Wetting Reveals the Surface Dynamics of Polymer Glasses

The temperature dependence of the dynamic wetting of an immiscible ionic liquid on surfaces of polystyrene (PS), poly(4-methylstyrene) (P4MS), and poly(tert-butylstyrene) (PtBS) films was investigated in this work. Profound stick–slip motions of the droplets were observed near the glass transition temperature (Tg) of the polymers, where energy dissipation is significant. Additionally, an unexpected stick–slip motion with a small amplitude was observed at ∼60 K below Tg for PtBS and P4MS. This small-but-discernible stick–slip motion below Tg was attributed to the α-relaxation of the mobile surfaces of the polymer glasses. The surface activation energy, derived from the frequency dependence of the low-temperature stick–slip motion, was ∼150 kJ/mol, which is substantially lower than the activation energy for bulk polymers. However, no stick–slip motion was observed on PS films below Tg, likely due to the lowered surface mobility and/or reduced surface layer thickness of PS compared to PtBS and P4MS, implying a greater enhancement on the surface mobility for polymer glasses bearing bulkier side groups. Our results reinforce the idea of the presence of a mobile surface layer on polymer glasses and demonstrate that dynamic wetting is a sensitive indicator for surface relaxation of polymer glasses.