an anomaly and the dielectric relaxation time showed a critical slowing down phenomenon. A change in crystal lattice spacing associated with remanent strain and an anomalous specific heat were observed. These results are consistent with a ferroelectric phase transition. Comparison is made between observation and prediction by phenomenological and simple dipolar theories for a order-disorder type transition.

Ferroelectric Behavior in the Copolymer of Vinylidenefluoride and Trifluoroethylene

The linear and nonlinear dielectrical and depolarization processes of the copolymer of vinylidene fluoride and trifluoroethylene have been measured. The D vs. E hysteresis loop below room temperature indicates the ferroelectricity of this copolymer. The dielectric anomaly and the disappearance of remnant polarization at T0=70°C suggest that T0 is the phase transition temperature from the ferroelectricity to the paraelectricity phase. Results are analyzed on the basis of Weiss-type semimolecular theory.

Investigation of switching characteristics of vinylidene fluoride/trifluoroethylene copolymers in relation to their structures

Ferroelectric switching characteristics of VDF/TrFE copolymers have been investigated over the entire range of composition for the samples subjected to drawing and/or annealing. Switching phenomena were most remarkably observed in the copolymers containing 50–80 mol% VDF. The as-cast copolymer exhibits no switching behavior because of high a depolarization field associated with low crystallinity. Draw-induced orientation of the chain axis parallel to the film surface causes an increase in the amount of reversed polarization. Annealing at a temperature between the Curie point and the melting point results in a significant increase in the degree of crystallinity and the amount of reversed polarization. Extrapolating to 100% crystallinity allows an estimation of spontaneous polarization, which is 120–135 mC/m2 for the 73/27 mol% copolymer. This value suggests a small Lorentz factor in crystals of VDF/TrFE copolymers.

Crystalline phase transition in the copolymer of vinylidenefluoride and trifluoroethylene

The measurement of the temperature dependence of elastic, dielectric, and piezoelectric constants for the film of 55/45% VDF‐TrFE copolymer suggests that the Curie temperature exists at about 70 °C. The lattice spacing sharply changes at a temperature range from 55 to 75 °C. The remanent polarization is also determined from the D versus E hysteresis loop below 75 °C. It is found that the spontaneous strain in crystal lattice is linearly related to the square of remanent polarization. The transition is ascribed to electrostriction due to dipolar orientation.

Nanosecond Switching in Thin Fims of Vinylidene Fluoride/Trilluoroethylene Copolymers

High-field switching characteristics have been investigated in the spin-coated thin films of vinylidene fluoride/trifluoroethylene copolymers containing 52, 65, 73 mol% of the former. The switching times of three copolymers are identical af fields from 150 to 500 MV/m, following an exponential dependence on the reciprocal field. The observed fastest switching time is 100 nsec which is shorter than the dielectric relaxation time associated with crystalline dipole motions. The result indicates that total reorientation of crystalline dipoles occurs faster than the correlation time of their natural fluctuations.

Dielectric relaxations in copolymers of vinylidene fluoride

Abstract Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene have been investigated by means of broad band dielectric spectroscopy (10 mHz-1 GHz) at temperatures covering the ferroelectric, paraelectric and molten phases as well as the glassy and rubbery states. The low-temperature relaxation spectra near Tg (∼ -40°C) consist of two processes; a WLF-type segmental mode and an Arrhenius type local mode. These processes are unified near room temperature to be transferred continuously to the high-temperature process. After showing a peak at the Curie point Tc (70–140°C), the relaxation strength decreases gradually in the paraelectric phase until the melting point Tm (∼ 150°C) is reached, where it decreases rapidly. It is found that the relaxation time in the molten phase lies on the Arrhenius line extended from the low-temperature local mode process. This implies that related molecular motions arise from a common elementary process. Departure from this Arrhenius lin...

Piezoelectricity of chiral polymeric fiber and its application in biomedical engineering

Poly-L-lactic acid (PLLA), which is a type of chiral polymer, exhibits a high shear piezoelectric constant. To realize a higher shear piezoelectric constant, we spun PLLA resin into fibers. We succeeded in controlling the piezoelectric motion of a PLLA fiber by applying a dc voltage and ac voltage, similar to the control of a piezoelectric actuator. On the basis of this experimental result, we designed a catheter using a PLLA fiber (PLLA fiber catheter) and tweezers using a pair of PLLA fibers (PLLA fiber tweezers), controlled by adjusting the applied voltage. Then, using the PLLA fiber tweezers or catheter, we successfully picked up and removed small samples, such as a thrombosis in a blood vessel.

Effects of Thickness on Ferroelectricity in Vinylidene Fluoride and Trifluoroethylene Copolymers

The effects of thickness on ferroelectricity in vinylidene fluoride and trifluoroethylene copolymers have been investigated mainly through switching measurements. Switching phenomena as a result of ultimately 180° rotations of individual chain molecules around their axes were observed in the copolymer films with thicknesses in the range of 0.03-1 µ m. With decreasing film thickness, the switching times increase, the switching curve becomes broad and the permittivity at the Curie point decreases. However, the amount of reversed polarization is nearly independent of film thickness. These results are generally reproducible if we assume a surface layer with thickness less than 0.01 µ m, low permittivity, and absence of ferroelectric switching even in the presence of an applied electric field.

Microactuators with Piezoelectric Polylactic Acid Fibers—Toward the Realization of Tweezers for Biological Cells

The poly-L-lactic acid (PLLA) shows a high shear piezoelectric constant. We fabricated PLLA fiber samples with large degree of crystallinity and high orientation of crystallites using high-speed spinning. If the electric field is applied to the PLLA fiber in the direction perpendicular to the PLLA fiber axis, the PLLA fiber must be driven by the piezoelectric effect. Therefore, we devised an effective method of applying the electric field to the PLLA fiber. We also developed equipment for measuring the piezoelectric motion of the PLLA fiber, which uses a CCD camera and divided photodiodes. As a result, we could drive the PLLA fiber by the shear piezoelectric effect under the application of ac voltage, and observed the bending motion of the entire PLLA fiber. Finally, we attempted to release and grasp the bead sample using a pair of PLLA fibers like tweezers, and succeeded.

Electrical properties of biodegradable polylactic acid film

We measured the basic electrical insulation characteristic of biodegradable polylactic acid (PLA), and the following results were obtained. The volume resistivity, dielectric constant, and dielectric loss tangent measured at room temperature were almost the same as those of crosslinked polyethylene (XLPE) currently used as insulating material for cables and electric wires. The mean impulse breakdown strength of PLA was about 1.3 times that of XLPE. Also, space charge accumulation in PLA and XLPE was measured. The amount of space charge accumulation in PLA was one-half that in XLPE when a DC voltage was applied for a short time.