Aggregation of a hydrophobically modified poly(propylene imine) dendrimer

The poly(propylene imine) dendrimer DAB-dendr-(NH2)(8) was hydrophobically modified with dodecanoyl end groups. The modified dendrimer was deposited onto mica by adsorption from solution and observed by atomic force microscopy. With the decrease of adsorption time, the modified dendrimer varied from continuous film to scattered islands. For the adsorption time of 20s the dendrimer formed a sub-monolayer thin film that contained many fractal aggregates of fractal dimension 1.80 that were > 1 mu m in diameter and no more than 0.8nm thick. After 5 months at 25 degrees C, the initial fractal aggregates transformed into disks and other less-branched shapes with average heights of the domains of 0.6 nm and 0.4 nm, respectively. Formation of the fractal aggregates is explained by diffusion-limited aggregation. The slow reorganization of dendrimer molecules in the fractal aggregates occurs at a temperature well above the T-g of the dendrimer.