A Quantum Chemistry Based Force Field for Perfluoroalkanes and Poly(tetrafluoroethylene)

An ab initio quantum chemistry investigation of conformational energetics of C4F10, C5F12, and C6F14 revealed that the relative energies of gauche and ortho conformers relative to anti conformer were molecular weight independent for C4F10−C6F14 whereas the gauche ↔ ortho and ortho ↔ anti barriers monotonically increased with increasing molecular weight. An ab initio quantum chemistry based force field has been developed for perfluoroalkanes and poly(tetrafluoroethylene) (PTFE). The same set of torsional parameters accurately described conformational energetics of C4F10, C5F12, and C6F14 including gauche and anti splits, indicating transferability of the developed force field to longer perfluoroalkanes. Molecular dynamics (MD) simulations of various perfluoroalkanes ranging from C4F10 to C20F42 using the developed force field yielded densities within 2% of experimental values, predicted enthalpies of vaporization within 10% of experimental data, and viscosities within 25% of experimental values. The static structure factor for C9F20 at 295 and 373 K from MD simulations was found to be in excellent agreement with experimental X-ray measurements. The combined fraction of gauche and ortho conformers for C5F12−C7F16 between 15% and 18% at 298 K predicted by the developed force field is in good agreement with the fraction of 19−24% from the analysis of D-LAM Raman bands of liquid perfluoroalkanes ranging from C9F20 to C20F42 at 300 K. In contrast to the developed force field, the general OPLS-AA force field predicted a twice as low gauche fraction of only 7.2% for C6F14. The rotational isomeric state (RIS) model based on conformational populations of C5F12 from MD simulations accurately reproduced the mean-square end-to-end distance for C20F42 for the developed force field and the OPLS-AA force field. The RIS model for the developed force field predicted a PTFE characteristic ratio of 15, higher than the value of 8 ± 2.5 from the light scattering experiments, whereas the general OPLS-AA force field yielded a much higher characteristic ratio of 28.