Chain Reorientation in Poly(tetrafluoroethylene) by Mobile Twin-Helix Reversal Defects

Abstract

Analysis of measurements of the static scattering function by electron and X-ray diffraction, of the dynamic scattering law by neutron scattering, and of infrared and Raman spectra points at the important role of twin helix-reversal defects for the molecular dynamics in the intermediate phase and high-temperature phase (modifications IV and I) of poly(tetrafluoroethylene). In contrast to single helix-reversal defects, these twin defects constitute a local perturbation which can move along the chain without affecting remote units. One species, the ''coherent'' twin defect, possesses a particularly low formation energy, since it retains the twist long-range order along the chain. Formation of the defects occurs mainly in the intermediate phase. In the high-temperature phase a saturation is observed. The steep increase in the defect concentration in the intermediate phase followed by the equilibration in the high-temperature phase can be understood as a transition between a disordered state controlled by intra- and intermolecular forces to one which is only determined by the intramolecular potentials. In spite of the disorder in the helices, an orientational short-range order is retained for sequences in adjacent chains. The degree of order is related to the defect concentration. The reorientation of the CF2 group resulting from the formation and motion of the twin defects occurs with rates in the range of 10-100 GHz. Orientational short-range order leads to a slowing down.