Interfacial structures and mechanical response of highly viscous polymer melt on solid surfaces investigated by atomic force microscopy

The interfacial structures and mechanical response of the highly viscous poly(dimethylsiloxane) (PDMS, 9750 mPa･s, Mn ~ 59000) on solid surfaces were investigated by frequency-modulation atomic force microscopy (FM-AFM) using a quartz tuning fork sensor. A layered density distribution on a PDMS/mica interface was visualized on a 29 h-settling sample by two-dimensional frequency shift (∆ƒ) mapping, and the result exhibited that the layered structure was metastable. It was also found that the damping coefficient of tip vibration (∆γ) increased in the region of ~1 nm from the solid surface, which suggests the limited mobility of the liquid molecules closest to the solids. After settling for more extended time or heating the sample, the layered density distribution was suppressed, and the conservative repulsive force near the solid surface in 2-3 nm were observed over longer distances by settling for a more extended time or heating. The suppression of the layered density distribution showed the similar temperature dependence with the bulk viscoelastic relaxation. In contrast, the elongation of conservative repulsive force near the solid surface showed steeper temperature dependence than the bulk, which suggests that it was rate-limited by the attraction between the solid surface and the closest liquid molecules.