Molecular Dynamics Simulation of the Condis State of Polyethylene

Using a realistic model for polyethylene (PE), the molecular dynamics technique is used to simulate atomic motion in a crystal. The calculations reveal conformational disorder above a critical temperature. The customarily assumed rotational isomers model is found to be a poor description of the crystal at elevated temperature. The rate of isomerization computed from molecular dynamics is compared to transition-state theory and leads to an activation energy of 23 kJ/mol, 6.3 kJ/mol above the single-bond rotation value. Classical heat capacities have been calculated and result in negative and positive deviations from 3R, as was also observed experimentally. The negative expansion coefficient of the fiber axis is in good agreement with the experiment. A movie of the dynamics of the polyethylene crystal has been produced.