Density-functional calculations of the elastic properties of some polymer chains

The force constants and elastic properties of some polymers are calculated with a full-electron, crystal-orbital density functional method. Calculations are carried out on single helical chains of polyethylene, polytetrafluoroethylene (teflon), polyglycine (nylon-2), and nylon-3. The longitudinal elastic moduli are obtained both from the Hessian and by direct elongation relaxations. The different degrees of freedom of the polymers are ranked according to their contributions to the elastic response. It is shown, that a proper choice of the internal coordinates makes it possible to keep some of them frozen during the process without a considerable loss of elasticity.