Quasielastic scattering of neutrons by a liquid crystal substance with flexible molecules

Abstract

When interpreting quasielastic neutron scattering (QNS) spectra of liquid crystals with complicated and flexible molecules, it seems absolutely necessary to take into consideration the internal reorientations. The present QNS study concerns TCDCBPh/di-(4-n-butyloxyphenyl) trans-cyclohexane-1,4-dicarboxylate). It contains a cyclohexane ring which possesses a high internal mobility. As a probing parameter we use the ‘excess elasticity’ p which in a way represents the elastic form-factor. We make drastic assumptions: we equalize all partial correlation times, we allow each hydrogen atom to participate in one reorientation only, and we ignore possible couplings between reorientations. Then we analyse the behaviour with temperature of the p parameter for eleven models in which either all specified molecular fragments reorient or some of them reorient and others are at rest. In the crystal, far below melting, all internal stochastic mobilities seem to be at rest. However, still 30°C below melting, the cyclohexane ring starts large angle internal deformations which have a stochastic character. It seems that some fragments in the molecules (e.g., benzene rings and/or alkyl terminal groups) start to reorient a few degrees before melting. This is connected with the formation of a ‘mobile’ crystal phase, just before melting. Then all internal reorientations become free, and one cannot observe much difference between the smectic A phase and the nematic phase. This study shows that the cyclohexane ring is the most mobile sub-entity in the TCDCBPh molecule. Observations by polarized microscopy seem to corroborate our claim for the existence of a “mobile” phase just before melting. The literature information about the phase sequence is C 107°C S_A 156°C N 209°C I.