Molecular stiffness of individual hyperbranched macromolecules at solid surfaces

The elastic properties of dendritic (hyperbranched) molecules with dimensions below 3 nm have been probed with atomic force microscopy (AFM), which allows for the micromapping of the surface stiffness with nanoscale resolution. To anchor dendritic molecules with hydroxyl terminal groups and reduce tip-molecule interactions, a modification of the silicon surface with an amine-terminated self-assembled monolayer (SAM) and AFM tips with methyl-terminated SAMs was used. The nanomechanical response was analyzed in the terms of sequential deformation of dendritic molecules and alkyl-silane monolayers. We observed higher elastic modulus of individual dendritic molecules of the fourth generation in comparison with the corresponding third generation (350 vs 190 MPa). This difference is caused by more shape persistent properties of dendritic molecules with denser shells. Higher stiffness was also revealed for molecules within long-chain aggregates as compared to individual molecules and small aggregates. We speculate that this is caused by additional lateral constraints due to the presence of densely packed neighboring "border" molecules tethered to the supporting substrate.