Disordered Chain Conformations of Poly(tetrafluoroethylene) in the High-Temperature Crystalline Form I

Abstract

Model conformations of polytetrafluoroethylene (PTFE) chains containing various amounts of helix reversal defects suitable for the high-temperature form I of PTFE have been modeled using semiempirical methods implemented in the Gaussian package, which makes use of the PM3 Hamiltonian. In these disordered conformations, ordered portions of chains in right- and left-handed 15/7 helical conformation succeed each other statistically along the chain. The Fourier transform of model chains containing various amounts of helix reversal defects obtained through the QM approach are then compared with the experimental X-ray fiber diffraction patterns of PTFE. Straight and slim model chains of PTFE containing helix reversal defects may be obtained at a low cost of internal energy and with small lateral encumbrance. For the minimum energy conformers the defect is always localized into a small region involving only 4 −CF2− units, with the internal variables placed at the junction between the two enantiomorphic portions of chain, deviating only slightly from their average values in the defect-free portions of chains. Two consecutive helix reversals do not interact with each other if they are separated by more than three dihedral angles, and their contributions to internal energy can be considered as additive. The energy cost is ∼2.3 kcal/mol for each reversal. The disordered conformations of PTFE chains account for the X-ray fiber diffraction patterns of form I of PTFE at temperatures higher than 30 °C. With increasing temperature an increase of the frequency of helix reversal defects is observed according to predictions of QM energy and Fourier transform calculations.