Nine polyesters were synthesized by polycondensation of silylated
hydroquinone with
substituted terephthaloyl chlorides. Three mono(alkylthio),
three bis(alkylthio), and three
tetra(alkylthio)terephthaloyl chlorides were used as reaction partners. The length
of the alkyl side chains was varied
among 8, 12, and 16 carbons. WAXS powder patterns indicate that
these polyesters form three different
types of sanidic layer structures in the solid state depending on the
number of side chains per repeating
unit. All three layer structures have an interdigitating of the
alkyl side chains in common. As evidenced
by 13C NMR CP/MAS spectroscopy and DSC measurements, the
longer side chains form crystalline paraffin
domains between the stacks of the main chains. The monosubstituted
polyesters form two different LC
phases, a biaxial nematic phase with layer structure and at higher
temperatures a normal nematic phase.
The disubstituted polyesters exclusively form a biaxial nematic
melt. Increasing length and number of
the side chains reduce the isotropization temperature of all
polyesters, and the tetrasubstituted polyesters
form an isotropic melt directly upon melting of the crystallized side
chains. This result demonstrates
that rigid rod polymers are not necessarily mesogenic. The failure
to form a LC phase and the good
solubility in many common solvents is attributed to poor attractive
electronic interactions between the
main chains. A more detailed interpretation was achieved by
computer modeling.
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