Molecular simulations of crosslinking process of thermosetting polymers

We use molecular dynamics (MD) with a procedure to describe chemical reactions to predict the atomic structure and properties of the thermosetting polymer epoxy EPON-862 and curing agent DETDA. The DREIDING force field is employed with environment-dependent atomic charges obtained self consistently during the dynamics. We propose a computationally efficient method to describe charge evolution based on the observation that atomic charges evolve significantly only during chemical reactions and in a repeatable manner. Two chemistry models with different relative rates for primary and secondary amine reactions are used to mimic the curing process in two extreme cases of processing conditions. The simulations show that differences in chemical reaction rates affect properties for intermediate conversion degrees (∼40-70%) but not for the higher conversion rates of interest in most applications. We also find that performing the polymerization at high temperatures leads to networks with lower internal strain energy due to increased molecular mobility. The predicted density, coefficient of thermal expansion, glass transition temperature and elastic constants of the resulting polymers are in excellent agreement with experiments.

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