Quantitative determination of Young's modulus on a biphase polymer system using atomic force microscopy

Abstract

Atomic force microscopy (AFM) is currently used to investigate polymer surface morphology, to obtain roughness parameters or to map the qualitative differenceces of surface properties. Some previous studies have attempted to determine quantitatively the elastic surface properties, but the difficulty with AFM is that the contact geometry is not very well known, due to the complexity of the mechanical system composed of the cantilever–tip set and the solid surface. We propose here a relative method for measuring the Young's modulus E of a polymer surface by AFM indentation, involving a calibration step obtained from a set of standards constituted by pure polymers with known modulus. Contact stiffness, indentation at peak load and shape of unloading curve are obtained for each reference polymer, leading to a linear relationship between E and a function of these parameters. This calibration curve allows the unknown Young's modulus values of the different phases at the surface of a biphase polymer system to be determined. The force volume mode was used to record force curves. Compared to classical indentation techniques, the force volume mode gives the advantage of imaging surface domains exhibiting elasticity differences. Thus, the elastic modulus can be quantified with a spatial resolution on a nanometric scale. Copyright © 2000 John Wiley & Sons, Ltd.