Bottom-up Ultra-coarse-graining of Homopolymers for Inhomogeneous Systems

We coarse-grain homopolymers into single spheres, focusing on simulations of thin films and droplets. Accurately representing these inhomogeneous systems by a coarse-grained (CG) model is a difficult task due to the large density variations at the interfaces. Using CG models interacting only via pair potentials parametrized in the bulk leads to unstable systems, because the surface tension is not preserved. We follow an alternative approach, including higher order interactions either through an additional three-body potential or a local density-dependent potential. We parametrize the two- and three-body potentials via force matching, and the local density-dependent potential through relative entropy minimization. While the CG models with three-body potentials fail to reproduce stable polymer films and droplets, CG simulations with a local density-dependent potential are able to do so. A detailed analysis of the film/droplet morphologies reveals some (minor) quantitative differences between the reference and the CG simulations, namely a slight broadening of the interfaces accompanied by a smaller surface tension in the CG simulations. These differences are due to the deformation of polymers near the interfaces, which can not be resolved in the CG representation.