Chemistry Reference
In-Depth Information
FIGURE 10.15
Molecular 3-D structures of different dendrimers. The structure of the
PAMAM dendrimer (left) with optimization of the molecular dynamics calculations is from
the literature. The structure of the phenylazomethine dendrimer (right) with optimization of the
semiempirical orbital calculations is from the literature.
component. This observation is due to the spherical morphology that allows the least
contact between individual molecules. As an example of the phenylazomethine
dendrimer, the 3-D structure obtained through a molecular modeling calculation is
shown in Figure 10.15. The peripheral terminals of the DPA are hardly bent to the core
side, thus the molecule retains its star shape. The nanospace in the dendrimer
molecule is greater than that of the flexible PAMAM dendrimers [3,171-173]. This
characteristic also appeared in the experimental values obtained by a hydrodynamic
measurement in the solution phase (THF). The hydrodynamic radii (R
h
) of the DPAs
with PPh and Por cores are 2.0 and 2.2 nm, respectively. These values are consistent
with the estimated terminal-to-terminal length (
5 nm) based on the molecular
modeling of
DPAG4-Por
. The intrinsic viscosities of the DPAs ([
Z
]) increase from
G1 (generation 1) to G3, corresponding to the density decrease ([
[M]/V
h
). They
reach a constant value for dendrimers larger than G3. This behavior is much different
from that of
Z
]
a
the flexible dendrimers such as the polybenzylether
(Fr
echet
type) [126,127] dendrimers or PAMAMs [3]. The [
] values of these flexible
dendrimers decrease as the generation number increases after reaching the maximum
value at G3. These results reflect the fact that the backbones of the DPAs are not folded
back to the core even in a THF solution (Figure 10.15).
The
Z
-conjugation in the DPAs contributes not only to the rigidity but also to
the stability of the chemical bonding. Durability of the DPAs against heat decom-
position is nearly equal to that of engineering plastics. The temperatures for a
10% weight decrease under a N
2
atmosphere (T
d10%
) exceeds 500
C for each DPA
(Table 10.2) [72,174].
The encapsulating effect of the DPA shell has been clearly observed. The DPAs
having a redox active core (porphyrin, triphenylamine) showed the attenuation effect
p
