Polytetrafluoroethylene structure

If a polymer molecule has a sufficiently regular structure it may be capable of some degree of crystallisation. The factors affecting regularity will be discussed in the next chapter but it may be said that crystallisation is limited to certain linear or slightly branched polymers with a high structural regularity. Well-known examples of crystalline polymers are polyethylene, acetal resins and polytetrafluoroethylene. 

Ethylene vinyl acetate has also found major applications in drug delivery. These copolymers used in drug release normally contain 30-50 wt% of vinyl acetate. They have been commercialized by the Alza Corporation for the delivery of pilocarpine over a one-week period (Ocusert) and the delivery of progesterone for over one year in the form of an intrauterine device (Progestasert). Ethylene vinyl acetate has also been evaluated for the release of macromolecules such as proteins. The release of proteins form these polymers is by a porous diffusion and the pore structure can be used to control the rate of release (3). Similar nonbiodegradable polymers such as the polyurethanes, polyethylenes, polytetrafluoroethylene and poly(methyl methacrylate) have also been used to deliver a variety of different pharmaceutical agents usually as implants or removal devices. 

Polytetrafluoroethylene (Tf) is a polymeric fluorine compound that consists of a -C2F4- molecular structure,ini which contains a mass fraction of fluorine of 0.75. Tf is insoluble in water and its specific mass is in the range 3550-4200 kg nr in peUe-... 

The ignition temperature of a mixture of Ti and C is relatively high compared with those of other pyrolants. When a small amount of polytetrafluoroethylene (Tf) is added to a Ti-C pyrolant, the ignition temperature is significantly lowered due to the exothermic reaction between Ti and Tf Since Tf consists of a -C2F4- chemical structure, the oxidizer gas, F2, is formed by thermal decomposition of Tf according to ... 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

The general structure of this class of materials can, therefore, be summarized as a fine dispersion of metal oxide in a polymer matrix very similar to plasma polytetrafluoroethylene and in principle any metal should be able to be incorporated. Clearly, if the films are protected from the atmosphere, for metals which form involatile fluorides having a relatively weak metal-fluorine bond strength, it should be possible to produce films having metal atoms dispersed in the matrix. It is expected that these films will have many interesting chemical, optical, electrical and magnetic properties., ... 

Polytetrafluoroethylene (PTFE) is an attractive model substance for understanding the relationships between structure and properties among crystalline polymers. The crystallinity of PTFE (based on X-ray data) can be controlled by solidification and heat treatments. The crystals are large and one is relieved of the complexity of a spherulitic superstructure because, with rare exceptions, spherulites are absent from PTFE. What is present are lamellar crystals (XL) and a noncrystalline phase (NXL) both of which have important effects on mechanical behavior. 

Conformational energy estimates are employed to determine the conformational characteristics of polyfvinyl fluoride) (PVF), polyfluoromethylene (PFM), and polytrifluoroethylene (PTF3). Effects of stereoconfiguration and, in the case of PVF and PTF3, the presence of head-to-head tail-to-tail (HH TT) defect structures are considered. The calculated results are compared to corresponding values found for polyfvinylidene fluoride), polytetrafluoroethylene, and polyethylene, and the equilibrium flexibilities of PVF, PFM, and PTF3 are discussed on this basis. 

Infra-red dichroism has been used in studies of polymer specimens in which the chain molecules are parallel to each other, to give evidence on the orientation of particular atomic groups. In nylon and polyvinyl alcohol (Ambrose, Elliott, and Temple, 1949), Terylene (Miller and Willis, 1953), and polytetrafluoroethylene (Liang and Krimm, 1956) the results are consistent with structures already established by X-ray methods. Turning to more complex structures not yet solved in detail by X-ray methods, infra-red dichroism has indicated that in cellulose... 

Polytetrafluoroethylene (PTFE) has a chemical structure which can be designated by (CF2)k. From its resemblance to the chemical structure of polyethylene it might be thought that the spectra of these two polymers should be quite similar. They do in fact resemble each other, but there are also important differences. This is a consequence of the fact that the PTFE chain configuration is quite different from that of polyethylene, and also the intramolecular forces are undoubtedly significantly different in the two cases. As we shall see, the spectrum is moderately well understood, but not in quite as great detail as that of polyethylene. This is primarily a result of the lack of Raman data on the polymer and certain key polarization data in the infrared. 

Fig. 8 a—c. Structure of polytetrafluoroethylene. a) Twisted carbon backbone, b) Side and end views of molecule, c) Symmetry relations between CF2 groups [Liang and Krimm (11 /)]... 

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