J. Baschnagel – 10 septembre

We present results of molecular-dynamics (MD) simulations for the dynamics of glass-forming polymer melts. After a general introduction to the glass transition and the simulation model we analyze the dynamics of single-monomer trajectories. Upon cooling toward low temperature the single-monomer trajectories display long periods of localization interrupted by “fast moves”. This observation suggests a modeling by a continuous-time random walk (CTRW), i.e. by a series of random jumps separated by random waiting times. We introduce an algorithm allowing to filter the “fast moves” so as to retain only those “moves” which comply with the conditions of a CTRW [1]. These moves are called “jumps” in the following; the remaining analysis is based on them. As a function of temperature we then examine key distributions of the CTRW: the jump length distribution (JLD) and the waiting time distribution (WTD) for the jumps [1,2]. For the equilibrium (polymer) liquid under consideration the WTD and JLD suffice to model the single-monomer dynamics by the CTRW. For the mean-square displacement (MSD) of a monomer the results of this modeling are compared with the underlying MD data [2]. The MD data exhibit two regimes of subdiffusive behavior, one for the early alpha-process and another at later times due to chain connectivity. By contrast, the analytical solution of the CTRW yields diffusive behavior for the MSD at all times. Empirically, we can account for the effect of chain connectivity in Monte Carlo simulations of the CTRW. The comparison between MD and CTRW simulations is then extended to the self-part of the van Hove correlation function, revealing the strengths and the weaknesses of the CTRW to account for the spatial dependence of the van Hove function [3].

[1] J. Helfferich, F. Ziebert, S. Frey et al., Physical Review E 89, 042603 (2014).
[2] J. Helfferich, F. Ziebert, S. Frey et al., Physical Review E 89, 042604 (2014).
[3] J. Helfferich, J. Brisch, H. Meyer et al., European Physical Journal E 41, 71 (2018).