Page 158 - 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

666 Polymers, Ferroelectric

can be achieved. The longitudinal strain S 3 measured at defects into a VDF/TrFE copolymer chain. The result-
room temperature and 1 Hz frequency reaches −4% at ing VDF/TrFE/CTFE terpolymers are completely solution
150 MV/m. The irradiation significantly reduces the hys- and melt processable, and exhibit high dielectric constant
teresis in the polarization loop. However, the polarization and electrostrictive response at ambient temperature.
was also significantly reduced, and the sample becomes
completely insoluble in common solvent because of a A. Synthesis
severe crosslinking side reaction during the high-energy
Prior to our research, there were some reports discussing
irradiation.
the terpolymerization reactions involving VDF, TrFE,
The increase in hardness of the copolymer due to
and a small amount of termonomer, such as hexaflu-
crosslinking was also revealed in the electromechanical
oropropene (HFP), chlorotrifluoroethylene (CTFE), or
response. A very high field was required to get high strain
tetrafluoroethylene (TFE), by conventional free radical
response. It appears that the irradiation process produces
polymerization processes. The ter-polymerization was
many undesirable side reactions that increase the amor-
usually carried out in the emulsion or suspension pro-
phous phase content and diminish the processability of
cess using inorganic or organic peroxides as initiator at
the sample.
elevated temperatures.
Recently, we have developed a bulk (room temperature)
C. Poly(vinylidene fluoride–tetrafluoroethylene) polymerization process initiated by the oxidation adducts
(VDF/TFE) Copolymer of the organoborane molecule, as illustrated in Fig. 4.
Upon exposure to a controlled quantity of oxygen,
The second mostly studied copolymer is the VDF/TFE
asymmetrical alkylborane (I) is selectively autoxidized
random copolymer, which can be viewed as a PVDF poly-
at the linear alkyl group to produce ethylperoxylborane
mer containing an increased amount of head-to-head and
(II). The peroxyborane (II) behaves very differently from
tail-to-taildefects.ThesterichindrancecreatedbytheTFE
regular benzoyl peroxides and consequently decomposes
unit effectively stabilizes the trans conformer of the VDF
to alkoxy radical (C O∗) and a borinate radical (B O∗)
chain. As the content of TFE units increases, the trans con-
(III) that is relatively stable due to the back-donating of
formation is stabilized overwhelmingly and the generation
electron density to the empty p orbital of boron. In the
of the gauche bonds is suppressed. The copolymers of
presence of fluro-monomers, the homolytic cleavage of
VDF/TFE with at least 7% are crystallized in the β form.
peroxide occurs even at very low temperatures (−30 C).
◦
The Curie transition was clearly observed in 18–23 mol%
The alkoxyl radical is very reactive and initiates the rad-
TFE. The crystal structure and phase transition behavior
ical polymerization at ambient temperature. On the other
of copolymers have been studied by X-ray, IR, and Raman
hand, the borinate radical forms a weak and reversible
spectroscopies.
bond with the growing chain end, which assures the “sta-
Piezoelectric and pyroelectric studies of VDF/TFE
ble” radical polymerization. During the propagating reac-
copolymers showed highly inhomogeneous polarization
tion, a coordination intermediate (IV) may form due to
across the thickness of the copolymer films. Ferroelectric
the B F acid–base complex between the active site and
studies revealed a hysteresis loop, but no Curie transition
the incoming monomer. Such an interaction will regulate
in the temperature-dependent dielectric curve. However,
there was some evidence showing that the Curie transition
occurs in the vicinity of the melting temperature.
III. NEW PIEZOELECTRIC VDF/TRFE/CTFE
TERPOLYMERS
Inourlaboratory,wehaveadoptedanewchemicalstrategy
of altering crystalline domains and creating relaxor ferro-
electric behavior of VDF/TrFE copolymer, with the objec-
tive of achieving a processable polymer with controllable
phase transition temperature, high dielectric constant, and
fast and large electromechanical response at ambient tem-
perature. The chemistry involves homogeneous incorpo-
FIGURE 4 An equation showing selective auto-oxidation of
ration of a small amount of bulky ter-monomer units, organoborane and terpolymerization of VDF, TrFE, and CTFE
such as chlorotrifluoroethylene (CTFE), as the crystalline monomers.