Nanoindentation of Polymers

Small-scale polymers are increasingly being applied to a variety of fields, making it important to understand the behavior of polymers at small length scales. Indentation size effects (ISE) is the apparent increase in material-specific properties at very low indentation depths. These effects are observed for a wide range of materials, including polymers and metals. Unlike in metals, ISE in polymers is neither well understood nor well documented. The ISE in properties such as universal hardness and elastic modulus were investigated using nanoindentation. For this study, polydimethylsiloxane (PDMS) was chosen as a common polymer. Surface detection criteria were experimentally determined, and a consistent Berkovich indenter tip was used for all tests. The results for a cross-link density of 5% indicate two characteristics: first, there is a clear increase in universal hardness and elastic modulus with decreasing indentation depths, and second, the hardness and elastic modulus reach transient values as indentation depth increase. This illustrates the drastic ISE in PDMS at low indentation depths. The research group members and I will be combining the results for different cross-link densities to examine the effect of cross-link density on ISE. These findings will be incorporated into a manuscript to be submitted for publication.