PVF2

Polymer Ferroelectrics. In 1969, it was found that strong piezoelectric effects could be induced in the polymer poly(vinyhdene fluoride) (known as PVD2 or PVDF) by apphcation of an electric field (103). Pyroelectricity, with pyroelectric figures of merit comparable to crystalline pyroelectric detectors (104,105) of PVF2 films polarized this way, was discovered two year later (106.)... 

Poly(viaylidene fluoride) [24937-79-9] is the addition polymer of 1,1-difluoroethene [73-38-7], commonly known as vinylidene fluoride and abbreviated VDF or VF2. The formula of the repeat unit in the polymer is —CH2—CF2—. The preferred acronym for the polymer is PVDF, but the abbreviation PVF2 is also frequently used. The history and development of poly(vinyhdene fluoride) technology has been reviewed (1 3). 

Fig. 4. The effect of temperature on the pie2oelectric strain constant, for A, nylon-11 B, nylon-7 and C, poly(vinyhdene fluoride) (PVF2) films (35).

PMMA-PVFj, PEMA-PVFj better chemical and uv resistance than PMMA or PEMA, better clarity than PVF2 outdoor film, decorative striping on automobiles 45,325,44... 

The most chemical-resistant plastic commercially available today is tetrafluoroethylene or TFE (Teflon). This thermoplastic is practically unaffected by all alkahes and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260°C (500°F). Chlorotrifluoroethylene or CTFE (Kel-F, Plaskon) also possesses excellent corrosion resistance to almost all acids and alkalies up to 180°C (350°F). A Teflon derivative has been developed from the copolymerization of tetrafluoroethylene and hexafluoropropylene. This resin, FEP, has similar properties to TFE except that it is not recommended for continuous exposures at temperatures above 200°C (400°F). Also, FEP can be extruded on conventional extrusion equipment, while TFE parts must be made by comphcated powder-metallurgy techniques. Another version is poly-vinylidene fluoride, or PVF2 (Kynar), which has excellent resistance to alkahes and acids to 150°C (300°F). It can be extruded. A more recent development is a copolymer of CTFE and ethylene (Halar). This material has excellent resistance to strong inorganic acids, bases, and salts up to 150°C. It also can be extruded. 

Bauer, F. (1982), Behavior of Ferroelectric Ceramics and PVF2 Polymers Under Shock Loading, in Shock Waves in Condensed Matter—1981 (edited by W.J. Nellis, L. Seaman, and R.A. Graham) American Institute of Physics, New York, pp. 251-267. 

Even though potential memory exists in all TPs, polyolefins, neoprenes, silicones, and other cross-linkable TPs are example of plastics that can be given memory either by radiation or by chemically curing. Fluorocarbons, however, need no such curing. When this phenomenon of memory is applied to fluorocarbons such as TFE, FEP, ETFE, ECTFE, CITE, and PVF2, interesting high-temperature or wear-resistant applications become possible. 

Fluorocarbons (PTFE, FEP, PVF2) Powder, emulsions Excellent high temperature properties. TFE to 500 F. FEP is easier to mold, but maximum use temperature is 400 F. Nearly inert chemically. Nonflammable. Loading with conductive filler improves creep resistance. Low coefficient of friction. High-temperature cable shielding, gaskets, heat-shrinkable tubing. 

The fraction of head-to-head linkages in the poly(fluoro-olefms) increases in the series PVF2 < PVF PVF3 (Tabic 4.2). This can be rationalized in terms of the propensity of electrophilic radicals to add preferentially to the more electron rich end of monomers (i.e, that with the lowest number of fluorines). This trend is also seen in the reactions of trifluoromethyl radicals wilh the fluoro-olefins (see 2.3). 

A specific set of experiments which must be mentioned, being directly associated with the main topic of this paper, is the work of Bergman, et. al. (22) dealing with the second-order nonlinear optical properties of polyvinylidene fluoride (PVF2). Nonvanishing the second-order nonlinear electric dipole susceptibility, is expected in PVF2 since it exhibits other properties requiring noncentrosymmetric microscopic structure. These properties appear... 

In table I we present the molar Kerr constants and mean square dipole moments of three fluorinated polymers, poly (trifluoroethylene) (PFjE), polylvinylidene fluoride) (PVF2) and poly(fluoromethylene) (PFM), dissolved in p-dioxane. The results show the sensitivity of mK to the degree and type of fluorination varying over an order of magnitude and also changing sign. Calculations of mK and for comparison are in progress (5). 

Ferroelectricity has also been found in certain copolymer compositions of VF2 with trifluoroethylene, F3E, [6-11] and tetrafluoroethylene, F4E, [12-15] and in nylon 11 [16]. Specifically, copolymers of vinylidene fluoride and trifluoroethylene (VF2/F3E) are materials of great interest because of their outstanding ferroelectricity [9,17-18], together with a parallel strong piezo- [7] and pyroelectricity [19]. These copolymers exhibit, in addition, an important aspect of ferroelectricity that so far has not been demonstrated in PVF2 the existence of a Curie temperature at which the crystals undergo reversibly a ferroelectric to a paraelectric phase transition in a wide range of compositions [9, 17-18],... 

Fig. la, b. Schematic depiction of the two most common crystalline chain conformations in PVF2 a. tg+ tg and b. all-trans. The arrows indicate projections of the -CF2 dipole directions on planes defined by the carbon backbone. (Figure and caption from Ref. [32])... 

Copolymers of VF2 with trifluoroethylene are randomly added copolymers. Those containing a mole fraction of VF2 of 50-80% have been widely studied. Since they contain a greater proportion of the comparatively bulky fluorine atoms than PVF2 their molecular chains cannot accommodate the tg+tg conformation and crystallize at room temperature in the ferroelectric phase with the extended all-trans planar conformation [37] with small statistical deviations away from that plane, i.e. copolymers of VF2 with F3E crystallize essentially with the same conformation as P-PVF2. 

Fig. 2. Projection of four crystal structures of PVF2 viewed along the molecular axes. Dipole moments are shown by arrows. Large circles represent fluorine, smaller circles represent carbon. Hydrogen atoms are not shown. Figure and caption from Ref. [55])...

Fig. 3. Unit cell of (S-PVF2 (top) and of the ferroelectric phase Of a VF2/F3E copolymer with mole fractions of 73/27 (bottom), projected along the molecular axis (Figure and caption from Ref. [65])...

Fig. 4. Typical appearance of a-PVF2 spherulites. Sample cast from dimethyl formamide, molten at 200CC and recrystallized at 156°C for 12 hours. Scale bar, 25 pm...

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

Table 1 summarizes the unit cell dimensions at room temperature reported in the literature as a function of the VF, content For composition of VF2 higher than 80% the a-phase of PVF2 is obtained. In the range of VF2 compositions between 50 and 80% a predominant phase (orthorhombic or monoclinic) with the chains in the polar trans-conformation similar to that of the P-phase of PVF2, giving rise to ferroelectric crystals, is observed (see Fig. 3). In the case of the 55/45 copolymer [60], the dimensions of the two coexisting unit cells - the ferroelectric and non-ferroelectric one - at room temperature are given in Table 1. Figure 7 shows the coexistence of the [110] lattice spacings corresponding to the two phases (ferroelectric and non-ferroelectric) over the whole...

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