MD SIMULATION STUDY OF GLASS-TRANSITION AND SHORT-TIME DYNAMICS IN POLYMER LIQUIDS

Molecular dynamics simulations have been performed on models of polyethylene (and n-alkane) liquids at their realistic bulk densities. The united atom approximation has been adopted. On stepwise cooling of the system under a constant pressure, the temperature coefficient of specific volume changes abruptly at a temperature mimicking the glass transition phenomenon observed in laboratory. The results of the simulation runs, lasting for the order of nanoseconds, were analyzed to investigate the short time dynamics of bond reorientation. The distribution of bond reorientation angle is much broader than is expected from a rotational diffusion with a single diffusion coefficient. Similarly the time-correlation function of bond reorientation is non-exponential and can be fitted well by the stretched exponential function. Attempts to explain these behaviors by assuming a superposition of rotational diffusion processes is met with many contradictions. Explanation is instead offered on the basis of the observed anisotropy of the bond reorientation motion, i.e., the finding that the chain axis reorients much more slowly than a vector attached perpendicular to the chain axis. Such anisotropy results from the intramolecular and intermolecular constraints to the motion of the chain imposed by the segments in the neighborhood.