Статьи на перевод PVDF_P(VDF-TrFE) / Long-Term Dielectric Response of Corona Charged

I. INTRODUCTION

The structure of polyvinylidene uoride (PVDF) in which the dipole moments are directly attached to the main chain allows a variety of conformations. The all-trans (TTTT) conformation is characteristic of the PVDF-I polymorph which exhibits ferroelectric properties [1,2]. PVDF I is usually obtained by stretching and poling melt-crystallized PVDF II polymorph (TGTG conformation). The all-trans polar conformation can be, however, melt-extruded when a certain amount of tri uoroethylene (TrFE) is added to PVDF [3]. P(VDF/TrFE) copolymers of TrFE contents ranging from 20 to 50 mol% are of especially great interest as they exhibit high remanent polarization and ferroelectric-paraelectric phase transition [4{6]. Polymer lms, however, exhibit another complexity on the crystalline level: they are semicrystalline and the degree of crystallinity and the crystallite sizes can both be a ected by thermal annealing and electric eld [7,8]. The semicrystalline structure is, in turn, the origin of the dielectric heterogeneity, since poling eld-induced compensating charges are piled up at the crystalline phase of a semicrystalline polymer. The internal bias related to this e ect is responsible for stabilization of the dielectric polarization of ferroelectric polymers [9{12] and a ects the dielectric dispersion [13,14].

Multiple relaxation processes for P(VDF/TrFE) copolymers were described by Koizumi in a wide temperature range [15,16] but in the frequency range above 1 Hz. Schenk et al. reported on dielectric dispersion in P(VDF/TrFE)(70/30) [17]; the samples, prepared by spin coating, were not polarized. Dielectric dispersion and absorption at 378 K was tted to the three Debye

type processes: the low frequency process ( 1 s) was ascribed to space charge movement related to local compensation of polarization, whereas the high frequency relaxation ( 10 8 s), characterized by an activation energy of 0.56 eV, has been associated to ferroelectric properties of the material. The third process, recognized in the spectra, have not been discussed. Here we would like to report some results on the dielectric response of the corona charged P(VDF/TrFE)(75/25) lm. We are interested in dielectric behaviour of the copolymer in the lower frequency range.

II. RESULTS

Dielectric dispersion and the long time depolarization was studied in biaxially oriented, 14 m thick P(VDF/TrFE) lms, polarized by corona discharge (Piezotech SA). Measurements of the low frequency dispersion up to 104 Hz were performed with computer aided Solatron Frequency Responce Analyzer 1250 working with Chelsea Dielectric Interface and in the range from 10 Hz to 10 MHz by using HP-4192 ALF impedance analyzer. The lms were characterized by di erential scanning calorimetry measurements (DSC) with Netzsch DSC-20 equipment. Since the Curie temperature Tc of the copolymer measured on heating is very close to its melting temperature Tm, thermal energy which is su - cient to destroy the ferroelectric ordering is absorbed by the sample in the same temperature range as the melting heat and the process is not resolved in DSC thermogram [13]. On cooling the sample at a rate of 5 K/min the heat anomaly related to the recrystallization occurs at

Fig. 1. Room temperature dielectric dispersion and absorption of corona charged P(VDF/TrFE)(75/25).

Trc = 409:8 K ( Hr = 31:2 J/g), whereas the long-range electric ordering arises at Tcc = 355:8 K ( Hc = 23 J/g). The result corresponds to the Curie temperatures obtained by us in the dielectric measurements.

Fig. 1 shows room temperature dielectric dispersion and absorption of the copolymer. The broad absorption

Fig. 2. Low frequency dielectric absorption 1- at 325 K, 2- 353 K and 3- 372 K.

Fig. 3. Discharging current at various temperatures.

band in the MHz region can be ascribed to a wide angle torsional oscillation of polar groups followed by their rotation involving the main chain cooperation [18]. Thermal annealing (heating to 435 K and cooling to room temperature at a rate of 6 K/min) shifts the absorption towards lower frequencies [14]. This e ect can be understood as a hindering of the relaxation by compensating

Fig. 4. Arrhenius plot of the time 1 at which the shortest process is nished.

charges transferred by thermal annealing from the nearsurface region and piled up at the crystalline phase of semicrystalline polymer. The low frequency dispersion occurs in the range below 102 Hz and, as follows from the results obtained at higher temperatures (Fig. 2), is more complex.

To get more information we performed long term measurements of the discharge current at various temperatures. Circular samples ( = 12 mm) with electrodes were placed in a thermostat the temperature of which was stabilized to within 0.1 K. The voltage Up = 10 V was applied at a given temperature T for time tp = 6 h and suddenly decreased to 0 in 1 ms. The polarization and depolarization currents were measured by using a computer controlled MMS1 set within tm = 2 h after the electric eld was on and o , respectively. The phase heterogeneity of the samples was monitored by means of IR spectroscopy. IR spectra were obtained with Perkin Elmer FT-IR 1725X spectrometer at room temperature and the ratio of the maximum absorption of the band at 532 cm 1, related to CF2 bending of TGTG conformation, to that of the band at 508 cm 1, characteristic of the CF2 bending of all-trans conformation, was measured. It was found that the value I532=I508 0:4 is not a ected by the measuring procedure.

Fig. 3 shows time variation of discharging current in log-log scale at various temperatures. Four slopes can be distinguished in the curves obtained at high temperatures; thus I t m with four di erent m-values, each

m corresponding to the well de ned time range. The m value, characteristic of the I(t) dependence in the shortest time range, amounts to 0.5. At room temperature the depolarization current was described by I t 1=2 in the time range 10 2 s t 103 s, whereas at 368 K in the range it is merely 10 2 s t 1 s. The power law with m = 1=2, characteristic of charge carrier di usion, [19] is followed by the I t 1 process extending over a decade on the time scale (at temperatures higher than 310 K). Next in the current-time dependence we can distinguish a kink (well observable at 343 at 368 K) which could be related to the thermal activation of charges trapped at the crystalline-amorphous interfaces of the semicrystalline polymer. The kink is characterized by m 0:2 which is very close to that observed in polyvinyl carbazole (Fig. 6.13 in Ref. [18]) and related to the dispersive transport mechanism. The fourth, long time process, observed only above 330 K and characterized by the power law with m = 0:6, could be associated to the changes in both ferroelectric and real charge polarization characteristic of polymer systems. The shortest decay process was observed to complete within 1 time at all temperatures. From the Arrhenius dependence we evaluated the activation energy (Fig. 4) to 0.92 eV.

III. CONCLUSIONS

Dielectric relaxation in polarized ferroelectric polymerlms is rather complex due to the heterogeneity of the material both on molecular and crystalline level and the dielectric heterogeneity. In fact we are dealing with the interaction between ferroelectric dipoles ordered in the crystalline phase of the polymers, and real charges trapped not only in the near surface levels of the lm but also at the crystallite surfaces in the semicrystalline polymer which result in the stabilization of the polarization. Our long time-scalestudies of the dielectric response allow to distinguish processes which can be attributed to several mechanisms namely the charge carrier di usion, the dispersive transport connected with the semicrystalline structure of the polymer and the I t 0:6 process. The latter can be related to the internal bias due to the interaction of real charges and ferroelectric dipoles [9,20]. The interaction between the thermally detrapped charges and the ferroelectric dipoles is responsible for the m value higher than that of charge career di usion (m=1/2).

ACKNOWLEDGMENTS

The paper was supported by the Grant 2 PO2B 198 08 from the Committee of Scienti c Research in Poland.

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