In atomic force microscopy, tapping-mode (also called intermittent contact mode) operation is known for its ability to image soft materials without inducing severe damage. For soft materials, the determination of the relative contributions of the topography and the local mechanical properties to the recorded image is of primary importance. In this paper, we report a systematic comparison between images and approachretract curve data. We show that this experimental comparison allows the origin of the contrast that produces the image to be straightforwardly evaluated. The method provides an unambiguous quantitative measurement of the contribution of the local mechanical response to the image. To achieve this goal, experimental results are recorded on a model system, a triblock copolymer, with a nanophase separation between elastomer and glassy domains. In this particular case, we show that most of the contrast in the height and phase images is due to variations of the local mechanical properties.

This work is an attempt to investigate viscosities at the nanometer scale. To do so, the tapping mode atomic force microscopy is used on a triblock copolymer exhibiting a well-defined periodic structure at the nanometer scale. Variations of the oscillator amplitude and phase delay as a function of the tipsample distance are recorded on the glassy and rubbery domains of the copolymer. The experimental data are compared to analytical expressions derived from Stokes law. In the present study, among the different possible expressions of the viscous forces depending on the tip shape and on the experimental length scale, only a force proportional to the indentation depth is able to describe the experimental data. In this particular case, quantitative measurements are possible. Finally, the oscillator is shown to be sensitive to local variations of the viscosity within few nanometers.

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The dynamical behaviour of an entangled network, a melt made of long polymer chains, with an oscillating nanotip, is investigated to provide evidence for a close relationship between the spatial scale and dynamics of the network, from the molecular to the mesoscopic scale. The experimental results show how a dynamic force microscope is able to discriminate between slow and fast relaxation processes related to molecular motion at different scales. In particular, it is shown that within a very small variation of the tip indentation depth, a few ångströms, the relaxation times involved in the process of dissipation vary by more than three orders of magnitude.