ORDER FLUCTUATIONS OF THE DIRECTOR IN NEMATIC THERMOTROPIC LIQUID-CRYSTALS STUDIED BY NUCLEAR-MAGNETIC-RESONANCE DIPOLAR RELAXATION

Larmor frequency dependent NMR studies of dipolar order relaxation in liquid crystals have seldom been tried in the past. Using conventional static magnetic field techniques, the experiment cannot be extended t o the low Larmor frequency (nu(L)) regime due to limitations in the si gnal-to-noise ratio of the dipolar echo. In this work, we present an e xperimental study of the dipolar relaxation time in the frequency rang e 10(3)-7 x 10(7) Hz in nematic thermotropic liquid crystals. To exten d the study to such low frequencies, we used the Jeener-Broekaert puls e sequence combined with fast field-cycling NMR technique. For frequen cies higher than 10(5) Hz, the dipolar relaxation time T-1D(nu(L)) fol lows the nu(L)(1/2)-law that is characteristic of order fluctuations o f the director (OFD) in nematics. In contrast, the Zeeman relaxation i s driven by faster and less correlated motions, specially in the MHz f requency range. The relaxation of dipolar energy was measured to be re markably faster than the one predicted by the usual semiclassical mode l of isolated spin pairs. Conceivably, the failure of the usual two-sp in model should be sought in the absence of multispin interactions and multispin correlations. We propose that the OFD are the dominant rela xation mechanism for the dipolar order, even in the MHz regime. This r esult turns T-1D(nu(L)) experiment in a useful NMR technique for the s tudy of slow molecular dynamics in mesophases. (C) 1998 American Insti tute of Physics.