This article examines the correlation between the pyroelectric response in typical films of ferroelectric polymers and the value of the residual ferroelectric polarization. The pyroelectric response was obtained by the thermal pulse method proposed by Collins. The magnitude of the residual polarization was measured depending on the magnitude of the applied electric field during primary electrification (poling) and during its switching. It is shown that the used method has the high sensitivity. It was found that the magnitude of the pyroelectric response depends significantly on the polarized state, which is determined by the values of the electric field during electrification and polarization switching and on the duration of exposure to this field. The identity of the graphs of the dependence of the residual polarization on the field and time and the corresponding graphs of the magnitude of the pyroelectric response led to the final conclusion about the proportionality of the magnitude of the response to the residual polarization. Thus, it is concluded that the pyroelectric response method can be used to estimate the magnitude of polarization in ferroelectric polymers

The vinylidene fluoride copolymer with tetrafluoroethylene P(VDF-TFE) is a typical ferroelectric polymer that has not been sufficiently studied so far in comparison with pure polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene. The purpose of this work was to reveal the features of forming ferroelectric polarization and its switching in thin films of P(VDF-TFE) under various conditions, such as the level of the applied electric field and the duration of its action within 8 orders of magnitude from 10 μs to 100 s. The sequence of voltage pulses applied during poling and polarization reversal allowed to reveal the characteristics of the polarization switching dynamics. To increase the resolution of the measurements, two complementary methods of recording electrical induction were used. The features of polarization and space charge relaxation over time have been studied by thermally stimulated depolarization method. A new phenomenon of gradual separation of the relaxation processes associated with polarization and space charge was observed. Both components were accompanied by trapped charges that compensated the depolarizing field. Several practical recommendations have been formulated regarding the desirable values of poling parameters and additional processing to increase the ferroelectric polarization stability.

Peculiarities of the polarization build-up and its relaxation in corona poled PVDF films meant for manufacturing of pyro- and piezoelectric sensors have been revealed from the complex experimental investigation of the polymer. It has been found that the polarization and the space charge are interrelated forming a self-balanced system. Its stability depends mainly on charges trapped in the volume. It has been shown that the charge trapping occurs in macroscopic zones positioned at the borders of completely polarized parts of the volume. The trapped charges play an important role in stability of the ferroelectric polarization, since they neutralize the depolarizing field.

Experimental evidence is provided on close relation between polarization and space charge in corona poled PVDF films. It is shown that the depolarizing field is compensated by charges trapped in macroscopic transition zones, by which polarized parts of the volume are separated from non-polarized ones. The compensating charges are either injected in the bulk, or created inside due to the thermal emission and then trapped when the ferroelectric polarization in crystallites is formed.

Four modifications of the corona triode are described for charging polar polymers with ferroelectric or non-linear optical properties. Advantages of the constant current modification of corona poling are illustrated and discussed.

The procedure has been developed for extracting homocharge and heterocharge currents from experimentally measured thermally stimulated depolarization currents of corona poled PVDF. Application of different depolarization modes supplemented with the isothermal currents allowed to obtain such parameters of relaxation processes, as activation energies, characteristic frequencies and time constants.

Two modifications of the Thermally Stimulated Depolarization Current method are proposed to improve resolution and sensitivity of the method by connecting either a real capacitor, or an additional resistor in series with the sample. It is shown experimentally that high sensitivity of the TSDC method with an air gap can be obtained, if the gap is substituted by the capacitor, while all advantages of the method remain in force. It has been found that in one experiment it is possible not only to measure the TSD current, but also to obtain data on the Thermally Stimulated Conductivity, if the properly selected additional resistor is periodically switched on and off.

Using short circuit and open circuit modifications of the thermally stimulated depolarization current technique, relaxation currents have been measured in corona poled copolymer of vinylidene fluoride with tetrafluoroethylene in samples stored after poling for either 1 day, or 16 month. Two well structured peaks observed in aged samples were attributed to relaxation of electret and ferroelectric components of the remnant polarization. In fresh samples the two components were mixed forming one broad peak. Relaxation of the space charge caused inversion of the current in the open circuit mode both in fresh and in aged samples. Space charge peaks extracted from the total current by appropriate calculations have shown that the trapped charges were more stable than the polarization. It was suggested that both the electret and the ferroelectric components of the remnant polarization were accompanied by either space, or surface charges.

Two modifications of the thermally stimulated depolarization current (TSDC) method are proposed to improve resolution and sensitivity of the method by connecting either a real capacitor or an additional resistor in series with the sample. It is shown experimentally that the high sensitivity in the TSDC method with the air gap can be obtained if the gap is substituted by the capacitor, while all advantages of the method remain in force. It was found that one could obtain in one experiment the data on the thermally stimulated conductivity (TSC) along with the TSD current, if the additional resistor is periodically switched on and off.

Anomalous decrease of the apparent conductivity has been observed during sandwich electrode and corona poling of PVDF and PT:P(VDF-TrFE) representing heterogeneous ferroelectric polymers and polymer/ceramic composites. It has been shown that the phenomenon accompanies the build-up of the ferroelectric polarization in the materials and explained by a massive trapping of charge carriers at boundaries of the ferroelectric species and at the transition zones where a gradient of polarization exists. The trapped charge contributes to stability of the ferroelectric polarization, since it compensates the depolarizing field.

In this work, we applied constant current corona poling in order to study potential buildup and polarization switching in uniaxially and biaxially stretched PVDF. Relaxation processes have been studied by the thermally stimulated depolarization (TSD) current method. It has been confirmed that the poling and switching processes consisted of three stages. A simple technique has been suggested to find the P(E) hysteresis curves and obtain static and dynamic dielectric constants, as well as the coercive Ec and saturated Es fields in one sequence of experiments. The method can be used for any material with the hysteresis-like field dependence of polarization. It has been found that there are three components of the residual polarization in PVDF, namely the stable ferroelectric component, the slowly relaxing one and the unstable component. Two TSD peaks have been observed immediately after poling, but two more peaks appeared in the samples stored for a long time. It follows from our results that very slow redistribution of charges and polarization takes place in PVDF until the electret and ferroelectric components of polarization are finally separated, both accompanied by the trapped charges.

The laser-induced pressure pulse probing shiwed that a two-zone structure is formed in the films of vinylidene fluoride-tetrafluoroethylene copolymer under the action of defocused partially penetrating 20 keV electron beam. In the irradiated zone, there is no residual polarization because of the radiation-induced conductivity, while in the non-irradiated zone, there appears a nonuniform polarization across the film thickness. The trapped charges are distributed with nonuniform: density within a layer of finite thickness to form a virtual injecting electrode. A;comparison of the thermostimulated depolarization currents measured immediately upon irradiation and after 14-month storage indirectly confirmed the existence of both ferroelectric and dipolar electret polarization components as well as of the space charges of two types. related to these components.

Abstracts are not published in this journal

Summary form only given. Corona poling of ferroelectric polymers is one of the main methods to provide for a high residual polarization in the materials, but there were no systematic studies of corona poled P(VDF-TFE) copolymer especially on the spatial distribution of polarization in the films. Our study was performed in order to understand the origin of polarization non-uniformity in corona poled P(VDF-TFE) and to find its relation to the space charge.

Summary form only given. Several methods have been developed recently to study spatial distribution of polarization in ferroelectric polymers, but all measurements were performed in static conditions on already poled samples, reflecting only the final state of polarization. In this paper we present data on dynamics of polarization profiles obtained by `in situ' measurements on samples placed in the poling field