PVF2, copolymers

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. 

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],... 

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. 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])...

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...

Fig. 9. Phase diagram showing the temperature versus the copolymer composition. The dashed areas correspond to the transition region. The frontiers among the different regions depend on sample preparation as well as on sample thermal and processing history. The fraction of ferroelectric phase, Ff, increases with increasing PVF2 content...

Several plastics, with high resistance to chemical attack and high temperatures, deserve special mention for process designers of inherently safer plants. For example, tetrafluoroethylene (TFE), commonly called Teflon brand TFE, is practically unaffected by all alkalies and acids except fluorine and chlorine gas at elevated temperatures, and molten metals. It retains its properties at temperatures up to 260°C. Other plastics that have similarly excellent properties (but are different enough that they each have their niche) include chlorotrifluoroethylene (CTFE) Teflon FEP, a copolymer of tetrafluoroethylene and hexafluoropropylene polyvinylidene fluoride (PVF2) (also... 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Last but not least, PVF2 has also been melt-blended successfully with another engineering polymer, like Noryl (PPE + PS) using a PS-PMMA diblock copolymer (41), since it is known that PPE and PS are miscible (6). Scanning... 

Extrusion-Applied Insulations. The polymers used in extrusion applications can be divided into two classes low-temperature applications and high-temperature applications. Polymers in the first category are poly(vinyl chloride), polyethylene, polypropylene, and their copolymers along with other elastomers. Polymers in the second category are mainly halocarbons such as Teflon polytetrafluoroethylene (which requires special extrusion or application conditions), fluoroethylene-propylene copolymer (FEP), perf luoroalkoxy-modified polytetrafluoroethylene (PFA), poly(ethylene-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVF2) (borderline temperature of 135 °C), and poly(ethylene-chlorotrifluoroethylene). Extrusion conditions for wire and cable insulations have to be tailored to resin composition, conductor size, and need for cross-linking of the insulating layer. 

Fig. 11.6 SEM images of submicron poly(vinylidene fluoride) (PVF2) particles on a polished silicon substrate (silicon wafer, left) and a polyester copolymer (right).

Candidates. The only commeroially available oriented films known at this time which fit the weather resistance requirements are polyvinylidene fluoride (PVF2)i polyvinyl fluoride (Tedlar), polymethyl methacrylate (PMMA), and polybutyl aorylate/methyl methacrylate copolymer (PBA/MMA). PVF2 is currently expensive. PBA/MMA is inexpensive but in clear form does not appear to be sufficiently oxidatively stable for our purposes. It is also too water sensitive and too easily softened in many laminating processes. PMMA appears to be somewhat more chemically stable than PBA/MMA and is also relatively Inexpensive, but has the same dimensional stability problems at 150°C, the normal pottant processing temperature. Both acrylics maintain excellent optical clarity on heat aging, however. 

The list of polymers known to respond satisfactorily to permanganic etching is now long and continually growing. It consists of linear and branched polyethylene, four isotactic polyolefins (polypropylene, polystyrene, poly(4-methylpentene-l) and poly(butene-l)), related atactic polymers, poly(vinylidene fluoride) (hereafter denoted PVF2), PEEK, and poly(ethylene terephthalate) (PET), together with various copolymers and others such as ethylene propylene rubbers and ethylene-propylene-diene (EPDM) terpolymer. 

First ferroelectric polymer - polyvinilidene fluoride (PVDF or PVF2) - was discovered in 1969. Extensive research has been focused on this substance and their copolymers withtrilluoroethylene (TrFE) since that time. Due to its resistivity to the harmful chemical substances is this polymer used in stractural coatings to prevent damage. Another excellent functional property is a veiy low value of the acoustic impedance, which allows for the better acoustic matching to water environment. Due to this property P(VDF/TrFE) copolymer is being applied mostly in hydrophones (Nalwa 1995) and ultrasound imaging transducers. PVDF polymer and its blends with TrFE are commercially available in the market. 

It is now known that a wide array of polymers can be etched using potassium permanganate [273] although some care must be taken to limit the effect of artifacts. The list includes linear and branched PE, PP, PS, poly(4-methylpentene-l), poly (butene-1), PVF2, PEEK, PET and various copolymers such as EPDM terpolymers [273]. More recent work has shown that even liquid crystalline polymers can be etched by a variation of this method. Controls and complementary microscopy are essential to ensure that the experimentalist is not led astray imaging artifacts, hills and valleys or nussing fine structure, lost in the wash baths. 

In the addition to homo-PVF2, a large number of copolymers have also been synthesized which allow to optimize the mechanical properties of fluoropolymers. Most common are copolymers with vinyl fluoride, trifluoroethylene, tetrafluoroethylene, hexafiuoropropy-lene, hexafluoroisobutylene, chlorotrifluoroethylene, and pentafiuoro-propene [521,535, 559-562]. Copolymerization with nonfluorinated monomers is possible [563] in principle but has not yet found commercial use. Fluorocarbon monomers that can help to retain or enhance the desirable thermal, chemical, and mechanical properties of the vinylidene structure are more interesting comonomers. Copolymerization with hexafluoropropylene, pentafluoropropylene, and chlorotrifluoroethylene results in elastomeric copolymers [564]. The polymerization conditions are similar to those of homopoly(vinylidene fluoride) [564]. The copolymers have been well characterized by x-ray analysis [535], DSC measurements [565], and NMR spectroscopy [565,566]. 

Poly(vinylidene fluoride)(PVDF or PVF2) and Its Copolymers. 511... 

Piezoelectric and ferroelectric polymers have been recognized as a new class of electroactive materials when the significant piezoelectricity in polyvinyhdene fluoride (PVDF or PVF2) was discovered by Kawai (1969). Since then, a variety of new piezoelectric polymers have been developed including copolymers of vinylidene fluoride and trifluoroethylene, P(VF2-TrFE), odd-numbered nylons, composite polymers, etc. These materials offer options of material selections for sensor and actuator technologies that need lightweight electroactive materials. 

Copolymerization with other vinylic monomers (i.e., styrene, vinyl acetate) allows even further modification. The ease of structural modification to yield desired blend properties (miscibility) is well-documented in the experimental literature. The common acrylate polymer is PMMA and has been noted to be miscible with various other polymers noted in this chapter (PVF2, styrene copolymers, PVC, PVPh, PEO) and thus will not be discussed in this section. 

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