Page 143 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/MBS P2: GQT Final Pages
Encyclopedia of Physical Science and Technology En012j-597 July 26, 2001 11:8

650 Polymers, Electronic Properties

magnitude of the effects in polyvinylidene fluoride point in PVF 2 . The fact that quite efficient poling occurs at
(PVF 2 ), which is the most widely studied and commer- temperatures of ∼130 C, compared with transition tem-
◦
cially used piezo- and pyroelectric polymer. peratures of β to α phase of 180 C and the α-phase melt-
◦
◦
Substantial piezo- and pyroelectricity can be perma- ing point of ∼200 C, suggests that dipole orientation does
◦
nently induced by heating stretched films to ∼100 C, fol- not occur in the crystalline phase. Instead, it is believed
lowed by cooling to ambient temperature with a strong to occur in disordered regions that have conformational
−1
dc electric field (∼300 kV cm ) applied. This treatment similarity to the β form but with lateral disorder between
is called “poling.” Such polarization, attributed to redis- chains.
tribution of electronic or ionic charges within the solid The structure–property relationship for PVF 2 is an ex-
or injected from electrodes, characteristically vanishes on tremely complex question that remains an area of some
exceeding some polarization temperature, T p . The effect controversy and contention. There is some evidence that
in PVF 2 is totally different in that the induced polariza- other effects, such as charges injected during the poling
tion is thermally reversible and polarization currents are process, in addition to dipole reorientation, may also be
produced on either heating or cooling. important. Other evidence has been found for nonuniform
Polyvinylidene fluoride is a crystalline polymer that ex- pyroelectric constants and internal electric field after pol-
ists in at least three and possibly more crystalline phases. ing. There seems little question that internal space charges
Since the polymorphism is a critical ingredient in deter- in the films can have a significant influence on the poling
mining the piezo- and pyroelectric properties, it is worthy process.
of some discussion. Phase I (β form) has a planar zigzag In summary, it is perhaps wise to highlight the fact that
conformation and ordered C–F bonds, giving a large unit piezoelectric and pyroelectric properties can be quite sam-
dipole of 2.1 debye (D). This large moment and the facility ple dependent in terms of purity, morphology, and detailed
with which it can orient, because of the small atomic vol- processing procedure. It thus becomes very difficult to
ume of fluorine, accounts for the high permittivity. Phase identify a specific mechanism when in fact several inter-
II (α form) has a conformation with two chains of opposite related ones may be operative.
dipole moment per unit cell. Melt-crystallized samples be-
low 150 C are largely phase II, although phase I films can
◦
be made by casting from appropriate solvents and by high- V. PHOTOELECTRONIC PROPERTIES
pressure techniques. Induced strain by biaxial stretching OF PENDANT-GROUP POLYMERS
at temperatures around 60 C can result in α-to β-phase
◦
transformations. Contrary to early expectations, phase II is The weak intermolecular interactions in molecular solids
not nonpolar under all circumstances but can have unit cell lead to very narrow energy bands, even for ordered molec-
dipole moments of ∼1.5 D after orientation of poled films. ular crystals such as anthracene. This results in low mo-
More recent studies have revealed the existence of a third bilities for electronic charge carriers. If, in addition to this
phase, called γ ,in films cast from dimethylformamide and weak interaction between the molecular building blocks,
◦
annealed at ∼170 C. The γ phase is uncertain, but it has one introduces the feature of disorder, rather dramatic ef-
been proposed that it is a modified phase II with a double fects on carrier transport may be expected. Pendant-group
repeat unit. polymers, as we have seen, are examples of polymers
The necessity of poling to induce permanent piezo- in which the optical properties are dominated by those
and pyroelectric behavior and the intuition that these phe- of the pendant-group molecules themselves. Extensive
nomena are intimately associated with the polymorphic studies of the photoelectronic properties of such poly-
form have induced several workers to attempt to detect mers have verified these features to a dramatic degree.
structural changes due to poling. Reversible changes were It is therefore found that dynamic charge transfer, in-
detected in the polarized infrared absorption at 510 and cluding both photogeneration and transport, is determined
445 cm −1 when poling and depolarization were repeated. by charge exchange between neighboring, essentially iso-
A minor change in the X-ray diffraction when β-phase lated molecules.
PVF 2 was poled was reported, and variations in dielectric The full range of these effects has been examined, but
constant and loss have also been reported. On the basis of not by studying pendant-group polymers themselves, al-
these observations, one might conclude that the origin of though in the case of PVCA this has been done. Rather,
spontaneous polarization in PVF 2 is due to dipole orien- the concept of molecular doping of polymers has been
tation in β crystallites and that the orientation can be in employed as a powerful way of exploring the intuitive
part reversed by electric field; that is, PVF 2 has some as- concepts. A typical example of a molecularly doped
pects of ferroelectric behavior. However, direct evidence polymer is the molecular dispersion of N-isopropyl car-
has yet to be found for the existence of domains or a Curie bazole (NIPCA) in a polycarbonate polymer. The doping