Dynamic force microscopy analysis of block copolymers: beyond imaging the morphology

Dynamic force microscopy is known for its ability to image soft materials without inducing severe damage. For such 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 show that a systematic comparison between images and approach–retract curve data allows the origin of the contrast 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 symmetric triblock copolymer, which possesses a lamellar morphology due to 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. As a step further, the analysis of the variation of the phase is carried out as a function of the tip–surface distance. Local variations of the phase can be linked to dissipative processes between the tip and the soft sample. When the tip touches the surface, viscous forces acting against the tip motion contribute to the phase lag. Depending on the tip apex geometry and on the nature of the sample, the relationships between the phase variations and the tip–surface distance can be derived. On that basis, we propose an approach to evaluate the viscosity at the nanometer scale.