Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material

The atomic force microscope (AFM), apart from its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well-assessed technique while the characterization of the viscoelastic mechanical properties remains the challenge. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e. on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelastic features which cannot be identified by the force curve alone. For this reason, a viscoelastic characterization, and thus the study of the effects of indentation rate and temperature, was done on model materials where such additional phenomena are not observed. Another time dependence originating from the instrument itself has also been identified and decoupled. In fact, the viscoelastic behavior has been found to be reproducible even if one changes the experimental set-up as far as the preliminary determinations concerning AFM nanoindentations are well performed. The effects of temperature and time scales on the mechanical behavior have also been undertaken. A check on time-temperature superposition is also attempted through the WLF equation and the apparent activation energies for the elementary motions in the rubbery and in the glass transition regions are in good agreement with the expected values.