Journal
CHEMICAL PHYSICS
Volume 297, Issue 1-3, Pages 99-110Publisher
ELSEVIER
DOI: 10.1016/j.chemphys.2003.09.017
Keywords
field-cycling NMR relaxometry; rotating-frame spin-lattice relaxation; Aerosil; liquid crystal; order director fluctuations; Monte Carlo simulation
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We present a study of the molecular dynamics in an octylcyanobiphenyl (8CB)-Aerosil complex above the bulk isotropization temperature. Using proton nuclear magnetic relaxation experiments in the laboratory frame (T-1(-1)) and in the rotating-frame (T-1rho(-1)), we found a notable increase of the relaxation rates in the kHz frequency range as compared to the bulk 8CB liquid crystal at the same temperature. The field-cycling technique was used for the laboratory frame experiments while a conventional apparatus was used for the rotating frame method. The observed behavior is analyzed with the aid of Monte Carlo simulations on the basis of a two-phase fast-exchange model distinguishing surface-ordered and bulk phases. Two processes affecting the low frequency relaxation could be identified: reorientation mediated by translational displacements, accounting for molecular reorientations, and exchange losses of molecules from the surface to the bulk. (C) 2003 Elsevier B.V. All rights reserved.
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