Atomic force microscopy of elastomers: morphology, distribution of filler particles, and adhesion using chemically modified tips

The microdispersion of silica and carbon black-based filler particles in unvulcanized and vulcanized ethylene-propylene-diene terpolymer (EPDM) rubbers was investigated by atomic force microscopy (AFM). Tapping mode phase imaging was found to be particularly useful for imaging of the filler aggregates and for the visualization of single primary filler particles. It was demonstrated that the use of silane coupling agents significantly improves the microdispersion of silica filler in the rubber matrix, as compared to (a) silica without coupling agent, and (b) to carbon black. These results correlate very well with the observed mechanical properties of the materials. In addition, adhesion imaging and the analysis of measured pull-off forces allowed us to differentiate between the filler particles and the rubber matrix, as well as between different types of filler particles. The application of chemically modified AFM tips in pull-off force measurements allowed us to monitor the increase of the hydrophilicity as a result of plasma treatment of the surface of crosslinked poly(dimethylsiloxane), and as a result of chlorination of butyl rubber.