Use of Atomic Force Microscope as a Nanoindenter to Characterize Elastomers

Abstract

Abstract The atomic force microscope (AFM) in the force mode has been used as a nanoindenter for determining the crosslink density and modulus of rubber on a fine scale. The force—distance curves obtained as the sample is moved against the tip can render information about mechanical properties of the sample. The technique for this purpose has been established. Several instrumental and technical difficulties encountered during the development of the technique have been identified and solved. The difficulties include surface roughness, thickness variation of the sample, hysteresis of the scanner, determining the stiffness of the cantilever, determining the geometry of the tip, converting the sensor output to force and determining the penetration depth of the tip. The determination of the effective tip geometry requires use of a calibration material. Force—distance measurements were made on a series of styrene-butadiene rubbers (SBR) with different crosslink densities. The modulus determined by AFM agreed with the modulus measured with dynamic mechanical analysis. The results of force/penetration depth show a linear relationship with crosslink density. The AFM results are shown to correlate with those from NMR. The effects of rubber oxidation in air were examined. The results show that both homogeneous and heterogeneous oxidation can be detected by this method.