By: João Figueirinhas
From: IST, Lisbon
At: Complexo Interdisciplinar, B3-01
The search for thermotropic liquid crystalline materials exhibiting the biaxial nematic phase attracts currently considerable interest, in part due to its potential technological application. In recent years, this phase has been detected in a number of liquid crystalline thermotropic systems. Among those, organosiloxane tetrapodes with side-on mesogenic groups have been studied by different experimental techniques, which consistently allowed for the identification of that phase in such materials.
Deuterium NMR spectroscopy is generally considered one of the key techniques for the identification of biaxial nematic ordering in liquid crystalline systems. The application of this technique, using different experimental methods, involves a special approach in the simulation of the spectral data when molecular movements with time-scale similar to the NMR observation times are present. Evidence for the effect of such slow molecular motions has been found in the analysis of deuterium NMR spectra of organosiloxane tetrapodes at low temperatures. The experimental results are particularly sensitive to such effect when a technique involving the continuous rotation of the NMR sample is applied. In the present work, the slow motions are associated to collective modes in both the uniaxial and biaxial nematic phases. The relevance of that type of molecular movements generally observed in liquid crystals at a timescale corresponding to the NMR observation time has been confirmed by relaxometry for dendrimers and in particular for organosiloxane tetrapodes. In this study it is observed that the collective modes have a very significant impact on the line shape of the spectra recorded with rotating samples and are responsible for a significant decrease of intensity of the NMR signal when the director is not aligned with nor orthogonal to the static B0 field. The model applies to NMR results previously reported, obtained both for static and rotating samples. The simulations yield consistent values for the order parameters of the system investigated through two independent sets of NMR experiments, with particular emphasis for the asymmetry parameter, characteristic of the biaxial nematic phase.