using surfactants. A number of reviews that discuss different aspects of both
approaches to the generation of polymeric nanostructures have appeared recently
[1–14]. Block copolymers form nanometer-size aggregates of various sizes and
shapes, due to the intrinsic property of structurally dissimilar macromolecular
segments to phase separate in bulk. They also form a variety of mesophases,
ranging from micellar aggregates to more complex bicontinuous gyroid phases,
when dissolved in an appropriate solvent (or solvent mixture), which functions as a
block-selective solvent, i.e. a medium that serves as a good solvent for one block and
a poor solvent for the other. Utilizing these phase-separated morphologies, in conjunction with domain-specific crosslinking strategies, a wide range of polymeric
nanostructures have been generated. The second approach is based on the utilization of the rich variety of aggregate morphologies formed by surfactants. Here
again, approaches either rely on direct fixation by polymerization of ordered nanometer-size phases formed by appropriately designed polymerizable surfactants, or
they utilize selective partitioning of monomers within specific domains followed by
polymerization to generate structures that mirror the original surfactant aggregate
morphology. Both approaches, using block copolymers and surfactants, have received considerable attention during the past decade and will form the bulk of the
discussions that follow.
Block copolymers, as will become evident, form one of the main underlying
structural motifs in a variety of strategies to prepare nanostructured polymeric
systems. This being the case, the first part of this chapter will discuss the underlying principles behind the preparation of block copolymers, together with a
brief introduction to living polymerization methods, which are the most important
synthetic tool for the generation of block copolymers. Different strategies for utilizing bulk phase-separated morphologies to generate nanostructures, ranging
from nanofibers, nanotubes and nanochannels, will be highlighted. Similarly, approaches that utilize block-selective solvents to generate a rich variety of lyotropic
phases, and approaches for their permanent fixation to generate macromolecular
objects such as nanospheres and nanobubbles, will be discussed. Towards the end,
exciting developments in surfactant-based strategies for the preparation of hollow
nanospheres and nanoporous polymeric systems will be described.
15.2
Macromolecular Structural Control
Unlike the organization of small molecules into crystalline lattices, polymeric
molecules typically do not organize in an orderly fashion to form a macroscopic
solid. This is due to the innumerable conformations of similar energies which long
polymer chains can adopt. In turn, this leads to a topologically heterogeneous collection of molecular entities that cannot readily organize into a typical crystalline
molecular solid. The problem is further compounded by yet another intrinsic constraint while dealing with synthetic polymers, which is the molecular structural
heterogeneity that arises from the distribution of molecular weight (chain lengths),
15.2 Macromolecular Structural Control 477
approaches to the generation of polymeric nanostructures have appeared recently
[1–14]. Block copolymers form nanometer-size aggregates of various sizes and
shapes, due to the intrinsic property of structurally dissimilar macromolecular
segments to phase separate in bulk. They also form a variety of mesophases,
ranging from micellar aggregates to more complex bicontinuous gyroid phases,
when dissolved in an appropriate solvent (or solvent mixture), which functions as a
block-selective solvent, i.e. a medium that serves as a good solvent for one block and
a poor solvent for the other. Utilizing these phase-separated morphologies, in conjunction with domain-specific crosslinking strategies, a wide range of polymeric
nanostructures have been generated. The second approach is based on the utilization of the rich variety of aggregate morphologies formed by surfactants. Here
again, approaches either rely on direct fixation by polymerization of ordered nanometer-size phases formed by appropriately designed polymerizable surfactants, or
they utilize selective partitioning of monomers within specific domains followed by
polymerization to generate structures that mirror the original surfactant aggregate
morphology. Both approaches, using block copolymers and surfactants, have received considerable attention during the past decade and will form the bulk of the
discussions that follow.
Block copolymers, as will become evident, form one of the main underlying
structural motifs in a variety of strategies to prepare nanostructured polymeric
systems. This being the case, the first part of this chapter will discuss the underlying principles behind the preparation of block copolymers, together with a
brief introduction to living polymerization methods, which are the most important
synthetic tool for the generation of block copolymers. Different strategies for utilizing bulk phase-separated morphologies to generate nanostructures, ranging
from nanofibers, nanotubes and nanochannels, will be highlighted. Similarly, approaches that utilize block-selective solvents to generate a rich variety of lyotropic
phases, and approaches for their permanent fixation to generate macromolecular
objects such as nanospheres and nanobubbles, will be discussed. Towards the end,
exciting developments in surfactant-based strategies for the preparation of hollow
nanospheres and nanoporous polymeric systems will be described.
15.2
Macromolecular Structural Control
Unlike the organization of small molecules into crystalline lattices, polymeric
molecules typically do not organize in an orderly fashion to form a macroscopic
solid. This is due to the innumerable conformations of similar energies which long
polymer chains can adopt. In turn, this leads to a topologically heterogeneous collection of molecular entities that cannot readily organize into a typical crystalline
molecular solid. The problem is further compounded by yet another intrinsic constraint while dealing with synthetic polymers, which is the molecular structural
heterogeneity that arises from the distribution of molecular weight (chain lengths),
15.2 Macromolecular Structural Control 477