Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus

With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.