Molecular dynamics computer simulations are presented for polyethylene crystals containing ethyl branches. The crystals are simulated using an all-atom (explicit hydrogen) molecular mechanics force field. The effect of the branches in expanding the crystalline unit cell is demonstrated for a range of branch densities. We compare the behaviour of two types of model, each consisting of arrays of 48 chains. In the first, the polyethylene chains are effectively infinite in length, by virtue of the periodic boundary conditions, which link the polymer chains across the faces of the simulation box. In the second model, we simulate long n-alkanes. Two different chain lengths are considered, containing 24 or 48 carbon atoms. By examining the individual torsional angles and the setting angles of each segment of each chain, it is possible to demonstrate that branches are incorporated into the unit cell without the introduction of gauche defects in the polymer backbones. The effect of large numbers of branches is to expand the cell to such an extent that a mobile rotator phase is induced i.e. the system forms a dynamic rotationally disordered crystal in which chain sliding occurs readily. Although such high branch densities in the crystalline phase are not accessible experimentally, the prediction of a mesophase is interesting, because it may have implications for crystallisation. For example, the mesophase could occur transiently during crystallisation, as has been suggested for linear chains, and it would fulfil the dual role of allowing the growing crystals to thicken, and providing the branches with the opportunity to diffuse out of the crystal. (c) 2005 Elsevier Ltd. All rights reserved.