Molecular dynamics in poly- and oligotetrafluoroethylenes

Abstract

Poly- and oligotetrafluoroethylenes exhibit at room temperature a high molecular mobility in the crystalline state. A detailed analysis of the molecular motion is possible by measuring the static and dynamic structure factor using x-ray and neutron scattering. The onset of diffusive molecular dynamics coincides in poly- and oligotetrafluoroethylenes with an orderdisorder phase transition. — In PTFE the prevalent relaxational mode is a double helix reversal. Occurrence of helix reversals leads to a characteristic change in the static structure factor S(q), indicating a loss of azimuthal correlation within the chain. A quasieelastic line in the dynamic structure factor S(q, ω) can be associated with motion of helix reversals. Six-fold symmetry (around the chain axis) of the intensity distribution of the diffusive scattering indicates an intermolecular correlation of rotational motion. — In C20F42 the phase transition at 200 K is mainly connected with the onset of rotational motion. Additionally, longitudinal diffuse motion sets in at the phase transition. This mode shows an increasingly collective character on approaching the phase transition.