Incoherent Neutron Quasi-Elastic Scattering (NQES) as a Tool to Study Molecular Ordering in Liquid Crystals

Abstract

It is shown that high resolution incoherent neutron quasi-elastic scattering (NQES) may be used, via the measurement of the elastic incoherent structure factor (EISF), as a new tool to study molecular ordering in liquid crystals. The EISF is a bilinear combination of all the order parameters (P₁ ) and its experimental value can be deduced from an intensity measurement. As an example, we measure from the NQES spectra, the EISF of powder smectic A TBBA at 184 [ddot]C. These data are analysed in terms of two microscopic models (molecules rotating around their long axis and (i) fluctuating about the director: model 1, (ii) precessing on a cone about the director: model 2). It is shown that from the powder results only, we cannot discriminate between these two models, but that model 2 can explain other published experimental results (in particular some Raman scattering of light data) whereas model 1 cannot. Some conjectures about model 2 are made.