Polarization dynamics of VDF-TrFE copolymers

Abstract

The polymer PVDF and ita copolymers with TrFE show a strong piesoelectric effect ane r poling in high e lectric ficldl ( 100 MV / m). In order to . tudy the dynamiu of the poling proce •• we have applied HV impubes of deftnite duration to the polymer. The dielectrie di.placement during the HV impul.e i. r e corded. Aner tbe impul.e tbe remanent pollll'iBation under . bo rt eircuit condit ion i. mea.ured. Thu. it i. po .. ible to obtain the minimum poling time dependent on the applied field strength nece .. ary to I tabilise the remanent polarillation. By comparing the time development of t he dielectric di.placement with the corre.ponding r emanent polaris.tion we find. time delay between the orientation o f the dipole. and their .tabili.ation. Therefore we conclude that the procell o f orientation of the dipole. ihelfi. not .uffteient to le ad to a r emane nt polarization. An additional interaction between trapped cblll'g"" and the orie nted dipoles can explain the . tability of the remanent pola risation and the ob.erved time d e lay. INTRODUCTI ON KAWAI di aeovered piuoelectricity in the polymer of polyvinylidenefluoride (PV OF) in 1969. Later it wu shown that the copoly m~rl of vi nyl idenefluoride (V OF) with trifluoroethylene (TrFE) alto ex hi bit piezoelect rici ty TlFE) the cry.taUite di poles can be permanently aligned. Unpoled filma .how no piesoeleetricity. The pieloeleetrie reapon ... can be deaeribed mainly by a ehange in the num· ber density or aligned crYltallite dipole moment. under strain. While PVDF crYl tallize8 in the unpolar o·phMe, the P (VOF / TrFE) copolymer cry. taUilu directly in the ,8-phase, if the TrFE portion i. greater than 15 mole% [6J. o·PVOF it Iran.rormed to the polar ,8·ph3le by stretch· ing the fi lma.[21· By poling with lower field strength spatial inhomoge_ neOUI polarilation distributions are found in PVDF. The invest igation of t he development of these inhomogeneous distributions during t he poling process gives strong evi_ dence that injee~ion and migration of lpace charges dete rmine the polarisation distribution [9].Another point is the surprising stability of the remanent polarisation in both polymers al room temperature for many year •. In addi tion, the cocrdtive field Ilrength is th ree orders of magnitude latger than in other ferroelt<:triel [3]. In Figure 2 the expected behavior il.ho",n for the case of polarisation rever .... l of pre polarised filma. At t.) the di· polel begin to invert under t he field and the displacement I t arts to grow as in the ease of unpoled films (compare Figure 1). At t\ all dipole. are inverted under the field and the displlLCement attain. itl maximum. For long er poling time. it remain. conltant. Under .hort circuit the remanent polari1ation in the original dir ection fi rst decreuel to ,eTO li nd then growl in the new, opposite direction. The remanent polatilation in the opposi te direetion latu ratu again o.t h.Since the values of the remanent polarization are the lame (but in oPP01lite direction) before the application ...

Time development of multiple polarization zones in PVDF

“…In addition the acoustic impedance of PVDF is of the same order of the acoustic impedance of watt'r and the most organic Simultaneously to the dipole orientation charges are injected from the electrodes during the pol· ing process [4]. Experiments with blocking electrodes [5J and the fact that the time development of the remanent polarization is delayed by a factor 3 ... 5 compared to the dipol alignment under field [6,7] indicate that the polarization is stabilized by Coulomb interaction of deeply trapped charges at the crystallite surfaces. These charges are ions which are created by electrochemical reactions at the electrodes [8J.…”

Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures

“…Although, the presented measurements yield no further evidence for such a process, at this point, charge migration throug the insulator can not entirely excluded. 8,24 (b) It was proposed by Naber et al 75,80 that since no inversion layer forms in the p-type P3HT there is a lack of minority electrons to compensate the ferroelectric polarization leading to a partial depolarization of the P(VDFTrFE). However, our measurements confirm that the ferroelectric polarization is stable.…”

The electric moment contribution to the piezoelectricity of PVDF

“…The interface can control the development of polarization by a nucleation and growth scheme when the phase transition starts predominantly from the crystal surface [13]. The polarization, on the other hand, can be stabilized by means of trapped charges at the crystal surface, the mobility of which in turn is influenced by the structure and the dynamics of the mentioned interface [14]. Further processes are possible.…”