Structural manipulation of the frictional properties of linear polymers in single molecular layers

Manipulation of the frictional properties of self-assembled monolayers containing internal molecular scaffolding through subtle variations in the :,alkyl structure is demonstrated using atomic force microscopy. Photopolymerizable diacetylene groups are incorporated within the alkyl chains of the monolayer, resulting in noncovalent electronic interactions between diacetylene groups, in the unpolymerized form. Under these conditions, discrete transitions in the friction vs load curves are observed, indicating partial collapse of the monolayer at specific loads. Subsequent photopolymerization by UV irradiation results in covalent cross-linking between molecules and-the creation of a linear conjugated polymer backbone. This significantly alters the frictional properties of the monolayers by inhibiting lateral displacement within the monolayer. The presence of domains due to the directionality of the polymer backbone leads to a corresponding directionality in the frictional properties of the assembly that is not observed for the unpolymerized monolayers.