United Atom and Coarse Grain Force Fields for Crosslinked Polydimethylsiloxane with Applications to the Rheology of Silicone Oils

Chemical crosslinking plays a determining role in the mechanical properties of silicone materials comprised of polydimethylsiloxane (PDMS). Classical molecular dynamics (MD) is an invaluable tool to explore, understand, and optimize materials performance, but requires accurate force fields (FFs). We develop a hierarchical pair of FFs for crosslinked PDMS. We first extend an existing united-atom FF to include crosslinking terms and then use it together with iterative Boltzmann inversion to train a coarse grain FF where each monomer is represented by a single particle. MD simulations with both FFs show that crosslinking alters the rheology of silicone oils, leading to systematic increases in density and shear viscosity. The viscosity of crosslinked oils follows an empirical linear trend with number-average molar mass. The FFs developed here serve as a bridge to upscale insights on siloxane chemistry to enable quantitative bottom-up predictions for curing- and age-induced effects.