Fluoropolymer chains

Other researchers have attempted to coat aluminum particles directly with an acid, creating fluoropolymer chains that attach to the alumina shell surrounding the aluminum core particle, as illustrated in Figure 15.1 [15]. The acid shell accounts for a small percentage of the total mixture such that the mixture also requires another oxidizer to balance the reaction. This approach is referred to as surface functionalization of aluminum particles and has shown great success in controlling the reactivity of a mixture [15]. 

The types of polymers that are used as release coatings include silicone networks, silicone containing copolymers, polymers with long alkyl or fluoroalkyl side chains, fluoropolymers, and polyolefins. These polymers have surface energies that are less than the surface energies of commonly used PSAs, an important feature of release materials. 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Since fluorocarbon elastomers being discussed here contain hydrogen in their molecules, they have the tendency to cross-link in addition to scission, common in fluoropolymers when exposed to radiation. The cross-linking predominates, but there is still a significant degree of chain scission. ... 

Teflon is the brand name for the chemical compound called polytetrafluoroethylene (PTFE). One molecule of PTFE contains two carbon atoms bonded to four fluorine atoms. Many molecules of PTEE can form polymer chains known as fluoropolymers. 

The shortcomings of the force field notwithstanding, these preliminary molecular dynamics simulations indicate that modeling of the chain motions of crystalline fluoropolymers and their interactions is sure to be quite rewarding. Further, the results suggest that with additional refinement of the force field. 

A force field for solid state modeling of fluoropolymers predicted a suitable helical conformation but required further improvement in describing intermole-cular effects. Though victory cannot yet be declared, the derived force fields improve substantially on those previously available. Preliminary molecular dynamics simulations with the interim force field indicate that modeling of PTFE chain behavior can now be done in an all-inclusive manner instead of the piecemeal focus on isolated motions and defects required previously. Further refinement of the force field with a backbone dihedral term capable of reproducing the complex torsional profile of perfluorocarbons has provided a parameterization that promises both qualitative and quantitative modeling of fluoropolymer behavior in the near future. 

Most types of polymerizations can be performed in liquid and supercritical C02. The two major types of polymerizations, chain-growth and step-growth, have been demonstrated in C02. Reviews in the literature (Canelas and DeSimone, 1997b Kendall et al., 1999) have described numerous polymerizations in C02, many of which will not be discussed in this chapter. Since only amorphous or low-melting fluoropolymers and silicones show appreciable solubility at relatively mild temperatures and pressures (T< 100 °C, P<400 bar), only these two classes of polymers can be synthesized by a homogeneous polymerization in C02. All other types of polymers, including semicrystalline fluoropolymers and lipophilic or hydrophilic polymers, must be made by heterogeneous methods, such as precipitation, dispersion, emulsion, and suspension, since the polymers are insoluble in C02 (when T< 100 °C and P<400 bar). Some semicrystalline fluoropolymers and hydrocarbon polymers can be dissolved at more extreme temperatures and pressures and are discussed in Chapter 7 of this book. 

This chapter is divided into three main sections fluoropolymer synthesis, nonfluorinated polymer synthesis, and polymer characterization in C02. In the fluoropolymer synthesis section, solution, precipitation, biphasic, and continuous polymerizations will be described. Several examples of nonfluorinated heterogeneous chain-growth polymer syntheses will be followed by step-growth polymerizations. A brief summary of polymer characterization methods in C02 will conclude the chapter. 

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