Two-dimensional deuteron nuclear magnetic resonance of a polymer dispersed nematic liquid crystal

Abstract

The two-dimensional nuclear magnetic resonance (NMR) spectra and relaxation rates of polymer dispersed 4′-pentyl-4-cyanobiphenyl droplets deuterated in the β position of the hydrocarbon chain have been measured in the isotropic and nematic phases. A typical Pake power line shape pattern characteristic for a system of ‘‘bipolar’’ droplets with a random distribution of their symmetry axes in the external magnetic field has been found in the ω2 domain whereas the homogeneous linewidth was observed in the ω1 domain. The existence of a weakly orientationally ordered surface layer above the nematic–isotropic transition has been demonstrated. Whereas the deuteron T1/T2≂4.3 in the isotropic phase, T1/T2≂188 in the nematic phase for confined droplets. This effect, which is absent in the bulk, shows that T2 is here mainly determined by translationally induced rotation in the nonuniformly oriented director field inside the cavity. T1, on the other hand, is determined by the slowing down of local molecular reorientations at the surface of the confined droplet as well as by local molecular fluctuations and rotations within the randomly oriented droplets.