Comparison of Various Correlation Times in Polymer Melts by Molecular Dynamics Simulation

This investigation presents data from prolonged molecular dynamics simulations of dense monodisperse polymer melts of linear polymer chains of length between N = 3 and N = 255. Hear N is number of chain coarse-grained segments. The study aim is to observe the crossover from Rouse-like dynamics for short chains to reptation-like dynamics for long chains and its influence on various translational and orientational correlation times. To address the problem, we calculate a variety of different quantities: standard mean-square displacement of inner monomer and of the chain mass centre, autocorrelation function of chain end-to-end vector, orientational autocorrelation functions of segment and chain end-to-end vector. The last two functions define NMR relaxation in polymer melts. The chain length dependences of all large-scale correlation times reveals clear crossover from non-entangled to entangled dynamics near N-C approximate to 35. Additional primitive path analysis of the same data gives statistically consistent entanglement value N-E approximate to 14, i.e., the longest chain investigated is well entangled and displays explicit motional anisotropy. The full data assembly explains qualitatively strong dependence of NMR transverse relaxation time for Rouse-like regime and weak one in reptation regime.