Study of nematic and smectic order fluctuations in the nematic phase through proton magnetic relaxometry

摘要

Proton Magnetic relaxation dispersion (PMRD) measurements were made, at several temperatures, in the nematic phase of the liquid crystal 8OCB employing field cycling NMR technique. PMRD data show that pretransitional effects dominate the relaxation mechanism within about two degrees from the isotropic - nematic transition temperature. The nematic director fluctuations essentially determine the relaxation mechanism at lower temperatures. The presence of short range smectic organizations, and a rapid increase in the size of these clusters at the nematic-smectic transition, could be observed through these PMRD studies. While in the mid-nematic phase these clusters are relatively smaller and isotropic in shape, their size grows by more than an order of magnitude and they become anisotropic in shape as nematic - smectic transition is approached. The information regarding the other dynamic processes, such as molecular self-diffusion and reorientations, which also contribute to the spin relaxation, is also obtained through these studies.