Adhesive forces significantly affect elastic modulus determination of soft polymeric materials in nanoindentation

The present study investigated the effects of adhesion on the elastic modulus determined from nanoindentation curves for soft polydimethylsiloxane (PDMS) elastomers with five different crosslink concentrations. Indentation load-displacement curves were obtained for samples of all concentrations at four different peak loads. All load–displacement curves were nearly linear, resulting in load independent contact stiffnesses (p < 0.003) for the range of loads tested. As a result, elastic modulus calculated from nanoindentation curves with the Hertz contact model exhibited significant differences (p < 0.004) both at different peak loads for a single PDMS concentration and between different PDMS concentrations at a single peak load (p < 0.001). The differences for different peak loads were attributed to the presence of substantial adhesive forces at the tip–sample interface. By taking these adhesive interactions into account with the Johnson, Kendall, Roberts (JKR) contact model, the differences in elastic modulus at different peak loads could be reconciled. Significant differences (p < 0.001) in moduli between different PDMS concentrations were still present. The results highlight the importance of considering adhesive forces in nanoindentation analyses of low modulus polymeric materials.