situation arises in polysoaps, a specific class of macromolecules that possess
equally spaced ionic moieties that tend to cluster in hydrophobic solvents, in a situation akin to aggregation of surfactants to form micelles [28]. Since, in the case of
polysoaps, the surfactant molecules are strung together to form a single polymer
chain, the aggregation is present even at very dilute polymer concentrations, unlike
the case of simple surfactants which exhibit a critical micelle concentration (CMC).
Effective strategies to fold synthetic macromolecules, therefore, will need to preclude such aggregation, which occurs due to the isotropic nature of the interaction
between the segments. Instead, by the introduction of a strong directionality to the
interaction between the equi-spaced units along the polymer chain, the chains can
be made to fold (see Figure 15.2). For instance, a design wherein the interacting
segments along the polymer backbone are flat disk-like aromatic units (with high
aspect ratio) could lead to the generation of a folded structure that results from
stacking of the disk-like segments. Such a directional aggregation could be termed
intra-chain self-assembly. An interesting example of such an assembly was recently
demonstrated in H-bonded polymeric systems based on disk-shaped units [29]. A
few other examples of such assembly that utilize other types of interactions, such
as donor–acceptor interactions, p-stacking interactions and solvophobic effects,
often in tandem, have been reported in oligomeric systems [30].
To summarize, folding synthetic macromolecules into topologically well-defined
molecular entities remains a major challenge. The significant strides that have
been taken in supramolecular science [31], which deals with the assembly of small
molecules using a variety of non-covalent interactions, would serve as an ideal
launching pad for the development of precisely folded macromolecules. It is evident that several weak interactions will need to work in tandem to overcome the
entropic penalty that accompanies the folding process. Several steps have been
taken in the right direction and many more are needed before we can accomplish
even a small fraction of what nature has perfected.
15.4
Morphology of Block Copolymers
The majority of polymers are immiscible and, in bulk, they phase separate to form
domains of varying sizes and shapes, depending on their relative volume fraction.
This happens because of the very low entropy of mixing in the case of large polymeric molecules. Therefore, unless there is a large favorable enthalpic contribution, most polymers do not form molecularly miscible systems. The same is true for
block copolymers, in which the length of each block exceeds a certain critical value.
As mentioned earlier, block copolymers are systems wherein two (or more) different types of homopolymers are linked to each other at the chain end(s); diblock
copolymers, represented as (A) m –(B) n , are systems in which two homopolymers
are linked to each other at one end, while triblock copolymers, represented as
(A) m –(B) n –(C) p , are systems in which one central homopolymer block is linked at
either end with two other homopolymers. The values m, n and p, represent the
15 Nanostructured Polymers
484
equally spaced ionic moieties that tend to cluster in hydrophobic solvents, in a situation akin to aggregation of surfactants to form micelles [28]. Since, in the case of
polysoaps, the surfactant molecules are strung together to form a single polymer
chain, the aggregation is present even at very dilute polymer concentrations, unlike
the case of simple surfactants which exhibit a critical micelle concentration (CMC).
Effective strategies to fold synthetic macromolecules, therefore, will need to preclude such aggregation, which occurs due to the isotropic nature of the interaction
between the segments. Instead, by the introduction of a strong directionality to the
interaction between the equi-spaced units along the polymer chain, the chains can
be made to fold (see Figure 15.2). For instance, a design wherein the interacting
segments along the polymer backbone are flat disk-like aromatic units (with high
aspect ratio) could lead to the generation of a folded structure that results from
stacking of the disk-like segments. Such a directional aggregation could be termed
intra-chain self-assembly. An interesting example of such an assembly was recently
demonstrated in H-bonded polymeric systems based on disk-shaped units [29]. A
few other examples of such assembly that utilize other types of interactions, such
as donor–acceptor interactions, p-stacking interactions and solvophobic effects,
often in tandem, have been reported in oligomeric systems [30].
To summarize, folding synthetic macromolecules into topologically well-defined
molecular entities remains a major challenge. The significant strides that have
been taken in supramolecular science [31], which deals with the assembly of small
molecules using a variety of non-covalent interactions, would serve as an ideal
launching pad for the development of precisely folded macromolecules. It is evident that several weak interactions will need to work in tandem to overcome the
entropic penalty that accompanies the folding process. Several steps have been
taken in the right direction and many more are needed before we can accomplish
even a small fraction of what nature has perfected.
15.4
Morphology of Block Copolymers
The majority of polymers are immiscible and, in bulk, they phase separate to form
domains of varying sizes and shapes, depending on their relative volume fraction.
This happens because of the very low entropy of mixing in the case of large polymeric molecules. Therefore, unless there is a large favorable enthalpic contribution, most polymers do not form molecularly miscible systems. The same is true for
block copolymers, in which the length of each block exceeds a certain critical value.
As mentioned earlier, block copolymers are systems wherein two (or more) different types of homopolymers are linked to each other at the chain end(s); diblock
copolymers, represented as (A) m –(B) n , are systems in which two homopolymers
are linked to each other at one end, while triblock copolymers, represented as
(A) m –(B) n –(C) p , are systems in which one central homopolymer block is linked at
either end with two other homopolymers. The values m, n and p, represent the
15 Nanostructured Polymers
484