Vibrational Analysis of Polytetrafluoroethylene

Abstract

With the aid of newly obtained Raman data, a normal coordinate analysis has been performed on the polytetrafluoroethylene (PTFE) molecule. Calculations were made for three different helix models including the 13/6, 15/7, and planar zigzag. The frequencies corresponding to the 15/7 helix were fitted by adjusting the force field. The conformation was changed, and the changes in the frequencies for the 13/6 helix and the planar zigzag were calculated using the same force field. Changes predicted for the 13/6 helix from the normal coordinate calculations are not in agreement with those actually observed at the 19°C transition. It was therefore hypothesized that the changes in the infrared and Raman spectra at this transition were due to Raman thermal defects as suggested by Brown. The potential‐energy distribution was obtained for the optically active modes of the 15/7 helix, and the eigenvectors were presented for the motions of the A modes. The dispersion curve for the 15/7 helix is presented, and the coupling which occurs between the chain stretch and the CF stretching modes is discussed.