Among several triblock copolymer systems, wherein only the terminal group of
the rod segment was changed, keeping all other structural features the same, it
is surprising that only the CF 3 -terminated system generated macromolecular objects with precisely controlled size having a very narrow size distribution. All other
triblock systems either generated insoluble bulk-crosslinked material or soluble
systems with much larger aggregate size and very broad size distributions (polydispersity > 8). This suggests that only in the case of the CF 3 -terminated derivative, was successful steric isolation of the coil PB segment during the crosslinking
process achievable. Interestingly, none of the dimensions of the aggregate varied
significantly after the crosslinking process; the average diameter of the aggregate
was ca. 2 nm, while the thickness of the layers formed when these objects were
packed was about 8 nm. Most importantly, films cast from solutions of the crosslinked macromolecular objects retained identical packing to those prepared directly
from the nascent triblock copolymers, confirming that little change had occurred
to the size and structure of the nano-aggregate upon crosslinking [70].
It is clear that while dealing with relatively smaller molecules, small changes in
the molecular structure can lead to dramatic differences in the nature and size of
the supramolecular aggregates formed. Terminating the rod-segment with a dendron (a branched segment) as in molecule D (in Figure 15.18), led to an unprecedented gelation in organic solvent at concentrations as low as 0.2 wt% [71]. By
varying the length of the biphenyl segment and the functionality at the termini,
it was established that the strongest gel was formed when 4 biphenyl segments
were present in the rod-segment which at the same time possesses at least four
hydroxy groups that enable the formation of an H-bonded structure. TEM pictures
clearly established the formation of nanoribbons with aspect ratios as high as 1000
(Figure 15.20b). From the width of the nanoribbon (ca. 6.5 nm) and its thickness
(2 nm), it is clear that bimolecular packing resulting from H-bonding between the
terminal dendrons could explain the formation of such nanostructures. Changing
the size of the coil segment, either by significantly increasing or decreasing its
length, led to either simple precipitation without gel formation or complete absence of gelation. This suggests that an optimum balance of rod–coil size was
critical for the generation of nanoribbons [71]. More recently, these nanoribbons
have been used to scaffold polystyrene that was prepared by polymerization of the
gel formed by dissolution of the dendron rod–coil system in styrene monomer.
The most interesting aspect of this study was that, despite the very low concentrations of the nanoribbons (@1 wt%), the entire solid polystyrene sample exhibited
birefringence, suggesting the enormous contact surface area between the nanoribbons and the matrix [72].
One of the major motivations for the study of nano-aggregate formation, either
in bulk or in solution, is because it provides an opportunity to construct macromolecular objects with dimensions significantly larger than molecular dimensions,
and therefore would pave the way to further organize them well into the macroscopic domain. Thus, this serves as a step-wise approach to organizing macromolecules in the bulk phase, with a control over the molecular organization at each
step along the way. The fixation of the mushroom-shaped supramolecular ag15 Nanostructured Polymers
506
the rod segment was changed, keeping all other structural features the same, it
is surprising that only the CF 3 -terminated system generated macromolecular objects with precisely controlled size having a very narrow size distribution. All other
triblock systems either generated insoluble bulk-crosslinked material or soluble
systems with much larger aggregate size and very broad size distributions (polydispersity > 8). This suggests that only in the case of the CF 3 -terminated derivative, was successful steric isolation of the coil PB segment during the crosslinking
process achievable. Interestingly, none of the dimensions of the aggregate varied
significantly after the crosslinking process; the average diameter of the aggregate
was ca. 2 nm, while the thickness of the layers formed when these objects were
packed was about 8 nm. Most importantly, films cast from solutions of the crosslinked macromolecular objects retained identical packing to those prepared directly
from the nascent triblock copolymers, confirming that little change had occurred
to the size and structure of the nano-aggregate upon crosslinking [70].
It is clear that while dealing with relatively smaller molecules, small changes in
the molecular structure can lead to dramatic differences in the nature and size of
the supramolecular aggregates formed. Terminating the rod-segment with a dendron (a branched segment) as in molecule D (in Figure 15.18), led to an unprecedented gelation in organic solvent at concentrations as low as 0.2 wt% [71]. By
varying the length of the biphenyl segment and the functionality at the termini,
it was established that the strongest gel was formed when 4 biphenyl segments
were present in the rod-segment which at the same time possesses at least four
hydroxy groups that enable the formation of an H-bonded structure. TEM pictures
clearly established the formation of nanoribbons with aspect ratios as high as 1000
(Figure 15.20b). From the width of the nanoribbon (ca. 6.5 nm) and its thickness
(2 nm), it is clear that bimolecular packing resulting from H-bonding between the
terminal dendrons could explain the formation of such nanostructures. Changing
the size of the coil segment, either by significantly increasing or decreasing its
length, led to either simple precipitation without gel formation or complete absence of gelation. This suggests that an optimum balance of rod–coil size was
critical for the generation of nanoribbons [71]. More recently, these nanoribbons
have been used to scaffold polystyrene that was prepared by polymerization of the
gel formed by dissolution of the dendron rod–coil system in styrene monomer.
The most interesting aspect of this study was that, despite the very low concentrations of the nanoribbons (@1 wt%), the entire solid polystyrene sample exhibited
birefringence, suggesting the enormous contact surface area between the nanoribbons and the matrix [72].
One of the major motivations for the study of nano-aggregate formation, either
in bulk or in solution, is because it provides an opportunity to construct macromolecular objects with dimensions significantly larger than molecular dimensions,
and therefore would pave the way to further organize them well into the macroscopic domain. Thus, this serves as a step-wise approach to organizing macromolecules in the bulk phase, with a control over the molecular organization at each
step along the way. The fixation of the mushroom-shaped supramolecular ag15 Nanostructured Polymers
506