Proton spin–lattice relaxation in nematic microdroplets

Abstract

The frequency and temperature dependences of the proton spin–lattice relaxation rate have been studied in nematic droplets embedded in a solid polymer matrix. The comparison of these data to those for bulk nematic and pure polymer samples shows that cross-relaxation between liquid crystal and polymer protons dominates the liquid crystal relaxation in the MHz frequency region, while at low frequencies the translationally induced molecular rotation becomes important. The cross-relaxation rate is found to be ≈103 s−1 giving an order of magnitude estimate 10−4–10−3 s for the time for which a liquid crystal molecule is bonded to the surface.

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

“…Proton [11,15,16] and deuteron [17,18] NMR relaxation studies have been used to analyze the influence of the large pore-wall interface and the restricted geometry on the molecular dynamics. Specific relaxation channels arise in confined liquid crystals in addition to the relaxation mechanisms existing in the bulk sample.…”

“…Moreover, the conventional relaxation mechanisms are modified. In a polymer-based matrix the process of proton cross-relaxation at the liquid crystal-polymer interface has been shown to be important [1,15,16,19]. Molecular anchoring at liquid crystal-pore interface influences the near-surface molecular dynamics.…”

“…This effect is particularly pronounced at the temperatures above the nematic-isotropic liquid transition [17]. Director orientational nonuniformity, induced by confinement, results in a relaxation mechanism known as translational diffusion-induced molecular reorientation (TR) [1,11,15,17]. Besides that, the change of molecular mobility at the pore surface causes a change in the spectrum of the order director fluctuation (ODF): wavelengths longer than the pore size are eliminated [15,20].…”

Rotational friction at the molecular level

“…All these experimental studies probe average dynamic properties. Some experiments probe the dynamics of the surface director [2,4,7,17,18] while others probe the dynamics of liquid crystals confined in pores with techniques that are not surface specific [3,5,6,[8][9][10][11][12][13][14][15][16]. These confined systems also exhibit dynamic effects due to the confinement geometry [8][9][10]19] making the effect of the surfaces themselves difficult to distinguish.…”

“…The ratio between surface and bulk relaxation times varies from 2 to 10 3 [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and the value of the surface viscosity associated with the reorientation of the so-called surface director, i.e. the mean molecular axis just outside the interface between the limiting wall and the bulk (see fig.…”