Molecular modeling of elastic properties of thermosetting polymers using a dynamic deformation approach

This paper employs fully atomistic molecular dynamics simulations to characterize relationships between structural and elastic properties of thermosetting polymers both in glassy and rubbery state. The polymer system investigated consists of epoxy resin DGEBA and hardener DETDA. An effective cross-linking procedure that enables generation of thermoset structures containing up to 35000 atoms with realistic structural characteristics was used. A dynamic deformation approach has been used that takes into consideration both potential energy and thermal motions in the structure. Small uniaxial, volumetric and shear deformations were applied to the systems to obtain elastic moduli. A method to independently determine Poisson's ratio was proposed that reduces statistical errors and circumvents the time scale limitations of molecular dynamics simulations. The influence of variables such as extent of curing and length of epoxy strands on elastic response at various temperatures was explored. Expected trends in the dependence of the elastic constants on these practical process parameters were shown. The relationship between the four independently calculated elastic constants was seen to comply with those predicted by the classical theory of linear elasticity in an isotropic medium, which provides confidence in the validity of these simulations. Moreover, the elastic properties obtained are also in good agreement with experimental data reported in the literature. Close agreements between predicted elastic constants and experimentally measured values underscore the ability of the approaches used in this study to provide realistic predictions of the mechanical response of thermosetting polymers, both in glassy and rubbery states. These results show significant improvement over earlier studies based on a static approach which takes into account the potential energy contribution to the elastic response but ignores temperature effect.