Probing Glass Transitions in Thin and Ultrathin Polystyrene Films by Stick–Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces

A novel method was developed to detect the glass transition of thin and ultrathin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick–slip behavior on their surfaces during dynamic wetting of glycerol or oligo-poly­(ethylene glycol) droplets. The peak temperature (<i>T</i>_jm) obtained from the jumping angle–film temperature curve, in which the jumping angle Δθ was employed to scale the stick–slip behavior, was nearly identical to the corresponding <i>T</i>_g (or <i>T</i>_α) of the PS film. This was confirmed by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The change of the measured <i>T</i>_jm with film thickness and substrate chemistry (SiO₂–Si and H–Si) further confirmed that the developed method is very sensitive for detecting the dynamics of ultrathin polymer films