equilibrium-type BCP nanostructures, controlled nonequilibrium-type structure
formation processes leading to structural asymmetries, as well as formation of
hierarchical BCP materials with control over nanoscale and macroscale structures.
Bottom-up BCP SA efforts are described for various materials classes, including
ceramics, semiconductors, and metals. Lastly, after introducing the toolbox for
fabrication of novel nanomaterials, we describe efforts in several potential application areas that may benefit from BCP-derived nanomaterials. Some of the
applications have already been demonstrated, while others are still hypothetical.
We hope that this review can thus provide new impetus for future polymer research
directions.
2 Block Copolymer Self-Assembly
A block copolymer is a macromolecule composed of two or more chemically
distinct polymer blocks that are connected by a covalent bond. The polymer blocks
are sometimes thermodynamically incompatible, like oil (hydrophobic) and water
(hydrophilic). They can be covalently connected in different ways, resulting in a
variety of polymer architectures such as linear, star, and graft block copolymers
(see Fig. 1). Due to repulsive interactions between the incompatible polymer
blocks, all blocks of the same kind try to mix while staying separated from all
other blocks. Such behavior leads to formation of ordered structures, with a
periodicity comparable to the size of the BCP chain as measured, e.g., by its
end-to-end distance.
In order to better understand the SA behavior, it is instructive to look at entropy
and enthalpy changes in the formation of an ordered state. The Helmholtz free
energy of an AB diblock copolymer (di-BCP) is approximately given by:
F ¼ ÀlnQ þ χ AB Nf A f B
where F is the normalized Helmholtz free energy; Q is the single molecule partition
function relevant to conformational entropy of the BCP; f A and f B are volume
fractions of blocks A and B, respectively; χ AB is the Flory–Huggins interaction
parameter between A and B monomers; and N is the number of monomers along
the chain. The conformational entropy, given by the first term of the equation,
leads to mixing of the two blocks, while the enthalpic interactions, given by the
second term, usually favor separation of these blocks. The two competing effects
result in an ordered periodic structure, for example, if χ AB N > 10.5 with f A ¼ f B ,
whereas a disordered (DIS) bulk phase is formed otherwise.
The resulting structure is controllable by varying the block ratio and the degree
of polymerization of BCPs, as shown in Fig. 1b. For di-BCPs, the structure changes
from close-packed spheres (CPS, referring to face-centered or hexagonally close
packed spheres) to body-centered cubic (BCC), hexagonal cylinder (HEX), double
gyroid (GYR), and lamellar phases (LAM) (note that the bicontinuous O70
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
263
formation processes leading to structural asymmetries, as well as formation of
hierarchical BCP materials with control over nanoscale and macroscale structures.
Bottom-up BCP SA efforts are described for various materials classes, including
ceramics, semiconductors, and metals. Lastly, after introducing the toolbox for
fabrication of novel nanomaterials, we describe efforts in several potential application areas that may benefit from BCP-derived nanomaterials. Some of the
applications have already been demonstrated, while others are still hypothetical.
We hope that this review can thus provide new impetus for future polymer research
directions.
2 Block Copolymer Self-Assembly
A block copolymer is a macromolecule composed of two or more chemically
distinct polymer blocks that are connected by a covalent bond. The polymer blocks
are sometimes thermodynamically incompatible, like oil (hydrophobic) and water
(hydrophilic). They can be covalently connected in different ways, resulting in a
variety of polymer architectures such as linear, star, and graft block copolymers
(see Fig. 1). Due to repulsive interactions between the incompatible polymer
blocks, all blocks of the same kind try to mix while staying separated from all
other blocks. Such behavior leads to formation of ordered structures, with a
periodicity comparable to the size of the BCP chain as measured, e.g., by its
end-to-end distance.
In order to better understand the SA behavior, it is instructive to look at entropy
and enthalpy changes in the formation of an ordered state. The Helmholtz free
energy of an AB diblock copolymer (di-BCP) is approximately given by:
F ¼ ÀlnQ þ χ AB Nf A f B
where F is the normalized Helmholtz free energy; Q is the single molecule partition
function relevant to conformational entropy of the BCP; f A and f B are volume
fractions of blocks A and B, respectively; χ AB is the Flory–Huggins interaction
parameter between A and B monomers; and N is the number of monomers along
the chain. The conformational entropy, given by the first term of the equation,
leads to mixing of the two blocks, while the enthalpic interactions, given by the
second term, usually favor separation of these blocks. The two competing effects
result in an ordered periodic structure, for example, if χ AB N > 10.5 with f A ¼ f B ,
whereas a disordered (DIS) bulk phase is formed otherwise.
The resulting structure is controllable by varying the block ratio and the degree
of polymerization of BCPs, as shown in Fig. 1b. For di-BCPs, the structure changes
from close-packed spheres (CPS, referring to face-centered or hexagonally close
packed spheres) to body-centered cubic (BCC), hexagonal cylinder (HEX), double
gyroid (GYR), and lamellar phases (LAM) (note that the bicontinuous O70
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
263