Chain Motions in Solid State Poly Tetrafluoroethylene

Disordering Chain Motions in Solid State Poly(Tetrafluoroethylene) 

Two temperatures were simulated 273 K at which PTFE is in Phase II and 298 K at which it is in Phase IV The canonical ensemble (constant temperature, constant volume) was used. Owing to the large number of atoms (-3500 including dummy atoms), the cell multipole method was chosen for the calculation of long-range van der Waals and electrostatic interactions. For the latter, a dielectric constant of 1.0 was used. 

The simulation results indicate that helix reversals form and migrate easily in PTFE crystal structures. Their motion in neighboring chains can be coupled. In simulations at both temperatures, the reversal bands formed mostly at chain ends where torsional motion was relatively unrestricted, but also were observed to form in the center of some of the helices. This latter point of origin, though more energetically and mechanistically difficult (because it involves coordinated motion 

Reorientation about the chain axes has been postulated previouslyand has been observed by NMR and neutron scattering techniques. jg 

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. 

---