The interfacial viscoelastic behavior of thin polymer films deposited on silicon substrate was investigated by nanoscale dynamic mechanical analysis involving modulation of the nanoindentation load in the frequency range of 10-200 Hz. Significant differences were observed between interfacial and bulk viscoelastic properties. Both the storage modulus and the loss modulus increased with the indentation depth due to the effect of the substrate stiffness. In the low-frequency range, the interfacial viscous behavior was greatly suppressed, presumably due to changes in the molecular chain conformation at the film/substrate interface and/or dynamic confinement of the molecular chains between the indenter and the substrate. Both effects significantly restricted small-scale mobility of the main chains. In the high-frequency range, main chain rearrangement was inhibited and short-range side group relaxation was the dominant mode.
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