CONFORMATIONAL CHARACTERISTICS OF POLY(TETRAFLUOROETHYLENE) CHAINS BASED UPON AB-INITIO ELECTRONIC-STRUCTURE CALCULATIONS ON MODEL MOLECULES

Conformational characteristics of poly(tetrafluoroethylene) (PTFE) chains have been studied in detail, based upon ab initio electronic structure calculations on perfluorobutane, perfluoropentane, and perfluorohexane. The conformational energy contours confirm the well-known minima of the trans states at +/- 17-degrees but also show that the gauche states split as well, with minima at +/- 124-degrees and +/- 84-degrees, in order to relieve steric crowding. The directions of such split distortions from the perfectly staggered states are strongly coupled for adjacent pairs of bonds in a manner identical to the intradyad pair of poly(isobutylene) chains. These conformational characteristics are fully represented by a six-state rotational isomeric state (RIS) model comprised of t+, t-, g++, g+-, g-+, and g-- states, located at the split energy minima. The resultant 6 X 6 statistical weight matrix is described by two first-order interaction parameters and two second-order interaction parameters; the ab initio estimated first-order interaction energies are ca. 0.6 kcal/mol for the g++ states and ca. 2.0 kcal/mol for the g+- states, while the second-order interaction energies are ca. 0.6 kcal/mol for the g++g++ states and ca. 1.0 kcal/mol for the g++g-+ states, according to MP2 level ab initio calculations using a D95+* basis set. This six-state RIS model, with no adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions, and the fraction of gauche bonds as a function of temperature, in good agreement with available experimental values. Compared with the previous four-state RIS model of Bates and Stockmayer, our six-state model differs significantly in that it fully takes into account the strong coupling of conformations for adjacent bond pairs, which leads to important differences in the nature and the magnitude of the interactions involving the gauche conformations between the two models.