Proton NMR study of CBOOA, NBOOA, and their chain-deuteriated homologues in smectic A_d and nematic mesophases

Abstract

The nature of the smectic A_d to nematic phase transition and the modes of molecular motion were investigated for the homologous mesogens, CBOOA, NBOOA, and their chain-deuteriated compounds. In NBOOA, the S_{A d}[sbnd]N phase transition was found to be first order with a pronounced pretransitional decrease of the orientational order parameter, <P ₂>, both above and below the transition point, while in CBOOA the anomaly in <P ₂> was very small. Despite the different behaviours in terms of the orientational order, the dynamical aspects of molecular motion, probed by the-NMR relaxation, are quite similar. The motional mode characteristic of the N phase is order director fluctuation, but translational self-diffusion becomes the dominant relaxation mechanism in the S_{A d} phase. It is also recognized that the relative importance of these mechanisms changes in a continuous manner as a function of temperature.