Supramolecular Polymerizations

526 A. Ciferri
Figure 19. (a) Micelles of coil-coil diblock copolymers in a
selective solvent undergoing supramolecular polymerization.
Hexagonally packed morphology as determined by means of
TEM. (b) Bilayers of rod-coil diblock copolymers in a solvent
selective for the coil block, growth directions within the plane of
the layer and perpendicular to it are shown. Layered morphology
[48, 112]
as determined by means of TEM.
cept has been used to describe solid arrays displaying
complex and ordered structurizations such as interpenetrating
nets. [111] The present author had suggested [1, 77] that
the three-dimensional solid state morphologies described
above, originating from molecular recognition of similar
blocks, ought to be described in terms of supramolecular
polymerization. The approach requires the identification
of unimers with proper functionality and of their one- or
multidimensional growth mechanism. [112] Relevant to this
end are the considerations set forth in Section 4.2 regarding
the separation of the modes of longitudinal and lateral
growth.
In particular, the formation of the hexagonal phase for
copolymers that cannot be crystallized could be described
as an essentially one-dimensional growth of micellar
unimers according to the mechanism of growth-coupledto-orientation.
By analogy with the processes illustrated
in Figure 13, leading to the hierarchical sequences 5 and
6, it has been suggested that the growth process schematized
in Figure 19a describes the formation of the hexagonal
phase of coil-coil block copolymers. [112] Here it is suggested
that, as for conventional surfactants, a block copolymer
micelle can assume an elongated form playing the
role of a bifunctional unimer. Upon increasing the concentration,
the latter undergoes linear growth simultaneously
with the development of nematic orientation.
This is followed by the hexagonal columnar phase (the
intermediate nematic phase may not appear for suitable
combinations of contact forces and persistence length) at
even higher concentration. The hexagonal phase should
be viewed as an embryo of the final morphology in the
condensed state. The coiled segments may dangle over
the lateral surface in a disordered fashion rather than
interdigitate regularly. Note that while the approach discussed
above does not have the predictive features of the
mean-field theory, it does predict a role of the persistence
length for the primary length scale, which is not predicted
by the latter theory.
The expectation that linear growth controls the formation
of hexagonal columnar mesophases is not limited to
coil-coil copolymers, or to systems with long molecular
axes normal to the growth direction. Bifunctional unimers
unable to grow along the lateral dimensions should in
general be candidates for linear growth. In the case of
keratin fibers, considering that the length of the microfibrils
by far exceeds the length of constituent chains and
falls in the range of the persistent length reported for
similar systems, [1] it is plausible that the assembly of the
fibril is also directed by the growth-coupled-to-orientation
mechanism. The following hierarchical assembly
sequence has therefore been suggested: extrusion of the
low sulfur protein into extracellular fluids e head-to-tail
assembly of shorter unimers into individual microfibrils
to a length related to the persistence length with simultaneous
orientation in the mesophase e stabilization of the
microfibril by internal 1S1S1 bonding and the two non
a-helical terminals e crosslinking of the sulfur-rich component
in the narrow interfibrillar space. [112]
The formation of lamellar structures could also be
described qualitatively in terms of the self-assembly of
specifically designed building blocks. Considering the
case of the single lamella schematized in Figure 19b,
growth akin to crystallization can occur along two perpendicular
in-plane directions: functionality is larger than
two. This mode of growth is at striking variance with the
case of the uni-dimensional growth of cylindrical, bifunctional
unimers considered above. It remains, however, to
be considered how the ordered polymerization along the
direction perpendicular to the lamellar plane is achieved.
It is possible that, as disoriented lamellae grow, a critical
axial ratio is attained at which purely hard interactions
stabilize a nematic phase of the discotic type. This type
of order has been experimentally evidenced during the
graphitization of organic materials when large and growing
planar rings are formed. [113] The coiling segments protruding
from the lamellae may dangle over the surface, as
in the case of cylindrical assemblies.
An alternative assembling mode along the direction
perpendicular to the lamellar plane could be based on
attractive forces, or interdigitation, among the coiled segments.
This interaction does not appear relevant to the
class of copolymers considered above, but was evidenced
for the comb-like poly(b-l-aspartate) with paraffinic side
chains. This system showed nematic order based on clusters
of helices correlated by interdigitation of partly molten
side chains. [106] The polymer formed a complete 3D
structure based on a layered organization of helices with
side chains crystallized in a separate hexagonal lattice.
Applications. Block copolymers are known to represent
an important class of materials allowing a desirable blend