Amorphous fluoropolymer properties

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

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,... 

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... 

Perfluorinated dioxole monomers have been used to prepare a series of amorphous fluoropolymers such as Teflon AF and Hyflon AD. A third amorphous fluoropo-lymer, Cytop contains perfluorotetrahydrofuran and perfluorotetrahydropyran rings, but is prepared in a cyclopolymerization process from an acyclic monomer. These amorphous fluoropolymers retain the outstanding chemical, thermal, and surface properties associated with perfluorinated polymers while also having unique electrical, optical, and solubility characteristics. 

Hyflon AD amorphous fluoropolymer is used in optical devices, pellicles in semiconductor manufacture, as a dielectric and as a separation membrane. Small amounts of TDD have been used as a modifier in ethylene-chlorotrifluoroethylene polymers to increase stress crack resistance. Minute amounts of TDD are used also as a modifier in polytetrafluoroethylene to improve elastic modulus, reduce creep and permeability and increase transparency. It has been suggested that the much higher reactivity of TDD and other fluorinated dioxoles relative to other modifiers gives a more uniform distribution of the modifier in the polymer chain that results in a greater increase in the desired properties at lower concentration of modifier in the polymer. 

All three commercial amorphous fluoropolymers. Teflon AF, Hyflon AD, and Cytop posses a unique set of properties. All dissolve in fluorinated solvents and thus may be spin coated to produce thin hlms and coatings. The polymers may also be extruded and molded using traditional polymer processing techniques. Note that the polymers are not soluble in hydrocarbon solvents or water and retain the chemical and thermal stability of perfluorinated polymers such as Teflon . These polymers have lower density than the well-known semicrystalline perfluorinated polymers such as pTFE that results in lower refractive index, lower thermal conductivity, higher gas permeability, and lower dielectric constant. The polymers are transparent and have excellent mechanical properties below their Tg due to their amorphous character. The presence of a heterocyclic ring in the polymer backbone of these materials is key... 

Copolymers of tetrafluoroethylene and 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) are perfluorinated amorphous polymers and possess unusual combination of properties. They retain the outstanding chemical, thermal, and surface properties of perfluorinated polymers in addition to having excellent electrical and optical properties and have solubility at ambient temperature in a normal fluorosolvent. This family of copolymers is manufactured by DuPont and sold under the trade name of Teflon AF, amorphous fluoropolymers. 

These amorphous fluoropolymers are chemically as well as thermally stable, soluble in fluorinated solvents, have low dielectric constants, and the films are transparent. They have unique properties compared to traditional fluoropolymers. The amorphous polymers have high potential in many applications. The following are representative examples that are being pursued polymer waveguides [24,25], pellicles used in the photolithographic reproduction of semiconductor integrated circuits [26], insulators and hydrophobic surfaces for electrowetting [27,28], polymer optical fibers [29,30], and membranes for gas separations [31-33], Here, we describe two examples of the use of the amorphous perfiuorinated polymers optical fibers and gas separation membranes. 

A number of fluoropolymers have been extensively investigated since Roy Plunkett discovered Teflon in 1937. Polytetrafluoroethylene (PTFE) exhibits remarkable electric, chemical, thermal, and mechaiucal properties. Most PTFE-based fluoropolymers arc either crystalline or semicrystalline. However, the Teflon AF series is a family of amorphous polymers that was reported in the literature (Resnick, 1976) because these polymers also showed the desirable electric, chemical, thermal, and mecharucal properties similar to semicrystaUine fluoropolymers. These amorphous fluoropolymers have uruque physical properties such as high... 

Several recent studies report gas permeation, sorption, and free volume characteristics of these two amorphous fluoropolymers. Nemser and Roman (1991 Nemser, 1993) reported permeability properties of Teflon AF polymer membranes prepared by a melt-press method. Similar to PTMSP, these polymers are permeable to permanent gases. For example, oxygen... 

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

The irradiation of fluoropolymers at elevated temperatures has been explored for the development of materials with better mechanical properties [35]. This arises because of the radiation-induced crossUnking of chains and subsequent higher network density in the resultant polymer [36]. Here, the irradiation is accomplished at a temperature higher than the melting point of the polymer. In the molten state, the polymer behaves as an amorphous matrix and the mobility of molecular chains is considerably enhanced. This promotes the mutual recombination of radicals, i.e., crossHnking involving chain end radicals and chain alkyl radicals [37]. 

The physical properties of these perfluoro polymers are summarized in Table 16.2. These perfluoropolymers are completely amorphous and contain no hydrogen atoms, show excellent chemical and thermal stability, and are soluble in fluorinated solvents such as hexafluorobenzene (HFB) and perfluorohexane. The dielectric constants of these fluoropolymers are considerably low, and they are almost unaffected by humidity. 

---