A comparison of computer models for the simulation of crystalline polyethylene and the long n-alkanes.

We address the problem of simulating the orthorhombic crystal structures of polyethylene (PE) and the long n-alkanes. Experimentally, the lattice parameters of these materials are known to depend on the level of disorder in the crystals. This introduces complications when developing reliable force field parameters for computer models. In this paper we compare the behaviours of three different atomistic computer models, which possess varying degrees of disorder. The first model consists of chains, which are infinite, by virtue of the periodic repeat of the simulation cell along the chain axis. The second model consists of finite chain stems, whose lengths are determined by the z-dimension of the simulation box. The third model, known as the jogged chain model, attempts to reproduce the effect of defects and chain folds, by allowing a finite chain to pass through the simulation box more than once. Jogs are introduced into each chain, preventing it from intersecting with periodic images of itself. The same force field parameters are used in each case. For the force field chosen, it is found that the jogged chain model provides a better match with the a-parameter of the experimental unit cell of PE than the other two models. In the infinite and finite chain models, it is found that the chains librate as rigid rotors and the formation of dynamic defects is rare. On the other hand, the extra free volume resulting from the defects in the jogged chain model, means that gauche defects form readily in this case. We conclude that, for the force field chosen, the jogged chain model is better suited to the simulation of chain-folded PE crystals than the more ordered models. Our results are compared with experimental results and previous simulations of PE from the literature.