Gradient in Chain Fluidity of Polystyrene near Solid Substrates

The dynamics of polymer chains near solid interfaces influence the properties of composites with solid fillers because the solid interfacial region occupies a large fraction of the material. While the dynamics at the segmental scale have been widely studied, the larger spatial and longer time-scale motion of polymers near the solid interface remains less well understood. In this study, the fluidity of polystyrene (PS) chains in close proximity to silicon substrates was investigated using nano-creep tests, which constantly apply a small force normal to the PS film surface by the interfacial tension of a liquid droplet. Nano-creep tests revealed that a fluidity gradient existed at larger spatial and longer time scales beyond the segmental regime, along the direction normal to the interface in the film. PS chains in the upper and middle regions of the film surface flowed at similar or slower rates than in bulk. PS chains located within several nanometers of the substrate, called adsorbed chains, flowed in the plane. This was supported by nano-creep tests conducted exclusively on the adsorbed chains, which showed in-plane flow. These results significantly advance the understanding of polymer dynamics near solid interfaces.