Effects of excluded volume and bond length on the dynamics of dense bead-spring polymer melts

We consider the effects of the ratio of average bond length, l(0), to excluded volume bead diameter, d(0), on the dynamics of dense liquids of freely jointed bead-spring polymer chains. Monodisperse (N=50) systems for which 0.73<l(0)/d(0)<1.34 were simulated using molecular dynamics in the NVT ensemble at a monomer number density rho=0.85 sigma(-3). The monomer friction coefficient, zeta, increased by a factor of similar to20 for a factor similar to2 increase in l(0)/d(0). This is due to the enhancement of sphere-packing homogeneity in the melts for l(0)/d(0)>1, which in turn increases the volume density and optimizes packing. Friction coefficients depend on local density (i.e., density of nearest neighbors around any monomer) in a manner akin to the empirical relationship between diffusivity and total number density in simple Lennard-Jones fluids [Levesque and Verlet, Phys. Rev. A 2, 2514 (1970)]. This work demonstrates the degree of interplay between structure and dynamics in macromolecular liquids. (C) 2002 American Institute of Physics.