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

We study the coarse-graining of homopolymers into single spherical beads, focusing on simulations of thin polymer films and droplets. Accurately representing these inhomogeneous systems by a coarse-grained model is challenging due to the large density variations at the interfaces. Using models interacting solely through pair potentials parametrized in the bulk typically leads to unstable systems. Hence, we follow an alternative approach, where we include higher order interactions through a local-density-dependent potential parametrized using relative entropy minimization. We defined the local density as the overlap integral of the monomer distributions around the centers of mass, effectively incorporating a density dependent contact interaction. Our coarse-grained model reproduces stable polymer films and droplets. There are some (minor) quantitative differences compared to the reference simulations, namely a slight broadening of the interfaces accompanied by a smaller surface tension, which are due to the (fixed) spherical polymer shape in the coarse-grained model. We further look into substituting some of the model parameters obtained via reference simulations by analytical expressions, reducing the computational effort necessary for parametrizing the coarse-grained model.