Among the various
ordered morphologies self-assembled from block
copolymers, the spherical packing phases are particularly interesting
because they resemble the familiar atomic crystals. The commonly observed
spherical morphology of block copolymers is the body-centered-cubic
phase. Recently, a number of novel spherical packing phases, i.e.,
the complex Frank–Kasper phases originally obtained in metallic
alloys, have been observed in block copolymer melts. Theoretical studies
have revealed that conformational asymmetry of the different blocks
provides a key mechanism to stabilize the Frank–Kasper phases.
Furthermore, local segregation of different copolymers in blends of
diblock copolymers and copolymer architectures provides additional
mechanisms to enhance the stability of the complex ordered phases.
In this Viewpoint we summarize recent advances in our understanding
of the formation of the nonclassical spherical packing phases in AB-type
block copolymers, emphasizing the formation mechanisms of these fascinating
complex ordered structures.
The
thermodynamics of dislocations in thin films of lamella-forming
diblock copolymers and their climb and glide motions are investigated
using single-chain-in-mean-field (SCMF) simulations and self-consistent
field theory (SCFT) in conjunction with the string method. The glide
motion of a defect perpendicular to the stripe pattern is characterized
by large free energy barriers. The barriers not only stem from altering
the domain topology; an additional barrier arises from a small-amplitude
but long-range domain displacement. In contrast, the climb motion
along the stripes does not involve a free energy barrier in accord
with the continuous translational invariance along the stripe. Thus,
the perpendicular distance (“impact parameter”) between
a pair of defects is approximately conserved. Dislocation pairs with
opposite Burgers vectors attract each other and move toward each other
(“collide”) via climb motion. We find that the forces
between apposing defects significantly depend on system size, and
the Peach–Koehler force in smectic structures only becomes
accurate for extremely large system sizes. Moreover, we observe in
SCMF simulations that the defect annihilation time qualitatively and
nonmonotonously depends on the defects’ perpendicular distance
and rationalize this finding by the collective kinetics along the
minimum free energy path (MFEP) and the single-chain dynamics in an
inhomogeneous environment.
The self-assembly
of amphiphilic macromolecules into various mesocrystals has attracted
abiding interest. Although many interesting mesocrystals have been
achieved, mesocrystals of a low coordination number (CN) such as simple
cubic are rarely reported. Here we purposely design an AB-type multiblock
copolymer to target exotic spherical phases of low CNs. Self-consistent
field theory reveals that two sophisticated mechanisms are realized
in the copolymer, that is, stretched bridging block and released packing
frustration, synergistically leading to the formation of three spherical
phases with extremely low CNs, including the simple cubic spheres
(CN = 6), the cubic diamond spheres (CN = 4), and normally aligned
hexagonal-packing spheres (6 < CN < 8) in a considerable parameter
region. Moreover, we demonstrate that these exotic phases are hard
to be stabilized by either of the two mechanisms individually.
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