Modeling of helix reversal defects in polytetrafluoroethylene I. Force field development and molecular mechanics calculations

A force field suitable for modeling fluoropolymers and oligomers in the solid state has been derived from MOPAC semiempirical molecular orbital calculations on perfluorohexadecane. A conformational energy profile was generated using the PM3 Hamiltonian, and then valence parameters of a molecular mechanics (MM) energy expression, including a six-term cosine dihedral potential, were adjusted with a nonlinear least squares fitting algorithm to reproduce the profile. Minimum energy helical conformations of 48/22 and 13/6 were obtained when the geometries of C60F122 molecules in isolation and in a crystalline cluster, respectively, were optimized using the refined force field. The X-ray diffraction pattern calculated from the crystalline cluster indicated an equatorial d-spacing of 4.9685 Angstrom. These intra and intermolecular structural characteristics for the cluster are in agreement with experimental X-ray diffraction data. Energy penalties of helix reversal defects in isolated chains and in a crystalline environment were also investigated. (C) 1999 Elsevier Science Ltd. All rights reserved.