Deuterium NMR and molecular ordering in the cholesteryl alkanoates mesophases

Abstract

Deuterium NMR of the smectic, cholesteric, and the blue phases of two members of the cholesteryl alkanoate series, viz. cholesteryl nonanoate and cholesteryl myristate, are reported. Deuterium spectra of probe molecules dissolved in these liquid crystals are also discussed. The measurements were made at 63 kG (42 MHz). At this high field both the smectic and cholesteric phases can be fully or partially aligned with the director oriented predominantly perpendicular to the magnetic field. The results provide information on the order parameter in the various phases and on the ordering process of the helical axis. It is found that the smectic mesophase forms well oriented domains, but that the cholesteric phase consists of two types of domains: (i) uniformly oriented domains with the helical axis parallel to the magnetic field and (ii) randomly oriented domains apparently with focal conic texture. The relative abundance of the two types of domains depends on the thermal history of the sample and other parameters. The spectralline shapes of the randomly oriented cholesteric samples depend on the molecular diffusion rate and on the cholesteric pitch length. A theory of the magnetic resonanceline shape due to reorientational diffusion of the director about the cholesteric pitch axis is presented and used for the quantitative interpretation of the experimental results.