Characterization of liquid crystals in porous materials by means of NMR of probe atoms and molecules

• Main Author: Tallavaara, P. (Pekka)
• Format: Doctoral Thesis
• Language: English
• Published: University of Oulu 2008
• Subjects: ¹²⁹Xe; CPG; chemical shift anisotropy; liquid crystal; methane; methyl iodide; nuclear magnetic resonance spectroscopy; porous material; shielding anisotropy
• Online Access: http://urn.fi/urn:isbn:9789514287848; http://nbn-resolving.de/urn:isbn:9789514287848

Abstract The present thesis describes a method for characterization of liquid crystals in confined spaces by means of NMR of probe atoms and molecules. 129Xe isotope enriched xenon gas and 13C isotope enriched methyl iodide and methane were used as probes. Behavior of solutes and liquid crystals confined to porous materials was investigated using 129Xe and 13C NMR spectroscopy. Uniaxial nematic liquid crystals Phase 4 and ZLI 1115 were used as a medium. Controlled pore glass with well defined and known properties was used as a porous material. The behavior of liquid crystals and solutes in various different size pores, temperatures and magnetic fields at different solute concentrations was explained. The average pore diameter of the material varied from mesopores to macropores. The studied temperature range covered solid, nematic and isotropic phases of bulk liquid crystals, and the highest magnetic field was 2.5 times stronger than the lowest one used (4.70 T). The chemical shifts, intensities, and line shapes of the resonance signals from the solutes were observed to contain lots of information about the effect of confinement on the state of the liquid crystals. Especially the line shape of the 13C resonances of methyl iodide was observed to be very sensitive to the liquid crystal orientation distribution in the pores. By varying experimental conditions the relative contribution of field and the surface forces of pore walls to the orientation of liquid crystal molecules inside the pores was seen to change quite drastically. In addition, it was also observed that when the sample is cooled very rapidly, xenon atoms do not squeeze out from the freezing medium but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. Furthermore, because solutes experienced on average isotropic environment inside the smallest pores, isotropic value of the shielding tensor could be determined at exactly the same condition and temperature as anisotropic counterpart between the pore particles. Thus, for the first time in the solution state, shielding anisotropies could be determined as a function of temperature.