Dendrimer Shape

There has been little experimental data on the generation dependence of molecular size to discriminate between the two models until recently. Some 

Molecular modeling techniques are a powerful tool to obtain a very detailed insight in the three-dimensional structure of dendrimer molecules at the atomic level. They have been applied to calculate sizes of the polypropylene imine) dendrimers and radial density profiles in order to estimate the free volume inside the dendrimers, as well as to make predictions about de Gennes dense-packed generations. The molecular modeling work by Coussens and co-workers [20] was focused on the generations 1-5 of the DAB-dendr-(CN)n and DAB-dendr-(NH2)n (n = 4, 8, 16,32, 64). 

The sizes of the dendrimers have been determined by calculating the molecular volumes, as defined by the van der Waals radii of the atoms, and by calculating the radii of gyration for several configurations of the dendrimers, as obtained from a molecular dynamics simulation at room temperature. The solvent influence on the calculated radii was estimated by scaling the nonbonded interactions between the atoms. Molecular volumes and average radii for ensembles of 500 conformations of the BAB-dendr-(NH2)D dendrimers have been collected in Table 26.2. 

The calculated radii with all interactions included are somewhat smaller than the radii measured with SANS, whereas the radii obtained with only the van der Waals repulsions taken into account are somewhat larger. As could be anticipated, the sizes of the dendrimers are dependent on the pH of the solution. Since both the primary and the tertiary amine groups may be protonated, repulsions begin when the pH of the solution is decreased. 

Mhe hydrodynamic radius ( = (5/3)1/2 Rg) was used for calculating this volume. cOnly repulsive van der Waals interactions taken into account. 

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The doubling or amplification that is inherent in dendrimer chemistry is the dominant process that controls the dendrimer shape [1], With each generation, the number of terminal units usually doubles. Each shell (generation) enhances at approximately a constant value, whereas the total molecular mass approximately doubles with each generation as does the number of branch points. While the dendrimer mass doubles with generation, the space to fit the units increases at a much slower rate. The contour length of any chain from the core to the terminal units is proportional to the number of chemical bonds and hence the number of... 

Controlling the size, shape and ordering of synthetic organic materials at the macromolecular and supramolecular levels is an important objective in chemistry. Such control may be used to improve specific advanced material properties. Initial efforts to control dendrimer shapes involved the use of appropriately shaped core templates upon which to amplify dendritic shells to produce either dendrimer spheroids or cylinders (rods). The first examples of covalent dendrimer rods were reported by Tomalia et al. [43] and Schluter et al. [44], These examples involved the reiterative growth of dendritic shells around a preformed linear polymeric backbone or the polymerization of a dendronized monomer to produce cylinders possessing substantial aspect ratios (i.e. 15-100) as observed by TEM and AFM. These architectural copolymers consisting of linear random... 

The spin coating technique of preparing PAMAM dendrimer samples for AFM is the best method to maintain undistorted dendrimer shape allows the visualization of isolated single dendrimer molecules. In spin coating, a dendrimer solution is rapidly spread across the sample surface and the majority of particles separate from one another. 

Corresponding electron-transport materials have been made by replacing the amino substituents in the first shell by oxadiazole groups (29) [70, 61] or phenyl-quinoxalines (30) [60]. As in the case of the amines, dendrimer-shaped structures are obtained by repeating the substitution pattern in a second shell (31) [76], The Tg was increased from 142°C in 29 to 222° C in 31. 

Figure 7. Divergent synthetic strategy to obtain polynuclear metal complexes of dendrimer shape. - ° Each deprotected compound of the dive ent synthetic approach can be used as a core in convergent synthetic processes. Some of these routes starting from a tetranuclear core are illustrated in Figure 8.

Dendrimer Shape Change A Nanoscale Molecular Morphogenesis. 344... 

Fig. 30 Congestion-induced dendrimer shape changes (/, //, III) with development of nanocontainer properties for a family of [core l,2-diaininoethane] (4 2) dendn-poly (amidoamine)-(NH2)z (G = 0-10) PAMAM dendiimers with core multiplicity = 4 and branch cell multiplicity A/b = 2. Distances between Z surface groups are shown as a fimction of generation [138]...

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