Page 163 - 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 671

2
of nylon-5 is ∼135 mC/m . Odd-numbered nylons have annealing temperature increases, indicating the rearrange-
polymorphs. Nylon-11 has at least five different crystal ment of the hydrogen bonding structure. The mechanism
structures: triclinic α form I, monoclinic form II, pseudo- of ferroelectric polarization in odd nylons has not been ad-

hexagonalγ formIII,δ phase,γ and phase.Theα formof dressed, except for nylon-11. The ferroelectricity of poled
nylon-11 is polar in nature, with dipoles that are hydrogen- nylon-11 is related to the hydrogen bond breaking, fol-
bonded and aligned in the same direction, whereas the lowed by the reorientation of the amide groups toward
γ form is nonpolar and has amide groups that lie in the the electric field’s direction and the re-forming of hydro-
plane perpendicular to the chain axis. The crystal phase gen bonds in a new direction. The orientation is retained
transformation of nylon-11 occurs between a triclinic α in the glassy state below T g even after the electric field
form and a pseudo-hexagonal γ form at high tempera- is removed. Ferroelectricity has also been observed in
ture (>90 C) through the randomization of the hydrogen polyamides containing m-xylylenediamine, aliphatic di-
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bonding. Polymorphs of nylon-9, nylon-5,7, and nylon- carboxylic acids, and fluorinated odd–odd nylons.
7,7 have also been prepared. The α form seems to be the Ferroelectricity in aliphatic odd nylons has been
most common polymorph in many odd-numbered nylons. reported recently. Unlike nylons with ring systems, in
which the ferroelectric nature arises from the orientation
of the amide dipoles in the amorphous region, the fer-
2. Ferroelectricity and Related Properties
roelectric polarization of aliphatic odd nylons originates
Nylons exhibit very interesting dielectric behavior in that from the crystalline phases. A slightly higher density of
the dielectric constant changes significantly with changes amide dipoles contributes to a larger value for the rema-
in temperature and frequency. The dielectric constants of nentpolarization,asseeninothernylonsystems.Recently,
poled and annealed nylons are relatively low (about 2.5–3) a new class of ferroelectric and piezoelectric polymers,
◦
at various frequencies below 0 C, and increase rapidly nylon-11/PVDF laminated films prepared by a co-melt-
above the glass transition. The peak position, magnitude pressing method, has been reported. The laminated films
of dielectric constant, and relaxation process in nylons exhibit a typical ferroelectric hysteresis loop with high
change with poling conditions and the presence of ab- remanent polarization, higher than that observed in PVDF
sorbed water. Several articles have been focused on the homopolymers or nylon-11 under the same measurement
high-frequency properties that are of interest for ultrasonic conditions. The film’s piezoelectric stress constant (d 31 =
transducers. 41 pC/N) and piezoelectric strain constant (e 31 = 109 mC/
2
The pyroelectricity and piezoelectricity of the α and γ m ) are significantly higher than those of PVDF
phases of nylon-11 films with variation of poling condi- and nylon-11. Another new class of ferroelectric polya-
tions have been reported. The γ phase nylon films show mides recently reported is the ferroelectric polyamide
much higher piezoelectric response than α-phase films blends. The D–E hysteresis curves were observed
under the same poling conditions. This characteristic dif- in all blends of nylon 6I/6T copolymers (6 = hexamethy-
ference was interpreted as the breaking and re-forming of lenediamine, I = isoththalic acid, T = terephthalic acid)
hydrogen bonds under a high electric field. The γ phase and m-xylylenediamine-6 (MXD6). It was concluded
has a more regular arrangement of dipoles and has stronger that the intermolecular exchange of hydrogen bonding in
hydrogen bonding, and thus is more strongly influenced the amide groups is responsible for the ferroelectricity.
by an applied electric field. However, it is found that a
mixture of α phase and γ phase, rather than pure α phase
B. Cyanopolymers
or γ phase, has the highest piezoelectric constants. The
piezoelectricity and pyroelectricity of odd nylons were Cyanopolymers discussed here include polyacrylonitriles
also affected by anisotropy, absorbed water, orientation, (PAN), poly(vinyl cyanide), and cyanocopolymers. The
and annealing temperature. cyano group (C CN) has the unique feature of a large
Ferroelectric properties of nylons have been investi- dipole moment (3.5 D) and the ability to form complexes
gated by several research groups. Polarization reversal with transition metals. The polymerization can occur
was found to be complete in a few tens of milliseconds through free radical and ionic polymerization, leading
under 140 MV/m at 20 C. This fast polarization reversal to nonstereoregular (atactic) cyano groups in the poly-
◦
andtherectangularD–Ehysteresisloopareevidenceofthe mer structure. Thus, cyanopolymers have no clear melting
dipoles’ origin. The effect of annealing on the ferroelectric temperature because of their high cohesive force and low
behavior of nylon-11 and nylon-7 has been investigated. thermal stability. It has been accepted that the strong in-
The remanent polarization decreases with increasing an- teraction and repulsion of the cyano groups force poly-
nealing temperature and disappears at an annealing tem- mer chains to adopt a helical conformation. However, the
◦
perature of 185 C, whereas the coercive field increases as all-trans conformation is more stable.