Molecular dynamics simulation of local chain motion in bulk amorphous polymers. I. Dynamics above the glass transition

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Abstract
The local chain dynamics of bulk amorphous polymers above the glass transition temperature is studied by means of molecular dynamics simulations in the time scale up to 10−9 s. A model of polyethylene having a realistic density is used where a fully vibrational treatment is applied to the chains subject to nonbonded interactions between segments. Also used for comparison is a freely-rotating chain model with no torsional potential, but otherwise having the same features. Time-correlation functions of various vectors embedded in the polymer backbone are evaluated and the result shows a strong dependence of the relaxation on the direction of the vector in both models. The chain reorientation process is then analyzed in terms of the two separate components of the motion: one, the reorientation of the chain axis and the other, the rotation of the chain around it. On a short time scale, the reorientation process is affected directly by the conformational transitions. On the longer time scale, however, the relaxation of the chain axis is influenced significantly by the presence of surrounding chains. The surrounding chains act as a rigid ‘‘pipe’’ constraining the relaxation of the chain axis even after the correlation of the dihedral angles is lost. The chain is thus rapidly tumbling around the chain axis, while the chain axis itself slowly changes its direction according to the relaxation of the local structure.