Molecular dynamics calculation to clarify the relationship between structure and mechanical properties of polymer crystals: the case of orthorhombic polyethylene

The molecular dynamics (MD) technique was used to calculate the temperature dependence of the structure, molecular motion, and mechanical property of the orthorhombic polyethylene (PE) crystal. The potential functional parameters reported by Karasawa et al. (J Phys Chem, 95 (1991) 2260) were refined further so that the vibrational frequencies of infrared and Raman bands, measured by us at ultra-low temperatures for the normal and fully deuterated PE, could be reproduced well. The flip-flop motion around the chain axis and the torsional motion of the skeletal chains were found to start above ca. 350 K and increase the amplitude of these motions progressively. Coupling these two types of chain motion resulted in a steep increase of the thermal vibration parameters or the mean-square-displacements of carbon and hydrogen atoms, corresponding well with the X-ray data. The lattice constants and the related linear thermal expansion coefficients were also found to be in good agreement with the observed data. The calculated Young's modulus along the chain axis decreased gradually with the increasing temperature: 330 GPa at 0 K to 280 GPa at room temperature. The latter was in good agreement with the value of 280–305 GPa evaluated from the Raman measurement of the longitudinal acoustic mode. Young's modulus was found to relate intimately with the chain contraction caused by the skeletal torsional motion. Only 0.3% contraction of the chain resulted in the reduction of the modulus by ca. 35%. A similar behavior was also seen in the trigonal polyoxymethylene and nylon 6 α forms.