Vinylidene Fluoride-trifluoroethylene Copolymer Research Articles

Relaxor ferroelectric behavior of γ-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

We have conducted a systematic investigation on \ensuremath{\gamma}-irradiated poly(vinylidene fluoride-trifluoroethylene) equimolar samples. The appearance of relaxor features is clearly demonstrated for this copolymer, for gamma-irradiation doses comparable to those of electron irradiated samples. This material constitutes a new class of relaxor ferroelectrics, since its structure has no direct relationship with perovskite or tungsten-bronze structures and it is also the first non-displacive relaxor ferroelectric. Dielectric, calorimetric, structural, and spectroscopic data allowed us to draw a picture of the chemical and morphological modifications that cause the appearance of the relaxor behavior of the material. In particular, the loss of stability of the ferroelectric domains is attributed to the freezing of the amorphous phase, which forbids the establishment of long-range ferroelectric order.

Glassy dynamics in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer system

The dynamic processes in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], copolymer with a VDF content of 55 mol % have been studied by measurements of the temperature and frequency-dependent linear $({\ensuremath{\varepsilon}}_{1})$ and third-order nonlinear $({\ensuremath{\varepsilon}}_{3})$ dielectric constants. Analysis of the complex linear dielectric response by a temperature-frequency plot has revealed that the longest relaxation time diverges at $T=277\mathrm{K},$ while the bulk of the relaxation times remains finite below this freezing temperature. Such an asymmetric behavior is, together with the temperature dependence of the static field-cooled dielectric constant, very similar to that observed in classical relaxor systems, such as lead magnesium niobate and lanthanum lead zirconate titanate, and is reminiscent of the dynamic behavior observed in various spin glasses. In addition, a paraelectric-to-glass crossover in the temperature dependence of the dielectric nonlinearity ${a}_{3}={\ensuremath{\varepsilon}}_{3}/{\ensuremath{\varepsilon}}_{0}^{3}{\ensuremath{\varepsilon}}_{1}^{4},$ typical for relaxors, has been observed.

Influence of the annealing conditions on the polarization and electromechanical response of high‐energy‐electron‐irradiated poly(vinylidene fluoride trifluoroethylene) copolymer

AbstractIn this study, we investigated the influence of annealing conditions before irradiation on the ferroelectric and electromechanical properties of uniaxially stretched high‐energy‐electron‐irradiated poly(vinylidene fluoride trifluoroethylene) (HEEIP) copolymer (68/32 mol %) films. For films annealed at one fixed temperature before the irradiation (one‐step annealing), the highest crystallinity, which was highly desirable for enhancing the electromechanical response, was obtained only for films annealed between 132 and 136 °C. In addition, annealing over 10 h in this temperature window resulted in a large increase in the crystal lamellar thickness, which was required for reducing the polarization hysteresis to a minimum in the HEEIP samples. For improvements in the mechanical qualities of the uniaxially stretched films, a two‐step annealing procedure was investigated; that is, before the irradiation, the films were first annealed at a lower temperature to release the mechanical stress in the films due to the stretching and then were annealed in the high‐temperature window to raise the crystallinity and crystalline size. The experimental results indicated that this approach could produce uniaxially stretched HEEIP films with much improved mechanical qualities. Furthermore, the uniaxially stretched HEEIP films with this two‐step annealing exhibited the same electromechanical response as or an even higher one than that from the one‐step‐annealed HEEIP films. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 797–806, 2003

Ultrasonic transducers using electron-irradiated vinylidene fluoride-trifluoroethylene copolymers

Single-element, planar transducers have been fabricated using electron-irradiated poly(vinylidene fluoride-trifluoroethylene) 80/20 mol% copolymers with different electron dosage. Electrical field-induced strain response of copolymer film with 100 Mrad dosage has been studied at 5 kHz and the electrostrictive coefficient was calculated. The transmitting response of the air-backing and epoxy-backing transducers was evaluated with the application of high DC bias voltages. Clear ultrasonic amplitudes and high frequency spectrum (>20 MHz) were observed when driven from a standard ultrasonic voltage source through a decoupling circuit. It has also showed that larger generation of ultrasonic waves will be induced under high DC bias field, which is due to the increase of induced d 33 piezoelectric coefficient. Two different polar bias voltages, positive and negative, were applied to the transducers and inverse waveforms were received, which was coincident with the theoretical analysis of the strain response of electrostrictive film.

Dielectric Behaviour and Polarization Response of Proton Irradiated Ba0.65Sr0.35TiO3/P(VDF-TrFE) Composites

Ba0.65Sr0.35TiO3/P(VDF-TrFE)70/30 composites have been prepared by dispersing Ba0.65Sr0.35TiO3 powder in a vinylidene fluoride-trifluoroethylene copolymer (with 70 mol% vinylidene fluoride) matrix. The effects of irradiation with 3 MeV protons on the dielectric properties and polarization response of the composites have been investigated. At a dosage of 100 Mrad, both the copolymer and the composites exhibit a broad peak in the relative permittivity whose location shifts upwards in temperature with increasing frequency. This result implies that the proton irradiation breaks up the coherent polarization domains in the normal ferroelectric copolymer phase into nanopolar regions, thereby converting the material to a relaxor ferroelectric. The polarization-electric field hysteresis loop becomes slimmer after irradiation, particularly for the composites with low ceramic volume fraction. For the unirrdiated samples, the slight increase of remnant polarization with ceramic volume fraction indicates that the remnant polarization of the ceramic phase is not much higher than that of the copolymer phase. After irradiation, the remnant polarization of the copolymer drops sharply. The remnant polarization of the irradiated composites rises steeply with increasing ceramic volume fraction and is only 20% lower than that of the unirradiated composites at a ceramic volume fraction of about 0.5.

Longitudinal and transverse piezoelectric coefficients of lead zirconate titanate/vinylidene fluoride-trifluoroethylene composites with different polarization states

Composite films comprising lead zirconate titanate (PZT) ceramic particles dispersed in a vinylidene fluoride-trifluoroethylene copolymer matrix have been prepared by compression molding. The ceramic and copolymer phases of the composite films are polarized separately, resulting in samples with three different polarization states: only the ceramic phase polarized, both phases polarized in the same direction, and two phases polarized in opposite directions. The effect of polarization state on the longitudinal and transverse piezoelectric coefficients (d33 and d31) of the composite film has been investigated as functions of ceramic volume fraction φc. When the ceramic and copolymer phases of a composite film are polarized in the same direction, their piezoelectric activities partially cancel each other, thereby giving almost zero piezoelectric activity at φc∼0.4. On the other hand, when the phases of a composite film are polarized in opposite directions, their piezoelectric activities reinforce. However, depolarization of the ceramic phase is observed at high φc, leading to a decrease in the piezoelectric activity. The observed d33 and d31 values for the composite films agree well with theoretical predictions.

Second-harmonic generation in vinylidene fluoride-trifluoroethylene copolymers doped with donor-acceptor molecules

The temperature dependence of the second-harmonic generation in a vinylidene fluoride-trifluoroethylene ferroelectric copolymer doped with 5 wt % of a noncentrosymmetric 4-anilino-4′-nitroazobenzene chromophore is analyzed. The second-order susceptibility χ(2) in the ferroelectric phase of these copolymers appears to be twice as high as in the poly(methyl methacrylate) with the same chromophore content. The effect is attributed to the presence in ferroelectrics of a local electric field associated with the spontaneous polarization. The intensity of the second-harmonic generation in both ferroelectric and paraelectric phases correlates with the variation of the surface potential.

Interface Effect on Polarization Switching and Critical Thickness of Crystallization in P(VDF-TRFE) Copolymer Thin Films

In poly(vinylidene fluoride-trifluoroethylene) copolymer spin cast films, it has been observed that the polarization switching time increases as the film thickness is reduced. We will show that this change with thickness can be divided into two thickness regimes, those above a critical thickness and below this critical thickness. For films above the critical thickness, the change is in the switching behavior is mainly due to interface effect. While for films below the critical thickness, there is an additional and very large increase of the switching time with reduced film thickness, which is caused by a large drop of the crystallinity in this thickness range. It has been observed that this critical thickness, below which the crystallization in the polymer films is strongly hindered, depends on the film annealing conditions and is directly related to the thermodynamic stable crystallite thickness.

Formula omitted] NMR characterization of electron beam irradiated vinylidene fluoride–trifluoroethylene copolymers

Electron beam irradiated poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymers have been recently shown to exhibit “giant” electrostrictive stains. In this work, 19 F nuclear magnetic resonance (NMR) spectroscopy was used to probe structure of P(VDF–TrFE) copolymers before and after irradiation in order to elucidate the chemistry associated with the development of outstanding electroactive properties. The two-dimensional solution-state 19 F NMR of as-synthesized copolymers reveals strong (∼300 Hz) geminal indirect coupling ( 2J ) and the resulting assignments have some differences from the previous literature. The most frequently occurring sequence containing TrFE is CH 2CF 2CHFCF 2CH 2, followed by CF 2CH 2CHFCF 2CF 2. This reveals a strong preference for the TrFE units to be isolated and a slight preference for the head-to-tail addition. Both sequences contain VDF polymerized in the same direction around the central TrFE monomer unit. Resolution of bonding environments in the insoluble irradiated P(VDF–TrFE) required the use of fast (25 kHz) magic angle spinning (MAS) solid-state 19 F NMR at elevated temperature. Of the resolved peaks formed by irradiation, the side group CF 3CH<, was present in the highest concentration. Such CF 3 pendant groups are likely to reduce crystallinity and thus impact electroactive properties. Also observed was significant formation of CF 3CF 2, which can be either a pendant group or a side group. Several types of CF 3 end groups were observed: CF 3CHF, CF 3CH 2, and CF 3CH. The latter, indicates the formation of some unsaturated bonds upon irradiation. Small concentrations of CHF 2 and CF 2H were also found. Cross-linking is indirectly evidenced by broadening of the NMR spectra of the irradiated material. In the unirradiated copolymer, two conformation; of sequence CH 2CF 2CHFCF 2CH 2 are observed in the solid-state. In the solution-state in the post-irradiated solid-state, only one of these conformations exists.

Pyroelectric and dielectric properties of spin‐coated thin films of vinylidene fluoride–trifluoroethylene copolymers

AbstractThis work emphasizes the use of vinylidene fluoride and trifluoroethylene copolymer P(VDF‐TrFE) as a pyroelectric sensor. The pyroelectric and dielectric properties of the copolymer have been investigated in the temperature interval 150–350 K. The samples were prepared by using a spin‐coating technique with 70/30 mol% VDF/TrFE copolymer. The final film thickness of the samples, which is mainly determined by the concentration of the copolymer, spinning rate and spin time, was measured with a surface profiler. The samples were annealed at 150 °C for 10 min to improve the crystallinity of the copolymer. The crystallinity of the annealed and non‐annealed samples was compared by IR spectroscopy. The most effective process by which to improve the pyroelectric response of the material is to pole the sample with huge poling field‐strengths at elevated temperatures. Both pyroelectric and dielectric activities of the samples were measured after each successful poling process. It was observed that while the pyroelectric activity of the material increases, the dielectric activity decreases, so the figure‐of‐merit of the material, which shows the sensor capability of the material, was increased by a significant amount. It was found that the pyroelectric coefficient of VDF/TrFE (70/30 mol%) copolymer is 68.7 µC m−2 K−1 at 300 K.© 2001 Society of Chemical Industry

Poling study of PZT/P(VDF–TrFE) composites

Thick-film composites with 0.2 and 0.6 volume fractions of lead zirconate titanate (PZT) powder dispersed in a vinylidenefluoride–trifluoroethylene copolymer [P(VDF–TrFE)] matrix were prepared by solvent casting followed by compression moulding. The composites were poled by various procedures to produce samples with only the ceramic phase poled, only the copolymer phase poled, or both the ceramic and copolymer poled. The polarization profiles of the poled composites have also been obtained using the laser intensity modulation method (LIMM).

Dielectric study of the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer system

The high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), copolymer exhibits many features resembling the relaxor ferroelectric behavior. In polymer systems, there are local dipolar motions at the monomer or unit cell scale, which manifest themselves as various relaxation processes. In this paper we investigate the relationship between the relaxor ferroelectric behavior, especially, Vogel-Fulcher (V-F) behavior and these local dipolar relaxation processes in irradiated P(VDF-TrFE) 65/35-mol % copolymer. In order to cover the change in polarization dynamics of the copolymer system, the dielectric behavior of copolymer is measured over a broad frequency (0.01 Hz--10 MHz) and temperature (-40 to 80 \ifmmode^\circ\else\textdegree\fi{}C) range. The results indicate that there is an increased coupling among the local dipolar motions with reduced temperature in the crystalline region. On the other hand, the randomness introduced in the irradiation prevents the formation of a polar phase, on both the macroscale and the microscale, in the polymer. The observed relaxor behavior is a consequence of the competition of these two effects. The results further show that the V-F process of the irradiated copolymer system is different from the glass transition, which occurs in the amorphous phase of the copolymer.

Critical thickness of crystallization and discontinuous change in ferroelectric behavior with thickness in ferroelectric polymer thin films

We report on the observation of the critical thickness of crystallization of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer thin films, which were solution spun cast on platinum coated silicon wafer. The effect occurs at about 100 nm thickness, which is significantly above any currently known spatial dimensions of the polymer, so that for films at thickness below about 100 nm, the crystallization process is strongly hindered, resulting in a low crystallinity in these films. This low crystallinity leads to a large and discontinuous change of the dielectric constant and ferroelectric polarization in the films below the critical thickness.

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers.

Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and 'smart' ceramics such as lead zirconate titanate. But the strains achievable in these materials are small--less than 0.1 per cent--so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material--the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV x m(-1) have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV x m(-1). This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

High Electromechanical Coupling Factor and Electrostrictive Strain over Broad Frequency Range in Electrostrictive Poly(vinylidene fluoride-trifluoroethylene) Copolymer Films

Electromechanical coupling factor is one of the most important parameters for measuring the performance of materials for electromechanical transduction applications. In this paper, we will show that a transverse electromechanical coupling factor k31 of more than 0.45 can be achieved in poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer under certain electron irradiation treatment conditions. In addition, the effective piezoelectric coefficients of the irradiated copolymer have been found to increase markedly in comparison to non-irradiated copolymers. Experimental evidences also indicate that the improved electrostrictive strains in irradiated copolymer films can be maintained over a broad frequency and temperature range.

Comparison of aluminum and sodium doped poly(vinylidene fluoride-trifluoroethylene) copolymers by x-ray photoemission spectroscopy

The chemical interaction between the simple metals, aluminum and sodium, and crystalline copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%), has been studied using x-ray photoemission spectroscopy. Aluminum and sodium metalize the polymer differently and different binding sites for the two metals can be inferred from the corresponding core level shifts. Aluminum leads to enhanced screening of final photoemission states associated with the polymer, while sodium doping strongly influences the fluorine, but perturbs the carbon backbone only slightly.

Effect of Interface and Crystallinity on the Ferroelectric Properties of Poly(Vinylidene Fluoride-Trifluoroethylene) Copolymer Thin Films

ABSTRACTFerroelectric polymer thin films are attractive for a wide range of applications such as MEMS, IR sensors, and memory devices. We present the results of a recent investigation on the thickness dependence of the ferroelectric properties of poly(vinylidene fluoridetrifluoroethylene) copolymer spin cast films on electroded Si substrate. We show that as the film thickness is reduced, there exist two thickness regions. For films at thickness above 100 nm, the thickness dependence of the ferroelectric properties can be attributed to the interface effect. However, for thinner films, there is a large change in the ferroelectric properties such as the polarization level, the coercive field, and polarization switching speed, which is related to the large drop of the crystallinity in the ultrathin film region (below 100 nm). The results from Xray, dielectric measurement, and AFM all indicate that there is a threshold thickness at about 100 nm below which the crystallinity in the film reduces abruptly.

Piezoelectric properties of samarium-modified lead titanate/polymer 0–3 composites

Composites containing 30 volume % of samarium-modified lead titanate (PST) powder dispersed in a vinylidene fluoridetrifluoroethylene copolymer [P(VDF-TrFE)] matrix were prepared. The PST powders had been prepared using a sol-gel process and annealed at 600°C, 900°C, 1000°C and 1200°C, respectively. The crystallite diameters of the powders were about 70 nm, 139 nm, 417 nm and 1 μm, respectively. The PST phase of the composites was polarized under various dc electric fields for 2 h at 60°C and 120°C, respectively. The piezoelectric and dielectric properties of the composites were investigated.

Piezoelectric properties of samarium-Modified lead titanate/polymer 0-3 composites

Abstract Composites containing 30 volume % of samarium-modified lead titanate (PST) powder dispersed in a vinylidene fluoridetrifluoroethylene copolymer [P(VDF-TrFE)] matrix were prepared. The PST ...

PCLT/P (VDF-TrFE) nanocomposite pyroelectric sensors.

Thin composite films consisting of 12 vol% of nanosized lanthanum and calcium-modified lead titanate (PCLT) powder embedded in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix were deposited on silicon (Si) substrates to form pyroelectric sensors with three different configurations. The influences of a thermal buffer layer and back etching of the silicon substrate on the current and voltage responsivities of the sensors were investigated theoretically and experimentally. The specific detectivity of the sensors was also calculated from the measured voltage responsivity and noise.