CHAIN REORIENTATION IN POLY(TETRAFLUOROETHYLENE) BY MOBILE TWIN-HELIXREVERSAL DEFECTS

Analysis of measurements of the static scattering function by electron and X-ray diffraction, of the dynamic scattering law by neutron scatt ering, and of infrared and Raman spectra points at the important role of twin helix-reversal defects for the molecular dynamics in the inter mediate phase and high-temperature phase (modifications IV and I) of p oly(tetrafluoroethylene). In contrast to single helix-reversal defects , these twin defects constitute a local perturbation which can move al ong the chain without affecting remote units. One species, the ''coher ent'' twin defect, possesses a particularly low formation energy, sinc e it retains the twist long-range order along the chain. Formation of the defects occurs mainly in the intermediate phase. In the high-tempe rature phase a saturation is observed. The steep increase in the defec t concentration in the intermediate phase followed by the equilibratio n in the high-temperature phase can be understood as a transition betw een a disordered state controlled by intra- and intermolecular forces to one which is only determined by the intramolecular potentials. In s pite of the disorder in the helices, an orientational short-range orde r is retained for sequences in adjacent chains. The degree of order is related to the defect concentration. The reorientation of the CF2 gro up 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.