Page 156 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology en012f-594 July 26, 2001 11:9

664 Polymers, Ferroelectric

an electric field of about 0.5 MV/cm at high temperature with about 50% crystallinity and perfect alignment. The
(80–100 C), followed by cooling in the presence of the direction of P r can be reversed by subsequent application
◦
applied field. Other methods of poling include corona dis- of the field in the opposite direction. This phenomenon,
charge, plasma, and poling during orientation. called ferroelectric switching, has been investigated exten-
For many years, there was debate over the origins of sively to elucidate the mechanism of polarization reversal.
the piezoelectric and pyroelectric properties in PVDF. Although PVDF exhibits strong piezoelectric and pyro-
The arguments seem to have reached the conclusion that electric properties, it is necessary that the polymer film be
the properties primarily arise from the dipole orientation, subjected to mechanical stretching and electrical poling
rather than from trapped space charge. The discovery of to get the β phase. Such procedures include, for exam-
the enhancement of piezoelectric activity in PVDF by ple, subjecting the ferroelectric polymer to mechanical
Kawai led to the revelation of others properties, such as py- deformation, electron irradiation, uniaxial drawing, crys-
roelectricity and ferroelectricity. Although there is no ob- tallization under high pressure, and crystallization under
vious evidence of a Curie transition in PVDF, the existence high electric field. It is tempting to speculate about how
of polarization loops together with polarization reversal much improvement of the dielectric, piezoelectric, and py-
and the switching phenomenon is generally accepted as roelectric properties may yet be achieved by modification
proof of ferroelectricity in PVDF. Figure 2 shows the D–E of the chemical structure of the polymer. Some improve-
◦
hysteresis loops at various temperatures. Even at −100 C ment has been achieved by synthesizing copolymers of
a square-shape hysteresis loop is clearly observed with a vinylidene fluoride with trifluoroethylene (TrFE), tetraflu-
2
remanent polarization P r about 60 mC/m , which does not oroethylene (TFE), or vinyl fluoride (VF), and, indeed,
change with temperature. However, the coercive field E c , some of these copolymers exhibit even higher piezoelec-
which is the electric field used for neutralizing polariza- tric and pyroelectric properties that will be discussed in
tion in the material, is temperature-dependent. The value is the next section.
about 50 MV/m at room temperature and remains almost
constant above the glass transition temperature (−50 C), B. Poly(vinylidene fluoride–trifluoroethylene)
◦
but increases sharply at lower temperatures. (VDF/TrFE) Copolymer
The remanent polarization P r , which is the polariza-
P(VDF/TrFE) is the most studied copolymer. Lando et al.
tion after the field has been removed, is dependent on the
and Yagi et al. initially studied the properties and struc-
crystallinity. For PVDF, the calculated macroscopic polar- ture of this copolymer. The randomly distributed VDF and
2
ization for 100% alignment of all dipoles is 130 mC/m , TrFE units form the cocrystalline phase in the whole com-
2
and the measured polarization of 60 mC/m is consistent
position range of the copolymers. The greater proportion
of bulky trifluorine atoms in PVDF prevents the molec-
+
ular chains from accommodating the tg tg conforma-
−
tion. Therefore, copolymers crystallize at room temper-
ature into a ferroelectric phase with the extended planar
zigzag (all-trans) conformation, whose crystalline phase
is similar to the β phase of PVDF homopolymer.
1. Ferroelectric–Paraelectric Phase Transition
Probably the solid evidence for the ferroelectricity in this
copolymer is the existence of the ferroelectric to paraelec-
tric (F–P) phase transition or Curie temperature T c . The
Curie temperature of synthetic polymer was discovered in
1980 by Furukawa et al. At this temperature, the dielec-
tric constant shows a maximum value, the polarization and
piezoelectric constants go down to zero, and the Young’s
modulus and elastic constant decrease. The phase transi-
tionofcopolymershasbeenfoundtobeaffectedbyseveral
factors, especially the VDF content. As shown in Fig. 3,
copolymers with VDF content below 82 mol% exhibit a
phase transition below the melting point.
FIGURE 2 The D–E hysteresis loops of PVDF at various electric The lowest Curie temperature of the copolymer is about
◦
fields. 60 C, and this phase temperature increases linearly with