Molecular and relative segmental order in the nematic and smectic C phases: A nuclear magnetic resonance study

Abstract

Order parameters expressing the molecular orientational order as well as the time average conformation of the molecule are measured, some for the first time, in the nematic, N, and smectic C, Sc, phases. Deuterium spectral splittings from the compounds 4-n-octyl-d17-oxybenzoic acid-d1 (80BA-d18) and 4-n-heptyl-d15-oxybenzoic acid-d1 (70BA-d16) are interpreted in terms of parameters which are time averages of spherical harmonic functions of order 2. There are three parameters measured for each deuterated molecular segment which describe its orientational order relative to one particular segment of the molecule. The time average conformation of the molecule is therefore described in terms of the relative order of the individual segments and hence these conformation parameters. These parameters show that the wagging motion of the hydrocarbon chain is not cylindrically symmetric. In addition, the conformation parameters are shown to be temperature independent within experimental error. The temperature dependences of the observed spectral splittings are governed instead by that of three molecular order parameters describing the orientational order of the molecule as a whole. Two molecular order parameters are measured in the nematic phase which give the degree of order of the molecular long axis and the anisotropy in that order, respectively. The onset of a third molecular order parameter is observed at the N–Sc phase transition. The data appear to be consistent with a molecular model which is, on the time average, rigid and exhibits a hindered rotation about its long axis in both the N and Sc phases.