How Well Can Coarse-Grained Models of Real Polymers Describe Their Structure? The Case of Polybutadiene

Coarse-graining of chemical structure of macromolecules in the melt is investigated using extensive molecular dynamics simulation data which are based on a united atom force-field model of polybutadiene. Systematically increasing the number, n, of the united atoms approximated by an effective coarse-grained monomer, we study the influence of degree of coarse-graining on the structure functions such as the segment-segment intermolecular and intramolecular correlation functions. These results are compared to Monte Carlo simulations of the corresponding coarse-grained bead-spring model and Chen-Kreglewski potential for chain molecules. In contrast to the atomistic chemically realistic model of polybutadiene, the bending and torsional potentials are not included into the coarse-grained models. Nevertheless, for a range of intermediate values of n a good qualitative agreement between intra- and intermolecular coarse-grained correlations of the atomistic model and the coarse-grained bead-spring model is found on large and intermediate length scales, but deviations occur on length scales well below one nanometer. The structure functions obtained for the Chen-Kreglewski chains exhibit many artificial features.