Chemistry Reference
In-Depth Information
Figure 11.12 Snapshots of the calculated structure of a poly(amido amine) dendrimer at high
and low pH. The size was predicted to increase by 13% upon progression from high to low pH.
studies revealed that the dendrimer swelling is more accurately attributed to the incor-
poration of water molecules within the dendrimer interior along with a localization
of counterions at the periphery, rather than merely a pH effect (Fig. 11.12; Maiti
and Goddard 2006).
A more recent SANS study revealed a 4% increase in dendrimer volume with
decreasing pH (Chen et al. 2007). Although these structural variations were signifi-
cantly less than predicted by the MD studies, this study confirmed the occurrence
of both counterion condensation and water penetration of the dendrimer interior.
11.3. SELF-ASSEMBLY AND AGGREGATION
11.3.1. Dendrimers and Dendrons
The phase segregation of polar and nonpolar segments of amphiphilic dendrimers that
produces structural features resembling unimolecular micelles actually creates a
strong propensity for self-association. High-generation dendrimers with high spheri-
cal symmetry achieve phase separation via an intramolecular folding process.
However, when intramolecular segregation is structurally impeded, intermolecular
aggregation produces assemblies consisting of a core of insoluble segments sur-
rounded by a solvated shell of solvophilic segments. The tendency for intermolecular
phase segregation is most pronounced for lower generation dendrimers exhibiting
highly dynamic conformations ranging from globular to disklike (Fig. 11.13).
For example, PPI dendrimers functionalized with azobenzene chromophores
appended with aliphatic side chains assemble into large spherical vesicles in water
below pH 8 (Fig. 11.14; Tsuda et al. 2000).
Cryo-transmission electron spectroscopy (TEM), scanning electron spectroscopy,
and confocal laser scanning microscopy studies indicated the presence of large,
