<title>Study of nematic and smectic order fluctuations in the nematic phase through proton magnetic relaxometry</title>

Abstract

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.

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation

“…On the contrary, in the crystal phase, the relatively higher activation energy associated with reorientations about the short axes thus seems to correlate with the existence of collective modes with higher cutoff wavelengths as reported in the case of nematics with underlying smectic organizations. 42,43 Such a finding supports the collective-like organizations in the crystal phase of 6CHBT with the mobility of the methyl group as is evident from the relatively sharp and narrow lines gauzed by high-field 1 H NMR spectra (Figure 6A). Furthermore, the observation of the ratio R greater than unity suggests the importance of the interaction of methyl protons with other protons stabilizing the steric hindrance of molecules 22 and augments the possibility of collective organizations probed via FFC NMR.…”

“…Rich polymorphism exhibited by thermotropic liquid crystals provides a challenge to probe complex motions in isotropic, nematic, smectic, and crystal phases. ,, Molecular dynamical processes in thermotropic liquid crystals encompass a frequency range from ultraslow (kHz) to ultrafast motions (MHz). , The ultraslow motions are in general collective motions that contribute to nuclear spin relaxation due to the orientation fluctuations of the local nematic order above the nematic–isotropic transition temperature ( T NI ), long-range order parameter fluctuations just below T NI , order director fluctuations (ODF) in the midnematic phase, local smectic organizations in the nematic phase, i.e., cybotactic clusters ( T > T AN ), layer undulations in the smectic-A phase, and slow molecular reorientations in solid phases, − while ultrafast motions cover noncollective motions such as translational self-diffusion and molecular reorientations about their short and long axes, including end chain rotations. − The cumulative effect of all these dynamical processes has a bearing influence on a nuclear spin probe, leading to specific frequency and thermal signatures on nuclear spin–lattice relaxation rates. − The molecular motions covering the kHz to MHz regime are in general amenable to the NMR relaxometry window tasked by fast field cycling (FFC) NMR (10 kHz–30 MHz) and high-field NMR (∼1 GHz). − Sizable work on molecular dynamics of isotropic, nematic, and smectic phases by employing NMR relaxometry is available elsewhere. − …”