Atomistic Molecular Dynamics Simulation of Polydisperse Linear Polyethylene Melts

Well-relaxed atomistic configurations of polydisperse, linear polyethylene (PE) melts, obtained with the end-bridging Monte Carlo algorithm, have been subjected to detailed molecular dynamics simulations in both the canonical ( NVE ) and microcanonical ( NVT ) ensembles. Three different systems have been investigated, characterized by mean molecular lengths C 24, C 78, and C 156, and by the same polydispersity index I of about 1.09. Results are presented for the static and (mainly) dynamic properties of these melts at P = 1 atm and T = 450 K. The diffusion coefficient D, determined for various chain lengths, N, is in very good agreement with experimentally measured values. The friction coefficient ζ D is extracted from D by invoking the Rouse model; it is seen to increase from a relatively small value characteristic of short alkanes to a chain-length-independent plateau, reached in a region of N = 60−80. The friction coefficient ζ τ is also obtained by analyzing the decay of the time autocorrelation function for the normal modes X p at various chain lengths; the values thus extracted are consistent with those obtained from D for N above 40. Although the decay of the autocorrelation function of the end-to-end vector is very well described by the Rouse model, individual Rouse modes show some deviation from theoretical predictions. Even for chains sufficiently long to be in the asymptotic ζ regime, only the first two normal modes fully conform to Rouse theory in terms of their squared amplitudes and correlation times. Zero-shear viscosities computed from ζ D values by means of the Rouse model are in excellent agreement with available experimental data for N = 90.