Percec-type dendrons, as shown in Chart 5 (Percec et al. 1998a). The corresponding
monodendron, 12G2-AG-COOH, exhibits a cubic mesophase arising from the
assembly of nanospheres. Each nanosphere consists of 12 conic 12G2-AGCOOH molecules. When DP of a corresponding polymer is lower than the threshold
value (~20 and 15 for 12G2-AG-PS and 12G2-AG-PMA, respectively), the
self-assembly is dominated by the conical dendrons and leads to the formation
of nanospheres and a cubic mesophase with a random-coil main-chain.
However, when the DP is greater than the threshold value, the space-demanding
dendrons force the polymer chains into rigid-rods and form a Col h phase where
dendrons themselves deform to a flat-tapered shape. X-ray diffraction analysis
indicated that the PS chain is in an almost fully extended conformation in the
Col h phase.
Rigid-Rod Polymers
This class of polymers forms a columnar LC phase due to its intrinsically rigid mainchain. Flexible side-groups are usually needed to increase their molecular mobility.
Rigid polysaccharides are capable of exhibiting a columnar LC phase when
equipped with proper flexible side-chains. For instance, a Col h phase was observed
in triacylated celluloses (CTA-m) for m = 8–18 (Chart 6) (Yamagishi et al. 1991).
Very recently, a series of nearly monodisperse columnar phase-forming polyamidosaccharides (PAS-n in Chart 6), polysaccharides with an amide linkage
replacing one ether linkage, have been reported (Ghobril et al. 2014). The phase
behavior of the polymer is MW dependent. PAS-22 only shows a smectic phase,
while PAS-55 and PAS-100 exhibit a Col h phase between the room temperature
smectic phase and the isotropic phase. The lack of Col h phase in PAS-22 can be
explained by the insufficient aspect ratio.
Phthalocyanine polymers with chemical bonds connecting center ions in adjacent
phthalocyanine units are very rigid. When equipped with long, flexible alkyl chains, a
columnar LC phase has been observed in those polymers with Sn
4+ (Sirlin et al. 1987)
or Si
4+ (Yoneda et al. 2015) as the center ion. Silicon-centered phthalocyaninatopolysiloxanes (PcPSiOs in Chart 6) are capable of forming a Col h phase with highly
ordered and tight phthalocyanine π-stacks. Due to covalent bonds connecting adjacent aromatic cores, no isotropization transition can be detected. Most of polymers
exhibit a narrow diffraction at ~0.33 nm indicating well-ordered phthalocyanine
π-stacks enforced by Si-O-Si bond. The importance of the Si-O-Si is also supported
by the fact that some phthalocyanine monomers do not show LC behaviors, when the
corresponding polymers organize in a columnar LC phase.
A rectangular columnar phase has also been observed in the complex of a cationic
rigid-rod polyparaphenylene and a fluorinated amphiphile, as shown in Chart 6.
There are two columns in each cell, with three to four polyparaphenylene chains
bundled together in each column which is bound ionically to the fluorinated matrix.
The most probable orientation of the polymer chain is along the column direction
(Thünemann et al. 2000a).
5 Columnar Phase-Forming Polymers
131
monodendron, 12G2-AG-COOH, exhibits a cubic mesophase arising from the
assembly of nanospheres. Each nanosphere consists of 12 conic 12G2-AGCOOH molecules. When DP of a corresponding polymer is lower than the threshold
value (~20 and 15 for 12G2-AG-PS and 12G2-AG-PMA, respectively), the
self-assembly is dominated by the conical dendrons and leads to the formation
of nanospheres and a cubic mesophase with a random-coil main-chain.
However, when the DP is greater than the threshold value, the space-demanding
dendrons force the polymer chains into rigid-rods and form a Col h phase where
dendrons themselves deform to a flat-tapered shape. X-ray diffraction analysis
indicated that the PS chain is in an almost fully extended conformation in the
Col h phase.
Rigid-Rod Polymers
This class of polymers forms a columnar LC phase due to its intrinsically rigid mainchain. Flexible side-groups are usually needed to increase their molecular mobility.
Rigid polysaccharides are capable of exhibiting a columnar LC phase when
equipped with proper flexible side-chains. For instance, a Col h phase was observed
in triacylated celluloses (CTA-m) for m = 8–18 (Chart 6) (Yamagishi et al. 1991).
Very recently, a series of nearly monodisperse columnar phase-forming polyamidosaccharides (PAS-n in Chart 6), polysaccharides with an amide linkage
replacing one ether linkage, have been reported (Ghobril et al. 2014). The phase
behavior of the polymer is MW dependent. PAS-22 only shows a smectic phase,
while PAS-55 and PAS-100 exhibit a Col h phase between the room temperature
smectic phase and the isotropic phase. The lack of Col h phase in PAS-22 can be
explained by the insufficient aspect ratio.
Phthalocyanine polymers with chemical bonds connecting center ions in adjacent
phthalocyanine units are very rigid. When equipped with long, flexible alkyl chains, a
columnar LC phase has been observed in those polymers with Sn
4+ (Sirlin et al. 1987)
or Si
4+ (Yoneda et al. 2015) as the center ion. Silicon-centered phthalocyaninatopolysiloxanes (PcPSiOs in Chart 6) are capable of forming a Col h phase with highly
ordered and tight phthalocyanine π-stacks. Due to covalent bonds connecting adjacent aromatic cores, no isotropization transition can be detected. Most of polymers
exhibit a narrow diffraction at ~0.33 nm indicating well-ordered phthalocyanine
π-stacks enforced by Si-O-Si bond. The importance of the Si-O-Si is also supported
by the fact that some phthalocyanine monomers do not show LC behaviors, when the
corresponding polymers organize in a columnar LC phase.
A rectangular columnar phase has also been observed in the complex of a cationic
rigid-rod polyparaphenylene and a fluorinated amphiphile, as shown in Chart 6.
There are two columns in each cell, with three to four polyparaphenylene chains
bundled together in each column which is bound ionically to the fluorinated matrix.
The most probable orientation of the polymer chain is along the column direction
(Thünemann et al. 2000a).
5 Columnar Phase-Forming Polymers
131