Use of atomic force microscope as a nanoindenter to characterize elastomers

The Atomic Force Microscope in the force mode has been used as nanoindenter for determining the crosslink density and modulus of rubber on a fine scale. In the force mode, the force-distance curve is obtained as the sample is moved up against the tip. The slope of the force-distance curve can render information about mechanical properties of the sample examined.;The technique for this purpose has been established. Several instrumental and technical difficulties encountered during the development of the technique have been identified and solutions have been found. Cryogenic microtoming and a special sample preparation method have been used to: solve the problem of surface roughness and thickness variation of the sample. The hysteresis of the scanner was greatly reduced by using a scanner with a strain gauge attached. The stiffness of the cantilevers was determined by using beam theory and finite element analysis. Also the geometry of the tip was characterized and an empirical method developed to determine the force-penetration depth relationship. The conversion factor of sensor output to force was determined by using a sapphire as the sample to detect the cantilever deflection. Also the penetration depth was determined using the force-distance curve on the sapphire surface.;Force-distance measurements were made on a series of styrene-butadiene rubbers with different crosslink densities. The results of the modulus determined by AFM agreed with the modulus measured independently with dynamic mechanical analysis. The results of force/penetration depth show a linear relationship with the crosslink density. The AFM results also are shown to correlate linearly with the results from NMR.;The effects of rubber oxidation in air were also examined by the A technique. The results show that both homogeneous and heterogeneous oxidation can be detected by this method.