Measuring the Surface and Bulk Modulus of Polished Polymers with AFM and Nanoindentation

Abstract

A new method to determine the elastic modulus of a material using the atomic force microscope (AFM) was proposed by Tang et al. [Nanotechnology 19, 495713 (2008)] and is used in this study. This method models the cantilever and the sample as two springs in a series. The properties of both the spring and cantilever are determined on two reference samples with known mechanical properties and these properties are then used to find the elastic modulus of an unknown sample. The indentation depth achieved with AFM is in the nanometer range (30–130 nm in this study); and hence when this technique is performed on polymers, whose surface structure is different from their bulk structure, AFM gives a measure of the surface elastic modulus. In the present study, after employing AFM to measure the surface modulus of five polymers, traditional depth-sensing nanoindentation, with penetration depths of about 1 μm, was used to determine the elastic modulus in the bulk. The mean values for elastic modulus from the AFM were within 5–50% of the nanoindentation results, suggesting the existence of a surface modulus for polished polymers.