Examination of butadiene/styrene-co-butadiene rubber blends by tapping mode atomic force microscopy.: Importance of the indentation depth and reduced tip-sample energy dissipation in tapping mode atomic force microscopy study of elastomers

Tapping mode atomic force microscopy (TMAFM) measurements were performed for blends of two elastomers, cis-1,4-butadiene rubber (BR) and styrene-co-butadiene rubber (SBR) containing silica filler particles. To help interpret the TMAFM phase and height images of the BR/SBR blends, transmission electron microscopy (TEM) measurements were carried out for the BR/SBR blends, and dynamic mechanical analysis (DMA) as well as frequency-sweep/force-probe TMAFM measurements were carried out for BR and SBR homopolymers. TEM images show that silica filler particles of BR/SBR blends are present mainly in the SBR component, and DMA results reveal that BR has a lower glass transition temperature than does SBR. In the phase images of BR/SBR blends the less stiff component BR is brighter than is the stiffer component SBR. For the rational interpretation of TMAFM phase images of viscoelastic materials, it is crucial to consider the indentation depth of the tip into samples as well as the reduced tip-sample energy dissipation, not the total tip-sample energy dissipation. At a given set-point ratio the indentation depth is smaller on the stiffer component SBR than on the less stiff component BR, but at a given indentation depth the phase shift is larger on the stiffer component SBR. The phase shift increases almost linearly with increasing the reduced tip-sample energy dissipation. The reduced tip-sample sample energy dissipation is larger for SBR than for BR in agreement with DMA results.