Scanning viscoelasticity microscopy (SVM) enables one to gain direct access to local viscoelastic properties on solid surfaces. In this study, it was demonstrated how deep the stimulus displacement by a cantilever tip propagated through a film. The glassy polymer/rubbery polymer bilayer prepared on silicon wafer was examined as a model system. Monodisperse polystyrene (PS) and polyisoprene (PI) were used as a glassy polymer for the outermost layer and a fully rubbery one for the underneath phase, respectively. The surface dynamic storage modulus, E′, decreased with decreasing thickness of the upper PS layer due to the contribution from the soft underneath PI layer once the upper layer thickness fell short of 70 nm. On the contrary, surface E′ was invariant for the bilayer with a thicker upper layer. These results indicate that the stimulus displacement imposed propagates to the depth of, at least, 70 nm along the surface normal at room temperature. The SVM measurement was also applied to monodisperse PS thin films with the number-average molecular weight of 140K spin-coated on silicon wafer with native oxide layer. In this case, while surface E′ increased with a decrease in the thickness ranging from 80 nm down to 50 nm on account of the contribution from the hard substrate, it started to decrease for a thinner PS film. This result might imply that the PS shrface and/or whole film started to soften up with decreasing thickness, in the case of a PS film thinner than 40 nm.
All Science Journal Classification (ASJC) codes
- Materials Chemistry