ABSTRACT
Beach et al [150] measured pull-off forces between hexadecanethiol monolayers, self-assembled on gold-coated silicon nitride cantilever tip and silicon wafer, using AFM. The authors concluded that the AFM technique appeared to be a very useful tool in the examination of surface free energy of engineered materials. The surface energy of the self-assembled monolayer of hexadecanethiol was calculated to be in the range 24.28 ± 6.61 to 26.93 ± 9.57 mJ/m2 using the measured pull-off force values. These values are between the values reported in the literature from contact angle and force curve measurements. Duwez and Nysten [136] used tips modified with methyl-and hydroxyl-terminated alkanethiols and showed that AFM tips functionalized with alkanethiol SAMs could be utilized to map the distribution of adhesion forces on polypropylene (PP) surfaces (Fig. 7). The image in Fig. 7 shows the lateral distribution of pull-off forces. The authors also found evidence for additives migrating toward the surface and modification of additive distribution on the surface due to material aging, utilizing laterally resolved adhesion force maps [151].
