Experimental Results and Spectra Simulations

° rotation technique and spectra of a static distribution of nematic domains

The 90° rotation technique was used in a sample of the organosilox- ane tetrapode Tas, mixed with the nematic deuterated probe 7CBad2. The chemical structure of Tas and of the probe and the phases' sequence of the mixture (15% weight) is shown in Fig. 8.3. The isotropic-nematic phase transition is detected both by polarizing optical microscopy and differential scanning calorimetry (DSC). The Nu-Nb phase transition in the mixture is observed by deuterium NMR spectroscopy, as discussed below (Cruz et al., 2008; Figueirinhas et al., 2005). The phases' sequence of the pure compound Tas is shown in Fig. 9.4 (Filip et al., 2010).

The deuterium spectra, obtained experimentally by means of the 90° rotation technique, on the Tas compound, using the deuterated probe are presented in Fig. 8.10a,b. The respective theoretical simulations, that allow for the determination of the most relevant physical parameters in the problem (vq and n), are also shown in the figure (red line).

In the case of the 90° rotation technique the simulated spectra can be described by the function

with given by Eq. 8.4 and P (^) (^ = (в, L(rn) is an appropriate function that describes the NMR line shape (typically a Lorenzian function):

where A is the half-width at half-maximum.