Molecular dynamics simulations of the hexagonal structure of crystals with long methylene sequences

An unconstrained polymethylene crystal, consisting of 9600 CH₂ groups, in which each CH₂ is permitted to carry out stretching, bending, and torsional motion, has been studied at various temperatures using molecular dynamics simulations. Information about the atomistic details of the dynamics and structure of these crystals is presented. A significant disorder exists at tempratures well below the melting point. Close to melting, the disordered crystals have about 2% of gauche bonds that are distributed mainly at positions close to the surface of the crystal. The major disorder consists, however, of a collective twisting of the chains leading to a hexagonal crystal structure. The hexagonal structure of the symmetric motifs is caused by a dynamic multidomain arrangement of the chains. © 1993 John Wiley & Sons, Inc.