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Behavior of Liquid Crystals Confined to Mesoporous Materials as Studied by ¹³C NMR Spectroscopy of Methyl Iodide and Methane as Probe Molecules

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Pekka Tallavaara and Jukka Jokisaari*

Department of Physical Sciences, NMR Research Group, University of Oulu, P. O. Box 3000, FIN-90014 University of Oulu, Finland

J. Phys. Chem. B, 2008, 112 (3), pp 764–775

DOI: 10.1021/jp076840i

Publication Date (Web): January 1, 2008

Section:

Crystallography and Liquid Crystals

Abstract

The behavior of thermotropic nematic liquid crystals (LCs) Merck Phase 4 and ZLI 1115 confined to mesoporous controlled pore glass materials was investigated using ¹³C nuclear magnetic resonance spectroscopy of probe molecules methyl iodide and methane. The average pore diameters of the materials varied from 81 to 375 Å, and the temperature series measurements were performed on solid, nematic, and isotropic phases of bulk LCs. Chemical shift, intensity, and line shape of the resonance signals in the spectra contain lots of information about the effect of confinement on the state of the LCs. The line shape of the ¹³C resonances of the CH₃I molecules in LCs confined into the pores was observed to be even more sensitive to the LC orientation distribution than, for example, that of ²H spectra of deuterated LCs or ¹²⁹Xe spectra of dissolved xenon gas. The effect of the magnetic field on the orientation of LC molecules inside the pores was examined in four different magnetic fields varying from 4.70 to 11.74 T. The magnetic field was found to have significant effect on the orientation of LC molecules in the largest pores and close to the nematic−isotropic phase transition temperature. The theoretical model of shielding of noble gases dissolved in LCs based on pairwise additivity approximation was utilized in the analysis of CH₄ spectra. For the first time, a first-order nematic−isotropic phase transition was detected to take place inside such restrictive hosts. In the larger pores a few degrees below the nematic−isotropic phase transition of bulk LC the ¹³C quartet of CH₃I changes as a powder pattern. Results are compared to those derived from ¹²⁹Xe NMR measurements of xenon gas in similar environments.

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History

Published In Issue January 24, 2008
Received August 27, 2007
Revised November 2, 2007

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Article

Behavior of Liquid Crystals Confined to Mesoporous Materials as Studied by ¹³C NMR Spectroscopy of Methyl Iodide and Methane as Probe Molecules

Abstract