Self-assembly that occurs at more than one length scale is known as hierarchical
self-assembly. From the technological point of view, hierarchical structures are of
particular interest since they can blend the complexity into the structure and lead to
multiple functional systems. One of the critical challenges that nanotechnology faces
is the transfer of the novel properties obtained at nanoscale into a higher length scale
for application reasons. Well-correlated structure at different length scales offers one
of the ideal strategies to fulfill this goal. Due to their long-chain-nature, polymeric
materials are known to be suitable for creating hierarchical self-assembly systems.
As a typical biopolymer, natural protein possesses a specific amino acid sequence
(primary structure), leading to its unique secondary, tertiary, and quaternary structures occurring at different length scales. These precise structure orderings also lead
to their specific function(s). Compared to the precision and elegant complexity of
biological systems, synthetic polymers are, in contrast, known as possessing relatively simple structures. However, linking different functional groups into synthetic
polymer backbones may lead to the synthetic hierarchical structures possessing
multiple functionalities that can mimic the complicated biopolymer structures and
functions.
To design a self-assembled molecular system with hierarchical structures, one
needs to achieve structural ordering at different length scales. Structural ordering
processes result from the competing molecular interactions (e.g., interactions
between hydrophobic versus hydrophilic components, van der Waals, Coulombic,
and hydrogen bonding). A number of different hierarchical assembly systems have
been successfully developed by employing one or more of these interactions. Liquid
crystalline (LC) block copolymers (BCPs) play a major role in creating hierarchical
self-assembled structures. LC molecules typically consist of an anisotropic aromatic
mesogen (with rod or disc shape) and aliphatic tails. In this chapter, we shall discuss
the hierarchical structure and assembly behaviors of LC BCPs in bulk states. We
shall first introduce BCP, followed by LC and LCPs. We will then summarize the
hierarchical structure of LC BCPs.
Phase Behavior of Block Copolymers
A BCP is a single phase macromolecular system formed by covalently combining
two or more polymers. The enthalpic and entropic factors associated with covalently
linking dissimilar polymers leads to intriguing phase behavior in BCPs (Bates 1991;
Bates and Fredrickson 1990; Bates and Fredrickson 1999; Hamley 1998;
Muthukumar et al. 1997). The thermodynamic interaction between two dissimilar
molecules A and B is given by the Flory-Huggins interaction parameter, χ AB , which
is the enthalpy term. The magnitude of χ AB for two polymers is usually positive
indicating that their mixing is not favored. In a typical blend of two polymers A
and B, this chemical incompatibility drives the system to undergo macrophase
separation leading to A-rich and B-rich domains, having characteristic dimensions
in the micron length scale, such that there is minimal segment-segment contact at the
interface. In an AB BCP, the chemical bond between the blocks prevents such
7 Structure and Assembly of Liquid Crystalline Block Copolymers
175
self-assembly. From the technological point of view, hierarchical structures are of
particular interest since they can blend the complexity into the structure and lead to
multiple functional systems. One of the critical challenges that nanotechnology faces
is the transfer of the novel properties obtained at nanoscale into a higher length scale
for application reasons. Well-correlated structure at different length scales offers one
of the ideal strategies to fulfill this goal. Due to their long-chain-nature, polymeric
materials are known to be suitable for creating hierarchical self-assembly systems.
As a typical biopolymer, natural protein possesses a specific amino acid sequence
(primary structure), leading to its unique secondary, tertiary, and quaternary structures occurring at different length scales. These precise structure orderings also lead
to their specific function(s). Compared to the precision and elegant complexity of
biological systems, synthetic polymers are, in contrast, known as possessing relatively simple structures. However, linking different functional groups into synthetic
polymer backbones may lead to the synthetic hierarchical structures possessing
multiple functionalities that can mimic the complicated biopolymer structures and
functions.
To design a self-assembled molecular system with hierarchical structures, one
needs to achieve structural ordering at different length scales. Structural ordering
processes result from the competing molecular interactions (e.g., interactions
between hydrophobic versus hydrophilic components, van der Waals, Coulombic,
and hydrogen bonding). A number of different hierarchical assembly systems have
been successfully developed by employing one or more of these interactions. Liquid
crystalline (LC) block copolymers (BCPs) play a major role in creating hierarchical
self-assembled structures. LC molecules typically consist of an anisotropic aromatic
mesogen (with rod or disc shape) and aliphatic tails. In this chapter, we shall discuss
the hierarchical structure and assembly behaviors of LC BCPs in bulk states. We
shall first introduce BCP, followed by LC and LCPs. We will then summarize the
hierarchical structure of LC BCPs.
Phase Behavior of Block Copolymers
A BCP is a single phase macromolecular system formed by covalently combining
two or more polymers. The enthalpic and entropic factors associated with covalently
linking dissimilar polymers leads to intriguing phase behavior in BCPs (Bates 1991;
Bates and Fredrickson 1990; Bates and Fredrickson 1999; Hamley 1998;
Muthukumar et al. 1997). The thermodynamic interaction between two dissimilar
molecules A and B is given by the Flory-Huggins interaction parameter, χ AB , which
is the enthalpy term. The magnitude of χ AB for two polymers is usually positive
indicating that their mixing is not favored. In a typical blend of two polymers A
and B, this chemical incompatibility drives the system to undergo macrophase
separation leading to A-rich and B-rich domains, having characteristic dimensions
in the micron length scale, such that there is minimal segment-segment contact at the
interface. In an AB BCP, the chemical bond between the blocks prevents such
7 Structure and Assembly of Liquid Crystalline Block Copolymers
175