Molecular simulation, fluoropolymers

The impressive development in molecular simulation stems from important improvement in both codes and computers. It made available accurate atomistic simulation of fluorine derivatives. Depiction of the SPF is most commonly achieved by quantum calculations. The major difficulty in describing interactions involving fluorine atoms actually lies in a correct description of the electrostatic effects [38]. Crystal unit cell dimensions [39,40] and the thermal behavior of fluoropolymers [41 3] were thus originally difficult to reproduce. Dihedral potential energy (Equation 6.3) that plays a central role in the backbone dynamics was also incorrectly depicted. In this section, illustrative examples of force fields specifically dedicated to fluoropolymers and more transferable force fields are reviewed. 

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. 

Litinskii et al. investigated the contact between the fluoropolymer SKF 32 (fluorine-containing rubber) and three oxide surfaces molybdenum oxide and solid solutions of molybdenum-niobium oxides and molybdenum-technetium oxide [115], Contact appeared when a bond was created between a metal atom and a carbon that lost its fluorine atom. Molecular orbitals on clusters simulating the contact between the fluoropolymer and the oxide surfaces have been computed using the DFT B3LYP/3-21g. Nine contacts between the three available metal atoms (molybdenum, niobium, and technetium) and the three carbon atoms of the fluoropolymer have been studied. Electronic spectra revealed that the molybdenum atom leads to the most favorable contact with the SKF 32 fluoropolymer. A similar analysis has been carried out with ferric and nickel oxides [116]. 

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