Zero-shear stress relaxation and long time dynamics of a linear polyethylene melt: A test of Rouse theory

Results of united atom molecular dynamics simulations of a n-C120H242 melt at 450 K are presented. It is shown that the results of mean square displacement, dynamic structure factor, end-to-end vector autocorrelation, and shear relaxation modulus can consistently be described by the Rouse model with a single set of fit parameters, provided the length scales involved are larger than the statistical segment length b = 1.2 nm. On smaller length scales the stiffness of the chain becomes prominent, and the results deviate increasingly from the Rouse predictions. The shear relaxation modulus G(t) is determined from the stress autocorrelation function from both atomic and molecular points of view. The integrals G(t)dt are found to be identical after 1 ps and a Rouse description is shown to coincide for time scales larger than 0.4 ns. Compared to experimental values, the measured diffusion coefficient is overestimated by 63% and the viscosity is underestimated by 38%, consistent with molecular dynamics simulations of small molecules.