偏氟乙烯/三氟乙烯铁电共聚物的非晶介电弛豫

用溶液成膜、液氮淬火、熔融慢冷和热处理4种方法制得不同结晶度的VDF/TrFE共聚物试样。在已改进的介电弛豫谱仪(精度由原来的1％提高到2‰)上测得复数介电常数。从结晶度为20％的淬火试样室温以下的介电频率谱中发现了非晶介电弛豫,并由此得到了玻璃化温度与共聚物组分含量的关系。 关键词：     

Influence of phase transitions on the spontaneous voltage in P(VDF/TrFE) copolymers

Spontaneous voltages were observed in a structure made of poly(vinylidene fluoride-trifluoroethylene) copolymers films sandwiched by metals evaporated onto both surfaces. Open voltages and short-circuit currents were measured as a function of the temperature using different combinations of evaporated metals as electrodes: aluminum, gold, nickel and copper. The ferroelectric-to-paraelectric transition was also observed in such structures for 60:40, 70:30 and 80:20 of VDF/TrFE molar ratios, including the temperature hysteresis characteristic of the first-order transition. Received: 9 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

VDF/TrFE共混物晶区相容性的热释电流研究

采用热释电流 (TSDC)技术 ,结合差热分析和X射线衍射方法 ,对由共聚物VDF(5 6) /TrFE(4 4)和VDF(77) /TrFE(2 3 )共混形成的不同摩尔比的VDF/TrFE共混物进行了研究 .发现共混物显示二个居里转变温度 ,其结晶区域由二个共聚物组分的晶区构成 ,体现出共混物中二共聚物组分的晶区是不相容的 .不同极化温度的热处理共混物的TSDC结果还证明 ,空间电荷对剩余极化有一定的稳定作用 .     

Dielectric dispersion of ultrathin polymer Langmuir-Blodgett films

Dielectric dispersion is studied in films made of vinylidene fluoride/trifluoroethylene copolymer [P(VDF/TrFE)], which are obtained by the Langmuir-Blodgett technique in the temperature range from −90 to +132°C. The thickness of the films studied is ∼7 nm. A glass transition is observed in the temperature range from −40 to −50°C; this indicates the presence of an amorphous phase in the ultrathin Langmuir-Blodgett films.     

Radiation-Induced Changes in the Dielectric Response of Poly(Vinylidene Fluoride) Type Polymers

High and dispersive dielectric response, characteristic of the relaxor state, was observed in ferroelectric P(VDF/TrFE)(50/50) copolymers irradiated with 1.0 MeV and 1.5 MeV electrons. Relaxor-like behaviour of the copolymer is a result of overlapping of the dielectric anomaly characteristic of the glass transition and that related to the Curie point, which is shifted downwards by electron irradiation. The results of ESR, IR and NIR Raman spectroscopy studies of the radiation damage to P(VDF/TrFE)(50/50) show that radiation-induced irreversible transformation of the ferroelectric copolymer to the relaxor state is related to the existence of polar clusters, consisting of a variety of short range coherence of trans-conformation, stabilized by random fields of C=C and conjugated C=C bonds.     

Ferroelectric properties of vinylidene fluoride copolymers

Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (～ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylidene fluoride at temperatures from 70 to 140°C. The transition accompanies cooperative dipole motions associated with elementary trans-gauche transformations. Changes in various properties as well as in structures in the vicinity of the Curie point are consistent with an order-disorder transition of first order. Detailed characteristics specifying their ferroelectricity are summarized and discussed in relation to the molecular, crystalline, and higher-order structures inherent to ferroelectric polymers.     

Adsorbate/absorbate interactions with organic ferroelectric polymers

We discuss the interactions of adsorbates with the organic ferroelectric copolymer poly(vinylidene fluoride (PVDF)–trifluoroethylene (TrFE)). Range of molecular adsorbates is discussed from the smaller polar molecules like water, which is small enough to both adsorb and absorb, to the larger macrocyclic metal–organic metal phthalocyanines. The changes in local dipole orientation may affect the strength of the coupling between adsorbate or absorbate and the copolymer poly(vinylidene fluoride–trifluoroethylene). The interface dipole interactions may also affect device properties. The dipole interactions are implicated at the interface between copper phthalocyanine and poly(vinylidene fluoride with trifluoroethylene) affecting the band offsets and the diode properties.     

Structural phase transition in ferroelectric fluorine polymers: X-ray diffraction and infrared/Raman spectroscopic study

The structural phase transition has been investigated by X-ray diffraction and infrared and Raman spectroscopic measurements for ferroelectric fluorine polymers, including poly(vinylidene fluoride) and its copolymers with trifluoroethylene or tetrafluoroethylene. One of the most characteristic features of this ferroelectric transition is the large conformational change of the molecular chains between the trans and gauche rotational isomers, quite different from the structural change observed generally in the usual ionic ferroelectric materials. The crystallization and transition behaviors depend sensitively on the monomer composition in the copolymers as well as on the sample preparation conditions. The roles of the optic and acoustic phonons in the ferroelectric phase transition have been discussed based on the temperature dependences of the far-infrared spectra and the ultrasonic velocity.     

Pyroelectric conversion—Effects of P(VDF–TrFE) preconditioning on power conversion

Large amounts of low-grade heat are emitted from various industries and wasted into the environment. This heat energy can be used as a free source for pyroelectric power generation. We first discuss the principle of pyroelectric conversion based on the differences in crystal structures of copolymer. Then we discuss the pyroelectric conversion of waste heat using copolymer films containing 60% vinylidene fluoride (VDF) and 40% trifluoroethylene (TrFE) and the impact of the preconditioning of the pyroelectric films on the net power output. We discovered that the pre-polarization (poling) of pyroelectric films has a significant beneficial effect by decreasing internal conduction during power conversion. This in turn increased net power output. We were able to reach 95–165 J/L of copolymer used in each cycle even including the internal leakage. We also showed it is critically important to operate the power conversion near the phase transition where electron discharge is the highest.     

The DIVAM sequence: selective excitation of signals from both rigid and mobile domains in a fluoropolymer

Simulations have been carried out on z-magnetizations produced by (and hence of the resulting spectra from) the dipolar filter (DF) and the recently suggested "Discrimination Induced by Variable-Amplitude Minipulses" (DIVAM) pulse sequences [S. Ando et al., Polymer 42 (2001) 8137; Magn. Reson. Chem. 40 (2002) 97]. The strengths of dipolar interactions have been modelled by introducing different values for transverse relaxation times. The DF case has been extended by allowing the pulse angles to be smaller than 90 degrees. The pulse intervals have also been used as variables. For the DIVAM case, the variables are similarly the minipulse nutation angles and minipulse intervals. The computations show that DIVAM is superior to DF in terms of selectivity for spectra of heterogeneous materials such as semi-crystalline polymers. The effects of the pulse sequences on 19F spectra of poly(vinylidene fluoride) (PVDF) and of a copolymer of vinylidene fluoride and trifluoroethylene (p(VDF/TrFE)) are presented, together with fits of the experimental results by the simulations.     

Properties and characteristics of P(VDF/TrFE) transducers manufactured by a solution casting method for use in the MHz-range ultrasound in air

Highly effective piezoelectric polymer transducers operating in air at high frequencies have been successfully made by casting a solution of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) P(VDF/TrFE) directly on a backing metal plate, and their performance has been evaluated. By utilizing this method, it has been possible to develop the three kinds of transducers that operate respectively at 4, 6 and 10 MHz in air. For precise evaluation of the performance of the P(VDF/TrFE) transducers, the absorption loss in air was measured up to 10 MHz. It was confirmed that the empirical formula obtained from the measured absorption values in air at high frequencies was in alignment with its theoretical value. In addition, a high lateral resolution acoustic image of a ROM-Chip (amplitude-image) at 6 MHz in air was successfully displayed using an air coupled concave type P(VDF/TrFE) transducer by bonding an epoxy adhesive.     

The polymorphism of poly(vinylidene fluoride) V. The effect of hydrostatic pressure on the melting behavior of copolymers of vinylidene fluoride

The high-pressure melting behavior of samples of vinylidene fluoride copolymerized with vinyl fluoride, trifluoroethylene, or tetrafluoroethylene indicate that the copolymers have a lower entropy of melting than the poly(vinylidene fluoride) homopolymers in the same phase. As the comonomer size increases, the entropy of melting decreases. High-pressure crystallization of copolymers of 91-9 mole% vinylidene fluoride-trifluoroethylene and 93-7 mole% vinylidene fluoride-tetrafluoroethylene results in a high-melting form of phase I (planar zig-zag).     

A van der Waals density functional theory study of poly(vinylidene difluoride) crystalline phases

Ferroelectric polymers, such as poly(vinylidene difluoride) (PVDF), have many potential applications in flexible electronic devices. PVDF has six experimentally observed polymorphs, three of which are ferroelectric. In this work we use density functional theory to investigate the structural properties, energetics and polarisation of the stable α-phase, its ferroelectric analogue, the δ-phase, and the β-phase, which has the best ferroelectric properties. The results from a variety of exchange and correlation functionals were compared and it was found that van der Waals (vdW) interactions have an important effect on the calculated crystal structures and energetics, with the vdW-DF functional giving the best agreement with experimental lattice parameters. The spontaneous polarisation was found to strongly correlate with the unit cell volumes, which depend on the functional used. While the relative phase energies were not strongly dependent on the functional, the cohesive energies were significantly underestimated using the PBE functional. The inclusion of vdW interactions is, therefore, important to obtain the correct lattice structures, polarisation and energetics of PVDF polymorphs.     

VDF/TrFE铁电共聚物极化分布与退极化过程的激光强度调制方法研究

建立起激光强度调制法测量装置及其测量与数据处理方法,研究了VDF/TrFE铁电共聚物的极化分布和退极化过程.发现25μm厚的VDF(80)/TrFE(20)薄膜在距表面8μm左右薄层热电系数显示极大值,而12μm厚的则在中心处,两者的热电系数皆以两表面呈对称分布.在145℃热处理产生退极化.随着时间的增长,热电系数减小,退极化由试样的一面向另一面逐渐进行,2h后由强电场形成的极化基本消失.直流极化30μm厚的VDF(70)/TrFE(30)薄膜,热电系数分别在离两表面6,8μm处达恒定值,基本以两表面呈反 关键词：     

Poly(vinylidene fluoride) Crystallization Behavior and Membrane Structure Formation Via Thermally Induced Phase Separation with Benzophenone Diluent

Microporous poly(vinylidene fluoride) (PVDF) membranes were prepared by thermally induced phase separation (TIPS) at different quenching temperatures with benzophenone as the diluent. The crystallization behavior and crystal structure of PVDF in PVDF/benzophenone systems were investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The different PVDF concentrations had a remarkable effect on PVDF crystallization behavior and resulted in different membrane structures. Spherulitic structures were vague when the PVDF/benzophenone solution was quenched to ?8°C; however, discernable spherulitic structures were obtained when quenched to 34 and 49°C. Additionally, two phase separation mechanisms (solid–solid (S–S) and solid–liquid (S–L) phase separation) were observed during membrane preparation. It was revealed by scanning electron microscopy (SEM) that microporous membranes had more discernable spherulitic structures formed by S–L phase separation than by S–S phase separation, which induced macrovoids and irregular pores on the fracture surfaces of membranes.     

Ultrasonic imaging using air-coupled P(VDF/TrFE) transducers at 2 MHz

A reflection non-contact ultrasonic microscope system working both in amplitude and phase difference modes at 2 MHz has been developed using an air-coupled concave transducer made of piezoelectric polymer films of poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)]. The transducer is composed of three 95 μm-thick P(VDF/TrFE) films stacked together, each of which is activated electrically in parallel by a driving source. The transducer has a wide aperture angle of 140° and a focal length of 10 mm. The measured two-way transducer insertion loss is 80 dB at 1.83 MHz. Despite 20 dB higher insertion loss than that estimated from Mason’s equivalent circuit, we have obtained clear amplitude acoustic images of a coin with transverse resolution of 150 μm, and clear phase difference acoustic images of the rough surface of a paper currency bill with depth resolution of sub-micrometer. Using two planar transducers of P(VDF/TrFE), we have also successfully measured in through-transmission mode the sound velocity and absorption of a 3 mm-thick silicone-rubber plate. The present study proves that, owing to its low acoustic impedance and flexibility, P(VDF/TrFE) piezoelectric film is very useful for high frequency acoustic imaging in air in the MHz range.     

Thermal and dielectric investigations of the curie transition in Poly(vinylidene fluoride—trifluoroethylene) copolymers

The ferroelectric-to-paraelectric phase transition in poly(vinylidene fluoride-trifluoroethylene) copolymers has been investigated by differential scanning calorimetry and dielectric techniques. The transition appears to be very dependent on the crystallization kinetics and thermal history. The anomalies accompanying the transition show that the structural transformations are complex. In particular, the 30 mol % TrFE sample presents the most remarkable effects. The results are discussed in terms of the existence of a possible intermediate crystalline phase. The role of defects and of the anchored amorphous phase have also been considered.     

Ferroelectric characteristics of MFIS structure with P(VDF–TrFE)/BaTiO3 nanocomposite as ferroelectric layer

A metal–ferroelectric–insulator–semiconductor (MFIS) structure has been made using poly(vinylidene difluoride–trifluoroethylene)/barium titanate [P(VDF–TrFE)/BaTiO₃] nanocomposite as ferroelectric layer, on silicon/silicon dioxide (Si/SiO₂) substrate. Different concentrations of BaTiO₃ were added to P(VDF–TrFE) polymer using bath sonication method, and the films were prepared using spin coating method. The structure was annealed to 120 °C for 2 h and then the top aluminium electrode was deposited by thermal evaporation method. Capacitance–voltage shows an increase in accumulation capacitance as the BaTiO₃ nanoparticle concentrations increases. Dielectric constant was estimated from the capacitance voltage (C–V) characteristics and found to be changing as the concentration of BaTiO₃ is varied. Polarization–electric field analyses show hysteresis behaviour of the nanocomposite. A comparison of MFIS and metal–ferroelectric–semiconductor structures was done with varying ferroelectric film thicknesses. All these results suggest that this polymer nanocomposite can be a promising material which can be used in non-volatile memory devices.     

Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM

Scanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.