Energy dissipation during nanoscale indentation of polymers with an atomic force microscope

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Abstract
Nanometer size indentations on polypropylene and polymethylmethacrylate were made with the atomic force microscope and were 200-nm wide and 20-nm deep for a tip penetration depth of 75 nm. To image the indentations with the same tip used for writing, the tip was retracted at high speed, thus detaching any polymer sticking on it. Nanomechanical properties of polymer are studied in the limit of shallow indentations and slow penetration speed. From the hysteresis of the force versus tip motion curve, the dissipated energy during the indentation and the inelastic deformation of the polymer surface can be measured. We find that the measured dissipated energy is proportional to the volume of the indentation times the activation energy needed to excite chain segments motion during the deformation.