The proton spin-lattice relaxation time T1, in the nematic liquid crystal 4-pentyl-4′-cyanobiphenyl confined in a glassy porous matrix has been measured in a wide Larmor frequency range of 1 · 102−2 · 107 Hz employing the fast field-cycling NMR technique. A strong influence of the restricted geometry on the character of the T1 dispersion was found. Our investigation clearly demonstrates the importance of the translationally induced molecular reorientations in inhomogeneous director field for the relaxation in the samples with 200 and 80 nm mean pore size. The experimental results are in a good agreement with the theoretical predictions. In the sample with 7 nm pore size the main contribution to the relaxation is ascribed to the slowing down of the molecular motion in the near-surface layer. Zero-field 1H NMR spectra of a microconfined liquid crystal are reported for the first time.

A field-cycling NMR study of nematic 4-pentyl-4′-cyanobiphenyl confined in porous glasses

Liquid crystals (LCs) confined to structures of submicrometer characteristic lengths exhibit many interesting phenomena related to their mesoscopic size and large surfaceto-volume ratio [1, 2]  They represent an important field of current research both from the fundamental point of view and for applications in displays, electro-optical shutters, and telecommunications Since the discovery of polymer dispersed liquid crystal materials (PDLCs) in 1986 [3], the interest in studying microconfined liquid crystals (first initiated by applications in electro-optical devices) has been steadily increasing However, a rich variety of unexplained phenomena remains a driving force in this field of research which currently is undergoing rapid development

Low-Frequency NMR Relaxometry of Spatially Constrained Liquid Crystals

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, T1 1 , measured above the nematic-isotropic transition TNI. We show that the relaxation rate T1 1 - when induced by uniform molecular translational diffusion in a spherical cavity - depends on the strength of the rotating magnetic field as T1 1 1 where 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 T1 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. © 2003 Elsevier Science Inc. All rights reserved.

Invited Lectures Low-frequency proton NMR relaxometry of a polymer dispersed liquid crystal above T NI

Measurements of the proton cross-relaxation rate in the polymer dispersed 4-pentyl4-cyanobiphenyl (5CB) droplets were performed by the selective magnetization inversion NMR technique. This method makes possible direct determination of the magnitude of the fast cross-relaxation rate in theMHz regime where the standard spin-lattice relaxation measurements only indicate its presence. In the isotropic phase, the cross-relaxation rate is found to be more than ten times larger in the droplets of pure 5CB than that reported earlier for the liquid crystal mixture E7 in the same polymer. The difference is discussed in terms of dynamic processes in the surface layer.

O. Jarh

Papers

A note on proton cross-relaxation in polymer dispersed liquid crystals