Strained fluorocarbon

An excellent overview on the unusual and unique chemical phenomena of highly strained fluorocarbons and fluoroorganic reactive intermediates Lemal, D.M. (2004) /. Org. Chem., 69, 1-11. 

Potential oxidizers based on highly strained fluorocarbon compounds have been investigated by the author earlier [8, 9]. From this, the fluorofullerene C60F48 should be the most interesting compound with respect to both energy content and reactivity. The author has predicted stratified carbon compounds with alternating NF2-substitutents as possible oxidizers in pyrolants [10]. 

FIGURE 31.13 (a) Plot showing the stress-strain behavior of various irradiated rubbers, (b) Plot showing the variation of tensile strength and modulus of rubbers irradiated with different doses, (c) Plot showing the variation of hysteresis loss, set, and elongation at break of irradiated fluorocarbon rubbers. (From Banik, I. and Bhowmick, A.K., Radial. Phys. Chem., 54, 135, 1999. With permission.)... 

Here is another instance of the extreme model dependency of strain energies in the fluorocarbon series. A strainless C(F)2(C)2 group increment of - 104.9 kcal mol" had earlier been derived ". In principle, one might expect that this increment would be applicable to calculating the strain energy of hexafluorocyclopropane. Applying this value, we concluded that the total strain in the molecule was 80.9 kcal mol" This was even higher than the value calculated by Bernett some years earlier 68.6 kcal mol" We were buoyed by the fact that the endothermicity of equation 18 indicated a strain of... 

Koroniak, H. The Thermoisomerisation of Fluorocarbons Containing Strained Three and Four Membered Rings Adam Mickiewicz University Press Poznan, 1986 Ser. Chem. 51, pp76-78. 

As the hydration of the individual exchange sites decreases with heavier cations, the elastic strain of the fluorocarbon matrix associated with hydration will also decrease. This will make it possible for additional clustering to occur, with an associated increase in elastic strain, until thermodynamic equilibrium is achieved with the external solvent. 

In addition, highly perfluoroalkyl-substituted molecules are kinetically and thermodynamically stable due to sterical protection by the bulky perfluoroalkyl groups from any external attacks. Comparison of the half-lives of thermal isomerization of the perfluoroalkylated strained molecules (28 and 30) with those of the parent hydrocarbons (29 and 31) reveals an exceptionally high stability of these fluorocarbons (Scheme 1.38) [10]. Both the strengthening C—C bond and the steric protection of the inner carbon skeleton by perfluoroalkyl groups are responsible. 

The C—F bond energy is indeed very high (486 kJ mol-1 cf. C—H 415, and C—Cl 332 kJ mol"1), but organic fluorides are not necessarily particularly stable thermodynamically rather, the low reactivities of fluorine derivatives must be attributed to the impossibility of expansion of the octet of fluorine and the inability of, say, water to coordinate to fluorine or carbon as the first step in hydrolysis, whereas with chlorine this may be possible using outer d orbitals. Because of the small size of the F atom, H atoms can be replaced by F atoms with the least introduction of strain or distortion, as compared with replacement by other halogen atoms. The F atoms also effectively shield the carbon atoms from attack. Finally, since C bonded to F can be considered to be effectively oxidized (whereas in C—H it is reduced), there is no tendency for oxidation by oxygen. Fluorocarbons are attacked only by hot metals, e.g., molten sodium. When pyrolyzed, fluorocarbons tend to split at C—C rather than C—F bonds. 

The stress-strain diagram is used to find the modulus of elasticity in the lateral direction in ferroelectret Aims. The modulus of elasticity is determined as the slope of the initial portion of the curves. Since flie modulus of elasticity depends on the strain, it is customarily found for strains not exceeding 0.5 %. Figure 3 shows the stress-strain curves of laminated fluorocarbon ferroelectret films in lateral direction at various temperatures measured by a DMA instrument (TA Instruments DMA Q800) (Sunetal. 2011). 

Figure 7.9 Applied strain energy density, L dc, at onset of crack growth versus reciprocal of the crack length, from simple-extension test specimens [103]. (a) Cross-linked SBR/fluorocarbon copolymer tested at 100 °C. (b) Crosslinked SBR/poly(ethylene terephthalate) tested at 50 °C. 

Many years of experience with sliding plate rheometers have revealed several phenomena that limit their utility under certain conditions. First, the normal stress differences create a pressure gradient in the sample that tends to pump melt in from the ends of the sample and out toward the edges. [ 147-149]. This flow can be prevented by the use of fluorocarbon side- rails. More serious limitations are imposed by slip, cavitation and rupture, which interrupt experiments at sufficiently high strains and strain rates. At the same time, however, sliding plate rheometers have been found to be useful tools for the study of melt slip [150]. Elastomers are particularly resistant to shearing deformations and even the use of deep grooves in the plates does not ensure their adherence [146]. 

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