Investigating the origin of the core-shell structure of polymeric nanofibers during fabrication process at the atomistic scale

Polymeric nanofibers demonstrate core-shell structure when their diameter drops below the nanoscale. Under-standing the corresponding mechanism at the atomistic scale paves the way for achieving the long-term goal of adjusting their microstructure and controlling their properties purposefully. To explain the current contradictory views on the origins of the nanofiber core-shell structure, a molecular dynamics (MD) study is performed. In the MD simulation, cold-drawing and hot-drawing methods are implemented to investigate the role of evaporation rate, stretching force and external temperature on the formation of polymeric nanofiber's core-shell structure. The results show that the distribution of the solvent atoms inside the as-formed nanofiber is strongly associated with the evaporation rate. The increase of the stretching force during the hot-drawing or cold-drawing process has a significant influence on the polymeric chain orientation inside the nanofiber. The external temperature has little influence on nanofiber formation. The final microstructure of nanofibers relies on the interplay between the stretching force and the evaporation rate. When the evaporation is dominant, fiber tends to form a tabulated structure with a less dense core embedded in a denser shell layer. On the other hand, when the stretching force is dominant, the fiber may form a denser structure with chains more aligned.