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

Abstract

We present coarse-grained molecular dynamics simulations of linear polyethylene (PE) melts, ranging in chain length from C80 to C1000. 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 Ns, which is smaller than the entanglement length Ne. The effective segmental friction rapidly increases around Ns leading, at constant density, to a transition in the scaling of the diffusion coefficient from D∼N−1 to D∼N−2, a transition in the scaling of the viscosity from η∼N to η∼N1.8, 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 C120. Full entanglement effects are observed only above C400, 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 η∼N3.6, the tube diameter is d≈5.4 nm, the entanglement molecular weight is Me≈1700 g/mol, and the plateau modulus is GN0≈2.4 MPa, all in good agreement with experimental data.