A study of the atomistic details of structure and dynamics of polymethylene crystals via molecular dynamics simulations

A series of molecular dynamics simulations on a supercomputer of the molecular motion of pentacontylene (-C₅₀H₁₀₀-) crystals containing 9600 (implicit hydrogen model) to 28,800 (explicit hydrogen model) atoms have been carried out for different temperatures and under various conditions of constraint. Bending, stretching, and torsional motions are allowed in these simulations. Such a system is sufficiently large to serve as a model of a macroscopic crystal. It can also serve as a model for polyethylene. The details in trajectory analyses and the links between microscopic and macroscopic properties are described. Reasonable agreements with experimental data on density, defect concentration, heat capacity increase, melting temperature, and speed of sound have been reached. The large volume of detailed information of the structure and motion permits a new level of understanding. Information about the atomistic details of the structure and dynamics of these crystals, such as crystal appearance, pseudo- symmetric crystal structures, picosecond time-scale formation of conformational defects, collective twist of chains, mass transport, longitudinal and transverse vibration of the crystal, dynamic multi-domains, and the beginning of the melting process at instantaneous superheating, is presented. Chaos in the polymethylene crystals and the applicability of potential functions for explicit hydrogen interactions are also discussed.