Proton spin-lattice relaxation in a liquid crystal-Aerosil complex above the bulk isotropization temperature

E. ANOARDO, F. GRINBERG, M. VILFAN, R. KIMMICH

CHEMICAL PHYSICS

Año: 2004 vol. 297 p. 99 - 110

0301-0104

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 ($T_{1 ho}^{-1}$), 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 (RMTD), accounting for molecular reorientations, and exchange losses of molecules from the surface to the bulk.