Molecular-dynamics simulation of structural and conformational properties of poly(propylene oxide)

Molecular-dynamics simulations of poly(propylene oxide) (PPO) have been performed in the temperature range 300 K less than or equal to T less than or equal to 450 K using a newly developed atomistic force field. Chains with 1, 11 and 45 repeat units have been considered. We find excellent agreement for the short and intermediate range order as deduced from x-ray and neutron diffraction measurements with deviations (excluding quantum mechanical effects for the nuclear motion) lying within experimental uncertainties. The static structure factor S(q) shows a first sharp diffraction peak due to interchain correlations which is shifted to smaller wave vectors with increasing temperature. The molecular weight dependence of the short and intermediate range order is found to be weak and the obtained density and thermal expansivity compare favorably with experimental data. The backbone dihedral angle distributions are essentially identical for PO11 and PO45, but deviate slightly from those for PO1, and the probability for gauche conformations is quite large resulting in a small characteristic ratio. The dihedral and bond angle distributions obtained from the present MD study are more closely related to experiments than those that have been achieved previously by combining experimental data with reverse Monte Carlo modeling. (C) 2000 American Institute of Physics. [S0021-9606(00)50721-1].