Polytetrafluoroethylene repeating unit

A very important chain polymer from the vinylidene class [195,197] is polytetrafluoroethylene (PTFE), which is widely applied in cooking pots, as a sealing material, and also has other uses. In Figure 2.37, the repeating unit in the case of PTFE is shown [8],... 

Polymers with Tg/Tm ratios below 0.5 are highly symmetrical and have short repeating units consisting of one or two main-chain atoms each, carrying substituents consisting of only a single atom (polymethylene, polyethylene, polytetrafluoroethylene, polymethylene oxide). They are markedly crystalline. 

Like polyethylene, polytetrafluoroethylene (PTFE) has a very simple chemical repeat unit, (-CFz-CF2-)n, and its discovery is a great story that in large part depended on Lady Luck, as opposed to design. Dr. Roy Plunkett, who obtained his Ph.D. from The Ohio State University, was hired by DuPont in 1936 and assigned to work on a new Freon -type refrigerant CClF2CHFr In order to synthesize this compound, he needed to prepare about 100 pounds of tetrafluoroethylene gas (TFE or CF2=CF2) as an intermediate. This he did with the aid of his technician,... 

Figure 5.1. Molecular structures of the chemical repeat units for common polymers. Shown are (a) polyethylene (PE), (b) poly(vinyl chloride) (PVC), (c) polytetrafluoroethylene (PTFE), (d) polypropylene (PP), (e) polyisobutylene (PIB), (f) polybutadiene (PBD), (g) c/5-polyisoprene (natural rubber), (h) traw5-polychloroprene (Neoprene rubber), (i) polystyrene (PS), (j) poly(vinyl acetate) (PVAc), (k) poly(methyl methacrylate) (PMMA), ( ) polycaprolactam (polyamide - nylon 6), (m) nylon 6,6, (n) poly(ethylene teraphthalate), (o) poly(dimethyl siloxane) (PDMS).

Polymers with Tm>2 Tg are highly symmetrical and have short repeat units consisting of one or two chain backbone atoms, each carrying either no substituents at all or single-atom substituents. These polymers can manifest very high crystallinity. Examples include polyethylene, polyoxymethylene and polytetrafluoroethylene. 

When Y is not H, X and Y may be the same type of atom or group, as with poly(vinylidene chloride) (X and Y are Cl), or they may differ, as in poly-(methyl methacrylate) (X is —CH3, Y is —COOCH3) and poly(a-methyl styrene) (X is —CH3, Y is — ). When the substituents are small, polymerisation of a tetra-substituted monomer is possible, to produce a polymer such as polytetrafluoroethylene (PTFE), with the repeat unit )CF2—CF2, but if large substituents are present on both carbon atoms of the double bond there is usually steric hindrance to polymerisation, i.e. the substituents would overlap each other if polymerisation took place. 

The polymers formed from symmetric monomer units, such as polyethylene, (CH2CH2) and polytetrafluoroethylene, (CF2CF2) are an exception to this notation. Although the simplest repeat units are the —CHj— and —CF2— groups, we show two methylene groups and two difluoromethylene groups because they originate from ethylene (CH2= H2) and tetrafluoroethylene (CF2= F2), the monomer units from which these polymers are derived. 

The chemistry of polyimides has previously been reported but the repeat unit of polytetrafluoroethylene is based upon the structure... 

Fluoropolymers have outstanding chemical resistance, low coefficient of friction, low dielectric constant, high purity, and broad use temperatures. Most of these properties are enhanced with an increase in the fluorine content of the polymers. For example, polytetrafluoroethylene, which contains four fluorine atoms per repeat unit, has superior properties compared to polyvinylidene fluoride, which has two fluorine atoms for each repeat unit. Generally, these plastics are mechanically weaker than engineering polymers. Their relatively low values of tensile strength, deformation under load or creep, and wear rate require the use of fillers and special design strategies. 

The size bf the dynamical matrix, hence the order of the secular equation, to be solved for each k, is tractable for simple polsrmers (Polyethylene). But the size becomes soon untractabie for just slightly more complicates systems. For instance, the crystallographic repeat unit of isotactic polypropylene contains three monomer units and the resulting secular determinant has dimension 81. For polytetrafluoroethylene in the most stable 15/7 structure the dispersion relation contains 135 branches 144 are the branches for isotactic polystyrene. Each of the secular equation... 

Polytetrafluoroethylene (Teflon, TFE) with the repeat unit —(CF2—CF2)— is another example of a thermosetting material, as the close packing and extensive secondary bonding of the main chains prevents flow when the polymer is heated. 

Reese and Tucker (196 uMasured the 1 to 4.5° K Ireat cai ity of this pol3m)er. Ho nan (1952) measured the higgler temperature heat capacity between 270 and 520° K. The repeating unit of polychlorotri-fiuoroethylene is CFCICF2, with a formula wdg t of 116.47 g, so that chemically its relation to polytetrafluoroethylene k similar to that of poly(vinyl chloride) to pcdyethylene. 

Draw repeat units for polyethylene, poly(vinyl chloride), polytetrafluoroethylene, polypropyl ene, and polystyrene. 

Figure 14.2 Repeat unit and chain structures for (a) polytetrafluoroethylene,...

Polytetrafluoroethylene is a completely fluorinated polymer manufactured by free-radical polymerization of tetrafluoroethylene. With a linear molecular structure of repeating -CF2—CF2- units, PTFE is a crystalline polymer with a melting point of 326.7°C. Its specific gravity is 2.13—2.19. Polytetrafluoroethylene has exceptional resistance to chemicals. Its dielectric constant (2.1) and loss factor are low and stable across a wide range of temperature. It has useful mechanical properties from myogenic temperatures to 260°C. In the United States, PTFE is sold as Halon, Algoflon, Teflon, Fluon, Hostaflon, and Polyflon. ... 

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