Periodic and High-Temperature Disordered Conformations of Polytetrafluoroethylene Chains: An ab Initio Modeling

Abstract

We report here the main results of a successful attempt to predict some macroscopic properties of representative polymers of technological relevance both in regular and disordered forms by using first principle quantum mechanical approaches at microscopic level. Until now, the prediction of the structural and thermal properties of those polymers has been mostly a domain of molecular mechanics methods. To overcome the limits of those classical computational tools whenever physical properties are significantly influenced by stereoelectronic effects (e.g., electron rich substituents), we employed methods rooted in the Density Functional Theory (DFT). A general computational strategy including the proper choice of periodic boundary conditions (PBC), functional, basis set, and model system size, has been tested and validated for saturated polymers such as polyethylene and isotactic/syndiotactic polypropylenes. On the basis of these results, a comprehensive study of poly(tetrafluoroethylene) (PTFE) chains in both regular periodic and disordered conformations has been performed. A statistical approach has been next applied to obtain the thermal concentration of defects and to reproduce the thermal behavior of the investigated polymer. At the end, a very good agreement with experimental X-ray diffraction and IR results has been achieved, definitely reaching a good understanding of the widely studied disorder phenomenon determining the main technological properties of poly(tetrafluoroethylene) (the trade Teflon) and, at the same time, identifying the proper computational tools to investigate perfluoro-compounds and other complex polymeric systems.