Molecules

Some of the most prominent examples of the synthesis of 3D molecules with a well-defined shape are spherical dendrimers, arborescent-graft polymers, monodendron-jacketed linear chains, and cylindrical brushes (Fig.1.). The shape of these molecules is controlled by the steric repulsion of its branches and by the branching symmetry.

Fig.1.Height image of single star-like macromolecules on mica. Scan size 300nm.

The dense grafting of these molecules gives them a well-defined shape. With a well-chosen monomer unit, the molecules can undergo conformational changes with changes in their environment. This functional behavior could be used to design stimuli-responsive objects that could work as tiny springs or even motors.

Molecular visualization provides a unique opportunity for the characterization of polymers. First, it gives direct evidence of their architecture. Second, it allows for accurate measurements of the number average molecular weight when combined with an appropriate sample preparation technique. Third, it enables size measurements of the individual branches separately from the size of the whole molecule.

For example, you can see images of single polymer molecules with minidendritic groups in Fig. 2. Due to the minidendritic groups, the molecules become thicker (about 7nm in diameter) so that they can be visualized using standard silicon AFM probes.

Fig. 2. Height (left) and phase (right) images of single macromolecules of polymer with minidendritic groups obtained in the tapping mode.Scan size is 320nm. Image courtesy of S. Magonov.

Single polymer molecules above T_g are delicate samples that require very gentle imaging conditions in tapping mode due to small size and weak adhesion to the substrate. General shape and average dimensions of the particles can be acquired using standard AFM tips with R<10nm. Imaging of soft delicate samples requires soft AFM cantilevers.

When fine features need to be resolved, the same AFM cantilevers with Hi'Res-C AFM tips should be used for further reduction of tip-sample interaction forces and geometrical dilation effects. When using Hi'Res-C AFM probes, "light tapping" conditions are preferable, which implies low resonance amplitude (start at 0.2V up to 1.2V) and a set point ratio about 0.9 to 1. Scan rate should start at below 1Hz. Scan size should start out at 50nm (250nm maximum). See high-resolution imaging page for details.