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. Calculatio ns are carried out on single helical chains of polyethylene, polytetrafluor oethylene (teflon), polyglycine (nylon-2), and nylon-3. The longitudinal el astic moduli are obtained both from the Hessian and by direct elongation re laxations. The different degrees of freedom of the polymers are ranked acco rding to their contributions to the elastic response. It is shown, that a p roper choice of the internal coordinates makes it possible to keep some of them frozen during the process without a considerable loss of elasticity.