Temperature-dependent formation of dendrimer islands from ring structures.

Previously unobserved high surface mobility and phase transformation phenomena in condensed, micron-scale dendrimer structures are documented using atomic force microscopy. Stratified dendrimer rings (a unique morphology resulting from microdroplet evaporation of dendrimer-alcohol solutions on mica) undergo dramatic temperature, time, and dendrimer-generation-dependent morphological changes associated with large-scale molecular rearrangements and partial melting. These transformations produce ring structures consisting of a highly stable first monolayer of the scalloped structure in equilibrium with spherical cap shaped dendrimer islands that form at the center of each pre-existing scallop (creating a "pearl necklace" structure). A generation-dependent critical temperature for dendrimer melting is determined. As-evaporated structures can be stabilized against thermally driven rearrangements by holding them at room temperature before annealing. Analysis of the dendrimer island shapes reveals a dependence of island contact angle on contact line curvature (island size) that varies systematically with dendrimer generation. A negative line tension, tau, is deduced in these systems. The morphological transformations in this system indicate the potential for creating complex, dendrimer-based multilevel structures and macroscopic-scale arrays using, for example, droplet-on-demand or dip pen nanolithography techniques, coupled with appropriate annealing and stabilizing treatments.