Coarse grain forces in star polymer melts

An analysis is presented of forces acting on the centers of mass of three-armed star polymers in the molten state. The arms consist of 35 Kremer-Grest beads, which is slightly larger than needed for one entanglement mass. For a given configuration of the centers of mass, instantaneous forces fluctuate wildly around averages which are two orders of magnitude smaller than their root mean square deviations. Average forces are well described by an implicit many-body potential, while pair models fail completely. The fluctuating forces are modelled by means of dynamical variables quantifying the degree of mixing of the various polymer pairs. All functions and parameters in a coarse grain model based on these concepts are obtained from the underlying small scale simulation. The coarse model reproduces both the diffusion coefficient and the shear relaxation modulus. Ways to improve the model suggest themselves on the basis of our findings.