Page 164 - 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

672 Polymers, Ferroelectric

TheferroelectricityofpoledandannealedPANhasbeen enylmethane (MDA)] have been reported. The dielectric
studied by X-ray and IR spectroscopy. It is believed that constant of MDA-rich films of poled and annealed
the ferroelectricity of PAN is related to the dipole ori- P(MDA/MDI) are relatively constant up to 200 C,
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entation and kinking of the chain above the glass transi- whereas the MDI-rich films have a low dielectric constant
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tion temperature. The piezoelectric and pyroelectric prop- but show good stability up to 200 C. The dielectric
erties of PAN were initially studied by Ueda and Carr, constants of the balanced stoichiometric films increase
and were proved by Von Berlepsch using copolymer with with increasing temperatures above 100 C. Pyroelectric
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methylacrylate. The D–E hysteresis loop of a stretched and piezoelectric constants are also large for the balanced
film of this copolymer has been measured at 68 C, which composition films. This is due to the fact that a crosslinked
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is below the glass transition temperature. The copolymer network structure does not form in balanced film the way
showed very low remanent polarization and high coercive it does in unbalanced films. The high-molecular-weight
field at low temperature. However, an alternating copoly- molecules are oriented or crystallized in a local region
mer of PAN, poly(allycyanide[35%]/acrylonitrile[65%]), under a high poling field and form a semicrystalline struc-
showed very high remanent polarization, 200–700 mC/m 2 ture; thus, stabilized residual polarization is produced and
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at 105 C. gives rise to high piezoelectric and pyroelectric constants.
Another alternating copolymer that has received more The pyroelectric and piezoelectric properties of alip-
attention as an amorphous copolymer is vinylidene hatic polyureas display similar features to those seen in
cyanide/vinyl acetate copolymer. The dipole moment of nylons, i.e., there is no observation of these activities if the
the repeating unit is 4.5 D in trans conformation, thus number of carbon atoms between the urea bond is even.
contributing to a large piezoelectric constant after poling. This is because alternating urea bond dipoles arrange in
The piezoelectricity (d 31 ) of drawn and poled films is antiparallel direction, and the dipole moments cancel out.
comparable to that of PVDF in the temperature range 20– Strong hydrogen bonds are possible in cases where the
100 C. Interestingly, this copolymer shows a very large parallel orientation of dipoles in a planar zigzag chain
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dielectric strength of more than 100, one of the largest gives rise to high glass transition and melting tempera-
values among polymers. It is concluded that the large tures. Dielectric peaks above 125 C have been observed in
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dielectric relaxation strength originates from the coopera- poly(heptamethylene/nanomethyleneurea). The relatively
tive motion of 10 or more CN dipoles. The exceptionally large dielectric constant is believed to be related to the
high dielectric peaks occur near the glass transition crystal transition from the mobile dipole state to the rigid
temperature, similar to a ferroelectric transition. A most dipole state corresponding to the different hydrogen bond-
unique feature of this copolymer is its low density, which ing states. The value of remanent polarization obtained
results in low acoustic impedance, close to the levels seen from D–E hysteresis of the same sample is as high as
2
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in water and the human body. This makes this copolymer 200–440 mC/m at 90 C, which may be attributed to the
very useful in medical applications. effect of the ionic currents of impurities. The remanent po-
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larization disappears above 110 C, which suggests the re-
arrangement of the hydrogen bonding at this temperature.
C. Polyureas and Polythioureas
Polythioureas are known pyroelectric, pyroelectric,
Polyureasandpolythioureasareaminoresins,andarether- and ferroelectric polymers. Their chemical structure is
mosetting polymers. They are usually synthesized by con- H 2 N CS NH 2 . Ferroelectric thiourea polymers have
densation polymerization, and their products are mostly been prepared by condensation reaction of thiourea
in the form of powder. The preparation of thin films was and formaldehyde under different conditions. Thiourea
not possible due to their insolubility until a technique of [SC(NH 2 ) 2 ] has a large dipole moment of 5.4 D. Ferro-
vapor deposition polymerization was developed. The syn- electricity in polythioureas was initially studied in 1978.
thesis and characterization of different kinds of thin-film The sharp dielectric constant of thiourea-formaldehyde
polyureas have been reported. A typical polymerization (PTUBFB) observed at 145 C is as high as 320. Recently,
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consists in vapor deposition of monomers onto the sur- an odd-numbered aliphatic polythiourea, polythiourea-9,
face of a substrate in a vacuum chamber. The monomers has been synthesized. The odd number of carbons was
diffuse on the surface and react with each other to form chosen to ensure a polar chain and polar packing. Large
urea bonds between an amino (NH 2 ) group and an iso- dielectric relaxation as well as a large dielectric constant
cyanate (CNO) group. The urea bond (NHCONH) has a were observed in the glass transition region. According
dipole moment of 4.9 D and is responsible for high piezo- to the theoretical data of Meakins et al., the dielectric
electric and pyroelectric properties. relaxation and dielectric loss increase with any increase
The dielectric constants of aromatic polyureas [P(4,4 - in hydrogen bonding. As a result, the hydrogen-bonded

diphenylmethane diisocyanate (MDI)/4,4 -diamino diph- thiourea dipoles in the intermolecular chains are easily