Amorphous fluoropolymer

Table 13.3 Typical properties of Teflon AF amorphous fluoropolymers...

Vinyl ethers constitute a third class of monomers which have been cationically polymerized in C02. While fluorinated vinyl ether monomers such as those described in Sect. 2.1.2 can be polymerized homogeneously in C02 because of the high solubility of the resulting amorphous fluoropolymers, the polymerization of hydrocarbon vinyl ethers in C02 results in the formation of C02-insoluble polymers which precipitate from the reaction medium. The work in this area reported to date in the literature includes precipitation polymerizations and does not yet include the use of stabilizing moieties such as those described in the earlier sections on dispersion and emulsion polymerizations (Sect. 3). 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

Teflon AF A Family of Amorphous Fluoropolymers with Extraordinary Properties... 

Teflon AF is truly a family of amorphous fluoropolymers with an extraordinary combination of properties. All of the excellent properties of die existing fluoropolymers have either been retained or improved upon and properties arising from the amorphous nature and the presence of microvoids in the AF family of polymers have been added. The similarities and differences of AF and other Teflon polymers are summarized in Table 2.3. This unique combination of properties of Teflon AF amorphous fluoropolymers makes them well suited for applications that had previously precluded polymeric materials,... 

It has been noted that C02 behaves very much like a hydrocarbon solvent with regards to its ability to dissolve small molecules consequently, many monomers exhibit a high solubility within C02. On the other hand, most high-molar-mass polymers are scarcely soluble in C02, and the only polymers that show good solubility under relatively mild conditions (T < 373 K, P < 35 MPa) are amorphous fluoropolymers, silicones, and polyether polycarbonate copolymers. 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

FEP and PFA despite being melt-processible are crystalline (between 50 and 70%). The crystallinity results in poor optical properties (low clarity) and a very poor solubility in organic solvents. The latter makes the preparation of thin optical coatings exceedingly difficult.10 TEFLON AF, an amorphous fluoropolymer, contains in its molecule a bulky dioxole ring, which hinders crystallization. As a result, the polymer has an exceptionally high clarity and excellent optical properties. Its refractive index is the lowest of any plastic.11... 

Resnick, P. R. and Buck, W. H., Teflon AF Amorphous Fluoropolymers, in Modem Fluoropolymers (Scheirs, J., Ed.) John Wiley Sons, New York, Chapter 22 (1997). 

Teflon AF Teflon amorphous fluoropolymer - Dupont trademark... 

Supercritical carbon dioxide is a very good solvent for small molecules, but a poor solvent for most high molecular weight polymers at mild conditions (T<100 °C, P<350 bar). Amorphous fluoropolymers and silicones are the only polymers known to be soluble in CO2 at mild conditions [6]. This difference in solubilities is an advantage for C02-based polymerizations, as it can be used to reduce the energy requirements necessary to separate and purify a polymer after synthesis. Consider, for example, a batch precipitation polymerization in... 

Two classes of polymeric materials, amorphous fluoropolymers and silicones, are the only commercially available polymers to exhibit appreciable solubility in SCCO2 at readily accessible temperatures and pressures (Figure 4.3). It has been proposed that this results from a special interaction between fluorine and carbon dioxide due to the polarity of both species. Silicones are also thought to dissolve because they have weak intermolecular interactions and flexible backbones. 

Transition metal catalyzed, ring opening polymerization Dispersion, cationic polymerization Homogeneous/precipitation, cationic polymerization Homogeneous, free radical/cationic polymerization Precipitation, free radical polymerization Dispersion, free radical polymerization Norbornene polymer, polycarbonate Isobutylene polymer Vinyl ether polymer Amorphous fluoropolymers Vinyl polymer, semicrystalline fluoropolymers Polyvinyl acetate and ethylene vinyl acetate copol5Tner... 

Very saline samples and/or the addition of organic solvents to the samples were used in early applications in order to provide high refractive indices [88], but the applicability of the LCW flow cell was limited because, in general, only dilute aqueous samples with low refractive indices have been analysed. The development of different kinds of amorphous fluoropolymers (Teflon AFs) with refractive indices lower than that of pure water enabled the design and overall acceptance of flow cells relying on LCWs [89,90]. 

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