Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-12-05
2002-03-12
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S195000, C526S196000, C526S197000, C526S249000, C526S250000, C526S253000, C526S254000
Reexamination Certificate
active
06355749
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a new class of polymeric materials that generate exceptionally high dielectric constant and high electromechanical response at ambient temperature. More particularly, the invention relates to a class of semicrystalline ferroelectric terpolymers comprising vinylidene fluoride (VDF), trifluoroethylene (TrFE), and at least one bulky monomer, such as chlorotrifluoroethylene (CTFE) or hexafluoropropene (HFP) or the like, prepared by borane/oxygen initiation in bulk reaction conditions.
BACKGROUND OF THE INVENTION
Ferroelectric materials that generate mechanical actuation induced by external electric field have attracted a great deal of attention and have been recognized for applications in a variety of transducers, actuators and sensors. Most of the current commercial applications for ferroelectric materials are based on piezoceramics and magnetostrictive materials, despite the fact that they exhibit many deficiencies, such as low strain levels (<0.2%), brittleness, heavy weight, high processing temperatures and processing difficulties when producing parts having complicated shapes. In sharp contrast, ferroelectric polymers exhibit many desirable properties, such as flexibility, light weight, high mechanical strength, an ability to be processed readily into large area films, and an ability to be molded readily into a variety of configurations. However, despite these advantages over ceramic materials, most ferroelectric polymers suffer the disadvantage of having a low electric field sensitivity, in terms of dielectric constant, piezoelectric coefficient, electromechanical coupling coefficient and field induced strain, which limit their applications.
One of the phenomena in ferroelectric polymers that has a great potential in generating high strain with high force level and broad frequency bandwidth is the phase transformation between ferroelectric (polar) and paraelectric (nonpolar) crystalline domains. The crystalline phase change produces large lattice strain and large change in sample dimension. Electrostriction refers to a coupling effect between the strain and the square of polarization, and is a desirable mechanism for achieving a large electric-induced mechanical response. It is interesting to note that the electrostrictive response due to crystalline phase transition is very different from electrostatic force in dielectric elastomers [R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, Science, 287, 836, 2000], which can produce a large strain but a very weak force.
In the past decade, most of the research activities involving ferroelectric polymers have focussed on ferroelectric fluorocarbon polymers, especially semicrystalline VDF/TrFE copolymers. Many research efforts have been devoted to a general goal of reducing the energy barrier for ferroelectric-paraelectric (Curie) phase transition, and for generating a large and fast electric-induced mechanical (piezoelectric) response at ambient temperature. Although VDF/TrFE copolymers (stretched film poled at 120° C.) exhibit a relatively high piezoelectric constant (on the order of from about 10 pC/N to about 25 pC/N; pC=10
−12
coulomb and N=newton) [K. Koga, and H. Ohigashi, J. Appl. Phys., 59, 2142,1986], the response of the dipoles to an electric field is very slow at ambient temperature, and the polarization hysteresis loop (polarity vs. electric field) of the copolymer is very large. As shown in
FIG. 3
, a VDF/TrFE copolymer comprising 55 mole % VDF and 45 mole % TrFE, which exhibits the narrowest polarization hysteresis loop and lowest Curie temperature of the copolymers in the VDF/TrEF family [Y. Higashihata, J. Sako, and T. Yagi, Ferroelectrics, 32, 85, 1981], still exhibits a significantly wider hysteresis loop than those exhibited by the VDF/TrEF/bulky monomer terpolymers of this invention.
The close connection between crystalline structure and electric properties led to many attempts to alter copolymer morphology by creating non-equilibrium states; and a number of such attempts resulted in ferroelectric polymers that exhibit somewhat improved electric responses. Such attempts have included, for example, subjecting ferroelectric polymers to mechanical deformation [K. Tashiro, S. Nishimura, and M. Kobayashi, Macromolecules, 21, 2463, 1988, and 23, 2802, 1990], electron-radiation [B. Daudin, and M. Dubus, J. Appl. Phys., 62, 994, 1987], uniaxial drawing [T. Furukawa, and N. Seo, Japanese Journal of Applied Physics, 29, 675, 1990], crystallization under high pressure [T. Yuki, S. Ito, T. Koda, and S. Ikeda, Jpn. J. Appl. Phys., 37, 5372, 1998], and crystallization under high electric field [S. Ikeda, H. Suzaki, and S. Nakami, Jpn. J. Appl. Phys., 31, 1112, 1992].
Zhang et al. [Science, 280, 2102, 1998 and WO99/26261] recently reported their work involving electron-radiation treatment of ferroelectric polyvinylidene fluoride polymers. The polymers that were generically disclosed in their work include polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trifluoroethylene-hexafluoropropylene and polyvinylidene fluoride-hexafluoropropylene. However, the only polymers actually prepared and studied were 50/50 and 65/35 copolymers of vinylidene fluoride/trifluoroethylene (VDF-TrFE). The Zhang et al work, which included a systematic study of the radiation conditions, such as dosage, temperature, inert atmosphere, stretching sample, etc., revealed an exceptionally high electrostrictive response (~4%) of the irradiated copolymer that behaves like a relaxor ferroelectric with fast electric-induced mechanical response at ambient temperature. Their work also revealed that the polarization hysteresis loop of the irradiated copolymer became very narrow at room temperature, compared with the hysteresis loop of a sample of the copolymer before irradiation. However, the polarization of the irradiated copolymer also was significantly reduced and the irradiated copolymer became completely insoluble because of the severe crosslinking side reaction that had occurred during the high-energy radiation. The increase of hardness of the irradiated copolymer sample due to the crosslinking also was revealed in its electric response, i.e., a very high electric field (150 MV/m) was required before the irradiated copolymer exhibited a high strain response (~4%). Thus, it appears that irradiating a ferroelectric copolymer not only reduces the polar crystalline domain size, but also produces many undesirable side reactions that increase the amorphous phase and diminish the processibility of the irradiated copolymer. At page 11, lines 25-27 [in WO 99/26261], Zhang et al, in a quite off-handed manner, suggested that the effects achieved by irradiation can be accomplished chemically, by adding a bulky side group to the main polymer chain which operates as an internal plasticizer. Zhang et al provided no examples of bulky side chain additions and made no further reference to chemically modified polymers.
As will be apparent from the following description of the invention, the present approach for altering the crystalline domains, and creating relaxor ferroelectric behavior, of VDF/TrFE copolymers is to introduce into the copolymer structure a controlled amount of bulky monomer units, such as chlorotrifluroethylene (CTFE) and hexafluoropropene (HFP) units, with a homogeneous fashion. The resulting terpolymers are solution and melt processible and form a desirable film morphology with uniform nano-crystalline domains that have Curie (polar-nonpolar crystalline phase) transition at about ambient temperature. Therefore, these terpolymers exhibit exceptionally high dielectric constant at ambient temperature and fast and high electro-mechanical response induced by external electric field.
Prior to the present invention, there have been several reports that discussed VDF/TrFE/CTFE terpolymers with significan
Chung Tze-Chiang
Petchsuk Atitsa
DeLaurentis Anthony J.
The Penn State Research Foundation
Zitomer Fred
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