NMR Investigation of Order Fluctuations, Self-Diffusion and Rotational Motions in the Nematic Liquid Crystal MBBA by T1-Relaxation Spectroscopy

Abstract

Abstract We report on measurements of the proton spin T1 relaxation dispersion at various temperatures in the nematic phase of the liquid crystal MBBA in the Larmor frequency range from 2 kHz to 270 MHz, which exceeds previous studies by more than 3 orders of magnitude. The new results cannot be interpreted in terms of either order fluctuations or self-diffusion as recently proposed by Doane et al.1 and Blinc et al.2, respectively. Instead, the dispersion and its temperature dependence indicate the significance of at least three relaxation mechanisms, namely order fluctuations (OF), self-diffusion (SD) and rotation of the molecular ellipsoids about the short axis (R). The combined OF-SD-R model presented in this work allows a quantitative analysis of the experimental findings! The correlation times and activation energies of the three molecular reorientations evaluated from the T1 dispersion by means of a computer optimization procedure are in essential agreement with data provided by other spectroscopic methods [light scattering, tracer technique, dielectric relaxation], but differ from former NMR conclusions.