On thresholds in the indentation size effect of polymers

Length scale-dependent deformation in polymers has been observed by various research groups in the last years. Here, the indentation size effect of polymers is studied where the hardness increases with decreasing indentation depth. As a model polymer, polydimethylsiloxane (PDMS) of two different cross-link densities is studied as it has been found in previous studies to exhibit very strong indentation size effects. It is found that there is an upper threshold in the indentation depth range of 17.9 and 41.6 μm, respectively, below which the indentation size effect can be observed and above which the hardness does not change with the indentation depth. Similar phenomena can be determined for other polymers, however, at much smaller length scales.