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

J. T. Padding, W. J. Briels
2002 Journal of Chemical Physics  
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more » ... r polyelectrolyte chains We present coarse-grained molecular dynamics simulations of linear polyethylene ͑PE͒ 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 DϳN Ϫ1 to D ϳN Ϫ2 , a transition in the scaling of the viscosity from ϳN to ϳN 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 ͑entanglement͒ 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 ϳN 3.6 , the tube diameter is dϷ5.4 nm, the entanglement molecular weight is M e Ϸ1700 g/mol, and the plateau modulus is G N 0 Ϸ2.4 MPa, all in good agreement with experimental data.
doi:10.1063/1.1481859 fatcat:hp74dfkcijbjrditutm32xpmta