Computer simulation of polyethylene crystals

Abstract

The computer programs described in Part 1 [9] have been used to investigate the structure and energy of stacking-faults and coherent twin boundaries in model polyethylene crystals. They are particularly relevant to the deformation behaviour of polyethylene, and it is explained how simulation studies can supplement experimental observations. Generalized translation faults have been simulated on the low-index planes of orthorhombic and monoclinic crystals constructed from rigid molecular chains of infinite length, and the stable faults have been identified. In the former structure, the only faults of significance are on the {110} planes, and three different configurations exist, each of energy ∼ 10 mJ m⁻². In the monoclinic phase, two stable faults occur on each of the planes (100) and (010), and in both cases the most stable fault has an energy ∼ 3 mJ m⁻². The possible implications of these findings for chain-axis and transverse slip in the two crystal structures are discussed. The structures of the coherent boundaries of {110} and {310} twins in orthorhombic and (100) and (010) twins in monoclinic crystals have been investigated, and in all cases several stable configurations are possible. The {310} interfaces have high energy (≳ 30 mJ m⁻²), whereas the others are similar in energy to the stacking faults. The results indicate the need to simulate incoherent boundaries. All the stable structures observed are found to have a simple geometrical explanation, and are not sensitive to the interatomic potentials used.