entry presents an overview of recent advances in the chemistry of polymeric DLCs
with a particular focus on their self-assembly which is useful for the preparation of
new functional soft materials. The mesomorphic behavior of polymeric DLCs is
described with a particular focus directed to the development of fundamental
understanding of structural-mesophase morphology relationship. A meticulous
attention is given to the dependence of mesomorphism on the influence of position
of discotic mesogens and connectivity in determining the supramolecular organization of these compounds. We have categorized all the discotic polymers into six
categories, and each of the category is further subdivided based on the type of core
structure of the discotic molecules. This entry will serve as general overview of
polymeric DLCs and their structure-property relationship based on each core.
Structure-Property Relationships
Discotic Main-Chain Polymers
In discotic main-chain polymers, discotic mesogens are incorporated into the main
chain of the polymer. The main chain of these polymers is generally made up of
polyether- or polyester-based moieties. Depending on the discotic cores, these polymers can be further classified into five categories.
Discotic Main-Chain Polymers Based on Rufigallol
In the hexahydroxyanthraquinone 1, the hydroxyl groups at 1- and 5-positions are
hydrogen bonded and therefore are less reactive. Under milder conditions, the hydrogen
bonded 1- and 5-positions do not get alkylated, and thus 1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone forms. Utilizing this strategy, Raja et al. (1998) have
prepared two series of thermotropic main-chain discotic liquid crystalline polymers
based on 1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone. These polymers can
be prepared in two ways. First, these polymers were prepared by reacting
1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone 2 (Scheme 1) and equimolar
amount of α,ω-dibromoalkane with aq. K 2 CO 3 in o-dichlorobenzene solvent under
nitrogen atmosphere at 90
C for 14 days in the presence of a phase transfer catalyst. In
the second method, these polymers were prepared by heating an equimolar mixture of
dihydroxy-tetraalkoxyanthraquinone 2 and α,ω-dibromoalkane in o-dichlorobenzene at
90
C in the presence of cesium carbonate for 10 days.
The length of the spacer in the polyethers was systematically varied. Gas phase
chromatography (GPC) studies revealed that all the polymers have moderate molecular weights between 5400 and 17,000. It can be seen from Table 1 that spacer to
peripheral alkyl chain length ratio highly influenced thermal behavior of these
polymers 3.1–3.8. The isotropization temperature inversely depends on spacer
length, when ratio is more than two. However, trend is opposite when ratio is less
than two. Also, the polymers exhibit the hexagonal columnar (Col h ) mesophase
when the ratio is more than two, while a rectangular columnar (Col r ) mesophase has
been seen when ratio is less than two.
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S. Setia et al.
with a particular focus on their self-assembly which is useful for the preparation of
new functional soft materials. The mesomorphic behavior of polymeric DLCs is
described with a particular focus directed to the development of fundamental
understanding of structural-mesophase morphology relationship. A meticulous
attention is given to the dependence of mesomorphism on the influence of position
of discotic mesogens and connectivity in determining the supramolecular organization of these compounds. We have categorized all the discotic polymers into six
categories, and each of the category is further subdivided based on the type of core
structure of the discotic molecules. This entry will serve as general overview of
polymeric DLCs and their structure-property relationship based on each core.
Structure-Property Relationships
Discotic Main-Chain Polymers
In discotic main-chain polymers, discotic mesogens are incorporated into the main
chain of the polymer. The main chain of these polymers is generally made up of
polyether- or polyester-based moieties. Depending on the discotic cores, these polymers can be further classified into five categories.
Discotic Main-Chain Polymers Based on Rufigallol
In the hexahydroxyanthraquinone 1, the hydroxyl groups at 1- and 5-positions are
hydrogen bonded and therefore are less reactive. Under milder conditions, the hydrogen
bonded 1- and 5-positions do not get alkylated, and thus 1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone forms. Utilizing this strategy, Raja et al. (1998) have
prepared two series of thermotropic main-chain discotic liquid crystalline polymers
based on 1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone. These polymers can
be prepared in two ways. First, these polymers were prepared by reacting
1,5-dihydroxy-2,3,6,7-tetraalkoxy-9,10-anthraquinone 2 (Scheme 1) and equimolar
amount of α,ω-dibromoalkane with aq. K 2 CO 3 in o-dichlorobenzene solvent under
nitrogen atmosphere at 90
C for 14 days in the presence of a phase transfer catalyst. In
the second method, these polymers were prepared by heating an equimolar mixture of
dihydroxy-tetraalkoxyanthraquinone 2 and α,ω-dibromoalkane in o-dichlorobenzene at
90
C in the presence of cesium carbonate for 10 days.
The length of the spacer in the polyethers was systematically varied. Gas phase
chromatography (GPC) studies revealed that all the polymers have moderate molecular weights between 5400 and 17,000. It can be seen from Table 1 that spacer to
peripheral alkyl chain length ratio highly influenced thermal behavior of these
polymers 3.1–3.8. The isotropization temperature inversely depends on spacer
length, when ratio is more than two. However, trend is opposite when ratio is less
than two. Also, the polymers exhibit the hexagonal columnar (Col h ) mesophase
when the ratio is more than two, while a rectangular columnar (Col r ) mesophase has
been seen when ratio is less than two.
62
S. Setia et al.