The effect of drawing on the dielectric properties and on the remanent polarization of alpha-PVDF was investigated. Films were drawing at 150 degrees C and a draw ratio of 4. Calorimetric analyses and X-ray diffraction results indicated that drawing increases the degree of crystallinity and reduces the crystal-amorphous interphase. As a consequence an increase in stable polarization and in the dielectric properties was observed, as well as a strong reduction in the metastable polarization. To explain these results it was suggested that the crystal-amorphous interphase might be responsible for both the metastable polarization and the alpha(c) relaxation of the polymer.

The influence of uniaxial stretching on the dielectric properties and remanent polarization of α-PVDF was determined. Commercial films supplied by Bemberg Folien GmbH were used. Some films were uniaxialy stretched at 140oC and with draw ratio (R) of 4, resulting in α-phase oriented films. Morphological effects of orientation were investigated by differential scanning calorimetry (DSC) and infrared spectroscopy (FTIR). It was verified that the orientation increases the crystallinity by the reduction of the amorphous-crystalline interface and, consequently, increase the stable remanent polarization. It was verified also a strong reduction of the metastable polarization and of the permittivity at 5x10 -5 Hz and an increase on the the permittivity at 1 kHz. These results allow supposing that the amorphous-crystalline interface is the responsible by the origin of the metastable polarization and the dielectric αcrelaxation. Poly(vinylidene fluoride) (PVDF) has remarkable properties leading to electro-optics, electromechanical and biomedical applications. In particular, its piezoand pyroelectric properties provide possibilities for many technological applications. The crystalline phase in PVDF contains at least four polymorphic modifications (α, β, γ, and δ) [1,2]. The apolar α-phase, is the most common, being easily obtained by melt crystallization or from dimethylformamide or dimethylacetamide solutions at temperatures above 120o C. In this study, changing of morphological, dielectric and ferroelectric properties with the drawing in αPVDF was investigated. The samples were stretched at 140oC and R=4, obtained oriented α-PVDF films. The crystalline phase, degree of crystallinity and dielectric properties of the samples, oriented and unoriented, were determined by Fourier transformed infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and dielectric spectroscopy (DS), respectively. The hysteresis loops of these films were determined by the ramp voltage technique [3,4]. In this method the linear ramp voltage is applied in a PVDF sample. The ramps are applied in discontinuous cycles and with the short-circuit time between the consecutive cycles. The number of the ramps voltage, of same polarity or reverse, it is controlled, allowing the application of an arbitrary sequence of ramps. In this way the stable and metastable remanent polarization can be determined separately. The correlation between structural, dielectric and ferroelectric characteristics of the oriented and unoriented samples shows that the stretching increases the stable polarization due the increase of the degree crystallinity and chains orientation. An increase of the permittivity at 1kHz was observed, probably caused by the better molecular ordination and consequent larger dipoles density in the amorphous phase. A decrease of the permittivity at 5x10 -5 Hz and a strong decrease in the metastable polarization were also observed and it can be relationship with the reduction of the amorphous-crystalline interface. The existence of this interface between the crystalline and amorphous phases was shown firstly by Flory [5]. Recently Ozkazanc et al [6] verified that the dielectric loss maxima, corresponding to the αc-relaxation transition in αPVDF films, decrease and still disappears with the draw ratio. Our results, supported by the work of Ozkazanc et al, allow us to infer that the dielectric relaxation is also associated to the amorphous-crystalline interface.

The ferroelectric and the dielectric behaviors of binary blends formed by an equimolar Poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE)] and Poly(methyl methacrylate) [PMMA] were investigated, for several PMMA compositions. For 40 wt% or more PMMA contents, the blends are completely amorphous. Below this value, they crystallize in the usual Cm2m polar structure of P(VDF-TrFE). The ferroelectric switching characteristics and the dielectric response of the blends demonstrate the formation of dynamically stable ferroelectric domains. Moreover, the blended films are highly transparent in the optical region. Therefore, thin films of these binary blends are good candidates as host materials for nonlinear optical applications.

The pulsed electric acoustic technique, PEA, have been usually applied to probe space charge profiles in polymers. In this work we show preliminary results obtained with lead zirconate-titanate and niobium, PZTN, ferroelectric ceramic samples. Experiments showed that induced charge densities on sample electrodes are mainly due to the ferroelectric polarization of the sample. We present results of the typical PEA response and the procedure to deconvolute the signal in order to obtain the charge densities and the electric field profiles. The PEA setup allows us to show a non-uniform polarization during ferroelectric switching.

The pulsed electric acoustic technique, PEA, has been usually applied to probe space charge profiles in polymers. Preliminary PEA results using a ferroelectric ceramic are presented. If the reverse applied electric field is of the order of the coercive field the switching polarization process occurs in a period larger than hundreds of seconds. Such a slow process allows one to use the PEA setup to follow the polarization switching dynamics and determine the electric field profile. The PEA signal obtained in the lead zirconate-titanate doped with niobium ceramic, PZTN, indicates that the polarization distribution and field are not uniform during the switching period. We were also able to observe that the acoustic wave velocity and attenuation depends on the stage of the polarization switching, which agrees with results obtained using the ultrasonic method.

The study of the stable and the metastable ferroelectric polarization of poly(vinylidene fluoride), PVDF, was performed using two successive equal sign ramp voltages, mediated by a short-circuit period. Rates from 10 V/s up to 0.7 MV/s were used. Results showed that they follow different formation kinetics; that the stable part decreases for higher ramp voltage rates and its apparent coercive field increases.

This article assesses the use of the constant current (CC) method for characterizing dielectric films. The method is based on charging the sample with a constant current (current stress) and measuring the corresponding voltage rise under the closed circuit condition. Our article shows that the CC method is an alternative to the constant voltage stressing method to study the electric properties of nonpolar, ferroelectric, and polar polymers. The method was tested by determining the dielectric constant of polytetrafluoroethylene, and investigating the electric conduction in poly(ethylene terephthalate). For the ferroelectric polymer poly(vinylidene fluoride), it is shown that hysteresis loops and the dependence of the ferroelectric polarization on the electric field can be obtained.

The ferroelectric switching characteristics of binary blends of an equimolar P(VDF-TrFE) copolymer with PMMA have been investigated, for PMMA contents ranging from 0 to 40 wt.%. The results were correlated with thermal and dielectric responses of the materials. The data revealed that the ferroelectric reversal polarization follow a "master-response" curve, when considered the crystallinity variations with blending and poling. Thermal treatment and/or poling stabilize the ferroelectric phase, even for the samples with the lowest degree of crystallinity investigated. These results imply that ferroelectric stable domains were formed for all compositions. The blended amorphous phase participates of the establishment of the stable domains.

It is shown that the constant current method, in which a dielectric is charged with a constant current while the voltage is monitored, allows one to determine the dependence of the stable ferroelectric polarization with the electric field. The determination is based on two successive experiments separated in time by a short-circuit period: a charging process in which polarization switching occurs followed by a recharging with the same current polarity. Analysis of the recharging experiments for poly(vinylidene fluoride), PVDF, shows that the polarization appearing in it is a metastable ferroelectric polarization, due to the reorientation of ferroelectric polarization lost during the short-circuit period. The method was applied to measure the ferroelectric polarization in PVDF samples with different β-phase contents and in an exploratory way for a few other ferroelectric polymers.

A new method is proposed to obtain hysteresis loops in ferroelectric materials in which the sample is submitted to a constant electric current. This technique was applied on poly(vinylidene fluoride), PVDF, biaxially stretched samples and the hysteresis loops of the electric displacement and electric polarization versus the electric field compared well with those obtained with the Sawyer–Tower method. The constant current method is advantageous in that no measuring amplifiers are needed and it is insensitive to prebreakdown noise.