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 displac ement, dynamic structure factor, end-to-end vector autocorrelation, and she ar relaxation modulus can consistently be described by the Rouse model with a single set of fit parameters, provided the length scales involved are la rger than the statistical segment length b approximate to1.2 nm. On smaller length scales the stiffness of the chain becomes prominent, and the result s deviate increasingly from the Rouse predictions. The shear relaxation mod ulus G(t) is determined from the stress autocorrelation function from both atomic and molecular points of view. The integrals integralG(t)dt are found to be identical after 1 ps and a Rouse description is shown to coincide fo r time scales larger than 0.4 ns. Compared to experimental values, the meas ured diffusion coefficient is overestimated by 63% and the viscosity is und erestimated by 38%, consistent with molecular dynamics simulations of small molecules. (C) 2001 American Institute of Physics.