Page 157 - 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

Polymers, Ferroelectric 665

During the transition from the ferroelectric to the para-
electric phase, structural changes in the crystal lattice and
the domain size are clearly demonstrated. The phase tran-
sition occurs through the trans–gauche conformational
change, where all-trans molecular chains change their
conformation to a disordered sequence of conformation
isomers (tg , tg , tt), resulting in the nonpolar unit cell
+
−
structure of hexagonal packing, with unit cell dimensions
˚
˚
˚
of a = 9.96 A, b = 4.96 A, and c = 4.64 A.
Interestingly, during the phase transition, there is large
strain change associated with the phase transformation.
For 65/35 mol% VDF/TrFE copolymers, respective lat-
FIGURE 3 The thermal transition temperature in VDF/TrFE
tice strains as high as 10% and 7% in the crystalline phase
copolymers.
along (001 reflection) and perpendicular to (200, 110 re-
flections) the polymer chain have been detected during the
increasing VDF content, which allows an extrapolation of phase transition. Therefore, for a high-crystalline (>50%
the Curie transition temperature of PVDF homopolymer crystallinity) copolymer, these strains can be transformed
◦
to about 215 C. Other widely studied factors that affect into macroscopic strains; indeed, a thermal strain of more
the Curie transition are hydrostatic pressure, tensile stress, than 6% in 65/35 copolymer has been observed. More
external electric field, annealing temperature and time, importantly, for a ferroelectric polymer, the phase trans-
mechanical drawing, poling, irradiation, solution history, formation can be controlled by an external electric field,
thermal history, and crystallization conditions. hence the high-field induced strain can be achieved by
exploring the lattice strain at the phase transition.

2. Crystal Structures
3. Ferroelectricity and Related Properties
A number of experimental techniques such as differential
scanning calorimetry (DSC), dielectric constant determi- Copolymers were demonstrated to possess ferroelectric-
nation, ferroelectric measurement, and X-ray diffraction ity over a wide composition range. Piezoelectric and py-
have been employed to investigate structural change dur- roelectric properties of these copolymers have also been
ing the phase transition. It is not clear whether the double reported. Since the electrical properties originate from the
peak at the ferroelectric transition of 70/30 VDF/TrFE crystal units, chain orientation by drawing, crystallization
copolymer is associated with the formation of different by annealing, and CF 2 dipole orientation by poling are im-
sizes of ferroelectric domains or the two-step phase trans- portant for achieving high piezoelectric and pyroelectric
formation. Tashiro suggested the existence of two types constants. The dipole orientation can result in a change
of ferroelectric phases: a low-temperature phase (LT) con- of chain conformation, chain packing, crystallinity, and
sistingofaparallelarrangementofdipolesinplanarzigzag crystal size. Recently, the structural and crystal changes
(all-trans) chain (as seen in the β phase of PVDF ho- of copolymers have been reported as functions of poling
mopolymer) and the cooled phase (CL) consists of long conditions and high-pressure crystallization. The polar-
trans segments connected by irregular gauche linkages ization reversal of copolymers strongly depends on their
along the chain axis (depending on the VDF content of the thermal and mechanical treatment. The polarization rever-
copolymer as well as the sample preparation conditions). sal of quenched copolymers proceeds over several decades
The transition behavior of the copolymer change with if annealed above the Curie temperature T C , whereas it is
change in the VDF content in the copolymer. For instance, completed within one decade if annealed below T C .
in a copolymer with a VDF content of 70–80 mol%, a first- The phase transition T C of all VDF/TrFE copolymers
ordertransitionbetweentheLTandHTphasesathightem- occurs at high temperatures (>60 C) and the transition
◦
perature is observed, whereas a second-order transition is relatively sharp. In addition, the early experimental re-
between the CT and HT phases occurrs in copolymer with sults showed a large hysteresis at the phase transition,
a VDF content of <50 mol%. The 50–60% VDF samples which is not desirable for practical applications. There-
show more complicated transitional behavior. The trans fore, many attempts have been devoted to broadening and
zigzag chains transform to the irregular trans form of the reducing the phase transition to room temperature and
LT phase, which easily changes to the random gauche con- minimizing or eliminating the hysteresis. Zhang et al.
formation. Recently, the phase transition from the LT to found that by systematic study of the irradiation condi-
CL phases has been confirmed. tions of the copolymer, high electromechanical response