Multistage Self-Assembly Events of Bisurea in a Mixed-Solvent Medium Resolved by United-Atom and Coarse-Grained Molecular Dynamics Simulations

Multistage self-assembly events of a high-performance bisurea (hexamethylene diisocyanate-benzylamine, HDI-BA) in a mixed-solvent medium (polyester/o-xylene = 1:1 in weight), wherein the polyester oligomer serves as a cooling agent to sufficiently slow down the solvent dynamics and thereby sustain the entire self-assembly process, are captured using united-atom molecular dynamics (UAMD) and coarse-grained molecular dynamics (CGMD) simulations. While the UAMD simulation captures the formation of primal bisurea chains via regular hydrogen bonding between folded HDI-BA molecules, the subsequent CGMD simulation provides compelling evidence of consecutive intertwining events of the HDI-BA chains that produce increasingly wider, stiffer, and more twisted fibrous objects mimicking the uniform and helical-like micrometer-fibers observed in recent experiments. Overall, this appears to be the first report revealing that bisurea may simultaneously capitalize on regular molecular packing in the initial stage and consecutive polymer-like intertwining in subsequent stages to foster highly regular self-assemblies with dimensions in the micrometer range.