Fine scale characterization of crosslinking with the atomic force microscope

Fine-scale variations of the crosslink density of elastomers are important scientifically and technologically. We have shown that the atomic force microscope (AFM), used in the force/distance mode, is sensitive to the crosslink density. In this mode, the sample is pushed against the tip in a manner similar to that for standard hardness testing. However, the end of the tip of the AFM is only 100 to 500 Angstrom in radius. Further, it can be moved in a controlled manner in Very fine increments both from point to point over the surface and into the surface of a sample. As a result, it can sample much more finely spaced and smaller volumes of material than either the typical hardness tester or the usual methods of measuring crosslink density. Samples of a styrene-butadiene copolymer were compounded with varying amounts of dicumyl peroxide and cured under compression molding at 16 degrees C. Crosslink density was measured by swelling. The results show a significant Variation of force/distance penetrated with crosslink density. A plot of experimental value of force/square of distance penetrated vs. crosslink density is comparable to a theoretical one calculated with the crosslink density data and the half angle of the conical probe tip. Measurements at a spacing of 170 nm along straight lines are reported as well as measurements which show the technique to be sensitive to oxidation at short times. The effects of Variables such as sample roughness, canting of the tip with respect to the sample surface, tilting of the sample surface, penetration depth, sample oxidation, and other Variables are considered.