Liquid Crystals: VII. Smectic-Nematic Transition Temperature as a Function of Alkyl End Group Length in p -Phenylene Di- p-n -alkoxybenzoates

The Sm-N transition temperatures of the C1-C12 homologs of some p-phenylene di-p-n-alkoxybenzoates were studied. For three homologous series of these esters (unsubstituted, methyl-substituted, and chlorosubstituted on the central phenylene ring), the transition temperature was found to decrease at first on ascending the series, pass through a minimum at the C4 homolog, and then increase regularly. The data suggest that intermolecular attraction in the nematic and smectic mesophases and molecular order in the nematic mesophase increase modestly with increasing alkyl chain length, but molecular order in the smectic mesophase rises sharply on going from the C1 to the C4 homolog and then much more gradually. This would result in a large increase of ΔS transition relative to ΔH transition on first ascending the series and, therefore, a drop in the transition temperature T = ΔH/ΔS. From the C4 to the C12 homolog, ΔH apparently increases faster than ΔS and T rises.