A NMR study on zero electric field gradient nematic liquid crystals
Creators: name: Chandrakumar, Thambirajah
Publisher: name: University of British Columbia
Year: 2009

description: The role of intermolecular forces in describing the orientational nature of liquid crystals is not well understood. Previous studies using dideuterium as a solute in liquid crys- tals have demonstrated the importance for orientation of the interaction between the solute molecular quadrupole moment and the average electric field gradient present in liquid crystals. With the aim of learning aboilt additional orientational mechanisms, we have studied the orientation of solutes in special mixtures of liquid crystals, where the contribution from the environment to the average electric field gradient at the ²H nucleus of dideuterium is negligibly small. In order to understand the role of shortrange forces in such special mixtures, orientational studies have been undertaken in the mixtures 55 wt% ZLI- 1132(1132)/N- (-4-ethoxybenzylidene)-4’-butylariiline (EBBA), 56.5 wt% 1132/EBBA and 70 wt% 4-n-pentyl-4’-cyanobiphenyl (5CB)/EBBA. As a starting point, the C2v and D2h symmetry solutes meta dichlorobenzene, ortho dichlorobenzene, para dichlorobenzene, ortho dicyanobenzene, furan, tetrathiofulvalene and fiuorobenzene have been studied in the special mixtures 56.5 wt% 1132/EBBA at 323K and 70 wt% 5CB/EBBA at 316K, using proton NMR. The measured electric field gradient for these two mixtures has been found to be zero. The order parameters ob tained from an analysis of the NMR spectra indicate that the solutes experience a similar anisotropic potential in both mixtures. The results are interpreted in terms of a model for the short-range anisotropic potentials experienced by the solutes. To further explore the investigation of the short-range forces in zero electric field gradient mixtures, the temperature dependence of the solutes meta dichlorobenzene, ortho dichlorobenzene, 1,3-bromochlorobenzene, benzene and 2-butyne has been studied in the special mixtures 55 wt% 1132/EBBA and 70 wt% 5CB/EBBA, using proton NMR. These solutes vary from each other in symmetry and shape. The aim was to see how the different shaped solutes experience the short-range forces in the liquid crystal mixtures. The results indicate that the solutes experience a similar anisotropic potential in both mixtures. The biaxial order parameters measured for the solutes meta dichlorobenzene, ortho dichlorobenzene, and 1,3-bromochlorobenzene have also been analysed to magnify the differences between the mixtures. To extend our understanding on the intermolecular forces among constituent liquid crystal molecules, a temperature dependence study of the liquid crystal 5CB — d19 as so lute has been undertaken in the three liquid crystal mixtures: 55 wt% 1132/EBBA, 56.5 wt% 1132/EBBA and 70 wt% 5CB/EBBA, using ²H — NMR. The study of SCB — d19 as solute has been used to compare the short-range interactions in these special mixtures. The spectra of 5CB — d19 in the two 1132/EBBA mixtures are equivalent, but are dif- ferent from those in the 5CB/EBBA mixture. The spectra in 55 wt% 1132/EBBA and 70 wt% 5CB/EBBA have been analysed using two different models for the short-range potential, and parameters of the models have been used to compare the potentials in the different mixtures. It has been shown that, for a given spectral splitting of the chain C1 deuteron, the reduced short-range potential is the same in all three mixtures studied. The spectral differences observed are a consequence of different nematic-isotropic phase transition temperatures combined with the effect of trans-gauche isomerization in the hydrocarbon chain.