Thermodynamically consistent coarse graining the non-equilibrium dynamics of unentangled polymer melts

A thermodynamically consistent strategy of coarse-graining microscopic models for complex fluids is illustrated for low-molecular polymeric melts subjected to homogeneous flow fields. The systematic coarse-graining method is able to efficiently bridge the time- and length scale gap between microscopic and macroscopic dynamics. A projection operator derivation is employed within the framework of nonequilibrium thermodynamics. From an alternating Monte-Carlo-molecular dynamics iteration scheme we obtain the thermodynamic building blocks of the macroscopic model. We investigate a number of imposed shear and elongational flows of interest and find that the coarse-grained model predicts structural as well as material functions beyond the regime of linear response. The elimination of fast degrees of freedom gives rise to dissipation, which we analyse in terms of the Rouse model. The results are in quantitative agreement with those obtained via standard nonequilibrium molecular dynamics (NEMD) simulations for planar shear and elongation. The coarse-graining method is able to deal with more general flows, which are not accessible by standard NEMD simulations.