Time and length scales of polymer melts studied by coarse-grained molecular dynamics simulations

We present coarse-grained molecular dynamics simulations of linear polyethylene (PE) melts, ranging in chain length from C-80 to C-1000. The employed effective potentials, frictions, and random forces are all derived from detailed molecular dynamics simulations, leaving no adjustable parameters. Uncrossability constraints are introduced in the coarse-grained model to prevent unphysical bond crossings. The dynamic and zero-shear rate rheological properties are investigated and compared with experiment and other simulation work. In the analysis of the internal relaxations we identify a new length scale, called the slowing down length N-s, which is smaller than the entanglement length N-e. The effective segmental friction rapidly increases around N-s leading, at constant density, to a transition in the scaling of the diffusion coefficient from Dsimilar toN(-1) to Dsimilar toN(-2), a transition in the scaling of the viscosity from etasimilar toN to etasimilar toN(1.8), and conspicuous nonexponential relaxation behavior. These effects are attributed to strong local kinetic constraints caused by both chain stiffness and interchain interactions. The onset of nonlocal (entanglement) effects occurs at a chain length of C-120. Full entanglement effects are observed only above C-400, where the shear relaxation modulus displays a plateau and the single chain coherent dynamic structure factor agrees with the reptation model. In this region the viscosity scales as etasimilar toN(3.6), the tube diameter is dapproximate to5.4 nm, the entanglement molecular weight is M(e)approximate to1700 g/mol, and the plateau modulus is G(N)(0)approximate to2.4 MPa, all in good agreement with experimental data. (C) 2002 American Institute of Physics.