Low-frequency proton NMR relaxometry of a polymer dispersed liquid crystal above TNI

Using proton NMR relaxometry in the kilohertz frequency range, we study dynamics of 5CB liquid crystal molecules dispersed in the form of spherical microdroplets in a PDLC material. The focus of the study is the spin-lattice relaxation in the rotating frame, T-1rho(-1), measured above the nematic-isotropic transition T-NI. We show that the relaxation rate T-1rho(-1) - when induced by uniform molecular translational diffusion in a spherical cavity - depends on the strength of the rotating magnetic field as T-1rho(-1) proportional to omega(1)(-alpha) where alpha varies between 0.7 and 1, depending on the thickness of the ordered surface layer. This relaxation mechanism governs mainly the transverse spin relaxation, whereas the measurements of the frequency and temperature dependence of T-1rho(-1) indicate a strong effect of slowing-down of molecular translational diffusion in contact with the polymer surface and yield the average dwell-time of molecules at the surface of the order 10(-5) s. (C) 2003 Elsevier Science Inc. All rights reserved.