Simulation of chain folding in polyethylene: A comparison of united atom and explicit hydrogen atom models

We performed a systematic study to show that the nicely folded lamellar structure of polyethylene chains derived from previous molecular simulations is a consequence of the use of the united atom model, which treats the CH2 groups as a hard sphere. Simulations presented here with explicit hydrogen atoms show that the fold is not uniform, and that regularly aligned stems do not result. While it is known that polyethylene crystallizes with chain folded morphology, the irregular folding seen here is due to the neglect of the interactions between the hydrogen atoms attached to a carbon atom Ci and the atom Ci+2 in the united atom model. We present this paper to point out how the choice of force fields is important for polymers with a high degree of crystallinity. We hope that this paper stimulates further work on developing united atom models that take into account terms that we show to dictate the polymer conformations, such as the interactions between atoms attached to carbon atom Ci and the atom Ci+2 in polyethylene.