Structure formation during the crystallization induction period of a short chain-molecule system: A molecular dynamics study

A molecular dynamics simulation has been performed for a short chain-molecule system to study structure formation during the crystallization induction period. A model simulating n-alkane having 20 methylene units was used for the short-chain molecule. A model system containing either 250 or 2000 chains was quenched from a high temperature, and the structure formation was examined at constant temperature and pressure. The model system containing 250 chains eventually forms an ordered phase after the induction period, during which macroscopic quantities, such as volume, remain almost constant. In spite of these small changes in the macroscopic quantities, development of local parallel order is significant during the induction period. We also found that the development of local parallel order causes density fluctuations which appear as a weak small-angle peak in the structure factor. A characteristic length scale corresponding to the density fluctuations becomes larger as time elapses in the induction period. These features are qualitatively in accord with recent experimental findings on a different polymer system [M. Imai ct al., Phys. Rev. B 52, 12 696 (1995)]. (C) 1998 America Institute of Physics. [S0021-9606(98)51037-9]