MOLECULAR-DYNAMICS SIMULATIONS OF THE HEXAGONAL STRUCTURE OF CRYSTALSWITH 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 molec ular dynamics simulations. Information about the atomistic details of the dynamics and structure of these crystals is presented. A significa nt disorder exists at temperatures well below the melting point. Close to melting, the disordered crystals have about 2% of gauche bonds tha t are distributed mainly at positions close to the surface of the crys tal. The major disorder consists, however, of a collective twisting of the chains leading to a hexagonal crystal structure. The hexagonal st ructure of the symmetric motifs is caused by a dynamic multidomain arr angement of the chains. (C) 1993 John Wiley & Sons, Inc.