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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 (T 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 T g (or T α) of the PS film. This was confirmed by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The change of the measured T 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.

The surface structures of poly(vinyl alcohol) (PVA) films with four different degrees of hydrolysis after immersion in ethanol were investigated using sum frequency generation (SFG) vibrational spectroscopy and contact angle (CA) goniometry. The result showed that the surface chemical structure of the PVA films was strongly dependent on the degree of hydrolysis. The vinyl acetate (VAc) units in the PVA chains resulting from incomplete hydrolysis segregate to the film surface and strongly affect the adsorption behavior of ethanol molecules on their surfaces. The surface hydrophilicity decreased greatly for PVA films with relatively high hydrolysis degrees (i.e., 99% and 97.7%), in which the water contact angle increased by 20°, and increased for PVA with relatively low hydrolysis degrees (95.1% and 84%) after immersion in ethanol. It was found that ethanol molecules adsorb from solution onto a PVA film surface in an ordered and cooperative way governed by hydrogen bonding when the hydrolysis degrees of PVA were higher than 98%. When the hydrolysis degree of PVA was lower than 96%, the surface structure obtained by surface reconstruction dominated after immersion in ethanol, with fewer ethanol molecules adsorbed on the surface, resulting in a decrease of its water contact angle.

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