about by keeping the f
coil constant and changing the coil chemistry from PEO to PPO
due to more space requirements of the bulkier PPO coil. In a system with PPO as the
coil block and the rod made of two biphenyl units with constant length of the rod
block, the authors observed SmA and SmC structures of the rods at symmetric f
coil ,
forming layered structures. An isotropic bicontinuous cubic phase and hexagonally
closed packed C of the rods were observed with an increase in the asymmetry ( f
coil )
(Fig. 8a, b) (Lee et al. 2001). Within the minority domains, the rods formed
aggregates with square cross-section. Further increase in f
coil led to the formation
of discrete spherical micellar structures. In all these phases, the rods in the minority
blocks possess a bilayered Sm structure. A slight increase in the rod length by using
an extra phenyl linkage showed remarkably different phase structures stabilized by
improved LC ordering. Unique honey-comb like layered structures, where PPO
formed perforations of 6.5 nm arranged 10 nm apart with hexagonal in-plane
symmetry (Fig. 8c) (Lee et al. 2001). Furthermore, attaching PPO coils on either
end of the rod resulted in the formation of oblate-shaped aggregates arranged in a
body-centered tetragonal (T) lattice (Fig. 8d) (Lee et al. 2001). Based on X-ray
scattering data, the authors calculated 84 molecules per aggregate in the T packed
structure (diameter 5 nm and length 3 nm). Based on theoretical theoretical calculations, the authors propose that the oblate shape of the aggregates is responsible for
the unique T arrangement as this structure imparts maximum packing density to the
system. Further increasing the length of the rod by introducing a phenyl group into
the molecule resulted in a hexagonally ordered mesophase that transformed into a T
micellar phase only at high temperatures. Further increasing the length of the rod
completely suppressed the formation of the T phase and resulted in L morphologies
with in-plane hexagonal symmetry in the rod block. The strong effect of coil
stretching penalty was demonstrated by increasing the grafting density of polymer
chains per interface. This was achieved by serial combination of (rod-coil) n units
with varying n values. In bulk, when n = 1, L crystalline structures were observed
and when n = 2 and n = 3, the morphology changed to 2D rectangular crystalline
and hexagonal columnar structures, respectively. As n increased, the grafting density
per interface increased and this resulted in the transformation of sheet like rod
domains into individual aggregates arranged in a 2D lattices so as to reduce the
stretching penalty of the coils.
Rod blocks made up of conjugated molecules have also been used to form
RCBCPs. Conjugated molecules consist of rigid cores that impart LC character to
the molecule. Aggregation of these molecules to form LC phases helps in orientation
of the delocalized π electrons making them ideal candidates for photonic and
electronic applications. Organizing these materials into ordered macroscopic structures is necessary to exploit their properties in the bulk. RCBCPs formed by π
conjugated oligomers and polymers (like oligo- or poly- p-phenylenevinylene
(PV), ( p-phenylene) (PP), thiophenes, phenyl quinolines (PQ), phenylene
ethynylene (PhE)) thus led to the formation of materials with structural and functional hierarchy. PPP and PPV have potential applications as components in light
emitting diodes (LEDs) or solid state light emitting cells (SECs), photovoltaic cells
etc. (Hide et al. 1996; Sirringhaus et al. 1998). Using these polymers as rod blocks in
7 Structure and Assembly of Liquid Crystalline Block Copolymers
189
coil constant and changing the coil chemistry from PEO to PPO
due to more space requirements of the bulkier PPO coil. In a system with PPO as the
coil block and the rod made of two biphenyl units with constant length of the rod
block, the authors observed SmA and SmC structures of the rods at symmetric f
coil ,
forming layered structures. An isotropic bicontinuous cubic phase and hexagonally
closed packed C of the rods were observed with an increase in the asymmetry ( f
coil )
(Fig. 8a, b) (Lee et al. 2001). Within the minority domains, the rods formed
aggregates with square cross-section. Further increase in f
coil led to the formation
of discrete spherical micellar structures. In all these phases, the rods in the minority
blocks possess a bilayered Sm structure. A slight increase in the rod length by using
an extra phenyl linkage showed remarkably different phase structures stabilized by
improved LC ordering. Unique honey-comb like layered structures, where PPO
formed perforations of 6.5 nm arranged 10 nm apart with hexagonal in-plane
symmetry (Fig. 8c) (Lee et al. 2001). Furthermore, attaching PPO coils on either
end of the rod resulted in the formation of oblate-shaped aggregates arranged in a
body-centered tetragonal (T) lattice (Fig. 8d) (Lee et al. 2001). Based on X-ray
scattering data, the authors calculated 84 molecules per aggregate in the T packed
structure (diameter 5 nm and length 3 nm). Based on theoretical theoretical calculations, the authors propose that the oblate shape of the aggregates is responsible for
the unique T arrangement as this structure imparts maximum packing density to the
system. Further increasing the length of the rod by introducing a phenyl group into
the molecule resulted in a hexagonally ordered mesophase that transformed into a T
micellar phase only at high temperatures. Further increasing the length of the rod
completely suppressed the formation of the T phase and resulted in L morphologies
with in-plane hexagonal symmetry in the rod block. The strong effect of coil
stretching penalty was demonstrated by increasing the grafting density of polymer
chains per interface. This was achieved by serial combination of (rod-coil) n units
with varying n values. In bulk, when n = 1, L crystalline structures were observed
and when n = 2 and n = 3, the morphology changed to 2D rectangular crystalline
and hexagonal columnar structures, respectively. As n increased, the grafting density
per interface increased and this resulted in the transformation of sheet like rod
domains into individual aggregates arranged in a 2D lattices so as to reduce the
stretching penalty of the coils.
Rod blocks made up of conjugated molecules have also been used to form
RCBCPs. Conjugated molecules consist of rigid cores that impart LC character to
the molecule. Aggregation of these molecules to form LC phases helps in orientation
of the delocalized π electrons making them ideal candidates for photonic and
electronic applications. Organizing these materials into ordered macroscopic structures is necessary to exploit their properties in the bulk. RCBCPs formed by π
conjugated oligomers and polymers (like oligo- or poly- p-phenylenevinylene
(PV), ( p-phenylene) (PP), thiophenes, phenyl quinolines (PQ), phenylene
ethynylene (PhE)) thus led to the formation of materials with structural and functional hierarchy. PPP and PPV have potential applications as components in light
emitting diodes (LEDs) or solid state light emitting cells (SECs), photovoltaic cells
etc. (Hide et al. 1996; Sirringhaus et al. 1998). Using these polymers as rod blocks in
7 Structure and Assembly of Liquid Crystalline Block Copolymers
189