Orientational order and thermodynamic properties of mainchain trimer liquid crystals: a combined use of 2H NMR, PVT and high-pressure differential thermal analysis

The trimer compounds (4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls) (CBA-Tn, n=9, 10) with n indicating the number of carbon atoms in the spacer exhibit a nematic order on cooling from the melt. The orientational characteristics of the central and terminal mesogenic cores were separately investigated in the liquid-crystalline (LC) state for CBA-T10 by the 2H NMR method. The ratio of the quadrupolar splittings was found to remain nearly constant over the entire range of the nematic state, suggesting that the conformation of the intervening spacer is also stable. The pressure-dependence of the phase boundary curves has been studied for transitions such as crystal↔nematic LC↔isotropic melt by the high-pressure differential thermal analysis. The transition entropies have been obtained according to the Clapeyron relation. A combined use of the PVT data led to an estimate of the constant-volume transition entropies, which is in reasonable agreement with the conformational transition entropies estimated previously by the rotational isomeric state analysis of the spectroscopic data.