Mobility Gradient of Polymer Chains in an Interfacial Region with a Solid*

Polymer composites have been widely used in a variety of engineering fields. The performance and functionality of the composites are closely related to the quality of the interface between polymer and filler. Thus, it is important to study polymer behavior at solid interfaces. We first present how synthetic polymer chains adsorb onto a solid surface. We closely followed the trajectory of a single polymer chain on the surface as a function of temperature using atomic force microscopy. Combining the results with a full-atomistic molecular dynamics simulation revealed that the chain became more rigid on the way to reaching a pseudo-equilibrium state, accompanied by a change in its local conformation from mainly loops to trains. Then, we turn to the relaxation behavior of polymer chains in direct contact with a substrate using interface-sensitive sum-frequency generation (SFG) spectroscopy. The characteristic temperature, at which interfacial chains started to relax, was much higher than the bulk glass transition temperature. The interfacial relaxation dynamics of chains was directly probed as a function of distance from the solid surface using time-resolved evanescent wave-induced fluorescence anisotropy, dielectric relaxation spectroscopy and SFG spectroscopy. We found the presence of the dynamics gradient of chains in the interfacial region with the solid surface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders.