Phase Behaviors of Mainchain Liquid Crystals. 2H NMR and Pressure–Volume–Temperature Analysis of Dimer and Trimer Model Compounds

Our previous studies on the structure-property relation of dimer liquid crystals (LC), α,ω-bis(4-cyanobiphenyl-4′-yloxy)alkanes (CBA-n, n=9, 10), have been extended to include trimer compounds, 4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls (CBA-Tn, n=9,10), comprising three mesogenic units jointed by two intervening spacers. These nematic-LC-forming compounds were chosen as a model for the mainchain-type polymer LCs on the assumption that the spatial configuration and thermodynamic roles of the flexible spacer are nearly identical as long as the chemical structure of the repeating unit is similar. The 2H NMR method was extensively used to elucidate the orientational characteristics of these molecules in the mesophase. The analysis of binary mixtures with a low molar mass (monomer) LC gave an important information regarding the mesophase structure of CBA-Tn as well as CBA-n. The pressure–volume–temperature (PVT) relation was determined to estimate the constant–volume transition entropies at the Interphase such as crystal/nematic LC/isotropic melt. The results were found to be favorably compared with the conformational entropy changes derived on the basis of the rotational isomeric state analysis of the 2H NMR data. Finally, an important role of the flexible spacer was emphasized in determining the molecular ordering as well as thermodynamic properties of mainchain-type LCs.